What Happens if You Don’t Put Enough Yeast in Beer? (With Fixes)

You’ve just pitched a new batch and realize that you might have used too little yeast. Now you’re wondering what’s going to happen because you didn’t put in enough.

Putting too little yeast in your homebrew – also called underpitching – will force it to work harder over a longer period of time. The more stress the yeast is put under, the sooner it will expend itself, resulting in a higher likelihood of off-flavors in the final beer. The extended brew time increases the chance of the beer becoming infected.

Keep reading to dive into the possible side effects of underpitching as well as tips on how to make sure you add enough yeast to your brew to start with.

What if you don’t use enough yeast in homebrew beer?

When home brewing beer, you should ensure that you are using the correct amounts of each ingredient to maintain a consistent, palatable result. An incorrect ratio can drastically change the taste of your beer.

Using too little yeast in your beer can lead to fermentation starting later than normal. This can lead to off-flavors that you didn’t want or expect. Depending on your goal and tastes, this can either lead either to terrible beer or an interesting experiment.

If your homebrew beer has gone bad, don’t just pour it down the drain! Check out this article for tips on what you can do with it.

It is important to note that if the yeast is not healthy, it won’t start fermenting. If the yeast is too old, it may not work at all. Even healthy yeast needs a properly aerated wort with enough nutrients at a viable temperature to have the best chance.

Healthy or not, when there isn’t enough yeast in your beer it forces the yeast to work harder. Essentially, the yeast cells take longer in creating the necessary enzymes they need to move on to the next phase: budding. The reproduction phase will also take longer because the yeast needs to grow more to reach the point where it can fully consume the sugars present.

This longer window in the first two phases does two things.

First, it creates a higher chance of the beer becoming infected. In a properly pitched batch, the wild yeasts and bacteria that cause infections have more competition from the chosen yeast. These infections feed on the same nutrients so they have a harder time gaining a foothold in the beer if the nutrients are being used up by the cultivated yeast.

Second, it stresses the yeast cells. This stress speeds up the wear and tear on the cells. Stressed yeast doesn’t last as long and has a higher likelihood of producing off-flavors.

When your brew finally reaches the budding phase, the mother cells create as many daughter cells as possible before becoming worn out.

In an underpitched brew, there will be a lot more reproduction going on to transform the sugars into ethanol. This increase in reproduction can cause an increase in ester production.

Does under-pitching yeast cause off-flavors?

With the increase in esters comes additional (often unwelcome) flavors.

Esters primarily create fruity flavors and aromas, but higher concentrations can create a solvent-like taste. While you can’t account for taste it is a safe assumption that no one wants a solvent beer.

One homebrewing trick to reduce the production of esters in this stage is to reduce the temperature. Yeast gets sluggish at colder temps and will slow down its reproduction.

Unfortunately, this can cause problems even if you stay in the ideal temperature range for yeast. If there wasn’t enough yeast in the initial pitch this could stop the fermentation process entirely.

Other sources of off-flavors include wild yeasts and bacteria. Your beer could naturally become infected at various stages but is most commonly caused by unclean equipment. While the beer will still be drinkable as the alcohol will kill anything harmful, it won’t taste the way you intended.

In extreme cases, infected beer can taste rotten. If the infection isn’t dominating the flavor, it can add sour notes. At this point, it is a matter of opinion on whether it’s a problem – some infected beers can actually taste great, especially if you prefer sour brews.

Can you add more yeast if your beer isn’t fermenting?

If your beer isn’t fermenting properly, you can add more yeast. However, if done incorrectly it can be a waste of time and ingredients.

If the fermentation process has already begun but has stopped, most added yeast will die. In this case, the wort’s nutrients will have been mostly used up so the new yeast will have nothing to get started with. In order to avoid this, you would need to use a yeast starter before repitching so that the yeast can continue fermenting.

If fermentation has not begun and you want to add yeast it is still helpful to do so with a yeast starter. That being said, adding yeast is not always the solution when your beer isn’t fermenting.

The yeast that controls fermentation is affected by factors other than quantity. The temperature of your brew can have a significant effect on the yeast: too cold and it won’t start; too hot and the yeast could die. Yeast has a range of temperatures that it is most comfortable in. Be sure to check it on the yeast packaging before brewing.

Yeast is further affected by the sanitizer used. If there is enough sanitizer residue left behind after cleaning it can damage or kill the yeast.

Sometimes it could be that the batch is fermenting, but it is not showing the usual signs. If the airlock isn’t fully closed or has a leak there won’t be any bubbling like usual.

Other times, the yeast could be flocculating or gathering in clumps, inactive. In this case, give the brew a good stir to reactivate it.

How to add the correct amount of yeast to homebrew beer

By now it is quite clear how important yeast is to homebrew beer. Below are a few tips and tricks to make the process as painless as possible.

The best ways to ensure you are getting the most out of homebrew yeast are:

  • Follow a recipe
  • Use a pitch calculator
  • Use the right strain for the job
  • Make a yeast starter
  • Check expiration dates
  • Always have extra yeast on hand

Follow a recipe

People have been brewing beer since humans started settling down as farmers. Over that time we’ve created some pretty great beer. There are plenty of good recipes out there.

Whether you’re new to this hobby or an experienced brewer, recipes are a very helpful starting point.

A good recipe will tell you exactly how much yeast you’ll need depending on the size of the batch. Most even tell you what yeast strain to use.

Use a pitch calculator

If you want to try your hand at making your own recipe, but don’t know how much yeast to use there are calculators that will help.

There are also tables that break it down for you.

Use the right strain for the job

Different strains of yeast are better for different beers.

Some strains of yeast are better for lagers while others are better for ales.

What sets these yeast strains apart are their viable temperature ranges and whether they are top or bottom fermenters. 

Lager yeasts ferment on the bottom and prefer cooler temperatures. Ale yeasts are top fermenters and prefer warmer temperatures.

All yeasts also have different flavors and aromas that they create in a beer.

Make a yeast starter

Yeast starters can ensure that the yeast you use is healthy and ready to begin fermenting your wort.

They are a good way to increase the cell count of the yeast you have before pitching it into your wort.

Check expiration dates

This may seem obvious but it can be easy to overlook.

Expired or old yeast will have lost a lot of its viability. If it is well past expiration it likely won’t work at all.

When yeast is old it becomes less adaptable to non-perfect environments. Old yeast will become stressed much easier.

Always have extra yeast on hand

In case you need to repitch it is very helpful to have extra yeast just in case.

Homebrewing can be fickle and you may need to throw out a batch for one reason or another.