But my understanding has always been that the amount of viable yeast in a dry back is MASSIVE compared to the amount available in a smackpack.
There are roughly 220 million cells, 86% of which are viable at packaging (on average) for dry yeast compared to 100 million cells with 99% viable at packaging. If you sprinkle on top, you are pitching roughly the equivalent of a tube of liquid yeast.
No. Rehydrating does not increase the number of yeast cells, it just lets the dehydrated cells take in water and "regain their composure" (for lack of a better term) so when they hit the wort they are rehydrated and ready to work their magic.
A starter actually grows more yeast cells. Liquid yeast vials/packets contain much fewer cells than a dry packet, and you therefore need to put that liquid yeast through a full growth and reproduction cycle so you can have an adequate number of viable cells at pitch time. This is especially true with high gravity beers. The starter wort allows the provided liquid yeast an opportunity to go through the growth and reproduction cycle in a short period of time, increasing the cell count significantly saving the cost of purchasing multiple vials/packs of liquid yeast.
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Thank you, I didn't realize that. I'll have to look into rehydrating my yeast from now on.