Submitted to: Biotechnology Letters
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 17, 1997
Publication Date: N/A
Interpretive Summary: Lactic acid is commonly used as a food additive for preservation, flavor, and acidity. Recently, it has gained popularity for the manufacture of the biodegradable plastic, poly-lactate. It has been estimated that the lactic acid market for the U.S., which is at production levels of about 50,000 tons/yr, could exceed one million tons within a decade. Lactic acid can exist as one of two different chemical structures, called L- and D-forms that are mirror images of each other. Fermentations with the fungus Rhizopus are one of the few cost-effective ways of producing high-grade lactic acid that is only in the L-form. This is especially important for food use where biologically metabolizable L-lactate must be used. The properties of the poly-lactate plastic are also greatly improved if the lactic acid is only in the L-form. We have isolated mutants of Rhizopus with improved abilities to produce lactic acid under oxygen limiting conditions. Rhizopus does not normally produce lactic acid in the absence of oxygen, while our mutant was able to produce nearly 40 grams per liter in 70 hrs. Eliminating the need for aeration could represent considerable savings over current Rhizopus fermentation methods.
Technical Abstract: Rhizopus oryzae produces lactic acid during aerobic growth from lactate dehydrogenase mediated reduction of pyruvate, while the primary fermentation product during anaerobic conditions is ethanol. A mutant was isolated that expressed only 5% of the wild type Adh activity under oxygen limiting conditions and produced nearly 40 g lactic acid/L in 70 hrs. This is almost a 10-fold increase when compared to production by the parent strain.