Read this very informative article over on Brodies Castle beer forum.
"Dry Yeast Rehydration, its critical!
By Moritz Kallmeyer
Chief Brewer Draymans Microbrewery, Silverton Pretoria, March 2005
Each brand of ADY (active dry yeast) specify its own optimum rehydration temperature, ranging from 35C to 40C. As you drop the strike temp from 40C to 15C the yeast will leach out progressively more of its insides, damaging each cell. The yeast viability thus drops proportionally. At 40C there is 100% recovery of the viable dry yeast while at 15C there can be as much as 60% dead cells. The dried yeast cell wall is very fragile and it is essentially in the first minute, even seconds of rehydration that warm temperature is critical while it is reconstituting its cell wall structure. During these first initial minutes of rehydration, the yeast cell wall can not differentiate what passes through the wall. Materials which is toxic to the yeast at this stage like sugars, hop products and SO2 that the yeast normally can selectively prevent from passing through its cell wall, rush right in and seriously damage the cells. The moment the cell wall is properly reconstituted the yeast can regulate what goes in and out of the cell. This is why the suppliers warn against rehydration in wort, instead of water.
The water should be normal tap water ideally with 250-500ppm hardness present. The hardness is essential for a good recovery. This is the reason why distilled or de-ionized water should never be used. I further prefer the water to be carbon filtered to remove any chlorine and impurities, then boiled to sterilize and force chilled down to the required rehydration temperature. Ideally if you have access to it, the warm rehydration water should contain 0.5-1.0% yeast extract like Go-ferm from Lallamand.
Active dry yeast is dormant or inactive, not inert, so it should be kept refrigerated at all times at around 4C. It will only loose 4% of its activity in a year if kept at this temperature. Remember to remove the package from the refrigerator early on brew day so it can naturally attemperate to the rehydration temperature. This prevents one stressor, temperature shock. I prefer to do the rehydration process in a hygienic polyethylene bucket. The yeast should be sprinkled into 10 times its own weight of rehydration water and gently folded in with a spoon to wet them. Oxygen is not needed at this stage so stirring should be avoided. After sitting for the recommended 15 minutes give it a vigorous whirl then again close the lid of the bucket lightly and leave it for 5 more minutes at the specified rehydration temperature.
Built into each cell by the manufacturer is a large amount of glycogen and trehalose reserves that give the yeast a burst of energy to kick off the growth cycle when it is added to the wort. It is quickly metabolized and used up by the yeast within about 30 minutes of rehydration. There is no damage done to the yeast if it is not added to the main batch of wort within this period of time you just do not get the benefit of that sudden burst of energy and lag times are likely to increase. The rehydrated yeast, being warm, should now be cooled to within 4C of the wort before pitching. This attemperation is done over a brief period by adding in increments, a small amount of cold wort (removed earlier from the kettle and chilled) to the rehydrated yeast container. Rehydrated warm yeast pitched into cold wort will cause many of the yeast cells to produce petite mutants that will never grow or ferment properly and will cause them to produce H2S.
Some ADY manufacturers recommend slightly lower than normal wort aeration while others argue that most ADY yeast actually require no O2 addition for successful, average gravity wort fermentation. There are enough lipids built into the cell at the yeast factory. It will however need O2 addition on the next re-pitching."
Personally, I try & rehydrate whenever possible, especially if brewing a big beer - but sprinkling direct on the wort has never given me a problem.
Cheers Ross
