Trans-Fat Formation in Frying Oils

TSUKUBA, JAPANIn response to health experts concerns, the food-manufacturing and foodservice industries have been reducing the amount of trans-fatty-acid containing hydrogenated fats used. But there are worries that some trans fats might be created during the frying process.

Current dietary recommendations say that trans fat intake should be as low as possible. Partial hydrogenation of vegetable oils has been the major source of dietary trans fatty acids, unsaturated fatty acids with at least one double bond in the trans configuration. Naturally occurring trans fats in meats and dairy products from cows, sheep, and other ruminants are consumed in smaller amounts (about 0.5% of total energy intake). But the high heat conditions encountered during frying is known to alter the fatty acid composition of frying oil, so there are concerns that trans-fat formation during cooking and frying might be another significant source of trans fatty acids (TFAs). To assess the impact of heated edible oils on intake of trans fat, Japanese researchers estimated the formations of TFAs during cooking by using a frying and heating model system.

To test the effect of frying, they used sliced raw potatoes (at a level of 10% of the frying oil on a weight basis) fried in commercially available unhydrogenated canola oil at 160, 180 and 200°C, for ten frying cycles. They then measured the TFAs in both the fried potatoes and in the frying oils by gas chromatography (GC).

The initial fat content of the raw potatoes was about 0.1%, and their TFA content was negligible. The fried potatoes contained 8.8% to 9.2% fat and their fatty-acid composition mostly correlated with that of the frying oil. The TFAs of potatoes fried by the tenth frying operation was between 0.99 to 1.05 grams/100 grams fat. They estimated that, for each 100 grams fried potatoes consumed, the TFA intake was less than 0.1 grams.

After ten frying operations, researchers measured the TFA content, acid values and peroxide values of the frying oils and compared them to those of the corresponding heated canola oils that were not used to fry food. They found that the amounts of trans 18:1 fatty acids in both the frying oil and the heated oil were less than the quantifiable limit (0.047 grams/100 grams oil). They measured increases of 0.02 grams of trans 18:2 fatty acids per 100 grams oil and 0.05 grams of 18:3 fatty acids per 100 grams in the frying oil, The trans 18:2 fatty-acid formation in the heated oil was slightly less than that in the frying oil.

To determine TFA formation in various edible oils during cooking, six kinds of commercially available edible vegetable oils were heated to 180 °C in glass test tubes. Small changes in TFA amounts were observed after four hours heating. According to the researchers, the results suggest that a regular frying process using unhydrogenated edible oils should have little impact on TFA consumption from edible oils.