It seems that this topic been brought to the attention of Fermentis. They reached out to me yesterday, and I've exchanged some emails, covering several issues.
They dispute the "contamination" idea, based on the assertion that no brewing yeast, liquid or dry, is 100% pure, nor does it need to be for brewing purposes. Further they suggest that in triangle tests, panels cannot distinguish beers fermented with liquid or dry yeast, but when informed that a beer is from dry yeast, will then claim to detect tartness. All very interesting (and as I said I may be deluding myself as to the taste because I've seen the pH readings) but of course, valid or not, that's all off topic, as what I'm curious about is the actual measured pH value.
very interesting is this:
They provided an EBC paper (1997) reporting comparisons of dried yeast and second generation brewery yeast, both prepared from the same lab culture of a lager
strain, using pilot scale fermentation. They found that rehydration vs direct pitching of dried yeast made no significant difference in fermentation performance, including cell count during fermentation. There was no significant difference between the liquid and dried yeasts in fermentation performance. They also found no significant analytic difference between the beers made with the liquid and dried yeasts, including finished beer pH, and tasting panels could not distinguish them. This is a scanned image, not a web page, but here's a link to it on my Google Drive if anyone's interested. https://drive.google.com/file/d/1zh2KW1pgHP-KM6CU5sxEOukTHm7uFdNM/view?usp=drivesdk
Now, this experiment involved a lager
yeast. I mentioned above in the thread that I have only one example of a dried yeast lager fermentation to offer, and no comparison with a repitch, but that my dried yeast fermentation was seemingly consistent with my experience with liquid cultures of the same original strain (W-34/70.) So also consistent with the results in the paper. Where I have
seen the low pH with first generation dried yeast is in ales,
specifically Nottingham and S-04. So it would still be interesting if the community could provide more data. If you repitch, do you see a measured pH difference in subsequent fermentations, with either liquid or dried yeasts? If you don't repitch, do you see such a difference between liquid and dried yeasts on a group average? Does rehydration matter? We may be able to do one of two things: Either show that there is something happening that needs an explanation, or that my observations are statistically insignificant or unreproducable. Either would be interesting.
(Whether I am able to do a starter with dried yeast this week will depend on my schedule. I may direct pitch again on brew day, either Nottingham or S-04. Either way, it will be another data point. )