The acid hydrolysis method is mainly divided into one-step method and two-step method. The main process flow of two-step preparation of gallic acid raw material → hot water extraction → filter residue → tannin aqueous solution concentration to about 20% → acid hydrolysis → cooling crystallization → centrifugation to obtain crude product → crude product dissolution and charcoal decolorization → after filtration Cooling and crystallization → centrifugation → drying → finished product of gallic acid. The one-step process for preparing gallic acid eliminates the need for one step of leaching cyanine compared with the two-step process. It is directly added with acid hydrolysis, eliminating the need for crushing, leaching, concentration and other processes, shortening the production cycle, and the process route and equipment design are reasonable Feasible, the development of its products has achieved good economic benefits, and found a way out for forest resources in mountainous areas.
However, the sulfuric acid used in the acid hydrolysis method is a strong acid, which corrodes the equipment to varying degrees. Although both the reaction filter and the freezer are made of stainless steel, due to the higher reaction temperature and higher acid concentration, the corrosion is obvious, which directly affects the service life of the equipment.
Alkaline hydrolysis is to hydrolyze the raw material extract, namely, the aqueous tannin solution under alkaline conditions, and then neutralize and acidify with acid to generate gallic acid.
The main process flow raw materials → hot water extraction → alkali hydrolysis → acid neutralization → cooling crystallization → centrifugation to obtain crude product → crude product dissolution and charcoal decolorization → filtration and crystallization → centrifugation → drying → gallic acid product.
Compared with the acid hydrolysis method, the alkaline hydrolysis method is less corrosive to the equipment and greatly reduces the depreciation of the equipment, but the process is relatively more complicated than the acid hydrolysis method. Most domestic gallic acid production uses alkaline hydrolysis. 
The fermentation method uses microorganisms for fermentation in an aqueous solution containing tannins, and uses glucose in tannins as a carbon source for the growth and reproduction of microorganisms. The biological enzymes induced by microorganisms catalyze the hydrolysis of tannins.
Process flow Grind raw materials to a diameter of less than 10mm → screen out insect powder → immerse in water to raise 30% tannin solution → add black mold species → fermentation for 8-9 days → filtration → washing → crude gallic acid → dissolving and recrystallization → industrial gallic acid.
The main problem in the fermentation method is that the formation of biological enzymes and the hydrolysis of tannins are carried out in the same reaction vessel, and the process conditions are difficult to reach the optimal state, resulting in a longer reaction cycle (more than 3 days), incomplete hydrolysis of tannins, and residual tannins Up to 15% ~ 20%.
In view of the shortcomings of the fermentation method, researches on new enzymatic processes have been developed at home and abroad. The key to the enzymatic method is to screen and prepare highly efficient biological enzymes. Tanninase is an acetyl hydrolase, which is an extrasporal induced acyl hydrolase, which can efficiently, specifically, and cleavage the ester bond, depsil bond and glycosidic bond in tannin molecules to generate gallic acid. Under appropriate conditions, various molds and inducer tannins can produce tannase. The commonly used strain is Aspergillus niger.
Process flow Enzyme seed cultivation → fermentation enzyme production → (adding raw materials) hydrolysis → filtration → concentration → coarse crystallization → separation → decolorization → primary crystallization → secondary crystallization → drying → crushing → finished gallic acid.
Compared with the fermentation method, the enzymatic method has greatly shortened the reaction time, the tannin hydrolysis conversion rate is over 98%, and the consumption index and production cost are significantly reduced.
Post time: Feb-23-2021