Oenology - Louis Pasteur, yeast and bacteria

Nothing in the world elicits such thing as wine.

Louis Pasteur

The oenology is the art of knowing the wine, from the planting stage to consumption stage via harvesting, winemaking, farming and tasting. Whereas the sommelier is the art of serving wine and managing wine cellars, i.e. restaurants.

The first treaty of Oenology dates back to 2000 BC, however it was only after the studies of Louis Pasteur (1855-1860) that it became possible to better control the fermentation process and the transformation of grapes into wine.

Louis Pasteur

Louis Pasteur broke the mysterious code of fermentation. Wikimedia

Wine is not only the result of the yeast fermentation but equally the lactic acid bacteria of fresh and mash grapes. It is a stage between grape juice and vinegar. If the winemaker does not stop the process through bottling (which avoids the contact of the wine with oxygen), the wine will naturally be transformed into vinegar.

During the winemaking process, the juice undergoes its first fermentation: alcoholic fermentation (AF) and it consists of the transformation of sugar into alcohol through the work of yeast naturally present on the grapes or added by the winemaker in dehydrated yeast (active dry yeast LSA). AF results in the simplified chemical formula as follows:

C6H12O6 --> 2 C2H5OH + 2 CO2 + heat

resulting in:

glucose moleculegives --> 2 alcohol molecule + 2 carbon dioxyde molecule + heat


Yeast cells under the microscope in full multiplication and the beginning of alcoholic fermentation. Wikipedia

In most cases and after the first stage, the wine undergoes a second fermentation: malolactic fermentation (MLF), which is the transformation of malic acid into lactic acid through the work of bacteria naturally present on the grapes or added by the oenologist in the form of dehydrated bacteria. The MLF results in the simplified chemical formula as follows:


resulting in:

1 malic acid moecule --> 1 lactic acid molecule + 2 carbon dioxyde molecule + heat

During the malolactic fermentation the wine is naturally deacidifies because the lactic acid is an acid weaker than malic acid (loss of an acid-COOH in the MLF).

bacteria cell

Prokaryotic bacterium that does not contain a core. Wikimedia

If the winemaker does not protect the wine from oxygen by bottling it, it naturally turns into vinegar. This is due to the work of acetic acid bacteria. Simplified chemical formula for this reaction is as follows:

C2H5OH + O2 --> CH3-COOH + H2O + heat

resulting in:

1 alcohol molecule + 1 oxygen molecule --> 1 acetic acid molecule + 1 water molecule + heat

Acetic acid is the main component of vinegar.


Pressing of Beaujolais Domaine de La Soufrandière © Guillaume Bodin

Oenology focuses on:

- The study of grapes

  • The composition of the grapes and the roles of its constituents in wine.
  • The importance of grapes’ quality for the development of wine
  • The knowledge of grape varieties and terroirs.
  • The maturity and its importance in wine production.

- Alcoholic and Malolactic fermentations

- Wine diseases

Basically everything that relates to the chemistry of wine.

Tags: wine


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