Laboratory-scale treatments of canola oils similar to deodorization were carried out by applying the following conditions: reduced pressure with nitrogen or steam stripping at different temperatures ranging from 210 to 270°C for 2-65 h. The formation of the group of trans linolenic acid isomers follows a firstorder reaction and the kinetic constant varies according to the Arrhenius' law. Similar results were observed for the trans isomerization of linoleic acid. Based on these experiments, a mathematical model was developed to describe the isomerization reaction steps occurring in linoleic and linolenic acids during deodorization. The calculated degrees of isomerization are independent of the composition of the oil but related to both time and temperature of deodorization. The degree of isomerization of linolenic acid is unaffected by the decrease of this acid content observed during the deodorization. Deodorization at about 220-230°C appears to be a critical limit beyond which the linolenic isomerization increases very strongly. The newly established model can be a tool for manufacturers to reduce the total trans isomer content of refined oils, and was applied to produce a special selectively isomerized oil for a European Nutritional Project.Paper no. J8867 in JAOCS 76, 73-81 (January 1999).

Degummed bleached rapeseed oil was heated at 210, 220, and 230°C for up to 86 h under reduced pressure with nitrogen stripping. No significant change of total linoleic acid content was found, but a decrease of the total linolenic acid content was observed under extreme conditions. The degradation rate of linolenic acid is described as a function of heating time and operating temperature. Linolenic acid degradation can be predicted for any set of conditions by the established model. No significant degradation of linolenic acid can occur under standard deodorization conditions. Vegetable oils are refined by means of alkali-or physical refining. In both industrial processes, the last step consists of heating the oil for 1 to 4 h at a high temperature (180-260°C), under low pressure (1-10 mbar), with steam or nitrogen injection. The effects of processing on minor components of oils, such as sterols, steryl esters, tocopherols, polymeric triglycerides and trans fatty acids, have been thoroughly investigated (1), but little information is available on degradation of polyunsaturated fatty acids during deodorization.Studies about refined oils that are subjected to long-term heat treatment mainly cover the user's point of view, e.g., heating in the presence of air and deep-frying of different foods. These papers focus on the measurement of free fatty acids, conjugated dimers and trimers, polymeric triglycerides, and trans fatty acid isomers (2-5). Also data related to fatty acid degradation can be found (6-9).In previous laboratory experiments at Lesieur Research and Development Center (Coudekerque-Branche, France), in addition to the isomerization of the polyunsaturated fatty acids, a decrease in the linolenic acid content was observed during long-term heating under vacuum, depending on time and temperature.To complete the study, special heating trials with extremely long residence time were carried out on a pilot scale for investigation of the degradation kinetics. EXPERIMENTAL PROCEDURESExperiments. The experiments were carried out in the pilot plant refinery of Cereol Group Research and Development Centre in Budapest (Hungary). Degummed and bleached oil was supplied by the Cereol physical refinery of Martfu (Hungary); the fatty acid composition of this treated oil was as follows: 16:0, 4.8%; 16:1, 0.2%; 18:0, 1.8%; 18:1, 60.1%; 18:2, 21.3%; 18:3, 8.3%; 20:0, 0.6%; 20:1, 1.3%; 22:0, 0.4%; 22:1, 0.4%; and unidentified, 0.8%. Heating of the oil was performed at 1.5 mbar pressure at 210, 220, and 230°C for 86, 48, and 24 h, respectively, in a pilot-scale batch deodorizer. In all three experiments, 100 L of oil were heated, under reduced pressure (<2 mbars) and nitrogen stripping, up to the operating temperature (±1°C). Continuous heating lasted from 24 to 86 h, depending on the temperature. Samples were taken for the analysis of the fatty acid composition every 2 h through a cooled sampling system.Determination of total fatty acid composition. Fatty acid methyl esters (FAME) were prepared according to the official method ...