What temperature should water be for active yeast?

30-40 degrees Celsius is fine. Different yeasts have different optimums. It is about body temperature

1) To restore their function yeast cells must reabsorb all of their cellular water. This step of rehydration is

perhaps the most critical phase in using dry yeast cultures. Only proper rehydration can ensure healthy cells

which retain good fermentation characteristics.

When dry yeast is exposed to water or aqueous solutions the cells rehydrate, absorbing the needed water within

seconds. If rehydration is not properly carried out, the cell can leak important cellular compounds through

the membrane, which is extremely permeable at the time of rehydration. Consequently, the yeast will lose

viability and the remaining populations will be unable to initiate a rapid fermentation. Difficulty will also be

experienced if the yeast are dispersed directly onto the must as the granules will clump and stick together. Also,

in some instances, a must may contain SO2 or residual fungicides which could be lethal during the rehydration

stage. Once rehydrated the cells can resist SO2 and low concentrations of fungicides, but not during water

uptake

Important points in proper dry yeast rehydration are:

• Allow 30 minutes for yeast to come to room temperature before rehydrating.

• Rehydrate in clean water (See Notes 1 & 2 below) rather than in must, and never use distilled water.

o Allows the cell to re-establish normal cell membrane functions more quickly - early in the

rehydration process the yeast will not be able to differentiate between good & toxic substances.

o In the first critical minutes of absorbing water, the yeast can take up micronutrients (if provided)

as well as water - largely due to the pH of the water being near neutral, which makes it less

stressful for the yeast to incorporate these nutritional elements

• Use the proper water temperature (99-105°F) - The rehydration temperature makes a big difference as

to how the yeast cells reconstitute from their dried state. The addition of dried yeast to cool water

(60°F), or must, can decrease cell viability by as much as 60%.

Rehydration should not exceed 30 min (20 min is ideal). Any longer and the yeast will exhaust their available

food source. Ref: WineMaker, Oct-Nov 2003, pgs 48-52

Rehydration with Go-Ferm®

Step 1: Weigh-out Go-Ferm® at the rate of 1.25 times the weight of dry yeast being used.

Step 2: Suspend the measured Go-Ferm® in 13.3 times its weight of clean tap water @ 110°F.

Step 3: Once the temperature has decreased to 104°F, rehydrate dried yeast in this Go-Ferm® solution – gently

stir-in to eliminate any clumping and then stop stirring.

Step 4: After 15 to 30 minutes, add this suspension to the must, (whose temperature should be ~75-80°F)

If I need to wait (to allow the must to cool), I mix-in ¼ tsp of sugar, and cover the rehydrated yeast with Saran

wrap. Within 10-15 minutes, the yeast should begin to show visible signs of viability (proofing the yeast).

2) Although the following information was intended for dry beer yeast, I believe it underscores some of the

points made above, and provides a bit more technical detail (source: Dr. Clayton Cone, rec.crafts.brewing

03/03/03):

“Every strain of yeast has its own optimum rehydration temperature - all of them range between 95 F to

105F (most of them closer to 105°F). 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 -

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 - 1.0% yeast extract.

Dry Yeast Rehydration

Rev 1 (2/14/07)

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