How does heat treatment of vegetables affect vitamin content?
It is true that different cooking methods alter the nutrient composition of fruits and vegetables, but this is not always a bad thing. Some studies show that while heat treating foods may degrade some nutrients, the availability of others may increase in the process.
Therefore, to say that there is a "best" form of eating plants, such as raw, is impossible.
Many people believe that raw vegetables contain more nutrients than cooked vegetables, but, again, this depends on the type of nutrient. One study in Germany involving 200 people who followed a raw food diet found that their beta-carotene levels were elevated, but their plasma lycopene levels were well below average. This is probably because fresh, raw tomatoes actually have lower lycopene levels than cooked or processed tomatoes. Thermal processing breaks down the thick cell walls of many plants, releasing the nutrients stored in them.
Water-soluble vitamins such as vitamin C and B vitamins, as well as a group of nutrients called polyphenols, are most vulnerable to processing and cooking. Canned peas and carrots lose 85 to 95 percent of their natural vitamin C. Another study found that frozen cherries lose up to 50 percent of their anthocyanins, the nutrients found in the dark pigment of fruits and vegetables. Cooking removes about two-thirds of the vitamin C from fresh spinach.
Depending on the cooking method used, the loss of vitamin C in home cooking can range from 15 to 55 percent, according to a review by researchers at the University of California, Davis. Interestingly, vitamin C levels are often higher in frozen foods compared to fresh foods -- probably because vitamin C is killed during storage and transportation of fresh foods.
Fat-soluble vitamins such as A, D, E and K and antioxidant compounds called carotenoids become more available after cooking and processing. An article in The Journal of Agriculture and Food Chemistry concluded that carrots, zucchini and broccoli are better cooked than steamed, fried or served raw. Roasting retains the least amount of nutrients.
But when it comes to cooking vegetables, there are always trade-offs to be made. You can increase the availability of one nutrient while eliminating another. For example, cooked carrots have significantly more carotenoids than raw carrots. However, raw carrots contain far more polyphenols, which disappear once you start cooking them.
To sum it all up, it's fashionable to say that no cooking method is perfect in terms of preserving 100% of the nutrients in vegetables. And since the best vegetables are the ones you will actually eat, taste should also be considered when deciding on a cooking method.
The best way to get the most out of your vegetables is to enjoy them in a variety of ways - raw, steamed, boiled, baked and roasted. If you eat a variety of fruits and vegetables regularly, you don't have to worry about how to prepare them.