Monitoring the growth of Lactobacillus species during a rye flour fermentation

The natural microbial community conducting an industrial sourdough fermenta tion was investigated by molecular biological methods using the following s trategy. strains were isolated and subjected to RAPD (randomly-amplified po lymorphic DNA) PCR. After computer-supported pattern analysis and clusterin g of the strains the 16S rDNA of members of each distinct cluster were part ially (530 bp) or completely (1570 bp) sequenced and identified by comparat ive sequence analysis. The predominant strains of this fermentation could b e allotted to the species Lactobacillus amylovorus, Lactobacillus pontis an d a species, which phylogenetically fakes an intermediate position between L. pontis and L. penis. Sporadically, strains were identified as L. reuteri . in a second step the effect of external factors was investigated under th e controlled conditions of a lab-scale process. Fermentations were carried our at 34 degreesC, 40 degreesC and 46 degreesC. The development of the flo ra was consistent in independent fermentations as proved by RAPD typing of randomly-picked colonies. The microbial community in these fermentations wa s identical to those found in an industrial scale. The qualitative composit ion of the flora was not affected by the temperature L. amylovorus was the dominant species. With increasing fermentation time, a shift toward the pre dominance of heterofermentative lactobacilli was observed. This finding was underlined by metabolic studies and stoichiometric calculations of the met abolic pathways. With increasing temperature the percentage of homofermenta tive organisms was reduced. Furthermore, the growth rate and th e metabolic activity increased, followed by an immediate decrease of the growth rate a t 46 degreesC and lower terminal values of lactate, acetate and ethanol, re spectively. (C) 2001 Academic Press.