Effets de la température sur la stabilité et la qualité de deux huiles végétales

摘要

Fats and oils may be used in human nutrition in the cold state without any problem of stability excepted their storage. But, at high temperature, during frying or cooking, they are exposed to thermoxydative damage generating various products and involving many changes in the oil characteristics. In this paper we relate main observations after experiments carried out with two types of vegetable oils (olive and soybean) used as frying oils at 200℃. The aim of this work was to compare the heating effects on the stability and the quality of these oils by determining fatty acid composition, peroxyde index, specific extinction at 232 and 270 nm and evaluating the thermoxydative products (named new chemical species) appearing after frying during various periods (2,4,6,8 and 10 hours). Results obtained show: 1. a total degradation of linolenic acid of olive and soybean oils after a total frying time of 10 hours 2. a reduction of the percentage of linoleic acid, the major fatty acid of soybean oil, from the initial 54.5% to 3.3% after 10 hours frying. The heating effect on linoleic acid of olive oil is moderate, so its proportion decreases from 11% to 4.9%. In another hand, soybean oil compared to olive oil presents during heating: 1. a more important hydroperoxydation degree 2. a high level of primary and secondary oxydative products determined by measuring the specific extinction index 3. an enhanced thermoxydation with progressive occurrence of new chemical species as esters, conjugated unsaturated fatty acids, oxyacids, polymers reaching 25% of initial oil weight. In short, during frying, olive oil exhibits a good thermal behaviour and a best stability contrary to soybean oil which can be used in preference for flavouring and dressing.