Nutritious food additives (12 points)
① Fruit and vegetable juices such as pear, apple, banana, and potato juice tend to discolor easily. Once discolored, they not only look unappealing but also lose their flavor. What can you do? Add a little lemon juice.
②Fruits and vegetables all contain varying amounts of polyphenolic compounds. Once the peel is removed, these compounds are exposed to air and undergo oxidation, forming quinone compounds. These quinones readily polymerize with one another to form “browning pigments,” causing the food to change color. Lemon juice, on the other hand, is rich in ascorbic acid, which can reduce quinones back to their original polyphenolic form; alternatively, ascorbic acid itself can be oxidized, thereby consuming the oxygen surrounding the polyphenols and protecting them from oxidative attack. In this way, ascorbic acid sacrifices itself to safeguard the vulnerable polyphenols, preserving the “fresh” color of fruits and vegetables.
③ In the food industry, based on the mechanism of action of lemon juice, ascorbic acid can be directly added. Many fruit and vegetable juices sold in supermarkets contain added ascorbic acid to maintain their appearance and flavor.
④The role of ascorbic acid extends beyond this. Nitrites are frequently added to cooked meat products for two main reasons: first, they react with myoglobin to produce an appealing red color; second, they inhibit bacterial growth, thereby providing a preservative effect. The addition of ascorbic acid can enhance the former reaction, thus accelerating the “color-development” process. Many people regard nitrites as “carcinogens.” In fact, nitrites themselves are not carcinogenic; only when they react with amino acids in meat to form nitrosamines do they become carcinogenic. However, if ascorbic acid is added to the meat, it inhibits this conversion, thereby reducing the “potential” carcinogenic risk associated with nitrites. Ascorbic acid is also sometimes added to meat products that do not require nitrites, because oxidation of the fat in meat can release off-flavors—commonly referred to as a rancid taste. When ascorbic acid is added, it rapidly consumes the surrounding oxygen, thereby protecting the fat from oxidation and helping to maintain the “fresh” flavor of the meat.
⑤ Ascorbic acid itself is readily oxidized to dehydroascorbic acid. However, dehydroascorbic acid is far from content to “lose” its electrons; it will snatch hydrogen atoms from other molecules in order to restore its own stability. Exploiting this property, ascorbic acid is often added during flour processing to improve dough performance and enhance dough elasticity and strength. Flour contains glutenin, which harbors numerous sulfhydryl groups—sulfur atoms bonded to a single hydrogen atom—and dehydroascorbic acid can strip these hydrogen atoms. During kneading, the hydrogen atoms in the sulfhydryl groups are removed by dehydroascorbic acid, leaving the sulfur atoms to link pairwise and form disulfide bonds. As numerous disulfide bonds are formed, the glutenin in the dough establishes an extensive three-dimensional network, thereby significantly increasing its elasticity and strength.
⑥ In most cases, food additives are used to enhance flavor and texture, improve product stability, and so on; they do not, in themselves, confer any nutritional value. However, ascorbic acid is an exception to this “majority.” When used as a food additive, it is typically referred to as “ascorbic acid,” yet it is also an essential nutrient for the human body—vitamin C. Vitamin C is inherently unstable: exposure to air, light, heat, or contact with metal containers can all lead to its loss of activity or decomposition. Paradoxically, this very instability gives it excellent antioxidant properties. Within the body, it protects cells from oxidative damage; when added to food, it willingly undergoes oxidation itself, thereby safeguarding the other ingredients in the product.
