The fungus Rhizopus orizae F-1030 cultivating mode on neutral sulfite liquor hydrolysates affects the lactic acid synthesis efficiency

The industrial lactic acid production is based on microbiological fermentation of sugar-containing feedstock. Generally, the price on the substrate controls almost half the cost of the entire production. The use of pure individual sugars as a carbon source for the microbiological lactic acid synthesis leads to a significant rise in production expenses. Lignocellulosic biomass is one of the most common natural renewable sources of carbon, the use of which in biotechnological production as a carbohydrate-containing substrate is limited mostly due to the lack of cellulolytic enzymes in most strains of microorganisms used on industrial scale. The filamentous fungus Rhizopus orizae is an industrial lactic acid producing microorganism capable of metabolizing lignocellulose. The growth of microbial mass, substrate consumption rate and lactic acid yield are controlled by the metabolic activity of the producer cells, the latter depending on the cultivation mode. The microbiological lactic acid production regulation is carried out by changing such factors of nutrient medium as temperature, pH, nutrient concentration in the substrate and the final product concentration. The work involved the conditions’ selection for fungus R. oryzae F-1030 cultivation on neutral sulfite liquor, which made it possible to attain the optimal ratio of biomass and the synthesized lactic acid yield. It has been shown that the nutrient medium acidification is affected by the physiological maturity of the fungus. The level of reducing substances in the nutrient medium is controlled by the growth rate of the fungus R. oryzae F-1030 cells and the activity of its cellulolytic enzymes. There were no statistically significant differences in the amount of synthesized lactic acid and biomass growth depending on the mode of the fungus R. oryzae F-1030 cultivation on the neutral sulfite liquor after treatment with externally added enzymes. The growth of fungus R. oryzae F-1030 biomass is influenced by both the type of hydrolyzing agent (acid or enzyme) and the composition of the nutrient medium, in particular the presence of nitrogen, phosphorus, sulfur, and potassium salts. The batch cultivation mode was demonstrated as optimal for the fungus R. oryzae F-1030 to grow on the neutral sulfite liquor after acid hydrolysis.

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