Studies On Xylitol Production By Microbial Fermentation Of Hemicellulosic Hydrolysate From Tea Seed Shells

In order to utilize comprehensively tea resources and increase economic value of teaplants to promote the development of tea industry, this paper performed a systematic study onxylitol production from the hemicellulosic hydrolysate of tea seed shells by fermentation ofCandida tropicals based on reviewing the references related to this subject. The major resultsattained were as followings.1. The production of xylose from tea seed shells with acid hydrolysis. In order toproduce xylose from tea seed shells, hydrochloric acid hydrolysis was applied with anultrasonic assisted pre-extraction. The productivity of xylose was monitored by3,5-dinitrosalicylic acid （DNS） colorimetry. The hydrolytic factors as pre-extraction time,particle size, hydrochloric acid concentration, hydrolysis time were investigated by means oforthogonal test. The results have demonstrated that the optimum factors are shell size of teaseeds at40mesh, ultrasonic assisted pre-extraction under ultrasonic frequency of50KHz for45min at25-30°C, hydrochloric acid concentration at0.6mol·L^-1, hydrolyzed at100°C for3hours. Under the optimum conditions, the highest yields of xylose were obtained which wasup to28.44%.2. Detoxification techniques on acid hydrolysate from tea seed shells. In order toutilize hemicellulose sufficiently and improve hydrolysate fermentability, the detoxificationtechniques on acid hydrolysate from tea seed shells were explored. After treatment ofover-neutralization on acid hydrolysate with limestone, the detoxification efficiency of twomethods as organic solvent extraction and activated carbon adsorption were compared. Theresults have shown that the method of activated carbon adsorption is more suitable fordetoxification. Furthermore, its parameters were optimized through orthogonal test and resultshave demonstrated that the optimum factors are pH4.0, addition of0.05g per milliliter ofactivated carbon, the temperature55°C and the time60minutes. Under those optimumconditions, the clearance rate of the lignin decomposition products reached up to74.25%whileretention rate of xylose was up to80.03%. Those research works have provided the underlyingbasis for the subsequent xylitol fermentation process.3. Xylitol production from tea seed shells hydrolysate fermented with Candidatropicals. Candida tropicals AS2.1776were applied to ferementation process for xylitolproduction. It is found that xylitol in the fermentation broth increased to the peak value at the 6th day after the consumption of the initial xylose at40g·L^-1. By means of orthogonal test, theoptimal fermentation process was that135mL of medium with15mL C. tropicals inoculumwas contained in a250mL flask at initial pH4.0, supplemented with5mL of nitrogen sourcesmixed with5g·L^-1of yeast extracts and5g·L^-1of tryptone.As a result, the xylose-to-xylitolbioconversion was up to63.80%and the maximum xylitol yields reached up to19.59g·L^-1.4. Separation and Purification of xylitol from fermentation broth. After fermentation,the decolorization process with activated charcoal was evaluated. The effects of pH, additiveamount of activated carbon, temperature and time on decolorization and xylitol recovery werestudied. Results demonstrated that the optimum decolorization conditions were at pH6.0,55°C for60min with the addition of20%（w/v）of activated carbon. As a result, theabsorbance at420nm of fermentation liquor declined from0.658to0.143, while the content ofxylitol decreased from14.81g·L^-1to9.77g·L^-1. The decolorization rate and xylitol recoverywere up to78.27%and65.97%, respectively. Subsequently, the decolorizated xylitol liquorwas desalted by732cation exchange resin and followed by7170anion exchange resin. Thedesalted liquor was precipitated with alcohol and freeze-dried under vaccum. Consequently,the crude products of xylitol crystals were obtained with the HPLC purity of78%.