During the summer vacation, I once gave a class to exchange students at Tokyo Agricultural University in Japan to introduce China's diet and nutrition. A girl majoring in nutrition raised her hand and asked: Why don't you Chinese lack vitamin C? Don't you cook all the vegetables? Isn't vitamin C very afraid of heat? Don't vitamin C decompose completely after cooking?

      The girls' question is not surprising, because many Chinese people also have the same idea that vitamin C is very afraid of heat. Vitamin C will completely decompose after tomatoes are fried and eaten, and lemon slices cannot be soaked in hot water. Some people want to eat everything raw and drink all fruits and vegetables with juice, thinking that this is the best way to get vitamin C.

      How afraid is vitamin C of heat? I'm afraid the truth of this problem is not exactly the same as people expected.

      From a chemical point of view, there is an olefin glycol structure in the vitamin C molecule, which is very vulnerable to oxidation. However, the rate of oxidation is also affected by environmental pH, water activity and protective substances. Vitamin C is very unstable in the presence of alkaline, light and oxidants. When it is exposed to the oxidase or catalyzed by metal ions, it will be more rapidly deactivated. However, in acidic conditions, anaerobic conditions, very dry food, or when there are protective factors such as starch, high concentration sugar, reducing substances, it can show a certain stability.

      This passage seems a bit difficult to understand. To put it simply, there are several important conditions to make vitamin C not afraid of heat and storage: (1) keep it in an acidic environment; (2) Remove oxygen and metal ions; (3) Do not contact with oxidase; (4) Little water; (5) Add a lot of starch, sugar or sugar alcohols; (6) Add other reducing substances. If these can be achieved, then vitamin C can also have a high preservation rate.

      If you look at the measured data, you will know what kind of food vitamin C is relatively stable. It is known that canning can cause serious loss of vitamin C, because canning requires twice heating, and the heating time for sterilization is relatively long. But why is the loss of vitamin C in canned tomatoes so small? It is because of its acidic environment. In contrast, spinach and carrots do not have such conditions. Polysaccharides exist in mushrooms, which have a certain protective effect on vitamin C, so the loss is relatively small. Just as the loss of vitamin C is small when potatoes are cooking, it is starch that protects vitamin C.

Table 1 Vitamin C loss in the processing of canned vegetables (%)

Data source: Endel Karmas and Robert S. Harris, Nutritional Evaluation of Food Processing, Avi, 1991

      If it is home cooking, the loss of vitamin C is smaller than that of canned food. Because cooking only needs to be heated once, and the time is relatively short. In contrast, the loss rate of vitamin C is much lower than 80% of fresh fruit and vegetable juice. In winter and spring, most of the vitamin C in our diet actually comes from cooked vegetables, rather than fruits such as apples, pears and bananas that have been stored for a long time.

Table 2 Nutrient C preservation rate of vegetables after domestic cooking (%)

Data source: Food composition table ( National representative value ) , Institute of Nutrition and Food Hygiene, Chinese Academy of Preventive Medicine, People's Health Press, one thousand nine hundred and ninety-one

      Since most of the vitamin C in the fried and boiled tomatoes is still there, lemons, which are more sour than tomatoes, will lose less vitamin C if soaked in hot water. If you don't like to drink cold water in winter, lemonade doesn't have to be soaked in cold water.

      At this point, we can also solve the problem of Japanese girls. The Chinese can also supply vitamin C by eating cooked vegetables. Although cooked vegetables must lose nutrients, as the intake of cooked vegetables is much larger than that of raw vegetables, you can get enough nutrients as long as you eat enough.

      The saying that nutrients are completely destroyed after cooking probably refers to the way Jia's family makes eggplant in A Dream of Red Mansions:

      "Peel off the eggplant that has just been picked. Just clean the meat, cut it into dices, fry it in chicken oil, and then use the chicken breast meat with fragrant mushrooms, new bamboo shoots, mushrooms, five spice dried fruits and various dried fruits , cut them into dice, simmer them dry with chicken soup, collect the sesame oil, mix with the sesame oil, put them in a porcelain jar, seal them tightly, take them out when you want to eat, and mix them with fried chicken melon. "

      In fact, many so-called delicacies, such as fake swallows, cabbage meat rolls, and fermented green peppers, require vegetables to be heated for many times, and even some cooking methods need to squeeze out the vegetable juice from the vegetables. If you really cook like this, you can eliminate all the vitamin C and other health care ingredients in the dishes, and add a lot of saturated fat.