Inoculation is the process of adding yeast to juice/must to begin fermentation. This is typically done immediately following solids separation for white varietals, and after some period of cold soaking for red varietals. Ensuring that the juice/must is properly prepared for inoculation (see here) is essential for the growth of the yeast culture, and in turn, successful fermentation.
Most winemakers use one strain of active dry wine yeast for inoculation, though some use two or more strains symbiotically or use one symbiotically with indigenous yeast. Active dry wine yeast is a commercially produced product available from a wide array of manufacturers (see post I and II). These companies not only provide basic instructions for preparation and inoculation rates (typical recommended rate is 250 ppm, or 25 g of yeast per 100 L of juice/must; this equate to approximately 5 million cells per mL), but also basic fermentation needs of the yeast (temperature, nutrient needs, etc.) and suggested varietals with "typical" aroma/flavor profile seen after fermentation.
Rehydration of yeast is a simple but very important step in winemaking. As mentioned, manufacturers provide preparation guidelines and there are plenty of different ways to prepare yeast, but these are the basics:
- Weigh out the appropriate amount of yeast. If a yeast rehydration nutrient is to be used, weight it out as well.
- Measure out the appropriate volume of clean, warm water based on the inoculation rate; deionized water is ideal, but unchlorinated water works too. Manufacturers typically recommend water at 40° C (104° F); this is essential because temperature shock is one of the greatest killers of yeast. If a yeast rehydration nutrient is being used, I like to get the water temperature to just above 41° C (106° F) because the nutrients will surely bring down the water temperature.
- If a yeast rehydration nutrient is being used, mix it into the water until completely dissolved. Then, re-check the temperature of the mix to ensure it is at 40° C.
- Add the dried yeast and gently mix until all granules are dissolved (gently is key; if too much aeration occurs, it will foam tremendously).
- Allow the inoculum to rest for 15-20 minutes; don't let it sit too long, the yeast need food!
- Make sure the inoculum looks active and happy (should be foaming some or a lot, depending on yeast strain). At this stage, the inoculum can either be added directly to the juice/must as long as the temperature differential isn't too high (remember, temperature shock is bad!). If the temperatures are widely different, add a mixture of 50/50 juice and water to slowly bring the temperature down (this may take several tries, allowing it to sit another 15 minutes each time you add). Usually inoculum should be within 5° C (approximately 10° F).
- Once the inoculum is added to the juice or must, homogenize the juice/must gently. The juice/must will ideally be in a temperature controlled environment to ensure that fermentation proceeds at the desired rate.