Inoculation is the process of adding yeast to juice/must to begin fermentation. This is typically done immediately following solids separation for white grape varieties, and after some period of cold soaking for red varieties. Ensuring that the juice/must is properly prepared for inoculation is essential for the growth of the yeast culture, and subsequently a successful fermentation.
Most winemakers use active dry wine yeast to complete direct inoculation. By the process, the yeast culture is added directly to the juice to be fermented. Some winemakers choose to propagate yeast and complete what may be referred to as 'wet inoculation'.
Direct inoculation typically involves a single strain of yeast. Allowing indigenous yeast to begin fermenting juice/must and then inoculating with a commercial yeast strain is also rather common. Sometimes on purpose, but often because fermentation kicks off before inoculation! Some winemakers choose to ferment with two or more strains symbiotically. It is commonly believed that inoculating with two strains simultaneously is a waste as the stronger of the two will dominate immediately, so symbiotic ferments are usually inoculated in stages: inoculate with a weak strain initially, wait several days, then inoculate with a stronger strain.
Active dry wine yeast is a commercially produced product available from a wide array of manufacturers (read Selecting the Right Yeast). Most manufacturers provide basic fermentation needs for each yeast strain, including desired temperature and nutrient needs. This is often accompanied by a list of suggested varieties the particular strain will perform well fermenting, along with 'typical' aroma/flavor to be expected post-fermentation.
The most important information these companies provide is instructions for preparation because proper rehydration of active dry wine yeast is essential. Nevertheless, plenty of winemakers have their own 'tried and true' rehydration method. Choosing the inoculation rate is also quite important as this dictates the population of yeast expected to complete the fermentation. Manufacturers typically recommend a rate of 250 ppm, or 25 g of yeast per 100 L of juice/must. This should equate to approximately 5 million cells per mL of yeast upon inoculation.
Here are the basics of yeast rehydration. Weigh out the appropriate amount of yeast. If a yeast rehydration nutrient is to be used, weight it out as well.