Enhanced production of lactic acid from liquefied sorghum starch by Rhizopus oryzae mutants / Laktik asit üretimini sıvılaştırılmış sorgum nişastası kullanılarak arttırmak için Rhizopus oryzae mutantlarının seçimi

Abstract

Objective: Our aim was to find out Rhizopus oryzae mutants that are able to ferment a cost-effective substrate such as sorghum starch for enhanced lactic acid production.Methods: The fungal spores were either exposed to UV treatment at 254 nm, for 1 h at a distance of 20 cm from the source or 0.01 mol/l of diethyl sulfate from 0.2 to 1 ml in 20 ml sterile potassium phosphate buffer (pH 7.0) at 30°C for 0-40 min. Samples were withdrawn in 10 min intervals and inoculated on the single colony isolation medium, after 36 h of incubation the colonies were singled out and subjected to fermentation tests.Results: Isolated mutants UV-40 and DES-30 were selected by following colour-zone method. They varied morphologically from the wild type R. oryzae; and single colony isolation of the mutants was carried out by using sodium deoxycholate. The UV-40 mutant produced (76.42%) more lactic acid than the DES-30 mutant (74.08%) and the wild type (23.8%) in 150 g/l of substrate concentration. UV-40 and DES-30 mutants yielded more lactic acid as 0.96 g/g substrate, with a productivity of 1.59 g/l/h than the wild type, which yielded only 0.39 g/g of substrate, with a productivity of 0.6 g/l/h at 150 g/l of substrate. Fermentation results of the both mutants at 200 g/l of the substrate were not encouraging, since more residual substrate was left over in the medium.Conclusion: From results it was evident that UV mutant was producing consistently more lactic acid in 150 g/l of liquefied sorghum starch medium with a pH of 4.0. The mutants obtained in this study were also stable lactic acid producers up to 15 generations.