Garlic and onions, which are high in polysulfides, can transform and generate trans fatty acids when cooked at high temperatures.
- Natural sulfur compounds, isothiocyanates and polysulfides, are present in garlic and onions.
- The latter promote the formation of trans fatty acids during cooking at temperatures above 140°C.
- Isomerization (a molecular reconfiguration) induced by isothiocyanate can be strongly inhibited by antioxidants, but this is not the case for polysulfides.
Trans fatty acids are unsaturated fatty acids naturally present in foods or industrially produced, for example in margarine, appetizer biscuits or fried products. The latter constitute a major cause of cardiovascular diseases. Indeed, they accumulate along the arterial walls, which limits blood circulation and increases the risk of myocardial infarction and even death. According to the World Health Organization (WHO), “more than 278,000 deaths worldwide are attributable to the consumption of industrially produced trans fatty acids” every year.
Garlic, onion, leek, cabbage, broccoli… Simulate their real cooking processes
To reduce the amount of unsaturated fatty acids consumed, “Various industrial measures have been taken. However, research into trans fatty acids formed during cooking is limited,” indicated researchers from Meijo University (Japan). It has recently been shown that unsaturated fatty acids can undergo trans-isomerization, a molecular reconfiguration that transforms them into trans fatty acids when heated to 150°C or higher. Furthermore, “sulfur-containing compounds, such as isothiocyanates and polysulfides, found in many vegetables, are known to promote the geometric isomerization of carotenoids (e.g., lycopene in tomato), natural pigments that give foods their red, orange or yellow color.
Faced with these data, the team decided, within the framework of a study, to evaluate the role of isothiocyanates and polysulfides in promoting the trans-isomerization of plant unsaturated fatty acids during cooking. She first examined the effects of sulfur compounds in triacylglycerols in a system using reagents. Then, tests were carried out with ingredients, such as garlic, onion, leek, cabbage and broccoli sprouts, and vegetable oils (soy and olive) to simulate real cooking processes. “We were interested in the effects of temperature, reaction time, concentration of sulfur compounds, type of sulfur compounds and the addition of antioxidants on the isomerization of unsaturated fatty acids”explained Junji Obiwho led the work published in the journal Food Research International.
Cooking garlic and onions in oil above 140°C transforms unsaturated fatty acids
The results showed that sulfur compounds significantly promoted heat-induced trans-isomerization of unsaturated fatty acids in vegetable oils, especially when cooking temperatures are above 140°C. The addition of antioxidants significantly reduced the promotion of isomerization of unsaturated fatty acids by isothiocyanates, but the latter did not inhibit the promotion of isomerization by polysulfides.
“Under normal cooking conditions, the release of trans fatty acids should be minimal. Therefore, there is no need to exercise excessive caution. However, it is important to understand that cooking with ingredients high in Natural sulfur compounds may increase the risk of ingesting trans fatty acids. said the scientists. In detail, the research revealed that cooking garlic and onions, rich in polysulfides, in vegetable oil at high temperatures significantly promoted the trans-isomerization of unsaturated fatty acids.