The main constituents of grapevine berry juice and wine are:

• Water
• Carbohydrates
• Acids
• Alcohols
• Phenolics
• Nitrogenous compounds
• Inorganic substances

Carbohydrates

In must, these can be divided up into the simple carbohydrates (sugars) and more complex ones (pectins and glucans).
Sugars are the raw material that wine yeasts convert to alcohol and glycerol. The major sugars in grapes are glucose and fructose, present in approximately equal quantities. Technically, a dry wine contains less than 2g/l sugar, but in commercial terms, a dry wine might contain up to 10 g/l sugar.
Pectins are present in grapes in small quantities. High concentrations of pectins make it difficult to extract and/or clarify grape juice and filter wine. Pectins can be broken down by pectolytic enzymes added during processing.

Acids

The main acids present in grapes are tartaric and malic. These are ‘organic’ acids, as they contain carbon atoms. They are responsible for the relatively high acidity and low pH (hydrogen ion concentration) of grapes, compared with other fruits, and for the fresh crisp taste of wine. Grape must also contains low levels of citric, ascorbic and acetic acids.

Alcohols

Ethanol is produced during fermentation by yeast from sugar. Trace amounts of other alcohols are also produced and these contribute to the flavour of the wine. Glycerol (glycerine) has an effect on the viscosity and sweetness of wine. It can be produced in significant amounts when grapes are infected with Botrytis as noble rot.

Phenolics

Phenolics are a group of chemical compounds that affect the wine’s colour, texture, astringency and bitterness. Smaller phenolics can taste bitter, or develop bitter tastes on oxidation. Anthocyanins are responsible for red wine colour and are generally found in pulp cells directly underneath the skins of black grapes. Tannins largely determine the astringency and body of a wine and are important antioxidants and preservatives.

Nitrogenous compounds

These are substances containing nitrogen, such as proteins, amino acids and ammonium salts. Yeast and bacteria need nitrogen sources for growth. Most proteins in red wines are removed by the tannins, but, in white wines, certain proteins can cause hazes.

Inorganic Constituents

Most inorganic ions (e.g. metals and nitrates) are not very important in juice and wine. The potassium content is the exception as the potassium salts of the organic acids contribute to the wine’s acidity: the level of potassium is lower in wine due to the precipitation of potassium bitartrate.
Iron and copper may be dissolved in wine from metal surfaces containing these metals. They give rise to hazes in the wine called iron or copper ‘casse’.
The major anions present are phosphate, chloride and sulphate.

Other flavour compounds

There are many other flavour compounds present in juice and wine in trace amounts, and these are largely responsible for their characteristic aromas and flavours. The situation is very complex with levels of volatility and the synergy between combinations of compounds often as important as the individual compounds present. Winemaking is concerned with the extraction and conservation of desirable flavour compounds and the minimisation of undesirable flavour compounds.

The chemical composition of grapes is affected by many factors, include the following:

• Grape variety or cultivar
• Environmental factors such as climate and soil (‘Terroir’)
• Viticultural management
• Seasonal variations (The concept of ‘vintage’). This is because the three principal climatic parameters (light, heat, water) vary considerably from year to year.

The table below shows the typical concentration ranges of the major chemical components of grape juice and dry table wine:

*T = trace. The above table is reproduced with the permission of the publishers, Winetitles Pty Ltd from Viticulture Volume 1 - Resources 2nd Edition, by Ed. Bryan Coombe and Peter Dry, 2004.