| Summary: | 碩士 === 明新科技大學 === 化學工程與材料科技系碩士班 === 104 === Abstract
Lignocellulose is the most abundant biomass on the Earth. It contains cellulose and hemicellulose which release fermentable hexoses and pentoses upon hydrolysis. In the development of bioenergy and biorefinery, lignocellulose is one of the most important feedstock. In Taiwan, more than 173 tons of rice straw was produced in 2013, most of it was burned on site, leading to the problems of air pollution and energy waste.
In this study, rice straw was utilized in lactic acid fermentation by Lactobacillus rhamnosus. Sun dried rice straw was chopped, pretreated in 2 % NaOH solution in microwave oven at 100 °C for 10 minutes, neutralized and followed by simultaneous saccharification and fermentation (SSF). After alkali pretreatment, most of the lignin in rice straw was destroyed and removed. In a batch SSF, the remaining cellulose and hemicellulose were hydrolyzed easily hydrolyzed enzymatically, but the production of lactic acid seemed to slow down after 48 hours.
To find out the reason of slow lactic acid production, and to increase lactic acid concentration in fermentation broth, several attempts were tried. First, SSF was conducted in MRS media containing 0-120 g/L of lactic acid to investigate the inhibition effect of lactic acid. Secondly, fed-batch fermentations of different feeding strategy were conducted. The results indicate significant inhibition of lactic acid to Lactobacillus rhamnosus. When lactic acid concentration in fermentation medium increased from 0 to 120 g/L, the average lactic acid production rate in the first 12 hours of SSF dropped from 1.43 g/L/h to 0.08 g/L/h. The addition of extra portion of pretreated rice straw after 12 hours of SSF did not boost the production of lactic acid.
On the other hand, the addition of nitrogen rich nutrients, such as: yeast extract,
peptone and beef extract, increased lactic acid production to a limited extend. The addition of rice straw and MRS medium 12 or 24 hours after the beginning of SSF resulted in a higher lactic acid concentration. Applying 0.125-0.5 ml xylanase at the beginning of SSF and adding glucosidase and cellulase 12 or 24 hours later helped the utilization of xylose, and increased both acetic acid and lactic acid concentrations.
Keywords: rice straw, lactic acid, simultaneous saccharification and fermentation, Lactobacillus rhamnosus, fed-batch
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