Gallic acid preparation method

acid hydrolysis

Acid hydrolysis is mainly divided into one-step method and two-step method. The main process of preparing gallic acid by the two-step method is as follows: raw material → hot water extraction → filtration of filter residue → concentration of tannin aqueous solution to about 20% → acid hydrolysis → cooling crystallization → centrifugation to obtain crude product → dissolution of crude product and carbon decolorization → cooling crystallization after filtration → centrifugation → drying → gallic acid finished product. The one-step process for the preparation of gallic acid is to eliminate the one step of leaching tannin than the two-step process, directly add acid hydrolysis, eliminating the crushing, leaching, concentration and other processes, shortening the production cycle, the process route and equipment design are reasonable and feasible, the development of its products has achieved good economic benefits, and found a way out for the mountain forest resources.

However, the sulfuric acid used in the acid hydrolysis method is a strong acid, which has different degrees of corrosion to the equipment. Although the reaction filter and the freezing kettle are made of stainless steel, due to the high reaction temperature, the acid concentration is large, and the corrosion is obvious, which directly affects the service life of the equipment.

alkali hydrolysis method

Alkali hydrolysis method is the raw material extract that tannin aqueous solution under alkaline conditions, and then use acid to neutralize and acidify to generate gallic acid.

Main process flow: raw material → hot water extraction → alkali hydrolysis → acid neutralization and acidification → cooling crystallization → centrifugal to obtain crude product → crude product dissolution and carbon decolorization → crystallization after filtration → centrifugal → drying → gallic acid finished product.

Compared with the acid hydrolysis method, the alkali hydrolysis method is not very corrosive to the equipment, which greatly reduces the depreciation of the equipment, but the process is relatively more complicated than the acid hydrolysis method. Most of the domestic production of gallic acid is the use of alkaline hydrolysis.

fermentation method

Fermentation method is the use of microorganisms in aqueous solution containing tannin fermentation, with glucose in tannin as carbon source for microbial growth and reproduction. The biological enzymes produced by microorganisms are induced to produce catalytic hydrolysis of tannins.

Process flow: raw materials are ground to a diameter of less than 10mm → worm powder is screened out → 30% tannin solution is immersed in water → black mold strain is added → fermentation for 8-9 days → filtration → water washing → crude gallic acid → dissolution and recrystallization → industrial gallic acid.

The main problem of fermentation method is that the formation of biological enzyme and the hydrolysis of tannin are carried out in the same reaction vessel, and the process conditions are difficult to achieve the best state, resulting in the increase of the reaction cycle (more than 3 days), the incomplete hydrolysis of tannin, and the residual tannin reaches 15% ~ 20%.

enzymatic method

In view of the defects of fermentation method, the new process of enzymatic method has been developed at home and abroad. The key of enzymatic method is to screen and produce efficient biological enzymes. Tannase is an acetylhydrolase, which belongs to the acylhydrolase induced by the outside of the spore. It can efficiently, specifically and directionally cleave the ester bond, the phenol bond and the glycosidic bond in the tannin molecule to generate gallic acid. Under appropriate conditions, a variety of mold and inducer tannin, can produce tannase. The commonly used strain is Aspergillus niger.

Process flow: enzyme culture → fermentation enzyme → (adding raw materials) hydrolysis → filtration → concentration → crude crystallization → separation → decolorization → primary crystallization → secondary crystallization → drying → crushing → finished gallic acid.

Compared with the fermentation method, the reaction time of enzymatic method is greatly shortened, the conversion rate of tannin hydrolysis is more than 98%, and the consumption index and production cost are significantly reduced.