372 M. Ugliano and P.A. Henschke
Another poorly explored area concerns grape and wine phenolics. Although yeast
interaction with phenolics was recognizedseveral decades ago, again progress has
depended on development of suitable analytical techniques. Some understanding
of the interactions that yeast play in colour development of wines has emerged
(Hayasaka et al. 2007; Morata et al. 2006) but almost nothing is known about inter-
actions with tannins, which are responsiblefor flavour, astringency and bitterness
(Eglinton et al. 2005). The complex polymeric nature of phenolic compounds, and
the need for sensory evaluation techniques, has slowed progress on these fundamen-
tal charactersistics of wine.
The published literature on the effects of microbial activities on wine chemical
composition is now considerable. Understanding the significance of wine chemistry
is, however, heavily dependent on complex analytical strategies which combine
extensive chemical characterization and sensory descriptive analysis. However, sen-
sory analysis is extremely resource-intense, requiring many hours of panelists’ time.
This prevents widespread application of these powerful analytical tools. Advanced
statistical techniques have been developed that are closing the gap between chemical
and sensory techniques. Such techniques allow the development of models, which
should ultimately provide a sensory description based on chemical data. For exam-
ple, Smyth et al. (2005) have developed reasonable models which can reveal the
most likely compounds that relate to particular attributes that characterise the overall
sensory profile of a wine. For wines such as Riesling and Chardonnay, the impor-
tance of several yeast volatile compoundshas been indicated. Such information will
allow yeast studies to target key compounds better rather than just those that are
convenient to measure.
Discovery of several yeast-catalysedtransformation reactions of grape com-
pounds, often leading to enhanced flavour character and intensity, has opened
another area for research. At this time most effort has been focused on glyco-
sides and cysteinylated precursors. Glycosides were previously thought to be largely
hydrolysed by the mild acidic conditions of grape musts and wine, with yeasts play-
ing no significant role in the release of their volatile moiety. Although the mech-
anism of yeast catalysed hydrolysis is still not yet resolved, clear evidence now
exists that yeasts are the driving force behind the release of glycosidically-bound
aroma compounds during winemaking. Furthermore, some released aglycons can be
transformed by yeast enzymes, resulting in a change to their odour intensity and/or
quality. For example, geraniol can be reduced by yeast to citronellol, changing the
aroma character of wine (Ugliano et al. 2006; Loscos et al. 2007). The liberation
of 3-mercaptohexanol and its subsequent esterification to the more powerful odor-
ant 3-mercaptohexylacetate, which gives a tropical, passion-fruit aroma, is another
example of the key role of yeast transformation reactions in the appearance of highly
distinctive aroma characters during fermentation (Swiegers et al. 2006).
Selection of new flavour strains is dependent on the chemical nature of the odor-
ant of interest, when it is known. Flavour compounds that have convenient methods
for screening, such as H 2 S, which can be assessed by plating onto agar containing
bismuth salts, allow rapid and efficient selection. On the other hand, screening yeasts
for odorants for which a rapid screen is not available requires individual fermenta-