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Abstract:The cultivation of toxic lignocellulosic hydrolyzates has become a challenging research topic in recent decades. Although several cultivation methods have been proposed, numerous questions have arisen regarding their industrial applications. The current work deals with a solution to this problem which has a good potential application on an industrial scale. A toxic dilute-acid hydrolyzate was continuously cultivated using a highcell-density flocculating yeast in a single and serial bioreactor which was equipped with a settler to recycle the cells back to the bioreactors. No prior detoxification was necessary to cultivate the hydrolyzates, as the flocks were able to detoxify it in situ. The experiments were successfully carried out at dilution rates up to 0.52 h -1 . The cell concentration inside the bioreactors was between 23 and 35 g-DW/L, while the concentration in the effluent of the settlers was 0.32 ± 0.05 g-DW/L. An ethanol yield of 0.42-0.46 g/g-consumed sugar was achieved, and the residual sugar concentration was less than 6% of the initial fermentable sugar (glucose, galactose and mannose) of 35.2 g/L.

The production of ethanol via syngas fermentation obtained from lignocellulose gasification provides a method for completely utilizing all of the carbon content from lignocellulosic feedstock. The low mass transfer rate of less soluble CO and H2 gas to liquid has been considered a major bottleneck in the overall process; however, microporous membrane has been proposed as a gas diffuser to improve gas-to-liquid mass transfer. In this study, a liquid batch of syngas fermentation employing Clostridium ljungdahlii with continuous gas supply was obtained using the configuration of a bioreactor connected to microporous hydrophobic polypropylene hollow fiber membrane (HFM) as a gas diffuser. Liquid recirculation between the fermentation vessel and membrane module was applied to enhance the gas-liquid contact as well as cell-recycle. The fermentation performance with and without HFM was compared and evaluated by cell growth, CO utilization, ethanol yield, and productivity. A higher ethanol yield, 0.22 mol/mol, was achieved using the system of an HFM-supported bioreactor with a higher ethanol titer of 1.09 g/L and an ethanol-acetate molar ratio of 1.43 mol/mol. The obtained result demonstrates that an HFM-supported bioreactor is the best fermentation system compared to stirred tank reactor (STR) without a membrane.

Xylitol has beneficial health properties and can be found in nature albeit in small quantities. In commercial industries, xylitol is produced via chemical hydrogenation of xylose. This process, however, requires high purity of xylose as the raw material. Biotechnological process offers an alternative xylitol production process, using the hydrolysate of lignocellulosic material such as the agricultural waste oil palm empty fruit bunches (OPEFB) as raw material. This substances may contain glucose beside xylose. The presence of glucose as cosubstrate, in the fermentation medium is also a critical factor that regulates the xylitol production by yeasts. Glucose may repress the activity of the key xylose reductase enzyme involved in the xylose conversion into xylitol resulting in low yields of the product.The purpose of this study was to explore the ability of microorganism to produce xylitol from OPEFB hydrolysate. This paper describes the effect of glucose as the co-substrate in xylitol production by Debaryomyces hansenii ITBCC R85 and further the use of OPEFB hydrolyasate as substrate in xylitol production. This research showed that addition of co-substrate glucose affected the fermentation performance of D. hansenii in producing xylitol. Glucose concentration of 2.5 g/L or concentration ratio of glucose to xylose of 25 % gave the highest yield of xylitol. The fermentation using OPEFB hydrolysate containing glucose to xylose ratio more than 25 % gave lower xylitol yield, addressing the hydrolysis of OPEFB to be optimized further.

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