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Production of L-lactic acid from fresh cassava roots slurried with tofu liquid waste by Streptococcus bovis.

To reduce the production cost of biodegradable plastics, the fermentation performance of L-lactic acid for a new fermentation medium, fresh cassava roots (FCRs) as a substrate slurried with tofu liquid waste (TLW) as basal medium, was investigated by batch fermentation of Streptococcus bovis. The fermentation properties of the three substrates, namely, FCR, tapioca (cassava starch) and glucose, which were independently mixed with TLW, were compared with those independently mixed with the standard basal medium, trypto-soya broth (TSB). Experiments were conducted at various sugar concentrations of the substrates with CaCO(3) as a neutralizer. The maximum L-lactic acid concentrations (C(La)) obtained using the three substrates in TLW were about 75% of those obtained using TSB caused by less nutrients in the TLW. The L-lactic acid productivities (P(La)) and the specific growth rates of S. bovis (mu) in TLW were about 1/4 to 1/3 and 1/5 to 1/4 of those in TSB, respectively. The maximum C(La), P(La) and mu were obtained at 10% w/w sugar concentration. Total yields (eta) were nearly constant up to 10% w/w sugar concentration for TSB and TLW, that is, 80% to 85% and 50% to 60%, respectively. But their total yields decreased in more than 10% w/w sugar concentration in both basal media, because of substrate inhibition. The fermentation properties (C(La), P(La), mu, and eta) were found to be in the order of: FCR > tapioca > glucose for all concentrations of the three substrates. The fermentation properties for FCR and tapioca were higher than those for glucose, in TLW or TSB, because S. bovis in a medium containing starch (FCR and tapioca) has more amylase activity than in a medium containing glucose. The nutrients in FCR with poor nutrient basal medium (TLW) more strongly affected the fermentation properties than those in FCR with rich nutrient basal medium (TSB). The proposed fermentation medium of FCR slurried with TLW is worth studying in order to reduce production cost of biodegradable plastics.