Study Of Cruciate Polyvinyl Alcohol (PVC) Fibers In Production Of L-lactic Acid By Immobilized Rhizopus Oryzae

Lactic acid is precursor of poly lactic acid which is a kind of biomedical engineering materials.Recently, with the rapid development of biodegradable materials-poly lactic acid, there has been an high interest in lactic acid production. The main lactic acid production method is microorganism fermentation in which lactic acid bacteria and Rhizopus oryzae are commonly used. The nutritional requirement of Rhizopus oryzae is simple and its output is in high optical purity. However, it is difficult to control the fungal cell morphology and the dispersed mycelia would easily form clumps which will impose diffusion limitation and reduce lactic acid production. The immobilized-cell fermentation can easily control the mycelium morphology and shorten fermentation time, besides the immobilized-cell can be reused stably. Glucose and starch are often used as the carbon source of Rhizopus oryzae, but as the energy and food problem becoming more and more serious, it is urgent to find new raw materials which are inexpensive and easy to get. Nowadys, the agriculture waste such as corn straw are extensively studied as cellulosic raw material in lactic acid fermentation which was also investigated in this paper.This paper was divided into two parts. Firstly, polyving achohol fiber as absorbing material for cruciform matrix production was adopted, and the parameters, fermentation conditions and long-term stability of matrix were researched. At the same time, the fermentation production of lactic acid (free and immobilized cell) by using straw hydrolyzate as carbon source was studied.Firstly, using glucose as carbon source and determining the shape of matrix to be cruciform in the fermentation, with the matrix inner diameter of 2.5 cm, PVA quality of 0.12g/per matrix, spores concentration of 0.5×10⁶/ml and one matrix, as a result, the lactic acid concentration achieved 51.13 g/Lwith the yield of 63.90%. Compared with free fermentation, using immobilized fermentation in shake-flasks, the final lactic acid concentration was increased by 25% and the fermentation time was shortened by 33%.Secondly, the matrix conditions of shake-flasks fermentation were determined: matrix was put in at the beginning and culturing for 24h, with a rotation speed of 180r/min and adding 20g/L CaCO3 at the beginning of the fermentation at once, lactic acid concentration reached 54.32 g/L. The performance in both of shake-flasks and 1L bubble-column bioreactor operated in a repeated batch mode was indicated the long-term reusability. The overall yield of first 10 cycles in shake-flasks was 65%, corresponding to a volumetric productivity of 1.1g/L·h. Moreover, the highest lactic acid concentration in the bubble-column bioreactor was 55.68% with a yield of 69.60%.Finally, the fermentation with hydrolyzate of corn straw as carbon source was primary investigated. The pretreatment of corn straw adopted the method of combined steam explosion and physical-chemical process. The content of lignin was reduced; meanwhile the sensitivity of cellulase enzyme to corn straw was increased. The optimal condition for hydrolyze was developed by using Uniform Design theory. When the concentration of cellulose enzyme was 35FPU/g and substrate was 50g/L, the hydrolysis achieved 94.03%. During the study of hydrolyzate fermentation, an optimal medium of MgSO₄·7H₂O 0.35 g/L, ZnSO₄·7H₂O 0.20 g/L, KH₂PO₄ 0.15 g/L and urea 1g/L was used. The lactic acid yield was 52.36% in 48h by using free cell while the lactic acid yield was 30.19% by using immobilized cell and the fermentation time was shortened to 30h.