Exogenous Valine Reduces Conversion of Leucine to 3-Methyl-1-Butanol in Saccharomyces cerevisiae

Dietary Compounds Influencing the Sensorial, Volatile and Phytochemical Properties of Bovine Milk

The main aim of this study was to evaluate the volatile profile, sensory perception, and phytochemical content of bovine milk produced from cows fed on three distinct feeding systems, namely grass (GRS), grass/clover (CLV), and total mixed ration (TMR). Previous studies have identified that feed type can influence the sensory perception of milk directly via the transfer of volatile aromatic compounds, or indirectly by the transfer of non-volatile substrates that act as precursors for volatile compounds. In the present study, significant differences were observed in the phytochemical profile of the different feed and milk samples. The isoflavone formonoetin was significantly higher in CLV feed samples, but higher in raw GRS milk, while other smaller isoflavones, such as daidzein, genistein, and apigenin were highly correlated to raw CLV milk. This suggests that changes in isoflavone content and concentration in milk relate to diet, but also to metabolism in the rumen. This study also found unique potential volatile biomarkers in milk (dimethyl sulfone) related to feeding systems, or significant differences in the concentration of others (toluene, p-cresol, ethyl and methyl esters) based on feeding systems. TMR milk scored significantly higher for hay-like flavor and white color, while GRS and CLV milk scored significantly higher for a creamy color. Milk samples were easily distinguishable by their volatile profile based on feeding system, storage time, and pasteurization.

The effects of apple variety, ripening stage, and yeast strain on the volatile composition of apple cider

This work examined the degree of influence of apple variety, apple ripening stage, and yeast strain on the volatile composition of apple cider. Four apple varieties grown in Estonia were selected for the study – Antei, Melba, Kulikovskoye, and Orlovski Sinap. The must from the apples at various stages of ripening (unripe, ripe, overripe) underwent alcoholic fermentation using commercially available yeast strains. Gas chromatography - mass spectrometry was employed to assess the differences in volatile composition between the samples. Out of the variables analyzed in this work, apple variety turned out to be the primary attribute influencing the quality and aroma properties of apple cider. The effect of yeast strain and the maturity of the fruit was variety-specific where the volatile profiles of ciders made with Melba variety were the least influenced by the ripening stage of apples and yeast strains used for the fermentation.

Characterization and Synergistic Effect of Antifungal Volatile Organic Compounds Emitted by the Geotrichum candidum PF005, an Endophytic Fungus from the Eggplant

Plant-associated endophytes are recognized as sources of novel bioactive molecules having diverse applications. In this study, an endophytic yeast-like fungal strain was isolated from the fruit of eggplant (Solanum melongena) and identified as Geotrichum candidum through phenotypic and genotypic characterizations. This endophytic G. candidum isolate PF005 was found to emit fruity scented volatiles. The compositional profiling of volatile organic compounds (VOCs) revealed the presence of 3-methyl-1-butanol, ethyl 3-methylbutanoate, 2-phenylethanol, isopentyl acetate, naphthalene, and isobutyl acetate in significant proportion when analyzed on a time-course basis. The VOCs from G. candidum exhibited significant mycelial growth inhibition (54%) of phytopathogen Rhizoctonia solani, besides having mild antifungal activity against a few other fungi. The source of carbon as a nutrient was found to be an important factor for the enhanced biosynthesis of antifungal VOCs. The antifungal activity against phytopathogen R. solani was improved up to 91% by feeding the G. candidum with selective precursors of alcohol and ester volatiles. Furthermore, the antifungal activity of VOCs was enhanced synergistically up to 92% upon the exogenous addition of naphthalene (1.0 mg/plate). This is the first report of G. candidum as an endophyte emitting antifungal VOCs, wherein 2-penylethanol, isopentyl acetate, and naphthalene were identified as important contributors to its antifungal activity. Possible utilization of G. candidum PF005 as a mycofumigant has been discussed based upon its antifungal activity and the qualified presumption of safety status.

Growth of the salt-tolerant yeast Zygosaccharomyces rouxii in microtiter plates: effects of NaCl, pH and temperature on growth and fusel alcohol production from branched-chain amino acids

Zygosaccharomyces rouxii, a salt-tolerant yeast isolated from the soy sauce process, produces fusel alcohols (isoamyl alcohol, active amyl alcohol and isobutyl alcohol) from branched-chain amino acids (leucine, isoleucine and valine, respectively) via the Ehrlich pathway. Using a high-throughput screening approach in microtiter plates, we have studied the effects of pH, temperature and salt concentration on growth of Z. rouxii and formation of fusel alcohols from branched-chain amino acids. Application of minor variations in pH (range 3-7) and NaCl concentrations (range 0-20%) per microtiter plate well allowed a rapid and detailed evaluation of fermentation conditions for optimal growth and metabolite production. Conditions yielding the highest cell densities were not optimal for fusel alcohol production. Maximal fusel alcohol production occurred at low pH (3.0-4.0) and low NaCl concentrations (0-4%) at 25 degrees C. At pH 4.0-6.0 and 0-18% NaCl, considerable amounts of alpha-keto acids, the deaminated products from the branched-chain amino acids, accumulated extracellularly. The highest cell densities were obtained in plates incubated at 30 degrees C. The results obtained under various incubation conditions with (deep-well) microtiter plates were validated in Erlenmeyer shake-flask cultures.

Factors influencing leucine catabolism by a strain of Staphylococcus carnosus

The metabolism of leucine by resting cells of Staphylococcus carnosus 833 was studied according to three physicochemical factors: preculture condition (defined medium; complex medium), nitrate concentration (0% and 0.03%) and stirring condition (static or shaking). A factorial design was set up to test the effects of these factors, each at two levels. The results showed that resting cells of S. carnosus 833 produced 3-methyl butanal, 3-methyl butanol and 3-methyl butanoic acid from leucine. Whatever the incubation conditions, there was greater quantity of 3-methyl butanoic acid than 3-methyl butanal and 3-methyl butanol. The preculture and incubation conditions influenced the level of production of the 3 metabolites. The highest overall production of the 3 metabolites was observed when cells were incubated without nitrate in the reaction mixture. 3-methylbutanoic acid production was enhanced when S. carnosus 833 was precultivated in complex medium. 3-methylbutanal was only detected when cells were precultivated in defined medium. Stirring condition had no effect on leucine catabolism of S. carnosus 833.

Pyruvate decarboxylase catalyzes decarboxylation of branched-chain 2-oxo acids but is not essential for fusel alcohol production by Saccharomyces cerevisiae

The fusel alcohols 3-methyl-1-butanol, 2-methyl-1-butanol, and 2-methyl-propanol are important flavor compounds in yeast-derived food products and beverages. The formation of these compounds from branched-chain amino acids is generally assumed to occur via the Ehrlich pathway, which involves the concerted action of a branched-chain transaminase, a decarboxylase, and an alcohol dehydrogenase. Partially purified preparations of pyruvate decarboxylase (EC 4.1.1.1) have been reported to catalyze the decarboxylation of the branched-chain 2-oxo acids formed upon transamination of leucine, isoleucine, and valine. Indeed, in a coupled enzymatic assay with horse liver alcohol dehydrogenase, cell extracts of a wild-type Saccharomyces cerevisiae strain exhibited significant decarboxylation rates with these branched-chain 2-oxo acids. Decarboxylation of branched-chain 2-oxo acids was not detectable in cell extracts of an isogenic strain in which all three PDC genes had been disrupted. Experiments with cell extracts from S. cerevisiae mutants expressing a single PDC gene demonstrated that both PDC1- and PDC5-encoded isoenzymes can decarboxylate branched-chain 2-oxo acids. To investigate whether pyruvate decarboxylase is essential for fusel alcohol production by whole cells, wild-type S. cerevisiae and an isogenic pyruvate decarboxylase-negative strain were grown on ethanol with a mixture of leucine, isoleucine, and valine as the nitrogen source. Surprisingly, the three corresponding fusel alcohols were produced in both strains. This result proves that decarboxylation of branched-chain 2-oxo acids via pyruvate decarboxylase is not an essential step in fusel alcohol production.