Degradation Of Tocopherols Research Articles

Influence of deodorization temperature on formation of tocopherol esters and fatty acids polymers in vegetable oil

The paper describes laboratory deodorization of rapeseed oil (Ro), model sunflower (SoM), and model rapeseed (RoM) oils, its effect on the tocopherol content and the kinetics of tocopherol degradation. We now present a novel approach to analysis of alpha‐tocopherol degradation product as well as kinetic study esters of alpha‐tocopherol with fatty acids and alpha‐tocopheryl quinones. Alpha‐tocopherol esters were found to be the principal products of tocopherols degradation in high temperature experiment. A laboratory deodorization, lasting 2 h caused formation of 70–128 mg/kg of alpha‐tocopherol esters in model oils and 25 mg/kg of alpha‐tocopherol esters in rapeseed oil without modification. The commercially available sunflower and rapeseed oils contained 3–12 mg/kg of alpha‐tocopherol esters. The alpha‐tocopherol esters are formed at higher rate than alpha‐tocopheryl quinone, which was unstable at high deodorization temperature. Tocopherols took no part in non‐radical thermal reactions, taking place in second high‐temperature experiments with oxygen‐free atmosphere. We proved by DART TOF‐MS the formation of polymeric fatty acids via Diels–Alder reaction taking place in heated oils.Practical applications: Although an extensive research have been conducted in the area of high‐temperature degradation of tocopherols, this is the first report on formation of fatty acids esters of tocopherols during oil deodorization. The proof of their presence in deodorized oils, deodorization condensates, deep‐fried foods, and oils constitutes a breakthrough. We ingested them as yet unknowingly. Their degradation is not nearly as fast as that of tocopherol itself or its oxidation products. In the course of their hydrolysis in stomach tocopheryls of fatty acids could thus become the source of unoxidized tocopoherols.We found out that during laboratory deodorization of vegetable oils were formed esters of tocopherols with fatty acids. During heating of oils are free fatty acids the acid catalyst of esterification.

Effects of Temperature and Duration of Storage on the Stability of Antioxidant Compounds in Egg Yolk and Plasma.

Antioxidants help protect tissues from oxidative damage caused by reactive oxygen species. In view of the widespread interest in egg yolk and plasma antioxidants in relation to environmental and life-history variables, there is a need for knowledge on the necessary conditions for sample storage, which is currently lacking. In this study, our purpose was to examine the kinetics of the degradation of lutein, retinol, and tocopherol in egg yolk samples and the total antioxidant capacity in plasma samples stored at three different temperatures (-20°, -50°, and -80°C) for 24 mo. We found that yolk lutein was stable during the study period. Yolk retinol and tocopherol showed a steep early decline and then remained relatively stable, but retinol showed significant losses at the end of the study period too. In contrast to our expectations, there was no difference in the stability of antioxidant compounds of egg yolk samples stored at different temperatures. Plasma OXY level was stable during the first 6 mo, showed a slight decline between 6 and 12 mo, and declined more intensely after 12 mo of storage. We suggest that studies focusing on the analysis of egg yolk retinol or tocopherol concentrations and total plasma antioxidant capacity should analyze the samples in the first 6-7 mo after collection. For the analysis of yolk lutein, samples can be stored for 24 mo without significant degradation. The storage temperature of -20°C seemed to be sufficient, as a lower temperature did not significantly affect the slope of degradation of the samples.

Degradation of tocopherols during rapeseed storage in simulated conditions of industrial silos

Abstract The investigations consisted in laboratory simulation of conditions prevailing in the real ecosystem in an industrial rapeseed storage facility. The aim of the study was to assess the impact of temperature, moisture, and static pressure on degradation of tocopherols contained in rapeseed. Rapeseed varieties with high oil content were analysed in the investigations. Samples of seeds with 7, 10, 13, and 16% moisture levels were stored at a temperature of 25, 30, and 35°C in specially designed airtight pressure silos for storage in controlled conditions. During the storage, the seeds were subjected to overpressure in the range of 20-60 kPa. The seeds were stored in these conditions for 28 days. It was demonstrated that primarily moisture induced the greatest loss of the total content of tocopherol and its α-T and γ-T homologues, followed by temperature and, to a lesser extent, pressure. In addition, the results obtained showed that, in the case of seeds characterised by higher moisture levels (13 and 16%), an increase in the storage temperature in the range of 25-30°C rather than 30-35°C intensified tocopherol loss more efficiently.

Rosemary extract can be used as a synthetic antioxidant to improve vegetable oil oxidative stability

The protecting ability of rosemary extract as a plant-based antioxidant and an alternative to synthetic antioxidants against the oxidation of vegetable oils was investigated in the present study. The effect of rosemary extract on improving the oxidative stability of oils was evaluated using the Rancimat and Schaal oven tests. Oils with or without incorporation of rosemary extract or synthetic antioxidants were analyzed using the Rancimat method at 120°C to determine their induction periods (IP). The changes in antioxidant capacity, total phenolic content, peroxide value, fatty acid composition, and tocopherol content were measured under the Schaal oven test during storage at 62°C. The IP of oils incorporated with rosemary extract was significantly (p<0.05) higher than blank oils and oils with synthetic antioxidants. The incorporation of rosemary extract into oils effectively prevented the oils by increasing the antioxidant capacity and total phenolic content, decreasing the peroxide value, and delaying the degradation of tocopherols and polyunsaturated fatty acids of oils. Results of this study suggested the potential use of rosemary extract as an effective alternative to synthetic antioxidants.

Kinetics of tocopherol degradation during the storage of wheat germ oil

AbstractTocopherol degradation in wheat germ oil was studied during storage at different temperatures. Two types of wheat germ were used: heat‐treated germ (HT) and raw germ (RG). The oil was extracted (Soxhlet, n‐hexane) and stored at 25 and 47 °C (82 days) and at ‐20 and 5 °C (600 days). Tocopherols were determined by HPLC. Total tocopherol, α‐tocopherol, and β‐tocopherol decreased following first‐order kinetics. At 25 and 47 °C, the values of the degradation rate constants for total tocopherol were 5.00 × 10−3 days−1 (RG), 3.00 × 10−3 days−1 (HT), and 5.37 ×10−2 days−1 (RG), 2.57 × 10−2 days−1 (HT), respectively, indicating an increase in tocopherol degradation rate with increasing temperature. The reaction rates of total tocopherol were similarly affected at 5 and −20 °C, with lower rate constants (4 × 10−4 and 2 × 10−4 days−1, respectively). α‐tocopherol degraded faster than β‐tocopherol, presenting higher rate constants. Low temperatures favour the preservation of tocopherols.

Effective lipophilic antioxidant enzymatically derived from Canadian crabapple

Phloridzyl octadecanoate was prepared by enzymatic reaction between the native phloridzin in Canadian crabapple (Malus baccata) extract and octadecanoic acid in the presence of Candida antarctica lipase. The antioxidant activity of the modified extract was assessed in rapeseed oil during accelerated storage at 65°C and frying at 180°C. Addition of 0.1% modified extract to refined, bleached, and deodorized rapeseed oil significantly improved storage stability with 40.7% decrease in the formation of lipid hydroperoxides at the end of the 7‐day storage compared to rapeseed control sample without extract; however, no significant difference was observed in the potency of the modified extract and that of the native extract. On the contrary, the modified extract offered markedly better protection to the rapeseed substrate during frying, inhibiting polar component formation, and polymerization of triacylglycerol by up to 44%, compared to 26% by the native extract. At the end of the frying, French fries prepared in the presence of modified extract retained significantly more tocopherols than those prepared with the native extract indicating better protection of the oil's endogenous antioxidants by the modified phenolic extract.Practical applications: Poor lipophilicity is a major challenge to the application of natural polyphenolic compounds in edible oils. Equally challenging is the development of natural antioxidants that can maintain effectiveness under the stringent dynamic conditions of frying. Results from the current study demonstrated the possibility of overcoming these challenges, providing food processors with lipophilic natural antioxidants of practical importance in the frying process.Improving the lipophilicity of the polyphenolic components of Canadian crabapple's extract resulted in enhanced antioxidant and antipolymerization activities during frying, with consequent decreased in: Total polar materials (TPM); di‐ and polymerized triacylglycerols (DPTG); formation of non‐volatile carbonyl compounds assessed by the anisidine value (AnV); and rate of endogenous tocopherol degradation. However, no change in activity was observed under storage conditions.

Effects of blanching and drying on pigments and antioxidants of daraesoon (shoot of the Siberian gooseberry tree, Actinidia arguta Planchon)

Chlorophyll, carotenoid, total phenolic compound, and tocopherol contents of daraesoon (shoot of the Siberian gooseberry tree, Actinidia arguta Planchon) were determined after blanching and during drying in the dark (13 days) and under light (10 days), using HPLC and spectrophotometry. Blanching caused a significant (p<0.05) increase in the total chlorophyll content to 4,629.8 mg/kg from 2,932.5 mg/kg, and in the β-carotene content to 4,953.4 mg/kg from 4,590.5 mg/kg, compared with controls. Drying of blanched daraesoon caused a significant (p<0.05) loss of chlorophylls and carotenoids, and carotenoids were more affected by light than chlorophylls. Total phenolic compounds and tocopherols were also degraded during blanching and drying, with more degradation of tocopherols (up to 54.3%) than for total phenolic compounds (up to 15.8%). In spite of degradation of pigments and antioxidants during drying, dried daraesoon provides high levels of health-beneficial carotenoids, total phenolic compounds, and tocopherols.

Novel simple process for tocopherols selective recovery from vegetable oils by adsorption and desorption with an anion-exchange resin

A novel and simple low-temperature process was used to recover tocopherols from a deodorizer distillate, which is a by-product of edible oil refining. The process consists of three operations: the esterification of free fatty acids with a cation-exchange resin catalyst, the adsorption of tocopherols onto an anion-exchange resin, and tocopherol desorption from the resin. No degradation of tocopherols occurred during these processes. In the tocopherol-rich fraction, no impurities such as sterols or glycerides were present. These impurities are commonly found in the product of the conventional process. This novel process improves the overall recovery ratio and the mass fraction of the product (75.9% and 51.0wt%) compared with those in the conventional process (50% and 35wt%).

Synthesis and analysis of tocopheryl quinone and tocopherol esters with fatty acids in heated sunflower oil

This work focuses on degradation of α‐tocopherol and formation of α‐tocopherol degradation products in sunflower oil heated at frying temperature. We determined the content and measured the kinetics of α‐tocopherol, α‐tocopheryl quinone, α‐tocopheryl fatty acid formation in heated sunflower oil without aeration (So) and in aerated oil (air flow 20 L/h) (SoAir). During 10 h of So oil heating, the α‐tocopherol was depleted and the content of α‐tocopheryl quinone in So grew from 3 to 84 mg/kg, as did the content of tocopherol free fatty acid esters from the initial 5 to 185 mg/kg. In SoAir oil, the rate constants of formation and decomposition of tocopherol degradation products were determined. Accelerated oxidation in SoAir caused depletion of the degradation products of tocopherol after 2 h of heating. The reason for this was significant oxidation and polymerization of fatty acids in SoAir oil. The unique features of this work are the syntheses and spectral analysis of tocopherol oxidation products and the development of a suitable method for quantitative analysis of α‐tocopheryl fatty acids. For the first time, we confirmed formation and degradation of tocopheryl fatty acids during oil heating at frying temperature.Practical applications: All the results presented here, consisting of organic syntheses, qualitative and quantitative analysis of α‐tocopheryl fatty acids as well as oxidation and degradation products of tocopherols and fatty acids, will aid future research focused on the degradation of tocopherols. So far the content of α‐tocopheryl fatty acids has not been determined by GC‐FID or HPLC‐UV. This is why there has been little interest in α‐tocopheryl fatty acids, although they are formed both in culinary treatment of food and in oil refining.Free fatty acids (FFAs), fatty acids (FAs), oxidation [o], products (i) and products (ii) are unspecified products of α‐tocopheryl FAs and α‐TQ degradation. Degradation products of α‐tocopherol were formed in heated sunflower oil and further heating of oil caused decomposition of α‐tocopherol degradation products.

&lt;b&gt;Characterization of &lt;i&gt;Carya illinoiensis&lt;/i&gt; and &lt;i&gt;Juglans regia&lt;/i&gt; oils obtained by different extraction systems

It is extremely important to evaluate the extraction processes of vegetable oils used in food, considering that the nuts are rich sources of triacylglycerols. Thus, the present study aimed at comparing the methods of solvent extraction and extraction by pressing of the lipid fractions of walnut (Juglans regia) and pecan nut (Carya illinoensis), in order to elucidate their influence on the content of bioactive substances. The samples were analyzed regarding fatty acid profile, tocopherols, phytosterols, carotenoids, and phenolic compounds, presenting relevant quantities of these bioactive substances. It was found that the extraction of oils by pressing minimized degradation of tocopherol when compared to solvent extraction. However, solvent extraction is more efficient to extract bioactive compounds such as phytosterols, carotenoids, and phenolic compounds.

Water content and micelle size change during oxidation of sunflower and canola oils

The aim of this research was to monitor the formation of water and the changes in micelle size during the oxidation of sunflower oil and canola oil oxidized in purified and unpurified forms. In all oils, the water content and micellar size increased along with peroxide value (PV) until the end of the induction period (IP), and then the micelle size was reduced. After the induction period, water content continued to increase in unpurified sunflower but not in unpurified canola oil, which may be due to different pattern of degradation of their hydroperoxides.Practical application: This research allows better understanding of the initiation phase of lipid oxidation and for the first time provides data on the relation between water content, micellization, and hydroperoxide formation during the initial stage of lipid oxidation.Changes that take place during the oxidation of unsaturated fatty acids. The red line marks the induction period; □: tocopherol degradation; ○: ratio of unsaturated to saturated fatty acids; △: micelle size; ●: hydroperoxides; and ◊: water.

A high drying temperature causes degradation of sterols and tocopherols in yellow‐seeded Brassica napus oils

The aim of the conducted investigations was to determine the effect of drying temperature on the changes in free fatty acid, phytosterol and tocopherol (T) contents in yellow‐seeded Brassica napus oils. Seeds were dried in a thin layer at 40, 60, 80, 100 and 120°C. Changes in phytosterols were assessed using GC‐MS, while tocopherols were assessed by HPLC. Degradation of phytosterols and tocopherols and an increased content of free fatty acids followed the applied drying temperature. During drying at 40 and 60°C, changes were statistically non‐significant, while at 120°C, losses of phytosterols reached 29% and tocopherols 23%.Practical applications: Drying of rapeseeds is an important stage of its production. However, we need to remember that these seeds are biological material highly sensitive to thermal treatment. Thus we need to search for the most advantageous conditions for their drying, with special emphasis on the preservation of bioactive components affecting human health. The analyses conducted compared the range of losses of bioactive components—phytosterols, tocopherols and plastochromanol‐8 during drying.Yellow‐seeded Brassica napus were dried in a thin layer at 40, 60, 80, 100 and 120°C. The effect of drying temperature on the free fatty acid, phytosterol and tocopherol content changes in oils was determined. Degradation of phytosterols, tocopherols and the increase in free fatty acid content were related to the applied drying temperature.

Chemical changes in microwave heated vegetable oils

The aim of this study was to investigate the influence of microwave heating on sunflower and corn oil in two types of microwave oven. The microwave ovens had the same output power and varied mainly in time of dissipation. The oil samples were heated for 90% and 70% of the total heating time by the two types of ovens, named as the first and the second oven, respectively, and the remaining time was dissipation pause. It was observed that greater dissipation pause in second microwave caused degradation of oil almost two times lower than did heating in the first microwave oil. In microwave heated oils the focus was on analysis of primary and secondary oxidation products, the fatty acids content, the tocopherol content and tocopherol degradation kinetics. The rate of tocopherol degradation in oils heated in the first oven was on average 2‐times higher than in the second oven. Oils heated for 10 min in the second oven were found to contain twice the tocopherol content, three times lower peroxide value, three times lower conjugated dienes and aldehydes compared with oils heated in the first type of oven. This is the first report about gentle microwave heating of oils and its dependence on time of heating and dissipation time.Practical applications: In order to dissipate heat during heating, microwave ovens are designed with different default magnetron pause. It was proved that in oils heated by microwave oven with longer dissipation pause there was less degradation of oils and their antioxidants. For the manufacturer it is important to conveniently select the lengths of heating a heat dissipation periods. By judicious selection of these two time characteristics healthier microwave heated food with lower degradation of lipids and their antioxidants can be obtained, since overheating is largely avoided.Peroxide values and decrease in tocopherol contents determined in sunflower oil during heating in two types of microwave ovens.

Effect of roasting conditions on the fat, tocopherol, and phytosterol content and antioxidant capacity of the lipid fraction from cocoa beans of different Theobroma cacao L. cultivars

The effect of roasting conditions on the content of fat, tocopherol, and phytosterol and antioxidant capacity of the lipid fraction from cocoa beans of different Theobroma cacao cultivars was investigated. The raw cocoa beans were roasted at four different temperatures (ranging from 110 to 150°C) and three different air humidities (0.3 to 5.0%). The results of this study show that roasting affected significantly the photochemical composition and lipophilic antioxidant activity of cocoa beans. Roasting may lead to significant degradation of α‐tocopherol and phytosterols when compared to raw cocoa beans. This was not true in samples roasted at the lowest temperatures and evaluated humidity; these samples had a higher γ‐ and δ‐tocopherol content and antioxidant capacity compared to the raw cocoa beans and samples roasted at other conditions. The present study also revealed that the content of bioactive compounds and antioxidant activity of different Theobroma cacao L. cultivars vary markedly.Practical applications: Cocoa beans are the primary raw material used in the manufacture of chocolate products, which can be great source of health‐beneficial bioactive compounds such as tocopherols and phytosterols. The differences between varieties in the levels of these bioactive compounds revealed in this study show that it is possible to increase their content in cocoa derived products by choosing an appropriate cultivar. Thermal processing at relatively high temperature may lead to the degradation of tocopherols and phytosterols.Influence of roasting conditions on the tocopherols (A), phytosterols (B), and antioxidant capacity (C) of the lipid fraction from cocoa beans of different Theobroma cacao L. cultivars

Tocopherols in cocoa butter obtained from cocoa bean roasted in different forms and under various process parameters

Obtaining good quality chocolate strongly depends on raw material, i.e. cocoa beans. The processing of cocoa beans consists of some important steps, including fermentation, drying fermented beans and roasting. Traditionally roasting is performed on whole beans but currently, roasting crushed cocoa beans or cocoa liquor becomes more and more popular. Many biologically active compounds may be found in the cocoa beans, including tocopherols. This work investigates the influence of the constant or variable roasting process parameters (temperature, velocity and relative humidity of roasting air) on the tocopherol concentration in cocoa butter (CB) extracted from cocoa beans originating from Togo and roasted in two different forms, namely as whole and crushed beans. Whole cocoa beans were roasted to a 2% moisture content and crushed cocoa beans were firstly partially dried which further enables easier dehusking, then ground, dehusked and roasted until their humidity decreased to around 2%. Roasting resulted in lowering the content of individual tocopherols in analyzed material. The degree of degradation of tocopherols in CBs was different, depending on the form of roasted beans from which these CBs were extracted. Higher concentrations were determined in CBs extracted from beans roasted in the form of crushed samples comparing to CBs obtained from beans roasted as a whole. The study investigates different roasting parameters of crushed beans, none of which drastically lowered the concentration of tocopherols in extracted CBs. Their concentration in CBs extracted from whole beans was, on the other hand, influenced by roasting air parameters. In case of whole beans roasted under constant parameters, application of 150°C proved to be more favorable than roasting at 135°C, as well as application of “dry” air and 1m/s roasting air velocity. Discussing the variable roasting process parameters, in case of applied roasting temperature it is more favorable to change it from 150°C to 135°C, than the other way round. Changing the relative humidity of roasting air from 5 to 0.3%, lower degradation of tocopherols in CB occurs when the process is conducted at 150°C. It may be further concluded that a direct dependence between the velocity of roasting air varied during the process and the concentration of tocopherols in extracted CB may not be indicated. In conclusion, it is stated that the temperature of the air applied during the roasting process has the decisive influence on the tocopherol content in CBs extracted from cocoa beans subjected to the process.

Performance of palm olein and modified rapeseed, sunflower, and soybean oils in intermittent deep‐frying

The frying performance of rapeseed, soybean, and sunflower oils with modified fatty acid composition, and palm olein (PALMO) was compared during a rotational frying operation. The frying was conducted at 185 ± 5°C for 6 days where French fries, battered chicken, and fish sticks were fried in succession. At the end of the frying period, high‐oleic rapeseed and sunflower oils exhibited a significantly higher frying stability than PALMO and other modified oils, based on total polar components (TPC), polymers, and non‐volatile carbonyl compounds formation (anisidine value (AV)). The rate of TPC formation was 2.9, 2.9, 3.2, 3.2, and 3.4% per frying day for high‐oleic low‐linolenic rapeseed (HOLLRAP), high‐oleic sunflower (HOSUN), mid‐oleic sunflower (MOSUN), low‐linolenic soybean (LLSOY), and PALMO, respectively. Although the contents of free fatty acids (FFA) in the used oils were significantly below the regulatory discard level, in PALMO formation of these compounds was 1.7 times higher compared to the modified oils. Color component formation and tocopherol degradation were also observed to be the highest in palm olein. A 15‐member consumer panel awarded HOLLRAP and HOSUN the highest overall sensory acceptance scores, while for LLSOY and PALMO the lowest.Practical applications: Although several frying oils are available in today's market, only a few of them can deliver satisfactory performance during extended frying operation. Thus, the search for the ideal frying oils/fats is an ongoing task. The present study assessed frying performance in the quest for the appropriate frying oils/fats in order to deliver healthy fried products with optimized nutritional qualities.

Interaction of Light and Temperature on Tocopherols During Oxidation of Sunflower Oil

AbstractLight and temperature effects on tocopherols during the oxidation of sunflower oil were studied. The oxidation was performed at 40, 60, or 80 °C for 30, 15, and 6 days, respectively, in the dark or under 1,700 lux light. Oil oxidation was analyzed with peroxide values and conjugated dienoic acid contents, and tocopherols in the oil were separated and quantified by HPLC. The oxidation of sunflower oil was increased with temperature increase, and the light decreased the temperature dependence of the oil oxidation. Sunflower oil before oxidation contained tocopherols with a total of 737.96 mg/kg, with α‐ and γ‐tocopherol at 726.41 and 11.56 mg/kg, respectively, and the tocopherol contents decreased during the oil oxidation. Degradation of tocopherols increased with the temperature increase, and its dependence on the temperature was lower under light than in the dark. γ‐Tocopherol showed higher stability than α‐tocopherol during oxidation of the oil in the dark and under light. Residual amounts of tocopherols showed a relatively good correlation with the degree of oil oxidation, and the dependence of α‐tocopherol degradation on the oil oxidation was higher than that of γ‐tocopherol, and light decreased the dependence on the oil oxidation in both tocopherols.

Effect of UV Processing Treatments on Soy Oil Conjugated Linoleic Acid Yields and Tocopherols Stability

AbstractPhotoirradiation has been used to synthesize 20 % conjugated linoleic acid (CLA) in soy oil with an iodine catalyst. CLA yields are affected by ultraviolet (UV) irradiation time, Magnesol® adsorbent treatment, iodine concentration and mixed tocopherols. However, these factors in combination had not been studied. Therefore, the objectives of this study were to determine the effect of (1) a combination of photoirradiation time, Magnesol® adsorbent treatment and added mixed soy tocopherols on CLA yields and the oxidative stability of CLA‐rich soy oil, (2) UV light on mixed tocopherol stability, as tocopherols enhance CLA yields during photoirradiation. Soy oil was initially treated with 5 % Magnesol®. Iodine at 0 and 0.35 % was mixed with Magnesol®‐treated soy oil and irradiated for 12 and 6 h. The irradiated oil was again treated with Magnesol®, mixed with 0, 0.35 or 0.175 % iodine; 1400 MT and irradiated for 12 or 6 more hours. CLA content in soy oil was determined by conventional gas chromatography‐flame ionization detector. The oxidative stability of the oil was determined by measuring peroxide value (PV). The tocopherols stability was determined by high performance liquid chromatography. The results showed that increasing photoirradiation time increased CLA yields and lowered PV. Magnesol® adsorption produced highest CLA yield for all treatments by removing peroxides in RBD soy oil. The γ‐tocopherols exhibited highest stability during UV irradiation. The order of tocopherol degradation was α‐tocopherol &gt; δ‐tocopherol &gt; γ‐tocopherols.

Effect of roasting on oxidative and tocopherol stability of walnut oil during storage in the dark

AbstractThe effect of roasting on the oxidative stability of oil from walnut (Juglans sinensis Dode) was investigated by observing changes in the characteristics of oils from unroasted and roasted kernels during storage in the dark at 60°C. Walnut kernels were roasted at 160°C for 15 min prior to oil extraction with the solvent, hexane. Roasting of kernels increased the peroxide value (POV) and conjugated dienoic acid (CDA) value of the oil. The rate of increase in the POV was significantly lower in roasted than in the unroasted walnut oil during storage at 60°C (1.90 vs. 1.06 and 4.45 vs. 3.55 meq/kg/day during induction period (IP) and post‐IP, respectively). Roasting of kernels significantly increased the IP of walnut oil from 0.89 to 3.39 days during storage. The total tocopherol content in roasted walnut oil was lower as compared to that in unroasted one (277.77 vs. 314.88 µg/g). However, the rate of degradation of total tocopherol during storage was lower in roasted walnut oil compared to unroasted one (1.18 vs. 2.17%/day), which showed that the tocopherol retention was higher in roasted walnut oil. These results indicate that roasting of kernels increased the oxidative and tocopherol stability of oil during storage in the dark.

Synergistic effect of lecithins for tocopherols: lecithin-based regeneration of α-tocopherol

Phospholipids synergistically enhance the antioxidant effect of phenolic antioxidants. Mixtures of lecithins, ascorbic acid and tocopherols are known as strong antioxidants. In our study, the mechanism of the synergistic effect of phospholipids with tocopherols was investigated. The oxidative deterioration and the degradation of tocopherols were analyzed in degummed rapeseed oil or in mixtures with ethyl linoleate at 25 or 110 °C. It was shown that lecithins reduce the degradation of tocopherols in rapeseed oil or in ethyl linoleate model system at 25 or 110 °C. It was found that phosphatidylethanolamine (PE) and phosphatidylserine (PS) can regenerate α-tocopheryl quinone (α-TQ) to α-tocopherol at elevated temperatures. The regeneration could be catalyzed by acetic acid. The synergistic effect of PE or PS with α-tocopherol and l-ascorbic acid resulted from the regeneration of α-TQ to α-tocopherol, because PE, l-ascorbic acid and α-tocopherol show the same stabilization effect upon ethyl linoleate as PE, l-ascorbic and α-TQ. Phosphatidylcholine as a non-amine phospholipid does not regenerate α-TQ. However, it improves the regeneration of α-tocopherol from α-tocopheroxyl radicals with l-ascorbic acid in ethyl linoleate through its ability to acts as a phase transfer catalyst between the polar l-ascorbic acid and the unpolar α-tocopheroxyl radicals. A mechanism for the regeneration of α-TQ by PE or PS is suggested. The activation of α-TQ by PE or PS and the formation of α-tocopherones are the key steps of the mechanism.