Cooking garlic and onions at high heat can add trans fats to your meal; here’s how

Garlic and onions lend distinct flavour to soups and curries and in many households it is used almost on a daily basis. But many of us may be guilty of cooking them in a wrong way which could be sneakily adding harmful fats to the diet. The wrong cooking process can turn your nutritious home-cooked meal into an unhealthy one which could raise bad cholesterol levels in the bloodstream.
A new study says cooking garlic and onion at high temperatures can convert certain compounds in them into

trans fats

which make for a big risk factor for cardiovascular disease. This happens when you cook with ingredients rich in sulfur compounds at temperatures 140 °C or higher. Let’s understand what trans-fatty acids and how they affect heart health.

What are trans-fatty acids or TFAs

Not all fats are unhealthy but the most unhealthy category belongs to trans fats. As per Harvard Health, it is a byproduct of a process called hydrogenation that is used to turn healthy oils into solids and to prevent them from becoming rancid. They have no known health benefits and they have been officially banned in the United States.

What’s wrong with trans fats

A diet full of trans-fat acids is a sureshot way to damage heart health. Several countries and several cities in the United States have limited or banned the use of trans fats. Their low cost and high shelf value makes them attractive to those selling junk food and restaurants and many of them often reuse it multiple times making them further unsafe for consumption.

Cakes, cookies, pies, microwave popcorn, frozen pizza, fried foods like french fries, doughnuts and fried chicken, nondairy coffee creamer, and stick margarine have trans fats.
Trans fats raise the risk of heart attacks, stroke and type 2 diabetes and raise bad cholesterol levels while reducing good cholesterol.
According to The World Health Organization (WHO), trans fats are linked to over 278,000 deaths globally each year. WHO advises restricting TFA intake to less than 1% of total daily energy consumption.

While it’s usually used in industrial settings, TFAs are primarily created during the hydrogenation of vegetable oils—a process that saturates the oil with hydrogen. As per research, TFAs can also be produced during home cooking under certain conditions. This is important to note while preparing meals.

How trans fats form during cooking

Unsaturated fatty acids

(UFAs) get transformed into TFAs when heated at 150 °C or higher as they undergo trans-isomerization, a molecular reconfiguration. In case of sulfur-containing compounds like isothiocyanates and polysulfides found in many vegetables, geometrical isomerization of carotenoids – natural pigments that give vegetables their red, orange, or yellow color, can happen.
To explore whether these sulfur compounds also promote the trans-isomerization of UFAs in everyday cooking, a Japanese research team led by Mr. Junji Obi of Nissui Corporation and Dr. Masaki Honda of Meijo University studied the role of isothiocyanates and polysulfides in promoting the trans-isomerization of vegetable UFAs during cooking. The research was published in the journal Food Research International.
The researchers initially assessed the effects of sulfur compounds in triacylglycerols (TAGs) using a model system with reagents. Following this, they conducted tests with ingredients such as garlic, onion, leek, cabbage, horseradish, and broccoli sprouts, along with vegetable oils like soybean and olive oils, to replicate real cooking conditions.
“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 studied the role of antioxidants such as α-tocopherol in reducing the isomerization of UFAs in triglycerides like triolein and trilinolein.

What the study found

The study found that sulfur compounds in vegetables like garlic and onions promote the heat-induced trans-isomerization of unsaturated fatty acids (UFAs) in vegetable oils, particularly at temperatures above 140°C. Antioxidants reduced isomerization caused by isothiocyanates but had little effect on isomerization from polysulfides. Cooking polysulfide-rich vegetables at high temperatures can generate trans fatty acids (TFAs), though under normal cooking conditions, the increase in TFAs is minimal. While excessive caution is unnecessary, it’s important to be aware that cooking with sulfur-rich ingredients may slightly raise 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.”