336 M. Ugliano and P.A. Henschke
role in wine quality (Eglinton and Henschke 1999). Concentrations vary widely
(<0.2 to>2.0 g/L, expressed as acetic acid) and depend on wine type. Dry white
wines typically have lowest concentrations whereas sweet whites, especially when
prepared fromBotrytis-affected grapes, tend to have the highest concentrations.
Wine law controls the maximum concentration of volatile acids allowed in wine,
which in the European Community cannot exceed 1.5 g/L, expressed as acetic acid.
The flavour threshold for acetic acid depends on wine type and style, and ranges
from 0.4 to 1.1 g/L (Dubois 1994). At threshold concentration it provides warmth
to the palate and, as the concentration increases, it imparts a sourness/sharpness to
the palate and a vinegary odour at higher concentration. As the fatty chain length
increases, volatility decreases and the odour changes from sour to rancid and cheese
(Francis and Newton 2005). Sensory studies show that hexanoic, octanoic, and
decanoic acids can contribute to the aroma of some white wines (Smyth et al. 2005).
The branched-chain fatty acids can also contribute to the fermentation bouquet
of wine, with the concentration of 2-methylpropanoic acid typically exceeding its
odour threshold (Francis and Newton 2005).
8D.4.3.2 Metabolism
Acetic acid formation fulfils several metabolic roles including providing a precur-
sor for the synthesis of acetyl-CoA and aredox sink for anabolic and physiolog-
ical stress reactions. Acetate is formed by the action of aldehyde dehydrogenases
from acetaldehyde, which is derived by thedecarboxylation of pyruvate (Fig 8D.6).
Five genes (ALD2-6), encoding different isozymes, have been described and par-
tially characterised inSaccharomyces cerevisiae, but their physiological functions
under wine fermentation conditions are stillsomewhat unclear (Eglinton et al. 2002;
Meaden et al. 1997; Navarro-Avino et al. 1999; Remize et al. 2000; Saint-Prix
et al. 2004; Pigeau and Inglis 2007). Under dry wine fermentation conditions
(20% sugar; anaerobic conditions) Ald6p, a cytosolic NADP+-dependent aldehyde
dehydrogenase, is the major isozyme that produces acetic acid and provides redox
balance. Lesser contributions are made by the mitochondrial NADP+-dependent
Ald5p isozyme, and depending on the strain and conditions, also the mitochon-
drial NAD(P)+-dependent Ald4p isozyme. Under high osmotic stress conditions,
as imposed by Eiswein and Icewine fermentation,ALD3,encoding the cytosolic
NAD+-dependent aldehyde dehydrogenase, is differentially expressed, suggesting
a role in the higher production of acetic acid in these wines over and above that
produced by conventional wine fermentation. In this case,ALD3might provide a
redox role to balance the extra demand for NADH generated by the concomitantly
higher production of glycerol, which is induced in response to the high osmotic
stress imposed by very high sugar concentrations.
Straight-chain fatty acids (C 4 –C 12 ) are by-products of saturated fatty acid
metabolism. Malonyl-CoA is first synthesised from acetyl-CoA by acetyl-CoA car-
boxylase. Subsequent reactions are catalysed by the synthase enzyme complex,
which increases chain length sequentially by two C units. C 16 and C 18 fatty acids