Wine and vinegar making

Written by Dr. T. M. Wassenaar Tuesday, 23 December 2008

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Wine is fermented grape juice. The microorganism that converts the sugars in the juice to alcohol, and at the same time produces the desired flavors, is yeast (Sachharomyces cerevisiae). Unfortunately as this is not a bacteria, yeast is not included in this museum . So, although it is outside our scope, here are a few nice links for wine-making hobbyists: Yeast is not the only microorganism that likes fruit juice: molds and bacteria love to feast on it, too. To prevent their growth, and that of wild yeast, sulphur dioxide is used when making wine. That compound also inhibits oxidation of the wine. The main yeast and bacteria which spoil wine are widely distributed in nature and are present in almost every wine making situation. The wine must be stored in a condition unfavourable for microbial growth. The conditions which restrict the growth of micro-organisms in table wines are: low pH (which means slightly acidic), low levels of fermentable sugar, adequate sulphur dioxide, early cleaning up of wine to a bright condition, and absence of oxygen.
The most common bacterium causing trouble is Acetobacter which converts alcohol to acetic acid (vinegar). Oxygen encourages its growth. We will treat Acetobacter with more respect below. A surface-growing yeast, Mycoderma, grows strongly in the presence of air and low sulphur dioxide levels, forming a compound called acetaldehyde which imparts a green-apple-like taste and smell to the wine. This is a common form of spoilage in small wine making situations, particularly in partly filled containers.

One other bacterial species is of importance in wine making. In  Wine and wine making it says: "The winemaker may choose to allow a white wine to undergo a second fermentation which occurs due to malic acid in the grape juice. When malic acid is allowed to break down into carbon dioxide and lactic acid (thanks to bacteria in the wine), it is known as 'malo-lactic fermentation,' which imparts additional flavor to the wine. A 'buttery' flavor in some whites is due to this process. This process is used for sparkling wines."

The bacteria meant here are lactic-acid bacteria. See our species file on lactic-acid bacteria. The Century Wine website describes malo-lactic fermentation like this: "Enologists have known for some time that young wines frequently have a secondary evolution of carbon dioxide, occurring sometime after the completion of alcoholic fermentation. This results from malolactic fermentation, in which malic acid is broken down into lactic acid and carbon dioxide. The fermentation is caused by enzymes produced by certain lactic-acid bacteria.
Flavour by-products of unknown composition are also produced during this fermentation. Malolactic fermentation is desirable when new wines are too high in malic acid, as in Germany, or when particular nuances of taste and flavour are desired, as in the red wines of Burgundy and Bordeaux in France. In other regions, some producers may encourage malolactic fermentation, and others may discourage it, depending upon the particular character desired in the wine. In all regions, this second fermentation is somewhat capricious. One product, diacetyl (a flavour and aroma agent), is apparently beneficial at low levels and undesirable at higher levels."

In case your wine was not at all drinkable, you may have discovered the mystery of making vinegar! Again, bacteria do the job. In this case Acetobacter is the most likely workhorse. Check here how the biochemical conversion from sugar to alcohol to acid works.
Make the best of your wine: make vinegar!

Vinegar can be used for preservation of food, especially in combination with oil. Read how vinegar prevents dangerous bacterial growth in vegetables-in-oil.

And in case your vinegar isn't even drinkable, here is the final solution: use vinegar as a disinfectant in your kitchen!

Last Updated on Sunday, 01 March 2009