Considerable knowledge has been accumulated on the lactic acid bacteria (LAB) that affect the aroma and flavour of yogurt. This review focuses on the role of LAB in the production of flavour compounds during yogurt fermentation. The biochemical processes of flavour compound formation by LAB including glycolysis, proteolysis, and lipolysis are summarised, with some key compounds described in detail. The flavour-related activities of LAB mostly depend on the species used for yogurt fermentation, and some strategies have been developed to obtain more control of the flavourforming process. Metabolic engineering can be a powerful tool to reroute the metabolic flux towards the efficient accumulation of the desired flavour compounds with the knowledge of the complex network of flavour-forming pathways and the availability of genetic tools. Further progress made in the omics-based techniques and the use of systems biology approaches are needed to fully understand, control, and steer flavour formation in yogurt fermentation processes. ARTICLE HISTORY

dAlthough fructooligosaccharides (FOS) can selectively stimulate the growth and activity of probiotics and beneficially modulate the balance of intestinal microbiota, knowledge of the molecular mechanism for FOS metabolism by probiotics is still limited. Here a combined transcriptomic and physiological approach was used to survey the global alterations that occurred during the logarithmic growth of Lactobacillus plantarum ST-III using FOS or glucose as the sole carbon source. A total of 363 genes were differentially transcribed; in particular, two gene clusters were induced by FOS. Gene inactivation revealed that both of the clusters participated in the metabolism of FOS, which were transported across the membrane by two phosphotransferase systems (PTSs) and were subsequently hydrolyzed by a ␤-fructofuranosidase (SacA) in the cytoplasm. Combining the measurements of the transcriptome-and membrane-related features, we discovered that the genes involved in the biosynthesis of fatty acids (FAs) were repressed in cells grown on FOS; as a result, the FA profiles were altered by shortening of the carbon chains, after which membrane fluidity increased in response to FOS transport and utilization. Furthermore, incremental production of acetate was observed in both the transcriptomic and the metabolic experiments. Our results provided new insights into gene transcription, the production of metabolites, and membrane alterations that could explain FOS metabolism in L. plantarum. Prebiotics are defined as nondigestible food ingredients that selectively stimulate the growth and activity of beneficial microbial strains residing in the host gastrointestinal tract (GIT) (1). Among the sugars that are qualified as prebiotics, fructooligosaccharides (FOS) are fructose polymers of diverse lengths that can be either derivatives of simple fructose polymers or fructose moieties attached to a sucrose molecule (2). Because of the linkage configuration, FOS are not digested in the upper GIT and have been shown in vivo to beneficially modulate the composition of the intestinal microbiota by preferentially increasing the numbers of bifidobacteria and lactobacilli (3, 4).Despite considerable commercial and research interest in the beneficial effects of FOS, the molecular basis of FOS metabolism by specific members of the intestinal microbiota has only recently been examined. In order to understand the influence of environmental conditions on genome-wide gene expression levels, wholegenome DNA microarrays have often been used to survey the gene expression patterns of strains in the presence and absence of oligosaccharides (2, 5-8). On the basis of in silico analysis of the Lactobacillus acidophilus NCFM genome sequence, Barrangou et al. (2) identified a multiple-sugar metabolism (msm) operon that was involved in the metabolism of FOS. The msm operon encodes an ATP-dependent binding cassette-type transport system and a cytoplasmic ␤-fructosidase, which mediates FOS uptake and intracellular hydrolysis. Moreover, expression of the operon was i...

No abstract