Preservation of food has been a problem and challenge for the human race since prehistoric times, since natural supplies of food run short in the winter and few foods keep for long without some preservative measures being taken. Those that do, such as nuts, are preserved by animals. But—apart from a few special cases such as bees—the only animal that treats food to make it keep is man.
All fresh foods begin to spoil at once, quickly or slowly. As soon as a plant is harvested or an animal killed, substances in it which were essential to its life begin to break down. It may also be colonized by micro-organisms coming from outside, or be acted on by those already present, such as the bacteria in the intestines of animals and the yeasts which form the ‘bloom’ on fruit.
Food spoilage may be caused by enzymes. Various micro-organisms, notably yeasts and moulds and bacteria, are responsible for numerous other forms of spoilage. And some foods are spoilt simply by exposure to the air, without any intervention by enzymes or micro-organisms.
As will be clear from the following paragraphs, which deal in turn with each kind of agent of spoilage, they have beneficial aspects as well as harmful ones, and in many instances the beneficial aspects actually promote preservation of food. It would therefore be a mistake to think of them as ‘the enemies’ in this context; but they all can cause spoilage unless prevented and it is useful to know what measures of prevention are effective in each case.
Innumerable enzymes are at work in all living things. They control the chemical reactions of life processes, but they are not themselves alive; they can continue working after the death of their host.
Some enzyme action can have a beneficial effect up to a point, as in the ripening of fruit after picking and the increasing tenderness of meat when it is hung. But if these changes are allowed to go too far, the food softens and spoils. Some changes are purely deleterious, for instance rancidity in fats and oils; or normally undesirable, as in the browning of vegetables.
Enzymes can be destroyed by heating food, even briefly, to boiling point. They can also be prevented from working by simple preservative chemicals such as salt and acids. Cold slows their action, but even freezing does not stop it completely.
Yeasts and moulds may also have both good and bad effects. They release enzymes of their own into the food, breaking it down, and producing substances which cause ‘off’ flavours. But some yeasts and moulds cause useful fermentation, as in bread and alcoholic drinks. Moulds are used in cheese and in the making of tempe from soya beans.
Yeasts and moulds can be kept out of food by hygienic precautions, controlled by chemical methods or killed by irradiation—a controversial technique used mostly for spices. They can be slowed or stopped by refrigeration, freezing or drying—but some moulds can grow in a freezer at temperatures below −18 °C (0 °F).
Some bacteria are useful. To exploit them, while suppressing undesirable bacteria, it is usually necessary to create special conditions favouring their growth, and sometimes to introduce a starter culture of them. Both these practices have been carried out for millennia, long before anyone knew that bacteria existed. Procedures which gave a good result were presumably found out by trial and error. Sauerkraut provides an example: of the bacteria present on the cabbage leaves, those which produce the desired effect grow strongly in airless, salty conditions, which other bacteria cannot stand. Most milk products, including cheese, yoghurt, and butter, are produced with the aid of bacteria, as are sourdough bread and vinegar.
The fermentation in milk products, sauerkraut, and certain other foods such as olives and salami is a preservative process. It is brought about mainly by lactic acid-producing bacteria, which also give pleasantly sour flavours to the food.
Bacteria can be suppressed or controlled without great difficulty, by drying, refrigeration, pickling, or cooking; but there are a few species which can remain active at −5 °C (23 °F), and some which can form spores that survive the heat of normal cooking. Irradiation kills all bacteria. But there remains the problem of the toxins produced by certain species, such as those which cause botulism.
There are also changes brought about purely by chemical reactions; one of the most important is rancidity of fats caused by the oxygen in the air, or less often by water.
These are mostly ways of destroying enzymes or micro-organisms, or inactivating them (sometimes merely slowing down their activity very markedly); and of denying them access to the food, if they are not already present.
Cooking preserves food by denaturing protein—damaging its molecules by heat. Enzymes are proteins, and all are destroyed before the temperature reaches boiling point. Some bacteria can survive normal cooking temperatures by forming spores, which grow into new bacteria when the food cools. No moulds will withstand cooking.
Sealing food in a partial vacuum or an inert gas is usually done only after micro-organisms have first been killed by another method. It is a good way of preventing rancidity caused by oxygen in foods which otherwise keep well, such as nuts.
Canning and bottling almost always involve cooking to kill micro-organisms. The contents of the container remain sterile until it is opened. Air is also excluded.
Refrigeration is seldom more than a short-term method. The low temperature slows all processes, including spoilage.
Freezing can make some foods last for years. They must first be blanched to destroy enzymes and micro-organisms. Even the coldest freezers cannot arrest spoilage completely, because when watery liquids freeze they form pure ice, leaving dissolved substances in the remaining water which becomes a solution too concentrated to freeze. Fats continue to go rancid, though very slowly.
Drying paralyses micro-organisms of all kinds, which need water to operate. In most cases these remain alive and will start growing again as soon as the food is moistened. Drying destroys enzymes, whose protein structure is denatured when it loses water. But traditional, slow drying methods allow a good deal of enzyme action before humidity falls to that point. Many dried fruits are darkened by enzymes. Drying does not halt rancidity caused by oxygen in the air; oily foods are seldom dried.
Salting turns all water in food to a strong salt solution; it is often combined with drying, which further increases the strength of the solution. When salt—sodium chloride—dissolves in water its molecules split into electrically charged sodium and chloride ions, which interfere with chemical reactions and thus suspend the action of enzymes. The solution also creates a strong osmotic pressure (see osmosis) on the cell walls of bacteria and other micro-organisms, which prevents them from passing substances in or out. Both enzymes and micro-organisms will restart when the salt is removed or diluted.
Sugar creates an osmotic effect in the same way. It does not stop enzyme action; but most foods that are preserved by sugar, whether by bottling in syrup or by crystallization, are cooked, so that is not a problem. A sugar coating can also exclude air, an effect exploited in ‘torrefaction’, where coffee beans are given a sugar coating during roasting to keep their oils from going rancid.
Pickle and pickling are loose terms covering preservation in a strong vinegar solution, fermentation by lactic acid-producing bacteria, and (sometimes) the salt curing of meats. In both vinegar and lactic pickling the preservative is an acid, which prevents enzymes from working (again by flooding the environment with ions). Most bacteria that cause spoilage or food poisoning cannot work in strongly acid conditions. In vinegar pickling the acid is added to the food, usually combined with salt and often sugar as an additional safeguard. In lactic pickling, lactic acid-producing bacteria make the acid on the spot, thus paralysing their undesirable competitors. They are helped to get a head start by adding a little salt, which they can tolerate better than their rivals can.
Smoking is almost always combined with other preservation methods such as drying, salting, or, in hot smoking, cooking. It has a certain drying effect itself, but its most important outcome is to seal the surface of the food with an airtight, antiseptic coating.
Irradiation kills all micro-organisms by damaging their genetic material. The low doses used in treatment of food have little effect on enzymes. Food can be irradiated through a sealed container, sterilizing the contents.
Ralph Hancock is an encyclopedist with a special interest in food history and food science.
Shepard, Sue (2000), Pickled, Potted and Canned, London: Headline.
