Wizard responds: I went to the Wyeast Web site to do a little bit of recon before answering this question. I found the site very informative and did see much of the information described in your question. What I could not find was any reference to the general rule you cite in your question. However, there is a chart on pitching rate that you may find very useful. Although at first glance the information on the Wyeast Web site seems inconsistent with the assumptions you make in your question, pitching rate is not black and white and the “rule of thumb” pitching rate of 1 million cells/ liter/°Plato is a rule of thumb. If there is no room for a range of pitching rates companies like Wyeast could only satisfy one type of customer, for example the 12 °Plato group, with their yeast packets.
So here is the real scoop about pitching rate. Commercial brewers use a rate that ranges from about 5 million cells/mL to about 20 million cells/mL. The rule of thumb for a 12 °Plato wort dictates a pitching rate of 12 million cells/mL, which is in the middle of the range. In general, ale brewers who want ales with more pronounced esters pitch on the lower end of the range and ferment warmer. Lager brewers tend to pitch at higher rates and ferment cooler. And both ale and lager brewers increase pitching rate with their higher gravity beers. Remember that most of the beer brewed in the world is lager so the rule of thumb is heavily influenced by lager brewers.
It does seem that the minimum cell density of 100 billion cells per packet tends towards the low side of the typical range of pitching rates. But these packets also have an activator to get things metabolically rolling and this is factored into the amount of cells in each packet. Another item discussed on the Wyeast Web site is propagation. If you want to pitch at a higher rate you can propagate or add a second packet of yeast. Like most things in brewing you can spend a lot of time and effort trying to follow textbook rules and even monitor the process to verify that you are indeed following the rules. For example, if you had a microscope and hemocytometer you could do yeast counts to monitor pitching rate. And you can also taste your beer and ask the question, “did everything go OK?”
If everything did not go OK, and your beer suffers from faults that are too numerous to count, then you probably need to carefully evaluate your whole process. This is not normally the case and some problems are often suspected before finishing the beer and tasting. If you fear that you are under pitching there are several symptoms associated with under pitching that are pretty easy to spot. Slow, lagging fermentations are one common symptom and beers with odd aromas are another. If you are using these yeast packets without problems then I fear you have early signs of becoming a homebrewing hypochondriac. Not to worry, this is not an irreversible illness and recovery is possible. If you are having problems associated with under pitching, increase your pitching rate and see if the problems go away.
I think I have become more grounded with experience. In my younger and more boisterous days I was sometimes guilty of thinking bad thoughts such as, “those guys, I know what they’re doing, they’re trying to confuse the facts and trick me into thinking that [fill in the blank].” That line of thinking may work for some consumer products, but doesn’t hold much water with art supplies.
Yeast is an art supply for the brewing artist. Some of us make rock-solid beers by pitching 5 million cells per mL and others make rock-solid beers by pitching 20 million cells per mL (that must read like total geek speak to the non-brewer!). I don’t believe any yeast supplier does anything to confuse or “cheat” their customers and they pick a product to market that hopefully fits the needs of most of their customers. If your needs are different, use the information about the product to make the necessary medications.