Growth and Life Cycle
The process of turning grape juice into wine is dictated primarily by the microorganisms present, specifically the yeast. Thus, an understanding of the microbiology of wine yeasts can be a powerful tool for any winemaker. The basic yeast life cycle helps to determine the initial rate of inoculation (number of cells per volume) as well as understand the behavior of the fermentation.

Upon rehydration, the yeast enter what is known as the lag phase. During this time, the yeast focus only on building up food reserves. Once sufficient food has been stored, the yeast enter the exponential or growth phase, during which the yeast double in both size and number at relatively the same rate. Visually, this phase can be identified by the formation of a layer of foam over the fermentation.

At the beginning of the growth phase there is still oxygen present in the juice, which the yeast rapidly consume to create the anaerobic environment necessary for fermentation. Inevitably, one or more elements essential for continued growth will be depleted from the juice, causing the yeast to settle into a constant population. The size of this final population greatly contributes to the speed and success of the fermentation. After almost all of the nutrients (and hopefully sugar) have been depleted, the yeast begin to flocculate and settle to the bottom of the fermentor, this is the sedimentation phase. At this point, the cells are either dead or preparing for dormancy.

Nutrition
Grape juice is an ideal medium for yeast growth. It is rich in most all essential nutrients (Nitrogen among the most important) as well as carbohydrates, such as the sugars that will be broken down to form ethanol. To avoid stuck or incomplete fermentations, winemakers will often add Nitrogen rich supplements such as DAP (diammonium phosphate), or specialty fermentation supplements. Along with Nitrogen, several of these fermentation supplements also contain dead yeast cells that serve as extra food for the live cell culture. The “richer” the juice is, the larger and healthier the final biomass of yeast will be, and the continued viability of the yeast population dictates whether a fermentation is successful, or becomes stuck.

Aerobic vs. Anaerobic
Anaerobic conditions are essential for fermentation. If there is oxygen available to the yeast, they will metabolize the sugars using a more energy efficient pathway, resulting in the formation of acetic acid rather than the preferred ethyl alcohol. However, an anaerobic environment is much more difficult to survive in. The building concentration of ethyl alcohol is toxic to the yeast, leading to a much more rapid rate of cell death. If the starting population of yeast cells is not large or strong enough, the ethanol toxicity will kill off all the cells before all of the sugar has been converted, leaving you with a stuck fermentation.

Wild Yeasts, Wild Fermentations
Yeasts are found naturally on grape skins, as well as several other places in the vineyard and winery. This abundance of wild yeasts begs the question, why inoculate? There are many different strains of yeast, the most common species used in fermentations is Saccharomyces. Wild yeasts are generally non-Saccharomyces and are frequently not as hardy. They are more susceptible to ethanol toxicity and often result in stuck or sluggish fermentations. Due to the lack of control over the types of yeast present, wines made with wild yeasts have a higher chance of developing off aromas and flavors. Also, pure-culture inoculums come from closely monitored and controlled parent populations. Yeast have a very short life cycle, which can allow for rapid and dramatic change. For this reason, the results obtained with wild yeast are highly variable and unpredictable, where as inoculating with a familiar yeast allows for a degree of reproducibility and consistent high quality wine.

Additional info
Yeast Genetics: Background
Brewer’s Yeast: Yeast Life Cycle
Scholarly article on yeast life cycle
How to Rehydrate and Use Phyterra Yeast