L(+)-Lactic acid production from starch by a novel amylolytic Lactococcus lactis subsp. lactis B84

Abstract

A new Lactococcus lactis subsp. lactis B84, capable of utilizing starch as a sole carbon source and producing l(+)-lactate, was isolated from spontaneously fermented rye sourdough. Aiming at maximum lactic acid productivity, the components of the media and the cultivation conditions were varied. In MRS-starch medium (with absence of yeast and meat extracts), at 33 °C, agitation 200 rpm and pH 6.0 for 6 days complete starch hydrolysis occurred and 5.5 g l −1 lactic acid were produced from 18 g l −1 starch. The identification of strain B84 was based on genetic criteria. Amplified ribosomal DNA restriction analysis (ARDRA), PCR with species-specific primers and sequencing of the 16S rDNA proved its species affiliation. Four genes for enzymes, involved in starch degradation were detected in B84 genome: amyL, amyY, glgP and apu, coding cytoplasmic and extracellular α-amylases, glycogen phosphorylase and amylopullulanase, respectively. Reverse transcription PCR experiments showed that both genes, encoding α-amylases ( amyL and amyY) were expressed into mRNAs, whereas apu and glgP were not. Amylase activity assay was performed at different pH and temperatures. The cell-bond amylase proved to be the key enzyme, involved in the starch hydrolysis with maximum activity at 45 °C and pH 5.4.