Wine yeast are microscopically small, single-cell organisms. Like every living organism, yeast need energy to survive, and the necessary energy is obtained by metabolizing grape sugars. Ethyl alcohol is produced as an end product, but the yeast do not use the alcohol. They are only concerned with the energy produced when the sugars are converted into alcohol. Besides sugar, yeast must have access to many other materials to reproduce new cells, and yeast are sensitive to their environments.
The conversion of glucose into alcohol is a complicated, multi step, biochemical process. Several different enzymes are needed to convert the sugars, and yeast must have access to vitamins, minerals, oxygen, nitrogen, etc. to produce the required enzymes. Grape juice normally contains all the necessary materials, but sometimes fermentations will lack one or more of these critical growth factors, and the yeast cannot convert all of the grape sugars into alcohol. Most stuck fermentations are caused by deficiencies in the starting juice or by excessively high or low temperatures.
Yeast need Oxygen
Yeast reproduces rapidly when sufficient oxygen is available, and yeast populations can double in about an hour when conditions are just right. This rapid period of yeast growth is called the exponential growth phase, and an enormous yeast population (10 million cells per milliliter of juice)
can develop in less than 24 hours. Rapid cell growth occurs during the exponential growth phase, but little alcohol is produced. The situation is different when oxygen is restricted. With little oxygen, yeast cell reproduction is slower, but the yeast produces larger amounts of ethyl alcohol.
Having oxygen available early in the fermentation process is always desirable from a winemaking point of view. Yeast cells then multiply rapidly, and a large yeast population is produced quickly. Later in the fermentation, oxygen is deliberately restricted to promote alcohol production. This simple technique allows winemakers to encourage early yeast growth, and the large yeast population will convert the grape sugars in a dependable way.
A lack of oxygen is seldom a problem when wine is fermented under typical, home winemaking conditions. The quantity of oxygen needed by the yeast is small, and more than enough oxygen is introduced when grapes are subjected to the normal winemaking processes of crushing and pressing.
Yeast need Nitrogen
Yeast must have protein to make new cells, and yeast must have nitrogen to produce the protein. Normally, grapes contain enough nitrogen to meet the yeast needs. However, vineyards needing fertilization often produce fruit excessively low in nitrogen content, and then the yeast has problems producing the large numbers of cells needed to complete fermentation. Winemakers often add small quantities of diammonium phosphate (or other sources of nitrogen) to juices low in nitrogen. The diammonium phosphate gives yeast the nitrogen needed to produce new cells and complete the task of fermentation. The yeast and the winemaker are then happy.
Sometimes, a problem develops when nitrogen is added near the end of fermentation. A significant amount of alcohol has accumulated by this time. The alcohol seems to prevent the intake of nitrogen by the yeast cells, and sometimes the fermentation sticks. To avoid this problem, winemakers monitor their fermentations carefully and correct any nitrogen deficiencies early in the fermentation cycle before large amounts of alcohol accumulate.
Yeast need Nutrients
Yeast needs an assortment of vitamins, minerals and other growth factors. Yeast require very small quantities of these substances, so winemakers often call these materials “micro nutrients.” Normally, grapes contain adequate quantities of these nutrients, but some vineyards consistently produce grapes deficient in some particular growth factor. In these cases, winemakers try to avoid fermentation problems by adding a complete “yeast food” to the juice. Several commercial products such as Superfood are produced specifically to supply yeast with these necessary nutrients.
Yeast often produce excessive amounts of hydrogen sulfide when they lack pantothenic acid. Hydrogen sulfide produces the familiar rotten egg smell, and even small quantities of hydrogen sulfide can damage wine quality, and commercial and home winemakers routinely add minute quantities of pantothenic acid to fermentations. Pantothenic acid is a common vitamin, and it can be purchased in any drug store.
Handling Dry Yeast
Dry yeast looks almost indestructible. However, dry yeast consists of live cells, and it must be handled with care. Yeast weakened by mishandling often requires an unusually long time to start fermenting, and damaged yeast sometimes has trouble fermenting the juice to dryness. Figure 5 shows several ways yeast can be damaged.
Dry yeast will remain viable for at least two years when unopened packages are stored in a cool, dry place. However, once a package has been opened, the yeast should be used within a few months. Using open packages of dry yeast the next crush season is risky even when the open packages are stored under optimum conditions. Old, dry yeast should be saved. It is useful for fining wines containing ethyl acetate.
Rehydrating Dry Yeast
Nine times out of ten satisfactory fermentations can be started just by sprinkling dry yeast on the must. To avoid problems the tenth time, all yeast manufacturers recommend rehydrating dry yeast before it is added to the must. Yeast rehydration is a simple procedure. Just add the dry yeast to a small amount of warm water. However,
the temperature of the water is important, and a thermometer should be used to adjust the water temperature to 100 degrees. About one cup of water is needed for a tablespoon of dry yeast. Stir the yeast mixture until it is smooth and then let it rehydrate. After 20 to 30 minutes, pour the yeast mixture into the must or juice. Most winemakers use about one gram of dry yeast for each gallon of must.