Pasteur then focused on studying one of the key processes in winemaking: fermentation. Essentially, fermentation is the transformation of sugars, present in foods like fruits and grains, into other substances: alcohol, acids, and carbon dioxide. 

At that time, it was believed that this transformation was purely a chemical process, meaning it was driven by compounds without involving any living organisms. However, Pasteur proposed that fermentation was actually a biological process, requiring the action of a living organism.

Through years of experiments and microscopic observations, Pasteur concluded that microorganisms, later identified as yeasts (a type of fungus), were responsible for breaking down the sugar in fruit to produce alcohol (a vital componen of wine), but also acetic acid, which was precisely what gave the resulting wine its sour taste.

Louis Pasteur in his laboratory, 19th century engraving.

This new understanding of microorganisms' role in fermentation was another groundbreaking contribution linked to the germ theory, with more to follow. After identifying the problem of wine souring in French production, Pasteur also thought of a way to prevent this unwanted effect of fermentation.

Through experimentation, he observed that heating wine for a few minutes at temperatures between 50°C and 60°C could kill the yeast, stop its activity, and prevent the wine from turning sour. Seeing the success of this method, which is now known as pasteurization, Pasteur proposed that winemakers heat the wine before bottling. Initially, his suggestion met with resistance, as producers feared that the heat might cause other unwanted effects. They eventually tested it on barrels of wine, heating one and leaving the other untreated. After ten months, the pasteurized wine remained fresh, while the untreated wine had turned acidic, confirming the method's effectiveness.

Press article announcing Pasteur’s Work on Wine Fermentation

Additionally, as part of his research, Pasteur observed that the action of microorganisms during fermentation occurs more efficiently in the absence of oxygen. This phenomenon is now known as the "Pasteur effect," which explains how even minimal exposure to oxygen can stop or disrupt the fermentation process. This is why fermentation containers are designed to be airtight.

In 1865, at the age of 43, Pasteur patented his method for preserving and enhancing wine through moderate heating in anaerobic conditions (without oxygen). While this technique is not commonly used in winemaking today, Pasteur adapted versions of his process for milk and beer in 1871, where pasteurization remains essential for safe consumption.

With his new method, Pasteur rescued the French wine industry and boosted its reputation. For instance, in 1867, his work earned him the Grand Prize at the Universal Exhibition held in Paris that year. What followed would shift focus from wine to a worm, leading Pasteur and his discoveries about microorganisms into the field of medicine.