Which daily foods are better for the eyes? It contains more anthocyanins and vitamin A. ...

Natural pigments in food refer to the colored substances visible to the naked eye in fresh raw materials, or substances that are colorless and colored due to chemical reactions during processing. Natural pigments in food can be divided into animal pigments, plant pigments and microbial pigments according to their sources. Plant pigments are the most abundant in color and constitute the main body of food color. These pigments from different sources can be divided into fat-soluble pigments and water-soluble pigments according to their solubility. According to the type of chemical structure, it can be divided into pyrrole pigment, polyene pigment, phenol pigment and quinone pigment. (1) pyrrole pigment 1. Chlorophyll Chlorophyll is a green pigment contained in higher plants and all other photosynthetic organisms, which makes vegetables and immature fruits appear green. The biological function of chlorophyll is as a catalyst for photosynthesis. Organisms absorb solar energy through chlorophyll, fix carbon dioxide, and make it react with water to convert it into organic compounds. Chlorophyll is a pyrrole pigment. Chlorophyll is a diol ester composed of chlorophyllin, chlorophyllin and methanol. Green comes from the residue of chlorophyllin. Chlorophyll includes chlorophyll a, chlorophyll b, chlorophyll c and chlorophyll d. There are two kinds of chlorophyll in higher plants, A: B = 3: 1. Chlorophylls A and B are fat-soluble pigments, which are easily soluble in ethanol, acetone and chloroform. , but insoluble in petroleum ether, and all have optical activity. Chlorophyll combines with protein in living cells to form chloroplasts, and chlorophyll is released after cell death. Free chlorophyll is extremely unstable and sensitive to light and heat. Under acidic conditions, magnesium atoms in chlorophyll molecules can be replaced by hydrogen atoms, resulting in dark olive brown pheophytin. Heating can accelerate the reaction. At room temperature, chlorophyll is still stable in weak alkali. If heated, the ester will be partially hydrolyzed into chlorophyllin, methanol and water-active chlorophyllin, which is bright green and relatively stable. When the alkali concentration is high, the sodium salt or potassium salt of chlorophyllin is also green. If magnesium in chlorophyll is replaced by copper or iron, green salt is more stable. Chlorophyll-decomposing enzymes are contained in chloroplasts. When chloroplasts are destroyed, they show activity and can decompose chlorophyll into methyl chlorophyll and chlorophyllin. Methyl chlorophyll is also green. Food will cause changes in chlorophyll in different degrees during processing or storage. For example, during sterilization or cooking in canned food, the protein of chloroplast denatures under the action of heat, releasing phyllin, and at the same time, the organic acids in the cells are also released. This little acid is enough to demagnetize chlorophyll almost completely into pheophytin; Dehydrated food packed in transparent containers is prone to photosensitive oxidation and cracking into colorless products; The color of vegetables will change during freezing and frozen storage, which is influenced by blanching temperature and time before freezing. Chlorophyll in peas and kidney beans is degraded by lipoxygenase to produce non-chlorophyllin products, and lipoxygenase also produces free radicals that degrade chlorophyll. Chlorophyll and pheophytin in food will be degraded during γ -ray irradiation and storage after irradiation. In the process of lactic acid fermentation of cucumber, chlorophyll is degraded into pheophytin, defoliation chlorophyll and pheophytin methyl ester monoacid. Green vegetables can remain green if they are blanched with hot water at 60~75℃ to inactivate chlorophyll hydrolase before processing. When heated to the boiling point of chlorophyll, chlorophyll is easily oxidized. After blanching with hot water at 60~75℃, the oxygen in vegetable tissues can be eliminated, that is, it can still remain bright green after high temperature treatment because the chance of oxidation is reduced. Put spinach in water, vacuum at high temperature for a few minutes (remove oxygen from tissues), and then blanch it, which can also keep green. Another view is that blanching can reduce a considerable amount of acid in green vegetable tissues, and then high temperature treatment can reduce the interaction between chlorophyll and acid, so it is not easy to form pheophytin. In addition, at low temperature, chlorophyll hydrolase has strong activity and can hydrolyze chlorophyll into stable green methyl chlorophyllin acid. It is best to choose water with a monthly pH of pH6.5~7.0 or a little higher for blanching, which makes it easier to preserve the original bright green color of vegetables. The temperature and time of blanching have different requirements for various vegetables. If the temperature is too high and the time is too long, the green color will disappear easily, or pheophytin will be generated. Before processing green vegetables, increasing pH with lime water or magnesium hydroxide can reduce the formation of pheophytin and keep the vegetables bright green. However, too much alkali will destroy the tissue and flavor of plants, and Vc will be easily lost. Treating with dilute copper sulfate solution can form stable copper chlorophyll and keep green, but food containing copper is harmful to health. In addition, chlorophyll is stable at low temperature or dry state, so both vegetables stored at low temperature and dehydrated vegetables can keep their bright green. 2. Heme Heme is a pigment in animal blood and muscle. In organisms, it is the carrier of oxygen and carbon dioxide when breathing, and also the auxiliary group of myoglobin (Mb) and hemoglobin (Hb). Pigment chemistry of meat is actually heme chemistry. Heme is also a pyrrole pigment. Heme exists in the form of complex protein, myoglobin is a combination of globin and one heme molecule, while hemoglobin is a combination of globin and four heme molecules. Heme is a porphyrin compound consisting of an iron atom and a porphyrin ring. It is formed by valence bond combining with nitrogen atom on imidazole ring of histidine residue in globin molecule through ferrous atom. The four nitrogen atoms of heme are located on the same horizontal plane, and protein molecules are connected with iron atoms above the plane, while water or oxygen is connected with iron atoms below the plane. There is bound water on the iron atom of myoglobin and hemoglobin molecules. When encountering molecular oxygen, water molecules are replaced by oxygen, forming oxymyoglobin and oxyhemoglobin. This reaction is reversible. Oxymyoglobin and oxyhemoglobin are bright red, and the iron atom in heme is still divalent after the reaction, so this combination is not oxidation but oxygenation. When oxymyoglobin or hemoglobin is heated in the presence of oxygen, globin undergoes thermal denaturation. Fe2+ of red blood cells is oxidized to Fe3+, and yellow-brown myoglobin (MMb) or myochromatin is produced. However, when stored under anoxic conditions, Fe3+ was reduced to Fe2+ due to the weak oxidation of globin (in which -SH participated in the reduction), and then turned pink again. This phenomenon is called hemochromatosis. This phenomenon can be seen in the process of cooking meat or meat storage. At a certain pH and temperature, part of myoglobin will regenerate myoglobin. If reducing agent is added to muscle, the formation of myoglobin can be accelerated. It is of great significance to use the stability of reduced myoglobin to maintain the color of meat products during processing and storage. Heme can also combine with NO to produce bright pink nitrosoheme, and NO is also connected with ferrous atom in the form of valence bond. Nitrosohemoglobin or nitrosohemoglobin (nitrosohemoglobin) denatures after heating, and it is called nitrosohemoglobin at this time, and its color remains bright red. Therefore, in meat processing, in order to keep the fresh color of muscle, some colorants and reducing agents, such as nitrite, nicotinamide and ascorbic acid, are often added. However, excessive nitrite can react with secondary amines in meat to produce nitrosamine carcinogens. So don't use too much nitrite and nitrate in the hair color of meat products. (2) Polyene pigments Polyene pigments, collectively known as carotenoids, are a kind of pigments that are mainly distributed in organisms and present yellow, orange, red to purple. When chlorophyll exists, green is dominant, which often masks the expression of carotenoid color. However, once chlorophyll is decomposed, this pigment appears. Such as ripe fruits and maple leaves in autumn. Carotenoids are fat-soluble pigments, which are a kind of compounds with long-chain double bonds based on isoprene residues. Most natural carotenoids can be regarded as derivatives of lycopene. Carotenoids can be divided into two types according to their solubility: (1) carotenoids. It is an unsaturated hydrocarbon, which is soluble in petroleum ether, methanol and ethanol. (2) lutein. Carotene derivatives mostly exist in the form of alcohol, aldehyde, ketone and acid. Soluble in methyl ether, ethanol and petroleum ether, but insoluble in ether. Carotenoids are stable to the changes of pH value and heat, and only strong oxidants can destroy and fade. The destruction of carotenoids in food is mainly due to photosensitive oxidation, which leads to the break of double bonds and the loss of color. Especially when the pH value and moisture content are low, it is easier to be oxidized. The extracted carotenoids are sensitive to light, heat and oxygen, but they are quite stable when combined with protein in cells. This is probably related to the permeability of cells and protective components. (3) Phenolic pigment 1. Anthocyanin is a main water-soluble plant pigment. Many fruits, vegetables and flowers show bright colors because of the existence of this water-soluble compound in cell sap. Anthocyanin is a phenolic pigment, which is mostly combined with sugar and exists in the form of glycoside (called anthocyanin). Its structural mother nucleus is 2- phenylbenzopyran, which is the motif of color. The oxygen on pyran ring in anthocyanin molecule is tetravalent, which makes anthocyanin alkaline and the warp group on phenol ring acidic, thus making this kind of pigment have the characteristic of color change with the change of pH value of medium. Due to the change of pH value during ripening, fruits and vegetables show various colors. Anthocyanins can react with metal ions such as Ca2+, Mg2+, Mn2+, Fe3+ and Al3+ to generate purplish red, cyan and blue. , and is no longer affected by pH value. Therefore, fruits containing anthocyanins must be packed in special paint cans or glass bottles. Anthocyanins are extremely sensitive to light and temperature, and foods containing anthocyanins will turn brown quickly under light or high temperature. Both sulfur dioxide and ascorbic acid can make it fade. Anthocyanin glycoside is decomposed into sugar and anthocyanin under the action of glucosidase or phenolase and faded. Anthocyanins react with hydrochloric acid to produce colorless substances, which are called colorless anthocyanins. Colorless anthocyanins also exist in plant tissues in the form of glycosides, which can be converted into colored anthocyanins under certain conditions, which is one of the reasons why canned fruit pulp turns red and brown. 2.Anthoxanthin usually refers to flavonoids and their derivatives, which are a kind of water-soluble yellow substances widely distributed in flowers, fruits and stem and leaf cells of plants. Combined with glucose, rhamnose, Tang Yunxiang, etc., it exists in the form of glycoside. Its importance in food processing is only that under processing conditions, it will produce ugly color due to pH value and the existence of metal ions, which will affect the appearance quality of food. Flavonoids are 2- phenylbenzopyranone composed of benzopyran and benzene ring. Under natural conditions, the color of flavonoids varies from light yellow to colorless, and it is rarely obvious yellow, but it will become obvious yellow when it meets alkali. The mechanism is that the C-O bond between the carbon positions of benzopyranone, a flavonoid, opens into chalcone structure under alkaline conditions, and the colors of various chalcones range from light yellow to dark yellow. Under acidic conditions, chalcone reverts to closed-loop structure, so the color disappears. Flavonoids turn blue when they meet iron ions. The pH value of hard water is often as high as 8, and the pH value of water softened with NaHCO 3 is higher. Some foods such as potatoes, rice, wheat flour, asparagus, etc. Boiling with alkali will turn yellow, because flavonoids will change into chalcone structure when exposed to alkali. This phenomenon is particularly prominent when the water quality of onions, especially yellow-skinned species, is alkaline. Onions are pale yellow due to the dissolution of flavonoids, while soups are bright yellow, as are broccoli and cabbage. One of the purposes of adjusting the pH value of pre-cooked water with citric acid in fruit processing is to control the change of flavone pigments. If these pigments are left in the air for a long time, they are easy to oxidize and produce brown precipitate, which is one of the reasons why juice turns brown after long-term storage. Tannin is an astringent and tannin, which exists in many plants (such as pomegranate, coffee, tea, persimmon, etc.). ), also known as tannin or tannin. In food chemistry, food tannin refers to all substances with astringency, which can react with metal ions or produce black due to oxidation. In addition to real tannins, it also includes catechol and some phenolic acids (chlorogenic acid). Tannins in plants can be divided into two categories: water solubility and polymerization. Water-soluble tannins are macromolecules formed by ester bonds between tannin monomers, which are hydrolyzed into tannin monomers by dilute acid, enzyme or boiling under mild conditions. Polytannins are macromolecules connected by carbon-carbon bonds between monomer molecules. Under mild conditions, they will not decompose into monomer substances, but will further condense into macromolecules. For example, the diphenyl polymer tannin in grapes, apples, peaches, plums and tea leaves is polymerized into dimer, tetramer and octamer after heating. All tannins are deliquescent, and tannins react with metals to form insoluble salts, especially with iron to form blue-black substances. So you can't use iron utensils to process this kind of food. Tannin oxidizes in air to form dark black oxide, which oxidizes faster in alkaline solution. Tannin in fruit juice can react with gelatin to produce turbid liquid and precipitate, so tannin in fruit and vegetable juice can be removed with gelatin. When there are astringent tannins in immature fruits or fruits, there are many ways to remove astringency. For example, astringent persimmons can be soaked in warm water, alcohol, carbon dioxide modified atmosphere, and ethylene ripening. (4) quinone pigment 1. Monascus pigment Monascus pigment is a pigment secreted by monascus, which is colorless at the initial stage of culture and gradually turns bright red. It is a common edible pigment in China. Such as brewing red rice wine, making sauces, fermented bean curd, sausages, soy sauce, steamed meat and coloring of various cakes. The pigment has strong heat resistance and light resistance, is not affected by metal ions, is not easily affected by oxidants and reductants, and is insoluble in water. 2. Curcumin Curcumin exists in the rhizome of the perennial herb Curcuma longa. Curcumin is reddish brown in alkaline solution and yellow in neutral or acidic solution. Not easy to be reduced, easy to combine with iron ions to change color. Poor stability to light and heat. Good coloring, especially in protein. Curcumin is often used for coloring and flavoring curry powder, dried radish and other foods. 3. Beet Red Beet Red is a natural plant pigment existing in red beets. Sweet red is the general name of colored compounds in sugar beet. β -carotene is mainly β-glycoside, accounting for 75% ~ 95% of red pigment. Beet glycoside solution is red to purplish red, relatively stable at pH3.0~7.0, and most stable at pH4.0~5.0. When pH 7.0, the solution also changed from red to purple. When ph > l 0.0, the color of the solution turns yellow rapidly. Because the pH value of most foods is between 3.0 and 7.0, the color of foods containing betanin is relatively stable. Water activity has a great influence on the stability of beet pigment. With the decrease of water activity, the stability of beet pigment increases, indicating that beet pigment can be used as a colorant for low-moisture food. Beet red pigment can be used for coloring candy, cakes, refreshing drinks and some dairy products and meat products.