Every strain of yeast has its own optimum rehydration temperature. All of them range between 95 F to 105F. Most of them closer to 105F. The dried yeast cell wall is fragile and it is the first few minutes (possibly seconds) of rehydration that the warm temperature is critical while it is reconstituting its cell wall structure.
As you drop the initial temperature of the water from 95 to 85 or 75 or 65F the yeast leached out more and more of its insides damaging the each cell. The yeast viability also drops proportionally. At 95 â 105 F, there is 100% recovery of the viable dry yeast. At 60F, there can be as much as 60% dead cells.
The water should be tap water with the normal amount of hardness present. The hardness is essential for good recovery. 250 -500 ppm hardness is ideal. This means that deionized or distilled water should not be used. Ideally, the warm rehydration water should contain about 0.5 â 1.0% yeast extract
For the initial few minutes (perhaps seconds) of rehydration, the yeast cell wall cannot differentiate what passes through the wall. Toxic materials like sprays, hops, SO2 and sugars in high levels, that the yeast normally can selectively keep from passing through its cell wall rush right in and seriously damage the cells. The moment that the cell wall is properly reconstituted, the yeast can then regulate what goes in and out of the cell. That is why we hesitate to recommend rehydration in wort or must. Very dilute wort seems to be OK.
We recommend that the rehydrated yeast be added to the wort within 30 minutes. We have built into each cell a large amount of glycogen and trehalose that give the yeast a burst of energy to kick off the growth cycle when it is in the wort. It is quickly used up if the yeast is rehydrated for more than 30 minutes. There is no damage done here if it is not immediately add to the wort. You just do not get the added benefit of that sudden burst of energy. We also recommend that you attemperate the rehydrated yeast to with in 15F of the wort before adding to the wort. Warm yeast into a cold wort will cause many of the yeast to produce petite mutants that will never grow or ferment properly and will cause them to produce H2S. The attemperation can take place over a very brief period by adding, in increments, a small amount of the cooler wort to the rehydrated yeast.
Many times we find that warm water is added to a very cold container that drops the rehydrating water below the desired temperature.
Sometimes refrigerated, very cold, dry yeast is added directly to the warm water with out giving it time to come to room temperature. The initial water entering the cell is then cool.
How do many beer and wine makers have successful fermentations when they ignore all the above? I believe that it is just a numbers game. Each gram of Active Dry Yeast contains about 20 billion live yeast cells. If you slightly damage the cells, they have a remarkable ability to recover in the rich wort. If you kill 60% of the cell you still have 8 billion cells per gram that can go on to do the job at a slower rate.
The manufacturer of Active Dry Beer Yeast would be remiss if they offered rehydration instructions that were less than the very best that their data indicated.
One very important factor that the distributor and beer maker should keep in mind is that Active Dry Yeast is dormant or inactive and not inert, so keep refrigerated at all times. Do not store in a tin roofed warehouse that becomes an oven or on a window sill that gets equally hot.
Active Dry Yeast looses about 20% of its activity in a year when it is stored at 75 F and only 4% when refrigerated.
The above overview of rehydration should tell you that there is a very best way to rehydrate.
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