As fermentation proceeds and yeast continue to propagate, they pass along a certain amount of cellular material to future generations. In juice/must without proper yeast rehydration, a gradual reduction in cell membrane thickness and decreasing amounts of nutrient reserve transfer from generation to generation is common as fermentation continues. This had led many producers to include the use of yeast rehydration nutrients to help make their yeast ‘happy’ during the rehydration process, and lead to a healthy yeast population, essential to successful fermentation.
There are several proprietary yeast rehydration nutrients available today, including Dynastart, GoFerm, and PreFerm. They are created from autolyzed yeast cells, and provide many essential micronutrients (including membrane lipids and sterols) and vitamins (including biotin, niacin, and thiamine) that are readily absorbed by active yeast cells. Providing these during the rehydration process reactivates the yeast’s internal metabolism quicker and leads to a substantial increase in cell volume; the original structures of the yeast’s plasmatic membrane are modified, leading to better viability, increased membrane fluidity, increased resistance to ethanol (essential towards the end of ferment), increased resistance to osmotic shock due to high sugar concentration (essential at inoculation), and increased aroma production (essential for good tasting wine!). Yeasts prepared with yeast rehydration nutrients also maintain a steadier metabolic rate throughout fermentation. Less stress also means far less volatile acidity formation and negative sulfur-containing compounds (hydrogen sulfide, disulfides).
The use of yeast rehydration nutrients is recommended at a 5:6 ratio with yeast (5 parts yeast to 6 parts rehydration nutrients, usually 250 ppm yeast with 300 ppm nutrient). I tend to use the recommended rate only when I know that strenuous fermentation conditions are inevitable (high brix levels, low fermentation temperature, low turbidity juice, historically deficient juice, etc.) or during yeast starter culture propagation. For normal fermentation conditions, I tend to use a 1:1 ratio (usually 200 ppm yeast and 200 ppm yeast rehydration nutrients).Winemakers also need to be wary of legal dosages of particular ingredients (i.e. thiamine) when using yeast rehydration nutrients in conjunction with other fermentation nutrients.
Please see here for yeast rehydration preparation.