Tocopherol losses were studied during pan-frying of bread and during heating of vegetable oils. During the heating experiments, 16 g of sunflower and olive oil was heated on a pan (internal diameter 17 cm) at maximum oil temperature 180°C for 4, 8, and 12 min, as well as at 200 and 220°C for 4 min. The total tocopherol losses grew according to increasing heating time and temperature from 10 to almost 100% of the initial content. The absolute losses (in mg/kg) were higher in sunflower oil with a higher content of polyunsaturated fatty acids, while the relative losses (in % of the initial content) were higher in olive oil with a lower initial content of tocopherols. During the pan-frying, 16 g of oil (sunflower, olive, soybean, or rapeseed) was pre-heated on the pan for 4 min (at oil temperature 180°C) and then two slices of bread (80 g) were fried in this oil for 8 min, while the oil soaked into the fried bread. Under these conditions, tocopherol losses did not depend prominently on the used oil and did not exceed 30% of the initial content.Practical applications: Tocopherol losses during culinary processing of foods have not been studied in a sufficient range yet. Therefore, the content of vitamin E in heat-treated dishes estimated from data in food composition tables and similar tools is not correct in all cases. The presented results could enlarge our knowledge in tocopherol losses during pan-frying under different conditions as well as in content of vitamin E in fried foods. As a consequence, these results could be utilized to minimize tocopherol losses during culinary food processing and so to increase in some cases insufficient intake of vitamin E.

The oxidative stability of 12 edible fats and oils was determined at three different temperatures (80, 100, and 120°C) using the Oxipres apparatus, and compared with their characteristics (peroxide value, fatty acid composition, antioxidant capacity determined by the DPPH method and tocopherol content). Using a simple correlation analysis, oleic acid content was found to correlate most strongly with the oxidative stability of the analysed fats and oils (p < 0.01). Highly reliable models (p < 0.00001), defined the induction period as a function of the oleic/linoleic acid ratio, antioxidant capacity and peroxide value, were obtained by multivariate linear regression analysis at each of the three temperatures. In additional experiment, the effect of temperature on the induction period of sunflower oil, pork lard and extra virgin olive oil was studied in detail within the temperature range of 80–130°C. It was found that the logarithm of the induction period decreased linearly with increasing temperature (p < 0.01). However, the induction period of the different fats and oils decreased with temperature to a varying degree, i.e., the ratio between the induction periods of these fats and oils was affected by the temperature. Practical applications: Analyses to determine the oxidative stability of fats and oils are frequently required by food manufacturers, who use this information for the quality control of raw materials and, in particular, the selection of suitable fats or oils for food production. However, only a relatively small number of analytical laboratories are able to determine and interpret results obtained for the oxidative stability of fats and oils. On the other hand, determination of the composition of edible fats and oils (fatty acid composition, tocopherol content and other parameters) are routinely performed analyses. The ability to predict the oxidative stability of fats and oils from these commonly determined parameters can assist the selection and quality control of suitable raw materials for food manufacturers. The oxidative stability of selected edible fats and oils is determined at different temperatures using the Oxipres apparatus, and compare with their characteristics. Highly reliable models defined the induction period as a function of the oleic/linoleic acid ratio, antioxidant capacity and peroxide value, are obtained by multivariate linear regression analysis at each of the three temperatures.

Inadequate culinary conditions used for preparation of baked meat can contribute to an increased oxysterol intake in the diet. Frozen storage did not stop oxysterol formation. The inhibition effect of marjoram on sterols oxidation was not confirmed. © 2017 Society of Chemical Industry.

Réblová Z., Fišnar J., Tichovská D., Doležal M., Joudalová K. (2012): Effect of temperature and oil composition on the ability of phenolic acids to protect naturally present α-tocopherol during the heating of plant oils. Czech J. Food Sci., 30: 351-357.The ability of phenolic acids (ferulic, gallic, protocatechuic, and sinapic; 600 mg/kg) to protect naturally present a-tocopherol was tested during the heating of sunflower oil on a hot plate set at 120, 150, 180, 210, or 240°C, and during the heating of rapeseed, olive and soybean oils on a hot plate set at 180°C. In all the studied conditions, a-tocopherol was significantly protected only by gallic acid. This phenolic acid prolonged the half-life of a-tocopherol (calculated as the time needed for the a-tocopherol content to decrease to 50% of the original value) typically two-to four-fold. Hence the ability of phenolic acids to protect a-tocopherol in bulk oils does not markedly depend on the experimental conditions as is seen in antioxidant activity, i.e. in the ability of antioxidants to protect fatty acids.

In the scientific literature, there is not reliable information about the vitamin E content of commercially prepared fried foods. Therefore, tocochromanols were determined in 44 samples of french fries and 33 samples of potato chips and similar fried snacks. The total tocochromanol content of the french fries varied in the range of 1.7–96.9 mg/kg, α-tocopherol 0.3–76.1 mg/kg, and vitamin E (expressed in α-tocopherol equivalents) 0.6–76.4 mg/kg. The total content of tocochromanols in the fried snack products varied in the range of 39.9–204.6 mg/kg, α-tocopherol 20.4–133.7 mg kg, and vitamin E 29.8–134.6 mg α-tocopherol equivalent/kg. After a comparison of fat content, and taking into account the reference intake of fat and vitamin E, the french fries were generally a worse source of vitamin E than fat. The fried snack products were usually a better source of vitamin E than fat. In the both types of fried foods, the total content of tocochromanols was most influenced by the total content of fat. The content of α-tocopherol and the vitamin E content were mainly affected by the kind of fat (oil) used for frying.