Principals of Preservation
Applying one principle of preservation to a food will extend its shelf-life for a short time, applying multiple principles will ensure the safe shelf-life for a long time. There are four main principles required to preserve food.
1. Exclusion of air
Most microbes require oxygen to be active. By removing air from around the food, the environment becomes unfavourable and microbes become dormant. Some bacteria are anaerobic, and care must be taken with processing and packaging options for susceptible foods or food poisoning may result. Manufacturers exclude air from packaging such as cans and bottles by hot filling processes — the steam rising off the product before sealing forces air from the package, creating a vacuum when the product cools. Vacuum packing and gas flushing (inserting gases other than oxygen into the package) are among other methods manufacturers use to exclude air from packages. Vacuum packing does not prevent enzyme activity but it can minimise non-enzymic oxidative reactions. The substitution of nitrogen and other gases in modi ed atmosphere packages does reduce enzyme activity.
2. Removal of moisture
Processes which remove moisture from food, such as dehydration and evaporation, are effective in preventing the actions of both enzymes and micro-organisms. This is because both enzymes and microbes need water to be active — and not just any water: clean, liquid water. If the water is not pure or if it is ice, it is unavailable to microbes. Enzymes can still be active in freezing conditions, but their activity occurs very slowly — this is the reason why foods cannot be stored in the freezer inde nitely. Evaporated foods still contain quite a high moisture content and so may be susceptible to mould growth.
3. Addition of chemicals
Dissolving other substances in water, such as salt and sugar, chemically alters the water, making it unavailable to microbes and enzymes. Adding acids to foods can minimise spoilage by denaturing enzymes and destroying micro-organisms. The presence of acids also reduces the time and temperature of heat processing needed to preserve food. Additives, in the form of chemical preservatives, food acids and smoke, can be used to help retard or destroy micro-organisms depending on the concentrations used. Bacon, tomato salsa, jam and dill pickles are just a few of the foods eaten daily that use chemicals to inhibit microbe growth.
4. Control of temperature
Cooking a food by heating is probably the simplest and most commonly used method of food preservation. When a food is cooked, the process normally involves three specific stages.
1. The food is warmed as the cooking begins.
2. The food is held at its cooking temperature for a certain period of time.
3. The food is cooled or eaten hot.
As food is warmed, two important events occur. Enzymic reaction rates begin to increase and micro-organisms can be activated. As the temperature rises, enzymic reaction rates begin to slow, and then fall rapidly as the optimum temperature for that enzyme (20–40°C) is exceeded. Microorganisms have a similar optimum temperature, and when this temperature is exceeded their metabolism will slow; eventually the heat will kill them. Enzymes and microbes are damaged by temperatures over 60°C, and a few minutes at 100°C will destroy all enzymes and kill most forms of bacteria. However, some spore-forming micro-organisms, such as Clostridium botulinum, can survive at 100°C for up to five hours.
The effectiveness of the heat treatment depends not only on the correct temperature being achieved, but also on the pH level of the particular food. Generally, bacteria and their spores can resist more heat when the environment is neutral or near neutral in pH. The higher the acidity of a particular food is, the less time and the lower the temperature it will take to kill any bacteria present. Every part of the food must receive an adequate heat treatment to prevent spoilage and to kill food poisoning organisms, so the heat achieved at the centre of the food is critical. Factors that affect the rate of heat transfer to the centre 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 the pieces.
Cooling of the food after cooking is also an important factor in preventing the growth of bacteria. If the food is not eaten hot and is allowed to cool slowly, this can cause a serious food poisoning hazard. Even when a foodstuff is cooked to a point at which all the bacteria present are killed, recontamination through poor handling procedures can occur. For example, cross-contamination can occur from human handling, dirty utensils and exposure to airborne particles.
1. Exclusion of air
Most microbes require oxygen to be active. By removing air from around the food, the environment becomes unfavourable and microbes become dormant. Some bacteria are anaerobic, and care must be taken with processing and packaging options for susceptible foods or food poisoning may result. Manufacturers exclude air from packaging such as cans and bottles by hot filling processes — the steam rising off the product before sealing forces air from the package, creating a vacuum when the product cools. Vacuum packing and gas flushing (inserting gases other than oxygen into the package) are among other methods manufacturers use to exclude air from packages. Vacuum packing does not prevent enzyme activity but it can minimise non-enzymic oxidative reactions. The substitution of nitrogen and other gases in modi ed atmosphere packages does reduce enzyme activity.
2. Removal of moisture
Processes which remove moisture from food, such as dehydration and evaporation, are effective in preventing the actions of both enzymes and micro-organisms. This is because both enzymes and microbes need water to be active — and not just any water: clean, liquid water. If the water is not pure or if it is ice, it is unavailable to microbes. Enzymes can still be active in freezing conditions, but their activity occurs very slowly — this is the reason why foods cannot be stored in the freezer inde nitely. Evaporated foods still contain quite a high moisture content and so may be susceptible to mould growth.
3. Addition of chemicals
Dissolving other substances in water, such as salt and sugar, chemically alters the water, making it unavailable to microbes and enzymes. Adding acids to foods can minimise spoilage by denaturing enzymes and destroying micro-organisms. The presence of acids also reduces the time and temperature of heat processing needed to preserve food. Additives, in the form of chemical preservatives, food acids and smoke, can be used to help retard or destroy micro-organisms depending on the concentrations used. Bacon, tomato salsa, jam and dill pickles are just a few of the foods eaten daily that use chemicals to inhibit microbe growth.
4. Control of temperature
Cooking a food by heating is probably the simplest and most commonly used method of food preservation. When a food is cooked, the process normally involves three specific stages.
1. The food is warmed as the cooking begins.
2. The food is held at its cooking temperature for a certain period of time.
3. The food is cooled or eaten hot.
As food is warmed, two important events occur. Enzymic reaction rates begin to increase and micro-organisms can be activated. As the temperature rises, enzymic reaction rates begin to slow, and then fall rapidly as the optimum temperature for that enzyme (20–40°C) is exceeded. Microorganisms have a similar optimum temperature, and when this temperature is exceeded their metabolism will slow; eventually the heat will kill them. Enzymes and microbes are damaged by temperatures over 60°C, and a few minutes at 100°C will destroy all enzymes and kill most forms of bacteria. However, some spore-forming micro-organisms, such as Clostridium botulinum, can survive at 100°C for up to five hours.
The effectiveness of the heat treatment depends not only on the correct temperature being achieved, but also on the pH level of the particular food. Generally, bacteria and their spores can resist more heat when the environment is neutral or near neutral in pH. The higher the acidity of a particular food is, the less time and the lower the temperature it will take to kill any bacteria present. Every part of the food must receive an adequate heat treatment to prevent spoilage and to kill food poisoning organisms, so the heat achieved at the centre of the food is critical. Factors that affect the rate of heat transfer to the centre 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 the pieces.
Cooling of the food after cooking is also an important factor in preventing the growth of bacteria. If the food is not eaten hot and is allowed to cool slowly, this can cause a serious food poisoning hazard. Even when a foodstuff is cooked to a point at which all the bacteria present are killed, recontamination through poor handling procedures can occur. For example, cross-contamination can occur from human handling, dirty utensils and exposure to airborne particles.