Common Causes of Spoilage in Canned Foods and How to Avoid Them

Canned foods are designed to last for months or even years, thanks to heat treatment and hermetic sealing that keeps harmful organisms out. But when something goes wrong during processing or storage, canned food can spoil – sometimes with serious health consequences. Understanding the causes of spoilage in canned foods is essential for anyone involved in food preservation, food science, or simply trying to keep their pantry safe.

What is canned food spoilage? 🔗

Canned food spoilage occurs when the preserved contents of a sealed container deteriorate due to unwanted chemical reactions or microbial growth. The U.S. FDA notes that the incidence of spoilage in canned foods is generally low, but when it does occur, it must be investigated properly. Spoilage can produce visible changes – like swollen cans, off-odours, and discoloured contents – or, more dangerously, it can occur without any outward signs at all.

The canning process itself relies on two key principles: applying sufficient heat to destroy harmful microorganisms and creating an airtight (hermetic) seal to prevent recontamination. When either of these fails, the door opens for spoilage to begin.

Non-microbial causes of spoilage 🔗

Not all canned food spoilage involves living organisms. Chemical and enzymatic reactions within the can itself can lead to significant quality loss, even in properly sterilized products.

Hydrogen swell 🔗

Hydrogen swell is the most common form of chemical spoilage in canned foods. It happens when acids naturally present in the food react with the iron or tin of the can’s interior, producing hydrogen gas. This gas accumulates inside the container, causing it to bulge or swell. IASRI’s e-course on food microbiology explains that factors like high storage temperatures, internal scratches on the can lining, and high acidity of the food product all accelerate this reaction. High-acid foods such as tomatoes, citrus fruits, and pickled products are particularly prone to hydrogen swells.

Enzymatic changes 🔗

Some enzymes in food can survive heat treatment at low levels. Over time, these residual enzymes slowly break down proteins, fats, and carbohydrates, leading to changes in texture, colour, and flavour. For example, canned fruits may gradually soften beyond their intended consistency due to ongoing enzymatic activity, even though they remain microbiologically safe.

Other chemical reactions 🔗

Browning reactions, corrosion of the metal container, and interactions between food pigments and metal surfaces can all produce off-flavours, cloudiness in syrups or brine, and discolouration of the food product. Using lacquered or enamel-lined cans helps reduce these chemical interactions.

Microbial causes of spoilage 🔗

Microbial spoilage is the more serious concern in canned foods, and it can happen for three main reasons: inadequate heat treatment during processing, inadequate cooling after heat treatment (or high-temperature storage), and leakage in the can allowing outside microorganisms to enter. The type of microbial spoilage that occurs depends largely on the acidity (pH) of the food product.

How pH classifies canned foods 🔗

Canned foods are divided into categories based on their pH, and this classification directly affects which types of microorganisms can grow in them:

Low-acid foods (pH above 4.6) include meat, poultry, seafood, milk products, and most vegetables like corn, beans, and peas. These foods support the widest range of spoilage organisms, including dangerous pathogens.

Acid foods (pH 3.7 to 4.6) include tomatoes, pears, figs, and certain fruits. These carry a moderate spoilage risk.

High-acid foods (pH below 3.7) include pickled products, fermented foods, and jams. Their natural acidity inhibits many harmful bacteria.

Spoilage by thermophilic bacteria 🔗

Thermophilic (heat-loving) bacteria produce spores that can survive standard heat processing. When cans are not cooled properly after heating or are stored at high temperatures (above 43°C), these spores can germinate and cause spoilage. There are three major types:

Flat sour spoilage is caused primarily by Geobacillus stearothermophilus (formerly Bacillus stearothermophilus). These bacteria ferment carbohydrates and produce acid without generating gas. As a result, the can ends remain flat and show no visible signs of spoilage, making detection from outside impossible. The food inside, however, becomes noticeably sour. According to PubMed research, these spores are found widely in nature, from soil to food ingredients like sugar and starch.

Thermophilic anaerobe (TA) spoilage is caused by organisms like Clostridium thermosaccharolyticum. Unlike flat sour spoilage, TA spoilage produces large quantities of hydrogen and carbon dioxide gases, causing the can to swell and eventually burst if left long enough. The food develops a sour or cheesy odour.

Sulphide stinker spoilage is caused by Desulfotomaculum nigrificans, which produces hydrogen sulphide (H₂S). This gas reacts with the iron in the can to form dark-coloured iron sulphide deposits, giving the food a characteristic blackened appearance and a strong rotten-egg smell. Online Biology Notes explains that because H₂S is somewhat soluble in water, cans may not always swell despite this type of spoilage.

Spoilage by mesophilic bacteria 🔗

Mesophilic bacteria grow at moderate temperatures (20-45°C) and typically indicate that the heat treatment during canning was insufficient. Key organisms in this category include species of Clostridium and Bacillus.

Proteolytic species such as Clostridium sporogenes break down proteins in the food, producing foul-smelling compounds like hydrogen sulphide, mercaptans, and ammonia, along with gas that causes can swelling. Saccharolytic species like Clostridium butyricum ferment carbohydrates, producing volatile acids and gas.

Spoilage through leakage contamination 🔗

Damaged or improperly sealed containers allow environmental microorganisms to enter the can after heat processing. The FDA’s guide on examination of canned foods notes that contaminated cooling water is a common culprit – it can leak into the can through pinholes or poor seams. A mixed population of bacterial rods and cocci in the can typically indicates leakage-based contamination. Non-spore-forming bacteria like Pseudomonas, coliforms, and Enterococcus species found inside a can are strong evidence of post-processing contamination.

Yeasts and moulds 🔗

Yeasts and their spores are not heat-tolerant, so their presence in a canned product points to either under-processing or post-processing contamination through leakage. Fermentative yeasts produce carbon dioxide, which causes cans to swell. Moulds like Aspergillus and Penicillium are more common in home-canned products where both heating and sealing may not be done under fully controlled conditions.

The special danger of Clostridium botulinum 🔗

Among all spoilage and pathogenic organisms associated with canned foods, Clostridium botulinum poses the greatest risk to human health. According to the World Health Organization, this bacterium produces one of the most potent toxins known, and even tiny amounts can cause severe illness or death.

Why canned foods are vulnerable 🔗

C. botulinum is an anaerobic organism, meaning it thrives in environments without oxygen – exactly the conditions inside a sealed can. It is found naturally in soil and water, and its spores are extremely heat-resistant. The University of Florida IFAS Extension describes these spores as capable of surviving harsh environmental conditions in a dormant state for extended periods, germinating only when conditions become favourable – low oxygen, low acidity (pH above 4.6), adequate moisture, and warm temperatures.

Low-acid canned foods – vegetables, meats, poultry, and seafood – are particularly susceptible because their pH does not naturally inhibit C. botulinum growth.

Why it is so dangerous 🔗

What makes botulism especially alarming is that the toxin can be present in food that looks, smells, and tastes completely normal. The U.S. CDC warns that even a small taste of food containing botulinum toxin can be deadly. Symptoms typically appear within 12 to 36 hours and include blurred vision, difficulty swallowing, muscle weakness, and potentially respiratory failure.

Home-canned foods account for the majority of foodborne botulism outbreaks, primarily because the time and temperature achieved during home processing are often not sufficient to destroy C. botulinum spores.

How to prevent spoilage in canned foods 🔗

Preventing spoilage requires attention at every stage – from processing to storage. Here are the critical control points:

Adequate heat treatment 🔗

Proper thermal processing is the first and most important defence against microbial spoilage. For low-acid foods, the University of Maryland Extension states that temperatures of 240°F (about 116°C) or higher are needed for a specific duration to destroy C. botulinum spores. This can only be achieved with a pressure canner – a regular boiling water bath (which only reaches 212°F/100°C) is not sufficient for low-acid foods.

High-acid foods (pH below 4.6) can be safely processed using a boiling water bath, since the acidic environment prevents C. botulinum from growing even if spores survive.

Ensuring airtight seals 🔗

The hermetic seal is what keeps the sterile environment intact. Any defect – whether from poor seaming, physical damage during transport, or corrosion – can create microscopic openings that allow bacteria to enter. Regular inspection of cans for dents, rust, bulging, or damage is essential. In commercial settings, seam integrity testing is a standard part of quality control.

Proper cooling and storage 🔗

After heat treatment, cans must be cooled rapidly to prevent thermophilic bacteria from germinating. Slow cooling or storing cans at elevated temperatures (above 35-43°C) creates ideal conditions for thermophilic spore-formers to become active. The CDC recommends storing canned foods in a cool, dark place – ideally between 50°F and 70°F (10-21°C) – and never above 95°F (35°C).

Controlling initial microbial load 🔗

The number of microorganisms present in the raw food before canning directly affects how effective the heat treatment will be. Good manufacturing practices – proper washing, blanching, and hygiene throughout the processing facility – help reduce the initial microbial load. Ingredients like sugar, starch, and spices can also carry thermophilic spores into the product, so sourcing quality ingredients matters.

Acidification 🔗

For borderline foods like tomatoes (which are only mildly acidic), adding lemon juice, citric acid, or vinegar brings the pH below 4.6 and makes the product safe for water-bath canning. This simple step is critical – the Penn State Extension emphasizes that altering ingredients in a recipe can change the pH and heating characteristics, potentially compromising safety.

Signs of spoilage to watch for 🔗

Before consuming any canned food, check for these warning signs: bulging or swollen lids, leaking containers, spurting liquid when the can is opened, unusual odours (sour, cheesy, or putrid), cloudiness in liquid, and foam or bubbles inside. Any can showing these signs should be discarded immediately without tasting.

Home canning vs. commercial canning 🔗

Commercial canning operations use precisely calibrated retorts (industrial pressure cookers), standardised time-temperature protocols, and rigorous quality control testing. Home canning, while perfectly safe when done correctly, carries higher risk because of variable equipment, inconsistent temperatures, and the temptation to modify tested recipes.

The CDC and the USDA strongly recommend following only research-tested recipes for home canning – not family recipes passed down through generations, unless they meet current safety standards. Using a properly functioning pressure canner (not a pressure cooker or multi-cooker) for all low-acid foods is non-negotiable.

Combining preservation methods 🔗

To further extend shelf life and add layers of safety, canning can be combined with other preservation techniques. Refrigeration after opening slows microbial growth. Acidification lowers pH to inhibit dangerous pathogens. Controlling water activity through salt or sugar addition limits microbial growth. In commercial settings, techniques like radiation sterilisation (radappertisation) are sometimes used to sterilise the surfaces of canned containers and lids.

The key principle is the hurdle approach – using multiple barriers simultaneously so that even if one fails, others still prevent microbial growth.

Key takeaways 🔗

Canned food spoilage is caused by either chemical reactions (like hydrogen swells from acid-metal interactions) or microbial contamination (from inadequate heat treatment, poor cooling, or container leakage). Among microbial threats, Clostridium botulinum is the most dangerous because its toxin can be present without any visible signs of spoilage. Prevention comes down to proper heat treatment, airtight sealing, appropriate storage conditions, and following scientifically validated canning procedures.

What do you think? Have you ever encountered a swollen or suspicious-looking can and wondered whether it was safe? What steps do you take to ensure the safety of canned foods in your home or workplace?

References

1. https://www.fda.gov/media/183724/download
2. http://ecoursesonline.iasri.res.in/mod/page/view.php?id=5128
3. https://onlinelibrary.wiley.com/doi/abs/10.1002/9781119237860.ch22
4. https://pubmed.ncbi.nlm.nih.gov/30836554/
5. https://www.onlinebiologynotes.com/microbiological-spoilage-of-canned-foods/
6. https://www.who.int/news-room/fact-sheets/detail/botulism
7. https://ask.ifas.ufl.edu/publication/FS104
8. https://www.cdc.gov/botulism/prevention/home-canned-foods.html
9. https://extension.umd.edu/resource/clostridium-botulinum-food-safety-risk-home-food-preservation-fs-1031
10. https://extension.psu.edu/botulism-a-deadly-concern