Applying one principle of preservation to a food will extendits shelf-life for a short time, applying multiple principleswill ensure the safe shelf-life for a long time. There are fourmain principles required to preserve food.
1. Exclusion of air
Most microbes require oxygen to be active. By removingair from around the food, the environment becomes unfavourableand microbes become dormant. Some bacteriaare anaerobic, and care must be taken with processing andpackaging options for susceptible foods or food poisoningmay result. Manufacturers exclude air from packagingsuch as cans and bottles by hot filling processes — thesteam rising off the product before sealing forces air fromthe package, creating a vacuum when the product cools.Vacuum packing and gas flushing (inserting gases otherthan oxygen into the package) are among other methodsmanufacturers use to exclude air from packages.Vacuumpacking does not prevent enzyme activity but it can minimisenon-enzymic oxidative reactions. The substitution ofnitrogen and other gases in modi ed atmosphere packagesdoes reduce enzyme activity.
2. Removal of moisture
Processes which remove moisture from food, such asdehydration and evaporation, are effective in preventingthe actions of both enzymes and micro-organisms. This isbecause both enzymes and microbes need water to be active— and not just any water: clean, liquid water. If the water isnot pure or if it is ice, it is unavailable to microbes. Enzymescan still be active in freezing conditions, but their activityoccurs very slowly — this is the reason why foods cannotbe stored in the freezer inde nitely. Evaporated foods stillcontain quite a high moisture content and so may be susceptibleto mould growth.
3. Addition of chemicals
Dissolving other substances in water, such as salt andsugar, chemically alters the water, making it unavailableto microbes and enzymes. Adding acids to foods canminimise spoilage by denaturing enzymes and destroyingmicro-organisms. The presence of acids also reduces thetime and temperature of heat processing needed to preservefood. Additives, in the form of chemical preservatives,food acids and smoke, can be used to help retard or destroymicro-organisms depending on the concentrations used.Bacon, tomato salsa, jam and dill pickles are just a few ofthe foods eaten daily that use chemicals to inhibit microbegrowth.
4. Control of temperature
Cooking a food by heating is probably the simplest andmost commonly used method of food preservation. When afood is cooked, the process normally involves three specificstages.
1. The food is warmed as the cooking begins.
2. The food is held at its cooking temperature for a certainperiod of time.
3. The food is cooled or eaten hot.
As food is warmed, two important events occur. Enzymicreaction rates begin to increase and micro-organisms canbe activated. As the temperature rises, enzymic reactionrates begin to slow, and then fall rapidly as the optimumtemperature for that enzyme (20–40°C) is exceeded. Microorganismshave a similar optimum temperature, and whenthis temperature is exceeded their metabolism will slow;eventually the heat will kill them.Enzymes and microbes are damaged by temperaturesover 60°C, and a few minutes at 100°Cwill destroy allenzymes and kill most forms of bacteria. However, somespore-forming micro-organisms, such as Clostridium botulinum,can survive at 100°Cfor up to five hours.
The effectiveness of the heat treatment depends not onlyon the correct temperature being achieved, but also on thepH level of the particular food. Generally, bacteria and theirspores can resist more heat when the environment is neutralor near neutral in pH. The higher the acidity of a particularfood is, the less time and the lower the temperatureit will take to kill any bacteria present.Every part of the food must receive an adequate heattreatment to prevent spoilage and to kill food poisoningorganisms, so the heat achieved at the centre of the foodis critical. Factors that affect the rate of heat transfer to thecentre of a food are:
- The container (size, shape and type of material)
- Temperature of the food before heating starts
- Cooking temperature
- Consistency of the food, and the size and shape of thepieces.
Cooling of the food after cooking is also an importantfactor in preventing the growth of bacteria. If the food isnot eaten hot and is allowed to cool slowly, this can causea serious food poisoning hazard. Even whena foodstuff is cooked to a point at which all the bacteriapresent are killed, recontamination through poor handlingprocedures can occur. For example, cross-contaminationcan occur from human handling, dirty utensils and exposureto airborne particles.
