Fermentation is the most important step in the process of learning how to make wine, and definitely the most involved, I’ll go into further detail later on as well as the chemistry involved but I’ll go over the basics. Fermentation occurs in two steps, the primary stage is the aerobic stage, and secondary stage which is anaerobic. In general terms fermentation is a natural process in which the yeast in your must eats the dissolved sugars, (glucose and fructose) and as a byproduct creates alcohol (ethanol) in addition to aromas and flavors, carbon dioxide, and heat because it is an exothermic process. Heat is the most important thing to consider, if the must is allowed to overheat the distinct flavors and aromas will begin to break down.
Primary stage: The primary stage, or “aerobic stage” is a rapid fermentation stage and will usually last 3-5 days, during this first stage 65-75 percent of the fermentation activity will occur, and it is not uncommon to notice considerable foaming. Because this stage is the aerobic stage of the fermentation process, the vessel in which the must is contained in is left unsealed and open to the outside air, this plays an important role in the multiplication of the yeast cells which will multiply 100-200 times during this stage, some alcohol will be produced but most of the energy is being used to multiply the yeast cells and the lack of air will be detrimental to the process.
Secondary stage: In the secondary or “anaerobic stage” the vessel will need to be sealed, this can easily be accomplished using an air lock, sealing the container at this point will put the yeast into the anaerobic phase, decreasing and ultimately stopping the multiplication of the yeast cells and shifting the concentration into producing alcohol and breaking down the remaining sugars. The remaining 25-35 percent of the fermentation will occur in this stage. In this stage the fermentation slows and will take from 1-2 weeks depending on how much remaining sugars and nutrients are available for the yeast to consume.
Temperature of the must: It is essential in the fermentation phases to keep the must at 72 degrees plus or minus two degrees. If the must gets too cold fermentation slows, too warm and it may still ferment but the flavor and aromatic qualities of the wine will suffer.