Comparative Characterization of L-Lactic Acid-Producing Thermotolerant Rhizopus Fungi

Acid-producing Rhizopus fungi from loog-pang, a traditional Thai fermented food, was screened to investigate its potential for use in industrial lactic acid production from starch. A thermotolerant strain, TISTR 3518, was isolated and characterized by its morphological, physiological, genetic and fermentation properties, and compared with its mesophilic isolates, TISTR 3514 and TISTR 3523. TISTR 3518 was characterized by shorter sporangiophores and smaller sporangia than the other isolates; however, apparent differences between the mesophilic isolates and the strain could not be clarified. Moreover, TISTR 3518 grew at 45 degrees C, whereas the others did not. The three isolates showed different profiles of oligosaccharide assimilation and organic acid production. Their rDNA ITS sequences indicated that TISTR 3518 is a strain of Rhizopus microsporus, and TISTR 3514 and TISTR 3523 are strains of Rhizopus oryzae. TISTR 3523 and TISTR 3518 mainly formed I.-lactic acid from glucose, while TISTR 3514 primarily formed fumaric acid. Under thermotolerant conditions, R. microsporus TISTR 3518 showed higher glucoamylase activity than the others, suggesting this enzyme from TISTR 3518 is more thermostable than that from TISTR 3523. The strain formed higher amounts of L-lactic acid from starch at 40 degrees C compared to R. oryzae TISTR 3523. This is the first report on the production of optically active L-lactic acid from starch by a thermotolerant fungus and could potentially provide a good tool for transforming biomass resources to chemical materials.