Why Does Food Go Off?

Bacteria, yeasts and moulds are present almost everywhere, and given the right conditions they multiply rapidly. As they feed on the food they produce waste products that cause off smells, sliminess and sour tastes.

Bacterial growth is exponential, which is why food can seem fine and then be clearly spoiled a few hours later. A single bacterium dividing every twenty minutes becomes millions within a working day at room temperature.

Microbial growth requires a specific set of conditions, and preservation works by removing at least one:

• •Water, in a form that is available rather than bound or frozen
• •Nutrients, particularly sugars and proteins
• •A suitable temperature, with the danger zone roughly 5 to 60 degrees Celsius
• •A tolerable pH, since most spoilage organisms dislike strong acid
• •Often oxygen, though some bacteria thrive without it

Not all spoilage is microbial. Fats react with oxygen and turn rancid, producing the stale smell of old nuts or oil, and this happens even in dry foods where nothing can grow.

Enzymes naturally present in food also keep working after harvest, which is why fruit ripens and then goes soft, and why cut apples brown. Blanching vegetables before freezing exists specifically to deactivate these enzymes, since freezing stops microbes but not enzyme activity.

Every traditional method targets one of the requirements. Drying and salting remove available water. Freezing locks water into ice so microbes cannot use it and slows everything chemically.

Pickling adds acid below the tolerance of most spoilage organisms. Canning heats food to kill microbes then seals it against recontamination. Smoking adds antimicrobial compounds and dries the surface. Vacuum packing removes oxygen. Modern refrigeration simply slows everything down without stopping it, which is why the fridge buys days rather than months.

An important distinction. Spoilage organisms make food unpleasant but are usually not dangerous. Pathogens, the organisms that cause food poisoning, often produce no smell, taste or visible change at all.

This is why the smell test is unreliable for safety. Food that smells fine can still be hazardous, and food that smells bad is usually just unappetising. It is also why date labels distinguish between best before, which is about quality, and use by, which is about safety and should be respected.

Date labels cause enormous confusion and a substantial amount of avoidable food waste. The two main types mean quite different things.

Use by is a safety date, applied to foods where microbial growth poses a genuine risk, such as fresh meat, fish and ready meals. It should be respected. Best before is a quality date, indicating when the manufacturer expects the food to be at its best, and applies to things like dried pasta, tinned goods and biscuits. Food past its best before is generally safe though possibly less good, and judgement rather than the label applies.

Scientists measure how much of the water in a food microbes can actually use with a value called water activity. Pure water scores 1, and most bacteria need a value well above 0.9 to grow, while moulds and yeasts can manage in somewhat drier conditions. Honey sits far lower. Its sugars hold water so tightly that microbes landing in it lose water by osmosis instead of gaining it, and honey is also mildly acidic. That is why sealed honey keeps for years, although it can ferment if it absorbs moisture from damp air.

The same logic explains a less obvious hazard. Clostridium botulinum, the bacterium behind botulism, grows without oxygen in low acid conditions, and its spores survive ordinary boiling. Removing oxygen, which usually helps preservation, can therefore suit it. This is why home canning of low acid foods such as vegetables needs a pressure canner to reach temperatures above boiling, and why homemade garlic in oil should be refrigerated and used quickly. The toxin gives no warning smell or taste, a sharp example of the difference between spoiled food and unsafe food.

Cooked rice is a good example of a food that can become unsafe without seeming spoiled. Raw rice often carries spores of a bacterium called Bacillus cereus. Cooking kills the active bacteria but not all of the spores. If cooked rice is then left at room temperature, the spores can germinate and the bacteria multiply, and some strains produce a toxin that causes vomiting. This toxin is resistant to heat, so thoroughly reheating the rice later will not make it safe, even if it is piping hot.

The practical defence is speed. Cool leftover rice quickly, ideally within about an hour, by spreading it out in a shallow container, then refrigerate it and eat it within a day or so, reheating it only once. The same principle applies to large pots of stew or soup, which cool slowly in the middle and can spend hours in the temperature range where bacteria grow. Dividing them into smaller containers lets them chill faster. The food may look and smell perfectly normal, which is exactly why timing matters more than the smell test.

People preserved food successfully long before anyone knew microbes existed. In the early nineteenth century the French confectioner Nicolas Appert, responding to a government prize for better ways to feed armies and navies, sealed food in glass bottles and heated them in boiling water. He published his method in 1810, and it worked well, but Appert could not say why. A common belief was that contact with air itself caused decay, so excluding air seemed to be the secret. Tin cans soon replaced fragile glass, and canning spread decades before its scientific basis was understood.

The explanation came from Louis Pasteur in the early 1860s. In his best known experiment, he boiled broth in flasks with long, curved swan necks that let air in but trapped dust in the bend. The broth stayed clear for long periods. When a flask was tilted so that the broth touched the dust in the neck, it soon spoiled. Air alone was not the problem; living microbes carried on dust particles were. The finding undermined the old idea that life arises spontaneously from decaying matter, and it gave preservation methods such as canning, pasteurisation and refrigeration a clear target.

Some fruit spoils faster when it sits next to other fruit, and the reason is a gas. Ethylene is a simple plant hormone that triggers ripening. Fruits such as apples, bananas, pears, tomatoes and avocados keep ripening after harvest, and as they ripen they release more ethylene, which speeds up their own ripening and that of their neighbours. A damaged or overripe apple gives off extra ethylene, which is the chemistry behind the saying about one bad apple spoiling the barrel. Mould spores spreading from a rotting fruit add to the effect.

You can use this deliberately. Putting an unripe avocado in a paper bag with a banana traps the ethylene and speeds ripening, while storing ethylene-sensitive vegetables such as leafy greens away from apples helps them last longer. Not all fruit responds in the same way: citrus, grapes and strawberries do little ripening once picked, so they will not sweeten on the counter. The food industry exploits the same biology on a large scale. Bananas are shipped green and ripened with ethylene near where they are sold, and apples kept cold in low-oxygen storage can stay crisp for many months after harvest.

Food goes off because microbes multiply and because fats and enzymes react chemically, preservation works by removing water, warmth, oxygen or neutral pH, and the smell test tells you about spoilage rather than about safety.