Is Your Pitch Rate Calculator Lying to You?

You probably use an online starter and/or pitch rate calculator to determine the size of your starter. The conventional wisdom is that to achieve an optimal fermentation you should pitch at a rate of 0.5 to 2.0 million cells per mL per degree Plato depending on your starting gravity — and other details, like whether you’re brewing a lager or an ale. You input target pitch rate and batch size (and more) and it outputs the starter volume required to produce a total number of cells. For example, 1.3L of 10°P (1.040) wort and 150 million cells. 

Of all the inputs you need to enter, the yeast strain is not of them. That means the calculator is built to assume that all brewing yeast behave the same. Yeast strains have evolved down separate paths for centuries, so it would be unreasonable to expect them to all behave identically. So, why does it work?

We inoculated flasks containing 100 mL of 10°P (1.040) wort each with one of the more than 70 yeast strains we have in our collection and grew them up until they reached their maximum cell density. Then we counted the cells using a microscope and hemocytometer. In addition to counting cells, we measured biomass. Here are the results:

The maximum cell count varied significantly from strain to strain, even though each were grown in identical and ideal growth conditions. That means not all yeast produced the same amount of cells in the same environment when given the same resources. That’s a good indication that they’re using resources differently.

Hefeweizen Ale I yielded 98 million cells per mL and West Coast Ale I yielded about 50% more (146 million cells/mL). The Brett yielded quite a lot more, at 885 million cells/mL. And French Saison yielded 465 million cells/mL — almost five times more than the Hefeweizen.

If strains were all behaving the same, as the calculators assume, you’d expect the cell counts to be the same. You’d also expect the amount of yeast matter to be proportional to the cell count across strains. If that were true, a strain that produced four times the amount of cells would have shown four times the volume of biomass.

The French Saison (OYL-026) produced nearly five times the number of cells than the Hefeweizen (OYL-021), but only about half of the biomass; West Coast Ale I (OYL-004) and the Brett (OYL-203) produced amounts of biomass that are about equal, but the Brett had six times the number of cells. So, what is behind the difference in cell count versus biomass?

When looking under the microscope, it’s easy to spot: French Saison has very small cells and Hefeweizen has much, much bigger cells.

Remember, from yeast’s perspective, they are not here to make beer, they’re here to make more yeast. Think of them like little factories whose job is to make more factories. The components in the wort are the raw materials needed to make a new factory, and the factories will make more copies of themselves until they run out of raw materials. Consider things like ethanol, carbon dioxide, and the secondary metabolites (flavor compounds that we taste and smell) to be the ​“emissions” than these factories make.

Different yeast strains have different ploidy levels. The ploidy level refers to the number of sets of chromosomes a yeast strain has — a strain whose cells have one set of each chromosome is haploid, two sets is diploid, four sets is tetraploid, etc. Though there are other factors at play, it is fairly well established that the number of sets correlates to the size of the cell, meaning the higher the ploidy level, the larger the cell size.

The bigger factories use more wort nutrients to make a copy of themselves, so they end up yielding fewer numbers of factories. They might start with one factory and end up building four. Also, some factories are not very efficient. A large factory that uses its materials inefficiently might end up with two factories instead of four (fewer factories can mean more ​“emissions,” like alcohol). The smaller factories make many more factories than the big ones.

So, if large-celled strains use the same wort nutrients to produce fewer cells than small-celled strains, and conversely, if small-celled strains, though smaller, produce greater numbers of cells, you can see how biomass between strains can end up being remarkably similar, even though the cell count can be extremely different.

Understanding optimal pitch rate by cell number may be a misleading way to look at things biologically speaking when comparing different strains. Different yeast strains are going to grow to different densities in the same environment because of their genetic differences. If the cell count varies so widely by strain, but the biomass is relatively similar, our starter/pitch rate calculators seem to be doing a better job at measuring biomass and not, in fact, cell count. Because of this, we suggest that optimal biomass is more biologically relevant than cell count.

Brewers probably calculate by cell count the way we do now because it is an easier, more generalized concept. It could also be in part because a lot of the research has been based on lager strains where cell size may be less variable. Regardless, if you took your pitch rate calculator literally and actually pitched the number of cells it suggests, the amount of biomass would vary widely by strain.

In the end, your starter/pitch rate calculator has been lying to you, it’s just that it probably doesn’t matter. Go ahead and keep using it — just think of it as a biomass calculator instead!