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The dark side of yeasts: Their glucophilic nature
By Louis Nel of
www.smalloak.com
As winemakers we love our yeast and place great trust in our partners in the winemaking process. It is only when we expect it the least that these yeasties can turn nasty.
One of the little known properties of yeasts is their glucophilic nature. This is their tendency to preferentially utilise glucose above fructose. Stated in reverse the glucophilic nature of yeast indicates its inability to ferment fructose. This situation may occur more regularly than we realise and is often the cause of a stuck or sluggish fermentation.
This nature of yeasts to ferment glucose by preference is even exploited by winemakers when making sweet wine. When making Sauternes wines the fermentation stops amongst other reasons because the glucose is depleted (Falke & Nordstrom 2003).
Not fructose again!
Under stress conditions all yeast preferentially metabolise glucose. Gafner & Shütz (1995) found that a stuck fermentation is caused when the ratio of fructose to glucose is higher than ten. This situation only presents itself once 80% of the sugars have been fermented.
This problem can be diagnosed by the addition of glucose to a small amount of the fermentation (Zoecklein 2002). In my own experience addition of 1 gram/litre of glucose to a representative sample of the wine has very little effect and the most effective way is to measure the glucose and fructose ratio at a laboratory that can carry out such analysis.
Most grapes contain equal amounts of glucose and fructose, but Chardonnay for one is known to contain more fructose than glucose (SDWS, 2003). Very mature grapes can often have higher proportions of fructose (Jackson, 1994. Rankine,1996).
What to do?
The natural reaction for any winemaker worth his salt would be to reach for the industry standard for all stuck fermentations - that super, kick ass, takes-no-prisoners yeast Saccharomyces bayanus. The fact is that in this situation it would be the worst possible choice. Saccharomyces bayanus yeasts are three to five times more glucophilic than Saccharomyces cerevisiae (The Ohio Grape Web, 1999).
It is common practice for some winemakers to reinoculate Pinotage must after pressing with a Saccharomyces bayanus yeast. But if the possibility of stress presents itself then the yeast can actually fair worse than a Saccharomyces cerevisiae yeast.
Gafner & Shütz (1995) found that reinoculation of a stuck ferment which was caused by a glucose imbalance, was far more effective with a Saccharomyces cerevisiae strain than with a Saccharomyces bayanus strain. Saccharomyces cerevisiae yeasts are not created equal and differ in their ability to handle stress factors. Therefore if the must becomes stuck due to the yeast preferentially metabolising glucose, a strong fermenting cerevisiae yeast is recommended.
May we never stop learning
Through many years of winemaking we prepare ourselves for the worst and when it happens we are sometimes still at a loss. I must thank Loftie Ellis and Michael Liedtke for making me aware of this phenomenon.
References:
Falke, J & Nordstrom, K., 2003,
http://spot.colorado.edu/~schmidts/Teach...-2000.html
Gafner & Shütz, 1995, Impact of glucose-fructose ratio on stuck fermentations: Practical experiences to restart stuck fermentations, Presented at the Lallemand symposium, Geisenheim
Jackson, S.J., 1994, Wine Science- Principles and Applications, p182, Academic Press, Inc
Rankine, B., 1996, Making good wine, p26, MacMillan & Co. London, San Diego, California
SDAWS, 2003,
http://www.sdaws.org/grapes.htm
The Ohio Grape Web, 1999,
http://www.oardc.ohio-state.edu/grapeweb...an0399.htm
Zoecklein, B., 2002,
http://www.fst.vt.edu/Zoecklein/july-aug02.html
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