Originally Posted by bottlebomber
Well why do people often sprinkle it right into the fermenter? Is that poor practice?
According to Jamil's Yeast book, page 146:
Skipping rehydration of dry yeast kills about half of the yeast cells pitched... the dead cells immediately begin to break down and affect the beer flavor.
The yeast are vulnerable during the first few moments and are not able to regulate what passes through their cell membranes. Rehydrating with warm water before pitching reduces the shock they experience since warm water is less harmful to the yeast than sweet wort.
I've been sprinkling dry yeast onto my wort with good results but after reading the above I'm going to rehydrate the yeast in the future before pitching.