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Dry Yeast Viability

Background

One ongoing point of contention among brewers is what benefits, if any, result from rehydrating dry yeasts according to manufacturers’ recommendations, as opposed to simply adding the yeast directly to the fermenter. On the technical level, there would appear to be a consensus for a substantial reduction in viability when rehydrating in wort, with most sources claiming a reduction on the order of 50%. [1, 2, 3, 4] There exists a substantial body of anecdotal evidence, however, maintaining that rehydration offers no perceptible benefits. I already have some data on the effect of pitching rate on beer flavor, and I think it’s reasonable to infer that a similar reduction in pitching rate would result in similar effects, regardless of whether that yeast was originally a dry or liquid culture. To that end, I decided to do a series of basic viability estimates using dry yeast rehydrated under various conditions.

Experimental Setup

Four cell counts were conducted, using Safale US-05 yeast obtained from a 500 g sachet that had been stored cold and was opened the same day. The yeast, according to the stamp on the packaging, was 176 days old at that point. Assuming that it was initially nearly 100% viable cells, and lost 4% per month, its viability should therefore have been roughly 75-80%. One level one-third teaspoon measure (approx. 1.3 g) was rehydrated in 100 mL of each of four different media: tap water and wort at both typical ale pitching temperature (~18°C) and rehydration temperature (~33°C). The water used is fairly low in mineral content; it was tested the following day at 50 ppm CaCO3 total alkalinity and 100 ppm CaCO3 total hardness. The wort used had a gravity of 11.5°P.

After resting loosely covered for half an hour, each sample was capped and shaken to homogenize it. 1.00 mL was then pipetted into 49.0 mL of a 0.02% methylene blue solution and agitated for one minute before a drop was placed on a Neubauer cytometer plate for counting. With the exception of the methylene blue concentration, I use the White Labs protocol for counting – I’ve found a 0.1% solution to be far too dark for effective discrimination of viable and non-viable cells.

Results

With an average total count of 122 cells, the cellular density for the dry yeast works out to 22.9 billion/gram, which correlates well with the established figure of 20 billion/gram considering that the yeast was dispensed by volume rather than mass. The viability estimates for the samples rehydrated in water are also remarkably close to the predicted values, although interestingly enough, the rehydration temperature did not appear to have a substantial effect on viability in either medium. Setting aside temperature as a variable, then, the average viability after rehydrating in wort is 56% of that for the samples rehydrated in water.

Based on these results, I’m confident that the conventional wisdom regarding dry yeast is essentially correct, and that a substantial reduction in the effective pitching rate would result from rehydrating in wort. Additional trials would need to be conducted to determine conclusively whether or not the rehydration temperature is as critical as the manufacturers’ recommendations suggest.

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