CONTRIBUTION OF THE MICROBIAL MASS TO THE NITROGEN PROFILE OF WHEAT BREAD DOUGHS STARTED WITH PURE AND MIXED CULTURES OF YEAST AND LACTIC-ACID BACTERIA

This paper describes work investigating the contribution of low-molecular-mass nitrogen metabolites (free amino acids and peptides) to the microbial mass of pure strains of fermentation microorganisms (Saccharomyces cerevisiae, P38; Lactobacillus brevis, B33; L. plantarum, B23 and Enterococcus faecium, B12) when they are incorporated as starters individually and in mixed cultures to wheat bread doughs. Under the experimental conditions used, the microbial mass of P38 releases mainly gamma-aminobutyric acid, proline, valine, lysine, isoleucine, glycine and alanine, which account together for less than 5% of the amino acid content of the corresponding bread dough; amino acids in the microbial mass of lactic acid bacteria, mainly glycine and alanine, amount to 1-2% of the dough. The peptide contribution of microbial mass to bread dough is higher and dependent on the strain. Mixing ingredients induces an increase of 65% in the total amino acid content of dough started with P38, particularly of glutamic acid, glutamine, isoleucine, ornithine, lysine, histidine and tyrosine, and a significant rise in peptide levels of all individual doughs with respect to the values obtained for mixtures of flour, water and salt. Starting bread dough with mixtures of yeast and lactic acid does not increase low-molecular-mass nitrogen levels, the amount of peptides being similar to those of dough containing yeast P38 alone. Addition of B33 induces a remarkable increase in the amino acid content of bread dough, mainly of dicarboxylic acids and their amides, and proline, similar to the composition of free amino acids of dough starter with B33.