Unlike in our chapter on the art of the tasting, we are talking here not of sensory analysis but of the chemical analysis of a wine, of its composition.
This small part of our site is intended for people who wish to know a little more about the chemical composition of a wine! Of course, we do not aim to turn you into expert oenologists, but this information aims to give you an overview of the chemical composition of a wine.
Yes, wine means tradition, an attachment to a local region, people, tasting, the aromas, the flavours and a plethora of varied sensations, but wine can also be described by its chemical characteristics!
So together, let’s find out something about the Science of Wine!
There is no wine without chemistry! Oenology is the science of wine and therefore involves a chemical knowledge of the product! Oenology studies include, moreover, many hours of chemistry and biochemistry!!!
The Cleebourg Cellar has an infrared analyser, a Fourier Transformed InfraRed Spectrometer or IRTF. This modern device allows the winemaker to follow the fermentation and vinification closely. Indeed, in 2 minutes, the winemaker obtains a complete analysis of his sample (must, fermenting wine, finished wine):
- sugar content
- alcohol content
- total acidity
- volatile acidity
- malic acid and lactic acid
- tartric acid
- gluconic acid
- assimilable nitrogen
Each of these parameters provides information on the quality of the product analysed:
The sugar content
This is an essential indicator of vinification. Measured during maturity inspections, the sugar content allows the date of the harvest to be anticipated. In fact, it takes approximately 16.83 g/l of sugar to for a yeast to produce 1% volume of alcohol! It also allows the alcoholic fermentation to be monitored (therefore the transformation of the sugar into alcohol), and it can give the quantity of residual sugars in a wine. The residual sugars contribute to the balance of a wine.
The alcohol content
We talk of potential ABV or percentage of alcohol per volume and acquired ABV. The potential ABV is obtained by dividing the sugar content by 16.83! It represents the quantity of alcohol which could be provided by the yeast in optimum conditions of fermentation.
The acquired ABV represents the quantity of alcohol actually present in the wine during fermentation or in the finished wine. The alcohol content is also an indicator, the gustatory approach to a wine with 11% alcohol by volume (% vol.) will be different to a wine with 14% vol. But note that the alcohol content alone is not an indicator of the quality of the wine! A wine with 11% vol. can be better or less good than a wine with 14% vol.. Everything is a question of balance with the other parameters (sugars, acidity…) and material! We talk of structure and balance!
This is the sum of the volatile acidity and the fixed acidity. The wine contains a certain number of mineral and organic acids. Some of these acids in the wine are entirely combined with alkali; they are in salt form and therefore are not involved in the perception of the acidity of the wine.
However, certain organic acids are only partially saturated by alkali or turned into salts. Some of their molecules are in salt form, others are free. The sum of the free acid functions and the partly free acids constitutes the acidity of the wine.
The three main acids in wine are tartaric, malic and citric acid. Tartaric acid is the specific acid of the grape and of wine; it is found very little in natural form except in vines. Malic acid is an organic acid which is very common in the plant world. It is the main acid in many fruits, and citric acid exists in grapes of every variety and in greater quantity in must concentrated by noble rot or the raisining of the grapes.
The acidity of Alsatian wines generally varies between 4.5 to 5 for Riesling, a grape variety that naturally vinifies “dry” and at least 2.5 for Gewurztraminer!
Parallel to the total acidity, the free acid functions are partially dissociated or ionised and release H+ ions into the liquid, which represent the “real” acidity, whose concentration is expressed as a pH. It depends to a large extent on tartaric acid. In Alsatian wines, the pHs are generally situated between 3.0 for Riesling and 4.0 for Gewurztraminer or Pinots Gris.
The volatile acidity of a wine consists of the part of the fatty acids belonging to the acetic series, which is found in wine either in the free state or in salt form. Volatile acidity gives the wine its bouquet, but when the dose of acid is too high, the wine is said to be “sour”, characterised by notes of glue or vinegar. A wine from an AOC will be “saleable” if its volatile acidity does not exceed 0.9 g/L expressed in H2SO4, namely 1.35 g/L for tartaric acid and 1.1 g/L for acetic acid.
Malic acid and lactic acid
Malic acid is an organic acid naturally present in grapes (it is a very common acid in the plant world) and in wines before malolactic fermentation. The malic acid is degraded into lactic acid and CO2 by lactic acid bacteria in the cases where the wine undergoes malolactic fermentation.
This a is an acid characteristic of the grape. It is rarely found in foods. It is also present in green tea.
This is an acid that is present in very small quantities in the grape berry, and sometimes not at all. Its presence during the analysis of a must or grape juice is characteristic of the development of rot for Botrytis cinerea is capable, by means of an enzyme, glucose-oxidise, of forming gluconic acid from glucose. Gluconic acid is therefore a good indicator of the sanitary quality of a grape.
The nitrogen content is especially useful when analysing the must. Nitrogen is a nutritional element indispensable to the good development of yeast and therefore a good alcoholic fermentation process. If the nitrogen content in the must is insufficient, it will be necessary to make up for it by adding “fermentation activators” to the must, containing natural sources of nitrogen.