Relation among quality traits of chicken breast meat during cold storage: correlations between freshness traits and torrymeter values

Relation among quality traits of chicken breast meat during cold storage: correlations between... ABSTRACT The aim of this study was to evaluate the relation among the quality and freshness traits of chicken breast meat during storage for 12 d at 4°C. In addition, correlations between freshness traits and torrymeter values were examined. The L* and a* value of the color was constant; however, the b* value increased on day 5 of storage (P < 0.05). The shear force value significantly decreased, whereas the 2-thiobarbituric acid reactive substances value significantly increased during storage. Regarding the sensory properties, the off-odor and the drip loss increased along with the decrease in color and overall acceptability evaluated by the panelist. The pH, total microorganism counts, and volatile basic nitrogen (VBN) value of chicken breast significantly increased following storage, whereas the torrymeter value and overall acceptability of chicken breast meat significantly decreased from 12.80 to 4.53 and from 6.14 to 1.86, respectively. The VBN value was positively correlated with the pH and total microorganism counts, and the pH value was positively correlated with the total microorganism counts. The torrymeter value was associated with the VBN (−0.918, P < 0.01), pH (−0.973, P < 0.001), total microorganism counts (−0.975, P < 0.001), and overall acceptability (0.884, P < 0.01). These results suggest that the tested traits highly correlate with torrymeter value and it can be used as an indicator of freshness of chicken meat. INTRODUCTION The production of poultry meat has increased worldwide significantly and it is predicted to be the largest meat sector at 130.7 million tons by 2023 (Skarp et al., 2016). Chicken meat is considered to be the healthiest animal-derived food because it is rich in protein and low in fat and cholesterol. Moreover, chicken meat is cheaper than other meats, such as beef, lamb, and pork. Salinas et al. (2012) reported that the plain taste of chicken meat is acceptable in many different countries and cultures, and it is used in prepared meals as a cheap source of protein. However, chicken meat also presents certain disadvantages, such as a short shelf life because of its high content of unsaturated fatty acids that are susceptible to the oxidation process (Marcinkowska-Lesiak et al., 2016). In addition, the presence of bacteria derived from the original microbiota of broiler and meat processing conditions (Salinas et al., 2012) may contribute to the short shelf life of chicken meat. The freshness of chicken meat is important because it determines the consumer decision to purchase the meat. During storage, the freshness of chicken meat decreases and a deterioration in quality may occur. Deterioration of meat quality results in spoilage, making products undesirable for human consumption owing to organoleptic changes, including the appearance of slime, discoloration, or the development of off-odors. Microbial analysis is the most common method used to determine meat spoilage. However, it is time consuming and takes 1 or 2 d for colony formation. Some chemical compounds can also be used as spoilage indicators, such as acetate, alcohols, H2S, acetone, dimethyl sulfide, or dimethyl disulfide (El Barbri et al., 2008). However, the spoilage determination using chemical compounds as indicators in meat requires extensive sampling, extraction, and analysis. Therefore, a faster, real-time meat spoilage detector is needed. Many studies have been performed to evaluate the simpler methods for determination of meat freshness such as electronic nose, optoelectronic nose, litmus paper, and torrymeter. The electronic nose (Boothe and Arnold, 2002) and volatile basic nitrogen (VBN) (Chae et al., 2011) can be used to measure the volatile compound produced by meat during the storage. The optoelectronic nose measures meat freshness by measuring the color change of meat through a colorimetric sensor array (Salinas et al., 2012). Kuswandi et al. (2015) reported that litmus paper can be used to assess the change in pH due to meat spoilage. However, a drawback of using litmus paper as an indicator is that it does not provide an exact value of the measurement. Meanwhile, the torrymeter can be highly sensitive, portable, and easy to use for measuring meat freshness (Kruk et al., 2011; Bae et al., 2014). The torrymeter measures changes in electrical properties of the meat tissue during storage. In fact, the Korea Institute of Animal Products Quality Evaluation uses torrymeters to evaluate the freshness of chicken carcasses at slaughterhouses for reference data. However, there is still a lack of information regarding the relation among quality traits and the freshness of chicken meat measured using a torrymeter. Thus, the aim of this study was to evaluate the relation among quality and freshness traits, including those represented as the torrymeter value, of chicken breast during storage at 4°C. MATERIALS AND METHODS Sample Preparation A total of 35 chicken breast meat samples were purchased fresh from a local slaughterhouse, immediately placed onto white styrofoam trays, and wrapped with low-density polyethylene. Then the samples were directly stored in the dark in a refrigerator at 4°C for 12 d. Analyses were conducted on day 1, 3, 5, 6, 7, 9, and 12 of storage. On each sampling day, 5 trays were selected randomly for analysis. pH Value Determination After blending 10 g of sample with 90 mL of distilled water for 60 s in a homogenizer (Polytron R PT-2500 E, Kinematica, Lucerne, Switzerland), the pH values of the homogenates were determined using a pH meter (Orion 230 A, Thermo Fisher Scientific, Waltham, MA). Instrumental Color Determination Meat color was measured using a colorimeter (Chroma Meter CR-300, Minolta Co., Osaka, Japan). Color values of L* (lightness), a* (redness), and b* (yellowness) were repeatedly measured using the same method and process. The standard white plate had a Y value of 93.60, x value of 0.3134, and y value of 0.3194. Shear Force Value Determination Before the shear force test, the sample was placed in a polyethylene plastic bag and heated in a constant temperature water bath until an internal meat temperature of 75°C ± 2°C was reached. After cooking and cooling, the samples were cut into 2 × 2 × 1 cm sized pieces. Shear force analysis was performed using a Texture Analyzer TA 1 (Lloyd Instruments, Berwyn, PA). The measurement traits of the texture analyzer were set to a 50-kg load cell, 50 mm/min trigger speed, 50 mm/min test speed, and 10 gf trigger forces. Microorganism Analysis Total microorganism and Escherichia coli counts were determined using Petrifilm according to the manufacturer's instructions (Aerobic Count Plates, Coliform/E. coli Count Plates, 3 M, St. Paul, MN). A 10 g sample of breast meat was placed in a sterile bag with 90 mL of sterilized saline and homogenized for 2 min using a stomacher (Bag Mixer 400; Interscience, Saint-Nom-la-Bretèche, France). The homogenate was serially diluted with sterilized saline, and 1 mL of diluted sample was inoculated into Petrifilm. After culturing at 37°C for 48 h, the number of colonies was counted. Sensory Evaluation Sensory evaluation was conducted by a consumer panel consisting of 24 students at the College of Animal Life Sciences, Kangwon National University, and the quantification of the sensory traits was performed according to the method described by Abdalhai et al. (2014). The chicken breast fillet was removed from the refrigerator on each sampling day and used directly for sensory evaluation. Sensory evaluation traits were quantified on a scale from 1 to 9, likely meat color (1 = very bad, 9 = very good), off-odor (1 = very low, 9 = very high), drip loss (1 = very low, 9 = very high), and overall acceptability (1 = very bad, 9 = very good). The drip loss was evaluated based on the level of water exudate around the chicken breast on the tray. VBN Determination The VBN content of chicken breast was analyzed using the microdiffusion method described by Chae et al. (2011) with a Conway unit. Briefly, 90 mL of distilled water was added to 10 g of sample and homogenized by a homogenizer (PolyTron PT-2500 E, Kinematica). The homogenate was centrifuged for 10 min at 800 × g, and the supernatant was filtered using a filter paper (Whatman No. 1). Next, 1 mL of 0.01 N boric acid and 50 μL of indicator (0.066% methyl red: bromocresol green = 1:1) were added to the inner section of the Conway microdiffusion cell. Then, 1 mL of filtrate and 1 mL of 50% potassium carbonate were added to the outer section of the Conway microdiffusion cell. The sealed Conway unit was incubated at 37°C for 2 h and titrated with 0.02 N sulfuric acid. The content of VBN was calculated as ammonia equivalents using the following equation:   \begin{eqnarray*} {\rm{VBN}}\left( {{\rm{mg}}\,\% } \right) &=& \left( {A-B} \right) \times F \times 0.02\nonumber\\ && \times {\rm{ }}14.007 \times 100/S, \end{eqnarray*} where A is the titration volume of sample (mL), B is the titration volume of blank (mL), F is the standardization index of 0.02 N sulfuric acid, and S is the sample weight (g). 2-Thiobarbituric Acid Reactive Substances (TBARS) Value Determination The 2-thiobarbituric acid reactive substances (TBARS) value was determined using the method described by Witte et al. (1970) with slight modification. First, 25 mL of 20% trichloro-acetic acid (in 2 M phosphoric acid) was added to 10 g of chicken meat sample and homogenized using a homogenizer (PolyTron PT-2500E, Kinematica) for 30 s. The homogenate was diluted with distilled water until the total amount of solution reached 50 mL and was then centrifuged (2500 × g, 4°C, 10 min). After centrifugation, the supernatant was filtered using a filter paper (Whatman No. 1). Next, 5 mL of the filtrate and 5 mL of 0.005 mM 2-thiobarbituric acid were mixed, and the mixture was maintained at 23°C for 15 h and then measured at 530 nm using a UV/VIS spectrophotometer (Molecular Devices, Sunnyvale, CA). The TBARS value was calculated using the following equation:   \begin{eqnarray*} &&{ {\rm{TBARS}}\left( {{\rm{mg}}\,{\rm{malondialdehyde}}/{\rm{kg}}\,{\rm{sample}}} \right)}\nonumber\\ && = \left( {{\rm{Absorbance}}\,{\rm{of}}\,{\rm{sample}}}\right.\nonumber\\ &&\left.-{\rm{Absorbance}}\,{\rm{of}}\,{\rm{blank}}\,{\rm{sample}} \right) \times 5.2. \end{eqnarray*} Torrymeter Value Determination A torrymeter (GR Torry Fish Freshness Meter, Distell Industries, West Lothian, UK) was used to directly measure the electrical properties of chicken breast meat stored at 4°C according to the manufacturer's instructions. Torrymeter sensors were placed firmly on the surface of each sample to eliminate air pockets and were cleaned between measurements. The temperature of the sample to be measured is between 0 and 10°C, with no ice crystals on the surface of meat. A torrymeter dispatches a low electric current under 1 mA to the sample, which helps to determine changes in the dielectric properties of the animal tissues (Bae et al., 2014). The torrymeter provides a response value ranging from 0.1 (very spoilt) to 18.5 (very fresh). Statistical Analysis All data were analyzed using a general linear model with the SAS program (ver. 9.2, SAS Institute, Cary, NC). Duncan's multiple range test was used to compare differences in sample mean values. The significance among the samples was tested at the P < 0.05. All measurements were carried out 5 times (n = 5). Additionally, correlation coefficients for VBN, pH, total microorganism count, overall acceptability, and torrymeter value were generated using Pearson's correlation analysis in SAS. RESULTS AND DISCUSSION pH Value Table 1 shows the physicochemical properties of chicken breast meat during storage for 12 d at 4°C. There was no significant change in the pH value through day 9. However, by day 12 of storage, the pH value had increased up to 6.20, which was significantly higher than pH values of chicken breast on day 1, 3, 5, and 6 (P < 0.05). The pH values of fresh chicken meat ranged from 5.69 to 6.13 (Bae et al., 2014). According to the Ministry of Food and Drug Safety, meat begins to spoil when the pH value reaches more than 6.20. Increases in pH during storage are due to the rapid proliferation of specific microorganisms (Marcinkowska-Lesiak et al., 2016). In this study, an increased pH value of 6.20 on day 12 may indicate that the prevalence of spoilage microorganisms had also increased in the chicken breast meat. This is supported by data from Knox et al. (2008), who found that the predominant spoilage organisms, such as Brochothrix thermosphacta and Shewanella putrefaciens, only grow at higher pH ranges (pH > 5.8). Other studies have also reported that the growth of psychrotrophic species of microorganisms is generally accompanied by an increase in pH due to the products of protein degradation (Rey et al., 1976). Table 1. Changes in physicochemical properties, microbial characteristics, and sensory properties of chicken breast meat during storage at 4°C.   Storage days    1  3  5  6  7  9  12  pH  6.10 ± 0.010b  6.09 ± 0.068b  6.11 ± 0.003b  6.13 ± 0.012b  6.15 ± 0.015a,b  6.15 ± 0.010a,b  6.20 ± 0.007a  Color   L*  59.06 ± 0.851a  59.17 ± 0.703a  58.30 ± 0.469a  58.85 ± 0.571a  58.55 ± 1.385a  58.75 ± 0.966a  58.81 ± 0.958a   a*  0.98 ± 0.032a  0.99 ± 0.093a  0.91 ± 0.134a  0.91 ± 0.127a  0.94 ± 0.236a  1.00 ± 0.361a  0.92 ± 0.206a   b*  3.66 ± 0.365c  4.55 ± 0.150b,c  5.73 ± 0.415a  5.04 ± 0.412a,b  5.24 ± 0.465a,b  5.02 ± 0.206a,b  5.89 ± 0.405a  Shear force (kgf)  4.80 ± 0.250a  3.51 ± 0.324b  3.08 ± 0.117b,c  2.94 ± 0.128c  2.54 ± 0.083 c,d  2.23 ± 0.106d  2.00 ± 0.191d  Total Microorganisms (log CFU/g)  4.43 ± 0.040e  4.48 ± 0.049e  4.72 ± 0.014d  4.99 ± 0.069c  6.09 ± 0.023b  6.12 ± 0.120b  7.83 ± 0.057a  E. coli (log CFU/g)  2.15 ± 0.065e  2.13 ± 0.066e  2.19 ± 0.118d,e  2.44 ± 0.182d  3.13 ± 0.062b  3.66 ± 0.027b  4.24 ± 0.063a  Sensory properties   Color  7.71 ± 0.184a  6.71 ± 0.184b  6.43 ± 0.297b,c  6.00 ± 0.000b,c  5.57 ± 0.202c  4.29 ± 0.286d  2.86 ± 0.738e   Off-odor  1.14 ± 0.143d  3.14 ± 0.340c  4.57 ± 0.369b  4.43 ± 0.369b  4.29 ± 0.360b  4.14 ± 0.261b  8.29 ± 0.286a   Drip loss  3.29 ± 0.360c  5.86 ± 0.404b  5.57 ± 0.571b  6.43 ± 0.297a,b  6.57 ± 0.202a,b  6.43 ± 0.297a,b  7.57 ± 0.481a   Overall acceptability  6.14 ± 0.340a  5.14 ± 0.261a,b  5.57 ± 0.429a,b  4.71 ± 0.184b,c  4.57 ± 0.481b,c  4.00 ± 0.309c  1.86 ± 0.261d    Storage days    1  3  5  6  7  9  12  pH  6.10 ± 0.010b  6.09 ± 0.068b  6.11 ± 0.003b  6.13 ± 0.012b  6.15 ± 0.015a,b  6.15 ± 0.010a,b  6.20 ± 0.007a  Color   L*  59.06 ± 0.851a  59.17 ± 0.703a  58.30 ± 0.469a  58.85 ± 0.571a  58.55 ± 1.385a  58.75 ± 0.966a  58.81 ± 0.958a   a*  0.98 ± 0.032a  0.99 ± 0.093a  0.91 ± 0.134a  0.91 ± 0.127a  0.94 ± 0.236a  1.00 ± 0.361a  0.92 ± 0.206a   b*  3.66 ± 0.365c  4.55 ± 0.150b,c  5.73 ± 0.415a  5.04 ± 0.412a,b  5.24 ± 0.465a,b  5.02 ± 0.206a,b  5.89 ± 0.405a  Shear force (kgf)  4.80 ± 0.250a  3.51 ± 0.324b  3.08 ± 0.117b,c  2.94 ± 0.128c  2.54 ± 0.083 c,d  2.23 ± 0.106d  2.00 ± 0.191d  Total Microorganisms (log CFU/g)  4.43 ± 0.040e  4.48 ± 0.049e  4.72 ± 0.014d  4.99 ± 0.069c  6.09 ± 0.023b  6.12 ± 0.120b  7.83 ± 0.057a  E. coli (log CFU/g)  2.15 ± 0.065e  2.13 ± 0.066e  2.19 ± 0.118d,e  2.44 ± 0.182d  3.13 ± 0.062b  3.66 ± 0.027b  4.24 ± 0.063a  Sensory properties   Color  7.71 ± 0.184a  6.71 ± 0.184b  6.43 ± 0.297b,c  6.00 ± 0.000b,c  5.57 ± 0.202c  4.29 ± 0.286d  2.86 ± 0.738e   Off-odor  1.14 ± 0.143d  3.14 ± 0.340c  4.57 ± 0.369b  4.43 ± 0.369b  4.29 ± 0.360b  4.14 ± 0.261b  8.29 ± 0.286a   Drip loss  3.29 ± 0.360c  5.86 ± 0.404b  5.57 ± 0.571b  6.43 ± 0.297a,b  6.57 ± 0.202a,b  6.43 ± 0.297a,b  7.57 ± 0.481a   Overall acceptability  6.14 ± 0.340a  5.14 ± 0.261a,b  5.57 ± 0.429a,b  4.71 ± 0.184b,c  4.57 ± 0.481b,c  4.00 ± 0.309c  1.86 ± 0.261d  Values are means ± SE (n = 5). Means within a row without a common superscript significantly differ at P < 0.05. View Large Table 1. Changes in physicochemical properties, microbial characteristics, and sensory properties of chicken breast meat during storage at 4°C.   Storage days    1  3  5  6  7  9  12  pH  6.10 ± 0.010b  6.09 ± 0.068b  6.11 ± 0.003b  6.13 ± 0.012b  6.15 ± 0.015a,b  6.15 ± 0.010a,b  6.20 ± 0.007a  Color   L*  59.06 ± 0.851a  59.17 ± 0.703a  58.30 ± 0.469a  58.85 ± 0.571a  58.55 ± 1.385a  58.75 ± 0.966a  58.81 ± 0.958a   a*  0.98 ± 0.032a  0.99 ± 0.093a  0.91 ± 0.134a  0.91 ± 0.127a  0.94 ± 0.236a  1.00 ± 0.361a  0.92 ± 0.206a   b*  3.66 ± 0.365c  4.55 ± 0.150b,c  5.73 ± 0.415a  5.04 ± 0.412a,b  5.24 ± 0.465a,b  5.02 ± 0.206a,b  5.89 ± 0.405a  Shear force (kgf)  4.80 ± 0.250a  3.51 ± 0.324b  3.08 ± 0.117b,c  2.94 ± 0.128c  2.54 ± 0.083 c,d  2.23 ± 0.106d  2.00 ± 0.191d  Total Microorganisms (log CFU/g)  4.43 ± 0.040e  4.48 ± 0.049e  4.72 ± 0.014d  4.99 ± 0.069c  6.09 ± 0.023b  6.12 ± 0.120b  7.83 ± 0.057a  E. coli (log CFU/g)  2.15 ± 0.065e  2.13 ± 0.066e  2.19 ± 0.118d,e  2.44 ± 0.182d  3.13 ± 0.062b  3.66 ± 0.027b  4.24 ± 0.063a  Sensory properties   Color  7.71 ± 0.184a  6.71 ± 0.184b  6.43 ± 0.297b,c  6.00 ± 0.000b,c  5.57 ± 0.202c  4.29 ± 0.286d  2.86 ± 0.738e   Off-odor  1.14 ± 0.143d  3.14 ± 0.340c  4.57 ± 0.369b  4.43 ± 0.369b  4.29 ± 0.360b  4.14 ± 0.261b  8.29 ± 0.286a   Drip loss  3.29 ± 0.360c  5.86 ± 0.404b  5.57 ± 0.571b  6.43 ± 0.297a,b  6.57 ± 0.202a,b  6.43 ± 0.297a,b  7.57 ± 0.481a   Overall acceptability  6.14 ± 0.340a  5.14 ± 0.261a,b  5.57 ± 0.429a,b  4.71 ± 0.184b,c  4.57 ± 0.481b,c  4.00 ± 0.309c  1.86 ± 0.261d    Storage days    1  3  5  6  7  9  12  pH  6.10 ± 0.010b  6.09 ± 0.068b  6.11 ± 0.003b  6.13 ± 0.012b  6.15 ± 0.015a,b  6.15 ± 0.010a,b  6.20 ± 0.007a  Color   L*  59.06 ± 0.851a  59.17 ± 0.703a  58.30 ± 0.469a  58.85 ± 0.571a  58.55 ± 1.385a  58.75 ± 0.966a  58.81 ± 0.958a   a*  0.98 ± 0.032a  0.99 ± 0.093a  0.91 ± 0.134a  0.91 ± 0.127a  0.94 ± 0.236a  1.00 ± 0.361a  0.92 ± 0.206a   b*  3.66 ± 0.365c  4.55 ± 0.150b,c  5.73 ± 0.415a  5.04 ± 0.412a,b  5.24 ± 0.465a,b  5.02 ± 0.206a,b  5.89 ± 0.405a  Shear force (kgf)  4.80 ± 0.250a  3.51 ± 0.324b  3.08 ± 0.117b,c  2.94 ± 0.128c  2.54 ± 0.083 c,d  2.23 ± 0.106d  2.00 ± 0.191d  Total Microorganisms (log CFU/g)  4.43 ± 0.040e  4.48 ± 0.049e  4.72 ± 0.014d  4.99 ± 0.069c  6.09 ± 0.023b  6.12 ± 0.120b  7.83 ± 0.057a  E. coli (log CFU/g)  2.15 ± 0.065e  2.13 ± 0.066e  2.19 ± 0.118d,e  2.44 ± 0.182d  3.13 ± 0.062b  3.66 ± 0.027b  4.24 ± 0.063a  Sensory properties   Color  7.71 ± 0.184a  6.71 ± 0.184b  6.43 ± 0.297b,c  6.00 ± 0.000b,c  5.57 ± 0.202c  4.29 ± 0.286d  2.86 ± 0.738e   Off-odor  1.14 ± 0.143d  3.14 ± 0.340c  4.57 ± 0.369b  4.43 ± 0.369b  4.29 ± 0.360b  4.14 ± 0.261b  8.29 ± 0.286a   Drip loss  3.29 ± 0.360c  5.86 ± 0.404b  5.57 ± 0.571b  6.43 ± 0.297a,b  6.57 ± 0.202a,b  6.43 ± 0.297a,b  7.57 ± 0.481a   Overall acceptability  6.14 ± 0.340a  5.14 ± 0.261a,b  5.57 ± 0.429a,b  4.71 ± 0.184b,c  4.57 ± 0.481b,c  4.00 ± 0.309c  1.86 ± 0.261d  Values are means ± SE (n = 5). Means within a row without a common superscript significantly differ at P < 0.05. View Large pH value is considered to be the main factor affecting all quality attributes, including color (Adzitey and Nurul, 2011), water holding capacity (Keenan et al., 2010), tenderness (Jayasena et al., 2013), and microbial growth (Knox et al., 2008). Instrumental Meat Color Meat color is the first trait used by the consumer to reach a decision regarding whether to purchase the meat; thus, meat color is an important trait of meat quality. As shown in Table 1, the L* (lightness) and a* (redness) values of the chicken breast meat were maintained during storage from 58.30 to 59.06 and 0.91 to 1.0, respectively, and no significant changes occurred until the end of storage. In contrast, the b* (yellowness) value of the chicken breast meat increased significantly on day 5 and remained constant until day 12. Similarly, Azlin-Hasim et al. (2015) reported that the L* and a* values of chicken breast meat were not affected by storage for 12 d at 4°C, and the L* value was approximately 54.1 to 55.1 and the a* value was approximately 1.6 to 2.0. They also reported that the b* value of the chicken breast was approximately 2.8 to 3.8 during storage. In this study, b* value was 3.66 on day 1 and increased to 5.73 on day 5, which is slightly higher than that reported by Azlin-Hasim et al. (2015). Young and Lyon (1997) reported that color deterioration can occur in meat due to myoglobin oxidation, where myoglobin is converted to metmyoglobin, resulting in the brown coloration of the meat. Lipid oxidation products such as hydroperoxides and other free radicals oxidize the ferrous ion (Fe2+) in oxymyoglobin into the ferric form (Fe3+) in metmyoglobin (Kim et al., 2013). However, poultry meat is not as dramatically discolored as beef and pork owing to its lower myoglobin content (Zhou et al., 2010). This may be the reason why the L* and a* values were not significantly changed during storage. Although the relation between instrumental color and other quality traits was not evaluated in the present study, Jung et al. (2015) reported that the color value has a relation with the ultimate pH and water holding capacity. The ultimate pH of the meat was negatively correlated with the L* value, and positively correlated with a* value of breast meat. In other words, meat with low ultimate pH will have a high L* value and low a* value. Low ultimate pH of meat is considered to be responsible for the pale meat color (Le Bihan-Duval et al., 2008), and highly related with poor water holding capacity in meat (Huff-Lonergan and Lonergan, 2005). Shear Force Value Shear force measurement is the most effective method for evaluating meat tenderness (Xiong et al., 2006). The shear force values of the chicken breast meat samples throughout the 12 d storage at 4°C are shown in Table 1. Shear force decreased significantly during storage from 4.80 kgf on day 1 to 2.00 kgf by day 12. This finding was consistent with the results reported by Young and Lyon (1997), who found that the shear force values of chicken breast meat decreased with prolonged storage time. Meat tenderness is determined by level of proteolysis of myofibrillar proteins, which serve to preserve the structural integrity of muscle fiber (Marcinkowska-Lesiak et al., 2016). Kruk et al. (2011) reported that degradation of proteins in meat can be caused by either bacterial or enzymatic processes. Analysis of Microorganisms The total microorganism counts in the chicken breast meat samples in this study increased gradually during storage at 4°C from day 1 to day 12 (Table 1). The total microorganism counts increased significantly on day 5, 6, 7, and 12, with values of 4.72, 4.99, 6.09, and 7.83 log CFU/g, respectively. This is consistent with the findings of Nowak and Krysiak (2005), who reported that the microorganism count increased in stored meat and contributed to the deterioration of the physicochemical properties of meat. In addition, the total microorganism counts in this study were similar to those in a study by Balamatsia et al. (2006), who reported that the total microorganism count in chicken meat during storage for 17 d exposed to air at 4°C ranged from 5.1 to 9.3 log CFU/g. The E. coli counts in the chicken breast meat samples are shown in Table 1. The population of E. coli in the chicken breast meat showed no significant changes from day 1 to 5 of storage. However, the E. coli count in chicken breast meat significantly increased on day 6, 7, and 12 with values of 2.44, 3.13, and 4.24 log CFU/g, respectively. The E. coli count of chicken breast on day 6, 7 and 12 was significantly higher (P < 0.05) than that on day 1 and 3 of storage. The rate of spoilage and increase in the number of microorganisms depend in part on the meat type, packaging treatment, and storage system after distribution to the market (Kozačinski et al., 2006). In terms of number of microorganisms, meat is considered spoiled if the total microorganism with an aerobic plate count at mesophilic temperature (25°C to 40°C) is 7 log CFU/g (Knox et al., 2008). Sensory Characteristics The sensory characteristic scores of the chicken breast meat samples undergoing cold storage are shown in Table 1. The color score significantly decreased from 7.71 on day 1 to 2.86 on day 12, in agreement with the study of Vaithiyanathan et al. (2011) who reported that the sensory color score of the chicken breast meat decreased from 6.8 (day 0) to 5.33 (day 12) during storage at 4°C. The off-odor score significantly increased from 1.14 on day 1 to 8.29 on day 12. Off-odor and off-flavor are also related to the lipid oxidation of the meat (Duan et al., 2017). In addition, protein degradation products released by microorganisms also lead to off-odor formation (Silva and Glória, 2002). Therefore, the meat spoilage can be determined by the presence of off-odors, off-flavors, or discoloration. The drip loss score significantly increased on day 3 and 12 with values of 5.86 and 7.57, respectively. The 3 major sensory properties that indicate meat quality are texture, flavor, and overall acceptability (Pelicano et al., 2003). Of these, overall acceptability is the most important, as it influences the consumer decision to purchase the product (Gray at al., 1996). In this study, the overall acceptability score decreased from 6.14 on day 1 to 1.86 on day 12. Based on the sensory evaluation results obtained from this study, after 12 d of storage, the chicken breast meat was considered inedible. VBN Value Volatile basic nitrogen is one of the traits used as an indicator of freshness, with higher VBN values indicating that chicken breast meat is less fresh. As shown in Table 2, the VBN value of the chicken breast meat on day 12 was significantly higher than that on day 7 and 9 (P < 0.05). During storage, VBN ranged from 17.06 mg% (day 1) to 27.99 mg% (day 12). These VBN values are slightly lower than those reported by Min et al. (2007), who found a VBN value of chicken breast meat stored at 4°C of 32.7 mg% on day 9. Increases in VBN may due to either bacterial or enzymatic degradation of meat proteins (Kruk et al., 2011). Spoilage microorganisms and natural enzymes in chicken meat break down proteins and produce VBN compounds, such as ammonia, trimethylamine, and dimethylamine (Cai et al., 2011). In Korea, for meat to be considered fresh, the VBN should be below 20 mg%. Based on the observed VBN values, the chicken meat in this study should be considered spoiled after 12 d of storage. Table 2. Volatile basic nitrogen (VBN), 2-thiobarbituric acid reactive substances (TBARS), and torrymeter values of chicken breast meat during storage at 4°C.   Storage days  Traits  1  3  5  6  7  9  12  VBN (mg%)  17.06 ± 0.329c  17.15 ± 0.320c  18.08 ± 0.163b,c  18.33 ± 0.350b,c  18.71 ± 0.665b,c  19.89 ± 0.531b  27.99 ± 1.380a  TBARS (mg MDA/kg)  0.012 ± 0.000f  0.016 ± 0.000e  0.040 ± 0.002d  0.057 ± 0.002c  0.056 ± 0.002c  0.093 ± 0.001b  0.105 ± 0.003a  Torrymeter value  12.80 ± 0.041a  10.85 ± 0.029b  9.10 ± 0.135c  8.13 ± 0.063d  6.80 ± 0.058e  6.53 ± 0.087f  4.53 ± 0.025g    Storage days  Traits  1  3  5  6  7  9  12  VBN (mg%)  17.06 ± 0.329c  17.15 ± 0.320c  18.08 ± 0.163b,c  18.33 ± 0.350b,c  18.71 ± 0.665b,c  19.89 ± 0.531b  27.99 ± 1.380a  TBARS (mg MDA/kg)  0.012 ± 0.000f  0.016 ± 0.000e  0.040 ± 0.002d  0.057 ± 0.002c  0.056 ± 0.002c  0.093 ± 0.001b  0.105 ± 0.003a  Torrymeter value  12.80 ± 0.041a  10.85 ± 0.029b  9.10 ± 0.135c  8.13 ± 0.063d  6.80 ± 0.058e  6.53 ± 0.087f  4.53 ± 0.025g  Values are means ± SE (n = 5). Means within a row without a common superscript significantly differ at P < 0.05. View Large Table 2. Volatile basic nitrogen (VBN), 2-thiobarbituric acid reactive substances (TBARS), and torrymeter values of chicken breast meat during storage at 4°C.   Storage days  Traits  1  3  5  6  7  9  12  VBN (mg%)  17.06 ± 0.329c  17.15 ± 0.320c  18.08 ± 0.163b,c  18.33 ± 0.350b,c  18.71 ± 0.665b,c  19.89 ± 0.531b  27.99 ± 1.380a  TBARS (mg MDA/kg)  0.012 ± 0.000f  0.016 ± 0.000e  0.040 ± 0.002d  0.057 ± 0.002c  0.056 ± 0.002c  0.093 ± 0.001b  0.105 ± 0.003a  Torrymeter value  12.80 ± 0.041a  10.85 ± 0.029b  9.10 ± 0.135c  8.13 ± 0.063d  6.80 ± 0.058e  6.53 ± 0.087f  4.53 ± 0.025g    Storage days  Traits  1  3  5  6  7  9  12  VBN (mg%)  17.06 ± 0.329c  17.15 ± 0.320c  18.08 ± 0.163b,c  18.33 ± 0.350b,c  18.71 ± 0.665b,c  19.89 ± 0.531b  27.99 ± 1.380a  TBARS (mg MDA/kg)  0.012 ± 0.000f  0.016 ± 0.000e  0.040 ± 0.002d  0.057 ± 0.002c  0.056 ± 0.002c  0.093 ± 0.001b  0.105 ± 0.003a  Torrymeter value  12.80 ± 0.041a  10.85 ± 0.029b  9.10 ± 0.135c  8.13 ± 0.063d  6.80 ± 0.058e  6.53 ± 0.087f  4.53 ± 0.025g  Values are means ± SE (n = 5). Means within a row without a common superscript significantly differ at P < 0.05. View Large TBARS Value Lipid oxidation is another factor used as an indicator of quality deterioration in chicken meat. Enhanced lipid oxidation processes in meat contribute to deterioration of meat quality. The TBARS assay measures malondialdehyde (MDA), which is the degradation product of lipid oxidation. The TBARS values of the chicken breast meat samples in this study significantly increased during storage, from 0.012 mg MDA/kg on day 1 to 0.105 mg MDA/kg on day 12 of storage at 4°C (P < 0.05, Table 2). However, these values were lower than those reported by Kruk et al. (2011), who measured TBARS at 0.28 mg MDA/kg on day 1 and 0.47 mg MDA/kg on day 7 of storage. Brewer et al. (1992) reported that TBARS values below 0.2 mg MDA/kg should be acceptable for consumers and indicate fresh meat. Another study reported that the off-odors and off-flavors can be detected by inexperienced consumers at TBARS values in the range of 0.6 to 2.0 mg MDA/kg (Chandra Mohan et al., 2017). The TBARS values observed in this study indicate that the meat should still be acceptable to consumers after 12 d of storage. However, based on the total microorganism count and sensory evaluation, the chicken breast meat appeared to be spoiled after 12 d of storage. This discrepancy may be due to differences among chicken parts and their lipid contents. Chicken breast contains a high amount of protein compared to thigh meat. Therefore, chicken breast may be spoiled more by protein degradation rather than by lipid oxidation. Jang et al. (2010) reported that the TBARS values for chicken thigh meat stored for 0, 3, and 7 d were 0.11, 0.17, and 0.48 mg MDA/kg, respectively, reflecting higher TBARS values than those observed in the present study. This result suggests that determination of meat freshness is based on not one specific trait, but several major freshness traits together. Thus, the determination of meat freshness should consider the TBARS value along with other traits, such as total microorganism count and sensory evaluation. As stated by Zhou et al. (2010), food spoilage can be affected by several factors, including storage temperature, packaging treatment, meat constituents, light intensity, initial microbial load, consumer perception, and endogenous enzyme levels. One of the mechanisms of lipid oxidation is autoxidation, the continuous reaction of the free-radical chain and the most important mechanism of lipid oxidation in meat (Cheng, 2016). Free radicals are atoms or molecules with an unpaired electron, and they are highly unstable and reactive. The lipid oxidation process generates lipid hydroperoxides from fatty acid autoxidation, which decompose into products, such as aldehydes, alkenes, ketones, and alcohols, that are responsible on off-odor and off-flavor formation (Brewer, 2011). There are many factors associated with lipid oxidation in meat, including heat and light, oxygen content and types of oxygen, catalysts, phospholipids, unsaturated fatty acids, processes that destroy muscle membranes, pre-slaughter conditions, and pH (Cheng, 2016). Torrymeter Value A torrymeter is generally used to measure the freshness of meat or fish by measuring the modified electrical properties of the tissue (Bae et al., 2014). Torrymeter values range from 0.1 (very spoiled) to 18.5 (very fresh). In this study, the torrymeter value decreased gradually (P < 0.05) with prolonged storage time (Table 2). The torrymeter value of chicken breast meat was 12.8 on day 1 and decreased to 4.53 by day 12 of storage. This is in agreement with the results of a study by Bae et al. (2014), who reported that the torrymeter value of chicken breast meat decreased significantly from 12.0 on the day 1 to 4.0 on the day 7 of storage at 4°C. Decreases in torrymeter values from storage may reflect decreases in the conductivity and permittivity of meat with prolonged storage times, resulting in a change in the electrical properties of the meat tissue (Ghatass et al., 2008). Torrymeters can be used for identifying meat freshness or spoilage in an accurate and sensitive manner. In addition, meat spoilage can be examined using either sensory evaluation or microbial counts. However, the major disadvantages of these methods are the high cost of training a human panel for sensory evaluation and the time required for microbiological analyses; in addition, these methods lead to the destruction of the tested products and do not provide a direct result (Najam ul et al., 2012). Thus, the use of a torrymeter is an effective method for determining the spoilage or freshness of meat. Correlations Among Freshness Traits of Chicken Breast Meat The correlations among VBN, pH, total microorganism count, torrymeter values, and overall acceptability of the sensory evaluation were analyzed (Table 3). The VBN was positively correlated with pH and total microorganism count with correlation coefficients of 0.901 and 0.924, respectively (P < 0.01). This indicates that an increase in VBN is caused by an increase in the microorganism count, as the microorganisms degrade the protein, eventually resulting in an increased VBN content (Kruk et al., 2011). In addition, pH and total microorganism count were positively correlated with a coefficient of 0.982 (P < 0.001). The increase in pH value that occurs during storage is caused by the end products of the metabolism of proteinaceous materials (Rey et al., 1976). A higher pH in meat makes conditions favorable for the growth of spoilage bacteria (Rey et al., 1976). Table 3. Correlation coefficients among volatile basic nitrogen (VBN), pH, total microorganism count, torrymeter value, and overall acceptability of chicken breast meat.       Total  Torrymeter  Overall  Trait  VBN  pH  Micro-organisms  value  acceptability  VBN  1  0.901**  0.924**  −0.918**  −0.945**  pH    1  0.982***  −0.973***  −0.929**  Total microorganisms      1  −0.975***  −0.951***  Torrymeter value        1  0.884**  Overall acceptability          1        Total  Torrymeter  Overall  Trait  VBN  pH  Micro-organisms  value  acceptability  VBN  1  0.901**  0.924**  −0.918**  −0.945**  pH    1  0.982***  −0.973***  −0.929**  Total microorganisms      1  −0.975***  −0.951***  Torrymeter value        1  0.884**  Overall acceptability          1  **P < 0.01, ***P < 0.001. View Large Table 3. Correlation coefficients among volatile basic nitrogen (VBN), pH, total microorganism count, torrymeter value, and overall acceptability of chicken breast meat.       Total  Torrymeter  Overall  Trait  VBN  pH  Micro-organisms  value  acceptability  VBN  1  0.901**  0.924**  −0.918**  −0.945**  pH    1  0.982***  −0.973***  −0.929**  Total microorganisms      1  −0.975***  −0.951***  Torrymeter value        1  0.884**  Overall acceptability          1        Total  Torrymeter  Overall  Trait  VBN  pH  Micro-organisms  value  acceptability  VBN  1  0.901**  0.924**  −0.918**  −0.945**  pH    1  0.982***  −0.973***  −0.929**  Total microorganisms      1  −0.975***  −0.951***  Torrymeter value        1  0.884**  Overall acceptability          1  **P < 0.01, ***P < 0.001. View Large The degradation of proteins in the meat is caused by either bacterial or enzymatic processes (Kruk et al., 2011). The torrymeter value was found to be negatively correlated with VBN (P < 0.01), pH (P < 0.001), and total microorganism counts (P < 0.001), with coefficients of −0.918, −0.973, and −0.975, respectively. The torrymeter value decreased during storage due to decreases in the conductivity and permittivity of the meat, resulting in changes in the electrical properties of the meat tissue (Ghatass et al., 2008). The degradation of proteins caused by autolytic and bacterial activity during meat storage thus resulted in increased permeability of the cell membranes, eventually destroying the meat tissue. Cell death produces mixed extracellular and intracellular fluids containing ions (Ghatass et al., 2008), thus modifying the electrical properties of the meat tissue and eventually resulting in a decrease in the torrymeter value. This explains why the torrymeter value was negatively correlated with the total microorganism counts, pH, and VBN and provided a good indicator of chicken breast freshness. The overall acceptability of the sensory properties was negatively correlated with the VBN (P < 0.01), pH (P < 0.01), and total microorganism counts (P < 0.001), with coefficients of −0.945, −0.929, and −0.951, respectively. In contrast, a highly positive correlation was observed between the overall acceptability of the sensory properties and the torrymeter value (P < 0.01), with a coefficient of 0.884. This explains that with the increase in freshness indicator (VBN, pH, and total microorganism counts) value, both the acceptability and the torrymeter value significantly decreased. This result suggests that the torrymeter can be used as an effective instrument for measuring meat freshness. 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Relation among quality traits of chicken breast meat during cold storage: correlations between freshness traits and torrymeter values

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Oxford University Press
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© 2018 Poultry Science Association Inc.
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0032-5791
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1525-3171
D.O.I.
10.3382/ps/pey138
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Abstract

ABSTRACT The aim of this study was to evaluate the relation among the quality and freshness traits of chicken breast meat during storage for 12 d at 4°C. In addition, correlations between freshness traits and torrymeter values were examined. The L* and a* value of the color was constant; however, the b* value increased on day 5 of storage (P < 0.05). The shear force value significantly decreased, whereas the 2-thiobarbituric acid reactive substances value significantly increased during storage. Regarding the sensory properties, the off-odor and the drip loss increased along with the decrease in color and overall acceptability evaluated by the panelist. The pH, total microorganism counts, and volatile basic nitrogen (VBN) value of chicken breast significantly increased following storage, whereas the torrymeter value and overall acceptability of chicken breast meat significantly decreased from 12.80 to 4.53 and from 6.14 to 1.86, respectively. The VBN value was positively correlated with the pH and total microorganism counts, and the pH value was positively correlated with the total microorganism counts. The torrymeter value was associated with the VBN (−0.918, P < 0.01), pH (−0.973, P < 0.001), total microorganism counts (−0.975, P < 0.001), and overall acceptability (0.884, P < 0.01). These results suggest that the tested traits highly correlate with torrymeter value and it can be used as an indicator of freshness of chicken meat. INTRODUCTION The production of poultry meat has increased worldwide significantly and it is predicted to be the largest meat sector at 130.7 million tons by 2023 (Skarp et al., 2016). Chicken meat is considered to be the healthiest animal-derived food because it is rich in protein and low in fat and cholesterol. Moreover, chicken meat is cheaper than other meats, such as beef, lamb, and pork. Salinas et al. (2012) reported that the plain taste of chicken meat is acceptable in many different countries and cultures, and it is used in prepared meals as a cheap source of protein. However, chicken meat also presents certain disadvantages, such as a short shelf life because of its high content of unsaturated fatty acids that are susceptible to the oxidation process (Marcinkowska-Lesiak et al., 2016). In addition, the presence of bacteria derived from the original microbiota of broiler and meat processing conditions (Salinas et al., 2012) may contribute to the short shelf life of chicken meat. The freshness of chicken meat is important because it determines the consumer decision to purchase the meat. During storage, the freshness of chicken meat decreases and a deterioration in quality may occur. Deterioration of meat quality results in spoilage, making products undesirable for human consumption owing to organoleptic changes, including the appearance of slime, discoloration, or the development of off-odors. Microbial analysis is the most common method used to determine meat spoilage. However, it is time consuming and takes 1 or 2 d for colony formation. Some chemical compounds can also be used as spoilage indicators, such as acetate, alcohols, H2S, acetone, dimethyl sulfide, or dimethyl disulfide (El Barbri et al., 2008). However, the spoilage determination using chemical compounds as indicators in meat requires extensive sampling, extraction, and analysis. Therefore, a faster, real-time meat spoilage detector is needed. Many studies have been performed to evaluate the simpler methods for determination of meat freshness such as electronic nose, optoelectronic nose, litmus paper, and torrymeter. The electronic nose (Boothe and Arnold, 2002) and volatile basic nitrogen (VBN) (Chae et al., 2011) can be used to measure the volatile compound produced by meat during the storage. The optoelectronic nose measures meat freshness by measuring the color change of meat through a colorimetric sensor array (Salinas et al., 2012). Kuswandi et al. (2015) reported that litmus paper can be used to assess the change in pH due to meat spoilage. However, a drawback of using litmus paper as an indicator is that it does not provide an exact value of the measurement. Meanwhile, the torrymeter can be highly sensitive, portable, and easy to use for measuring meat freshness (Kruk et al., 2011; Bae et al., 2014). The torrymeter measures changes in electrical properties of the meat tissue during storage. In fact, the Korea Institute of Animal Products Quality Evaluation uses torrymeters to evaluate the freshness of chicken carcasses at slaughterhouses for reference data. However, there is still a lack of information regarding the relation among quality traits and the freshness of chicken meat measured using a torrymeter. Thus, the aim of this study was to evaluate the relation among quality and freshness traits, including those represented as the torrymeter value, of chicken breast during storage at 4°C. MATERIALS AND METHODS Sample Preparation A total of 35 chicken breast meat samples were purchased fresh from a local slaughterhouse, immediately placed onto white styrofoam trays, and wrapped with low-density polyethylene. Then the samples were directly stored in the dark in a refrigerator at 4°C for 12 d. Analyses were conducted on day 1, 3, 5, 6, 7, 9, and 12 of storage. On each sampling day, 5 trays were selected randomly for analysis. pH Value Determination After blending 10 g of sample with 90 mL of distilled water for 60 s in a homogenizer (Polytron R PT-2500 E, Kinematica, Lucerne, Switzerland), the pH values of the homogenates were determined using a pH meter (Orion 230 A, Thermo Fisher Scientific, Waltham, MA). Instrumental Color Determination Meat color was measured using a colorimeter (Chroma Meter CR-300, Minolta Co., Osaka, Japan). Color values of L* (lightness), a* (redness), and b* (yellowness) were repeatedly measured using the same method and process. The standard white plate had a Y value of 93.60, x value of 0.3134, and y value of 0.3194. Shear Force Value Determination Before the shear force test, the sample was placed in a polyethylene plastic bag and heated in a constant temperature water bath until an internal meat temperature of 75°C ± 2°C was reached. After cooking and cooling, the samples were cut into 2 × 2 × 1 cm sized pieces. Shear force analysis was performed using a Texture Analyzer TA 1 (Lloyd Instruments, Berwyn, PA). The measurement traits of the texture analyzer were set to a 50-kg load cell, 50 mm/min trigger speed, 50 mm/min test speed, and 10 gf trigger forces. Microorganism Analysis Total microorganism and Escherichia coli counts were determined using Petrifilm according to the manufacturer's instructions (Aerobic Count Plates, Coliform/E. coli Count Plates, 3 M, St. Paul, MN). A 10 g sample of breast meat was placed in a sterile bag with 90 mL of sterilized saline and homogenized for 2 min using a stomacher (Bag Mixer 400; Interscience, Saint-Nom-la-Bretèche, France). The homogenate was serially diluted with sterilized saline, and 1 mL of diluted sample was inoculated into Petrifilm. After culturing at 37°C for 48 h, the number of colonies was counted. Sensory Evaluation Sensory evaluation was conducted by a consumer panel consisting of 24 students at the College of Animal Life Sciences, Kangwon National University, and the quantification of the sensory traits was performed according to the method described by Abdalhai et al. (2014). The chicken breast fillet was removed from the refrigerator on each sampling day and used directly for sensory evaluation. Sensory evaluation traits were quantified on a scale from 1 to 9, likely meat color (1 = very bad, 9 = very good), off-odor (1 = very low, 9 = very high), drip loss (1 = very low, 9 = very high), and overall acceptability (1 = very bad, 9 = very good). The drip loss was evaluated based on the level of water exudate around the chicken breast on the tray. VBN Determination The VBN content of chicken breast was analyzed using the microdiffusion method described by Chae et al. (2011) with a Conway unit. Briefly, 90 mL of distilled water was added to 10 g of sample and homogenized by a homogenizer (PolyTron PT-2500 E, Kinematica). The homogenate was centrifuged for 10 min at 800 × g, and the supernatant was filtered using a filter paper (Whatman No. 1). Next, 1 mL of 0.01 N boric acid and 50 μL of indicator (0.066% methyl red: bromocresol green = 1:1) were added to the inner section of the Conway microdiffusion cell. Then, 1 mL of filtrate and 1 mL of 50% potassium carbonate were added to the outer section of the Conway microdiffusion cell. The sealed Conway unit was incubated at 37°C for 2 h and titrated with 0.02 N sulfuric acid. The content of VBN was calculated as ammonia equivalents using the following equation:   \begin{eqnarray*} {\rm{VBN}}\left( {{\rm{mg}}\,\% } \right) &=& \left( {A-B} \right) \times F \times 0.02\nonumber\\ && \times {\rm{ }}14.007 \times 100/S, \end{eqnarray*} where A is the titration volume of sample (mL), B is the titration volume of blank (mL), F is the standardization index of 0.02 N sulfuric acid, and S is the sample weight (g). 2-Thiobarbituric Acid Reactive Substances (TBARS) Value Determination The 2-thiobarbituric acid reactive substances (TBARS) value was determined using the method described by Witte et al. (1970) with slight modification. First, 25 mL of 20% trichloro-acetic acid (in 2 M phosphoric acid) was added to 10 g of chicken meat sample and homogenized using a homogenizer (PolyTron PT-2500E, Kinematica) for 30 s. The homogenate was diluted with distilled water until the total amount of solution reached 50 mL and was then centrifuged (2500 × g, 4°C, 10 min). After centrifugation, the supernatant was filtered using a filter paper (Whatman No. 1). Next, 5 mL of the filtrate and 5 mL of 0.005 mM 2-thiobarbituric acid were mixed, and the mixture was maintained at 23°C for 15 h and then measured at 530 nm using a UV/VIS spectrophotometer (Molecular Devices, Sunnyvale, CA). The TBARS value was calculated using the following equation:   \begin{eqnarray*} &&{ {\rm{TBARS}}\left( {{\rm{mg}}\,{\rm{malondialdehyde}}/{\rm{kg}}\,{\rm{sample}}} \right)}\nonumber\\ && = \left( {{\rm{Absorbance}}\,{\rm{of}}\,{\rm{sample}}}\right.\nonumber\\ &&\left.-{\rm{Absorbance}}\,{\rm{of}}\,{\rm{blank}}\,{\rm{sample}} \right) \times 5.2. \end{eqnarray*} Torrymeter Value Determination A torrymeter (GR Torry Fish Freshness Meter, Distell Industries, West Lothian, UK) was used to directly measure the electrical properties of chicken breast meat stored at 4°C according to the manufacturer's instructions. Torrymeter sensors were placed firmly on the surface of each sample to eliminate air pockets and were cleaned between measurements. The temperature of the sample to be measured is between 0 and 10°C, with no ice crystals on the surface of meat. A torrymeter dispatches a low electric current under 1 mA to the sample, which helps to determine changes in the dielectric properties of the animal tissues (Bae et al., 2014). The torrymeter provides a response value ranging from 0.1 (very spoilt) to 18.5 (very fresh). Statistical Analysis All data were analyzed using a general linear model with the SAS program (ver. 9.2, SAS Institute, Cary, NC). Duncan's multiple range test was used to compare differences in sample mean values. The significance among the samples was tested at the P < 0.05. All measurements were carried out 5 times (n = 5). Additionally, correlation coefficients for VBN, pH, total microorganism count, overall acceptability, and torrymeter value were generated using Pearson's correlation analysis in SAS. RESULTS AND DISCUSSION pH Value Table 1 shows the physicochemical properties of chicken breast meat during storage for 12 d at 4°C. There was no significant change in the pH value through day 9. However, by day 12 of storage, the pH value had increased up to 6.20, which was significantly higher than pH values of chicken breast on day 1, 3, 5, and 6 (P < 0.05). The pH values of fresh chicken meat ranged from 5.69 to 6.13 (Bae et al., 2014). According to the Ministry of Food and Drug Safety, meat begins to spoil when the pH value reaches more than 6.20. Increases in pH during storage are due to the rapid proliferation of specific microorganisms (Marcinkowska-Lesiak et al., 2016). In this study, an increased pH value of 6.20 on day 12 may indicate that the prevalence of spoilage microorganisms had also increased in the chicken breast meat. This is supported by data from Knox et al. (2008), who found that the predominant spoilage organisms, such as Brochothrix thermosphacta and Shewanella putrefaciens, only grow at higher pH ranges (pH > 5.8). Other studies have also reported that the growth of psychrotrophic species of microorganisms is generally accompanied by an increase in pH due to the products of protein degradation (Rey et al., 1976). Table 1. Changes in physicochemical properties, microbial characteristics, and sensory properties of chicken breast meat during storage at 4°C.   Storage days    1  3  5  6  7  9  12  pH  6.10 ± 0.010b  6.09 ± 0.068b  6.11 ± 0.003b  6.13 ± 0.012b  6.15 ± 0.015a,b  6.15 ± 0.010a,b  6.20 ± 0.007a  Color   L*  59.06 ± 0.851a  59.17 ± 0.703a  58.30 ± 0.469a  58.85 ± 0.571a  58.55 ± 1.385a  58.75 ± 0.966a  58.81 ± 0.958a   a*  0.98 ± 0.032a  0.99 ± 0.093a  0.91 ± 0.134a  0.91 ± 0.127a  0.94 ± 0.236a  1.00 ± 0.361a  0.92 ± 0.206a   b*  3.66 ± 0.365c  4.55 ± 0.150b,c  5.73 ± 0.415a  5.04 ± 0.412a,b  5.24 ± 0.465a,b  5.02 ± 0.206a,b  5.89 ± 0.405a  Shear force (kgf)  4.80 ± 0.250a  3.51 ± 0.324b  3.08 ± 0.117b,c  2.94 ± 0.128c  2.54 ± 0.083 c,d  2.23 ± 0.106d  2.00 ± 0.191d  Total Microorganisms (log CFU/g)  4.43 ± 0.040e  4.48 ± 0.049e  4.72 ± 0.014d  4.99 ± 0.069c  6.09 ± 0.023b  6.12 ± 0.120b  7.83 ± 0.057a  E. coli (log CFU/g)  2.15 ± 0.065e  2.13 ± 0.066e  2.19 ± 0.118d,e  2.44 ± 0.182d  3.13 ± 0.062b  3.66 ± 0.027b  4.24 ± 0.063a  Sensory properties   Color  7.71 ± 0.184a  6.71 ± 0.184b  6.43 ± 0.297b,c  6.00 ± 0.000b,c  5.57 ± 0.202c  4.29 ± 0.286d  2.86 ± 0.738e   Off-odor  1.14 ± 0.143d  3.14 ± 0.340c  4.57 ± 0.369b  4.43 ± 0.369b  4.29 ± 0.360b  4.14 ± 0.261b  8.29 ± 0.286a   Drip loss  3.29 ± 0.360c  5.86 ± 0.404b  5.57 ± 0.571b  6.43 ± 0.297a,b  6.57 ± 0.202a,b  6.43 ± 0.297a,b  7.57 ± 0.481a   Overall acceptability  6.14 ± 0.340a  5.14 ± 0.261a,b  5.57 ± 0.429a,b  4.71 ± 0.184b,c  4.57 ± 0.481b,c  4.00 ± 0.309c  1.86 ± 0.261d    Storage days    1  3  5  6  7  9  12  pH  6.10 ± 0.010b  6.09 ± 0.068b  6.11 ± 0.003b  6.13 ± 0.012b  6.15 ± 0.015a,b  6.15 ± 0.010a,b  6.20 ± 0.007a  Color   L*  59.06 ± 0.851a  59.17 ± 0.703a  58.30 ± 0.469a  58.85 ± 0.571a  58.55 ± 1.385a  58.75 ± 0.966a  58.81 ± 0.958a   a*  0.98 ± 0.032a  0.99 ± 0.093a  0.91 ± 0.134a  0.91 ± 0.127a  0.94 ± 0.236a  1.00 ± 0.361a  0.92 ± 0.206a   b*  3.66 ± 0.365c  4.55 ± 0.150b,c  5.73 ± 0.415a  5.04 ± 0.412a,b  5.24 ± 0.465a,b  5.02 ± 0.206a,b  5.89 ± 0.405a  Shear force (kgf)  4.80 ± 0.250a  3.51 ± 0.324b  3.08 ± 0.117b,c  2.94 ± 0.128c  2.54 ± 0.083 c,d  2.23 ± 0.106d  2.00 ± 0.191d  Total Microorganisms (log CFU/g)  4.43 ± 0.040e  4.48 ± 0.049e  4.72 ± 0.014d  4.99 ± 0.069c  6.09 ± 0.023b  6.12 ± 0.120b  7.83 ± 0.057a  E. coli (log CFU/g)  2.15 ± 0.065e  2.13 ± 0.066e  2.19 ± 0.118d,e  2.44 ± 0.182d  3.13 ± 0.062b  3.66 ± 0.027b  4.24 ± 0.063a  Sensory properties   Color  7.71 ± 0.184a  6.71 ± 0.184b  6.43 ± 0.297b,c  6.00 ± 0.000b,c  5.57 ± 0.202c  4.29 ± 0.286d  2.86 ± 0.738e   Off-odor  1.14 ± 0.143d  3.14 ± 0.340c  4.57 ± 0.369b  4.43 ± 0.369b  4.29 ± 0.360b  4.14 ± 0.261b  8.29 ± 0.286a   Drip loss  3.29 ± 0.360c  5.86 ± 0.404b  5.57 ± 0.571b  6.43 ± 0.297a,b  6.57 ± 0.202a,b  6.43 ± 0.297a,b  7.57 ± 0.481a   Overall acceptability  6.14 ± 0.340a  5.14 ± 0.261a,b  5.57 ± 0.429a,b  4.71 ± 0.184b,c  4.57 ± 0.481b,c  4.00 ± 0.309c  1.86 ± 0.261d  Values are means ± SE (n = 5). Means within a row without a common superscript significantly differ at P < 0.05. View Large Table 1. Changes in physicochemical properties, microbial characteristics, and sensory properties of chicken breast meat during storage at 4°C.   Storage days    1  3  5  6  7  9  12  pH  6.10 ± 0.010b  6.09 ± 0.068b  6.11 ± 0.003b  6.13 ± 0.012b  6.15 ± 0.015a,b  6.15 ± 0.010a,b  6.20 ± 0.007a  Color   L*  59.06 ± 0.851a  59.17 ± 0.703a  58.30 ± 0.469a  58.85 ± 0.571a  58.55 ± 1.385a  58.75 ± 0.966a  58.81 ± 0.958a   a*  0.98 ± 0.032a  0.99 ± 0.093a  0.91 ± 0.134a  0.91 ± 0.127a  0.94 ± 0.236a  1.00 ± 0.361a  0.92 ± 0.206a   b*  3.66 ± 0.365c  4.55 ± 0.150b,c  5.73 ± 0.415a  5.04 ± 0.412a,b  5.24 ± 0.465a,b  5.02 ± 0.206a,b  5.89 ± 0.405a  Shear force (kgf)  4.80 ± 0.250a  3.51 ± 0.324b  3.08 ± 0.117b,c  2.94 ± 0.128c  2.54 ± 0.083 c,d  2.23 ± 0.106d  2.00 ± 0.191d  Total Microorganisms (log CFU/g)  4.43 ± 0.040e  4.48 ± 0.049e  4.72 ± 0.014d  4.99 ± 0.069c  6.09 ± 0.023b  6.12 ± 0.120b  7.83 ± 0.057a  E. coli (log CFU/g)  2.15 ± 0.065e  2.13 ± 0.066e  2.19 ± 0.118d,e  2.44 ± 0.182d  3.13 ± 0.062b  3.66 ± 0.027b  4.24 ± 0.063a  Sensory properties   Color  7.71 ± 0.184a  6.71 ± 0.184b  6.43 ± 0.297b,c  6.00 ± 0.000b,c  5.57 ± 0.202c  4.29 ± 0.286d  2.86 ± 0.738e   Off-odor  1.14 ± 0.143d  3.14 ± 0.340c  4.57 ± 0.369b  4.43 ± 0.369b  4.29 ± 0.360b  4.14 ± 0.261b  8.29 ± 0.286a   Drip loss  3.29 ± 0.360c  5.86 ± 0.404b  5.57 ± 0.571b  6.43 ± 0.297a,b  6.57 ± 0.202a,b  6.43 ± 0.297a,b  7.57 ± 0.481a   Overall acceptability  6.14 ± 0.340a  5.14 ± 0.261a,b  5.57 ± 0.429a,b  4.71 ± 0.184b,c  4.57 ± 0.481b,c  4.00 ± 0.309c  1.86 ± 0.261d    Storage days    1  3  5  6  7  9  12  pH  6.10 ± 0.010b  6.09 ± 0.068b  6.11 ± 0.003b  6.13 ± 0.012b  6.15 ± 0.015a,b  6.15 ± 0.010a,b  6.20 ± 0.007a  Color   L*  59.06 ± 0.851a  59.17 ± 0.703a  58.30 ± 0.469a  58.85 ± 0.571a  58.55 ± 1.385a  58.75 ± 0.966a  58.81 ± 0.958a   a*  0.98 ± 0.032a  0.99 ± 0.093a  0.91 ± 0.134a  0.91 ± 0.127a  0.94 ± 0.236a  1.00 ± 0.361a  0.92 ± 0.206a   b*  3.66 ± 0.365c  4.55 ± 0.150b,c  5.73 ± 0.415a  5.04 ± 0.412a,b  5.24 ± 0.465a,b  5.02 ± 0.206a,b  5.89 ± 0.405a  Shear force (kgf)  4.80 ± 0.250a  3.51 ± 0.324b  3.08 ± 0.117b,c  2.94 ± 0.128c  2.54 ± 0.083 c,d  2.23 ± 0.106d  2.00 ± 0.191d  Total Microorganisms (log CFU/g)  4.43 ± 0.040e  4.48 ± 0.049e  4.72 ± 0.014d  4.99 ± 0.069c  6.09 ± 0.023b  6.12 ± 0.120b  7.83 ± 0.057a  E. coli (log CFU/g)  2.15 ± 0.065e  2.13 ± 0.066e  2.19 ± 0.118d,e  2.44 ± 0.182d  3.13 ± 0.062b  3.66 ± 0.027b  4.24 ± 0.063a  Sensory properties   Color  7.71 ± 0.184a  6.71 ± 0.184b  6.43 ± 0.297b,c  6.00 ± 0.000b,c  5.57 ± 0.202c  4.29 ± 0.286d  2.86 ± 0.738e   Off-odor  1.14 ± 0.143d  3.14 ± 0.340c  4.57 ± 0.369b  4.43 ± 0.369b  4.29 ± 0.360b  4.14 ± 0.261b  8.29 ± 0.286a   Drip loss  3.29 ± 0.360c  5.86 ± 0.404b  5.57 ± 0.571b  6.43 ± 0.297a,b  6.57 ± 0.202a,b  6.43 ± 0.297a,b  7.57 ± 0.481a   Overall acceptability  6.14 ± 0.340a  5.14 ± 0.261a,b  5.57 ± 0.429a,b  4.71 ± 0.184b,c  4.57 ± 0.481b,c  4.00 ± 0.309c  1.86 ± 0.261d  Values are means ± SE (n = 5). Means within a row without a common superscript significantly differ at P < 0.05. View Large pH value is considered to be the main factor affecting all quality attributes, including color (Adzitey and Nurul, 2011), water holding capacity (Keenan et al., 2010), tenderness (Jayasena et al., 2013), and microbial growth (Knox et al., 2008). Instrumental Meat Color Meat color is the first trait used by the consumer to reach a decision regarding whether to purchase the meat; thus, meat color is an important trait of meat quality. As shown in Table 1, the L* (lightness) and a* (redness) values of the chicken breast meat were maintained during storage from 58.30 to 59.06 and 0.91 to 1.0, respectively, and no significant changes occurred until the end of storage. In contrast, the b* (yellowness) value of the chicken breast meat increased significantly on day 5 and remained constant until day 12. Similarly, Azlin-Hasim et al. (2015) reported that the L* and a* values of chicken breast meat were not affected by storage for 12 d at 4°C, and the L* value was approximately 54.1 to 55.1 and the a* value was approximately 1.6 to 2.0. They also reported that the b* value of the chicken breast was approximately 2.8 to 3.8 during storage. In this study, b* value was 3.66 on day 1 and increased to 5.73 on day 5, which is slightly higher than that reported by Azlin-Hasim et al. (2015). Young and Lyon (1997) reported that color deterioration can occur in meat due to myoglobin oxidation, where myoglobin is converted to metmyoglobin, resulting in the brown coloration of the meat. Lipid oxidation products such as hydroperoxides and other free radicals oxidize the ferrous ion (Fe2+) in oxymyoglobin into the ferric form (Fe3+) in metmyoglobin (Kim et al., 2013). However, poultry meat is not as dramatically discolored as beef and pork owing to its lower myoglobin content (Zhou et al., 2010). This may be the reason why the L* and a* values were not significantly changed during storage. Although the relation between instrumental color and other quality traits was not evaluated in the present study, Jung et al. (2015) reported that the color value has a relation with the ultimate pH and water holding capacity. The ultimate pH of the meat was negatively correlated with the L* value, and positively correlated with a* value of breast meat. In other words, meat with low ultimate pH will have a high L* value and low a* value. Low ultimate pH of meat is considered to be responsible for the pale meat color (Le Bihan-Duval et al., 2008), and highly related with poor water holding capacity in meat (Huff-Lonergan and Lonergan, 2005). Shear Force Value Shear force measurement is the most effective method for evaluating meat tenderness (Xiong et al., 2006). The shear force values of the chicken breast meat samples throughout the 12 d storage at 4°C are shown in Table 1. Shear force decreased significantly during storage from 4.80 kgf on day 1 to 2.00 kgf by day 12. This finding was consistent with the results reported by Young and Lyon (1997), who found that the shear force values of chicken breast meat decreased with prolonged storage time. Meat tenderness is determined by level of proteolysis of myofibrillar proteins, which serve to preserve the structural integrity of muscle fiber (Marcinkowska-Lesiak et al., 2016). Kruk et al. (2011) reported that degradation of proteins in meat can be caused by either bacterial or enzymatic processes. Analysis of Microorganisms The total microorganism counts in the chicken breast meat samples in this study increased gradually during storage at 4°C from day 1 to day 12 (Table 1). The total microorganism counts increased significantly on day 5, 6, 7, and 12, with values of 4.72, 4.99, 6.09, and 7.83 log CFU/g, respectively. This is consistent with the findings of Nowak and Krysiak (2005), who reported that the microorganism count increased in stored meat and contributed to the deterioration of the physicochemical properties of meat. In addition, the total microorganism counts in this study were similar to those in a study by Balamatsia et al. (2006), who reported that the total microorganism count in chicken meat during storage for 17 d exposed to air at 4°C ranged from 5.1 to 9.3 log CFU/g. The E. coli counts in the chicken breast meat samples are shown in Table 1. The population of E. coli in the chicken breast meat showed no significant changes from day 1 to 5 of storage. However, the E. coli count in chicken breast meat significantly increased on day 6, 7, and 12 with values of 2.44, 3.13, and 4.24 log CFU/g, respectively. The E. coli count of chicken breast on day 6, 7 and 12 was significantly higher (P < 0.05) than that on day 1 and 3 of storage. The rate of spoilage and increase in the number of microorganisms depend in part on the meat type, packaging treatment, and storage system after distribution to the market (Kozačinski et al., 2006). In terms of number of microorganisms, meat is considered spoiled if the total microorganism with an aerobic plate count at mesophilic temperature (25°C to 40°C) is 7 log CFU/g (Knox et al., 2008). Sensory Characteristics The sensory characteristic scores of the chicken breast meat samples undergoing cold storage are shown in Table 1. The color score significantly decreased from 7.71 on day 1 to 2.86 on day 12, in agreement with the study of Vaithiyanathan et al. (2011) who reported that the sensory color score of the chicken breast meat decreased from 6.8 (day 0) to 5.33 (day 12) during storage at 4°C. The off-odor score significantly increased from 1.14 on day 1 to 8.29 on day 12. Off-odor and off-flavor are also related to the lipid oxidation of the meat (Duan et al., 2017). In addition, protein degradation products released by microorganisms also lead to off-odor formation (Silva and Glória, 2002). Therefore, the meat spoilage can be determined by the presence of off-odors, off-flavors, or discoloration. The drip loss score significantly increased on day 3 and 12 with values of 5.86 and 7.57, respectively. The 3 major sensory properties that indicate meat quality are texture, flavor, and overall acceptability (Pelicano et al., 2003). Of these, overall acceptability is the most important, as it influences the consumer decision to purchase the product (Gray at al., 1996). In this study, the overall acceptability score decreased from 6.14 on day 1 to 1.86 on day 12. Based on the sensory evaluation results obtained from this study, after 12 d of storage, the chicken breast meat was considered inedible. VBN Value Volatile basic nitrogen is one of the traits used as an indicator of freshness, with higher VBN values indicating that chicken breast meat is less fresh. As shown in Table 2, the VBN value of the chicken breast meat on day 12 was significantly higher than that on day 7 and 9 (P < 0.05). During storage, VBN ranged from 17.06 mg% (day 1) to 27.99 mg% (day 12). These VBN values are slightly lower than those reported by Min et al. (2007), who found a VBN value of chicken breast meat stored at 4°C of 32.7 mg% on day 9. Increases in VBN may due to either bacterial or enzymatic degradation of meat proteins (Kruk et al., 2011). Spoilage microorganisms and natural enzymes in chicken meat break down proteins and produce VBN compounds, such as ammonia, trimethylamine, and dimethylamine (Cai et al., 2011). In Korea, for meat to be considered fresh, the VBN should be below 20 mg%. Based on the observed VBN values, the chicken meat in this study should be considered spoiled after 12 d of storage. Table 2. Volatile basic nitrogen (VBN), 2-thiobarbituric acid reactive substances (TBARS), and torrymeter values of chicken breast meat during storage at 4°C.   Storage days  Traits  1  3  5  6  7  9  12  VBN (mg%)  17.06 ± 0.329c  17.15 ± 0.320c  18.08 ± 0.163b,c  18.33 ± 0.350b,c  18.71 ± 0.665b,c  19.89 ± 0.531b  27.99 ± 1.380a  TBARS (mg MDA/kg)  0.012 ± 0.000f  0.016 ± 0.000e  0.040 ± 0.002d  0.057 ± 0.002c  0.056 ± 0.002c  0.093 ± 0.001b  0.105 ± 0.003a  Torrymeter value  12.80 ± 0.041a  10.85 ± 0.029b  9.10 ± 0.135c  8.13 ± 0.063d  6.80 ± 0.058e  6.53 ± 0.087f  4.53 ± 0.025g    Storage days  Traits  1  3  5  6  7  9  12  VBN (mg%)  17.06 ± 0.329c  17.15 ± 0.320c  18.08 ± 0.163b,c  18.33 ± 0.350b,c  18.71 ± 0.665b,c  19.89 ± 0.531b  27.99 ± 1.380a  TBARS (mg MDA/kg)  0.012 ± 0.000f  0.016 ± 0.000e  0.040 ± 0.002d  0.057 ± 0.002c  0.056 ± 0.002c  0.093 ± 0.001b  0.105 ± 0.003a  Torrymeter value  12.80 ± 0.041a  10.85 ± 0.029b  9.10 ± 0.135c  8.13 ± 0.063d  6.80 ± 0.058e  6.53 ± 0.087f  4.53 ± 0.025g  Values are means ± SE (n = 5). Means within a row without a common superscript significantly differ at P < 0.05. View Large Table 2. Volatile basic nitrogen (VBN), 2-thiobarbituric acid reactive substances (TBARS), and torrymeter values of chicken breast meat during storage at 4°C.   Storage days  Traits  1  3  5  6  7  9  12  VBN (mg%)  17.06 ± 0.329c  17.15 ± 0.320c  18.08 ± 0.163b,c  18.33 ± 0.350b,c  18.71 ± 0.665b,c  19.89 ± 0.531b  27.99 ± 1.380a  TBARS (mg MDA/kg)  0.012 ± 0.000f  0.016 ± 0.000e  0.040 ± 0.002d  0.057 ± 0.002c  0.056 ± 0.002c  0.093 ± 0.001b  0.105 ± 0.003a  Torrymeter value  12.80 ± 0.041a  10.85 ± 0.029b  9.10 ± 0.135c  8.13 ± 0.063d  6.80 ± 0.058e  6.53 ± 0.087f  4.53 ± 0.025g    Storage days  Traits  1  3  5  6  7  9  12  VBN (mg%)  17.06 ± 0.329c  17.15 ± 0.320c  18.08 ± 0.163b,c  18.33 ± 0.350b,c  18.71 ± 0.665b,c  19.89 ± 0.531b  27.99 ± 1.380a  TBARS (mg MDA/kg)  0.012 ± 0.000f  0.016 ± 0.000e  0.040 ± 0.002d  0.057 ± 0.002c  0.056 ± 0.002c  0.093 ± 0.001b  0.105 ± 0.003a  Torrymeter value  12.80 ± 0.041a  10.85 ± 0.029b  9.10 ± 0.135c  8.13 ± 0.063d  6.80 ± 0.058e  6.53 ± 0.087f  4.53 ± 0.025g  Values are means ± SE (n = 5). Means within a row without a common superscript significantly differ at P < 0.05. View Large TBARS Value Lipid oxidation is another factor used as an indicator of quality deterioration in chicken meat. Enhanced lipid oxidation processes in meat contribute to deterioration of meat quality. The TBARS assay measures malondialdehyde (MDA), which is the degradation product of lipid oxidation. The TBARS values of the chicken breast meat samples in this study significantly increased during storage, from 0.012 mg MDA/kg on day 1 to 0.105 mg MDA/kg on day 12 of storage at 4°C (P < 0.05, Table 2). However, these values were lower than those reported by Kruk et al. (2011), who measured TBARS at 0.28 mg MDA/kg on day 1 and 0.47 mg MDA/kg on day 7 of storage. Brewer et al. (1992) reported that TBARS values below 0.2 mg MDA/kg should be acceptable for consumers and indicate fresh meat. Another study reported that the off-odors and off-flavors can be detected by inexperienced consumers at TBARS values in the range of 0.6 to 2.0 mg MDA/kg (Chandra Mohan et al., 2017). The TBARS values observed in this study indicate that the meat should still be acceptable to consumers after 12 d of storage. However, based on the total microorganism count and sensory evaluation, the chicken breast meat appeared to be spoiled after 12 d of storage. This discrepancy may be due to differences among chicken parts and their lipid contents. Chicken breast contains a high amount of protein compared to thigh meat. Therefore, chicken breast may be spoiled more by protein degradation rather than by lipid oxidation. Jang et al. (2010) reported that the TBARS values for chicken thigh meat stored for 0, 3, and 7 d were 0.11, 0.17, and 0.48 mg MDA/kg, respectively, reflecting higher TBARS values than those observed in the present study. This result suggests that determination of meat freshness is based on not one specific trait, but several major freshness traits together. Thus, the determination of meat freshness should consider the TBARS value along with other traits, such as total microorganism count and sensory evaluation. As stated by Zhou et al. (2010), food spoilage can be affected by several factors, including storage temperature, packaging treatment, meat constituents, light intensity, initial microbial load, consumer perception, and endogenous enzyme levels. One of the mechanisms of lipid oxidation is autoxidation, the continuous reaction of the free-radical chain and the most important mechanism of lipid oxidation in meat (Cheng, 2016). Free radicals are atoms or molecules with an unpaired electron, and they are highly unstable and reactive. The lipid oxidation process generates lipid hydroperoxides from fatty acid autoxidation, which decompose into products, such as aldehydes, alkenes, ketones, and alcohols, that are responsible on off-odor and off-flavor formation (Brewer, 2011). There are many factors associated with lipid oxidation in meat, including heat and light, oxygen content and types of oxygen, catalysts, phospholipids, unsaturated fatty acids, processes that destroy muscle membranes, pre-slaughter conditions, and pH (Cheng, 2016). Torrymeter Value A torrymeter is generally used to measure the freshness of meat or fish by measuring the modified electrical properties of the tissue (Bae et al., 2014). Torrymeter values range from 0.1 (very spoiled) to 18.5 (very fresh). In this study, the torrymeter value decreased gradually (P < 0.05) with prolonged storage time (Table 2). The torrymeter value of chicken breast meat was 12.8 on day 1 and decreased to 4.53 by day 12 of storage. This is in agreement with the results of a study by Bae et al. (2014), who reported that the torrymeter value of chicken breast meat decreased significantly from 12.0 on the day 1 to 4.0 on the day 7 of storage at 4°C. Decreases in torrymeter values from storage may reflect decreases in the conductivity and permittivity of meat with prolonged storage times, resulting in a change in the electrical properties of the meat tissue (Ghatass et al., 2008). Torrymeters can be used for identifying meat freshness or spoilage in an accurate and sensitive manner. In addition, meat spoilage can be examined using either sensory evaluation or microbial counts. However, the major disadvantages of these methods are the high cost of training a human panel for sensory evaluation and the time required for microbiological analyses; in addition, these methods lead to the destruction of the tested products and do not provide a direct result (Najam ul et al., 2012). Thus, the use of a torrymeter is an effective method for determining the spoilage or freshness of meat. Correlations Among Freshness Traits of Chicken Breast Meat The correlations among VBN, pH, total microorganism count, torrymeter values, and overall acceptability of the sensory evaluation were analyzed (Table 3). The VBN was positively correlated with pH and total microorganism count with correlation coefficients of 0.901 and 0.924, respectively (P < 0.01). This indicates that an increase in VBN is caused by an increase in the microorganism count, as the microorganisms degrade the protein, eventually resulting in an increased VBN content (Kruk et al., 2011). In addition, pH and total microorganism count were positively correlated with a coefficient of 0.982 (P < 0.001). The increase in pH value that occurs during storage is caused by the end products of the metabolism of proteinaceous materials (Rey et al., 1976). A higher pH in meat makes conditions favorable for the growth of spoilage bacteria (Rey et al., 1976). Table 3. Correlation coefficients among volatile basic nitrogen (VBN), pH, total microorganism count, torrymeter value, and overall acceptability of chicken breast meat.       Total  Torrymeter  Overall  Trait  VBN  pH  Micro-organisms  value  acceptability  VBN  1  0.901**  0.924**  −0.918**  −0.945**  pH    1  0.982***  −0.973***  −0.929**  Total microorganisms      1  −0.975***  −0.951***  Torrymeter value        1  0.884**  Overall acceptability          1        Total  Torrymeter  Overall  Trait  VBN  pH  Micro-organisms  value  acceptability  VBN  1  0.901**  0.924**  −0.918**  −0.945**  pH    1  0.982***  −0.973***  −0.929**  Total microorganisms      1  −0.975***  −0.951***  Torrymeter value        1  0.884**  Overall acceptability          1  **P < 0.01, ***P < 0.001. View Large Table 3. Correlation coefficients among volatile basic nitrogen (VBN), pH, total microorganism count, torrymeter value, and overall acceptability of chicken breast meat.       Total  Torrymeter  Overall  Trait  VBN  pH  Micro-organisms  value  acceptability  VBN  1  0.901**  0.924**  −0.918**  −0.945**  pH    1  0.982***  −0.973***  −0.929**  Total microorganisms      1  −0.975***  −0.951***  Torrymeter value        1  0.884**  Overall acceptability          1        Total  Torrymeter  Overall  Trait  VBN  pH  Micro-organisms  value  acceptability  VBN  1  0.901**  0.924**  −0.918**  −0.945**  pH    1  0.982***  −0.973***  −0.929**  Total microorganisms      1  −0.975***  −0.951***  Torrymeter value        1  0.884**  Overall acceptability          1  **P < 0.01, ***P < 0.001. View Large The degradation of proteins in the meat is caused by either bacterial or enzymatic processes (Kruk et al., 2011). The torrymeter value was found to be negatively correlated with VBN (P < 0.01), pH (P < 0.001), and total microorganism counts (P < 0.001), with coefficients of −0.918, −0.973, and −0.975, respectively. The torrymeter value decreased during storage due to decreases in the conductivity and permittivity of the meat, resulting in changes in the electrical properties of the meat tissue (Ghatass et al., 2008). The degradation of proteins caused by autolytic and bacterial activity during meat storage thus resulted in increased permeability of the cell membranes, eventually destroying the meat tissue. Cell death produces mixed extracellular and intracellular fluids containing ions (Ghatass et al., 2008), thus modifying the electrical properties of the meat tissue and eventually resulting in a decrease in the torrymeter value. This explains why the torrymeter value was negatively correlated with the total microorganism counts, pH, and VBN and provided a good indicator of chicken breast freshness. The overall acceptability of the sensory properties was negatively correlated with the VBN (P < 0.01), pH (P < 0.01), and total microorganism counts (P < 0.001), with coefficients of −0.945, −0.929, and −0.951, respectively. In contrast, a highly positive correlation was observed between the overall acceptability of the sensory properties and the torrymeter value (P < 0.01), with a coefficient of 0.884. This explains that with the increase in freshness indicator (VBN, pH, and total microorganism counts) value, both the acceptability and the torrymeter value significantly decreased. This result suggests that the torrymeter can be used as an effective instrument for measuring meat freshness. 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Poultry ScienceOxford University Press

Published: Apr 14, 2018

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