Alcohol and yeast in wine........
The difference between a bottle of grape juice and a bottle of wine is the alcohol, but where does the alcohol come from?
Alcohol is the result of the fermentation process where a micro- organism (yeast) converts sugar into alcohol and carbon dioxide gas. Yeast is a living organism that is critical to winemaking. Without yeast, there would be no beer, wine or spirits, bread, yoghurt or cheese. 'God is Good' was how yeast was referred to prior to 1859 when Louis Pasteur discovered that a single cell organism was responsible for the conversion of sugars into alcohol and carbon dioxide. The Egyptians were the earliest recorded users of yeasts. They brewed wine and they baked bread.
Yeasts are around 6-8 microns in size (1 micron is 1 millionth of a metre), it takes about 20 billion of them to
make up 1 gram of yeast and the chemical reaction that happens is C6h32O6 + yeast = 2C2H5OH + 2CO2.
Three common winemaking yeasts are:
Yeast is grown commercially by starting with a known 'strain' of yeast, feeding it with molasses or other sugars so it multiplies and then harvesting it. Freeze dried yeast is the most popular form of yeast for wine making. It's kept in the fridge till required. The yeast is 're-hydrated' by adding it to water at 40°C and letting it stand for 20-30 minutes. This is then tipped into the wine must to start fermentation. There are hundreds of different yeasts available for making wine. Yeasts occur naturally on the skin of grapes but we kill those off before fermentation. These wild yeasts would give unpredictable results and who knows what flavour you would end up with. The winemaker decides what results he wants before the fermentation is started.
An example of one wine making yeast and the information available to the winemaker is shown below from the Lalvin company.
This strain was isolated from grapes grown in the Côtes-du-Rhône region of France by Dr. Dominique Delteil, head of the Microbiology Department, Institut coopératif du vin (ICV), in Montpellier. ICV D-47 strain was selected from 450 isolates collected between 1986 and 1990.
OENOLOGICAL PROPERTIES AND APPLICATIONS
The ICV D-47 is a low-foaming quick fermenter that settles well, forming a compact lees at the end of fermentation. This strain tolerates fermentation temperatures ranging from 10° to 30°C (50° to 86°F) and enhances mouthfeel due to complex carbohydrates. Malolactic fermentation proceeds well in wine made with ICV D-47. This strain is recommended for making wines from white varieties such as Chardonnay and Rosé. It is also an excellent choice for producing mead, however be sure to supplement with yeast nutrients, especially usable nitrogen.
Under Australian law a 'product' must include a statement of the percentage by volume of ethanol therein at 20°C, expressed
to the nearest first decimal place and accurate to within 0.5% of ethanol at 20°C and can be:
These definitions vary around the world. And it may not be just for health or marketing purposes that the alcohol content is listed. A higher alcohol wine attracts higher taxes in many parts of the world.
You may see the words 'proof' on some labels. Proof is simply the percentage of alcohol times 2. If a wine is 14% alcohol
it is 28 proof. This is mostly used on spirits rather than wines.
* Reproduced with permission from Peter Svans at The Gurdies Winery