Preparation of polylactic acid by two-step method
1. Preparation of lactic acid
We use the renewable resources of corn and potato as raw materials to prepare optically pure L-lactic acid or D-lactic acid by microbial fermentation. Moreover, L-lactic acid can be completely absorbed by the human body than D-lactic acid without any side effects.
Production of L-lactic acid, so we use the domestically used Rhizopus oryzae NAF-032.
(1) preparing a Rhizopus oryzae spore;
(2) preparing the Rhizopus oryzae spores into a suspension of Rhizopus oryzae spores;
(3) fixing the Rhizopus oryzae spore milk suspension to the immobilized carrier to obtain immobilized Rhizopus oryzae seeds;
(4) Immobilized Rhizopus oryzae seeds are inoculated into a fermentation medium for immobilization fermentation.
The method produces a high-yield strain of Rhizopus oryzae and fixes it on a cotton carrier to obtain immobilized Rhizopus oryzae seeds, and performs immobilized fermentation under suitable fermentation conditions. The potato starch has high conversion rate and the biomass of the fermentation product is high. The L-lactic acid yield is high, the cost is low, the steps are simple, and it is easy to control.
2. Acidification and purification separation of lactic acid
(1) The fermentation process produces a lactate because the pH of the fermentation is close to neutral. It is necessary to convert a certain lactate into lactic acid, and by directly adding sulfuric acid to the lactate solution, lactic acid can be obtained as a by-product of the crystallization of calcium sulfate dihydrate. It can be removed by filtration. Of course, calcium sulfate dihydrate can be used as a ground infusion gypsum, for example, as a raw material for drywall, cement and agricultural fields. Raw gypsum is a low-value salt produced during the production process, but this method is more cost-effective because the cost of calcium hydroxide and sulfuric acid is low and gypsum can be used for other industrial purposes. Other attempts have been made to link the two processes of alkalization and acidification, such as adjusting the pH with ammonia, acidifying with sulfuric acid, to obtain ammonium sulfate as a by-product, and ammonium sulfate as a fertilizer. Because ammonium salts are more expensive than calcium hydroxide, the high value of by-product ammonium sulfate just makes up for this gap, and ammonium sulfate is readily soluble in water relative to calcium salts, which facilitates separation.
(2) Cell removal
The choice of cell removal method depends primarily on the microorganisms used in the production. Rhizopus oryzae is 210-2500 μm in length and 5-18 μm in diameter, because the smaller cells can be removed by flocculation. Chitosan was added to the fermentation broth as a flocculant, the pH was adjusted to 6.8, the temperature was kept, and the flocculation was stirred. After the flocculation was completed, the supernatant was placed for 1.5 h, and the supernatant was taken in a centrifuge tube and centrifuged at 4000 r/min. At 20 min, a solid precipitate was isolated.
(3) Separation of residual sugar, residual medium and fermentation by-products
This project uses solvent extraction. After solvent extraction, the lactic acid solution passes through activated carbon, a cation exchange resin, and an anion exchange resin to obtain a yellowish deionized product.