Microbes in food: spoilage, fermentation and safety

Bacteria multiply by dividing: one cell becomes two, two become four, then eight, sixteen and so on. Under good conditions, such a doubling can take well under an hour for some species. That is why a few germs can become a great many within a few hours. Yeasts and moulds grow somewhat differently but follow the same basic rules.

Microbes need available water, nutrients and a suitable temperature. Some need oxygen, others manage without it or cannot tolerate it at all. Preserving means changing at least one of these conditions so that unwanted microbes can no longer grow: cold slows them down, heat kills many, drying and salt take away their water, acid inhibits them.

Many disease-causing bacteria multiply fastest between about 5 and 60 °C. The WHO calls this range the danger zone. Below it, in the fridge, most grow only slowly; well above it, when cooking, many die. According to WHO advice, cooked food should not stand at room temperature for more than about two hours.

In lactic acid fermentation – for example sauerkraut or sour pickles – lactic acid bacteria are the main actors. They live naturally on many vegetables. They eat sugar from the vegetables and turn it into lactic acid. As a result, the jar becomes more and more acidic, and many pathogens can no longer multiply.

Salt and the exclusion of air give the lactic acid bacteria a head start. Salt draws water out of the vegetables and forms a brine. Lactic acid bacteria tolerate the salt well, many unwanted microbes less so. Under the brine there is hardly any oxygen, which makes life hard for moulds. That is why the vegetables must stay completely under the liquid.

Other fermentations work similarly: yeasts turn sugar into carbon dioxide and alcohol – the gas makes bread dough rise. In yoghurt, bacteria sour the milk. What they all have in common is that the result is only reliably safe if the salt quantity, temperature and time are right. That is why you work with tested recipes and not by feel.

The bacterium Clostridium botulinum lives in soil and forms very hardy spores, a kind of resting form that can survive normal boiling. In airtight, low-acid foods at room temperature – such as home-canned vegetables, beans or meat – it can germinate and produce a deadly toxin. Often you can see, smell and taste nothing of it.

That is why low-acid foods may only be canned using tested methods that use pressure to reach higher temperatures than an ordinary pan. Acidic foods – experts draw the line at a pH of 4.6 – are less at risk, because the bacterium cannot grow in a sufficiently acidic environment. How acidic something is follows from the recipe, not from the taste.

Other germs also form toxins that are not destroyed by reheating. So it does not help simply to boil spoiled food up again. Bulging lids, gas in non-fermented preserves, unusual smells or mould mean: do not taste it, throw it away. If food poisoning is suspected – for example with blurred vision, difficulty swallowing or signs of paralysis – call the local emergency number (112 in the EU) immediately.

Cleanliness at the start gives you a head start: clean hands, clean jars, clean work surfaces. The fewer unwanted germs there are from the beginning, the easier it is for the wanted ones. Keep raw meat and ready-to-eat food apart so that germs do not travel from one to the other.

Keep food cold or hot, not lukewarm. Cool leftovers quickly and put them in the fridge. During a power cut keep the fridge door closed as much as possible so the cold lasts longer. Perishable food that has been warm for a long time is, when in doubt, no longer safe.

Dried and salted foods only keep as long as they stay dry. If dried fruit absorbs water in a damp environment, the risk returns. So store them airtight, cool and dry.

People have been fermenting food for thousands of years – long before anyone knew microbes existed. Only in the 19th century did Louis Pasteur show that fermentation is caused by living microorganisms. Since then we have understood why old household rules such as “salt it well” and “keep it under the brine” work.

A common misconception is: “If it smells fine, it is safe.” Many spoilage germs do smell unpleasant, but the most dangerous pathogens and toxins often do not. Conversely, well-made sauerkraut smells strong even though it is safe. The nose is a warning signal, not proof of safety.

This entry deliberately contains no recipes and does not help with identifying moulds or pathogens. Tested instructions are available from food safety and home preservation authorities.