As fermentation proceeds, yeast continue to propagate. Similar to reproduction in any species, certain attributes and amounts of cellular material are transferred to descents. Active dry wine yeast that is not properly rehydrated will show a gradual reduction in cell membrane thickness from generation to generation, coupled with decreasing amounts of nutrient reserves required for successful fermentation.
Several companies now offer proprietary yeast rehydration nutrients to help ensure healthy yeast during the rehydration process. These include products such as Dynastart, GoFerm, and PreFerm. These products are created primarily from autolyzed yeast cells. They contain many essential micro-nutrients, such as membrane lipids and sterols, and vitamins including biotin, niacin, and thiamine, that are readily absorbed by active yeast cells.
Providing such nutrients helps activate the yeast's internal metabolism quicker prior to inoculation. This leads to a substantial increase in cell volume as the original structures of the yeast's plasmatic membrane are modified. The benefits can make a significant difference to the yeast.
Most manufacturers recommend the use of yeast rehydration nutrients at a 5:6 ratio with yeast. Since yeast is recommended at 250 ppm, the associated nutrients would be 300 ppm.
I tend to use a 1:1 ratio, unless I know strenuous fermentation conditions are inevitable.
Like with any additive, winemakers should research the components of any rehydration nutrients they plan to use. They need to be wary of legal dosages of particular ingredients, such as thiamine, particularly when using in conjunction with other fermentation nutrients.