Biosynthesis and conversion of phenolics and taste active compounds in food fermentations

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Microbial metabolism determines and maintains the quality and safety of fermented foods. Lactobacilli are dominant representatives of fermentation microbiota in most fermented foods, including fermented cereal food. The applicant's research program generally aims to link the ecology and physiology of food-fermenting lactobacilli to their competitiveness in cereal fermentations and their impact on food quality. The long term aim of the proposed research programme is to characterize the metabolism of minor metabolites, particularly the formation of taste-active peptides and the conversion of phenolic compounds by lactobacilli, and to determine the impact of these metabolites on the sensory, nutritional, and textural quality of bread. Specifically, the proposed project aims to elucidate the genetic determinants of formation of ?-glutamyl peptides and conversion of phenolic acids, to understand the role of these metabolic pathways in the ecology of lactobacilli, and their contribution to improved bread quality. Taste active ?-glutamyl peptides impact the taste of foods by altering the perception of sweet, salty, and umami tasting compounds. This taste activity has been referred to as "kokumi" activity. ?-Glutamyl peptides accumulate to active concentrations in fermentation of soy sauce, long-ripened cheese, and in sourdough; however, their formation has typically been attributed to enzymes from the raw material rather than bacterial metabolism. Recent work of the applicant demonstrated that L. reuteri produces ?-glutamyl peptides in sourdough. Bioinformatic analyses identified ?-glutamate-cysteine ligases and glutaminases as enzymes that synthesise ?-glutamyl peptides.  The contribution of ?-glutamyl peptides to the taste of bread will be determined by quantification of these peptides in sourdough and sourdough bread, and by sensory analysis of bread produced with wild type strains and mutant strains with deletions in glutaminases or ?-glutamate-cysteine synthase. Sensory analyses will particularly aim to employ strains of L. reuteri that accumulate ?-glutamyl peptides to produce salt-reduced bread without reduction of consumer preference. Phenolic acids that in cereals, vegetables, or fruits modulate the composition of fermentation microbiota through their antimicrobial activity; moreover, phenolic acids also are precursor compounds for flavour volatiles. The contribution of phenolic acid metabolism of lactobacilli to ecological fitness in cereal fermentations and the texture and sensory quality of fermented cereal foods is unknown. Work in the proposed project aims to identify putative enzymes with activity on phenolic acids by comparative genomics. Phenolic acid decarboxylases and reductases will be characterized after heterologous expression, and by characterization of isogenic deletion mutants, and their role for sourdough biochemistry and bread quality will be determined.