Preservation is a suite of methods for lengthening the usability of ingredients beyond their natural life - increasing shelf life beyond minutes to a few hours, weeks or even a year or more. However, not only are they important for enabling humans to survive through the winter months and leaner times, but they are important food groups within cooking generally, and for regional cuisines - e.g. jams for fruits, pickles for fruit and vegetables, salting for fish and meat. Storing food is not a human prerogative, because various mammals store the food necessary for survival over the winter months, using ingredients that have dried naturally - seeds, grains and nuts.
As stores of energy and nutrients, foods are subject to deterioration from breakdown by their own enzymes as well as spoilage micro-organisms. Storage techniques are all designed to slow down spoilage and even largely eliminate this deterioration.
The application of science to this problem only began following the discovery by Louis Pasteur of the link between microorganisms and fermentation and spoilage. This causal link opened up the issue to scientific study and problem-solving, which has fundamentally changed how food is preserved and distributed - where would modern food culture be without its dependence (bad and good) on chilled and frozen food chains?
Nevertheless, not all preservation methods kill microorganisms, e.g. low temperatures (refrigeration and freezing) reduce microbial growth and activity, and dehydration reduces water activity that also slows down growth and activity. Methods like pasteurisation and sterilisation do, however, centre on killing microorganisms in food, or prevent other organisms from getting into ingredients. The most important methods for sterilisation is heating - something that arguably had been discovered by chance with the cooking of foods on a fire some 1.8 million years ago - but there are others that mainly involved chemical preservatives, notably salt. Microorganisms are also killed by radiation and ultraviolet light (even sunlight), however not only does radiation appear to alter food flavours but there are many justifiable ethical concerns over its safety, especially within European countries.
Unless sterilised food is sealed or preservatives added, it can then be re-infected. Early methods to seal foods include potting foods with a cap of fat or clarified butter (as with pâtés) or immersed in oils (as for artichokes, olives and sardines); however this does not work long-term because fats will go rancid, as will oils. The predominant methods for sealing, however, are bottling and canning.
The other traditional method of preservation is salting, which uses adding common salt, often mixed with saltpetre, either as brine or dry salting. It works by reducing water activity through osmosis, effectively preventing the spoilage and harmful microorganisms from accessing the water in food however, whereas it is toxic for some microorganisms, it is tolerated by others - and even used by humans to create new food flavours (kimchi and sauerkraut). If used at strong enough doses, salting will stop virtually all organisms and inhibit the actions of enzymes. Sugar can also be used to reduce water activity, e.g. in jam-making.
Smoking was traditionally also used for preservation, but nowadays is usually used for flavour and as an adjunct to salting and drying. Smoking covers the surface of meat or fish with a layer of chemicals that have antiseptic properties.
Industrially a few other methods are also used, e.g. filtration. Filtration works by removing microorganisms by passing liquids through very fine meshes. However, filtration is not possible at home because the equipment required is fickle, so is not best suited to the home cook.
- Ulmann, A. (2016) Louis Pasteur, Encyclopaedia Britannica, accessed 7 June 2016 
- Stobart, T. (1981) The Cook's Encyclopedia: ingredients & processes, Harper & Row. ISBN 0060141271.