檸檬酸，亦稱為枸櫞酸，化學式為 C6H8O7，是一種中強度有機酸，它包括3個羧基（R-COOH）基團。檸檬酸是自然在柑橘類水果中產生的一種天然防腐劑，也是食物和飲料中的酸味添加劑。在生物化學中，它是檸檬酸循環(huán)的重要中間產物，因此在幾乎所有生物的代謝中起到重要作用。此外，它也是一種對環(huán)境無害的清潔劑。
Citric acid, also known as citric acid, with chemical formula of C6H8O7, is a medium strength organic acid, which includes three carboxyl (R-COOH) groups. Citric acid is a natural preservative naturally produced in citrus fruits, and it is also an acid additive in foods and beverages. In biochemistry, it is an important intermediate product of citric acid cycle, so it plays an important role in almost all biological metabolism. In addition, it is also an environmentally friendly cleaner.
很多種水果和蔬菜，尤其是柑橘屬的水果中都含有較多的檸檬酸，特別是檸檬和青檸——它們含有大量檸檬酸，在干燥之后，含量可達8%（在果汁中的含量大約為47 g/L）。在柑橘屬水果中，檸檬酸的含量介于橙和葡萄柚的0.005 mol/L和檸檬和青檸的0.30 mol/L之間。這個含量隨著不同的栽培品種和植物的生長情況而有所變化。
Many kinds of fruits and vegetables, especially citrus fruits, contain more citric acid, especially lemon and lime - they contain a lot of citric acid, which can reach 8% after drying (about 47 g/L in juice). In citrus fruits, the content of citric acid is between 0.005 mol/L of orange and grapefruit and 0.30 mol/L of lemon and lime. This content varies with the growth of different cultivated varieties and plants.
性質
nature
在室溫下，檸檬酸是一種白色晶體粉末。它可以以無水合物或者一水合物的形式存在。檸檬酸從熱水中結晶時，生成無水合物；在冷水中結晶則生成一水合物。加熱到78 °C時一水合物會分解得到無水合物。在15攝氏度時，檸檬酸也可在無水乙醇中溶解。
At room temperature, citric acid is a white crystalline powder. It can exist in the form of non hydrate or monohydrate. When citric acid crystallizes from hot water, hydrate free substance is formed; Monohydrate is formed when crystallized in cold water. When heated to 78 ° C, the monohydrate will decompose to obtain hydrate free. At 15 ℃, citric acid can also be dissolved in absolute ethanol.
從結構上講檸檬酸是一種三羧酸類化合物，并因此而與其他羧酸有相似的物理和化學性質。加熱至175 °C時它會分解產生二氧化碳和水，剩余一些白色晶體。
Citric acid is a kind of tricarboxylic acid compound structurally, and therefore has similar physical and chemical properties with other carboxylic acids. When heated to 175 ° C, it will decompose to produce carbon dioxide and water, leaving some white crystals.
應用
application
1. 檸檬酸作為酸味劑廣泛應用于各種飲料、果汁、罐頭、糖果、果醬、果凍的 生產，使產品的酸味清爽可口，并有果味的香甜。檸檬酸本身是果汁的天然成分之一，不僅賦予飲料水果風味，而且具有增溶、緩沖、抗氧化等作用，能使飲料中的糖、香精、色素等成分交融協(xié)調，形成適宜的口味和風味。
1. As a sour agent, citric acid is widely used in the production of various drinks, fruit juice, cans, candies, jams, and jellies, making the products refreshing and tasty in sour taste and sweet in fruit taste. Citric acid itself is one of the natural ingredients of fruit juice. It not only gives the drink fruit flavor, but also has the functions of solubilization, buffering, antioxidant, etc. It can make the sugar, essence, pigment and other ingredients in the drink blend and coordinate to form a suitable taste and flavor.
2. 檸檬酸作蔗糖轉化劑
2. Citric acid as sucrose invertor
在蔗糖液中添加適量檸檬酸可使其轉化為糖，以提高蔗 糖的飽和度和粘度，增大滲透壓，從而有效地防止糖制品的蔗糖返砂，并提高制品的保藏性和改善其質地。
Adding an appropriate amount of citric acid in the sucrose solution can convert it into sugar, so as to improve the saturation and viscosity of sucrose, increase the osmotic pressure, effectively prevent the sucrose from returning to the sand, improve the preservation and improve the texture of sugar products.
3. 檸檬酸作果蔬原料的護色劑
3. Citric acid as color fixative for raw materials of fruits and vegetables
將果蔬原料放在1%～2%的食鹽和0.1%的檸檬酸 混合液中浸漬，可抑制果蔬原料酶褐變引起的變色。
The discoloration of fruit and vegetable raw materials caused by enzymatic browning can be inhibited by dipping them in a mixture of 1%~2% salt and 0.1% citric acid.
4. 檸檬酸作pH值的調整劑
4. Citric acid as pH regulator
檸檬酸在罐頭、果醬、果凍等制品中使用，可使其 pH值降低，抑制腐敗微生物的繁殖。一般當pH值小于5.5時，大部分腐敗細菌可被抑制。同時，用檸檬酸調整食品的pH值還可達到改善品質和風味的目的。如當葡萄或其他釀酒原料成熟過度而酸度不足時，可以用檸檬酸調節(jié)，以防止所釀造的酒口味單薄，同時還有抗氧化和保護色素的作用，以保護果汁的新鮮感和防止變色。
The use of citric acid in cans, jams, jellies and other products can reduce its pH value and inhibit the propagation of putrefactive microorganisms. Generally, when the pH value is less than 5.5, most putrefactive bacteria can be inhibited. At the same time, adjusting the pH value of food with citric acid can also improve the quality and flavor. For example, when grapes or other wine making materials are too mature and the acidity is insufficient, citric acid can be used to adjust them to prevent the thin taste of the brewed wine. At the same time, it has the effect of anti-oxidation and pigment protection to protect the freshness of the juice and prevent discoloration.
5. 檸檬酸作抗氧化劑的增效劑
5. Citric acid as synergist of antioxidant
如在切去皮后的果蔬原料中加入濃度為0.1%的 抗壞血酸溶液可防止原料氧化褐變。通常以檸檬酸作為增效劑，以控制酚酶的活力，防止酶褐變。如一分子結晶水檸檬酸可用作食用油的抗氧化劑。如添加檸檬酸可以改善冰激凌的口味，增加乳化穩(wěn)定性，防止氧化作用。在果醬和果凍中同樣可以增進風味，并有抗氧化作用。如各種肉類和蔬菜在腌制加工時，加入或涂上檸檬酸可以改善風味，除腥去臭，抗氧化。
For example, adding 0.1% ascorbic acid solution to peeled fruits and vegetables can prevent raw materials from oxidative browning. Citric acid is usually used as a synergist to control the activity of phenolase and prevent browning of the enzyme. For example, one molecule of crystal water citric acid can be used as an antioxidant of edible oil. For example, adding citric acid can improve the taste of ice cream, increase the emulsion stability and prevent oxidation. In jam and jelly, it can also enhance the flavor and has antioxidant effect. For example, when various meats and vegetables are pickled and processed, adding or coating citric acid can improve the flavor, eliminate fishy smell, deodorize and resist oxidation.
6. 檸檬酸作疏松劑的改良劑
6. Citric acid as an improver of loosening agent
在面食制品中用小蘇打作疏松劑時，制品往往堿度 大，口感差。若將檸檬酸和小蘇打同時使用，可使小蘇打分子在反應過程中產生的二氧化碳被吸收，不致使碳酸鈉積累，從而降低面食的堿度，改善風味。
When baking soda is used as a loosening agent in pasta products, the products tend to have high alkalinity and poor taste. If citric acid and baking soda are used at the same time, the carbon dioxide generated by baking soda molecules in the reaction process can be absorbed without causing the accumulation of sodium carbonate, thus reducing the alkalinity of pasta and improving the flavor.
7. 檸檬酸作緩沖劑，蔬菜、水果在加工時由于品種、產地、收獲期、存放時間不同會使它們的含酸量不同，給工業(yè)帶來不便，為此根據需要在控制溶液酸度，保證產品質量，還可抑制細菌生長、減少殺菌時間及降低殺菌溫度。
7. Citric acid is used as a buffer. Vegetables and fruits will have different acid content due to different varieties, places of origin, harvest periods and storage times during processing, which will bring inconvenience to industry. Therefore, the acidity of solution should be controlled according to needs to ensure product quality, inhibit bacterial growth, reduce sterilization time and reduce sterilization temperature.
8. 檸檬酸與脂肪酸單雙甘油酯合用作乳化劑 在人造奶油、冰激凌生產中添加這種乳化劑有利于脂肪的破乳、凝結，從而提高產品質量，使產品口感細膩。參考用量：10g/kg,冷飲10g/kg。
8. The combination of citric acid and fatty acid monoglyceride and diglyceride is used as emulsifier. Adding this emulsifier in the production of margarine and ice cream is conducive to the demulsification and condensation of fat, thus improving the product quality and making the product taste delicate. Reference dosage: 10g/kg, cold drink 10g/kg.

9. 檸檬酸作為香料及除味劑
9. Citric acid as spice and deodorant
檸檬酸可添加到軟飲料、冷飲、焙烤食品、糖果及 膠姆糖中。
Citric acid can be added to soft drinks, cold drinks, baked goods, candy and gums.
檸檬酸還可作為香料穩(wěn)定劑添加到許多食品包裝材料中去，起到保鮮除異味作用，如將添加1%檸檬酸等聚乙烯制成的膜用于包裝魚及其腌制品，三天后能明顯降低異味。此外，檸檬酸作為海產品、水產品及羊奶等制品的除腥、除膻劑也被廣泛使用，如海帶飲料中常用10%的檸檬酸和0.2%的鹽酸進行酸煮脫腥，沙丁魚在加工前浸泡在pH5.5的檸檬酸中30分鐘后可除去腥味。
Citric acid can also be added to many food packaging materials as a flavor stabilizer to keep fresh and remove odor. For example, the film made of polyethylene with 1% citric acid can be used to package fish and its pickles, which can significantly reduce odor after three days. In addition, citric acid is also widely used as a deodorant and deodorant for seafood, aquatic products, sheep's milk and other products. For example, 10% citric acid and 0.2% hydrochloric acid are commonly used in kelp drinks to remove fishy smell. sardine can remove fishy smell after being soaked in citric acid at pH 5.5 for 30 minutes before processing.
10. 檸檬酸對VC含量的影響
10. Effect of citric acid on VC content
所有甜橙汁樣品中儲藏過程中，維生素C含量都在 下降，但各自下降的特點有所不同。在對比試驗中，檸檬酸對維生素C的下降有一定的減緩作用。在新制備的甜橙汁新鮮樣品中，含有檸檬酸的樣品菌落總數(shù)下降速度比對照樣品快，且隨檸檬酸濃度的增加而加快。
The content of vitamin C in all samples of sweet orange juice decreased during storage, but the characteristics of their decline were different. In the contrast test, citric acid has a certain slowing effect on the decline of vitamin C. In the newly prepared fresh sample of sweet orange juice, the total bacterial count of the sample containing citric acid decreased faster than that of the control sample, and accelerated with the increase of citric acid concentration.
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