Pitching rate, the number of yeast cells used to inoculate a given volume of wort, influences several things in the brewing process. Higher pitching rates lead to faster fermentations — they start faster and finish faster. Higher pitching rates also lead to finishing gravities closer to what is predicted by a forced fermentation test. In other words, the yeast utilize all the carbohydrates that they can. In contrast, in severely underpitched beers, the yeast may quit early and leave fermentable carbohydrates behind, resulting in a higher final gravity (FG). Low pitching rates are frequently the cause of stalled or stuck fermentations.
For “characterful” yeast strains that produce plenty of fermentation byproducts, higher pitching rates are associated with “cleaner” beers. Some Belgian ale strains produce an estery, “spicy” aroma when slightly underpitched, but produce a cleaner beer when pitched at a higher rate (to a well-aerated wort). Temperature also plays a major role, with higher temperatures leading to more fermentation byproducts.
This is true of White Labs WLP530 (Abbey Ale) and Wyeast 3787 (Trappist High Gravity) yeast — larger than optimal pitches, thorough aeration, and low temperatures (within the usual ale fermentation range) lead to clean fermentations. It is also true that the “banana ester” level in German hefe-weizens can be manipulated this way.
Homebrewers who are concerned about their pitching rate generally consult a pitching rate calculator, then make a yeast starter of the suggested volume. However, if your brewday arrives and you haven’t made a starter, there is a way to “cheat” that may come in handy occasionally.
I discovered this trick to boosting your pitching rate by accident. Back in the early 2000s, I was brewing a lot of hefe-weizens. I was searching for a way to lower the overall amount of German wheat yeast aroma. At the time, I had not heard about the relationship between pitching rate and ester production. So, what I tried was pitching equal amounts of a German hefe-weizen strain and a neutral American ale yeast. The idea was to dilute the aroma of the German hefe-weizen yeast. However, the beer turned out smelling and tasting like a regular hefe-weizen. (I later tried pitching the ale yeast first, then the wheat beer strain a couple days later, and got the same results.)
Sometime after that, I had an opportunity to brew my porter. However, I had not made a yeast starter. Remembering the German wheat beer experiment, I went ahead and brewed, pitching one smack pack of English ale yeast (Wyeast 1968) and rounding out the pitch with half a packet of dried ale yeast (Fermentis US-05). The fermentation started quickly and produced a porter that was only slightly different than usual. (It was a little drier.)
So, the “trick” is to use dried yeast — which is fairly inexpensive — to boost your pitching rate when you haven’t made an adequately-sized yeast starter. You can do this when you haven’t made yeast starter at all, or if you don’t have a starter vessel large enough to hold the required volume.
For any ale, you can simply use a neutral ale strain to boost the cell count. If your other ale strain shows a lot of fermentation characteristics, the neutral ale strain will not dilute those characteristics — or, at least, it did not (to a noticeable degree) in the beers I’ve tried. The “characterful” ale strain still produces its esters and other fermentation byproducts. The neutral ale strain just ferments cleanly along side it. It does not “cover up” the products of the other yeast strain.
You can also use this approach to ferment with a favorite yeast strain, but get a higher degree of attenuation.
I have only done this a handful of times. Each time I have used Fermentis US-05 (although it was called US-56 back when I did this) as the dried yeast strain. I have not tested this yeast versus all of the liquid yeast strains available from White Labs and Wyeast (or any other supplier). So, it is possible that it may not work in some cases. Each time I tried this, the overall pitching rate was near the optimal level, with each strain comprising 50% of the pitch.
One potential problem with using this method involves killer yeast strains. Some yeast strains are killer yeast strains. These killer yeast strains secrete proteins into their growth medium to kill off competing yeast strains. Yeast strains vary in their sensitivity to these proteins, but a killer strain is never sensitive to its own killer factor. So, it is possible that some combinations of yeast strains may not work well together. As most beer is made from a single yeast strain, I’m not aware of any source that lists which strains secrete killer factors and which strains are sensitive to these factors.
A bigger problem is a potential mismatch in attenuation. The Fermentis US-05 strain is more attenuative than many popular ale strains. (In other words, it generally ferments to a lower FG.) Pairing this strain with another that typically finishes at a higher FG may produce a beer that is more attenuated than usual. This may or may not be a problem, depending on how much body and residual sweetness you want in your beer.
Mismatches in the flocculation characteristics of the yeast strains could also cause problems. If you are used to using a highly flocculant yeast strain, and you co-ferment with Fermentis US-05 (which is not very flocculent), it may take longer for your beer to clear than you are used to. On the other hand, it is possible that the more flocculant yeast strain may drop out early, and drag a less flocculant strain with it. This could potentially limit your attenuation.
Now that dried strains of lager yeast are available to homebrewers, it would be interesting to see if this works in a lager. I don’t see why it wouldn’t. However, I haven’t tried it.
Dried yeast is less expensive than liquid yeast. And many homebrewers keep a sachet or two of dried yeast in their fridge in case of a stuck fermentation. In a pinch, you can use a neutral dried ale yeast to increase your pitching rate for any ale fermentation. In the few times I’ve tried it, co-fermentation with a neutral ale yeast did not dilute the character of the non-neutral yeast. Co-fermentation may also allow you to achieve a higher degree of attenuation in some beers, while still using your favorite ale strain for its aroma profile.