A question that is commonly asked by home brewers when picking up a recipe kit for brew day. It’s really a good question too. Most home brewers understand that they need yeast to make beer, but too few really know just how much yeast is needed. Which means you need to think about the yeast cell count in your pitch. So why is the count so important? What happens when you under-pitch? What happens when you over-pitch? Not utilizing the correct number of yeast cells needed to carry out this process can have unfavorable results. There are a number of possible scenarios, so let’s cover a few.

 

When discussing proper cell count I like to use the analogy of soldiers in a battle. Never send a platoon into battle when you need a battalion to get the job done. Yeast are living organisms and as such, they behave accordingly. Overwhelming them with too much of a workload puts them under intense stress. Introducing them into an environment where there is little work for all of them to do will make them lazy. So either way the fermentation process will be compromised and the final product, your beer, will be less than ideal.

 

For instance, if you under pitch, you could notice high final gravities, a really long initial growth phase lasting 24 hours or longer, and overall slow fermentation. We get a lot of calls at the shop with questions like: “I pitched my yeast like two days ago and I still haven’t seen any bubbles in the airlock”. Other callers state “I measured my final gravity and it’s a lot higher than it should be”. We then ask questions to narrow down the cause and it’s usually because the proper number of yeast cells were not utilized. The caller didn't make a starter for a liquid yeast or maybe they didn't pitch enough dry yeast to handle the gravity. They find out the hard way that under pitched yeast can undergo stresses that can cause the cells to produce increased levels of diacetyl and other esters and off-flavors. Plus, a low yeast cell count can increase the risk of contamination by bacteria and wild yeasts. Something is going to grow in all that sugary wort, and you really want your brewer’s yeast to be it. The sooner you get your yeast up and running, the less likely you are of having an infection.

 

On the other side of the coin, over-pitching your yeast can also have negative effects. Too many yeast cells can both create off flavors and diminish some of the beneficial or desired esters that healthy, correctly pitched yeast bring to the table. Some styles, such as Belgian ales rely on ester production for their aroma and flavor characteristics, which would be lacking if the yeast was over pitched. Or if you are pitching a clean ale yeast, you might get flavors you normally wouldn't. This is in part due to the effects of autolysis which occur when there are an excess of yeast solids caused by inactive yeast cells that are not in the process of reproduction, making alcohol, or cleaning up after themselves. This is due to simply too many yeasts at the dinner table and not enough food to go around. Other noticeable characteristic of over pitched beers can be a thinner mouthfeel, body, and even diminished flavor due to over-attenuation.

 

So what is a proper pitch? Well typically about 0.75 billion cells for each gravity point per gallon of wort for an ale (or 4 billion cells for 5 gallons wort), and twice that for a lager. In other words, for a gravity of 1.050-55 or lower, we usually recommend one dry yeast packet or one pack of fresh liquid yeast (always check the date and make a starter if the pack is more than a month old). For gravities 1.055-60 to about 1.085 original gravity, you should use 2 dry yeast packs or two packs of liquid yeast. Beyond that, 3 packs of dry yeast or 3 or more packs of liquid with a starter, along with proper oxygenation. These values are approximate. To wrap it up, the best and most reliable way to go is to use a yeast calculator. There are many of these available and most offer options of entering a born on date for liquid yeast so you can calculate yeast viability based on age, etc. We like to use Mr. Malty.

 

Keep those yeast healthy and keep up the best practices you can!

--Pete Walden

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Pete is a consultant with Texas Brewing Inc. who has won multiple awards for his homebrew. He has a passion for brewing and enjoying great beer that he will always be glad to share. You can see more from Pete in the Pete's Extract Tips segment from our Come and Brew It video series and you can regularly hear more from Pete in our Come and Brew It Radio podcasts.