How do you know if a jar has botulism? (5 unmistakable signs)

In this article, we answer the following question: How do you know if a jar has botulism? We give you 5 unmistakable signs that something is wrong with the food that you are about to consume. We explain how botulism is produced and how to ensure the safety of your food.

How do you know if a jar has botulism?

One of the signs that a jar has botulism is its swollen lid, although this could be also due to accidental freezing, due to being left at sub-zero temperatures. In this case, the lid swells because the food inside has expanded when frozen. But it can also swell from contamination with Clostridium botulinum – one of the ten most dangerous foodborne pathogens – or other spoilage organisms. We must always reject a swollen can or jar of food.

Other signs that something is wrong:

  • When, when opening the jar, the food comes out as an explosion: It is normal for a small depressurizing sound to be produced when you open canned food, but if it goes further, there may be a problem. The accumulation of gas inside the can is a sign that there are anaerobic bacteria inside. This would also explain why the lid is bulging.
  • When there is a loud hiss when opening: it is a serious warning sign. A soft hiss when it is opened and air rushes into the can is normal, but not a loud one.
  • Presence of bubbles inside when opening: this is a sign of the presence of bacteria inside. Bubbles generally indicate some type of fermentation, a bacterial process that is fine if intentional, such as in pickles, but not if this process is not controlled.
  • Foam presence: it is an indicator of bacterial activity. Botulism and other harmful bacteria can survive, even spread, in environments with very little oxygen, such as canned foods. We are not referring to the foam that can be produced when washing canned, for example, legumes, but to the foam that appears in the jar.

Beware of botulism

One of the risks associated with canned food is botulism, a deadly food poisoning caused by Clostridium botulinum, which grows best under anaerobic conditions (lack of oxygen). Because the canning process expels air from food, bacteria can find a good place to grow in it and produce so-called botulinum toxin. As the US Centers for Disease Control and Prevention (CDC) warns, this toxin cannot be seen, smelled, or tasted.

Therefore, in the event of any doubt and any sign of those described, the can must be thrown away and disposed of safely. It is important that other foods are not contaminated during the disposal process, therefore, we will prevent them from coming into contact with food in good condition, from leaking to areas such as the countertop or the sink, and, at the end of everything, we must ensure that wash our hands well before handling food again.

Botulism is a disease caused by the toxin generated by a bacterium called Clostridium botulinum. Despite being a rare disease, we pay attention to it for the following reason:

It is a disease that can be fatal. Two methods of preserving food that we can do at home can lead to botulinum toxin poisoning: canned foods by boiling or in a water bath and vacuum foods. Other ways of transmitting botulism are canned food, meat products such as ham and sausages, inhalation of the toxin in terrorist attacks, ingestion of non-chlorinated water, wounds.

How is botulism produced?

When we talk about botulism, we are dealing with three terms that can cause us confusion when it comes to understanding the risk it poses: bacteria, toxins, and spores. 

Let’s clarify them:

Bacteria is the microorganism that causes botulism. But we do not get botulism by eating food with these bacteria, but by eating food with the toxin that this bacteria produces when it is active. 

The bacterium can also be found in its sporulated form or spores of Clostridium: it is the latent and resistant form of the bacterium. Clostridium botulinum spores can be in the ground, in water, or in food for years. If conditions become suitable, they multiply rapidly, giving rise to bacteria that can produce the deadly toxin within hours or days.

That is, if the environmental conditions are not suitable for the bacteria to live, it remains in the form of a spore while waiting for these conditions (like a seed waiting for the conditions that allow it to germinate). When the conditions are met, then the bacteria thrive and generate their toxin, which is the lethal part and that we really care about. In other words, there is no problem with eating food with spores or bacteria, the problem is eating food with botulinum toxin.

How to eliminate Clostridium bacteria, their spores, and their toxins

Bacteria and toxins are easy to eliminate. The problem is the spores, which are very resistant:

Botulinum toxin is destroyed:

  • When cooking food for 10 minutes (more than 80º for 10 minutes).
  • With prolonged exposure to oxygen.
  • In water, the toxin is destroyed by boiling the water or disinfecting it with a 0.1% solution of hypochlorite.
  • If the toxin is present in a food, the acidic pH alone does not inactivate it.
  • The spores are heat resistant, for that reason cooking or boiling food is not enough. To be sure that food does not contain spores, it must be sterilized (subjected to temperatures of at least 116ºC, which is achieved in a pressure cooker or through industrial pasteurizations), subjected to high pressure, or irradiated.

In summary

In this article, we answered the following question: How do you know if a jar has botulism? We gave you 5 unmistakable signs that something is wrong with the food that you are about to consume. We explained how botulism is produced and how to ensure the safety of your food.

  • For acidic foods, it is sufficient to sterilize at 100ºC.
  • Low acid foods should be autoclaved (or pressure cooker) to reach temperatures above 100ºC.
  • Refrigeration and freezing prevent the growth of Cl. Botulinum and the formation of spores, making it a correct alternative to heat preservation. As long as we consume the food in a suitable time (refrigerated no more than 2-3 days).
  • The presence of toxins is undetectable. Just because a preserve is unaltered and looks good does not mean it is toxin-free.

As always, the safety of the food we eat depends on the hygiene with which we handle it and that we apply good manufacturing practices.

Of course, botulism is worrisome. But knowing the risk and handling the products well, you can continue to enjoy canning safely.

Do you make homemade preserves? Do you find it difficult to sterilize in a pressure cooker at home? Please let us know if you have any tips, comments, or questions!


The US Centers for Disease Control and Prevention (CDC) at


Hi, I am Charlotte, I love cooking and in my previous life, I was a chef. I bring some of my experience to the recipes on this hub and answer your food questions.

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