Part 4 - Glycerol synthesis
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v Glycerol (CH2OH-CHOH-CH2OH or C3H8O3
)
Glycerol is the third major component of dry wines (after
water and ethanol). Glycerol is a liquid with a high viscosity. When
you swirl the glass, you can see the wine legs or tears
which are caused by glycerol and ethanol. Glycerol and
ethanol contribute body and sweetness to wine. In chemistry ethanol is an
alcohol with 1 –OH group, glycerol is an alcohol with 3 –OH groups. Another
name for glycerol is 1,2,3-propanetriol (triol = 3 -OH groups).
Glycerol is typically found at concentrations of 4 -10 g/L in
dry wine and in the case of the botrysized late harvest wines, levels in excess of 20 g/L are not uncommon (Ribéreau-Gayon et al., 1998).
v Glucose à 4% GAP + 96 % DHAP
Glucose, a 6-carbon molecule, after phosphorylation, is split
into two 3-carbon molecules of different structures: glyceraldehyde-3-phosphate (GAP)
and dihydroxyacetone
phosphate (DHAP), but at a ratio of 4% to 96%. The reason will be clear if we take a
look at the structure of the yeast cell membrane.
The cell membrane is constructed of
phospholipid. A phospholipid consists of a head of glycerol 3-p with a choline
and a tail of 2 fatty acids (lipid = fat ). See image above.
In the beginning of the
fermemtation a lot of glycerol 3-p will be needed for cell growth and multiplication.
For this purpose the DHAP will be reduced to glycerol 3-p. Hence the 96 %. See image below.
Two
–OH groups (of glycerol 3-p) link up with two fatty acids and one –OH group (of the phosphate
group) links up with a choline. Thus a phospholipid is formed. See right image above.
v DHAP à GAP -->
--> --> --> ethanol
When oxygen is running out, cell growth will slow down.
DHAP will be transformed to GAP (step 5) and then to ethanol (steps 6 to 12). This
is done for one purpose: to regenerate NAD, so it can be used again for
step 6.
v DHAP à Glycerol 3-P à Glycerol
A very small amount
of DHAP may be transformed to glycerol 3-p and then to glycerol during alcoholic
fermentation.
Glycerol formation will be increased when sulphite (or sulfite) is
formed during the fermentation. Sulphite combines with acetaldehyde which then
prevents NAD from regenerating. In this case extra DHAP will be reduced in
order to compensate for the NAD deficit. The glycerol 3-p will then be
dephosphorylated to become glycerol. That is why addition of sulphite will lead
to more glycerol formation.
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P.S.
Quiz: Why do botrysized late harvest wines, like Sauternes and Vin Jaune,
have much higher glycerol content than dry wines?
Coming next month, in Part 5, we’ll see what fatty acids
are, their formation and their influence on wine.
