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The aim of this study was to prove that Terrilactibacillus laevilacticus SK5-6, a novel D-lactate producer, exhibited a good fermentation performance comparing to the reference D-lactate producer Sporolactobacillus sp. Methods Glucose bioconversion for D-lactate production and the activity of five key enzymes including phosphofructokinase (PFK), pyruvate kinase (PYK), D-lactate dehydrogenase (D-LDH), L-lactate dehydrogenase (L-LDH), and lactate isomerase (LI) were investigated in the cultivation of T. laevilacticus SK5-6 and S. laevolacticus 0361 T. Results T. laevilacticus SK5-6 produced D-lactate at higher yield, productivity, and optical purity compared with S. laevolacticus 0361 T. T. laevilacticus SK5-6, the catalase-positive isolate, simultaneously grew and produced D-lactate without lag phase while delayed growth and D-lactate production were observed in the culture of S. laevolacticus 0361 T. The higher production of Dlactate in T. laevilacticus SK5-6 was due to the higher growth rate and the higher specific activities of the key enzymes observed at the early stage of the fermentation. The low isomerization activity was responsible for the high optical purity of D-lactate in the cultivation of T. laevilacticus SK5-6. Conclusion The lowest specific activity of PFK following by PYK and D/L-LDHs, respectively, indicated that the conversion of fructose-6-phosphate was the rate limiting step. Under the well-optimized conditions, the activation of D/L-LDHs by fructose-1,6-phosphate and ATP regeneration by PYK drove glucose bioconversion toward D-lactate. The optical purity of D-lactate was controlled by D/L-LDHs and the activation of isomerases. High D-LDH with limited isomerase activity was preferable during the fermentation as it assured the high optical purity.

Bacillus sp. BC-001 was first reported as a potent thermotolerant and homofermentative strain for an industrial-scale L-lactate production. In a flask culture, this isolate fermented both glucose and sucrose to lactate with high yield (0.96 and 0.87 g/g) and productivity (2.8 and 2.6 g/L h), respectively. The higher lactate production performance was obtained in the simultaneous saccharification and fermentation of liquefied starch (150.1 g/L final titer, 0.98 g/g yield, 3.2 g/L h productivity) and the fed-batch glucose fermentation (139.9 g/L final titer, 0.96 g/g yield, 2.9 g/L h productivity). Significant increase in lactate productivity (5.5-6.1 g/L h) was obtained from the high/heavy-inoculum seed in the stirred tank fermentor. Both calcium bases and monovalent bases were successfully employed for pH control during lactate fermentation by this isolate resulting in a versatile and simple operation. By the two-phase fermentation using the high/heavy-inoculum seed of BC-001, the fermentation reproducibility was acquired toward the pilot-scale fermentors.

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Rhizopus oryzae is becoming more important due to its ability to produce an optically pure L: -lactic acid. However, fermentation by Rhizopus usually suffers from low yield because of production of ethanol as a byproduct. Limiting ethanol production in living immobilized R. oryzae by inhibition of alcohol dehydrogenase (ADH) was observed in shake flask fermentation. The effects of ADH inhibitors added into the medium on the regulation of ADH and lactate dehydrogenase (LDH) as well as the production of cell biomass, lactic acid, and ethanol were elucidated. 1,2-diazole and 2,2,2-trifluroethanol were found to be the effective inhibitors used in this study. The highest lactic acid yield of 0.47 g/g glucose was obtained when 0.01 mM 2,2,2-trifluoroethanol was present during the production phase of the pregrown R. oryzae. This represents about 38% increase in yield as compared with that from the simple glucose fermentation. Fungal metabolism was suppressed when iodoacetic acid, N-ethylmaleimide, 4,4'-dithiodipyridine, or 4-hydroxymercury benzoic acid were present. Dramatic increase in ADH and LDH activities but slight change in product yields might be explained by the inhibitors controlling enzyme activities at the pyruvate branch point. This showed that in living R. oryzae, the inhibitors regulated the flux through the related pathways.