Chemistry Reference
In-Depth Information
9.2.4.2
Polyphenol properties
Polyphenols exhibit a wide range of properties, depending on their particular structures.
They include yellow, orange, red and blue pigments, as well as various compounds involved
in flavour. The major flavours associated with polyphenols are bitterness and astringency.
Other major polyphenol characteristics include their radical-scavenging capacity, which is
involved in antioxidant properties, and their ability to interact with proteins. The latter is
responsible for astringency perception (resulting from interactions of tannins with salivary
proteins), for formation of haze and precipitates in beverages.
9.2.4.3 Grape polyphenol localization
The polyphenols are distributed throughout the grape as follows:
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Grape seeds contain 60% of the grape polyphenols. This group is mainly constituted of
proanthocyanidins (monomeric flavan-3-ols, and their oligomers and polymers). The
lipid layer surrounding the seed prevents their extraction in maceration (with or without
enzymes).
Grape skin contains 30% of extractable grape polyphenols, mainly anthocyanins and resver-
atrol. Resveratrol is an antioxidant found in grapes and red wine, mulberries and peanuts.
It has been shown to have positive anti-aging benefits and has also demonstrated benefits
in diabetes, heart health, obesity and some cancers. For example, red wine of the pinot
noir variety contains 3.1 mg of resveratrol per litre. Grape pulp contains less than 10% of
total extractable grape polyphenols.
9.2.5 Wine varietal aromas and their precursors in grapes
In order to produce wines matching consumer expectations, that is with well-defined intense
aroma, winemakers must put everything in place to extract aroma and their precursors. Most
white varieties, aside from Muscats, do not contain odorous aroma but do contain heavy
hydrophilic aroma precursors. These precursors are converted into odorous aroma through
a series of chemical and enzymatic reactions naturally occurring during the winemaking
process. Once released, these molecules are responsible for the wine varietal aroma. There
are several families of molecules. Only the two main varietal aroma families are detailed
below but many more do exist including carotenoıd derivatives, dimethyl sulphide (DMS)
whose role in wine aroma is still being investigated.
9.2.5.1
Glycosylated compounds
Odourless aromatic terpenols precursors are present in the grape skin in a form linked to
sugars,
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their composition and content varies according to the grape variety. In
Vitis vinifera
,
monoterpenes precursors are bound to di-glycosides, glucose and another carbohydrate
residues such as arabinose, rhamnose or apiose. In varieties such as muscat or riesling, the
linalol, nerol and geraniol glycosylated precursors are the most abundant, their 'sugar' part
comprising ramnose-glucose for rutinosides, arabinose-glucose for arabinosides and apiose-
glucose for apiosides. Sequential hydrolysis of these sugars releases the highly odorous
terpenols. When released from their linked sugar, these compounds are responsible for a
wide range of aromatic notes from honey to fruity or flowery.
