How to know if yeast is spoiled

In this brief article, we will answer ‘’How to know if yeast is spoiled’. This article will also discuss the health consequences of eating spoiled yeast, what the shelf life of yeast is and how to store it in order to prolong its shelf life.

How to know if yeast is spoiled?

If your yeast has a moldy smell with visible mold growing on it with discoloration, then it is spoiled and it is better to discard it. 

You should never eat spoiled yeast, especially if it has visible mold growing on it. Molds can produce dangerous mycotoxins that can make you very sick (1)

It is therefore recommended that you always proof your yeast to ensure that it still has active ingredients and will fulfill the purpose. Instead of directly using yeast in flour, make sure that it is not spoiled by proofing!

To proof yeast pour a cup of warm water in a bowl and add some sugar. Slightly stir it and add the yeast that needs to be proofed. Check for foams and bubbles after 10 minutes. If there are no bubbles and it is just plain water with clumps then the yeast is spoiled.

What are the signs that yeast has spoiled?

If you are unsure whether your yeast has gone bad or not, look for the following symptoms:

  1. Discoloration: If you open the pack and see that your yeast has turned grayish to a dark brown then it is spoiled. Fresh yeast is a light brown color. 
  1. Clumping: Old yeast which does not activate starts to clump together which is a sign that the yeast is spoiled. Even if you use this old yeast, it won’t activate so it is better to open a new packet. 
  1. Moldy smell: Moldy smell is a sign that the yeast is spoiled. Look for bacteria growing to confirm but don’t take any risks with a yeast which smells bad. 
  1. Microbial growth: Yeast with green molds growing on it is spoiled and should not be used at all. The microbial growth will also create a very bad odor and will not let the yeast activate. 

Can you get sick from eating spoiled yeast?

Yes, eating spoiled yeast can potentially lead to foodborne illnesses caused by different pathogenic microorganisms (2-5). 

While yeast itself is not typically harmful when consumed in small quantities, if it becomes spoiled or contaminated with certain pathogens, it can pose health risks. 

Here are some examples of pathogenic microorganisms that can be found in spoiled yeast and the associated symptoms they may cause:

  • Salmonella: Salmonella is a type of bacteria commonly associated with food poisoning. If spoiled yeast is contaminated with Salmonella, it can cause symptoms such as nausea, vomiting, abdominal pain, diarrhea, fever, and headache (2).
  • Staphylococcus aureus: S. aureus is a bacteria that produces toxins causing food poisoning. If contaminated yeast is consumed, symptoms can include nausea, vomiting, stomach cramps, diarrhea, and occasionally fever (4-5).
  • Clostridium botulinum: C. botulinum is a bacterium that produces a potent neurotoxin causing botulism (6). 

If spoiled yeast is contaminated with C. botulinum and the conditions are favorable for toxin production (anaerobic environment), it could cause symptoms such as muscle weakness, blurred vision, difficulty swallowing, and respiratory problems (6).

You should know that the likelihood of encountering these pathogens in yeast is relatively low, as proper food handling and storage practices can prevent contamination.

However, if you suspect that you have consumed spoiled yeast or are experiencing any of these symptoms after eating it, it is essential to seek medical attention promptly.

What is the shelf life of the different types of yeast?

The shelf life of different types of yeast can vary. Here you can find an overview of the typical shelf life for active dry yeast and instant yeast:

  1. Active Dry Yeast:  The best-by date for active dry yeast is usually around 2 years from the manufacturing date. 

However, if the package remains unopened and is stored properly in a cool and dry place, it can often be used for a few weeks past its expiration date. 

It’s important to note that the effectiveness of the yeast may decrease over time, so it’s best to check for signs of activity before using it as described before.

  1. Instant Yeast: Instant yeast, also known as rapid-rise or quick-rise yeast, is already in its activated form and does not require prior activation. It is more finely ground and dissolves quickly in dough. 

The shelf life of instant yeast is typically shorter compared to active dry yeast. Unopened packages of instant yeast can usually be stored for about 2 years from the manufacturing date. Once opened, it is advisable to use it within 4 to 6 months for optimal results.

To maximize the shelf life of both types of yeast, it is very important to store them in airtight containers in a cool, dry place, away from moisture and direct sunlight. 

Additionally, as mentioned before, it is always recommended to check the yeast’s activity by proofing it before using it in recipes, especially if it is close to or past the expiration date.

Can you use active yeast after the expiration date?

Yes, you can use dry yeast after its expiration date if it still serves the purpose of raising the flour. You can check that before using it but if there is any moldy growth or moldy smell inside the packet, then discard it.

How to properly handle yeast to avoid spoilage?

You should always remember that proper handling and storage of yeast can help prevent spoilage and maintain its freshness. Here are some guidelines for handling yeast to avoid spoilage:

  • Storing Active Dry Yeast:
    • Store active dry yeast in a dark and dry place, such as a pantry or kitchen cabinet. Make sure the area is cool and away from direct sunlight. 
  • If you don’t have a pantry, you can store it in the refrigerator or freezer. In a cool environment, active dry yeast can last up to 2 years.
  • Once opened, the shelf life of active dry yeast reduces to around 4 months in the refrigerator. However, if you store it in the freezer, where the temperature is even lower, it can stay good for about a year.
  • Use a tight container or sealable bag to store the yeast, protecting it from excess moisture and humidity.
  • Storing Fresh Yeast:
    • Fresh yeast, also known as cake yeast or compressed yeast, should be stored in the refrigerator. It has a shorter shelf life compared to dry yeast, typically around 3 weeks.
  • Purchase fresh yeast shortly before you intend to use it to minimize the time it spends sitting around and losing its freshness.
  • If the fresh yeast is past its best-by date, it’s advisable to test its viability by proofing it before using it in recipes as previously described.
  • General Tips for Handling Yeast:
    • Always check the expiration date on the yeast package before purchasing or using it. Using yeast that has expired significantly reduces its effectiveness.
  • When measuring yeast for recipes, use a dry measuring spoon and level it off. Avoid tapping the spoon or packing the yeast, as this can affect the accuracy of the measurement.

By following these proper handling and storage practices, you can prolong the shelf life of yeast and minimize the chances of spoilage or loss of effectiveness.


In this brief article, we answered ‘’How to know if yeast is spoiled’. This article also discussed the health consequences of eating spoiled yeast, what the shelf life of yeast is and how to store it in order to prolong its shelf life.


1. Tola M, Kebede B. Occurrence, importance and control of mycotoxins: A review. [Internet]. 2016 [cited 2023 Jun 12];2(1). Available from: 

2. Carrasco E, Morales-Rueda A, García-Gimeno RM. Cross-contamination and recontamination by Salmonella in foods: A review. Food Res Int [Internet]. 2012 Mar 1 [cited 2023 Jun 3];45(2):545–56. Available from: 

3. Mccall CE, Collins RN, Jones DB, Kaufmann AF, Brachman PS. An interstate outbreak of salmonellosis traced to a contaminated food supplement. Am J Epidemiol [Internet]. 1966 Jul 1 [cited 2023 Jun 12];84(1):32–9. Available from: 

4. Halpin-Dohnalek MI, Marth EH. Staphylococcus aureus: Production of Extracellular Compounds and Behavior in Foods – A Review. J Food Prot [Internet]. 1989 Apr 1 [cited 2023 Jun 12];52(4):267–82. Available from: 

5. Le Loir Y, Baron F, Gautier M. Staphylococcus aureus and food poisoning. Genet Mol Res [Internet]. 2003 [cited 2023 Jun 12];2(1):63–76. Available from: 

6. Ting PT, Freiman A. The story of Clostridium botulinum: from food poisoning to Botox. Clin Med (Northfield Il) [Internet]. 2004 May 5 [cited 2023 May 3];4(3):258. Available from: 

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