Monitoring the growth of Lactobacillus species during a rye flour fermentation

The natural microbial community conducting an industrial sourdough fermentation was investigated by molecular biological methods using the following strategy: strains were isolated and subjected to RAPD (randomly-amplified polymorphic DNA) PCR. After computer-supported pattern analysis and clustering of the strains the 16S rDNA of members of each distinct cluster were partially (530 bp) or completely (1570 bp) sequenced and identified by comparative sequence analysis. The predominant strains of this fermentation could be allotted to the species Lactobacillus amylovorus, Lactobacillus pontis and a species, which phylogenetically takes an intermediate position between L. pontis and L. panis. Sporadically, strains were identified as L. reuteri. In a second step the effect of external factors was investigated under the controlled conditions of a lab-scale process. Fermentations were carried out at 34°C, 40°C and 46°C. The development of the flora was consistent in independent fermentations as proved by RAPD typing of randomly-picked colonies. The microbial community in these fermentations was identical to those found in an industrial scale. The qualitative composition of the flora was not affected by the temperature. L. amylovorus was the dominant species. With increasing fermentation time, a shift toward the predominance of heterofermentative lactobacilli was observed. This finding was underlined by metabolic studies and stoichiometric calculations of the metabolic pathways. With increasing temperature the percentage of homofermentative organisms was reduced. Furthermore, the growth rate and the metabolic activity increased, followed by an immediate decrease of the growth rate at 46°C and lower terminal values of lactate, acetate and ethanol, respectively.