γ－ Aminobutyric acid alias 4-aminobutyric acid（ γ- Aminobutyric acid (GABA) is an amino acid. GABA is white crystal powder without optical rotation. Melting point 195-204 ℃ (decomposition), miscible with water, slightly soluble in ethanol and acetone, insoluble in benzene and ether, water will be lost during decomposition to form pyrrolidone.
GABA often exists in the form of zwitterions (negatively charged carboxyl group and positively charged amino group) in solution. Due to the electrostatic interaction between positively and negatively charged groups, GABA can have both gaseous (folded) and solid (Extended) molecular conformations in solution, while the coexistence of multi molecular conformations in solution enables GABA to bind a variety of receptor proteins and play a variety of important physiological functions.
The content of GABA in plant tissue is very low, usually 0.3 ~ 32.5 μ Mol / g. It has been reported that the enrichment of GABA in plants is related to the stress response experienced by plants. When subjected to hypoxia, heat shock, cold shock, mechanical damage, salt stress and other stress pressures, it will lead to the rapid accumulation of GABA. After some stress treatment of plant food raw materials, or through microbial fermentation, the content of GABA in plant food raw materials is increased. Processing this raw material into GABA rich functional products has become a research hotspot. As a new functional factor, GABA has been widely used in the field of food industry. Foods developed from germinated brown rice, soybeans and broad beans rich in GABA have been put on the market.
Although the European Food Safety Agency (EFSA) allows GABA to be added to food and stipulates that the maximum dietary intake of GABA is 550mg / D, its main functional characteristics need to be supported by strict population test results. The U.S. Food and Drug Administration (FDA) pointed out that it is safe to add GABA to food according to the results of toxicological experiments. The scope of use includes beverages, coffee, tea and chewing gum, but it is not allowed to add GABA to baby food, meat products or meat containing products. China's Ministry of Health announced on December 12, 2009 that the intake of GABA should not exceed 500mg / D. the scope of use is beverages, cocoa products, chocolate and its beverages, candy, baked food and puffed food, but it can not be added to baby food.
GABA has been found in many animals, plants and microorganisms. It was first found in potato tubers in 1949 and found in mammalian central system in 1950. At the same time, it is considered to be a nerve inhibitor in the nervous system of mammals, insects or some parasitic worms, which has an important impact on the excitability of neurons. It is found that GABA is an active amino acid that plays an important role in the energy metabolism of human brain. It has many physiological functions, such as activating glucose metabolism in the brain, promoting acetylcholine synthesis, reducing blood ammonia, anticonvulsant, reducing blood pressure, improving brain function, mental stability, promoting growth hormone secretion and so on.
In the metabolism of microorganisms, GABA metabolism is completed through GABA branch. Using the high GAD activity in microorganisms, Glu is decarboxylated to form GABA. Then under the action of GABA-T and SSADH, GABA enters the downstream decomposition process to produce succinic semialdehyde and succinic acid, which participate in the physiological metabolism of microorganisms. Microbial enrichment of GABA is achieved by optimizing the culture medium and improving the strain to make it have high GAD activity, increasing the synthesis rate of GABA and reducing the decomposition rate. A large number of studies have proved that GAD exists in both prokaryotic and eukaryotic microorganisms. In addition, the decarboxylation of GAD in microorganisms to form GABA is not limited by resources, environment and space, which has significant advantages compared with other methods.
One of the methods of artificial synthesis of GABA - microbial fermentation method: select strains with excellent varieties, stability, non-toxic and harmless, and use these strains to prepare and produce GABA in the process of growth and reproduction. Although this method has strict requirements on the environment and equipment, the GABA produced by this method can be used as a natural food additive. Microbial fermentation production is one of the earliest and most extensive production methods in the food industry. The earliest microorganism used is Escherichia coli. GABA can be produced by using its decarboxylase. However, it has been unable to be directly used in the production of drugs or food due to some potential safety hazards.
With the development of science and technology, green food has attracted more and more attention. Later, researchers found that lactic acid bacteria, yeast, Aspergillus and other microorganisms can be used to replace Escherichia coli and catalyze the production of GABA. Moreover, under the condition of low cost, it also has the advantages of high output and good safety. This method has gradually developed to industrialized production.