F&B

Preservation is better than spoilage

Spoilage of food is a growing concern of the food and beverage department. Food preservation is one of the oldest techniques used by human beings. Chef Kaviraj Khialani enumerates the various ways in which it can be done

Preservation of food has been a problem and challenge for humans since pre-historic times, since natural supplies of food run short in winter and few food items stay for long without some preservative measures being taken.

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 air, without any intervention by enzymes or micro-organisms.

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.

Some bacteria are useful. To exploit them while suppressing undesirable bacteria, it is usually necessary to create special conditions favouring their growth and sometimes 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 and 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.

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 deg. C (23 deg. F) and some of which can, form spores that survive the heat of normal cooking.

Preservation techniques

There are many 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. 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 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.

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 crystallisation, are cooked, so that it 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.

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, sterilising the contents.

The writer is the head of department (Food Production) at the Kohinoor College of Hotel Management, Mumbai. He can be reached at kaviraj21@hotmail.com