Main food preservation methods
||Effect on microbial growth or survival
Refrigeration or chilling
||Low temperature to retard growth
||Low temperature and reduction of water activity to prevent growth
Drying, curing and conserving
Reduction in water activity sufficient to delay or prevent growth
Vacuum and oxygen free modified atmosphere packaging
Low oxygen tension in inhibit strict aerobes and delay growth of facultive anaerobes
|Carbon dioxide enriched modified atmosphere packaging
Specific inhibition of some micro-organisms by carbon dioxide
Addition of acids
Reduction of pH value and sometimes additional inhibition by the particular acid
Reduction of pH value in situ by microbial action and sometimes additional inhibition by the lactic and acetic acids formed and by other microbial products. (e.g. ethanol, bacteriocins)
||Cooking in high sucrose concentration creating too high osmotic pressure for most microbial survival.
Steeping or cooking in Ethanol produces toxic inhibition of microbes. Can be combined with sugar preservation
Compartmentalisation and nutrient limitation within the aqueous droplets in water-in-oil emulsion foods
Addition of preservatives such as nitrite or sulphite ions
||Inhibition of specific groups of micro-organisms
Pasteurization and appertization
Delivery of heat sufficient to inactivate target micro-organisms to the desired extent
Food irradiation (Radurization , radicidation and radappertization )
Delivery of ionising radiation
|Application of high hydrostatic pressure (Pascalization)
||Pressure-inactivation of vegetative bacteria, yeasts and moulds
Food preservation is the process of treating and handling food in such a way as to stop or greatly slow down spoilage to prevent foodborne illness while maintaining nutritional value, texture and flavor.
Preservation usually involves preventing the growth of bacteria, fungi and other micro-organisms, as well as retarding the oxidation of fats which causes rancidity. It also includes processes to inhibit natural ageing and discolouration that can occur during food preparation such as the polyphenoloxidase reaction in apples which causes browning when apples are cut. Some preservation methods require the food to be sealed after treatment to prevent re-contamination with microbes, other methods, such as drying, allow food to be stored without any special containment for long periods.
Preservation Processes include :-
- heating to kill or denature organisms (e.g. boiling),
- oxidation (e.g use of sulphur dioxide)
- toxic inhibition (e.g. smoking, use of carbon dioxide, vinegar, alcohol etc)
- dehydration (drying)
- osmotic inhibition ( e.g use of syrups)
- low temperature inactivation (e.g. freezing)
- many combinations of these methods.
Common methods of applying these processes include drying, freeze drying, freezing, vacuum-packing , canning, preserving in syrup, sugar crystalisation, food irradiation, adding preservatives or inert gases such as Carbon dioxide.
Other methods that not only help to preserve food, but also add flavor, include pickling, salting, smoking , preserving in syrup or alcohol , sugar crystalisation and curing.
One of the oldest method of food preservation is by drying, which reduces water activity sufficient to delay or prevent bacterial growth. Most types of meat can be dried and this is especially valuable in the case of pig meat since this is difficult to keep without preservation. Many fruits can also be dried and the process is often applied to apples, pears, bananas, mangos, papaya, coconut etc. Currants, Sultanas and Raisins are all form of dried Grapes. Drying is also the normal means of preservation for cereal grains such as wheat, maize, oats, barley, rice, millet and rye.
Probably as old as drying, many Arctic communities would preserve food in holes or larders dug into the ice. There is a tradition in Scandinavia of preserving fish and especially herrings in this way. The addition of urine at the time of packing the fish into the ice is said to assist in preservation and to improve the flavour of the food when eaten.
Freezing is also one of the most commonly used processes commercially and domestically for preserving a very wide range of food stuffs including prepared footstuffs which would not have required freezing in their unprepared state. I.e. Potato waffles are stores in the freezer but potatoes themselves require only a cool dark place to ensure many months' storage.
Cold stores provide large volume, long term storage for strategic food stocks held in case of national emergency in many countries.
Frozen food is subjected to very low air pressures which cause the water content to evaporate without passing through the water stage. This produces a very low water content food without addition of cooking flavours or presence of de-natured proteins. Freeze drying is commonly used to preserve small vegetables such as peas and beans and also a wide range of ready-made foods, such as soups and other convenience foods which only require the addition of hot water prior to consumption. Many foods intended for campers and hikers are freeze dried because of the considerable weight advantage over non freeze dried items. Freeze drying is also widely used for preparation of dried milk and whey products.
Smoking is sometimes done in conjunction with drying. Although not sufficient by itself to permit long term storage of food, smoking adds chemicals, particularly poly aromatic hydrocarbons (PAHs) that help inhibit the growth of micro-organisms.
Vacuum-packing stores food in a vacuum environment, usually in an air-tight bag or bottle. The vacuum environment strips bacteria of oxygen needed for survival, hence preventing the food from spoiling. Vacuum-packing is commonly used for storing nuts.
Curing draws moisture from the meat through a process of osmosis. Meat is cured with salt or sugar, or a combination of the two. Nitrates and nitrites are also often used to cure meat.
Sugar is used to preserve fruits, either in syrup with fruit such as apples, pears, peaches, apricots, plums or in crystalised form where the preserved material is cooked in sugar to the point of crystralisation and the resultant product is then stored dry. This method is used for the skins of citrus fruit (candied peel)), angelica and ginger. A modification of this process produces glacÚ fruit such as glacÚ cherries where the fruit is preserved in sugar but is then extracted from the syrup and sold, the preservation being maintained by the sugar content of the fruit and the superficial coating of syrup. The use of sugar is often combined with alcohol for preservation of luxury products such as fruit in Brandy or other sprits. These should not be confused with fruit flavoured spirits such as Cherry Brandy or Sloe gin
Pickling is a method of preserving food by placing it or cooking it in a substance that inhibits or kills bacteria and other micro-organisms, This material must also be fit for human consumption. Typical pickling agents include brine (high in salt), vinegar, ethanol and vegtable oil especially olive oil but also many other oils.
Most pickling processes also involve heating or boiling so that the food being preserved becomes saturated with the pickling agent.
A less-common form of pickling uses sodium hydroxide (lye) to make the food too alkaline for bacterial growth. Lye will saponify fats in the food, which will change its flavor and texture. Lutefisk uses lye in its preparation, as do some olive recipes.
Canning and Bottling
Canning involves cooking fruits or vegetables, sealing them in sterile cans or jars, and boiling the containers to kill or weaken any remaining bacteria. Various foods have varying degrees of natural protection against spoilage and may require that the final step occur in a pressure cooker. High-acid fruits like strawberries require no preservatives to can and only a short boiling cycle, whereas marginal fruits such as tomatoes require longer boiling and addition of other acidic elements. Many vegetables require pressure canning. Food preserved by canning or bottling is at immediate risk of spoilage once the can or bottle has been opened
Food may be preserved by cooking in a material that solidifies to form a gel. Such materials include gelatine, agar, maize flour and arrowroot flour. Some foods naturally form a protein gel when cooked such as Eels and elvers, and Sipunculid worms which are a delicacy in the town of Xiamen in Fujian province of China. Jellied eels are a delicacy in the East End of London where they are eaten with mashed potatoes. Potted meats in aspic, (the gel made from arrowroot flour) were a common way of serving meat off-cuts in the UK until the 1950s
A 1950s issue of Popular Mechanics details the impending arrival of "food irradiation". However, at the present time, the implications surrounding the irradiation of food are still not fully understood, and the technology is therefore still not in widespread use. However, irradiation of potatoes, strawberries, and meat is common in many countries where refrigerated facilities and trucks are not common. In 2002, the FDA permitted irradiation of meat and poultry to reduce the spread of E. coli and salmonella.
In the US and most of Europe irradiation of spices is common, as the only alternative (treatment with gas) has been shown to be potentially carcinogenic. The process is called "cold pasteurization" to avoid the reduced sales from negative implications of the term "irradiation".
Salad crops which are notoriously difficult to preserve are now being packaged in sealed bags with an atmosphere modified to reduce the oxygen concentration and increase the carbon dioxide concentration. There is concern that although salad vegetables retain their appearance and texture in such conditions, this method of preservation may not retain nutrient content, especially vitamins.
Grains may be preserved using carbon dioxide. A block of dry ice is placed in the bottom and the can is filled with grain. The can is then "burped" of excess gas. The carbon dioxide from the sublimation of the dry ice prevents insects, mold, and oxidation from damaging the grain. Grain stored in this way can survive five years in a moist garage.
Many root vegetables are very resistant to spoilage and require no other preservation other than storage in cool dark conditions, usually in field clamps .
Some foods, such as many traditional cheeses, will keep for a long time without use of any special procedures. The preservation occurs because of the presence in very high numbers of beneficial bacteria or fungi which use their own biological defences to prevent other organisms gaining a foot-hold.
Last updated: 05-07-2005 15:37:07
Last updated: 05-13-2005 07:56:04