Avoiding botulism: Understanding the hidden danger in homemade preserves

The scientific name clostridium botulinum refers to an anaerobic bacterium – meaning it thrives without oxygen – that produces a powerful toxin called botulinum toxin. This toxin can cause botulism, a serious form of food poisoning. The bacterium grows particularly well in low-oxygen environments, such as homemade preserves like tomato sauces and vegetables stored in oil.

To eliminate health risks, it is essential to take appropriate precautions when preserving food at home – above all, by following standard hygiene rules. In this guide, you’ll learn more about the dangers of botulism and how to safely preserve your food.

How does clostridium botulinum develop during home preserving?

Botulinum toxin can contaminate preserves both during preparation and storage, for instance via spores present in the environment or on food that are not destroyed during the preservation process. These heat-resistant spores thrive in low-oxygen environments and can produce the highly toxic botulinum toxin.

What are the health risks of botulism?

Botulism is a serious foodborne illness that can lead to neurological symptoms such as double vision, difficulty swallowing, and breathing problems. In severe cases, botulism can be fatal. However, there’s no need to worry: if you follow proper procedures and maintain good hygiene when preserving and pickling foods, the risk of contamination with botulinum toxin is very low.

What is botulism? Key rules for safe preserving at home

To minimise the risk of botulism from contaminated foods, observe the following basic rules when preserving:

• Use reliable recipes: Always use recipes from trustworthy sources. Ensure adequate heat treatment and add vinegar or lemon juice to your foods to inhibit the production of botulinum toxin.
• Only use fresh ingredients: Preserve only fresh produce. Overripe fruit, bruised vegetables, or leftover fish and meat should not be used, as the risk of bacterial contamination is too high.
• Sterilise jars, bottles, and lids: Wash jars, bottles, and glass lids thoroughly with hot water and a mild detergent. Wash rubber seals, preserving clips, and twist-off lids by hand without detergent. Sterilise all tools and your work surface to ensure safe preserving.
• Fill jars correctly: Leave an appropriate amount of headspace between the contents and the lid to prevent overflow or breakage during heat treatment.
• Seal jars airtight: Clean the rims of jars and bottles thoroughly after filling. Seal them with suitable lids, but do not overtighten.
• Preserve at the correct temperature: Carefully follow instructions for heat treatment (pasteurisation or sterilisation) in your recipe to virtually eliminate the risk of botulism.
• Store filled jars properly: Keep your preserves in a cool, dark place to protect them from environmental influences and maximise shelf life.

Botulism: which foods are most at risk?

Botulinum toxin can only develop under certain environmental conditions. Therefore, some foods are more susceptible to colonisation by clostridium botulinum – especially raw vegetables, meat, and fish. With hygienic and careful processing, however, even these products can be safely preserved.

Botulism in jams and fruit preserves

Jams and fruit preserves can also be contaminated with clostridium botulinum, as the bacterium can survive at relatively high temperatures and low acidity (pH > 4.5), which is typical for homemade fruit preserves.

Use equal parts fruit and sugar to stabilise jams and preserves. If you reduce the amount of sugar, add lemon juice to inhibit bacterial growth.

For pasteurisation, pour the freshly cooked jam into sterilised jars while still hot. Place the jars in a steriliser or large pot filled with hot water to avoid thermal shock. Heat-treat for at least 15 minutes at 90 °C to ensure your fruit preserves are safe and long-lasting.

Botulism: Recommendations from the Food Standards Agency

To reduce the risk of contamination and botulism, follow the guidelines from the Food Standards Agency. They provide a comprehensive overview of health risks and the correct preparation and preservation of foods.

Food Standards Agency tips for hygienic preserving and pickling

• Thorough sterilisation of tools and containers
• Use of tested recipes
• Correct heat treatment to stabilise preserves: Pasteurisation time varies depending on jar size and type of food. Generally, heating for 15–20 minutes is sufficient for jars containing 350 g to 400 g.
• Watch for signs of spoilage in preserved products: Discard homemade (and store-bought) preserves immediately if you notice any changes, to avoid any health risks from botulism.

Foods at higher risk of botulism

The most vulnerable products in home vacuum preservation are seafood, foods in brine, and fermented products. Herbs and vegetables in oil that are not properly chilled can also be contaminated with Clostridium botulinum. However, if you follow standard hygiene rules and use tested recipes, these foods can be safely preserved at home.

Pay special attention when preserving the following foods:

• Mushrooms
• Black olives in water
• Peppers
• Pickled aubergines and beans
• Meat and fish (especially tuna)

Note: Avoid preserving meat or fish at home. These products cannot be sufficiently acidified or treated with enough salt or sugar to fully inhibit botulinum toxin development. Truly safe heat treatment requires temperatures of 121 °C, which can only be achieved in industrial processes.

Safe Food Preservation: How to minimise the risk of botulism

Filling jars correctly

Before preserving, it is crucial to fill sterilised containers properly. Adequate headspace ensures the necessary vacuum in the jar. Depending on the type of preserve, choose a small headspace so the food is completely covered by liquid.

Pasteurising preserves – avoiding botulism

Pasteurisation is the most important heat treatment for making safe homemade preserves. The filled jars are fully submerged in boiling water for a set period, which varies depending on the type and size of the jars. Carry out this process with care to ensure effective pasteurisation and safe food.

Checking pasteurised jars

After pasteurisation, check the jars carefully: Are they sealed? Is there a vacuum? Twist-off lids should be slightly concave and not move when pressed. If a jar is not sealed or leaking, it must be heat-treated again. Alternatively, store the food in the fridge and consume it quickly.

Avoiding botulism: Food safety for homemade preserves

By following these guidelines, you can enjoy homemade vegetables, fruit, and more with confidence, avoiding botulism and protecting your health while making the most of your harvest in your own kitchen.

Further information on botulism

Food Standards Agency: Guidelines for safe preparation and preservation of foods at home

National Health Service (NHS): Information on botulism and its symptoms