L-cysteine, which has the formula C3H7NO2S, is a non-essential amino acid. It is an isomer of cysteine and can be converted from methionine.1. Cosmetics: Used in the production of hair perm powder, sunscreen, perfume, hair tonic, etc. This kind of advanced hair perm powder, cold perm powder with cysteine instead of thiocyanacetic acid, has the advantages of easy perm, easy to maintain hair style, soft but not messy, and can also prepare sunscreen and hair growth perfume; 2. In medicine: cysteine is mainly used in liver medicine, antidotes, expectorants and other medicines. Cysteine and its derivatives can be used for liver poisoning and detoxification, antipyretic analgesia, ulcer treatment, fatigue recovery, infusion and comprehensive amino acid preparation, especially for expectorant. Treatment of bronchitis and phlegm reduction. 3. In food: bread fermentation promoter, preservative. Cysteine is used as fermentation aid, antioxidant and stabilizer for milk powder and juice, and nutrition additive for pet animal food, etc. 4. L-cysteine can be used as raw material for the production of n-acetyl-L -, carboxymethyl cysteine and other cysteine derivatives. 5. Accelerate the formation of gluten and prevent aging; Used in natural juice can prevent the oxidation and Browning of VC; It can detoxify propylene acidosis and aromatic acidosis. Prevention of radiation damage. L-cysteine is a kind of amino acid with physiological function. It is the only amino acid that has the reducing group sulfhydryl (-SH) among more than 20 amino acids that make up proteins. It has been widely used in medicine, food additives and cosmetics. World demand for cysteine reached 4400-4600 tons in 2002 and is increasing by 2-3% per year. Demand in Western Europe is growing by 3-4% and in Japan by 2%. At present, the production of L cystine mainly depends on the extraction of L-cystine from human or animal hair by acid hydrolysis or alkali hydrolysis, and then by electrolytic reduction to produce L cystine. The method has low yield, high energy consumption, a lot of irritant gas is produced in the hydrolysis process, and the waste acid is difficult to be treated, causing serious environmental pollution. With the development of L cysteine production technology, L-cysteine production by microbial transformation has gradually replaced hair hydrolysis to produce L-cysteine. The preparation process of microbial transformation has attracted more and more attention due to its advantages such as mild reaction condition, strong specificity and friendly to environment.