What Happens If You Under-Pitch Yeast: Effects on Fermentation and Beer Quality

Yeast plays a critical role in the fermentation process by converting sugars into alcohol and carbon dioxide. The amount of yeast added to the wort, known as the pitching rate, significantly affects the fermentation process and the final quality of the beer. Under pitching refers to adding an insufficient amount of yeast to the wort. In this article, we will explore the effects of under-pitching yeast and its impact on fermentation and beer quality.

Slow or Stalled Fermentation

Under pitching yeast can result in a sluggish or stalled fermentation. An insufficient yeast population may struggle to effectively consume the available sugars, resulting in a slower fermentation process. This can extend the time required for primary fermentation and increase the risk of contamination by unwanted microorganisms.

Incomplete attenuation

Yeast is responsible for converting fermentable sugars to alcohol and carbon dioxide. If the yeast is under-utilized, it may not be able to fully ferment all of the available sugars, resulting in incomplete attenuation. This can result in a beer with higher residual sweetness and lower alcohol content than intended.

Off-flavors and aromas

Understimulated yeast can contribute to the production of off-flavors and undesirable aromas in the beer. When yeast is stressed by low pitching rates, it can produce higher levels of metabolic by-products such as esters, fusel alcohols, and sulfur compounds. These compounds can impart flavors and aromas ranging from fruity or spicy to harsh or solvent-like characteristics, negatively impacting the overall flavor profile of the beer.

Poor flocculation and clarity

Yeast plays a critical role in flocculation, the process by which yeast cells clump together and settle to the bottom of the fermenter. Inadequate yeast pitching can lead to poor flocculation, resulting in hazy or cloudy beer with reduced clarity. This can affect the visual appeal of the beer and may require additional clarification steps, such as fining or filtration, to achieve the desired clarity.

Increased risk of contamination

An inadequate yeast population can create an environment more susceptible to contamination by unwanted microorganisms, such as wild yeast or bacteria. These contaminants can cause off-flavors, spoilage and potential health risks. Understimulated yeast compromises the yeast’s ability to quickly establish dominance and outcompete unwanted microorganisms, increasing the risk of infection during fermentation.

Methods for Calculating the Proper Yeast Pitch Rate

Calculating the correct yeast pitch rate is critical to a successful fermentation. Here are some commonly used methods for determining the proper amount of yeast to pitch:

  1. Yeast Pitching Rate Calculator:
    Use online yeast pitching rate calculators or software designed specifically for brewers. These calculators take into account factors such as batch size, desired beer style, starting gravity, and fermentation temperature. They provide recommended pitch rates based on industry standards or specific yeast strain requirements.
  2. Brewer’s Rule of Thumb:
    A general rule of thumb is to pitch approximately 0.75 to 1 million cells per milliliter of wort per degree Plato (°P). The Plato scale is a measure of the concentration of sugars in the wort. For example, if you have a 20 liter batch with a starting gravity of 12°P, you would aim to pitch 150-200 billion yeast cells.
  3. Yeast Pitching Rate Charts:
    Reference yeast pitching rate charts provided by yeast manufacturers or brewing literature. These charts typically suggest pitching rates based on batch size and gravity. They can be a useful guide, especially when using specific yeast strains.
  4. Cell Counting:
    For more precise control of yeast pitching rates, you can perform a yeast cell count using a hemocytometer or specialized yeast cell counter. This method involves taking a sample of the yeast slurry, diluting it, and counting the cells under a microscope. This allows you to calculate the exact cell concentration and adjust the pitching rate accordingly.
  5. Repitching from a previous batch:
    If you are repitching yeast from a previous batch, you can estimate the pitching rate based on the volume of yeast slurry used. A common recommendation is to pitch 10-20% of the yeast cake from a previous batch, depending on factors such as the age and condition of the yeast.

It’s important to note that different yeast strains and beer styles may have specific pitching rate requirements, so it’s beneficial to consult yeast manufacturers’ recommendations, brewing literature, or experienced brewers for guidance. Adjustments to pitch rates may be necessary based on factors such as desired fermentation characteristics, oxygenation levels, and wort composition.


Proper yeast pitching is essential for successful fermentation and the production of quality beer. Under-pitching yeast can result in sluggish or stuck fermentations, incomplete attenuation, off-flavors, poor flocculation and an increased risk of contamination. Brewers should carefully calculate and adjust their yeast pitching rates to ensure a healthy fermentation process, optimal flavor development and desired beer characteristics. By understanding the impact of yeast pitching rates, brewers can take the necessary steps to ensure consistent and excellent beer production.


What happens if you under pitched yeast?

If you under-pitch, meaning you don’t add enough yeast to the cooled wort waiting inside your fermenter, the individual yeast cells may struggle to do more work than they can handle. They can reproduce too many times in order to compensate, which increases the chances of off-flavors.

Is it better to over pitch or under pitch yeast?

In general, underpitching affects flavor more, while overpitching negatively affects yeast health more over generations. However, both can result in a less than ideal fermentation with high levels of diacetyl, acetaldehyde, and low attenuation.

Can you Overpitch yeast?

Meaty flavours are typical in this case providing lingering harsh aftertastes. In addition, high levels of yeast autolysis can increase beer pH affecting your beer’s shelf life. Overpitching does, of course, produce more yeast in suspension which is likely to result in faster fermentation – albeit within limits.

Can I pitch more yeast after fermentation has started?

Once your wine has successfully fermented there is never any reason to add more yeast to the wine. The wine yeast you originally added at the beginning multiplies during the fermentation. If the fermentation went as it should, there should be about 100 to 150 times the amount of wine yeast you added, originally.

Can I pitch yeast twice?

Yes, you read that right: double yeast pitching.

Will more yeast make fermentation faster?

Adding more yeast should ferment faster. The risk is not so much off flavors but a lack of fermentation flavors – esters, etc. You might be able to pick a yeast that finished faster. Probably better to think of the whole process.

Can I pitch dry yeast?

You can pitch dry yeast straight into the wort. Or you can add it to water just prior to pitching. Dry yeasts have a longer storage life than liquid yeasts. Liquid yeasts must be stored by refrigeration means.

What happens if you over pitch yeast in beer?

Adding too many or two few viable yeast cells ruins a proper wort. Overpitching exhibits truncated respiration, a flash fermentation (12-24 hours), and stressed yeast.

How much yeast do I need to pitch?

A good rule of thumb is to pitch about twice as much yeast for a lager as for an ale: For ale, you need about 0.007 fresh liquid yeast vials or packs per gallon per gravity point. For lager, you need about 0.015 fresh liquid yeast vials or packs per gallon per gravity point.