8D Yeasts and Wine Flavour 319
Ta b l e 8 D. 2Summary of yeast metabolic interactions with grape compounds
Reaction Enzyme/metabolite Precursor(s) Product Ref.
Hydrolysis -Glucosidase Linalyl-glucoside Linalool 1
-Glucanasea Malvidin-3-O-glucoside Malvidin 2
Non-hydrolytic
cleavage
Carbon-sulfur lyase 3-(Hexan-1-ol)-L-cysteine 3-mercapto- 3a,b
hexanolReduction Alcohol Vanillin Vanillicalcohol 4
dehydrogenase Geraniol Citronellol 5
Decarboxylation Phenyl acrylic acid
decarboxylase p-Coumaric acid 4-Vinyl phenol 6
Esterification Alcohol 3-Mercapto-hexanol 3-Mercapto- 7
acyltransferase hexylacetate
Metabolite-
induced
condensation Acetaldehydea Catechin and Catechin-ethyl- 8
or malvidin-3-O-glucoside malvidin-3-glucoside
adduct reactions Pyruvic acida Malvidin-3-O-glucoside Vitisin A 9
aExtracellular reaction; References: (1) Ugliano et al. (2006); (2) Gil et al. (2005); (3a) Tominaga
et al. (1995); (3b) Howell et al. (2005); (4) Chatonnet et al. (1992a); (5) Gramatica et al. (1982); (6)
Clausen et al. (1994); (7) Swiegers et al. (2006); (8) Timberlake and Bridle (1976); (9) Fulcrand
et al. (1998)
Carbon-sulfur lyase, present inEubacterium limosumcell-free extracts, can lib-
erate volatile long-chainpolyfunctional thiols, such as 4-MMP and 3-MH, from
S-cysteine conjugates. Gene knock-out and expression studies in yeast support the
role of carbon-sulfur lyases (Tominaga et al. 1995; Howell et al. 2005; Swiegers
et al. 2007).
Grape compounds which can enter the yeast cell either by diffusion of the
undissociated lipophilic molecule or by carrier-mediated transport of the charged
molecule across the cell membrane are potentially subject to biochemical transfor-
mations by enzymatic functions. A variety of biotransformation reactions of grape
compounds that have flavour significance are known. One of the earlier studied bio-
transformations in yeast relates to the formation of volatile phenols from phenolic
acids (Thurston and Tubb 1981). Grapes contain hydroxycinnamic acids, which are
non-oxidatively decarboxylated by phenylacryl decarboxylase to the vinyl phenols
(Chatonnet et al. 1993; Clausen et al. 1994).
Reductive enzyme catalysed reactions of grape-derived compounds can result,
as they do for yeast metabolites, by interaction with dehydrogenases and reduc-
tases. Dehydrogenases, such as alcohol dehydrogenase appear to have wide speci-
ficity. Using the redox cofactor NADHto reduce acetaldehyde to ethanol, it is
likely that other compounds such as geraniol can be reduced to citronellol (Gra-
matica et al. 1982). Vanillin, a highly aromatic compound derived from oak wood,
is reduced to vanillic alcohol by yeast, accounting for the lower intensity of oak
aroma of wine fermented in oak wood compared to wine stored in oak wood after
fermentation (Chatonnet et al. 1992a). Reductive metabolism is favoured by yeast
under anaerobic conditions in order to enhance regeneration of NAD+, most likely
catalysed by alcohol dehydrogenase, to facilitate the fermentation of sugars.