8D Yeasts and Wine Flavour 317
8D.2.1 Nutrients
Saccharomycesspecies can grow on a minimal range of organic and inorganic
nutrients, with hexose sugars the preferred source of carbon and energy under
anaerobic conditions. Sources of nitrogen, phosphate, sulfate, various minerals K+,
Mg^2 +and Zn^2 +, and trace elements, provide the necessary nutrients for growth and
reproduction. SomeSaccharomyces cerevisiaestrains require several vitamins, typ-
ically biotin. In highly clarified (lipid-free) fermentations,Saccharomycesspecies
produce limited growth in the complete absence of oxygen, whereas most non-
Saccharomycesrequire oxygen. Exposure of fermentation to oxygen is normally
limited, to prevent unwanted oxidation reactions. However, small additions stimu-
late growth and fermentation by balancingmetabolic redox and increasing tolerance
to the inhibitory effects of ethanol by allowing the synthesis of sterols and unsatu-
rated fatty acids that improve membrane function (Rosenfeld et al. 2003). Oxygen
and exogenous lipids (grape solids) affect metabolism and, consequently, modulate
wine flavour.
Grape must typically contains sufficient essential nutrients to enable the ade-
quate growth of yeast, which is necessary to complete the fermentation of sugars in
an essentially oxygen-free environment. Supplementation experiments reveal that
most nutrients are not present in optimal amounts; sugars are a major exception.
Glucose and fructose, the predominant sugars, are fermented to ethanol and CO 2
to provide energy and carbon compounds for yeast growth. Sugar concentration
largely determines wine ethanol content, which can range from 8 vol.% to> 15
vol.%. The catabolism of sugars leads to formation of the non-volatile compounds,
polyols and carboxylic acids, and the volatile compounds, higher alcohols, ethyl
and acetate esters, carbonyls, volatile fatty acids, and volatile sulfur compounds.
Polymers, mainly mannoproteins, are also produced in small amounts (Table 8D.1)
(Swiegers et al. 2005).
Yeasts assimilate a variety of nitrogenous compounds, predominantly primary
amino acids, ammonium ions and small peptides. The types and concentrations of
extracellular nitrogenous compounds regulate yeast growth and metabolism, includ-
ing sugar, nitrogen and sulfur pathways, and therefore affect the production of non-
volatile and volatile metabolites, many of which have sensory implications. Nitro-
gen is a limiting nutrient in many wine regions, and consequently restricts growth,
and fermentation rate and completion. Inadequate nitrogen is often associated with
high concentrations of higher (fusel) alcohols and increased risk of sulfidic off-
flavours (principally H 2 S and mercaptans), whereas excessive nitrogen can lead to
estery off-flavour (Bell & Henschke 2005).
Because grape must contain little organic sulfur, in the form of S-amino acids
and glutathione, yeast assimilates sulfate, and sulfite when added, for the synthesis
of organic sulfur compounds. Sulfur metabolism is important in winemaking since,
depending on the yeast strain and nutrient composition, it is the source of mostly
unpleasant compounds, including H 2 S and mercaptans (Rauhut 1993).
Phosphate is an essential nutrient for energy metabolism and consequently reg-
ulates many metabolic pathways; however, no information is available on how