⚗️Basics of Wine Chemistry

The biochemical process that underpins viticulture

Glucose → 2 Ethanol + 2 Carbon Dioxide + Heat

Alcohol is fundamentally made from the chemical reaction of yeast breaking down sugar (i.e. glucose) into carbon dioxide and ethanol. This is called fermentation, and is an irreversible process. How much sugar the yeast is able to break down (i.e. metabolise), how much carbon dioxide is captured in the process and interactions with the other components of the origin of the sugar (i.e. juice) and environment affect the resultant style of the wine.

We can generalise a few principles based on this fundamental biochemical reaction:

1. Wine can be made as long as there is sugar. Juice of any fruit can technically be used if there is sufficient sugar and other components do not hinder yeast from metabolising it. There are other types of non-grape wines, such as those made from cherry, pineapple, plum and even banana — all natural choices because of their higher sugar content.

2. A wine with higher alcohol content results from (a) using juice with higher sugar content at the start and/or (b) more sugar being metabolised by the yeast in the process. This concept is important in understanding why some wines end up having the style they are produced in. It is worth highlighting that most yeast strains can only tolerate an alcohol concentration of 10 to 15%, which explains why the alcohol by volume (ABV) (i.e. how much alcohol there is) of most wines do not typically end up beyond 15% save for fortified wines.

3. Temperature control is important in fermentation. Fermentation is an exothermic reaction, which means heat is released into the surroundings during the process. The temperature of the winemaking vessel has to be kept cool at the optimum temperature of 25 to 30°C for yeast to do its work, but temperature can also be a useful tool to control the extent of sugar being metabolised by the yeast.

Some terms and parameters to know which would be useful to speak a common language and build an intuition for benchmarking in viticulture:

• Residual sugar: The amount of sugar left in a wine after fermentation has been completed and before any extra sugar has been added prior to subsequent processing or bottling the wine.

• Dry: A wine that is dry is one that is not sweet. The EU Commission dictates that dry wines with moderate acidity may contain no more than 9 g/L of residual sugar, except in cases where acidity is over 7 g/L.

• Alcohol by volume (ABV): The amount of alcohol in the wine, usually expressed as a percentage. WSET benchmarks an ABV of less than 11% as low, 11 to 13.9% as medium and 14% and above as high. For fortified wines, an ABV of 15 to 16.4% is low, 16.5 to 18.4% is medium and 18.5% and above is high.

Bonus read

• How sulfites affect a wine’s chemistry