A Conversation for An Introduction to Winemaking
OPAL_ideas Started conversation Apr 5, 2004
It is always a pleasure to read about winemaking in different parts of the world. Thank you for the article.
By the way, in southern Canada, where I am, we have a very active wine industry. (What, you say? Canada? Actually, I'm at the same latitude as northern California, and WAY further south than London.)
It may be instructive for those winemakers who want to move to a more advanced level, to consider the difference stages of fermentation. Knowing them, a problem batch can be saved.
The primary fermentation is the stage during which the yeast colony grows. As it happily eats through the sugar, it grows thickly throughout the container. Like fish, yeast come in "bottom-feeders", "top-feeders" and the regular mid-level fellows. A "stuck" fermentation at this point can be due to environmental conditions (as it usually is), or it may be a problem with the yeast colony, itself. Study the container, if you used a glass 25 litre carboy, or the larger demijohn, then you can see if the colony has started, or if the yeast has just formed a film on top. Checking the specific gravity, of course, will show if the colony has begun sufficiently to produce that byproduct, alcohol. A stuck fermentation is most usually due to incorrect temperature, but it may be the yeast, so adding a more robust yeast can get it going. However, it may be the yeast just doesn't like the sugar content - they are very particular little fellows.
The secondary fermentation is when the colony has fully occupied the must, and instead of growth mode, the colony is content to just lay back and eat sugar. A stuck fermentation at this time will usually not be helped the addition of more yeast. Yeast does not, surprisingly, tolerate alcohol very well, so this at stage the yeast is very susceptible to environmental changes. To them, alcohol is a toxic byproduct which can reduce their ability to tolerate changes. Depending on the specific gravity reading, and how cloudy the must has become, (and providing the temperature has remained constant and there have been no loud thunderstorms or other loud noises to frighten the poor things), it may be that the yeast can be coaxed into further work by siphoning out several litres into several smaller jugs, then adding fresh yeast into them. After it starts, pour one of the jugs back into the original must, stirring it up. Leave for a few days, then add the next jug. Do this until the must is topped back up. This works better for reductive (fruity) wines better than for oxidative wines (generally the more robust wines).
There's more! Not everybody knows about the tertiary fermentation stage. This is when the yeast has all died and gone to... the bottom. The alcohol conversion process still continues, feeding on whatever sugar is left. At this point, if the specific gravity doesn't show that you have 11-1/2 to 12% alcohol, and if you want to age the filtered wine in glass, you must use at least a 1 micron filter to clean out the sugar-converters. Yes, that means pulling out some flavour, but you run the risk of further fermentation bursting the glass. Or you can use champagne bottles.
No wonder it's taken us at least 6 thousand years to get this thing right!
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