Saturday, March 29, 2008
First, realize that for many drinkers, glycerine (glycerol) registers differently from residual pentoses and hexoses - it can be confused with fruit flavor, heightening the midpalate experience. Normally yeast produce glycerol in insignificant concentrations, but there are several known ways of inducing higher glycerol yields:
1. Increase the sugar. Yeast produce glycerol in response to hyper-osmotic environments, as a survival mechanism. However this is useless in winemaking, since the brix at harvest must be within a specific range.
2. Increase the pH to 7 or higher. However wine must be acidic (pH 3-4) to be palatable.
3. Increase the sugar and use special osmo-tolerant strains, that have an amplified glycerol response. Not feasible for the same reason as (1.). Also, using different strains of yeast alters the flavors of the wine.
4. Temporarily retard fermentation by fixing acetaldehyde with SO2. This causes a "jam" in the pathway, using Le Chatelier's principle to induce a glycerol "side-shunt" production pathway. The fermentation will resume naturally as the acetaldehyde is freed.
This last strategy struck me as especially innovative when applied to winemaking, and is used by at least one well-known vintner. To elaborate, I'm going to assume some knowledge of the glycolysis/fermentation pathway.
In the fourth stage of glycolysis, Fructose-1,6-bisphosphate is split into G3P and dihydroxyacetone-phosphate, the latter of which is normally isomerized enzymatically into a second G3P. The next stage (phosphorylation of G3P) yields 2NADH as a by-product. At the end of glycolysis, pyruvate is decarboxylated into acetaldehyde, then fermented, using up NADH which would (if aerobic) go towards oxidative phosphorylation. If acetaldehyde is blocked by SO2, there is a buildup of NADH, which "reaches back" (Le Chatelier's principle) and makes the phosphorylation of G3P unfavorable. As a result, more dihydroxyacetone-phosphate goes down the alternate glycerol-synthesis pathway (which uses up NADH, restoring balance). There may be another glycerol pathway, but I would hazard an educated guess that it's simply the G3P isomerization in reverse.
Since both alcohol and glycerol are produced from a fixed starting glucose concentration, by producing more glycerol, you sacrifice alcohol content - a major part of the wine's "body." However a higher glycerol concentration allows the yeast greater osmotic tolerance, so if you harvest really, really high brix grapes, the SO2 will cause more glycerol production, but alcohol production will still be high. In this way you get full-bodied, up to 16% wines that also have enough glycerol to fill the midpalate.
The wines I've had that used this method were very sweet, fruity, and alcoholic. I hate them, personally, but I think this is a very innovative biochemical strategy that will please many people's palates.