Microorganisms play a significant role in bioethanol production from lignocellulosic material. A challenging problem in bioconversion of rice bran is the presence of toxic inhibitors in lignocellulosic acid hydrolysate. Various strains of Zymomonas mobilis (ZM4, TISTR 405, 548, 550 and 551) grown under biofilm or planktonic modes were used in this study to examine their potential for bioconversion of rice bran hydrolysate and ethanol production efficiencies. Z. mobilis readily formed bacterial attachment on plastic surfaces, but not on glass surfaces. Additionally, the biofilms formed on plastic surfaces steadily increased over time, while those formed on glass were speculated to cycle through accumulation and detachment phases. Microscopic analysis revealed that Z. mobilis ZM4 rapidly developed homogeneous biofilm structures within 24 hours, while other Z. mobilis strains developed heterogeneous biofilm structures. ZM4 biofilms were thicker and seemed to be more stable than other Z. mobilis strains. The percentage of live cells in biofilms was greater than that for planktonic cells (54.32 ± 7.10% vs. 28.69 ± 3.03%), suggesting that biofilms serve as a protective niche for growth of bacteria in the presence of toxic inhibitors in the rice bran hydrolysate. The metabolic activity of ZM4 grown as a biofilm was also higher than the same strain grown planktonically, as measured by ethanol production from rice bran hydrolysate (13.40 ± 2.43 g/L vs. 0.432 ± 0.29 g/L, with percent theoretical ethanol yields of 72.47 ± 6.13% and 3.71 ± 5.24% respectively). Strain TISTR 551 was also quite metabolically active, with ethanol production by biofilm and planktonically grown cells of 8.956 ± 4.06 g/L and 0.0846 ± 0.064 g/L (percent theoretical yields were 48.37 ± 16.64% and 2.046 ± 1.58%, respectively). This study illustrates the potential for enhancing ethanol production by utilizing bacterial biofilms in the bioconversion of a readily available and normally unusable low value by-product of rice farming.

Background:The major challenges associated with the fermentation of lignocellulosic hydrolysates are the reduction in the operating cost and minimizing the complexity of the process. Zymomonas mobilis biofilm has been emerged to resolve these complexities. Biofilm has been reported to tolerate to the toxic inhibitors and easily manipulated toward the cell recycle through the cell immobilization. Results: Z. mobilis ZM4 and TISTR 551 were able to develop biofilms on DEAE cellulose under the differences in the morphologies. Z. mobilis ZM4 developed homogeneous biofilm that brought DEAE fiber to be crosslinking, while Z. mobilis TISTR 551 developed heterogeneous biofilm in which crosslinking was not observed. Ethanol production under batch and repeated batch fermentation of rice bran hydrolysate containing toxic inhibitors were compared between these two biofilms. TISTR 551 biofilm produced the maximum yield (Y P/S ) of 0.43 ± 0.09 g ethanol/g glucose (83.89% theoretical yield). However the repeated batch could not be proceeded due to the bacterial detachment. Z. mobilis ZM4 biofilm produced the maximum yield (Y P/S ) of 0.177 ± 0.05 g ethanol/g glucose (34.74% theoretical yield) in the batch culture and the biofilm remained intact to proceed along the repeated batch. The highest ethanol yield (Y P/S ) in the repeated batch of Z. mobilis ZM4 was 0.354 ± 0.07 g ethanol/g glucose (69.51% theoretical yield). Conclusions: Homogeneous biofilm structure of Z. mobilis provided more recycle beneficial over the heterogeneous biofilm structure for the ethanol production from lignocellulosic hydrolysate.

Purpose The purpose of this paper is to investigate the potential of fruit waste materials from fruit industry as sources of powerful natural antioxidants. Design/methodology/approach The peels of mango, rambutan, and santol were extracted and analyzed for their antioxidant activity. Pork ball samples were prepared and treated with different natural extracts at various concentrations, namely 0.05, 0.10, 0.15, and 0.20% (v/w), compared with the control (no natural extract), and then stored at 4°C. The samples were investigated at the zeroth, third, fifth, eighth and tenth day to find out the antioxidant activity of the total phenolic content and lipid oxidation including the evaluation of the change of rancid flavor and color during storage. All statistical results were analyzed by RCBD using SPSS at p⩽0.05. Findings During storage, the amount of phenolic compound in a pork ball by adding fruit extracts at various concentrations slightly decreased, while the lipid oxidation slightly increased. Mango extract showed the best efficiency to delay lipid oxidation in the pork ball for ten days followed by santol extract and rambutan extract. For sensory evaluation, the pork ball with fruit extract at various concentrations showed the lower rancidity development than control, and mango extract at 0.20% (v/w) showed the best delaying rancidity during ten-day storage. Thus, mango peel extract at 0.20% (v/w) showed the highest effectiveness of antioxidant activity against lipid oxidation in pork ball. Originality/value This study was continued from the previous research which investigated the best extraction condition for by-products of mango, rambutan, and santol. From that study, the authors found that the crude extracts need further research on their antioxidant property in foods. The research findings have provided information regarding the interesting new natural antioxidant that can be applied into lipid-containing foods to delay the rancidity and extend the shelf life.

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