Part 4 - Glycerol synthesis

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.