In the first part of our article series on alcoholic fermentation, Westgarth Wines wine specialist Maurizio Broggi discusses the important considerations of yeasts.

Alcoholic fermentation, sometimes known as primary fermentation, is the conversion of grape sugars into ethanol (alcohol), carbon dioxide, and heat by the action of yeasts. This is an anaerobic process as it occurs without the presence of oxygen. During alcoholic fermentation, other by-products are also produced, notably aromatic compounds, glycerol, acetaldehyde, and acetic acid, which all contribute to the overall profile of the wine.

Yeasts are an essential part of the fermentation process. Without yeasts, there would be no conversion of sugars into alcohol. Yeasts also contribute to the wine’s bouquet by producing aromatic compounds such as esters. The most common wine yeasts belong to the genus Saccharomyces cerevisiae. Saccharomyces cerevisiae is well suited to winemaking because of its tolerance to relatively high levels of alcohol as well as not being inhibited by sulfur dioxide.
In general terms, the winemaker may rely on spontaneous fermentation whereby ambient (also known as native) yeasts carry out the fermentation, or they may inoculate the must with commercially available cultured yeasts. Some winemakers prefer spontaneous fermentation because, although less predictable, it imparts a unique character and distinctiveness to the wine. Other winemakers favor inoculation with cultured yeasts because they are reliable, allow a rapid onset of fermentation, and provide specific characteristics to the wine.

Yeasts have specific requirements in order to grow and ferment successfully. These include sugars, temperature, oxygen, and nutrients. Yeasts need sugar for energy, without sugar there would be no alcohol. Roughly 17 grams of sugar per liter is necessary to produce 1% abv of alcohol. Sugars in grape must mainly consist of equal proportions of glucose and fructose. Yeasts tend to prefer glucose; thus, yeast begins to ferment glucose first and leaves fructose last. Typically, if there is any residual sugar left in wine at the end of the fermentation, it tends to be mostly comprised of fructose.

Saccharomyces cerevisiae grow and function within a limited range of temperatures. They usually cannot tolerate temperatures lower than 5 °C (41 °F) and begin to die above 35 °C (95 °F). At these extreme temperatures, fermentation slows down and even stops which results in a ‘stuck’ fermentation.

Although fermentation is essentially anaerobic, at the very beginning of fermentation, yeasts need oxygen to reproduce and grow to a sufficient level of population that will be able to ferment. Insufficient oxygen may also result in foul-smelling reduction odors (e.g. rotten eggs) due to the production of hydrogen sulfides. Besides sugars, yeasts also need nutrients to grow, reproduce, and function, notably nitrogen and vitamins. Lack of nitrogen in grape juice causes yeasts to starve which may prematurely stop fermentation and result in unpleasant reductive odors.

For healthy and rapid growth of the yeast population, oxygen is necessary at the beginning of fermentation. Once the yeast population has sufficiently grown, yeasts start fermenting anaerobically, converting sugars into alcohol.

In white winemaking, enough oxygen is provided during the destemming and crushing operations. In red winemaking, more oxygen is allowed to come into contact with the fermenting must thanks to the higher phenolic content present in grape skins. For this reason, in red winemaking, aeration of the fermenting must is allowed to take place at the beginning of the fermentation. This can be achieved by using open-top fermentation vessels and cap management techniques such as pump over or punch down, which allow optimal mixing of skins and must along with aeration of the fermenting must. Aeration of red must is also beneficial for color stability, thanks to the formation of more stable polymers between tannins and anthocyanins.

During fermentation, yeasts produce carbon dioxide as a by-product. Carbon dioxide at this stage plays a critical role as it forms a protective blanket over the fermenting wine preventing direct contact with oxygen, thus avoiding oxidation.

Once fermentation has finished, carbon dioxide is no longer produced, so the wine loses the blanket of carbon dioxide and needs to be effectively protected from oxygen. While very useful during fermentation, carbon dioxide is also extremely dangerous. It is colorless, odorless, and, being heavier than air, it collects at the bottom of empty vessels and in poorly ventilated spaces. Cellar operators should always be careful when entering areas or vessels where fermentation is active (or just finished) to avoid the risk of asphyxiation.

Want to read more? Take a look at some of our other blogs:

• • The art of timing the harvest
  • Grape processing: from picking to sorting
  • Grape School: Syrah