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UBC Theses and Dissertations

The efficacy of natural plant extracts (rosemary, sage, and thyme) in stabilizing hemp and soybean oils during storage Doi, Brenda Kathleen


Lipid oxidation of edible oils is a concern due to undesirable effects on the organoleptic, nutritional, and toxicological aspects of fats and oils. Vegetable oils are particularly prone to lipid oxidation due to the susceptibility of the unsaturated double bonds of the fatty acids to oxidation. Hemp oil is high in unsaturated fatty acids, including gammalinolenic acid, which is an essential fatty acid. Theoretically, hemp oil would be prone to lipid oxidation; however, scientific data on the subject could not be found. Of the common vegetable oils of North America, the fatty acid composition of soybean oil most closely resembles that of hemp oil; however, gamma-linolenic acid is absent in soybean oil. The following studies monitored the oxidative stability of hemp and soybean oils based on the peroxide values, p-anisidine values, totox numbers, UV absorption measured at wavelengths of 232 and 268 nm, and fatty acid (%) compositions. A 48-week shelf-life study was performed to compare the oxidative stability of hemp and soybean oils with and without 0.125% (w/w) of rosemary, sage, or thyme extracts and stored in the dark at 38°C. Also, a 14-day accelerated storage study determined the quality of hemp and soybean oils exposed to 180°C for 0,4, 8, 12, and 24 hours and the effects of subsequent storage at 38°C in darkness. A 24-day accelerated storage study was used to monitor oxidation of hemp and soybean oils that were heated at 180°C for 4 hours prior to storage at 38°C in darkness, in order to determine the antioxidant efficacy of 0.10% and 0.05% (w/w) of rosemary, sage, or thyme extracts, and 0.10% and 0.05% (w/w) extract binary mixtures of rosemary + sage, rosemary + thyme, or sage + thyme. During long-term storage (48 weeks), the levels of lipid oxidation products (i.e. hydroperoxides, aldehydes, conjugated dienes and trienes) in hemp and soybean oils exhibited some changes during storage, but generally remained low throughout the study. This showed that hemp and soybean oils were not inherently prone to lipid oxidation. Based on the thermally-induced accelerated oxidation conditions, lipid oxidation was apparent in heated hemp and soybean oil samples as detected by the p-anisidine, totox number, UV absorption methods, and fatty acid contents. The peroxide values remained low after hightemperature heating, but progressively increased during storage. The addition of various concentrations and mixtures of rosemary, sage and thyme extracts before the thermal treatment, did not prevent oil deterioration due to heating, but effectively inhibited the generation of hydroperoxides during subsequent storage.

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