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Effects of water activity on kinetics of simple and complex microbial bioconversions

The effects of water activity on the kinetics of "simple" (biooxidation of D-sorbitol into L-sorbose by means of Gluconobacter suboxydans S-22) and "complex" (biosynthetic conversion of D-sorbitol into oxytetracycline by means of Streptomyces rimosus R6) microbial processes were studied. Water activity was defined as a function of oxygen solubility. Experiments with shake flask cultures were used to study the kinetics of both processes. Growth of microorganisms, consumption of D-sorbitol and formation of products (L-sorbose and oxytetracycline) were monitored. Effects of water activity were more expressed in the process of oxytetracycline biosynthesis than in that of L-sorbose formation. Growth kinetics was affected more than product formation kinetics in both processes. The estimated critical values of water activity were 0, 35 (G. suboxydans S-22 growth), 0, 40 (S. rimosus R6 growth), 0, 21 (L-sorbose formation) and 0, 40 (oxytetracycline biosinthesis) respectively. With decreasing water activity the mass transfer phenomena were observed to become more and more important, especially when considering the liquid-microbial biomass interface as in the case of applying media with D-sorbitol concentration above 500 g/L.

water activity ; D-sorbitol bioconversion ; kinetics

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