While most commercial brewers rehydrate their dry yeast before pitching, many homebrewers just sprinkle the dry yeast on top of their wort. Perhaps they read it in a book, or their local expert told them rehydration was not necessary. Technically the beer will ferment if you pitch enough nonrehydrated yeast, but you are not giving the yeast an opportunity to make the best beer possible. Skipping rehydration kills about half the cells pitched. Besides having only half as much yeast as is needed, the dead cells immediately begin to break down and affect the beer flavour.
I always rehydrate my dry yeast, but I had no idea that if you pitched it directly into the wort without rehydration it kills half of the cells. I figured that I would pass this along here in case others weren't aware of this.
Also, making a starter with dry yeast is something that recently came up here in one of the threads. Here's another small snippet from the book regarding that.
Another case where you normally do not want to make a starter is with dry yeast. Dry yeast is inexpensive, and it is usually cheaper, easier, and safer to buy more dry yeast than to make a large starter. Many experts suggest that placing dry yeast in a starter just depletes the cell reserves that the yeast manufacturer tries to build into their product. For dry yeast, do a proper rehydration in tap water; do not make a starter.
Anyways, just wanted to past along to info from this book. Btw, it's a fantastic book on yeast and I'd highly recommend it. It provides very in depth information but in an easy to understand way where you don't need to be a scientist to understand it.
http://www.amazon.ca/Yeast-Practical-Gu ... 233&sr=8-1