How Does Fermentation Work?

Fermentation is a critical component in making good wine. In this article, I'll explain what happens during fermentation, identify the three basic products of fermentation, explain how fermentation affects the winemaking process, and explain the ideal conditions for fermentation to occur.

Fermentation starts with yeast, a naturally occurring, living single cell organism found everywhere in nature. In winemaking, we refer to these naturally occurring yeasts as wild yeasts, meaning that we have very little control over them, and the resulting fermentation can be unpredictable. Not every yeast is appropriate for winemaking.

In winemaking, a single strain of yeast is added to the grape juice. While other wild yeasts are present from contact with the air, the addition of the yeast creates a colony which is larger and therefore stronger than the others. Wild yeasts tend to die off as the population of the wine yeast increases and the alcohol content rises. The wine yeast becomes the dominant strain and takes over the fermentation process. Most varieties of wine yeast can survive in solutions up to 16% alcohol. There are a few specialized strains which will continue to thrive in fermentations up to 17% to 20%.

During fermentation yeast consumes the sugars, glucose, and fructose in the juice, and converts the carbohydrates into three products: citric acid, carbon dioxide, and ethanol alcohol. The easiest way to think about these three components is that they are waste products produced by the yeast as it consumes the sugar. The fermentation process also causes the yeast to interact with the composition of the grape juice to produce long chain polymers, which are the building blocks for each wines distinct flavor and aroma. The higher the sugar content in the juice or must, the more complex the flavor profile usually is, and the higher the potential alcohol levels if the wine is allowed to ferment until all the sugar is consumed.

The amount of sugar present in a wine must will most often determine how long fermentation takes. The higher the specific gravity of the must, the more sugar is present in the solution, and the longer fermentation will take. If your yeast is weak, that will also affect the length of time fermentation takes to finish. Some strains of yeast are more aggressive and more robust than others, so it is important to know the characteristics of the yeast that you are using.

A number of factors contribute to the fermentation process. As we have already discussed, temperature is important to the life of the yeast. If the must is too warm, the yeast will propagate quickly and will not give the wine time to develop the necessary flavors and aroma which distinguish each variety of wine. If it is too hot, the yeast will die. Too cold, and the yeast will go dormant. Generally speaking, a slower fermentation is better because it allows the yeast to interact with the compounds in the must.

The oxygen level in the must also contributes to the process. At the beginning of the fermentation process, the yeast needs oxygen in order to thrive and reproduce. As fermentation progresses, the oxygen levels in the must drop, and the yeast adapts to changing environment. The higher the levels of oxygen in the must, the greater chance that the yeast will produce citric acid rather than ethanol.

Because the yeast is a living organism, it also needs to be fed. Yeast thrives on minerals like ammonium phosphate and elements like nitrogen. Depending on the type of wine you are making, and the materials that you are using, you may need to add nutrients to your must to encourage fermentation. Feeding your yeast does not accelerate fermentation, but it ensures that your yeast remains healthy and performs its work at the optimum level.

Fermentation is a natural process which occurs any time you have the right conditions for yeast to consume sugar. Fermentation should not be confused with rotting or decay, which are created by molds and bacteria present in the air. Fermentation is the foundation of winemaking and is home winemakers we want to do everything we can to create an ideal environment for it to occur.

What questions do you have about fermentation? Be sure to post your questions in the comments section. In the next article, we will discuss how fermentation impacts the flavors and aromas of wine. If you're not already on the mailing list, sign up now to be notified of updates!