Scientific publications
Diana Isabella Serrazanetti
Maurice Ndagijimana
Andrea Gianotti
Matthias A. Ehrmann
Rudi F. Vogel
Aldo Corsetti
Maria Elisabetta Guerzoni
Archivio istituzionale della ricerca (Alma Mater Studiorum Università di Bologna)
In lactobacilli stress conditions can elicit overproduction of specific compounds or a metabolic shift, which may result in flavour improvement or off-flavour generation depending on the type of food. In particular in food-systems the organoleptic perception of isovaleric acid (IVA) depends on its concentration and interactions with other metabolites. IVA is a key intermediate in the branched-chain-fatty-acids biosynthesis and its role as signalling molecule in bacteria has been proven. \nThe mechanism for IVA production/overproduction by Lactobacillus sanfranciscensis under acid stress conditions was investigated upon exposure to pH3.6 in the presence of maltose or fructose and precursors of IVA. The metabolites released were analysed by GC-MS/SPME and HPLC. For the gene-expression analyses RT- PCR was used. \nThe results obtained demonstrated that the availability of the precursors is not limiting for IVA overproduction. IVA accumulation (>100 folds than the control) was associated with acid stress. In the samples at pH3.6 the concentration of IVA was particularly elevated in the presence of fructose. The maximal IVA production was obtained at pH3.6 with fructose combined with citrate, or leucine, or a-ketoglutarate. The gene-expression analyses were calculated for gluAT, bcAT, idh, which are involved in the IVA pathway, in respect to the control. The results revealed that, in the presence of maltose, leucine or glutamate, gluAT was overexpressed. Moreover gluAT and idh were overexpressed in the presence of fructose and citrate, or a-ketoglutarate, respectively. \nInteresting relationships between flavour compounds from branched-chain-amino-acid and fatty-acids biosynthesis were observed during analysis of the metabolic fate of deuterate-leucine. The control of the metabolic mechanisms and the effects of acid stress on overproduction of flavour compounds represent an important tool to optimise fermented-food production.
