A General Look At Yeast

If you enjoy wine and other fermented beverages, be thankful for the existence of yeast. It is yeast, by consuming sugar, that produce the alcohol in the wine. Of course, yeast also produces carbon dioxide (CO2 which we don’t want too much of, generally speaking, in our wine. Sparkling wines are an exception to this. Beer is also a fermented beverage where it is generally desirable to have an abundance of carbon dioxide present.

Traditionally, wine was made by chance in that it was left out to ferment and the winemaker would hope that whatever yeast were in the air would produce a good wine. You can imagine that leaving this up to chance meant that wines were often hard to predict how they would turn out or what percent of alcohol it would have. Today, most wine fermentation yeast is provided to the winemaking industry by four companies and each have their own standard varities. Using the same strain of commercially produced yeast ensures consistency from batch to batch.

Yeast feed on the sugar in the juice. They turn one molecule of sugar into two carbon dioxide molecules and two ethanol alcohol molecules. There are small amounts of other by-products as well including a few different acids that are produced in the fermentation process. Of course, they also convert energy into their own growth and reproduction.

All yeast do the same thing, regardless of what strain they are. Even bread yeast produce alcohol, and there are small amounts of alcohol in bread dough that has had yeast act upon it.

One of the major differences between yeast strains is alcohol tolerance. While some strains will die at low percentages of alcohol, other strains can tolerate alcohol percent of about 21%. Brewers Yeast can only tolerate 5% alcohol before it starts to die off.

Other differences in yeast strains include “flocculation,” the process by which they clump together and fall to the bottom of the fermentation vessel as sediment. Some strains of yeast, when the fermentation process completes will clump and fall at a faster rate than others. The speed in which sugar is converted to alcohol and CO2 is also a factor in different yeast strains. Slower fermenting yeasts are often desired for certain qualities of taste and aroma in some fruit or grape varieties, while others turn out better with faster fermentations.

Previously I’ve pointed out that John Iverson, reknowned expert on home winemaking doubted that yeast had much to do with the final taste of wine. He wrote in his book, Home Winemaking Step By Step,

“Much has been written about the different flavor characteristics that various strains of yeasts impart. Differences undoubtedly exist immediately after fermentation, and they matter greatly to a commercial winery not wanting to tie up its fermentation capacity too long and wanting its wines to be marketable as soon as possible. But the differences are minor from the home winemaker’s viewpoint.”

His view appears not to be accepted by many others and indeed, at least one study concludes that

“The occurrence of sulphurous off-flavours strongly depends on the yeast strain, the fermentation conditions, elemental sulfur residues and the nitrogen composition in grape musts.” (Source).

Most of the wine kits I have purchased have come with Lalvin EC1118 strain of yeast. It’s a low foaming yeast with an alcohol tolerance up to 18%, and is often the yeast of choice for a stuck fermentation. Considered to be mostly neutral on flavours, it’s a strain that many winemakers use exclusively. As well, it has a wide temperature range that it can grow in – between 45-95F.

For the home winemaker that wishes to experiment with their own fruit and juices, having some knowledge of the different characteristics of yeast strains can be helpful. Of course, nothing beats experimentation and comparison.

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