If you look on the manufacturers site(specifically fermentis) you will see they recommend rehydration, their pitch rates ARE based off of rehydrated yeast. Cell counts are often lower than quoted, and you kill roughly 50% from non-rehydration. So I know in the case of lager yeast. Most people are under pitching for sure.
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Even rehydrated... or the recommended cell counts of yeast pitching calculators are way too high.
I wouldn't say rehydrated its low, barring you follow the manufacturer yeast specs.
For us05:
FERMENTATION: ideally at 18-28°C (64-82°F)
PITCHING: 50 to 80 g/hl
So for for 18c the pitch rate 80g/HL or 15.2g per 5 gallons REHYDRATED.
For 34/70
FERMENTATION: ideally 12-15°C (53.6-59°F)
PITCHING: 80 to 120 g/hl for fermentation at 12°C – 15°C (53.6-59°F).
increase pitching for fermentation lower than 12°C (53°F), up to 200 to 300 g/hl at 9°C (48°F)
So for 12c pitch rate 120g/hl or 22.8g per 5 gallons REHYDRATED
IF you want to ferment it cold you are looking at
57g per 5 gallons REHYDRATEDThese are pitch rates at the low end of optimal, and moderate gravity beers....