Dried and preserved food: basics and limits

Microorganisms need water, nutrients, a suitable temperature and usually surroundings that are not too acidic. Every preservation method takes away at least one of these conditions. Drying removes the water. Salt and sugar bind it so strongly that germs cannot use it – they even draw water out of their cells. Acid inhibits the growth of many spoilage organisms.

Heat kills germs, and an airtight seal prevents new ones from getting in. In tins and jars both work together. Cold in a fridge or freezer only slows germs down; it usually does not kill them – which is why thawed food spoils quickly again.

Danger arises when a method only works partially. The bacterium Clostridium botulinum forms very resistant spores. In an airtight jar that is not acidic enough and has not been heated enough, they can survive, germinate and produce a potent nerve toxin. The lack of oxygen in the jar, which keeps other germs away, actually suits this bacterium.

For an emergency stock, commercially produced preserves and dry goods are the safest choice: tins, jars, dried fruit, pasta, rice, oats, pulses, milk powder. They are made by tested processes and labelled with a date.

Dry food must stay dry. Store dry goods airtight, cool and dark. Even moisture from the room air can be enough to make dried food go soft, mouldy or attract pests. Label decanted goods with contents and date.

Dried food has a catch: it often needs water and energy before you can eat it. Rice, pasta and pulses have to be cooked, lentils and beans usually for longer. So always include food that can be eaten without water and without cooking.

Botulinum toxin is one of the most potent known poisons and changes neither smell nor taste. The main risk comes from incorrectly canned low-acid foods such as vegetables, meat or fish, but also from home-made garlic or herb oils kept at room temperature. Preserve only according to tested instructions from official bodies and never taste suspect jars.

Jars with a domed lid, leaking seal, gas bubbles, cloudy liquid or an unusual smell are disposed of so that nobody – not even animals – can eat from them. Dried food that has become damp, mouldy or musty also goes.

If signs of paralysis, double vision, difficulty swallowing or speaking appear after eating, call the local emergency number (112 in the EU) immediately. This is an emergency, not something to wait out.

Drying at home is a good start for fruit and culinary herbs if you follow tested instructions. Lacto-fermented vegetables such as sauerkraut are preserved by acid and need no heat, but they also need hygiene and proven recipes. Drying meat requires an additional heat treatment and is not a beginner's method.

Canning is a craft for calm times, with time, equipment and current, tested instructions. In the middle of an emergency, without experience and a reliable heat source, improvised preserving is not a good idea. This entry deliberately gives no canning, smoking or curing instructions; regional authority recommendations take precedence.

Preserving is one of humanity's oldest cultural techniques. Modern food research has shown, however, that some traditional methods are not reliably safe – especially when canning low-acid foods. That is why specialist bodies such as the US National Center for Home Food Preservation publish tested procedures.

Botulism is rare but life-threatening. The simplest precaution for a household is: bought preserves and well-stored dry goods for the stock, home-made preserves only from tested instructions, and when in doubt, throw it out.