8D Yeasts and Wine Flavour 321
Hashizume and Samuta 1999). 3-Isobutyl-2-methoxypyrazine (IBMP) is the dom-
inant compound and is located in the grape skin, and being water soluble is
rapidly extracted into the must. Although there is unproven evidence that yeast
can metabolise this compound, sensory studies suggest that its aroma impact is
dependent on the presence of other compounds that partially mask its aroma, such
as phenyl ethanol and its ester, which are produced in higher amounts bySaccha-
romyces bayanus(Treloar and Howell 2006; Eglinton and Henschke, unpublished).
8D.3 Yeast Formation of Major Wine Compounds
8D.3.1 Ethanol
8D.3.1.1 Significance
Ethanol concentration of dry white and red wines can vary from 8 vol.% to 16
vol.% and signifies the style of wine and the degree of maturity of the grapes from
which the wine was prepared. The latter aspect indicates the extent to which a wine
might exhibit greener, ripe or over-ripe flavours. Ethanol content does in itself affect
the chemical, physical and sensory properties of wine. Ethanol content can impact
on perceived (alcohol) hotness, body and perceived viscosity, and lesser effects on
sweetness, acidity, aroma, flavour intensity and textural properties have been noted
(Gawel et al. 2007a, b). Ethanol concentration also affects the perception of wine
aroma compounds but not their partition coefficients (Guth 1997). Despite lower
perceived aroma intensities due to higherodour thresholds, thealcohol content of
wines has progressively increased by 1–1.5 vol.% over recent decades, especially in
New World wines (Godden and Gishen 2005). This trend has exploited the higher
flavour intensity of rich, ripe flavours that grapes harvested at higher sugar maturities
can produce.
8D.3.1.2 Metabolism
Under winemaking conditions, ethanol and CO 2 are the major products of alcoholic
fermentation of grapes sugars bySaccharomyces cerevisiae. Even when oxygen is
present, the high sugar concentration of grape must locks Crab-tree positive yeast
into the fermentative mode of metabolism. Some sugar-derived carbon is however
consumed in metabolic reactions required for generating ATP energy and biosyn-
thesis of cell mass (Verduyn et al. 1990a). Other quantitatively important metabo-
lites produced during fermentation includepolyols, especially glycerol and 2,3-
butanediol, and organic acids, such as succinic, keto and acetic acids. The numerous
minor metabolites (higher alcohols, esters, volatile fatty acids, carbonyls), many of
which contribute flavour-active properties, represent<1% of sugar carbon. Due to
these metabolic losses, the complete fermentation of hexose sugars only yields 94–
96% of the theoretical maximum ethanol yield. A widely adopted conversion figure