The present study was conducted to determine and compare the oxidative stability of soybean and sunflower oils using differential scanning calorimetry (DSC). These edible oils were enriched with marjoram ( Origanum majorana L.), thyme (Thymus vulgaris L.), and oregano (Origanum vulgare L.) extracts at three different concentrations and synthetic antioxidant (BHA). The fatty acid composition of studied oils was determined by gas chromatography mass spectrometry to evaluate the content of unsaturated fatty acids that are sensitive to oxidation process. Oil samples were heated in the DSC at different −1 heating rates (4.0, 7.5, 10.0, 12.5, and 15.0 °C min ) and oxidation kinetic parameters (activation energy, pre-exponential factor, and oxidation rate constant) were calculated. The results showed that the oxidative stability of sunflower oil samples enriched with oregano extracts and soybean oil supplemented with thyme extracts was improved compared to samples without the addition of herbal plant extracts and the synthetic antioxidant. Keywords Sunflower oil · Soybean oil · Herbal plant extracts · Oxidative stability · Differential scanning calorimetry Introduction reactive radicals, which initiate further reactions (Choe and Min 2006; Taghvaei and Jafari 2015). They cause a sequence Sunflower and soybean oils belong to the popular vegetable of unfavorable changes, mainly deterioration in the sensory oils used in the food, cosmetic, and pharmaceutical indus- properties, decrease in nutritional value, and give rise to tries (Rabasco Alvarez and González Rodríguez 2000). They chemical compounds that are harmful to the human health are a good source of the essential fatty acids and liposoluble (McClements and Decker 2000; Gramza-Michalowska et al. vitamins which are important components in human diet. 2007). The predominant fatty acids present in both oils are unsatu- The lipid oxidation may be inhibited by different ways rated ones such as oleic, linoleic, and linolenic acids. The including inactivation of enzymes catalyzing oxidation, addi- presence of double bound in these fatty acids can offer sev - tion of chelating agents or the use of suitable packaging. eral possibilities to carrying out of the chemical modification Another method is the addition of antioxidants, especially nat- of the structure to improve some of their properties (Naeli ural, because the prolonged usage of synthetic antioxidants has et al. 2017). On the other hand, they are prone to oxida- often been questioned due to potential toxicological concerns tive processes both when vegetable oils are stored at low (Nieva-Echevarría et al. 2015). Antioxidants added to edible temperature as well as when they are used in the kitchen oils should protect unsaturated fatty acids to increase their sta- for frying and cooking at high temperature. Autoxidation bility to thermal degradation as well as they should demon- of edible oils and fats can also be catalyzed by other factors strate good thermal stability. Extracts from herbal plants such such as exposure to light, heat, and transitional metals. This as thyme, rosemary, sage, marjoram, and oregano are a rich process is a free radical chain reaction, leading to increase in source of natural antioxidants (Oliveira et al. 2018; Chrpová et al. 2010; Generalić Mekinić et al. 2014). Their properties are determined by the presence of phenolic compounds which * Mariola Kozłowska extend the shelf-life of food, protect fats against autoxidation, firstname.lastname@example.org and also exhibit antimicrobial activity (Kozlowska et al. 2015). Faculty of Food Sciences, Department of Chemistry, Phenolic compounds may act as antioxidants by scavenging Warsaw University of Life Sciences (WULS-SGGW), of free radicals. They are active when are used at an optimal Nowoursynowska 159C St, 02-776 Warsaw, Poland Vol.:(0123456789) 1 3 2608 Chemical Papers (2018) 72:2607–2615 range of concentrations. The use of their in excessive amounts (Origanum majorana L.), thyme (Thymus vulgaris L.), and may result in pro-oxidant effect. Moreover, herbal plant oregano (Origanum vulgare L.) were purchased from a extracts contain many compounds that may interfere with one local food store in Warsaw. All the solvents (n-hexane, another and minor lipid components and also can influence on methanol, ethanol, acetone, diethyl ether, and chloroform) the autoxidation process. and reagents (potassium hydroxide, acetic acid, butylated To determine of the oxidative stability of oils, as well as hydroxyanisole—BHA, and certified fatty acids methyl the antioxidant effectiveness of spices and herbs many meth- ester reference standard mixture) were of analytical grade ods and techniques were adopted. The most popular methods and used without further purification. They were pur - are peroxide value and conjugated diene determinations, gas chased from Avantor Performance Materials (Gliwice, chromatography, chemiluminescence, Raman spectroscopy, or Poland) and from Sigma-Aldrich Chemicals (Poznań, Rancimat test (Farhoosh 2007; Carmona et al. 2014). Among Poland). them, also differential scanning calorimetry (DSC) is the most used analytical instrument for studies of physical properties and thermo-oxidative decomposition of native and inhibited Preparation of extracts fats and their blends (Thurgood et al. 2007; Kozłowska et al. 2014; Tengku-Rozaina and Birch 2016). Both isothermal The herbal plant extracts were prepared from marjoram, (constant temperature) and non-isothermal (linear increase thyme and oregano according to method described by in temperature) DSC techniques may be applied to obtain Kozłowska et al. (2010). 10 g of each dried plant material kinetic parameters of lipid oxidation in vegetable oils includ- was mixed with ethanol/water (7:3, v/v) in oil bath for 10 h ing activation energy (E ), the Arrhenius rate constant (k), at 45 °C. Next, the filtration was carried out to separate the pre-exponential factor (Z), and also induction time (τ). They plant residue, and then, the solvent was removed to dry- are calculated using the Ozawa–Flynn–Wall method (OFW) ness in a rotary evaporator at 40 °C. Obtained herbal plant after determination from DSC curves of the onset temperatures extracts were stored frozen until further use (− 20 °C). (t ) which are taken as a parameter characterizing the oxida- ON tive susceptibility of oils. Spice and herbs added to vegetable oils act as antioxidants mainly by increasing the onset tem- Determination of peroxide and acid values peratures. The correct use of the OFW method in DSC stud- ies is based on the comparison of systems at the same degree Analyses for peroxide and acid values (PV and AV, respec- of conversion (α) described as α = ΔH /ΔH , where ΔH tively) were carried out in triplicate according to the Stand- τ total τ is the heat evolved at a specific time τ and ΔH is the heat ards ISO 3960 (2009) and ISO 660 (2010). total evolved during the process. It may be assumed that the degree of conversion at the beginning of the oxidation process is low but constant (Guimarães-Inácio et al. 2018). The approach Determination of fatty acid composition proposed by Ozawa, Flynn, and Wall is often used during the evaluation of the stability of edible oils and researchers agree Fatty acid composition was analyzed by gas chromatography that the determination of a single activation energy at the start (GC) after derivatization to fatty acid methyl esters with of the oil degradation is sufficient to compare different samples 2 M methanolic solution of potassium hydroxide accord- that were subjected to the same experimental conditions (same ing to ISO 12966-2 (2011). A Shimadzu GC-17A gas chro- heating rates, oxidizing atmosphere) (Thurgood et al. 2007; matograph equipped with a flame ionization detector and Guimarães-Inácio et al. 2018). a BPX capillary column (30 m × 0.22 mm × 0.25 µm film The aim of this study was to evaluate and compare the thickness) was used. The analysis was performed using −1 oxidative stability of two vegetable oils with and without nitrogen (1 mL min ) as the carrier gas and applying the the addition of herbal plant extracts by DSC non-isothermal following temperature programme: 60 °C held for 1 min, measurements. The kinetic parameters were calculated and after which the temperature was increased to 170 °C at a −1 they also used for evaluation of plant antioxidant efficiency. rate of 10 °C min and from 170 to 230 °C at a rate of −1 3 °C min . The temperature was kept at 230 °C for another 15 min. The injector and detector temperatures were set at Experimental 225 and 250 °C, respectively. Individual fatty acids were identified by comparing their retention times with a certified Materials fatty acids methyl ester reference standard mixture (Supelco 37-Component Fame Mix, CRM47885, Sigma-Aldrich, St. Refined sunflower (SFO) and soybean (SBO) oils were Louis, MO, USA) and quantified as a percentage of the total bought from a local market. The dried leaves of marjoram fatty acids. 1 3 Chemical Papers (2018) 72:2607–2615 2609 Table 1 Parameters of sunflower (SFO) and soybean (SBO) oil sam- Preparation of oil samples ples The herbal plant extracts were added to sunflower and Parameter SFO SBO soybean oils as solutions in absolute ethanol in the fol- −1 a b Acid value (AV/mg KOH g ) 0.07 ± 0.01 0.35 ± 0.01 lowing quantities: 0.01, 0.03 and 0.07% (marjoram, thyme −1 a b Peroxide value (PV/mmol O kg ) 1.98 ± 0.12 2.50 ± 0.09 and oregano), 0.015 + 0.015% (mixture of oregano and Fatty acids/% thyme), 0.005 + 0.005% (mixture of thyme and BHA), a a Myristic (C14:0) 0.1 ± 0.0 0.1 ± 0.0 0.005 + 0.005% (mixture of oregano and BHA), and 0.01% a b Palmitic (C16:0) 6.7 ± 0.2 11.1 ± 0.3 of BHA. Afterwards, the samples were mixed and alcohol a a Oleopalmitic (C16:1) 0.1 ± 0.0 0.1 ± 0.0 was evaporated in a rotary evaporator at 40 °C. The oil a a Stearic (C18:0) 3.9 ± 0.2 4.1 ± 0.2 samples without extracts added were used as controls. All b a Oleic (C18:1) 27.7 ± 0.4 25.4 ± 0.5 the prepared oils samples were subjected to DSC oxidation b a Linoleic (C18:2) 60.2 ± 0.7 53.1 ± 0.6 measurements. a b Linolenic (C18:3) 0.5 ± 0.1 5.0 ± 0.2 a a Arachidic (C20:0) 0.0 ± 0.0 0.2 ± 0.0 a b Gadoleic (C20:1) 0.2 ± 0.0 0.4 ± 0.1 DSC analysis a a Dihomo-γ-linolenic (C20:3) 0.6 ± 0.2 0.5 ± 0.1 a b ∑ SFA/% 10.7 ± 0.4 15.5 ± 0.6 DSC measurements were conducted with a DSC 820 from b a ∑ MUFA/% 28.0 ± 0.6 25.9 ± 0.7 Mettler Toledo (Schwerzenbach, Switzerland) with air flow b a ∑ PUFA/% 61.3 ± 0.8 58.6 ± 0.7 −1 of 60 mL min . The oils samples with and without the addition of herbal plant extracts and BHA (4.5 ± 0.5 mg) Data represent mean ± SD (standard deviation) (n = 3). Values marked by the different lower case superscript letters within a row denote sta- were placed into aluminium pans, closed with lids with a tistically significant differences (P < 0.05) hole drilled in the centre to allow the samples to be in con- SFA sum of saturated fatty acids, MUFA sum of monounsaturated tact with the air stream. The aluminium reference pan as fatty acids, PUFA sum of polyunsaturated fatty acids identical as possible to the oil sample pan was left empty. The oil sample and reference pans were heated at the rates −1 of 4.0, 7.5, 10.0, 12.5, and 15.0 °C min . For each experi- Statistical analysis −1 ment and each programmed heating rates (β, °C min ) at least triplicate determinations were carried out. From the The analyses were conducted in triplicate and the results resulting oxidation exotherms, the onset oxidation temper- presented are the average of the values obtained. The multi- atures (t ) were determined as the intersection of the ple range least significant difference test (Duncan multiple ON, °C extrapolated baseline and the tangent line (leading edge). range test), with significance level at P < 0.05, was applied The t experimental values were recalculated on absolute to the results to test the significant difference. The Statgraph- ON onset temperatures (T , K) and it was found that for the ics plus 4.0 package (Statistical Graphics Corp., USA) was ON samples studied there is a linear correlation (r > 0.938) used for analysis. of the type: −1 log = a ⋅ T + b, (1) ON Results and discussion −1 where β is the heating rate (K min ), and a and b are adjustable coefficients [the slope coefficient from Eq. (1 ) Fatty acid composition for each oil samples studied]. The obtained data were used to calculate the apparent activation energy by the Vegetable oils are rich in unsaturated fatty acids, especially Ozawa–Flynn–Wall method from the following equation: monounsaturated and polyunsaturated fatty acids. They can make oils susceptible to oxidation (Mannekote and Kailas −1 E =− 2.19 ⋅ R ⋅ dlog∕dT , (2) ON 2012; Sarkar et al. 2015). It is known that the presence of −1 −1 where R is the universal gas constant (8.314 J mol K ). two double bonds in the fatty acids structure may cause −1 −1 Using the Arrhenius equation [k = Z·exp·(− E R ·T )], the 10–40 times faster oxidation than the in presence of one values of rate constants at 160 °C were calculated. Then, double bond (Szterk et al. 2010). Therefore, the fatty acid they were recalculated into induction time (τ) at this tem- composition of sunflower and soybean oils, expressed as perature. The detailed procedures for kinetic characterisation saturated, monounsaturated, and polyunsaturated, is sum- were reported elsewhere (Thurgood et al. 2007; Kozlowska marized in Table 1. The percentage content of saturated fatty et al. 2013). acids (SFA) in sunflower oil amounted to 10.7%, while the value obtained for soybean oil was slightly higher (15.5%). 1 3 2610 Chemical Papers (2018) 72:2607–2615 It can also be noted that the content of monounsaturated authors (Thurgood et al. 2007; Cordeiro et al. 2013; Sarkar (MUFA) and polyunsaturated (PUFA) fatty acids in soy- et al. 2015). On contrary, linoleic acid content in SFO stud- bean oil was slightly lower (25.9 and 58.6%, respectively) ies by Cordeiro et al. (2013) and Asnaashari et al. (2015) in comparison with their content in sunflower oil (28.0 and was lower than that obtained in the present study. Zambiazi 61.3%, respectively). et al. (2007) studying sunflower oils found smaller amounts These values were in agreement with Veronezi et al. of oleic acid (15.26 and 16.86%) and higher contents of (2014) studies. However, Chowdhury et al. (2007) studies linoleic acid (71.17 and 70.69%) compared to the results showed that the total content of MUFA and PUFA in SBO demonstrated in our research. Sunil et al. (2015) also dem- was similar to our results, but the percentage of monoun- onstrated the lower level of oleic acid and higher amount of saturated and polyunsaturated fatty acids in sunflower oil linoleic acid in SFO (23.0 and 66.2% for oleic and linoleic were different. They reported higher content of MUFA and acids, respectively) as compared to the present work. Die ff r - lower percentage of PUFA in sunflower oil if compared with ences in fatty acid composition of SFO and SBO are mostly our studies. Although the content of saturated fatty acids in determined by plant genotype and environmental conditions, vegetable oils is desirable to improve their oxidative stabil- particularly temperature, water supply, and more generally ity, but taking into account nutritional properties of oils, agro-climatic conditions (Castro and Leite 2018; Reena Rani their presence is undesirable, because they can contribute and Sharma 2017; Clemente and Cahoon 2009). The higher to increasing the concentration of low-density lipoproteins level of unsaturated fatty acids in SFO than SBO, especially and the plasmatic cholesterol (Wilke and Clandinin 2005). linoleic acid, makes it more susceptible to oxidation process. The major saturated fatty acids that were found in edible Therefore, the addition of antioxidants may be one of the oils studied were palmitic (C16:0) and stearic (C18:0). SBO way to inhibit oil autoxidation. contained higher levels of these fatty acids in comparison with SFO (11.1 and 6.7% of palmitic acid, and 4.1 and 3.9% Oxidative stability of sunflower and soybean oils of stearic acid, respectively). Among MUFA, oleic acid enriched with herbal plant extracts (C18:1) was the main representative and its content ranged from 25.4% in SBO to 27.7% in SFO. However, linoleic Selected DSC curves of non-isothermal oxidation registered −1 (C18:2) and linolenic (C18:3) acids were the dominant fatty at 4 °C min of heating rate for SFO enriched with BHA acids among PUFA. Linoleic acid amount in SFO was higher and herbal plant extracts are shown in Fig. 1. The same pro- than that of 53.1% which was determined in SBO, but lino- file was also observed for DSC exotherms describing the lenic acid was found in smaller amount in SFO. Regard- non-isothermal oxidation of SBO samples. There are usually ing SBO our results were similar to those reported by other two peaks present indicating that during oxidation at least Fig. 1 Example of DSC scan 5 of non-isothermal oxidation of sunflower oil (SFO) and SFO samples containing BHA (0.01%), MARJORAM (0.07%), SFO THYME (0.07%), OREG- ON ANO (0.07%), and THYME BHA (0.005%) + BHA (0.005%) at −1 heating rate β = 4 °C min t ON MARJORAM 0.07 % ON THYME 0.07 % ON OREGANO 0.07 % ON THYME + BHA ON Exo -5 050 100 150200 250300 Temperature/°C 1 3 Heat Flow/W/g Chemical Papers (2018) 72:2607–2615 2611 two processes take place. The onset point and first maxi- with the formation of peroxides (Qi et al. 2016). The sample mum peak are connected with initiation and formation of with a higher t at the same heating rate is more stable than ON primary auto-oxidation products, and the second maximum the one for which this parameter is lower. For both vegetable peak informs about further oxidation and decomposition of oils studied, an increase of the heating rate resulted in the oxidation products (Litwinienko et al. 2000). DSC curves higher values of t . In the case of SFO samples without the ON have similar shapes but are shifted towards higher tempera- addition of herbal plant extracts they were characterized by tures depending on types and concentrations of herbal plant lower values compared to SBO samples. extracts added. This observation may be due to the fact that sunflower The start of exothermic reaction temperature values meas- oil contained higher amount of unsaturated fatty acids, and ured as extrapolated onset temperature (t ) are summarized hence, it was more prone to oxidation process. The addition ON in Tables 2 and 3 for SFO and SBO, respectively. T was of herbal plant extracts and synthetic antioxidant to the both ON reported to be the most suitable parameter for lipid oxidation oils reduced the oxidation by lengthening t for antioxi- ON under non-isothermal oxidation as it is closely associated dant treated samples. Among herbal plant extracts added to Table 2 Extrapolated DSC thermoxidation onset temperatures (t /°C) measured at different heating rates (β) for SFO oil samples ON Heating rate SFO BHA MARJORAM THYME −1 β/ °C min 0.01% 0.01% 0.03% 0.07% 0.01% 0.03% 0.07% a cd b c c b c cd 4.0 145.86 ± 0.47 157.71 ± 0.56 152.04 ± 0.54 156.17 ± 0.57 156.22 ± 0.57 152.82 ± 0.61 156.68 ± 0.62 156.86 ± 0.84 a bc a a c ab c c 7.5 159.37 ± 0.52 164.53 ± 1.00 159.47 ± 0.97 161.90 ± 1.08 167.97 ± 1.12 162.84 ± 0.87 166.17 ± 0.89 166.93 ± 0.90 a ab a ab b a a a 10.0 167.84 ± 0,78 170.01 ± 0.60 168.74 ± 0.59 170.42 ± 0.91 172.30 ± 0.92 166.76 ± 1.13 168.02 ± 1.14 168.94 ± 1.15 a ab a a ab a ab ab 12.5 171.98 ± 0.56 175.58 ± 0.57 172.09 ± 0.56 172.42 ± 0.71 174.93 ± 0.72 172.45 ± 1.44 174.37 ± 1.45 175.34 ± 1.46 a a a a a a a a 15.0 176.08 ± 1.17 179.01 ± 1.24 176.10 ± 1.22 177.64 ± 1.23 178.14 ± 1.23 176.52 ± 1.22 177.11 ± 1.48 177.55 ± 1.48 OREGANO THYME + BHA OREGANO + BHA OREGANO + THYME 0.01% 0.03% 0.07% 0.005% + 0.005% 0.005% + 0.005% 0.015% + 0.015% c d d d b c 4.0 155.22 ± 0.42 158.44 ± 0.87 159.35 ± 0.69 159.01 ± 0.85 153.65 ± 0.72 155.18 ± 0.70 b bc b ab bc a 7.5 163.29 ± 0.58 164.04 ± 0.58 165.68 ± 0.82 162.62 ± 0.87 166.13 ± 1.06 159.72 ± 0.88 a b ab b b a 10.0 168.01 ± 0.90 169.45 ± 1.39 170.53 ± 1.16 173.88 ± 1.43 171.68 ± 1.16 167.73 ± 0.85 a a ab bc c a 12.5 172.41 ± 0.85 173.01 ± 0.95 175.48 ± 0.97 177.49 ± 1.48 180.03 ± 0.58 172.68 ± 1.10 a a ab ab b a 15.0 176.83 ± 1.00 179.05 ± 1.24 179.41 ± 1.34 180.22 ± 1.53 184.39 ± 1.49 176.09 ± 1.32 The results are mean ± standard deviation. Values marked by the different lower case superscript letters (a–d) within a row denote statistically significant differences (P < 0.05) Table 3 Extrapolated DSC thermoxidation onset temperatures (t / °C) measured at different heating rates (β) for SBO oil samples ON Heating rate SBO BHA MARJORAM THYME −1 β/°C min 0.01% 0.01% 0.03% 0.07% 0.01% 0.03% 0.07% a bc a c c c c c 4.0 157.05 ± 0.58 160.37 ± 0.82 158.07 ± 0.74 161.37 ± 0.50 162.15 ± 0.57 161.67 ± 0.50 162.18 ± 0.78 162.52 ± 0.78 a ab ab c c bc bc c 7.5 164.21 ± 0.67 167.31 ± 0.92 166.27 ± 0.94 171.67 ± 1.04 172.06 ± 1.12 169.16 ± 0.74 169.33 ± 0.74 171.72 ± 0.75 a b a b b a a b 10.0 170.62 ± 0.79 175.26 ± 0.57 173.06 ± 0.63 174.96 ± 0.64 176.72 ± 0.92 172.78 ± 0.93 173.07 ± 1.08 176.36 ± 0.97 a b ab b b b b b 12.5 175.89 ± 0.59 179.53 ± 1.37 178.26 ± 1.26 179.44 ± 1.22 180.37 ± 0.72 178.76 ± 0.91 179.46 ± 0.94 179.70 ± 0.94 a ab a ab ab ab ab ab 15.0 180.06 ± 1.17 182.35 ± 1.44 180.36 ± 1.28 182.59 ± 1.29 183.08 ± 1.23 181.59 ± 1.21 181.64 ± 1.26 182.13 ± 1.26 OREGANO THYME + BHA OREGANO + BHA OREGANO + THYME 0.01% 0.03% 0.07% 0.005% + 0.005% 0.005% + 0.005% 0.015% + 0.015% bc c c b ab c 4.0 160.98 ± 0.45 161.25 ± 0.50 161.77 ± 0.53 159.53 ± 0.85 158.75 ± 0.72 162.75 ± 0.70 c c c bc b c 7.5 170.17 ± 0.74 170.86 ± 0.79 171.86 ± 0.72 169.04 ± 0.87 168.01 ± 1.06 171.67 ± 0.88 b b b a a b 10.0 174.64 ± 1.09 175.65 ± 1.09 176.54 ± 0.97 172.24 ± 1.43 171.73 ± 1.16 177.84 ± 0.85 ab ab b a a b 12.5 177.05 ± 0.83 178.07 ± 0.88 179.58 ± 1.02 174.44 ± 1.48 174.75 ± 0.58 180.23 ± 1.10 ab ab ab a ab b 15.0 183.03 ± 1.42 183.09 ± 1.29 183.13 ± 1.22 180.11 ± 1.53 180.60 ± 1.49 185.75 ± 1.32 The results are mean ± standard deviation. Values marked by the different lower case superscript letters within a row denote statistically signifi- cant differences (P < 0.05) 1 3 2612 Chemical Papers (2018) 72:2607–2615 SFO, the best t values for samples enriched with oregano products in sunflower oil and oil-in-water emulsion. Oregano ON extracts at concentration of 0.03 and 0.07% were observed. extract was more active in oil than in emulsion. The extrapolated onset temperatures also were increased In the present study, the extrapolated onset temperatures for SFO samples supplemented with marjoram and thyme at different heating rates were used to evaluate the thermal- extracts, but they were shorter compared to that with the oxidative stability of SFO and SBO enriched with herbal addition of BHA. The significant differences between t plant extracts. The Ozawa–Flynn–Wall method and the ON −1 values at the heating rate of 4.0 °C min for all sunflower Arrhenius equation were applied for calculation of kinetic oil samples enriched with herbal plant extracts in compari- parameters of this process. The activation energy (E ) as son with SFO without the addition of herbal plant extracts well as pre-exponential factor (Z) and reaction constant were observed. However, they were not statistically signifi- (k) can be used for comparison of the oxidative stability cant (P <0.5) at the same heating rate when SFO samples of studied pure oils and oils enriched with antioxidants for containing BHA were compared to SFO samples with the evaluating of antioxidant activity of added antioxidants. In addition of herbal plant extracts. However, this observation all cases, the addition of herbal plant extracts and BHA to revealed that the addition of herbal plant extracts to sun- oils caused an increase in the values of activation energy flower oil may influence on the improvement of its oxidative (Tables 4, 5, respectively). At low concentration of herbal stability. plant extracts (0.01%), they were lower than at higher con- Similar conclusion is possible to make regarding soy- centration (0.07%). Activation energy values were 64.86 for −1 bean oil samples. In all SBO samples supplemented with SFO and 87.40 kJ mol for SBO, respectively. For SFO herbal plant extracts, the increase of t was observed com- enriched with herbal plant extracts, E values were in the ON a −1 pared to the SBO samples without the addition of herbal range of 66.40–101.34 kJ mol . In the case of SBO contain- plant extracts. The highest t values were shown for SBO ing marjoram, thyme, oregano extracts, and BHA, the range ON −1 enriched with thyme extracts. Moreover, when herbal plant of E values were 88.47–107.30 kJ mol . The highest values extracts were added to SBO samples at higher concentration of E were calculated for SBO samples enriched with thyme (0.07%), the resistance to oxidation was similar to that at extract and the mixture of thyme extract and BHA. In regard- concentration of 0.03%. It can also be noticed that the addi- ing to SFO samples, activation energy reached maximal −1 tion of mixture of natural and synthetic antioxidants to the value of 101.34 kJ mol when SFO was enriched with oreg- oil samples can cause the prolongation of t in comparison ano extract at concentration of 0.07%. Taking into account ON with oil samples with the addition of BHA and oil samples the induction time (τ) calculated at 160 °C, it can be seen that enriched with each herbal plant extracts separately. The best higher values were reported for oil samples with the addition results were obtained for SFO when it was supplemented of herbal plant extracts than for their counterparts without with thyme and BHA mixture. In the case, SBO oregano and additives. The efficiencies of natural and synthetic antioxi- thyme were the most active. dants used for protection of SFO oil samples from thermoxi- Herbal plant extracts used in the present study belong dation increased in the following order: control (SFO) < mar- to the Lamiaceae family, that is well known for their anti- joram 0.01% < thyme 0.01% < oregano + thyme < oregano oxidant activity due to the content of phenolic compounds. 0.01% < mar jor am 0.03% < or eg ano + BHA < t hyme Among phenolics identified by HPLC method in our previ- 0.03% < oregano 0.03% < thyme 0.07% < BHA < marjoram ously studies, rosmarinic and caffeic acids were the most 0.07% < thyme + BHA < oregano 0.07%. For SBO, the rank predominant phenolic compounds (Kozłowska et al. 2014; was in the following sequence: control (SBO) < marjoram Kozlowska et al. 2015). Phenolic antioxidants present in 0.01% < oregano + BHA < thyme + BHA < BHA < thyme herbal plant extracts have good free radical scavenging and 0 . 01 % < o r e g a n o 0 .0 1 % < t h ym e 0 . 03 % < o r e g a n o chelating properties. When they are incorporated to edible 0.03% < mar joram 0.03% < oregano 0.07% < mar joram oils help to improve their thermal stability and extend their 0.07% < thyme 0.07% < oregano + thyme. frying life (Bensmira et al. 2007; Kozlowska and Zawada Kowalski (1993) also evaluated oxidative stability of 2015). Combinations of different herbal plant extracts and edible oils after addition of various antioxidants using synthetic antioxidants may also have synergistic effects in pressure DSC. Jaswir et al. (2004) showed that addition of preventing hydroperoxides formation compared to samples pegaga leaves and limau purut to the oil reduced the oxida- containing only herbal plant extracts or only synthetic anti- tion by longer t of antioxidant-treated samples. Suja et al. ON oxidant (Rižner Hraš et al. 2000). In a research carried out (2004) demonstrated that sesame cake extract could be used by Ramalho and Jorge (2008), it was also noticed that rose- as a substitute for synthetic antioxidant to protect soybean, mary extract added to soybean oil showed a positive effect sunflower, and safflower oils. However, Litwinienko et al. on its oxidative and thermal stability. However, Abdalla and (1997, 1999) evaluated kinetic parameters of linoleic acid Roozen (1999) showed that thyme and lemon balm extracts thermoxidation in the presence of the phenolic compounds inhibited formation of both primary and secondary oxidation by DSC. The inhibitory effect varies with concentration 1 3 Chemical Papers (2018) 72:2607–2615 2613 2 −1 −1 Table 4 Regression analysis of DSC data (a, b, r ), activation energies (E , kJ mol ), pre-exponential factors (Z, min ), rate constants (k, −1 min ) and induction times (τ, min) for SFO samples Parameters SFO BHA MARJORAM THYME 0.01% 0.01% 0.03% 0.07% 0.01% 0.03% 0.07% a − 3.562 − 5.153 − 4.407 − 5.051 − 5.117 − 4.725 − 5.446 − 5.347 b 9.09 12.53 10.99 12.41 12.49 11.70 13.28 13.04 r 0.997 0.984 0.987 0.973 0.981 0.989 0.971 0.974 E 64.86 93.82 80.24 91.97 93.17 86.03 99.16 97.36 log Z 7.52 10.85 9.32 10.68 10.76 10.00 11.52 11.28 k at 160 °C 0.494 0.346 0.440 0.384 0.337 0.422 0.362 0.348 τ at 160 °C 2.024 2.890 2.272 2.604 2.967 2.369 2.762 2.873 OREGANO THYME + BHA OREGANO + BHA OREGANO + THYME 0.01% 0.03% 0.07% 0.005% + 0.005% 0.005% + 0.005% 0.015% + 0.015% a − 5.221 − 5.466 − 5.566 − 4.619 − 3.652 − 4.908 b 12.80 13.31 13.50 11.36 9.17 12.12 r 0.991 0.960 0.980 0.938 0.990 0.960 E 95.06 99.52 101.34 84.10 66.49 89.36 log Z 11.06 11.55 11.73 9.67 7.58 10.40 k at 160 °C 0.396 0.354 0.325 0.334 0.364 0.420 τ at 160 °C 2.525 2.824 3.076 2.994 2.747 2.380 2 −1 Parameters a, b, and r were obtained from plotting logβ vs. T for all samples ON 2 −1 −1 Table 5 Regression analysis of DSC data (a, b, r ), activation energies (E , kJ mol ), pre-exponential factors (Z, min ), rate constants (k, −1 min ), and induction times (τ, min) for SBO samples Parameters SBO BHA MARJORAM THYME 0.01% 0.01% 0.03% 0.07% 0.01% 0.03% 0.07% a − 4.800 − 4.941 − 4.859 − 5.455 − 5.475 − 5.641 − 5.677 − 5.811 b 11.79 12.02 11.88 13.15 13.17 13.59 13.67 13.93 r 0.985 0.989 0.995 0.990 0.996 0.981 0.978 0.997 E 87.40 89.96 88.47 99.32 99.69 102.71 103.36 105.80 log Z 10.08 10.31 10.17 11.38 11.40 11.82 11.89 12.14 k at 160 °C 0.348 0.286 0.318 0.255 0.239 0.271 0.264 0.240 τ at 160 °C 2.873 3.496 3.144 3.921 4.184 3.690 3.787 4.166 OREGANO THYME + BHA OREGANO + BHA OREGANO + THYME 0.01% 0.03% 0.07% 0.005% + 0.005% 0.005% + 0.005% 0.015% + 0.015% a − 5.406 − 5.411 − 5.437 − 5.893 − 5.408 − 5.140 b 13.06 13.05 13.09 14.21 13.13 12.40 r 0.982 0.990 0.994 0.972 0.979 0.992 E 98.43 98.52 99.00 107.30 98.47 93.59 log Z 11.30 11.29 11.33 12.41 11.37 10.66 k at 160 °C 0.270 0.256 0.246 0.295 0.313 0.236 τ at 160 °C 3.703 3.906 4.065 3.390 3.194 4.237 2 −1 Parameters a, b, and r were obtained from plotting logβ vs. T for all samples ON −1 of compounds added and the range 5–12 mmol mol of linoleic acid gave the best results. 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