Effects of tocopherols and their mixtures on the oxidative stability of olive oil and linseed oil under heating

The tested tocopherols (γ,δ,α) showed antioxidative activity at all levels of addition to the monounsaturated olive oil, the effects increased as a function of concentrations (maximum: +287% with 800 mg γ‐tocopherol/100 g oil compared to the control oil). In the highly unsaturated linseed oil, which contains 58 mg/100 g initial concentration of γ‐tocopherol, γ‐tocopherol showed antioxidative behavior up to the addition of 100 mg/ 100 g oil. Additions of more than the 100 mg/100 g affected the oil, resulting in a faster oxidation. Mixtures of γ/δ‐tocopherols in olive oil were found to protect more efficiently than both vitamins when added separately α‐tocopherol reduced effects of other tocopherols in both plant oils. The stabilizing effect of added tocopherols and their mixtures (100 mg/100 g oil each) in olive oil are γ/δ‐T>γ‐T>δ‐T>γ/α‐T>δ/α‐T>α‐T and in linseed oil γ‐T>γ/δ‐T>δ‐T>γ/α‐T>α‐T>α/δ‐T.     

Composition of organic and conventionally produced sunflower seed oil

The aim of the present study was to highlight the main differences between seed oils produced from conventionally cultivated crops and organically cultivated ones and processed using mild extraction procedures. The composition and the nutritional and health aspects of both types of sunflower seed oils were compared and were analytically tested to determine the macroscopic differences in proximate composition, the main differences in the minor components, the main quality parameters, the in vitro antioxidant activity, and the presence of trans-ethylene steroisomers in FA. No significant trends were found in the oil samples for TAG and FA composition, but remarkable differences were found in the composition of minor components and in the main chemical and analytical quality properties. The organically grown samples had a higher total antioxidant activity compared with the conventional samples. Trans FA were found only in the conventional oils.     

Oxidative degradation kinetics of lycopene, lutein, and 9-cis and all-trans β-carotene

The thermal and oxidative degradation of carotenoids was studied in an oil model system to determine their relative stabilities and the major β-carotene isomers formed during the reaction. All-trans β-carotene, 9-cis β-carotene, lycopene, and lutein were heated in safflower seed oil at 75, 85, and 95°C for 24, 12, and 5 h, respectively. The major isomers formed during heating of β-carotene were 13-cis, 9-cis, and an unidentified cis isomer. The degradation kinetics for the carotenoids followed a first-order kinetic model. The rates of degradation were as follows: lycopene>all-trans β-carotene≈9-cis β-carotene>lutein. The values for the thermodynamic parameters indicate that a kinetic compensation effect exists between all of the carotenoids. These data suggest that lycopene was most susceptible to degradation and lutein had the greatest stability in the model system of the carotenoids tested. Furthermore, there was no significant difference in the rates of degradation for 9-cis and all-trans β-carotene under the experimental conditions.     

Inhibition of oxidation in 10% oil-in-water emulsions by β-carotene with α- and γ-tocopherols

The effects of low concentrations of β-carotene, α-, and γ-tocopherol were evaluated on autoxidation of 10% oil-in-water emulsions of rapeseed oil triacylglycerols. At concentrations of 0.45, 2, and 20 μg/g, β-carotene was a prooxidant, based on the formation of lipid hydroperoxides, hexanal, or 2-heptenal. In this emulsion, 1.5, 3, and 30 μg/g of γ-tocopherol, as well as 1.5 μg/g of α-tocopherol, acted as antioxidants and inhibited both the formation and decomposition of lipid hydroperoxides. Moreover, at a level of 1.5 μg/g, γ-tocopherol was more effective as an antioxidant than α-tocopherol. At levels of 0.5 μg/g, both α- and γ-tocopherol significantly inhibited the formation of hexanal but not the formation of lipid hydroperoxides. Oxidation was effectively retarded by combinations of 2 μg/g β-carotene and 1.5 μg/g γ- or α-tocopherol. The combination of β-carotene and α-tocopherol was significantly better in retarding oxidation than α-tocopherol alone. While γ-tocopherol was an effective antioxidant, a synergistic effect between β-carotene and γ-tocopherol could not be shown. The results indicate that there is a need to protect β-carotene from oxidative destruction by employing antioxidants, such as α- and γ-tocopherol, should β-carotene be used in fat emulsions.     

The influence of antioxidants on the oxidation stability of biodiesel

Oxidation stability of bodiesel is an important issue because FA derivatives are more sensitive to oxidative degradation than mineral fuel. Therefore, in the most recent European Specifications for biodiesel, a minimum value of 6 h for the induction period at 110°C, measured with a Rancimat instrument, is specified. To guarantee this value at the filling station, the use of additional antioxidants will be necessary. In this paper we show the influence of different synthetic and natural antioxidants on the oxidation stability, using the specified test method. Biodiesel produced from rapeseed oil, sunflower oil, used frying oil, and beef tallow, both undistilled and distilled, was investigated. The four synthetic antioxidants pyrogallol (PY), propylgallate (PG), TBHQ, and BHA produced the greatest enhancement of the induction period. These four compounds and the widely used BHT were selected for further studies at concentrations from 100 to 1000 mg/kg. The induction periods of methyl esters from rapeseed, oil, used frying oil, and tallow could be improved significantly with PY, PG, and TBHQ, whereas BHT was not very effective. A good correlation was found between the improvement of the oxidation stability and the FA composition.     

The impact of antioxidants on biodiesel oxidation stability

The potential of 11 different synthetic phenolic antioxidants to improve the oxidation stability of biodiesel prepared from different feedstocks was investigated. Measurements of oxidation stability were carried out according to the European biodiesel specifications with a Rancimat instrument at 110 °C. At antioxidant concentrations of 1000 mg/kg, an improvement in oxidation stability could be achieved with all antioxidants tested. Especially the antioxidants DTBHQ, IONOX 220, Vulkanox ZKF, Vulkanox BKF, and Baynox were able to significantly improve the oxidation stability, leading to stabilization factors between 1.89 and 13.07. Variation of antioxidant concentrations between 100 and 1000 mg/kg showed that the efficiency of the antioxidants varied depending on the different types of biodiesel. When used as additives, Baynox showed good effects on rapeseed oil methyl ester (RME) stability, DBHQ on recycled cooking oil methyl ester (RCOME) stability, Vulkanox BKF on distilled RCOME (DRCOME) stability, and IONOX 220 on tallow methyl ester (TME) stability. Evaluation of the influence of the antioxidants on critical biodiesel fuel parameters showed no negative impacts on viscosities, densities, carbon residues, CFPP, and sulphated ash contents of the different biodiesel samples. However, in terms of acid values, a noticeable increase could be observed at antioxidant levels of 1000 mg/kg. At lower antioxidant concentrations, this increase was much lower and the values remained within the required limits.     

Comparative study on the stability of crude and refined rice bran oil during long‐term storage at room temperature

The effect of the full refining process on the stability of rice bran oil during storage at room temperature was studied. Crude and refined rice bran oil were kept in the dark and were exposed to light for 240 days, and every 10 days samples were drawn and analysed. The storage stability of crude and fully refined rice bran oil was determined and compared with respect to fatty acid composition, tocopherols, tocotrienols, sterols and γ‐oryzanol content. In addition, the oxidative status was evaluated by determining the concentration of polar compounds and the oil stability index (OSI). A good correlation between the decrease of total tocopherols and the OSI was found. α‐Tocopherol had the highest correlation coefficient (r² = 0.9653) in crude rice bran oil kept in the dark, and γ‐tocopherol showed the lowest in the refined sample (r² = 0.4722). The order of stability of tocopherols and tocotrienols in crude oil was completely different from that in refined oil. In comparison to tocopherols, sterols showed a better stability during the entire storage period. The exposure to daylight heavily affected the composition and the stability of both crude and refined rice bran oil.     

Determination of oxidation stability of soybean oil with the oxidative stability instrument: Operation parameter effects

Operation parameters of the oil stability index instrument were evaluated to determine their effect on the oxidative stability of commercial soybean oil. A factorial design was developed to evaluate the following three parameters, each at two levels, sample weight (2.5 or 5.0 g), conductivity tube temperature (20 or 30°C), and air flow rate (12 or 20 L/h), for a total of eight observations. Significance testing indicated that sample size and air flow rate affected oil oxidative stability independently (P<0.001), but not in combination. The conductivity tube temperature did not affect the oxidation stability index. Presented at the 1993 American Oil Chemists’ Society Meeting in Anaheim, California.     

Antioxidant activity of tocopherols and phenolic compounds of virgin olive oil

The antioxidant effects of hydrophilic phenols and tocopherols on the oxidative stability in virgin olive oils and in purified olive oil have been evaluated. Total hydrophilic phenols and the oleosidic forms of 3,4-dihydroxyphenolethanol (3,4-DHPEA) were correlated (r=0.97) with the oxidative stability of virgin olive oil. On the contrary, tocopherols showed low correlation (r=0.05). Purified olive oil with the dialdehydic form of elenolic acid linked to 3,4-DHPEA, an isomer of oleuropeine aglycon, and 3,4-DHPEA had good oxidative stability. A synergistic effect was observed in the mixture of 3,4-DHPEA and its oleosidic forms with α-tocopherol in purified olive oil by the Rancimat method at 120°C.     

Effect of fatty acids and tocopherols on the oxidative stability of vegetable oils

The oxidative stability of vegetable oils is determined by their fatty acid composition and antioxidants, mainly tocopherols but also other non‐saponifiable constituents. The effect of fatty acids on stability depends mainly on their degree of unsaturation and, to a lesser degree, on the position of the unsaturated functions within the triacylglycerol molecule. Vegetable oils contain tocopherols and tocotrienols, especially α‐ and γ‐tocopherols, as their main antioxidants. The antioxidant behavior of tocopherols represents a complex phenomenon as they are efficient antioxidants at low concentrations but they gradually lose efficacy as their concentrations in the vegetable oils increase. The “loss of efficacy” of tocopherols, sometimes referred to as a “pro‐oxidant effect”, is witnessed by an increase in the rate of oxidation during the induction period, despite elongation of this phase. The phenomenon is much obvious for α‐tocopherol, but is also evident for other tocopherols. In agreement with nature's wisdom, the tocopherol levels in vegetable oils seem to be close to the optimal levels needed for the stabilization of these oils. The presence of other antioxidants in the oils, e.g. carotenoids, phenolic compounds, and Maillard reaction products, may synergize with tocopherols and minimize this loss of efficacy.     

Frying stability of canola oil in presence of pumpkin seed and olive oils

Frying performance of canola oil (CO) was investigated in the presence of 5, 10, and 15% levels of virgin olive oil (VOO) and pumpkin seed oil (PSO) during frying of potatoes at 180°C. Acid value, carbonyl value, total polar compounds content, and total tocopherols content of the oil samples were determined during the frying process. VOO and PSO addition improved the frying stability of the CO. Frying performance of the CO increased more in the presence of PSO than in the presence of the VOO. The PSO levels higher than 5% exerted pro‐oxidant effects, indicating the necessity of investigation at lower levels. The better antioxidative effect of PSO was attributed to its probably different phenolic composition.     

Endogenous antioxidants and stability of sesame oil as affected by processing and storage

The effect of processing of coated and dehulled sesame seeds on the content of endogenous antioxidants, namely sesamin, sesamolin, and γ-tocopherol in hexane-extracted oils, was studied over 35 d of storage under Schaal oven test conditions at 65°C. Seeds examined were Egyptian coated (EC) and dehulled (ED) and Sudanese coated (SC) varieties. Processing conditions of raw (RW) seeds included roasting at 200°C for 20 min (R), steaming at 100°C for 20 min (S), roasting at 200°C for 15 min plus steaming for 7 min (RS) and microwaving at 2450 MHz for 15 min (M). The sesamin content in fresh oils from EC, ED, and SC raw seeds was 649, 610, and 580 mg/100 g oil, respectively. Corresponding values for the content of sesamolin in oils tested were 183, 168 and 349 mg/100 g oil, respectively. Meanwhile, the content of γ-tocopherol, the only tocopherol present in the oils, ranged from 330 to 387 mg/kg sample. The effect of processing on changes in the sesamin content in oils from coated seeds was low and generally did not exceed 20% of the original values. On the other hand, oils from dehulled seeds underwent a more pronounced decrease in their sesamin content than the oil from coated seeds after 35 d of storage at 65°C. The corresponding changes in sesamolin and γ-tocopherol contents were more drastic. The RS treatment, which would be the optimal to prepare sesame oil with better quality, was found to retain 86, 80 and 60% of the sesamin, sesamolin and γ-tocopherol, respectively, originally present in the seeds after the storage period. The loss in the content of endogenous antioxidants present in the oils paralleled an increase in their hexanal content.     

Effect of natural antioxidants in virgin olive oil on oxidative stability of refined,bleached, and deodorized olive oil

The factors influencing the oxidative stability of different commercial olive oils were evaluated. Comparisons were made of (i) the oxidative stability of commercial olive oils with that of a refined, bleached, and deodorized (RBD) olive oil, and (ii) the antioxidant activity of a mixture of phenolic compounds extracted from virgin olive oil with that of pure compounds andα-tocopherol added to RBD olive oil. The progress of oxidation at 60°C was followed by measuring both the formation (peroxide value, PV) and the decomposition (hexanal and volatiles) of hydroperoxides. The trends in antioxidant activity were different according to whether PV or hexanal were measured. Although the virgin olive oils contained higher levels of phenolic compounds than did the refined and RBD oils, their oxidative stability was significantly decreased by their high initial PV. Phenolic compounds extracted from virgin olive oils increased the oxidative stability of RBD olive oil. On the basis of PV, the phenol extract had the best antioxidant activity at 50 ppm, as gallic acid equivalents, but on the basis of hexanal formation, better antioxidant activity was observed at 100 and 200 ppm.α-Tocopherol behaved as a prooxidant at high concentrations (>250 ppm) on the basis of PV, but was more effective than the other antioxidants in inhibiting hexanal formation in RBD olive oil.o-Diphenols (caffeic acid) and, to a lesser extent, substitutedo-diphenols (ferulic and vanillic acids), showed better antioxidant activity than monophenols (p- ando-coumaric), based on both PV and hexanal formation. This study emphasizes the need to measure at least two oxidation parameters to better evaluate antioxidants and the oxidative stability of olive oils. The antioxidant effectiveness of phenolic compounds in virgin olive oils can be significantly diminished in oils if their initial PV are too high.     

Oxidative stability of healthful frying oil medium and uptake of inherent nutraceuticals during deep frying

Four healthful frying oil mediums have been formulated using sunflower (FOB-I), groundnut (FOB-II), mustard (FOB-III), and palm olein (FOB-IV) oils as base oils, and fortified with rice bran and crude sesame oils separately in the ratio of 60∶20∶20 (by vol). Oxidative stabilities have been ascertained by deep-frying potato bajji (potato slices sandwiched with Bengal gram flour) continuously for 60 min for three cycles with a gap of 7 d each. The product had moisture between 12.8 and 16.0% and absorbed fat between 32.5 and 38.1%, making the oil media vulnerable to oxidation. The p-anisidine values for leftover FOB-I and FOB-IV ranged from 10.8 to 24.4 and from 1.5 to 10.7, respectively, indicating that the former was a less and the latter a more stable combination. Hydroperoxide and conjugated dienes were assessed by UV spectrometry at λ_max 230 nm. The O.D. was maximal (1.4) for FOB-I samples for both leftover and absorbed oils for third-cycle experiments. That there was no absorbance for the FOB-III and-IV samples indicated their absence. Estimation of oryzanol and sesamol in oil left over after deep frying and in the oil absorbed by the products indicated that distribution was equal and there was no loss of these active factors during deep frying. The study indicated that sunflower oil blend was the least stable and the palm olein blend was most stable.     

Effects of antioxidants and humidity on the oxidative stability of microencapsulated fish oil

The effects of various antioxidants and RH on the oxidative stability of microencapsulated fish oil powder were investigated using PV and thiobarbituric acid tests. The micorencapsulation process provided high encapsulation efficiency (≥88% of extractable fish oil). Without antioxidants, the encapsulated fat was 10 times more stable against oxidation than the surface fat, as determined by PV. α-Tocopherol, which is a lipophilic antioxidant, showed a greater antioxidative effect in both surface and encapsulated fats than ascorbyl palmitate, which is an amphiphilic antioxidant. According to TBARS values, the longest lag period was observed at 0% RH. Addition of >200 ppm α-tocopherol in a 10–30% RH range prolonged the oxidative stability of the microencapsulated fish oil powder.     

Nutritional and potential disease prevention properties of carotenoids

Epidemiological studies have shown that people who consume diets with a high content of vegetables have a reduced risk of degenerative diseases such as specific cancers, cardiovascular disease, age-related macular degenerative disease (AMD), and cataracts. There is no convincing evidence that the protective role of vegetables against cancer and cardiovascular disease is due to carotenoids. However, there is a strong possibility that lutein and zeaxanthin present in food materials may prevent AMD and cataract formation. Increased use of cooked tomato products also has been shown to reduce prostate cancer risk as a result of increased bioavailability of cis-lycopene. One of the most important biochemical mechanisms underlying the cancer-preventive activity of carottenoids is the stimulation of intercellular gap junction communications. β-Carotene, canthaxanthin, and lutein are efficient inducers of intercellular gap junction communication, whereas α-carotene and lycopene are less active.     

Role of minor constituents in the photooxidation of virgin olive oil

Virgin olive oil was photooxidized at 2 and 40°C and at fluorescent light intensities of 0, 620, 2710, and 5340 lux. As expected, higher fluorescent light intensities induced higher peroxide formation in the oil. The thiobarbituric acid reactive substances (TBARS) were found to be good indicators of photooxidation during the early stage of the reaction. Pheophytin A and β-carotene were light- and temperature-sensitive, whereas α-tocopherol and total polyphenols were mostly affected by light. Pheophytin A functioned as a photosensitizer, resulting in rapid oxidation of the oil. β-Carotene was a strong natural inhibitor of photooxidation for all light intensities at 2°C, suggesting quenching properties for singlet oxygen. However, β-carotene antioxidant activity was reduced at 40°C because of its rapid destruction.     

Effect of synthetic antioxidants on cholesterol stability during the thermal-induced oxidation of a polyunsaturated vegetable oil

As a molecule with an unsaturated bond, cholesterol is prone to oxidation. Cholesterol oxidation products (COP) are found in many common foods and have been shown to be atherogenic, cytotoxic, mutagenic, and possibly carcinogenic. Efforts to reduce the formation of oxidation products are considered important during the manufacture and processing of foods. The effect of synthetic antioxidants on cholesterol oxidation has not been extensively studied. We assayed the effect of five commonly used antioxidants—BHT, BHA, the n-propyl ester of 3,4,5-trihydroxy benzoic acid (PG), TBHQ, and 6-ethoxy-1,2-dihydro-2,4-trimethylquinoline (EQ)—on cholesterol stability when oxidation is induced in a Rancimat 679 instrument by bubbling air through the sample at 150°C. The sample consisted of 200 mg cholesterol dispersed in 100 g of a polyunsaturated vegetable oil (soybean oil). Formation of six COP was measured at the induction period, and at the 50 and 100 μS conductivity values. Under the experimental conditions, BHT and TBHQ were the most effective inhibitors of cholesterol oxidation. BHA and EQ were less effective, and PG was unable to prevent cholesterol oxidation. Synthetic antioxidants were more effective in preventing COP formation at the nucleus of the cholesterol structure than at the lateral chain.     

The effect of oxygen concentration on oxidative deterioration in heated high-oleic safflower oil

High-oleic safflower oil was heated at 180°C in atmospheres with four levels of oxygen concentration (2, 4, 10, and 20%) modified with nitrogen gas, to assess the effects of atmospheric oxygen concentration on the oxidative deterioration of deep-frying oils. Acid value, carbonyl value, polar materials, linoleic acid, tocopherol contents, and oxidative stability were measured to evaluate the quality of heated oils. These values were found to be correlated with both heating time and oxygen concentration. Acid and carbonyl values and polar material content of oils heated at oxygen concentrations of 2 and 4% were lower than those at 10 or 20%. On the other hand, linoleic acid and tocopherols were hardly reduced in oils after heating for 30 h at 2% O₂, whereas they were decomposed according to the oxygen concentration and heating time. Oxidative stability was well maintained in oils heated at 2 and 4% O₂. These results suggest that the oxidative deterioration of heated higholeic safflower oil depends on oxygen concentration.