RESUMO
Higher alcohol formation by yeast is of great interest in the field of fermented beverages. Among them, medium-chain alcohols impact greatly the final flavour profile of alcoholic beverages, even at low concentrations. It is widely accepted that amino acid metabolism in yeasts directly influences higher alcohol formation, especially the catabolism of aromatic and branched-chain amino acids. However, it is not clear how the availability of oxygen and glucose metabolism influence the final higher alcohol levels in fermented beverages. Here, using an industrial Brazilian cachaça strain of Saccharomyces cerevisiae, we investigated the effect of oxygen limitation and glucose pulse on the accumulation of higher alcohol compounds in batch cultures, with glucose (20 g/l) and leucine (9.8 g/l) as the carbon and nitrogen sources, respectively. Fermentative metabolites and CO2 /O2 balance were analysed in order to correlate the results with physiological data. Our results show that the accumulation of isoamyl alcohol by yeast is independent of oxygen availability in the medium, depending mainly on leucine, α-keto-acids and/or NADH pools. High-availability leucine experiments showed a novel and unexpected accumulation of isobutanol, active amyl alcohol and 2-phenylethanol, which could be attributed to de novo biosynthesis of valine, isoleucine and phenylalanine and subsequent outflow of these pathways. In carbon-exhausted conditions, our results also describe, for the first time, the metabolization of isoamyl alcohol, isobutanol, active amyl alcohol but not of 2-phenylethanol, by yeast strains in stationary phase, suggesting a role for these higher alcohols as carbon source for cell maintenance and/or redox homeostasis during this physiological phase.
Assuntos
Bebidas Alcoólicas/microbiologia , Álcoois/metabolismo , Meios de Cultura/metabolismo , Aromatizantes/metabolismo , Glucose/metabolismo , Leucina/metabolismo , Oxigênio/metabolismo , Saccharomyces cerevisiae/metabolismo , Bebidas Alcoólicas/análise , Álcoois/análise , Brasil , Fermentação , Aromatizantes/análise , Nitrogênio/metabolismo , Proteínas de Saccharomyces cerevisiae/genética , Proteínas de Saccharomyces cerevisiae/metabolismoRESUMO
This study provides the first attempt to analyse the influence of ammonium supplements on sugar-cane juice fermentation and the flavour profile in a cachaça industrial process. The objective was to find a relationship between higher alcohol/ester content and the transcription levels of the main genes involved in production of these compounds under cachaça fermentation. Sugar-cane juice with a low amount of assimilable nitrogen (81 mg N/L), was further supplemented with mid-range or high concentrations of ammonium sulfate. Overall, higher alcohol production was reduced by ammonium supplementation, and this can be correlated with a general downregulation of genes encoding decarboxylases and dehydrogenases of the Ehrlich pathway. The production of acetate esters was enhanced by mid-range ammonium supplementation and the production of acyl esters by high ammonium supplementation. The acyl esters could be correlated with expression of alcohol acyl-transferase EEB1 and the acyl esterase IAH1.