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Nitrogen availability and utilization by Saccharomyces cerevisiae significantly influence fermentation kinetics and the production of volatile compounds important for wine aroma. Amino acids are the most important nitrogen source and have been classified based on how well they support growth. This study evaluated the effect of single amino acids on growth kinetics and major volatile production of two phenotypically different commercial wine yeast strains in synthetic grape must. Four growth parameters, lag phase, maximum growth rate, total biomass formation and time to complete fermentation were evaluated. In contrast with previous findings, in fermentative conditions, phenylalanine and valine supported growth well and asparagine supported it poorly. The four parameters showed good correlations for most amino acid treatments, with some notable exceptions. Single amino acid treatments resulted in the predictable production of aromatic compounds, with a linear correlation between amino acid concentration and the concentration of aromatic compounds that are directly derived from these amino acids. With the increased complexity of nitrogen sources, linear correlations were lost and aroma production became unpredictable. However, even in complex medium minor changes in amino acid concentration continued to directly impact the formation of aromatic compounds, suggesting that the relative concentration of individual amino acids remains a predictor of aromatic outputs, independently of the complexity of metabolic interactions between carbon and nitrogen metabolism and between amino acid degradation and utilization pathways.

Nitrogen availability and utilization by Saccharomyces cerevisiae significantly influence fermentation kinetics and the production of volatile compounds important for wine aroma. Amino acids are the most important nitrogen source and have been classified based on how well they support growth. This study evaluated the effect of single amino acids on growth kinetics and major volatile production of two phenotypically different commercial wine yeast strains in synthetic grapemust. Four growth parameters, lag phase, maximum growth rate, total biomass formation and time to complete fermentation were evaluated. In contrast with previous findings, in fermentative conditions, phenylalanine and valine supported growth well and asparagine supported it poorly. The four parameters showed good correlations for most amino acid treatments, with some notable exceptions. Single amino acid treatments resulted in the predictable production of aromatic compounds, with a linear correlation between amino acid concentration and the concentration of aromatic compounds that are directly derived fromthese amino acids. With the increased complexity of nitrogen sources, linear correlations were lost and aroma production became unpredictable. However, even in complex medium minor changes in amino acid concentration continued to directly impact the formation of aromatic compounds, suggesting that the relative concentration of individual amino acids remains a predictor of aromatic outputs, independently of the complexity of metabolic interactions between carbon and nitrogen metabolism and between amino acid degradation and utilization pathways. [ABSTRACT FROM AUTHOR]

Grape pomace, a by-product of the winemaking industry, is a potential source of animal feed but its nutrient and chemical composition is not sufficiently studied in South Africa. The current study investigated the changes in nutrient, amino acid and mineral composition, and in vitro digestibility of neutral detergent fiber of grape ( Vitis vinifera var. Pinotage, Shiraz and Sauvignon Blanc) pomace after three drying treatments: sun (7d), freeze (72 h) and oven drying (72 h at 60 °C). Oven-dried Shiraz had the greatest dry matter, crude protein and the least ash content ( P <  0.05), while sun-dried Shiraz had the highest neutral detergent fiber, acid detergent fiber and acid detergent lignin compared to other grape pomace treatments ( P <  0.05). Freeze-dried Sauvignon Blanc had the highest starch content, whereas freeze-dried Shiraz had the highest ether extract content relative to other grape pomace treatments ( P <  0.05). The in vitro digestibility of neutral detergent fiber at 24- and 48 h were highest in freeze-dried Pinotage followed by freeze-dried Shiraz and Sauvignon Blanc treatments ( P <  0.05). Freeze-dried Pinotage had the majority of amino acids in highest concentration, followed by freeze-dried Shiraz ( P <  0.05). Freeze- and oven-dried Pinotage exhibited higher contents of potassium, magnesium, sulfur, sodium, iron and aluminum than other treatments. Overall, sun- and oven-dried Shiraz had the higher dry matter, crude protein, neutral detergent fiber, acid detergent fiber and acid detergent lignin contents, freeze- and oven-dried Pinotage had the best mineral composition, while freeze-dried Pinotage had the best amino acid profile and in vitro digestibility of neutral detergent fiber at 24 and 48 h. [ABSTRACT FROM AUTHOR]