Regulation method for preparing γ-polyglutamic acid by sludge substrate fermentation

A regulation method for preparing γ-polyglutamic acid from sludge, comprising the steps of:

• (a) extraction of glutamic acid from the sludge by
• (i) pretreating the sludge with a solid content of 10%-15% in a high-pressure reactor to obtain a sludge slurry, wherein pretreating the sludge in the high-pressure reactor comprises pretreating at a temperature of 110° C.-220° C. in the reactor and a pressure of 0.2 MPa or 0.3 MPa in the reactor for 4-5 hr;
• and
• (ii) performing gravity pressure filtration on the sludge slurry obtained in step (a)(i) through a filter press to obtain a dried sludge with a moisture content of 40%-60% and a first supernatant, keeping the first supernatant, and recycling the dried sludge;
• (b) synthesis of the γ-polyglutamic acid by
• (i) mixing a standard Bacillus culture medium comprising a Bacillus strain and a fermentation medium comprising fermentation raw materials to obtain cultivated Bacillus;
• and
• (ii) inoculating the cultivated Bacillus obtained from step (b)(i) into the first supernatant of step (a)(ii), performing aeration of the inoculated first supernatant to ensure a sufficient oxygen supply, and adjusting fermentation conditions of the inoculated first supernatant to obtain a fermentation product containing the γ-polyglutamic acid;
• and
• (c) preparing pure γ-polyglutamic acid by
• (i) acidifying the fermentation product containing the γ-polyglutamic acid obtained in step (b)(ii) to adjust the fermentation product pH to precipitate cell mass and polysaccharides, centrifugation to separate the precipitated cell mass and polysaccharides, and collecting a second supernatant from the first centrifugation;
• (ii) adjusting the pH of the second supernatant obtained in step (c)(i) with an alkaline solution and using an organic solvent to precipitate the γ-polyglutamic acid, and a second centrifugation to isolate the precipitated γ-polyglutamic acid;
• (iii) dissolving the precipitated γ-polyglutamic acid obtained in step (c)(ii) in deionized water to obtain a γ-polyglutamic acid solution, dialyzing the γ-polyglutamic acid solution at 4° C. for 6-24 hr, removing small molecular organic substances and inorganic salt impurities by a dialysis bag with a molecular weight cut-off of 8-12 kDa to obtain a dialyzed γ-polyglutamic acid solution;
• and
• (iv) vacuum freeze-drying the dialyzed γ-polyglutamic acid solution obtained in step (c)(iii) for 24-72 hr to obtain the pure γ-polyglutamic acid.