This is an excerpt from the article ‘Why is ketchup so delicious? Science answers the big food questions’ by Dr Ali Khavandi and Dara Mohammadi.
How can something as tasty as fried food be so bad for you?
There’s an F-word even the bravest health bloggers daren’t mutter. It’s a word with such a bad image that even KFC rebranded itself to be free of its oil-splattered reputation. That word is fry. But what’s so unhealthy about the process? Is there anything we can do to make it less bad for us?
Whether fried, sautéed or roasted, we cook foods in fat because it makes them taste good. Fats reach higher temperatures than water, and at high temperatures chemical reactions happen that add texture and flavour – think seared meats, caramelised garlic, or crisp, battered fish.
If you cook like this occasionally, the extra fat you’ll eat is probably negligible to your health: it’s what you do to the fat when you heat it that counts. Like sending an astronaut into space, you need to choose the right vehicle – some fats will break down before they get to the temperatures you want. At temperatures below 180C (so sautéing, pan-frying or sweating-off vegetables) olive oil is a good choice. You’ll not get the full health benefits as in its raw state, but at these heats there’s no major risk of it degrading.
Polyunsaturated fats extracted from vegetables such as sunflower, corn, or soy oil are susceptible to degradation or oxidation when heated, and release toxic compounds such as the potentially carcinogenic aldehyde. A link to cooking with these oils and developing cancer has never been shown, but knowing a risk exists suggests it’s wise to steer clear. The same can be said about acrylamide – a chemical produced by excessive browning, or burning, of starchy foods such as bread or potatoes which has received recent attention after it was found that giving enormous amounts of it to mice gave them cancer. Such studies tell us little about its effect in a human diet. Still, a sensible approach would be to brown food and not burn it: a mantra any chef should be OK with.
On polyunsaturated fats, even when raw, some researchers think the high omega 6 content in these oils can lead to chronic disease of both body and mind. Get your protective polyunsaturated fats from whole foods high in omega 3 – nuts, seeds and fish.
For high-heat searing, roasting or deep-fat frying, where oil is heated to higher temperatures or for long periods, go for saturated fats like animal fats. These are more stable at higher temperatures and, as a tasty bonus, can get your food up to those stratospheric temperatures for taste: it’s why goose fat makes the crispiest potatoes. Rapeseed oil is a stable monounsaturated fat with a great theoretical profile for cooking, but we don’t know much about its long-term effect on health.
Before we free the clarified butter and lard from the shadowy corners of our cupboards, we should remember that frying is by no means healthy. High-heat cooking produces plenty of tasty chemical reactions but it might also produce some potentially unhealthy ones. In lieu of solid evidence about the health effects of dietary aldehyde and acrylamide, we should accept that there might be some risks but ameliorate them as much as possible by using the right fats.