Here some info from the source:
Dr Clayton Cone the Yeast Guru from Lallemand had an interesting discussion with Dan Litermann on the Homebrewers Digest ( hbd.org ) about rehydrating BACK IN 2000 .
Subject: Yeast Q's- Dan Listerman- Dr. Cone
Date: 2000-04-14 20:56:55 GMT
From: Dan Listermann <firstname.lastname@example.org
Subject: Yeast Hydration, Infusion Mashing and England
My question to Dr. Cone regards yeast rehydration. All the packages of
yeast contain instructions for rehydration yet they all ferment just fine
without it. I have to believe that such a procedure may be theoretically
beneficial, however it would seem to be margionally usefull at least on a
I own a home brew shop and a very common phone call is the " My beer is not
fermenting." problem. I go through the list of potential causes ( plastic
bucket lid leaks, too cold, ect.) About twice a week the caller will
indicate that he rehydreated the yeast. This is a strong signal that the
yeast has been damaged and will need to be replaced. I have come to the
conclusion that, since rehydration is not necessary to ferment beer
properly and there is a strong chance that the yeast will be damaged in a
botched rehydration, it is not desirable to recommend such a proceedure.
Just how important is rehydration and is it worth the risk?
Dan Listermann email@example.com firstname.lastname@example.org
I appreciate your dilemma It is a universal problem for those that market
Active Dry Yeast.
Let me give you some facts regarding rehydration and you can decide for
yourself where you want to compromise.
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 - 105 F, there is
100% recovery of the viable dry yeast. At 60F, there can be as much as 60%
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
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 immediatly 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 encrements, a small amount of the cooler wort to the rehydrated
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 intering 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
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. It should also tell you where you are safe in adapting
the rehydration procedure to fit your clients.