Anhydrous citric acid is a natural component and an intermediate product of physiological metabolism in animals and plants. It is also one of the most widely used organic acids in food, medicine, chemical industry and other fields. It is colorless, transparent or translucent crystals, or granular or microgranular powder, odorless, with a strong sour taste but pleasant, slightly astringent taste.

It gradually weathers in warm air and is slightly deliquescent in humid air. Distribution in Nature Citric acid is widely distributed in nature and is found in plants such as lemons, citrus, pineapples, currants, raspberries, grape juice and other fruits, as well as in the bones, muscles, and blood of animals. Synthetic citric acid is produced by fermenting sugar, molasses, starch, grapes and other sugar-containing substances. It can be divided into two types: anhydrous and hydrated. Pure citric acid is colorless transparent crystal or white powder, odorless and has an attractive sour taste. The translucent colorless crystals obtained from hot concentrated aqueous solutions are anhydrous and have a melting point of 153°C. The translucent colorless crystal obtained from cold water solution is monohydrate with a density of 1.542. It softens at 75℃ and melts at about 100℃. Monohydrate can lose water in dry air. It is a strong organic acid. Soluble in water, ethanol and ether. Used in the manufacture of medicines, soda drinks, candies, etc., and also used as metal cleaners, mordants, etc. Many fruits and vegetables, especially citrus fruits, contain large amounts of citric acid, especially lemons and limes – they contain large amounts of citric acid, and after drying, the content can reach 8% (the content in juice About 47g/L). In citrus fruits, the content of citric acid ranges from 0.005 mol/L in oranges and grapes to 0.30 mol/L in lemons and limes. This content varies with different cultivars and plant growth conditions.

Discovery History The discovery of citric acid began with the Iranian alchemist Jabir in the 8th century. In 1784 CW Scheler first isolated citric acid by crystallizing it from lemon juice. It is produced by adding lime milk to the fruit juice to form a calcium citrate precipitate. Natural citric acid is widely distributed in nature and is found in plants such as lemons, citrus, pineapples and other fruits, as well as in the bones, muscles, and blood of animals. Originally produced in California, Italy and the West Indies. Italy ranks first in production. The production of citric acid by fermentation began at the end of the 19th century. In 1893, Weimer discovered that Penicillium could produce citric acid from sugar. However, the industrial use of microorganisms to produce citric acid was not on the agenda until World War I hampered Italian lemon exports. In 1913, B. Zachowski reported that Aspergillus niger could produce citric acid. In 1916, Tom and Currie conducted experiments with Aspergillus species and confirmed that most Aspergillus species, such as Aspergillus awamori, Aspergillus oryzae, Aspergillus weinwei, Trichoderma viride and Aspergillus niger, have the ability to produce citric acid, while Aspergillus niger has the ability to produce acid. Stronger.

For example, Curry used Aspergillus niger as the test strain and fermented it in 15% sucrose culture medium, and the sugar absorption rate reached 55%. In 1923, the American Feizer Company built the world’s first factory to produce citric acid using the Aspergillus niger shallow plate fermentation method. Subsequently, Belgium, the United Kingdom, Germany, and the Soviet Union successively studied and successfully produced citric acid through fermentation. In this way, the method of extracting natural citric acid from citrus was gradually replaced by fermentation. Before 1950, citric acid was produced using the shallow pan fermentation method. In 1952, Myers Laboratory in the United States used the submerged fermentation method to produce citric acid on a large scale. Since then, the submerged fermentation method has gradually been established. Submerged fermentation has a short cycle, high yield, labor saving, small footprint, and easy instrument control and continuity. This manufacturing technology is still the most important manufacturing method at present. In this technique, Aspergillus niger is cultured in a medium containing sucrose or glucose to produce citric acid. Sources of sugar include corn steep liquor, molasses fermentation broth, corn flour hydrolysates, or other cheap sugar solutions. After the mold is removed, calcium hydroxide is added to the remaining solution to react the citric acid to form a calcium citrate precipitate. After the precipitate is separated, sulfuric acid is added to obtain citric acid. Application in the food industry Anhydrous citric acid is known as the first edible sour agent. my country’s GB2760-1996 stipulates that it is a permitted food acidity regulator. It is widely used in the food industry as sour agents, solubilizers, buffers, antioxidants, deodorizers, chelating agents, etc. Its specific uses are too numerous to mention. 1. Beverages According to domestic and foreign statistics, the consumption of citric acid in the beverage industry accounts for 75% to 80% of the total citric acid production. Citric acid is one of the natural components of fruit juice. It not only imparts fruit flavor but also has solubilizing, buffering, and antioxidant effects. It blends and coordinates sugar, flavors, pigments and other ingredients in beverages to form a harmonious taste and aroma, and can increase antimicrobial resistance. Antiseptic effect. Therefore, it is widely used in the preparation of various fruit-based soft drinks. For example, fruit-based sodas generally add acid content between 0.10% and 0.25%. For beverages that are not filled with CO2, such as water, juice, sugar, acid, pigments, and flavor mixtures, citric acid is used to adjust the pH to a certain acidity so that the titrated acidity of the product is between 0.25% and 0.40%. For pulp-type solid beverage powder, the added amount of citric acid can be increased to 1.5% to 5.0%. In recent years, with the continuous improvement of living standards of urban and rural people, the demand for various beverages has increased significantly. It can be predicted that the consumption of citric acid in the beverage industry will increase significantly in the future. 2. Jams and jelly The role of citric acid in jams and jelly is similar to that in beverages. It mainly adjusts the pH of the product and gives the product a sour taste. The pH must be adjusted to a narrow range that is most suitable for pectin coagulation. Depending on the type of pectin used, the pH can be limited to between 3.0 and 3.4. In jam production, it can improve the flavor and prevent the sanding defects of sucrose crystallization. 3. Candy Citric acid is added to candies to increase the sour taste and prevent the oxidation of various ingredients and the crystallization of sucrose.

Generally sour candies can contain 2% citric acid. After the sugar is boiled, citric acid, coloring matter, flavors, etc. are added together during the cooling process of the massecuite. In the production of pectin gummies, citric acid can adjust the sourness and enhance gel strength. Anhydrous citric acid is used in chewing gum and powdered foods. 4. Frozen food citric acid has the characteristics of chelating and adjusting pH, which can strengthen the antioxidant effect and inactivate enzymes, and can more reliably ensure the stability of frozen food. Citric acid, alone or in combination with erythorbic acid, helps extend the shelf life of frozen fish and shellfish by inactivating enzymes and chelating trace elements that cause fish spoilage and shellfish spoilage. Seafood is generally soaked in a solution of 0.25% citric acid and 0.2% erythorbic acid before freezing, which is beneficial to preservation and refrigeration. It is also effective in preserving fresh pork and preventing spoilage. Citric acid can also inhibit the deterioration of the color and aroma of frozen fruits. This is also a manifestation of inhibiting the enzyme system in the fruit and preventing the oxidation of trace elements. For example, the combination of 0.1% ~ 0.3% citric acid and 0.001% ~ 0.002% erythorbic acid is effective. Application in the pharmaceutical industry: Effervescence is a popular drug oral ingredient release system. Citric acid reacts with sodium carbonate or sodium bicarbonate aqueous solution to produce a large amount of CO2 (i.e., effervescence) and sodium citrate, which can quickly dissolve and enhance the active ingredients in the drug. Taste ability. For example, solubilizing laxatives and analgesics. Citric acid syrup is a refreshing drink for fever patients. It has the effects of flavoring, cooling and detoxification. Citric acid is widely used in various nutritional oral liquids to buffer pH 3.5 to 4.5, maintain the stability of active ingredients, and enhance the effect of preservatives. Citric acid is used in combination with fruit flavors to give people their favorite sour taste and mask the bitter taste of medicines, especially traditional Chinese medicine preparations. Adding 0.02% citric acid to liquid ingredients can form trace amounts of iron and copper complexes to delay the degradation of active ingredients. effect. Using 0.1% to 0.2% citric acid in chewable tablets can improve the flavor of the tablets and give them a lemon flavor. This information was edited and compiled by Xiaonan.

Application in industry 1. Metal purification The metal purification formula based on citric acid can effectively remove the oxides formed on the surface of ferrous metals and non-ferrous metals. As a weak organic acid, it can cause slight corrosion of the metal surface layer. However, as a metal ion chelating agent, it accelerates the migration of metal oxides. In many steel pickling systems, in order to eliminate stress cracks in metal oxides and minimize the erosion and embrittlement of high-strength steels, citric acid is used to remove rust to replace inorganic strong acids. In boilers, heaters, superheater tubes, unclean reactors and stainless steel equipment, citric acid is generally used to pre-wash and purify iron and copper oxides, especially in places where chlorides are not allowed to exist.

. The purification reaction of citric acid solution should be carried out when the temperature is higher than 37.8°C. The purification time should be prolonged when it is lower than 37.8°C. The amount of citric acid and the action conditions should be adjusted based on the estimated increase or decrease in the amount of precipitation. Citric acid is a highly efficient chelating cleaning agent that is effective against calcium, magnesium, iron, chromium, copper and other dirt. It is widely used for cleaning various metal surfaces. The wastewater after washing can be biodegraded without polluting the environment. In recent years, citric acid is suitable for chemical treatment of metal purification, and is also suitable for non-polluting ashing of waste solvents. The citric acid molecules in the ash are burned into CO2 and water vapor, and valuable metals can be recovered from the ash. , toxic or radioactive elements can be fully enclosed for chemical treatment. 2. The detergent sodium citrate can increase the decontamination performance. It can accelerate biodegradation in many household detergent products. It can be used as a substitute for phosphate and is widely used in laundry detergents and detergents. Sodium citrate It is very effective together with aluminosilicates to enhance the cleaning action of detergents. This type of product uses industrial grade 50% citric acid solution as raw material. In non-synthetic detergent solutions, citric acid is also used in hot water extraction carpet cleaners, bath soaks and textile softeners due to its buffering effect and ability to chelate trace metals. 3. Soilless cultivation agronomy. Soilless cultivation using plant nutrient solutions with trace elements has long been common abroad and has also begun to develop in China. In order to remove certain excess elements such as copper, zinc, manganese, and chromium from the soil, citric acid is generally used to chelate these elements. Using citric acid instead of phosphate can achieve satisfactory yield increase results. 4. In terms of minerals and pigments, citric acid has dispersing effect and can reduce the viscosity of some important slurry raw materials. In mining, for example, the viscosity problem of phosphate rock powder in hydraulic transportation is often the main reason for clay expansion. Citric acid treatment can control the expansion and make the slurry raw material easy to transport.