by OzKev » Sun Apr 13, 2014 8:39 am
I think it also comes down to pitch rates, temperatures and desired flavor outcomes (esters). Things like Lowans we pitch a very high rate, and then we are running it at a higher temperature. Even if we lost 1/2 of the viable cells with 4 times the pitch rate we still have twice the final viable pitch rate. Then with the increased temperature it significantly reduces the lag period. Hence not necessarily the same need for rehydration. That's why a lager yeast needs to be pitched at twice the rate of an ale yeast. Then say an ec1118 we are reducing the pitch rate and temperatures, so hydration for yeast viability starts to become more significant. When yeast reproduces in the log phase it produces a lot of the byproduct esters, which add desired character. Compare to the beer world, most the time the yeast is a lot more expensive and the pitch rate is a lot less to allow these byproducts to be produced. Pitching 10g of Lowans, or 100g of Lowans into a rum wash will still both produce the final product. They will take different amounts of time to get there, and there will be a difference in taste. How much difference of that taste carries over from the still, and how much is removed in the heads will vary.
More viable yeast cells also reduce the changes of infections taking hold. I believe all worts are infected. Don't believe me, check out a yeast msds. From SafSpirits American Whiskey, Acetic acid bacteria: < 1 x 10^3 / gramme. Lactobacillus: < 1 x 10^4 / gramme. So it's acceptable for some trace to be in the product. Then when the fermenter is open to the air during filling etc, you will get some bacterial sources in there. So a higher pitch rate increase the good cells Vs bad cells count in our favor. As long as the level of infection is not detectable by our taste buds we have succeeded in our ferment.
Personally from my >20years experience in brewing I don't believe the loss from pitching yeast dry is anywhere near 1/2. I think it should say up to 50% loss. The kill rate would be from many things. Thermal shock due to difference in temperatures between yeast and wort, actual wort temperature, wort pH, wort O2 levels, wort nutrient levels, initial yeast health, yeast clumping, amount of damage to the cells due to strong stirring, there's probably more but these are just of the top of my head. Even a drop of Olive oil on the wort can make a change to the overall speed of the ferment.
Remember bad hydration techniques can kill more cells than pitching dry. The main killer is time, leaving it for an hour or so in pure water would most likely result in less viable cells that pitching dry.
For the record I don't rehydrate any of my yeast anymore. My rum, as discussed prior, has a good enough pitch rate for me to pitch dry. My whiskey using the SafSpirit American Whiskey yeast I create a light starter. All this is a mix 50:50 chlorine free water and the wort it will be pitched into, at approx 30c. Allow the yeast to warm up close to the starter temperature, stir the starter mix and sprinkle in the yeast. Within 30mins you should see activity, and since it has a food source it can happily commence fermenting and be left for hours until you are ready to pitch it. There is a separate debate about using starters at lower gravity than the target wort, but I won't get into that.
Hope this helps 8-}