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Novel wine yeast with ARO4 and TYR1 mutations that overproduce 'floral' aroma compounds 2-phenylethanol and 2-phenylethyl acetate
It is well established that the choice of yeast used to perform wine fermentation significantly influences the sensory attributes of wines; different yeast species and strains impart different profiles of esters, volatile fatty acids, higher alcohols, and volatile sulphur compounds. Indeed, choice of yeast remains one of the simplest means by which winemakers can modulate the sensory characteristics of wine. Consequently, there are more than 100 commercially available Saccharomyces cerevisiae wine yeast strains available, mostly derived by isolation from vineyards and successful fermentations. Nevertheless, some desirable characteristics such as 'rose' and 'floral' aromas in wine are not present amongst existing strains. Such aromas can be conferred from the higher alcohol 2-phenylethanol (2-PE) and its acetate ester, 2-phenylethyl acetate (2-PEA). These metabolites of the aromatic amino acid phenylalanine are present at concentrations below their aroma detection thresholds in many wines, so their contribution to wine style is often minimal. To increase the concentration of phenylalanine metabolites, natural and chemically mutagenised populations of a S. cerevisiae wine strain, AWRI796, were exposed to toxic analogues of phenylalanine. Resistant colonies were found to overproduce 2-PE and 2-PEA by up to 20-fold, which resulted in a significant increase in 'floral' aroma in pilot-scale white wines. Genome sequencing of these newly developed strains revealed mutations in two genes of the biosynthetic pathway of aromatic amino acids, ARO4 and TYR1, which were demonstrated to be responsible for the 2-PE overproduction phenotype
Properties of Wine Polymeric Pigments Formed from Anthocyanin and Tannins Differing in Size Distribution and Subunit Composition
To
explore the effect of tannin composition on pigment formation,
model ferments of purified 3-<i>O</i>-monoglucoside anthocyanins
(ACN)
were conducted either alone or in the presence of two different tannins.
Tannins were isolated from grape seeds (Sd) or skins (Sk) following
exhaustive extraction in 70% v/v acetone. The Sd and Sk tannin fractions
had a mean degree of polymerization of 5.2 and 25.6, respectively.
The Sd fraction was highly galloylated, at 22%, but galloylation was
<2% in the Sk fraction. The Sk fraction was distinguished by a
high proportion of prodelphinidin, at 58%. After a 6 month aging period,
polymeric pigments were quantified and their color properties determined
following isolation by solid-phase extraction. Wine color and polymeric
pigment were highest in the treatment containing ACN+Sd and similar
in the ACN+Sk and ACN treatments. The same trend between treatments
was observed for total and polymeric nonbleachable pigments. Only
minor changes in tannin subunit composition were found following ACN
incorporation, but the size distribution of polymeric pigments determined
by gel permeation chromatography decreased, in particular for the
ACN+Sk treatment. Color incorporation in the higher molecular mass
range was lower for ACN+Sk wines than for ACN+Sd wines. Compositional
differences between the two tannin fractions may therefore limit the
incorporation of ACNs in the colored form. The results suggest that
in the ACN+Sk and ACN treatments, the formation of lower molecular
mass oligomeric pigments was favored. In polymeric pigments derived
from ACNs, the presence of ethyl- and vinyl-linked ACNs to the level
of trimers was identified using mass spectrometry