Trans-isomerization of unsaturated fatty acids in cooking with sulfur-rich vegetables

Trans-fatty acids (TFA) are a major cause of cardiovascular diseases. These harmful fats can accumulate along artery walls, restricting blood flow and increasing the risk of heart attacks. According to the World Health Organization (WHO), TFAs are responsible for over 278,000 annual deaths worldwide, and it recommends that TFA consumption should be limited to less than 1% of daily energy intake. Common sources of TFAs include fried (junk) foods and processed foods such as margarine, ghee, biscuits, cakes, etc. In processed foods, TFAs are produced through the hydrogenation of vegetable oil, a chemical process that saturates the oil with hydrogen. While the formation process of TFAs in processed foods is well-known, evidence suggests that they can also be created at home during cooking.

Studies indicate that unsaturated fatty acids (UFAs) can undergo trans-isomerization, a molecular reconfiguration that transform them into TFAs when heated at 150 °C or higher. On the other hand, sulfur-containing compounds, such as isothiocyanates and polysulfides, which are found in many vegetables, are known to promote geometrical isomerization of carotenoids (e.g., lycopene in tomato) -;natural pigments that give vegetables their red, orange, or yellow color. This raises the question that whether these sulfur compounds also promote the trans-isomerization of UFAs in everyday cooking. Understanding the impact of natural sulfur compounds on the cooking process can help regulate TFA intake, particularly for those who avoid processed foods.

To explore this, a Japanese research team led by by Mr. Junji Obi of Nissui Corporation and Dr. Masaki Honda of Meijo University assessed the role of isothiocyanates and polysulfides in promoting the trans-isomerization of vegetable UFAs during cooking. The research was published online on November 27, 2024, in the journal Food Research International.

The researchers first evaluated the effects of sulfur compounds in triacylglycerols (TAGs) in a model system using reagents. Then, tests were conducted using ingredients (garlic, onion, leek, cabbage, horseradish, and broccoli sprouts) and vegetable oils (soybean and olive oils) to simulate actual cooking processes.

"We wanted to understand the principal characteristics of UFA isomerization in TAGs promoted by sulfur compounds through a model system using reagent-grade sulfur compounds and triglycerides," explains Junji Obi, the first author of this paper. "We were interested in the effects of temperature, reaction time, sulfur compound concentration, the type of sulfur compounds, and addition of antioxidants on UFA isomerization."

The team also assessed the role of antioxidants such as α-tocopherol in reducing the isomerization of UFAs in triglycerides like triolein and trilinolein.

The study revealed that sulfur compounds significantly promote heat-induced trans-isomerization of UFAs in vegetable oils, especially when cooking temperatures is above 140°C. The addition of antioxidants significantly reduced the promotion of UFAs isomerization by isothiocyanates, whereas they did little to inhibit the promotion of isomerization by polysulfides. This explains that cooking polysulfide-rich vegetables such as garlic and onions in vegetable oil at high temperatures can generate TFAs. In fact, this study demonstrated that garlic and onions significantly promote the trans-isomerization of UFAs.

Under normal cooking conditions, the corresponding increase in the trans isomer ratio is expected to be minimal, at most a few percentages. Therefore, while excessive caution is not necessary, it is important to recognize that cooking with ingredients rich in sulfur compounds may increase the risk of TFA intake.

"Release of TFAs under normal cooking conditions is expected to be minimal. Therefore, excessive caution is unnecessary," concludes Dr. Honda, the corresponding author of this paper. "However, it is important to understand that cooking with ingredients rich in natural sulfur compounds may increase the risk of TFA intake."