TY - JOUR
AU - Zhang,, Xin-Rong
AB - Abstract Optimizing the ventilation design of packaging system is of crucial importance for improving the efficiency of the forced-air precooling process to maintain the quality of horticultural produce and extend the shelf life in food cold chain. Many efforts had been devoted to the study about the impact of ventilation design on airflow and temperature distribution inside ventilated packages. This paper reviews relevant research methods, commonly used quantities for the measurement of precooling effectiveness, attractive design parameters, and their impact on precooling effectiveness. These allow us to know exactly the characteristic and deficiency of each research method, identify dominant design parameters, and seek a promising way for the future improvement of the ventilated packaging system. horticultural produce, precooling effectiveness, ventilation design, forced-air precooling technique, packaging system Introduction As perishable foods, horticultural products are regional and seasonal. Consequently, postharvest handling is a very important step in maintaining the quality of agricultural produce and extending the shelf life (Dehghannya et al., 2010; Yan et al., 2019). Respiration and transpiration of horticultural produce are important causes for loss of organic material and moisture (Verboven et al., 2004b; Ambaw et al., 2013; Aghdam et al., 2018). There are many factors influencing the physiological and biological changes of horticultural produce (Brosnan and Sun, 2001; Aghdam et al., 2019; Xu et al., 2019), such as product thermal properties, temperature, concentration of ethylene, oxygen, carbon dioxide, etc. It is noteworthy that temperature is the most important environmental factor affecting the postharvest life of horticultural produce. For example, if fresh strawberries are left for 2 h in the 30°C heat, only 80% of the fruits are considered suitable for sale (Mitchell et al., 1972). Among the many postharvest technologies, precooling of horticultural produce is the most important technique for maintaining desirable, fresh, and marketable products. It can remove field heat from fresh agricultural produce, thereby slowing down metabolism and reducing deterioration before storage and transportation (Dehghannya et al., 2010). The average mass loss of the cold chain without precooling is about 23% more than that of the cold chain based on precooling (Wu and Defraeye, 2018). Forced-air cooling is one of the most widely used precooling techniques due to its advantages of rapid cooling rate, high efficiency, and low cost (Kader, 2002; Vigneault and Goyette, 2002; Delele et al., 2010). The schematic diagram of the forced-air precooling system is shown in Figure 1. This technique uses a fan to generate the necessary driving force to create a pressure difference, which forces the cold air through the inside of the box. The cold air removes heat from fruits and vegetables when passing through the ventilated packaging system, and thereby achieves the goal of rapid precooling. Figure 1. Open in new tabDownload slide Typical forced-air cooling system of horticultural produce (Delele et al., 2010). Figure 1. Open in new tabDownload slide Typical forced-air cooling system of horticultural produce (Delele et al., 2010). Cooling heterogeneity is a prevalent phenomenon during the forced-air precooling process (Alvarez and Flick, 1999a; Alvarez et al., 2003), and consequently the ventilation design of packaging system has been a research hotspot in the past 20 years. The adoption of packaging system keeps horticultural produce clean, protects horticultural produce from mechanical damage, minimizes moisture loss, and delays microbial decay (Ngcobo et al., 2012). However, the packaging material also hinders the direct contact between horticultural produce and the cooling airflow and consequently increases the resistance of the cooling airflow and the energy consumption of the precooling process (Delele et al., 2013a, 2013b). Therefore, the ventilation design of the packaging system should balance the relationship between the cooling performance and the mechanical strength of packages. At the same time, the complex internal structure of the package filled with trays and products complicates the airflow and heat transfer characteristics, resulting in serious cooling heterogeneity, such as local under-cooling and over-cooling of products. These heterogeneous phenomena threaten the quality of horticultural produce and the shelf life in food cold chain. Therefore, the optimization of the ventilation design of the packaging system is an active topic in the food industry. This review focuses on available studies about the influence of ventilation design (airflow rate, vent area, vent position, etc.) on the precooling effectiveness of horticultural produce. The rest of the review paper is organized as follows. In the Available Research Methods section, available research methods about the forced-air precooling process of horticultural produce in the ventilated packaging system are summarized, along with some representative researches and results. Both the merits and flaws of each method are described. In the Impact of Ventilation Design on Precooling Effectiveness section, available discussions about the impact of ventilation design on precooling effectiveness are elaborated. Finally, the research trends and future challenges about the ventilation design of packaging system are pointed out in the Conclusions section. Available Research Methods In order to ensure the quality and safety of horticultural produce and extend the storage and shelf life, the critical step in the postharvest cold chain is to rapidly precool horticultural produce by removing field heat (Han et al., 2015). Poor postharvest handling techniques (mainly poor temperature management) can affect the rate of respiration and thereby lead to decay of horticultural produce and postharvest losses (Saenmuang et al., 2012). The postharvest loss and waste in fruit and vegetable production supply chains even can be as high as 13%–38% before reaching consumers (Defraeye et al., 2015). Experimental measurement dominated the early research on the forced-air precooling of horticultural produce. Many efforts had been devoted to the experimental study about the impact of the ventilation design of packaging system on the precooling effectiveness of fresh fruits and vegetables. The most attractive design parameters include airflow rate, vent area, vent position, internal tray, line film, etc. The frequently encountered parameters defined to measure precooling effectiveness include cooling rate, cooling uniformity, air pressure drop, energy consumption, half-cooling time, seven-eighths cooling time, heat transfer coefficient, cooling efficiency, etc. Some representative experimental researches on the impact of ventilation design in the forced-air precooling process of horticultural produce are summarized in Table 1. In order to ensure the reliability of test data obtained in repeated experiments and in the meanwhile reduce experimental cost, simulators (Castro et al., 2004a, 2004b, 2005; Vigneault et al., 2005, 2006, 2007) were usually used in experimental studies rather than horticultural produce, such as solid polymer balls, polychlorinated vinyl balls filled with water, etc. In the rest few experiments (Leyte and Forney, 1999; Fikiin et al., 1999; Anderson et al., 2004; Vigneault et al., 2004a, 2004b; Ferrua and Singh, 2011; Ngcobo et al., 2012; Mukama et al., 2017; Wu et al., 2018), the precooling process of horticultural produce, such as table grapes, strawberries, blueberry, etc., was studied. In available experimental studies, the typical package types include carton, clamshell, wooden container, container formed by acrylic plates, etc. The main results obtained in some representative researches are summarized in Table 1. Due to the complex internal structure of ventilated packages filled with trays and products, some physical phenomena are difficult to measure with the high spatial and temporal resolution, such as airflow and heat transfer characteristics, convective heat transfer coefficient, and temperature change of the entire product. However, there were still many researchers who made unremitting efforts for accurate measurements. Vigneault et al. (2007) developed the Square Cross-Section Velocity (SCSV) measurement method and proved that SCSV can provide a more accurate measurement of airflow rates than the previously developed Circular Approach Velocity method. Ferrua and Singh (2011) used nonintrusive flow measurement techniques [Particle Image Velocimetry (PIV)] to determine the internal airflow field in a package with a container-to-product diameter ratio of less than 10. The use of PIV provided a valuable understanding and quantitative description of the local behaviour of fluid flow inside the packaging system. Nevertheless, experimental studies for improving ventilation design had been limited due to the fluctuations in physical properties of horticultural produce in repeated experiments, the difficulties in handling biological materials, and the inherent drawbacks of experimental studies, such as expensive, time-consuming, etc. Table 1. Representative experimental researches and results about the impact of ventilation design in the forced-air precooling of horticultural produce Reference . Produce or simulator used . Container or package type . Design parameters . Effectiveness parameters . Leyte and Forney, 1999 Highbush blueberry Plastic clamshell Vent position Cooling time Remark: Results indicate that the most rapid cooling occurs in the clamshells with vents on the top. While adding vents at the bottom has no effect on the cooling rate. Fikiin et al., 1999 Five horticultural produce Trays of pine wooden material Airflow rate Heat transfer coefficient Remark: Results indicate that the surface heat transfer coefficient of the fruit increases with the airflow rate. Anderson et al., 2004 Strawberry Different clamshell and tray combinations Vent area; clamshell and tray combinations Cooling time Remark: Results indicate that the integral optimization of the combination of clamshell and tray is an effective way to maximize the contact between the cold air and the product. Vigneault et al., 2004a, 2004b 25 different horticultural produce Wooden container Product size; product shape; vent position Air pressure drop Remark: Results indicate that both the size and shape of horticultural produce have a significant impact on the pressure drop. In order to achieve the goal of minimizing the pressure drop, the vents should be uniformly distributed on the sides (perpendicular to airflow) of the container. Castro et al., 2004a Solid polymer ball Container formed by acrylic plates Airflow rate; vent area Cooling efficiency; air pressure drop Remark: Results indicate that the increase of total vent area (0.67%, 2%, 6%) leads to an enhancement of cooling efficiency, while increasing the airflow rate (from 0.125 to 3.9 L/(s*kg)) results in a greater air pressure drop. In order to obtain a relatively satisfactory cooling efficiency, the opening area should be more than 6%. Castro et al., 2004b Polychlorinated vinyl ball filled with water/agar-agar solution Wooden tunnel Airflow rate; vent area Cooling time; cooling rate; cooling uniformity; air pressure drop; energy consumption Remark: Results indicate that the airflow rate is an important factor influencing the half-cooling time. At the maximum airflow rate, neither vent area nor vent position has significant impacts on half-cooling time. Using the ventilated package with the total vent area of 14% has the same cooling rate as the fully open structure. Although increasing airflow rate compensates for the negative effects of the low vent area, it also increases the pressure drop and energy consumption of the precooling process. Vigneault et al., 2005 Solid polymer ball Container formed by acrylic plates Vent area; vent position Cooling rate; cooling uniformity; energy consumption Remark: Results indicate that in the viewpoint of optimizing energy consumption, the optimal vent area should be between 8% and 16%, and the vents should be avoided as far as possible in the corner. Castro et al., 2005 Solid polymer ball Container formed by acrylic plates Vent area; vent position Cooling rate; cooling uniformity; air pressure drop Remark: Results indicate that cooling uniformity increases as the vents move from the corner of ventilated package to the centre of the package surface. Vigneault et al., 2006 Solid polymer ball Container formed by acrylic plates Slat width; airflow rate; vent area; vent position Cooling time; cooling uniformity; air pressure drop Remark: Results indicate that the effect of vent position on cooling uniformity is pronounced at low airflow rate, in comparison with vent area. The vent area and slat width are suggested to be larger than 2.4% and 200 mm, respectively. Vigneault et al., 2007 Solid polymer ball Container formed by acrylic plates Airflow rate; vent area Cooling rate Remark: This research developed the Square Cross Section Velocity measurement method, which can measure the airflow rate more accurately than the previously developed Circular Approach Velocity method. Ferrua and Singh, 2011 Strawberry Clamshell Vent area; design of tray Cooling rate; cooling uniformity; energy consumption Remark: Results indicate that the optimal design of tray has a considerable contribution to the improvement of precooling effectiveness. Ngcobo et al., 2012 Table grape Carton Different components in the package Cooling rate; airflow resistance Remark: Results indicate that the line films have the greatest resistance to airflow compared to other components of the grape package. Mukama et al., 2017 Pomegranate Carton Plastic liner; stack orientation Cooling rate; energy consumption Remark: Results indicate that the direction of the stack (relative to the direction of the cooling airflow) affects energy consumption. The plastic liner has the greatest impact on the precooling process, and the energy consumption is up to three times higher than the unlined stack. Wu et al., 2018 Citrus fruit Carton Packaging type; wrapping; fruit size Cooling rate; cooling uniformity Remark: Results indicate that cooling heterogeneity mainly occurs in the flow direction. Fruit wrapping significantly reduces the cooling rate and increases cooling heterogeneity. The Nova mandarin fruit in the open top carton cooled 24% faster on the inflow side of the pallet and 42% faster on the outflow side than the Eureka lemon in the Supervent carton. Reference . Produce or simulator used . Container or package type . Design parameters . Effectiveness parameters . Leyte and Forney, 1999 Highbush blueberry Plastic clamshell Vent position Cooling time Remark: Results indicate that the most rapid cooling occurs in the clamshells with vents on the top. While adding vents at the bottom has no effect on the cooling rate. Fikiin et al., 1999 Five horticultural produce Trays of pine wooden material Airflow rate Heat transfer coefficient Remark: Results indicate that the surface heat transfer coefficient of the fruit increases with the airflow rate. Anderson et al., 2004 Strawberry Different clamshell and tray combinations Vent area; clamshell and tray combinations Cooling time Remark: Results indicate that the integral optimization of the combination of clamshell and tray is an effective way to maximize the contact between the cold air and the product. Vigneault et al., 2004a, 2004b 25 different horticultural produce Wooden container Product size; product shape; vent position Air pressure drop Remark: Results indicate that both the size and shape of horticultural produce have a significant impact on the pressure drop. In order to achieve the goal of minimizing the pressure drop, the vents should be uniformly distributed on the sides (perpendicular to airflow) of the container. Castro et al., 2004a Solid polymer ball Container formed by acrylic plates Airflow rate; vent area Cooling efficiency; air pressure drop Remark: Results indicate that the increase of total vent area (0.67%, 2%, 6%) leads to an enhancement of cooling efficiency, while increasing the airflow rate (from 0.125 to 3.9 L/(s*kg)) results in a greater air pressure drop. In order to obtain a relatively satisfactory cooling efficiency, the opening area should be more than 6%. Castro et al., 2004b Polychlorinated vinyl ball filled with water/agar-agar solution Wooden tunnel Airflow rate; vent area Cooling time; cooling rate; cooling uniformity; air pressure drop; energy consumption Remark: Results indicate that the airflow rate is an important factor influencing the half-cooling time. At the maximum airflow rate, neither vent area nor vent position has significant impacts on half-cooling time. Using the ventilated package with the total vent area of 14% has the same cooling rate as the fully open structure. Although increasing airflow rate compensates for the negative effects of the low vent area, it also increases the pressure drop and energy consumption of the precooling process. Vigneault et al., 2005 Solid polymer ball Container formed by acrylic plates Vent area; vent position Cooling rate; cooling uniformity; energy consumption Remark: Results indicate that in the viewpoint of optimizing energy consumption, the optimal vent area should be between 8% and 16%, and the vents should be avoided as far as possible in the corner. Castro et al., 2005 Solid polymer ball Container formed by acrylic plates Vent area; vent position Cooling rate; cooling uniformity; air pressure drop Remark: Results indicate that cooling uniformity increases as the vents move from the corner of ventilated package to the centre of the package surface. Vigneault et al., 2006 Solid polymer ball Container formed by acrylic plates Slat width; airflow rate; vent area; vent position Cooling time; cooling uniformity; air pressure drop Remark: Results indicate that the effect of vent position on cooling uniformity is pronounced at low airflow rate, in comparison with vent area. The vent area and slat width are suggested to be larger than 2.4% and 200 mm, respectively. Vigneault et al., 2007 Solid polymer ball Container formed by acrylic plates Airflow rate; vent area Cooling rate Remark: This research developed the Square Cross Section Velocity measurement method, which can measure the airflow rate more accurately than the previously developed Circular Approach Velocity method. Ferrua and Singh, 2011 Strawberry Clamshell Vent area; design of tray Cooling rate; cooling uniformity; energy consumption Remark: Results indicate that the optimal design of tray has a considerable contribution to the improvement of precooling effectiveness. Ngcobo et al., 2012 Table grape Carton Different components in the package Cooling rate; airflow resistance Remark: Results indicate that the line films have the greatest resistance to airflow compared to other components of the grape package. Mukama et al., 2017 Pomegranate Carton Plastic liner; stack orientation Cooling rate; energy consumption Remark: Results indicate that the direction of the stack (relative to the direction of the cooling airflow) affects energy consumption. The plastic liner has the greatest impact on the precooling process, and the energy consumption is up to three times higher than the unlined stack. Wu et al., 2018 Citrus fruit Carton Packaging type; wrapping; fruit size Cooling rate; cooling uniformity Remark: Results indicate that cooling heterogeneity mainly occurs in the flow direction. Fruit wrapping significantly reduces the cooling rate and increases cooling heterogeneity. The Nova mandarin fruit in the open top carton cooled 24% faster on the inflow side of the pallet and 42% faster on the outflow side than the Eureka lemon in the Supervent carton. Open in new tab Table 1. Representative experimental researches and results about the impact of ventilation design in the forced-air precooling of horticultural produce Reference . Produce or simulator used . Container or package type . Design parameters . Effectiveness parameters . Leyte and Forney, 1999 Highbush blueberry Plastic clamshell Vent position Cooling time Remark: Results indicate that the most rapid cooling occurs in the clamshells with vents on the top. While adding vents at the bottom has no effect on the cooling rate. Fikiin et al., 1999 Five horticultural produce Trays of pine wooden material Airflow rate Heat transfer coefficient Remark: Results indicate that the surface heat transfer coefficient of the fruit increases with the airflow rate. Anderson et al., 2004 Strawberry Different clamshell and tray combinations Vent area; clamshell and tray combinations Cooling time Remark: Results indicate that the integral optimization of the combination of clamshell and tray is an effective way to maximize the contact between the cold air and the product. Vigneault et al., 2004a, 2004b 25 different horticultural produce Wooden container Product size; product shape; vent position Air pressure drop Remark: Results indicate that both the size and shape of horticultural produce have a significant impact on the pressure drop. In order to achieve the goal of minimizing the pressure drop, the vents should be uniformly distributed on the sides (perpendicular to airflow) of the container. Castro et al., 2004a Solid polymer ball Container formed by acrylic plates Airflow rate; vent area Cooling efficiency; air pressure drop Remark: Results indicate that the increase of total vent area (0.67%, 2%, 6%) leads to an enhancement of cooling efficiency, while increasing the airflow rate (from 0.125 to 3.9 L/(s*kg)) results in a greater air pressure drop. In order to obtain a relatively satisfactory cooling efficiency, the opening area should be more than 6%. Castro et al., 2004b Polychlorinated vinyl ball filled with water/agar-agar solution Wooden tunnel Airflow rate; vent area Cooling time; cooling rate; cooling uniformity; air pressure drop; energy consumption Remark: Results indicate that the airflow rate is an important factor influencing the half-cooling time. At the maximum airflow rate, neither vent area nor vent position has significant impacts on half-cooling time. Using the ventilated package with the total vent area of 14% has the same cooling rate as the fully open structure. Although increasing airflow rate compensates for the negative effects of the low vent area, it also increases the pressure drop and energy consumption of the precooling process. Vigneault et al., 2005 Solid polymer ball Container formed by acrylic plates Vent area; vent position Cooling rate; cooling uniformity; energy consumption Remark: Results indicate that in the viewpoint of optimizing energy consumption, the optimal vent area should be between 8% and 16%, and the vents should be avoided as far as possible in the corner. Castro et al., 2005 Solid polymer ball Container formed by acrylic plates Vent area; vent position Cooling rate; cooling uniformity; air pressure drop Remark: Results indicate that cooling uniformity increases as the vents move from the corner of ventilated package to the centre of the package surface. Vigneault et al., 2006 Solid polymer ball Container formed by acrylic plates Slat width; airflow rate; vent area; vent position Cooling time; cooling uniformity; air pressure drop Remark: Results indicate that the effect of vent position on cooling uniformity is pronounced at low airflow rate, in comparison with vent area. The vent area and slat width are suggested to be larger than 2.4% and 200 mm, respectively. Vigneault et al., 2007 Solid polymer ball Container formed by acrylic plates Airflow rate; vent area Cooling rate Remark: This research developed the Square Cross Section Velocity measurement method, which can measure the airflow rate more accurately than the previously developed Circular Approach Velocity method. Ferrua and Singh, 2011 Strawberry Clamshell Vent area; design of tray Cooling rate; cooling uniformity; energy consumption Remark: Results indicate that the optimal design of tray has a considerable contribution to the improvement of precooling effectiveness. Ngcobo et al., 2012 Table grape Carton Different components in the package Cooling rate; airflow resistance Remark: Results indicate that the line films have the greatest resistance to airflow compared to other components of the grape package. Mukama et al., 2017 Pomegranate Carton Plastic liner; stack orientation Cooling rate; energy consumption Remark: Results indicate that the direction of the stack (relative to the direction of the cooling airflow) affects energy consumption. The plastic liner has the greatest impact on the precooling process, and the energy consumption is up to three times higher than the unlined stack. Wu et al., 2018 Citrus fruit Carton Packaging type; wrapping; fruit size Cooling rate; cooling uniformity Remark: Results indicate that cooling heterogeneity mainly occurs in the flow direction. Fruit wrapping significantly reduces the cooling rate and increases cooling heterogeneity. The Nova mandarin fruit in the open top carton cooled 24% faster on the inflow side of the pallet and 42% faster on the outflow side than the Eureka lemon in the Supervent carton. Reference . Produce or simulator used . Container or package type . Design parameters . Effectiveness parameters . Leyte and Forney, 1999 Highbush blueberry Plastic clamshell Vent position Cooling time Remark: Results indicate that the most rapid cooling occurs in the clamshells with vents on the top. While adding vents at the bottom has no effect on the cooling rate. Fikiin et al., 1999 Five horticultural produce Trays of pine wooden material Airflow rate Heat transfer coefficient Remark: Results indicate that the surface heat transfer coefficient of the fruit increases with the airflow rate. Anderson et al., 2004 Strawberry Different clamshell and tray combinations Vent area; clamshell and tray combinations Cooling time Remark: Results indicate that the integral optimization of the combination of clamshell and tray is an effective way to maximize the contact between the cold air and the product. Vigneault et al., 2004a, 2004b 25 different horticultural produce Wooden container Product size; product shape; vent position Air pressure drop Remark: Results indicate that both the size and shape of horticultural produce have a significant impact on the pressure drop. In order to achieve the goal of minimizing the pressure drop, the vents should be uniformly distributed on the sides (perpendicular to airflow) of the container. Castro et al., 2004a Solid polymer ball Container formed by acrylic plates Airflow rate; vent area Cooling efficiency; air pressure drop Remark: Results indicate that the increase of total vent area (0.67%, 2%, 6%) leads to an enhancement of cooling efficiency, while increasing the airflow rate (from 0.125 to 3.9 L/(s*kg)) results in a greater air pressure drop. In order to obtain a relatively satisfactory cooling efficiency, the opening area should be more than 6%. Castro et al., 2004b Polychlorinated vinyl ball filled with water/agar-agar solution Wooden tunnel Airflow rate; vent area Cooling time; cooling rate; cooling uniformity; air pressure drop; energy consumption Remark: Results indicate that the airflow rate is an important factor influencing the half-cooling time. At the maximum airflow rate, neither vent area nor vent position has significant impacts on half-cooling time. Using the ventilated package with the total vent area of 14% has the same cooling rate as the fully open structure. Although increasing airflow rate compensates for the negative effects of the low vent area, it also increases the pressure drop and energy consumption of the precooling process. Vigneault et al., 2005 Solid polymer ball Container formed by acrylic plates Vent area; vent position Cooling rate; cooling uniformity; energy consumption Remark: Results indicate that in the viewpoint of optimizing energy consumption, the optimal vent area should be between 8% and 16%, and the vents should be avoided as far as possible in the corner. Castro et al., 2005 Solid polymer ball Container formed by acrylic plates Vent area; vent position Cooling rate; cooling uniformity; air pressure drop Remark: Results indicate that cooling uniformity increases as the vents move from the corner of ventilated package to the centre of the package surface. Vigneault et al., 2006 Solid polymer ball Container formed by acrylic plates Slat width; airflow rate; vent area; vent position Cooling time; cooling uniformity; air pressure drop Remark: Results indicate that the effect of vent position on cooling uniformity is pronounced at low airflow rate, in comparison with vent area. The vent area and slat width are suggested to be larger than 2.4% and 200 mm, respectively. Vigneault et al., 2007 Solid polymer ball Container formed by acrylic plates Airflow rate; vent area Cooling rate Remark: This research developed the Square Cross Section Velocity measurement method, which can measure the airflow rate more accurately than the previously developed Circular Approach Velocity method. Ferrua and Singh, 2011 Strawberry Clamshell Vent area; design of tray Cooling rate; cooling uniformity; energy consumption Remark: Results indicate that the optimal design of tray has a considerable contribution to the improvement of precooling effectiveness. Ngcobo et al., 2012 Table grape Carton Different components in the package Cooling rate; airflow resistance Remark: Results indicate that the line films have the greatest resistance to airflow compared to other components of the grape package. Mukama et al., 2017 Pomegranate Carton Plastic liner; stack orientation Cooling rate; energy consumption Remark: Results indicate that the direction of the stack (relative to the direction of the cooling airflow) affects energy consumption. The plastic liner has the greatest impact on the precooling process, and the energy consumption is up to three times higher than the unlined stack. Wu et al., 2018 Citrus fruit Carton Packaging type; wrapping; fruit size Cooling rate; cooling uniformity Remark: Results indicate that cooling heterogeneity mainly occurs in the flow direction. Fruit wrapping significantly reduces the cooling rate and increases cooling heterogeneity. The Nova mandarin fruit in the open top carton cooled 24% faster on the inflow side of the pallet and 42% faster on the outflow side than the Eureka lemon in the Supervent carton. Open in new tab With the rapid development of computer technology, numerical simulation has grown to be an alternative research method for the ventilation design of packaging system used in the forced-air precooling of fresh foods. Different from experimental studies, the numerical simulation method is capable of providing detailed airflow and temperature distribution inside the packaging system and avoiding the effect of the fluctuation in the physical properties of fresh foods in a series of tests. Generally, available numerical methods about the forced-air precooling process can be divided into two categories: the porous-medium approach and the direct computational fluid dynamics (CFD) simulation. Due to the complex internal structure of ventilated packages and the limitation of computation resources, the porous-medium approach was the dominant numerical method in the early stage. It allows the simplification of mathematical models by treating the complex structure inside ventilated packages, such as the area between the product air zone of the bulk package and that between trays of the layered package, as a porous media (Verboven et al., 2004b). Consequently, the complicated treatment about geometry details is easily avoided, and the computation time and simulation cost are dramatically reduced. In the past 20 years, many efforts had been devoted to the study about the forced-air precooling process by using the porous-medium approach. Some representative researches and results are summarized in Table 2. In the porous-medium approach, a pressure drop induced by the porous media is added to the momentum conservation equation. The general relationship between permeability and inertial loss coefficient is determined by the Ergun equation. For some products, the calculated pressure drop by the Ergun equation shows good agreement with the experimental results. For example, the results obtained by Van der Sman (2002) demonstrated the validity of the Ergun equation in the precooling of potatoes and that of oranges, with the deviation of the numerically predicted value from experimental measurement being less than 6%. However, due to the difference in individual characteristics, such as product shape, surface roughness, and porosity, it may also give poor predictions. The inconsistency between the experimental data and the prediction from the Ergun equation is attributed to the uncertainty of the porosity values caused by the relatively small confinement ratio of the packed bed. Verboven et al. (2004a) pointed out that the impact of confinement should be considered when the confinement ratio is less than 10. By taking into account the confinement ratio, the deviation of the numerically predicted pressure drop from the experimental measurement can be reduced from 65% to 20% (Verboven et al., 2004a). In addition, a spatial average method is required in the porous-medium approach, and the fluid velocity is generally characterized by a superficial velocity (Verboven et al., 2006). Therefore, the porous-medium approach is unlikely to predict the detailed flow behaviour around an individual product and the accurate temperature distribution inside ventilated packages. Table 2. Representative researches based on the porous-medium approach Reference . Material . Time dependency . Effectiveness parameters . Xu and Burfoot, 1999 Potato Transient Cooling time; water vapour concentration Remark: This paper presented a transient three-dimensional numerical model for the heat and mass transfer of porous bulk pellet foods. The Ergun equation was used to describe the interaction between the airflow and the porous media. The conservation equations were solved to predict the airflow, temperature, and humidity. Results indicate that the predicted temperature profile is highly consistent with the measured one, with the maximum difference of 1.4°C. Alvarez et al., 2003 PVC sphere Steady Heat transfer coefficient Remark: This paper presented a single equation model for a two-dimensional problem. The Darcy–Forchheimer equation was adopted to describe the influence of the porous media on the airflow. Results indicate that the discrepancy between the predicted heat transfer coefficient and the experimental measurement is about 6%. Verboven et al., 2004a Apple and chicory root Steady Air pressure drop Remark: This paper presented a modified numerical scheme, in which the impacts of the product shape, surface roughness, and confinement were taken into account. The precooling process of apples and that of chicory roots were numerically studied, respectively. Results indicate that the deviation of the calculated pressure drop from the experimental measurement is reduced from 65% to 20%. Zou et al., 2006a, 2006b Apple Steady and transient Airflow pattern; temperature profile Remark: This paper presented the numerical prediction about both the airflow pattern and temperature profile in the case of bulk products and layered products, respectively. Results indicate that the consistency between the model prediction and the experimental measurement is satisfactory. Dehghannya et al., 2008 Solid polymer ball Transient Cooling uniformity Remark: This paper presented the study about the impact of vent area on cooling uniformity. Results indicate that a more uniform airflow distribution is obtained by increasing the vent area from 2.4% to 12.1%, and the most non-uniform cooling occurs in the vicinity of vents. Delele et al., 2013c Table grape Transient Cooling time; moisture loss; relative humidity Remark: This paper presented the discussion about the impacts of internal packaging components (bunch carry bag and plastic liners) and product stacking modes on airflow, heat, and mass transfer characteristics. Results indicate that the presence of the bunch carry bag increases the half-cooling time and seven-eighths cooling time by 61.09% and 97.34%, respectively, compared to the cooling of the bulk grape bunch. Adding a plastic liner to bunch carry increases the half-cooling time and seven-eighths cooling time by 168.90% and 185.22%, respectively. Reference . Material . Time dependency . Effectiveness parameters . Xu and Burfoot, 1999 Potato Transient Cooling time; water vapour concentration Remark: This paper presented a transient three-dimensional numerical model for the heat and mass transfer of porous bulk pellet foods. The Ergun equation was used to describe the interaction between the airflow and the porous media. The conservation equations were solved to predict the airflow, temperature, and humidity. Results indicate that the predicted temperature profile is highly consistent with the measured one, with the maximum difference of 1.4°C. Alvarez et al., 2003 PVC sphere Steady Heat transfer coefficient Remark: This paper presented a single equation model for a two-dimensional problem. The Darcy–Forchheimer equation was adopted to describe the influence of the porous media on the airflow. Results indicate that the discrepancy between the predicted heat transfer coefficient and the experimental measurement is about 6%. Verboven et al., 2004a Apple and chicory root Steady Air pressure drop Remark: This paper presented a modified numerical scheme, in which the impacts of the product shape, surface roughness, and confinement were taken into account. The precooling process of apples and that of chicory roots were numerically studied, respectively. Results indicate that the deviation of the calculated pressure drop from the experimental measurement is reduced from 65% to 20%. Zou et al., 2006a, 2006b Apple Steady and transient Airflow pattern; temperature profile Remark: This paper presented the numerical prediction about both the airflow pattern and temperature profile in the case of bulk products and layered products, respectively. Results indicate that the consistency between the model prediction and the experimental measurement is satisfactory. Dehghannya et al., 2008 Solid polymer ball Transient Cooling uniformity Remark: This paper presented the study about the impact of vent area on cooling uniformity. Results indicate that a more uniform airflow distribution is obtained by increasing the vent area from 2.4% to 12.1%, and the most non-uniform cooling occurs in the vicinity of vents. Delele et al., 2013c Table grape Transient Cooling time; moisture loss; relative humidity Remark: This paper presented the discussion about the impacts of internal packaging components (bunch carry bag and plastic liners) and product stacking modes on airflow, heat, and mass transfer characteristics. Results indicate that the presence of the bunch carry bag increases the half-cooling time and seven-eighths cooling time by 61.09% and 97.34%, respectively, compared to the cooling of the bulk grape bunch. Adding a plastic liner to bunch carry increases the half-cooling time and seven-eighths cooling time by 168.90% and 185.22%, respectively. Open in new tab Table 2. Representative researches based on the porous-medium approach Reference . Material . Time dependency . Effectiveness parameters . Xu and Burfoot, 1999 Potato Transient Cooling time; water vapour concentration Remark: This paper presented a transient three-dimensional numerical model for the heat and mass transfer of porous bulk pellet foods. The Ergun equation was used to describe the interaction between the airflow and the porous media. The conservation equations were solved to predict the airflow, temperature, and humidity. Results indicate that the predicted temperature profile is highly consistent with the measured one, with the maximum difference of 1.4°C. Alvarez et al., 2003 PVC sphere Steady Heat transfer coefficient Remark: This paper presented a single equation model for a two-dimensional problem. The Darcy–Forchheimer equation was adopted to describe the influence of the porous media on the airflow. Results indicate that the discrepancy between the predicted heat transfer coefficient and the experimental measurement is about 6%. Verboven et al., 2004a Apple and chicory root Steady Air pressure drop Remark: This paper presented a modified numerical scheme, in which the impacts of the product shape, surface roughness, and confinement were taken into account. The precooling process of apples and that of chicory roots were numerically studied, respectively. Results indicate that the deviation of the calculated pressure drop from the experimental measurement is reduced from 65% to 20%. Zou et al., 2006a, 2006b Apple Steady and transient Airflow pattern; temperature profile Remark: This paper presented the numerical prediction about both the airflow pattern and temperature profile in the case of bulk products and layered products, respectively. Results indicate that the consistency between the model prediction and the experimental measurement is satisfactory. Dehghannya et al., 2008 Solid polymer ball Transient Cooling uniformity Remark: This paper presented the study about the impact of vent area on cooling uniformity. Results indicate that a more uniform airflow distribution is obtained by increasing the vent area from 2.4% to 12.1%, and the most non-uniform cooling occurs in the vicinity of vents. Delele et al., 2013c Table grape Transient Cooling time; moisture loss; relative humidity Remark: This paper presented the discussion about the impacts of internal packaging components (bunch carry bag and plastic liners) and product stacking modes on airflow, heat, and mass transfer characteristics. Results indicate that the presence of the bunch carry bag increases the half-cooling time and seven-eighths cooling time by 61.09% and 97.34%, respectively, compared to the cooling of the bulk grape bunch. Adding a plastic liner to bunch carry increases the half-cooling time and seven-eighths cooling time by 168.90% and 185.22%, respectively. Reference . Material . Time dependency . Effectiveness parameters . Xu and Burfoot, 1999 Potato Transient Cooling time; water vapour concentration Remark: This paper presented a transient three-dimensional numerical model for the heat and mass transfer of porous bulk pellet foods. The Ergun equation was used to describe the interaction between the airflow and the porous media. The conservation equations were solved to predict the airflow, temperature, and humidity. Results indicate that the predicted temperature profile is highly consistent with the measured one, with the maximum difference of 1.4°C. Alvarez et al., 2003 PVC sphere Steady Heat transfer coefficient Remark: This paper presented a single equation model for a two-dimensional problem. The Darcy–Forchheimer equation was adopted to describe the influence of the porous media on the airflow. Results indicate that the discrepancy between the predicted heat transfer coefficient and the experimental measurement is about 6%. Verboven et al., 2004a Apple and chicory root Steady Air pressure drop Remark: This paper presented a modified numerical scheme, in which the impacts of the product shape, surface roughness, and confinement were taken into account. The precooling process of apples and that of chicory roots were numerically studied, respectively. Results indicate that the deviation of the calculated pressure drop from the experimental measurement is reduced from 65% to 20%. Zou et al., 2006a, 2006b Apple Steady and transient Airflow pattern; temperature profile Remark: This paper presented the numerical prediction about both the airflow pattern and temperature profile in the case of bulk products and layered products, respectively. Results indicate that the consistency between the model prediction and the experimental measurement is satisfactory. Dehghannya et al., 2008 Solid polymer ball Transient Cooling uniformity Remark: This paper presented the study about the impact of vent area on cooling uniformity. Results indicate that a more uniform airflow distribution is obtained by increasing the vent area from 2.4% to 12.1%, and the most non-uniform cooling occurs in the vicinity of vents. Delele et al., 2013c Table grape Transient Cooling time; moisture loss; relative humidity Remark: This paper presented the discussion about the impacts of internal packaging components (bunch carry bag and plastic liners) and product stacking modes on airflow, heat, and mass transfer characteristics. Results indicate that the presence of the bunch carry bag increases the half-cooling time and seven-eighths cooling time by 61.09% and 97.34%, respectively, compared to the cooling of the bulk grape bunch. Adding a plastic liner to bunch carry increases the half-cooling time and seven-eighths cooling time by 168.90% and 185.22%, respectively. Open in new tab The direct CFD simulation has grown to be the most attractive method for the numerical study about the forced-air precooling process of horticultural produce in ventilated packaging system in the past 10 years. Different from the porous-medium approach, the direct CFD simulation allows an accurate description of explicit geometries of the complex internal structure of packaging system, including the shape and size of the product, the gap of neighbouring products, the location and material of trays, etc. Although the grid generation becomes complicated and the computation effort has a dramatic increase, the direct CFD simulation is capable of providing accurate local information about the detailed airflow and temperature distribution inside packages (Zhao et al., 2016). Furthermore, it is not limited by the confinement ratio (Ferrua and Singh, 2007). Therefore, many efforts had been devoted to the development of efficient CFD schemes for the forced-air precooling process of horticultural produce. Based on the measurements from some packaging or cold storage, a new zoning-model approach was developed to predict the heat and mass transfer processes in refrigerated horticultural packaging (Tanner et al., 2002a, 2002b, 2002c). By neglecting the buoyancy effect and assuming the airflow to be incompressible, the mass conservation equation can be decoupled from the energy conservation equation. On the basis of the decoupling method, Ferrua and Singh (2009a, 2009b, 2009c) performed a numerical analysis of the forced-air precooling process for strawberry packaging and reported that the difference between the average fruit temperature prediction and the experimental curve for each clamshell was less than 0.7°C. Delele et al. (2013a) developed a three-dimensional CFD model for airflow and heat transfer processes in packaged horticultural produce. The prediction results are in good agreement with the measured results, with the average relative errors of predicted pressure drop and product temperature being 13.80% and 16.27%, respectively. The SST k−ω turbulence model combined with the wall function had been demonstrated to perform best (Defraeye et al., 2013) and was widely used in the optimization design of ventilation packaging. Although respiration is the most important life activity of fruits and vegetables, available studies (Campanone et al., 1995; Gowda et al., 1997; Tanner et al., 2002c; Han et al., 2017a) indicated that respiratory heat is unlikely to significantly affect the precooling effectiveness. During typical precooling, the rate of heat generated by respiration is often low (about 0.5% of the total heat load). However, considering respiration heat can improve the accuracy of numerical models (Vigneault et al., 2007). Available results about the impact of ventilation design on precooling effectiveness, obtained by using the direct CFD simulation, are very rich. Some representative researches are summarized in Table 3. Table 3. Representative studies based on direct computational fluid dynamics (CFD) simulation Reference . Material . Time dependency . Effectiveness parameters . Delele et al., 2008 Sphere Steady Pressure drop Remark: In this study, a random stack of spherical products in a package was created by using discrete-element method. The impact of ventilation design on pressure drop was discussed. The involved parameters include confinement ratio, stacking pattern, product size, vent area, and randomness of filling. The numerical prediction is in good agreement with experimental measurement. Results indicate that the air resistance of random filling is much smaller. Tutar et al., 2009 Sphere Steady and transient Cooling uniformity; cooling time Remark: In this paper, a numerical scheme was proposed to predict the airflow pattern and temperature distribution in the stack of circular products in the precooling process. The influencing factors discussed are flow dimension, turbulence intensity, opening size, vent ratio, and airflow rate. Results indicate that the airflow rate has the greatest impact on precooling effectiveness. Properly increasing the opening area at the bottom or on side walls can considerably improve the vertical airflow. The intensity of imported turbulence is not an important factor, which only has a slight impact on the surface average heat flux of the product. Ferrua and Singh, 2009d Strawberry Steady and transient Cooling uniformity; cooling time Remark: This study provided quantitative results to emphasize the necessity of adding ventilation design. Results indicate that vent area has a significant impact on cooling time, but has no effect on cooling uniformity. Removing the vents on the outside of the clamshells results in the increase of cooling time by 20%. Dehghannya et al., 2011 Polymer sphere Transient Cooling uniformity Remark: In this study, the impact of the number of vents (1, 3, and 5) on temperature distribution was investigated under the condition that the area of a single vent was 2.4%. Results indicate that increasing the number of vents leads to a significant improvement in cooling uniformity. Dehghannya et al., 2012 Polymer sphere Transient Cooling time; cooling rate; cooling uniformity Remark: In this paper, the influences of vent area and vent position on precooling effectiveness were studied numerically. Results indicate that increasing the vent area beyond a certain level does not have a positive impact on cooling uniformity or cooling time. If vent holes are not properly distributed on the package wall, they can even increase the cooling time. Delele et al., 2013a, 2013b PVC ball filled with water Transient Cooling rate; cooling uniformity; air pressure drop; energy consumption Remark: In this paper, a three-dimensional CFD model of the precooling process of packaged horticultural products was developed, in which the effects of product respiration heat and buoyancy were considered. The numerical prediction is in good agreement with the experimental measurement, with the average relative errors of pressure drop and product temperature being 13.80% and 16.27%, respectively. Results indicate that the vent area has the greatest impact on precooling effectiveness. Defraeye et al., 2013, 2014 Citrus Steady and transient Cooling time; cooling uniformity; energy consumption Remark: In this paper, the direct CFD simulation was used to evaluate the cooling performance and energy consumption of the existing container and two new designs. Results indicate that the existing container is prone to chilling injury, while the adoption of the two new designs not only increases the cooling rate and cooling uniformity, but also reduces energy consumption. Berry et al., 2016 Apple Steady and transient Cooling rate; cooling uniformity; air pressure drop; energy consumption Remark: In this paper, the cooling performance and energy consumption of four packaging designs were evaluated numerically on the premise of the total vent area of 4%. The impact of trays on airflow resistance was also studied. Results indicate that the adoption of the two designs with trays not only improves the cooling uniformity between the fruit layers, but also reduces the energy consumption by 27% and 26%, respectively. O’Sullivan et al., 2016 Kiwifruit Transient Cooling rate Remark: In this paper, a three-dimensional CFD model was proposed to study the precooling process in a palletized polylined kiwifruit package. The model considers the effect of natural convection on both the flow and heat transfer characteristics in polyliners. Results indicate that the maximum volumetric flowrate through the pallet is much lower than the recommended flowrate of non-polylined produce. The continuous increase in flowrate results in an increasingly diminished reduction of cooling rate. Berry et al., 2017 Apple Steady and transient Cooling rate; cooling uniformity; air pressure drop; energy consumption Remark: In this study, the impacts of vent configuration, vent area, and corrugated fibreboard grade on cooling effectiveness and energy consumption were evaluated. Results indicate that the carton strength is negatively correlated with the vent area. As the total vent area increases, the cooling heterogeneity decreases. Han et al., 2017b Apple Transient Cooling rate; cooling uniformity; air pressure drop; energy consumption Remark: In this study, the cooling performances of 10 different apple box samples used in China were evaluated by using a three-dimensional numerical model. The maximum deviation between the predicted and experimental values of the apple surface temperature was 18.69%. Results indicate that the cooling rate increases with the airflow rate, and the optimal air-inflow velocity is 0.4 m/s. The uniform and symmetric distribution of ventilation holes has a more significant contribution to the improvement of precooling effectiveness than the increase of vent area, especially when handling packages with more than three layers. In addition, polyvinyl chloride foam has a relatively smaller airflow resistance. Gruyters et al., 2018 Apple Transient Heat transfer coefficient; air pressure drop Remark: In this study, the realistic product shapes were used in the numerical simulation of the forced-air precooling process of apples. Results indicate that the main factor contributing to the overall pressure drop is the packaging design rather than the shape of the product. However, the shape of the product has a significant impact on the local air velocity and the convective heat transfer coefficient. The real apple shape has a lower surface heat transfer coefficient. Han et al., 2018 Apple Transient Cooling rate; cooling uniformity; energy consumption Remark: In this study, the forced-air precooling process of apple was simulated by a three-dimensional numerical model, in which the heat of respiration and evaporation was taken into account. Results indicate that the optimal air-inflow rate is 2.31 L/(s*kg). Wang et al., 2019 Strawberry Transient Cooling rate; cooling time; cooling uniformity Remark: In this study, the forced-air precooling process of strawberry was simulated by a three-dimensional numerical model, in which the heat of respiration and evaporation was taken into account. Results indicate that the combination of 9.4% area of box vent and 8.5% area of clamshell vent in the commercial packaging system has the best precooling effect on strawberry. Reference . Material . Time dependency . Effectiveness parameters . Delele et al., 2008 Sphere Steady Pressure drop Remark: In this study, a random stack of spherical products in a package was created by using discrete-element method. The impact of ventilation design on pressure drop was discussed. The involved parameters include confinement ratio, stacking pattern, product size, vent area, and randomness of filling. The numerical prediction is in good agreement with experimental measurement. Results indicate that the air resistance of random filling is much smaller. Tutar et al., 2009 Sphere Steady and transient Cooling uniformity; cooling time Remark: In this paper, a numerical scheme was proposed to predict the airflow pattern and temperature distribution in the stack of circular products in the precooling process. The influencing factors discussed are flow dimension, turbulence intensity, opening size, vent ratio, and airflow rate. Results indicate that the airflow rate has the greatest impact on precooling effectiveness. Properly increasing the opening area at the bottom or on side walls can considerably improve the vertical airflow. The intensity of imported turbulence is not an important factor, which only has a slight impact on the surface average heat flux of the product. Ferrua and Singh, 2009d Strawberry Steady and transient Cooling uniformity; cooling time Remark: This study provided quantitative results to emphasize the necessity of adding ventilation design. Results indicate that vent area has a significant impact on cooling time, but has no effect on cooling uniformity. Removing the vents on the outside of the clamshells results in the increase of cooling time by 20%. Dehghannya et al., 2011 Polymer sphere Transient Cooling uniformity Remark: In this study, the impact of the number of vents (1, 3, and 5) on temperature distribution was investigated under the condition that the area of a single vent was 2.4%. Results indicate that increasing the number of vents leads to a significant improvement in cooling uniformity. Dehghannya et al., 2012 Polymer sphere Transient Cooling time; cooling rate; cooling uniformity Remark: In this paper, the influences of vent area and vent position on precooling effectiveness were studied numerically. Results indicate that increasing the vent area beyond a certain level does not have a positive impact on cooling uniformity or cooling time. If vent holes are not properly distributed on the package wall, they can even increase the cooling time. Delele et al., 2013a, 2013b PVC ball filled with water Transient Cooling rate; cooling uniformity; air pressure drop; energy consumption Remark: In this paper, a three-dimensional CFD model of the precooling process of packaged horticultural products was developed, in which the effects of product respiration heat and buoyancy were considered. The numerical prediction is in good agreement with the experimental measurement, with the average relative errors of pressure drop and product temperature being 13.80% and 16.27%, respectively. Results indicate that the vent area has the greatest impact on precooling effectiveness. Defraeye et al., 2013, 2014 Citrus Steady and transient Cooling time; cooling uniformity; energy consumption Remark: In this paper, the direct CFD simulation was used to evaluate the cooling performance and energy consumption of the existing container and two new designs. Results indicate that the existing container is prone to chilling injury, while the adoption of the two new designs not only increases the cooling rate and cooling uniformity, but also reduces energy consumption. Berry et al., 2016 Apple Steady and transient Cooling rate; cooling uniformity; air pressure drop; energy consumption Remark: In this paper, the cooling performance and energy consumption of four packaging designs were evaluated numerically on the premise of the total vent area of 4%. The impact of trays on airflow resistance was also studied. Results indicate that the adoption of the two designs with trays not only improves the cooling uniformity between the fruit layers, but also reduces the energy consumption by 27% and 26%, respectively. O’Sullivan et al., 2016 Kiwifruit Transient Cooling rate Remark: In this paper, a three-dimensional CFD model was proposed to study the precooling process in a palletized polylined kiwifruit package. The model considers the effect of natural convection on both the flow and heat transfer characteristics in polyliners. Results indicate that the maximum volumetric flowrate through the pallet is much lower than the recommended flowrate of non-polylined produce. The continuous increase in flowrate results in an increasingly diminished reduction of cooling rate. Berry et al., 2017 Apple Steady and transient Cooling rate; cooling uniformity; air pressure drop; energy consumption Remark: In this study, the impacts of vent configuration, vent area, and corrugated fibreboard grade on cooling effectiveness and energy consumption were evaluated. Results indicate that the carton strength is negatively correlated with the vent area. As the total vent area increases, the cooling heterogeneity decreases. Han et al., 2017b Apple Transient Cooling rate; cooling uniformity; air pressure drop; energy consumption Remark: In this study, the cooling performances of 10 different apple box samples used in China were evaluated by using a three-dimensional numerical model. The maximum deviation between the predicted and experimental values of the apple surface temperature was 18.69%. Results indicate that the cooling rate increases with the airflow rate, and the optimal air-inflow velocity is 0.4 m/s. The uniform and symmetric distribution of ventilation holes has a more significant contribution to the improvement of precooling effectiveness than the increase of vent area, especially when handling packages with more than three layers. In addition, polyvinyl chloride foam has a relatively smaller airflow resistance. Gruyters et al., 2018 Apple Transient Heat transfer coefficient; air pressure drop Remark: In this study, the realistic product shapes were used in the numerical simulation of the forced-air precooling process of apples. Results indicate that the main factor contributing to the overall pressure drop is the packaging design rather than the shape of the product. However, the shape of the product has a significant impact on the local air velocity and the convective heat transfer coefficient. The real apple shape has a lower surface heat transfer coefficient. Han et al., 2018 Apple Transient Cooling rate; cooling uniformity; energy consumption Remark: In this study, the forced-air precooling process of apple was simulated by a three-dimensional numerical model, in which the heat of respiration and evaporation was taken into account. Results indicate that the optimal air-inflow rate is 2.31 L/(s*kg). Wang et al., 2019 Strawberry Transient Cooling rate; cooling time; cooling uniformity Remark: In this study, the forced-air precooling process of strawberry was simulated by a three-dimensional numerical model, in which the heat of respiration and evaporation was taken into account. Results indicate that the combination of 9.4% area of box vent and 8.5% area of clamshell vent in the commercial packaging system has the best precooling effect on strawberry. Open in new tab Table 3. Representative studies based on direct computational fluid dynamics (CFD) simulation Reference . Material . Time dependency . Effectiveness parameters . Delele et al., 2008 Sphere Steady Pressure drop Remark: In this study, a random stack of spherical products in a package was created by using discrete-element method. The impact of ventilation design on pressure drop was discussed. The involved parameters include confinement ratio, stacking pattern, product size, vent area, and randomness of filling. The numerical prediction is in good agreement with experimental measurement. Results indicate that the air resistance of random filling is much smaller. Tutar et al., 2009 Sphere Steady and transient Cooling uniformity; cooling time Remark: In this paper, a numerical scheme was proposed to predict the airflow pattern and temperature distribution in the stack of circular products in the precooling process. The influencing factors discussed are flow dimension, turbulence intensity, opening size, vent ratio, and airflow rate. Results indicate that the airflow rate has the greatest impact on precooling effectiveness. Properly increasing the opening area at the bottom or on side walls can considerably improve the vertical airflow. The intensity of imported turbulence is not an important factor, which only has a slight impact on the surface average heat flux of the product. Ferrua and Singh, 2009d Strawberry Steady and transient Cooling uniformity; cooling time Remark: This study provided quantitative results to emphasize the necessity of adding ventilation design. Results indicate that vent area has a significant impact on cooling time, but has no effect on cooling uniformity. Removing the vents on the outside of the clamshells results in the increase of cooling time by 20%. Dehghannya et al., 2011 Polymer sphere Transient Cooling uniformity Remark: In this study, the impact of the number of vents (1, 3, and 5) on temperature distribution was investigated under the condition that the area of a single vent was 2.4%. Results indicate that increasing the number of vents leads to a significant improvement in cooling uniformity. Dehghannya et al., 2012 Polymer sphere Transient Cooling time; cooling rate; cooling uniformity Remark: In this paper, the influences of vent area and vent position on precooling effectiveness were studied numerically. Results indicate that increasing the vent area beyond a certain level does not have a positive impact on cooling uniformity or cooling time. If vent holes are not properly distributed on the package wall, they can even increase the cooling time. Delele et al., 2013a, 2013b PVC ball filled with water Transient Cooling rate; cooling uniformity; air pressure drop; energy consumption Remark: In this paper, a three-dimensional CFD model of the precooling process of packaged horticultural products was developed, in which the effects of product respiration heat and buoyancy were considered. The numerical prediction is in good agreement with the experimental measurement, with the average relative errors of pressure drop and product temperature being 13.80% and 16.27%, respectively. Results indicate that the vent area has the greatest impact on precooling effectiveness. Defraeye et al., 2013, 2014 Citrus Steady and transient Cooling time; cooling uniformity; energy consumption Remark: In this paper, the direct CFD simulation was used to evaluate the cooling performance and energy consumption of the existing container and two new designs. Results indicate that the existing container is prone to chilling injury, while the adoption of the two new designs not only increases the cooling rate and cooling uniformity, but also reduces energy consumption. Berry et al., 2016 Apple Steady and transient Cooling rate; cooling uniformity; air pressure drop; energy consumption Remark: In this paper, the cooling performance and energy consumption of four packaging designs were evaluated numerically on the premise of the total vent area of 4%. The impact of trays on airflow resistance was also studied. Results indicate that the adoption of the two designs with trays not only improves the cooling uniformity between the fruit layers, but also reduces the energy consumption by 27% and 26%, respectively. O’Sullivan et al., 2016 Kiwifruit Transient Cooling rate Remark: In this paper, a three-dimensional CFD model was proposed to study the precooling process in a palletized polylined kiwifruit package. The model considers the effect of natural convection on both the flow and heat transfer characteristics in polyliners. Results indicate that the maximum volumetric flowrate through the pallet is much lower than the recommended flowrate of non-polylined produce. The continuous increase in flowrate results in an increasingly diminished reduction of cooling rate. Berry et al., 2017 Apple Steady and transient Cooling rate; cooling uniformity; air pressure drop; energy consumption Remark: In this study, the impacts of vent configuration, vent area, and corrugated fibreboard grade on cooling effectiveness and energy consumption were evaluated. Results indicate that the carton strength is negatively correlated with the vent area. As the total vent area increases, the cooling heterogeneity decreases. Han et al., 2017b Apple Transient Cooling rate; cooling uniformity; air pressure drop; energy consumption Remark: In this study, the cooling performances of 10 different apple box samples used in China were evaluated by using a three-dimensional numerical model. The maximum deviation between the predicted and experimental values of the apple surface temperature was 18.69%. Results indicate that the cooling rate increases with the airflow rate, and the optimal air-inflow velocity is 0.4 m/s. The uniform and symmetric distribution of ventilation holes has a more significant contribution to the improvement of precooling effectiveness than the increase of vent area, especially when handling packages with more than three layers. In addition, polyvinyl chloride foam has a relatively smaller airflow resistance. Gruyters et al., 2018 Apple Transient Heat transfer coefficient; air pressure drop Remark: In this study, the realistic product shapes were used in the numerical simulation of the forced-air precooling process of apples. Results indicate that the main factor contributing to the overall pressure drop is the packaging design rather than the shape of the product. However, the shape of the product has a significant impact on the local air velocity and the convective heat transfer coefficient. The real apple shape has a lower surface heat transfer coefficient. Han et al., 2018 Apple Transient Cooling rate; cooling uniformity; energy consumption Remark: In this study, the forced-air precooling process of apple was simulated by a three-dimensional numerical model, in which the heat of respiration and evaporation was taken into account. Results indicate that the optimal air-inflow rate is 2.31 L/(s*kg). Wang et al., 2019 Strawberry Transient Cooling rate; cooling time; cooling uniformity Remark: In this study, the forced-air precooling process of strawberry was simulated by a three-dimensional numerical model, in which the heat of respiration and evaporation was taken into account. Results indicate that the combination of 9.4% area of box vent and 8.5% area of clamshell vent in the commercial packaging system has the best precooling effect on strawberry. Reference . Material . Time dependency . Effectiveness parameters . Delele et al., 2008 Sphere Steady Pressure drop Remark: In this study, a random stack of spherical products in a package was created by using discrete-element method. The impact of ventilation design on pressure drop was discussed. The involved parameters include confinement ratio, stacking pattern, product size, vent area, and randomness of filling. The numerical prediction is in good agreement with experimental measurement. Results indicate that the air resistance of random filling is much smaller. Tutar et al., 2009 Sphere Steady and transient Cooling uniformity; cooling time Remark: In this paper, a numerical scheme was proposed to predict the airflow pattern and temperature distribution in the stack of circular products in the precooling process. The influencing factors discussed are flow dimension, turbulence intensity, opening size, vent ratio, and airflow rate. Results indicate that the airflow rate has the greatest impact on precooling effectiveness. Properly increasing the opening area at the bottom or on side walls can considerably improve the vertical airflow. The intensity of imported turbulence is not an important factor, which only has a slight impact on the surface average heat flux of the product. Ferrua and Singh, 2009d Strawberry Steady and transient Cooling uniformity; cooling time Remark: This study provided quantitative results to emphasize the necessity of adding ventilation design. Results indicate that vent area has a significant impact on cooling time, but has no effect on cooling uniformity. Removing the vents on the outside of the clamshells results in the increase of cooling time by 20%. Dehghannya et al., 2011 Polymer sphere Transient Cooling uniformity Remark: In this study, the impact of the number of vents (1, 3, and 5) on temperature distribution was investigated under the condition that the area of a single vent was 2.4%. Results indicate that increasing the number of vents leads to a significant improvement in cooling uniformity. Dehghannya et al., 2012 Polymer sphere Transient Cooling time; cooling rate; cooling uniformity Remark: In this paper, the influences of vent area and vent position on precooling effectiveness were studied numerically. Results indicate that increasing the vent area beyond a certain level does not have a positive impact on cooling uniformity or cooling time. If vent holes are not properly distributed on the package wall, they can even increase the cooling time. Delele et al., 2013a, 2013b PVC ball filled with water Transient Cooling rate; cooling uniformity; air pressure drop; energy consumption Remark: In this paper, a three-dimensional CFD model of the precooling process of packaged horticultural products was developed, in which the effects of product respiration heat and buoyancy were considered. The numerical prediction is in good agreement with the experimental measurement, with the average relative errors of pressure drop and product temperature being 13.80% and 16.27%, respectively. Results indicate that the vent area has the greatest impact on precooling effectiveness. Defraeye et al., 2013, 2014 Citrus Steady and transient Cooling time; cooling uniformity; energy consumption Remark: In this paper, the direct CFD simulation was used to evaluate the cooling performance and energy consumption of the existing container and two new designs. Results indicate that the existing container is prone to chilling injury, while the adoption of the two new designs not only increases the cooling rate and cooling uniformity, but also reduces energy consumption. Berry et al., 2016 Apple Steady and transient Cooling rate; cooling uniformity; air pressure drop; energy consumption Remark: In this paper, the cooling performance and energy consumption of four packaging designs were evaluated numerically on the premise of the total vent area of 4%. The impact of trays on airflow resistance was also studied. Results indicate that the adoption of the two designs with trays not only improves the cooling uniformity between the fruit layers, but also reduces the energy consumption by 27% and 26%, respectively. O’Sullivan et al., 2016 Kiwifruit Transient Cooling rate Remark: In this paper, a three-dimensional CFD model was proposed to study the precooling process in a palletized polylined kiwifruit package. The model considers the effect of natural convection on both the flow and heat transfer characteristics in polyliners. Results indicate that the maximum volumetric flowrate through the pallet is much lower than the recommended flowrate of non-polylined produce. The continuous increase in flowrate results in an increasingly diminished reduction of cooling rate. Berry et al., 2017 Apple Steady and transient Cooling rate; cooling uniformity; air pressure drop; energy consumption Remark: In this study, the impacts of vent configuration, vent area, and corrugated fibreboard grade on cooling effectiveness and energy consumption were evaluated. Results indicate that the carton strength is negatively correlated with the vent area. As the total vent area increases, the cooling heterogeneity decreases. Han et al., 2017b Apple Transient Cooling rate; cooling uniformity; air pressure drop; energy consumption Remark: In this study, the cooling performances of 10 different apple box samples used in China were evaluated by using a three-dimensional numerical model. The maximum deviation between the predicted and experimental values of the apple surface temperature was 18.69%. Results indicate that the cooling rate increases with the airflow rate, and the optimal air-inflow velocity is 0.4 m/s. The uniform and symmetric distribution of ventilation holes has a more significant contribution to the improvement of precooling effectiveness than the increase of vent area, especially when handling packages with more than three layers. In addition, polyvinyl chloride foam has a relatively smaller airflow resistance. Gruyters et al., 2018 Apple Transient Heat transfer coefficient; air pressure drop Remark: In this study, the realistic product shapes were used in the numerical simulation of the forced-air precooling process of apples. Results indicate that the main factor contributing to the overall pressure drop is the packaging design rather than the shape of the product. However, the shape of the product has a significant impact on the local air velocity and the convective heat transfer coefficient. The real apple shape has a lower surface heat transfer coefficient. Han et al., 2018 Apple Transient Cooling rate; cooling uniformity; energy consumption Remark: In this study, the forced-air precooling process of apple was simulated by a three-dimensional numerical model, in which the heat of respiration and evaporation was taken into account. Results indicate that the optimal air-inflow rate is 2.31 L/(s*kg). Wang et al., 2019 Strawberry Transient Cooling rate; cooling time; cooling uniformity Remark: In this study, the forced-air precooling process of strawberry was simulated by a three-dimensional numerical model, in which the heat of respiration and evaporation was taken into account. Results indicate that the combination of 9.4% area of box vent and 8.5% area of clamshell vent in the commercial packaging system has the best precooling effect on strawberry. Open in new tab Impact of Ventilation Design on Precooling Effectiveness Optimized design of forced-air precooling is critical to maintaining quality and minimizing postharvest losses of horticultural produce (Castro et al., 2006). However, due to poor temperature management, serious cooling heterogeneity will occur in the process of forced-air precooling (Alvarez and Flick, 1999a; Alvarez et al., 2003). The products located behind blind walls have insufficient cooling, but others exposed to high airflow are over-cooled, causing chilling injury. The ventilation design of packaging system is the direct cause of heterogeneous airflow distribution (Vigneault and Goyette, 2003; Vigneault et al., 2005). The reasonably distributed venting holes allow the field heat of horticultural produce to escape, so that there are good airflow patterns, proper temperature, and relative humidity inside the package to ensure long shelf life and good quality of fresh produce (Opara and Zou, 2006). However, the impact of the ventilation design of packaging system on precooling effectiveness is complicated. Due to the complex structure inside the ventilated package filled with horticultural produce, the internal airflow and temperature distribution exhibit serious non-uniformity during the forced-air precooling process. Consequently, cooling heterogeneity is the most important unresolved issue, which threatens both the quality and shelf life of fresh fruits and vegetables seriously. Alvarez and Flick (1999a, 1999b) observed the non-uniform distribution of airflow in the ventilated packaging system through wind tunnel experiments, which resulted in very serious cooling heterogeneity, with the difference in local heat transfer coefficient as high as 40%. The non-uniform distribution of airflow was manifested on the large speed appearing at the inlet and outlet, and the zero-speed on the surface of horticultural produce (Ferrua and Singh, 2008). Using a coupled airflow and heat transfer model for aerodynamic and thermal analysis of the forced-air precooling process, Dehghannya et al. (2008) pointed out that the most non-uniform cooling appears in the vicinity of the inlet and outlet of a ventilated package. The numerical results by Delele et al. (2013a) indicated that the area near the package vent showed relatively high cooling air velocity and turbulence intensity. The typical airflow characteristics and temperature distribution are shown in Figures 2 and 3, respectively. The coldest place is behind the entrance vents. The pressure drops across the inlet and outlet are 51.1% and 45.2%, respectively. There is a good correlation between the airflow rate and the product temperature distribution. The coldest zone corresponds to the zone with the highest cooling air velocity. Therefore, optimization of the ventilation design of packaging system had been regarded as a promising way to improve precooling effectiveness and maintain the quality of horticultural produce in the postharvest cold chain. Figure 2. Open in new tabDownload slide Airflow characteristics inside a ventilated package for a superficial velocity of 0.3 m/s: (a) velocity vector, (b) velocity contour, (c) pressure contour, and (d) turbulent kinetic energy contour (Delele et al., 2013a). Figure 2. Open in new tabDownload slide Airflow characteristics inside a ventilated package for a superficial velocity of 0.3 m/s: (a) velocity vector, (b) velocity contour, (c) pressure contour, and (d) turbulent kinetic energy contour (Delele et al., 2013a). Figure 3. Open in new tabDownload slide Temperature distribution within a ventilated package after 2 h of cooling from an initial temperature of 21°C using air with a superficial velocity of 0.3 m/s and temperature of −5°C (Delele et al., 2013a). Figure 3. Open in new tabDownload slide Temperature distribution within a ventilated package after 2 h of cooling from an initial temperature of 21°C using air with a superficial velocity of 0.3 m/s and temperature of −5°C (Delele et al., 2013a). Impact of air-inflow rate It is commonly accepted that the inlet airflow velocity has the greatest impact on the cooling rate (Castro et al., 2004a; Tutar et al., 2009; Han et al., 2018) since the heat transfer coefficient of the fruit surface is related to the airflow rate. When the airflow rate is insufficient, the precooling process is decelerated, and consequently the rate of product quality deterioration increases (Verboven et al., 2004a). Increasing airflow rate can compensate for the negative effect of low open area. As the airflow rate increases, the surface heat transfer coefficient increases (Fikiin et al., 1999). Under the condition that the vent area is given constant, increasing airflow rate is an effective way to reduce the cooling time (Vigneault et al., 2006; Han et al., 2018). However, increasing airflow rate also has a negative effect on precooling effectiveness. For example, a high airflow rate leads to excessive water loss of fresh foods (Verboven et al., 2004a). Pressure drop also increases with the airflow rate, and consequently the energy consumption increases greatly (Verboven et al., 2004a). Furthermore, the impact of air-inflow rate on precooling effectiveness is in tight association with the location of vents. Any increase in airflow rate would result in an obvious pressure drop and airflow non-uniformity if the open area was formed by non-uniformly distributed vents (Vigneault et al., 2005). As the air-inflow rate increases to 2.31 L/(s*kg), the cooling rate and uniformity are reasonably improved. Any further increase in air-inflow rate will result in a relatively low increase in cooling rate and uniformity, but energy consumption and cooling damage will increase significantly (Han et al., 2018). Impact of vent area Vent area is an important factor in the optimization of ventilation design. To maximize cooling uniformity, vent area should be large enough not to restrict airflow. In practical application, however, vent area should have an upper limit to balance the relationship between the precooling effectiveness of horticultural produce and the mechanical strength of the packaging system. Many efforts had been devoted to the discussion about the impact of vent area on cooling rate. Generally, increasing the area of ventilation holes can accelerate the cooling rate of products (Vigneault et al., 2006; Ferrua and Singh, 2009d). For example, adding top vents for blueberry plastic clamshell can increase the cooling rates of both 6-oz and 1-pt clamshell by 10%–40% and 15%–55%, respectively (Leyte et al., 1999). When the area of a single vent is constant, increasing the number of ventilation holes from two to four halves the cooling time (Han et al., 2015). The ±20% change of vent area had a significant impact on both the central and near-exit area of a ventilated package rather than the near-entrance area (Opara and Zou, 2007). Vent area has an optimal value. The further increase of vent area beyond its optimal value has no obvious contribution to the cooling rate (Tutar et al., 2009). The wind tunnel experiment about the precooling of solid polymer balls (Castro et al., 2004b) indicates that the vent area should exceed 6%. As the total opening area exceeds 8%, increasing vent area has no significant impact on the cooling rate (Castro et al., 2004a). For strawberry, the cooling uniformity is best as the vent area is about 9.4% (Wang et al., 2019). Vent area also affects cooling uniformity and air pressure drop. A more uniform airflow distribution can be achieved by increasing the vent area from 2.4% to 12.1%, with the heterogeneity index decreasing gradually from 108% to 0% (Dehghannya et al., 2008). Similarly, increasing the number of vent holes could also improve cooling uniformity (Dehghannya et al., 2011). In addition, Van der Sman (2002) confirmed the hypothesis of the power-law relationship (⁠ Δptot∼O−1.5 ⁠) between the pressure drop and the box vent ratio (⁠ O ⁠). That is, as the vent area increases, the air pressure drop through the package decreases (Delele et al., 2008), and consequently the energy consumption is reduced (Ferrua and Singh, 2011). From the viewpoint of energy saving, the optimal vent area was suggested to be between 8% and 16% (Vigneault et al., 2005). Impact of vent position Vent position is also an important factor influencing precooling effectiveness. Changing the vent position will change the airflow pattern and consequently influence the heat transfer characteristic and temperature distribution inside the ventilated packaging system. Berry et al. (2016) pointed out that packages with multiple vents were more energy-efficient as the total vent area was given constant. Han et al. (2017b) compared 10 typical apple carton samples used in China and found that uniformly and symmetrically distributed vents were beneficial to the cooling uniformity of multilayer ventilated packaging, especially when handling more than three layers of packaging. Moreover, the position of ventilation holes is critical for cooling uniformity when the air velocity is low. When vent holes are not properly distributed, increasing the vent area may no longer be sufficient to increase the cooling rate (Dehghannya et al., 2012). The design of four vents distributed in corners resulted in poor cooling uniformity and at least 1.75 times more energy consumption in comparison with uniformly distributed vents (Vigneault et al., 2005). Therefore, optimizing vent position is of essential importance for cooling uniformity and energy saving. Impact of other factors The box material and internal packaging, such as tray, bunch carry bag, plastic liner, etc., also influence precooling effectiveness of horticultural produce. Ventilated package is the main cause for pressure drop compared to the product itself (Delele et al., 2013a, 2013b) since it hinders the direct contact of horticultural produce with the cooling airflow and increases the flow resistance. The comparison study by Han et al. (2017b) indicates that polyvinyl chloride foam plastics have a low airflow resistance in comparison with corrugated boards. In addition, Opara and Zou (2007) studied the impact of the gap width between the tray and the package wall on precooling effectiveness. Results indicate that the product centre temperature is very sensitive to the change in the gap width, especially at the product layer away from the airflow inlet. Internal packaging, such as bunch carry bag, plastic liners, polyliners, etc., was often used to prevent the moisture loss in the precooling process. However, these internal packaging materials had a significant contribution to the increase of both pressure drop and cooling time (Delele et al., 2013c). Quantitatively, the pressure drop produced by the non-perforated plastic liner accounts for 83.34% ± 2.13% of the total pressure drop, while the pressure drop of the grape itself only accounts for 1.40% ± 0.01% to 9.41% ± 1.23% (Ngcobo et al., 2012). The maximum volumetric flowrate in a palletized polylined kiwifruit package is 0.34 L/(s*kg), much lower than the recommended flowrate of non-polylined produce (O’Sullivan et al., 2016). That is, in order to achieve the same precooling effectiveness, a lot more energy is needed under the circumstances that complex internal packaging is adopted. In the precooling process, the quality loss of fruit is mainly affected by cooling time. The plastic liner increased the average 7/8 cooling time of the CT1 and CT2 stacks from 4.0 and 2.5 h to 9.5 and 8.0 h, respectively (Ambaw et al., 2017). In addition, energy consumption is up to three times higher than the unlined stack (Mukama et al., 2017). Conclusions This paper reviews available studies about the ventilation design of the packaging system for the forced-air precooling of horticultural produce. The function of packaging system is to protect horticultural produce from mechanical damage during storage and transportation, while vent holes are designed on the package wall to achieve the purpose of rapid and uniform removal of field heat by ensuring adequate airflow through the surface of horticultural produce. Relevant research methods are summarized, along with representative researches and results. Both the merits and flaws of each method are described. Based on the limited and valuable experimental results, developing reliable and efficient numerical simulation schemes seems to be the most hopeful direction of future research. Ventilation packaging plays a key role in the postharvest handling of horticultural produce. The ventilation design is of critical importance in the forced-air precooling process and consequently has been one of the research hotspots of the food cold chain. The impacts of the air-inflow rate, the vent area, the vent position, and other factors, such as the box material and internal packaging, on precooling effectiveness are summarized from the viewpoint of cooling rate, cooling time, cooling uniformity, air pressure drop, and energy consumption. It is found that the influence of ventilation design on precooling effectiveness is complicated. For example, increasing air-inflow rate can accelerate the cooling rate, but it will also increase energy consumption. Increasing vent area tends to increase the cooling rate and improve cooling uniformity, but it also leads to a reduction in the mechanical strength of the ventilated packaging system. Other factors, such as the variety in horticultural produce, the various options for box material and size, the internal packaging, etc., further complicate the optimization of ventilation design. Therefore, multi-parameter analysis seems to be a challenging but promising way for the future improvement of the ventilated packaging system. Funding This work is supported by the National Key R&D Program of China (Grant No. 2016YFD0400100). Conflict of interest statement. The authors certify that there are no conflicts of interest with any organization, financial, or other regarding the material discussed in the manuscript. References Aghdam , M. S. et al. . ( 2019 ). 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TI - Impact of ventilation design on the precooling effectiveness of horticultural produce—a review
JF - Food Quality and Safety
DO - 10.1093/fqsafe/fyaa004
DA - 2020-05-11
UR - https://www.deepdyve.com/lp/oxford-university-press/impact-of-ventilation-design-on-the-precooling-effectiveness-of-fIYeKKy4Pi
SP - 29
VL - 4
IS - 1
DP - DeepDyve
ER -