Every yeast strain has its own optimum rehydration process, but the basic procedure is as follows:
1. Warm the dry yeast to room temperature.
2. In a sanitized container, prepare an amount of sterile tap water at 105*F (41*C) equal to 10 times the weight of the yeast (10 ml/g of yeast).
3. Sprinkle the dry yeast on top of the water, trying to avoid setting up large, dry clumps. Let it sit for 15 minutes, then gently stir.
4. Once the yeast has reconstituted, gentle stir once again to form a cream, and let sit for another 5 minutes.
5. Carefully and slowly, adjust the temperature of the yeast to within 15*F (8*C) of the wort temperature.
6. Pitch the resultant cream into the fermentation vessel, ideally as soon as possible.
Controlling temperature is the most important part of the process. Rehydration generally ranges from 95 to 105* F (35 to 41* C) although some manufacturers suggest a lower temperature range. The ideal temperature for each dry yeast product can vary, and you should strive to find out from the manufacturer what temperature is optimal for their product. Do not attempt to rehydrate yeast in cold water. Warmth is critical to the cell during the first moments of reconstituting its fragile cell membrane. Lower temperatures result in more cell material leaching out of the cell during rehydration, which permanently damages the cell. At the optimal rehydration temperature, it is possible to recover 100 percent of the cells. Too cold a temperature can result in the death of more than 50 percent of the population. You should measure the temperature of the rehydration vessel just before adding the yeast. The temperature of the water can drop significantly if the vessel is colder than the water.
Most filtered tap water works well for rehydration. Ideally, the mineral content should range from 250 to 500 parts per million hardness. During the first few moments of rehydration, the cell cannot regulate what passes through the membrane. High levels of sugars, nutrients, hop acids, or other compounds can enter freely and damage the cells. This is why adding dry yeast directly to wort results in such a high percentage of dead ad damaged cells.
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