When bugs have you on the run
Box 2 | Food preservation
Methods of food preservation have been known for thousands of years. Preserved samples of food have even been discovered in ancient Greek urns.
The techniques of food preservation can be separated into two groups: physical and chemical.
Physical methods of preservation such as canning and freezing rely on killing the microorganisms present, or at least stopping their growth for long enough to allow the food to be safely consumed. Other physical methods include drying, gamma irradiation, ultraviolet or high intensity white light, ultra high pressure and filtration.
Chemical preservatives work either as direct microbial poisons or by reducing the pH to a level of acidity that prevents the growth of microorganisms.
Acetic acid, better known as vinegar, has also been used as a food preservative since ancient times. Salted, pickled or dried foods were about the only nourishment sailors were offered on long sea voyages before the invention of modern refrigeration and preservation techniques.
Chemicals used today
Two commonly used preservative chemicals are:
- nitrates and nitrites that are used to preserve meats such as ham and bacon;
- sulphites that are commonly used to prevent the browning of fruits and vegetables after they've been peeled, and to prevent fungal spoilage.
As important and useful as they are, preservatives have developed a bad name in Western societies such as Australia. Salt is now widely shunned because of its effects on blood pressure. Nitrites and sulphites can both cause asthma, nausea, vomiting and headaches in some people.
For these reasons, consumers have started to demand foods containing lower levels of chemical preservatives. The potential drawback of this is the reduced length of time before conditions favour the rapid multiplication of food-poisoning agents like Salmonella and Listeria. A number of food-processing techniques have been developed to prolong the shelf-life of foods and permit a reduction in preservative levels.
Natural preservatives
Scientists are putting increased efforts into the discovery and purification of natural compounds for use as safe alternatives to chemical preservatives.
The new breed of protective compounds are small proteins. They are called bacteriocins and are starting to be used in a wide variety of foods. Anyone who has eaten yoghurt has been protected by bacteriocins without knowing it. Bacteriocins are produced by some good bacteria to kill competing organisms such as Listeria monocytogenes. The whole bacteria that produces the bacteriocin, or the purified bacteriocin itself, can be added to foods such as soft cheeses to reduce the risk of pathogen growth. An example of protection using bacteriocins is the use of nisin in crumpets to restrict the growth of Bacillus cereus.
Long-life treatments
Modern technology has produced several new processing techniques for prolonging the shelf-life of perishable foods.
UHT
Ultra high temperature treatment (UHT), for example, involves the rapid heating of food to about 140ºC. This temperature is maintained for a few seconds to kill bacteria. The product is then cooled rapidly and placed in sterile, airtight containers to prevent recontamination. This treatment is used commonly to produce 'long-life' milk and fruit juices. A disadvantage of the high temperature treatment is that heat-sensitive vitamins such as vitamin C are destroyed. In fruit juices the vitamins are added back after treatment.
Food irradiation
Food irradiation is another sterilising technique in which foods are bombarded by high-energy rays called gamma rays or by fast-moving electrons to kill bacteria, fungi and insects and, in some cases, to delay fruit ripening. A major benefit of irradiation is that it can occur after food is packaged and sealed.
Although irradiation is effective in killing contaminating microorganisms, it may mask the fact that the food had high levels of spoilage or insect infestation prior to treatment: the microorganisms or insects are killed but their carcasses, faeces or toxins remain.
As with all preservation techniques, irradiation should be used to prolong the life of appropriate food which is of high quality prior to treatment.
Modified atmosphere packaging
As the name implies, modified atmosphere packaging (MAP) alters the mix of gases – notably oxygen, nitrogen and carbon dioxide - in the atmosphere in which foods are stored. The altered storage atmosphere can limit the growth of spoilage bacteria and fungi and extend the shelf life of food. The technique is used for bread, cakes, fruit, vegetables, meat and seafood, which are stored in special plastic bags that help maintain the atmosphere for a certain length of time.
The technique is also applied to bulk foods in especially equipped storage containers. This is how fruits such as apples can be supplied to consumers throughout the year, long after the apple harvest takes place. Although modified atmosphere packaging (MAP) limits the growth of spoilage microorganisms, it does not slow the growth of some harmful bacteria. For this reason, MAP is usually used in conjunction with other preservation techniques such as refrigeration.
Boxes
Box 1. Common microbial agents of food-poisoning
Box 3. Chemicals in food
Box 4. Safe cooking
Related sites
How
food preservation works (How Stuff Works, USA)
Preservation
of vegetables in oil and vinegar
(Food Science Australia)
Small
scale commercial preservation of acid foods
(Food Science Australia)
Active
packaging (Food Science Australia)
What's
the beef? Food irradiation (Scientific American, Explorations)
New
coatings improve food safety and quality (CSIRO media release)
Page updated October 2004.