Microbial ecology of sorghum sourdoughs: Effect of substrate supply and phenolic compounds on composition of fermentation microbiota

The choice of the cereal substrate determines sourdough microbiota, however, the substrate-associated ecological factors for this phenomenon have not been elucidated. This study investigated the competitiveness of Lactobacillus sanfranciscensis LTH 2590, a wheat sourdough isolate, and four isolates from sorghum sourdoughs (ting), Lactobacillus casei FUA3166, Lactobacillus harbinensis FUA3199, Lactobacillus parabuchneri FUA3169, and Lactobacillus coryniformis FUA3307, in sorghum sourdoughs, sorghum sourdoughs supplemented with maltose, or wheat sourdoughs. Fermentations were characterised by determination of cell counts, pH, and quantification of metabolites. Maltose was the main carbon source in wheat sourdoughs whereas glucose was the major carbon source in sorghum. L. coryniformis and L. parabuchneri produced 1,3- and 1,2-propanediol from glycerol and lactate, respectively, metabolites that were previously not observed in sourdough. To determine the competitiveness of strains, wheat and sorghum slurries were inoculated with equal cells counts of L. sanfranciscensis, L. parabuchneri, and L. casei fermented at 28 °C or 34 °C and propagated by back-slopping every 24 h. Lactobacilli in sourdough were quantified by plating and species-specific quantitative PCR (qPCR). Generally, sorghum and wheat sourdoughs inoculated with isolates from ting gave no appreciable differences in the metabolites produced during the fermentation process. L. sanfranciscensis grew in wheat but not in sorghum sourdoughs, or sorghum sourdoughs supplemented with 2% maltose, 1% tryptone, 0.1% l-cysteine and 2% sucrose. Furthermore, L.sanfranciscensis decreased progressively during propagation of sorghum sourdoughs but ting isolates were overgrown by L. sanfranciscensis after three propagations in wheat sourdoughs independent of the incubation temperature. The anti-microbial activity of four different types of sorghum extracts was tested against L. sanfranciscensis, L. parabuchneri, and L. casei to correlate the resistance to phenolic compounds to growth in wheat or sorghum sourdoughs. L. sanfranciscensis was inhibited by phenolic extracts from sorghum flours whereas ting isolates were resistant. In conclusion, microbiota of sorghum sourdough differ from wheat and rye because sorghum contains active concentrations of antimicrobial phenolic compounds, and offers glucose as major carbon source.

► Sorghum contains antimicrobial phenolic compounds and glucose as carbon source.
► Antimicrobial phenolics select for resistance of fermentation microbiota.
► Glycerol esters of phenolic esters in sorghum support glycerol fermentation by LAB.
► Results facilitate the selection of starter cultures in developing countries.