Formation of fatty acid geometrical isomers and of cyclic fatty acid monomers during the finish frying of frozen prefried potatoes

Frozen prefried french fries were deep fried in peanut and in soybean oils. Thirty frying operations were conducted at 180, 200 and 220 degrees C for 5 min. The quality of both the oil and the french fries was assessed by measuring the amount of polymers, polar components, fatty acid geometrical isomers and cyclic fatty acid monomers (CFAM). The structures of the CFAM were determined using gas-liquid chromatography coupled with mass spectrometry (GC-MS). The results indicated that the starting oils contained some geometrical isomers of linoleic acid (0.9% in soybean and 1.6% in peanut of the total 18:2). Soybean oil also contained the monotrans isomers of linolenic acid (9.2% of the total 18:3). The amounts of 18:2 Delta 9c,12t and 18:2 Delta 9t,12c only increased after 10 frying treatments at 220 degrees C while no differences were observed at 180 and 200 degrees C. Similarly, no differences were found for the 18:3 geometrical isomers content at 180 and 200 degrees C, while these increased at 220 degrees C. Furthermore, ditrans 18:3 isomers were formed above 200 degrees C. The amount of CFAM increased when increasing the temperature and the number of frying operations. No major differences in the quantity of CFAM were observed for products fried in peanut or soybean oils. However, the relative proportions of CFAM were different for a product fried in an oil containing an appreciable quantity of linolenic acid (soybean oil) compared with the french fries fried in peanut oil. The CFAM were distributed in both the nonpolar and the polar fractions. Copyright (C) 1996 Published by Elsevier Science Ltd on behalf of the Canadian Institute of Food Science and Technology