Food protection and food preservation have one aim in common they are intended to prevent contamination and spoilage of foods. Many of the methods of food protection and preservation used today are of ancient origin. Having an understanding of food microbiology (as you learned in Study Session 8) is important for food protection and preservation practice. In this study session, you will learn about the principles and methods of food protection and preservation, and also the details of safe ways of food processing and preparation.
When you have studied this session, you should be able to:
10.1 Define and use correctly all of the key words printed in bold. (SAQs 10.1 and 10.6)
10.2 Explain what is meant by food protection methods and what is achieved by using them. (SAQ 10.2)
10.3 Describe the requirements for safe food preparation. (SAQ 10.3)
10.4 Describe personal hygiene requirements for food handlers for preparing food safely. (SAQ 10.4)
10.5 Describe the basic principles and methods of food preservation. (SAQs 10.5 and 10.6)
Food protection methods are measures taken to protect food from being contaminated by any agent. All food must be protected at all times during storage and preparation from the following contaminants:
These factors either affect the food directly to make it unsafe (such as cigarette smoke or soot), or, like coughs and sneezes or insects, contaminate the food with microorganisms.
The most important way of preventing contamination is by adopting good food handlers’ hygiene. This is the term for a group of practices that should be followed at all times by anyone handling food at any stage of the food supply process. Food handlers’ hygiene in retail and commercial premises where food and drink is sold to customers is of critical importance and this is discussed further in Study Session 11.
The same principles also apply in domestic situations. The importance of promoting good food handlers’ hygiene is:
Anyone handling food should avoid bad habits such as scratching, touching the hair, nose or mouth, having unclean hair, unclean and long fingernails, smoking, and coughing or sneezing in food handling and preparation areas. They should always wash their hands before starting to prepare food, and after every interruption, particularly after using the toilet. People who have skin infections, diarrhoea or sore throats should avoid handling food.
There are other general principles for preventing food contamination:
One critical aspect of food protection is appropriate food storage. Food storage areas should be well ventilated and illuminated, and protected from overhead drips. Floors, wall surfaces and tables should be easy to clean, and the floors should be well drained. The storage area should be kept free from insects and vermin, by screening if possible.
Food should be obtained from approved sources and should come in its original container. It should be kept free from contamination once it has been received from the supplier. Processed foods are often safer than unprocessed foods, for example, pasteurised milk is safer than raw, untreated milk.
Whether in the home or in commercial premises, once in the food preparation area, food should be kept on shelves or clean racks. These should be sufficiently high off the floor, at least 50 cm, and be spacious enough to prevent contact spoilage or contamination. This is especially important for storing raw and cooked foods, which must never touch each other, because raw food can contaminate cooked food.
Why is it important to store foods on a shelf above the floor?
Food stored at floor level is more vulnerable to damage and contamination. For example, rats and mice are more likely to find it, and it may be accidentally knocked or kicked if people pass by.
Perishable and potentially hazardous foods that can be easily contaminated, such as milk and raw meat, should be stored at low temperature, preferably in a refrigerator at below 10ºC. Frozen foods should be stored in a freezer below -18ºC.
The general rule for food storage is to keep hot foods hot and cold foods cold. Cooked foods should be eaten immediately, but if there is a delay the foods should be kept at a temperature higher than 60ºC. Allowing cooked food to cool to room temperature allows microorganisms to start to grow and multiply; therefore, cooked food must be stored very carefully. If it cannot be eaten straight away, it should be kept as cold as possible, ideally in a refrigerator, to avoid growth of microorganisms.
If any food has to be reheated, this must be done thoroughly. If food is only warmed and not reheated properly, microorganisms will multiply in it, so you need to heat it enough to destroy them. Infant foods should not be stored at all, but must be used immediately.
Food is particularly vulnerable to contamination while it is being prepared for eating. It is important to remember food handlers’ hygiene and to ensure that all surfaces and utensils are clean.
Foods intended to be eaten raw, such as fruit and some vegetables, must be washed carefully in clean, safe water (Figure 10.2). Food that is to be cooked must be cooked thoroughly to kill all pathogenic microorganisms. All parts of the food must reach a temperature of at least 70oC. You cannot tell how hot the food is just by looking, so it is important to cook the food for long enough to make sure that it is all cooked through. Cooking, as well as being a very important part of food preparation, is also used for preserving food; this is the subject of the next section.
Food preservation includes a variety of techniques that allow food to be kept for extended periods of time without losing nutritional quality and avoiding the growth of unwanted microorganisms. There are three basic objectives for the preservation of foods:
For storing or preserving food, one or several of the living conditions needed for the growth of microorganisms have to be removed. Like humans, microorganisms need a source of food and water, and they also need a suitable pH and temperature to grow, so food preservation techniques aim to target these requirements. Food preservation depends on procedures which effectively manage the microbial content of foods and on processes that alter or delay the activities of enzymes in the food. The techniques may be applied separately or in combination. Their aims are to prevent contamination in the first place, to remove or reduce the numbers of contaminants, and to prevent microbial growth. We describe them below.
This technique simply means to prevent contamination of the food by spoilage agents or by contact with them. The word ‘aseptic’ means free from harmful bacteria, viruses etc.
The technique requires either using an artificial covering for the food, or keeping its natural protective covering if there is one. Examples of natural coverings are the shells of eggs, fat or skins in animals, and/or the skin or peel of fruits. Leaving the natural covering of the food intact, or applying a clean artificial cover, can prevent microorganisms from entering or dropping on to the food.
Microorganisms can be physically removed from food, or their numbers reduced, by techniques like washing, trimming, sieving and filtration. For example, vegetables and fruit should be washed in clean water; any damaged or dirty parts of vegetables should be trimmed off with a clean knife; flour can be sieved to remove any unwanted contaminants.
Heat is one of the oldest methods of destroying microorganisms in food processing and preservation. The greatest advance in food hygiene was inadvertently made when humans discovered the advantage of boiling, roasting, baking and other heat treatments of food, hence preserving the food for longer periods. Food is also rendered safe by the application of heat because most pathogenic microorganisms are comparatively heat-sensitive. Some of the methods of heat treatment used for food preservation are discussed below.
Boiling is the process of applying heat to water until the temperature reaches about 100°C. Boiling foods in water cannot completely destroy all microorganisms, but the vegetative cells of bacteria, yeasts and moulds are generally quickly destroyed at temperatures of 100°C or above. Spores of some bacteria are extremely resistant to heat and are not killed at this temperature, although their growth is prevented. For this reason, boiling food can rarely be relied upon to ensure complete destruction of all organisms. However, most pathogens are killed, provided that sufficient exposure time is maintained. Although the spores of Clostridium botulinum, which causes botulism, are extremely heat-resistant, the toxin produced by this organism is readily destroyed by boiling. However, some toxins produced by other bacteria such as staphylococci are not easily inactivated. Thermophilic (heat-loving) organisms may survive the effects of boiling and can cause food spoilage if environmental conditions are favourable for them.
Bacterial destruction by heat is affected by time and temperature variation. The higher the temperature, the more rapid is the destruction. On the other hand, as the temperature is lowered, the time of exposure (holding time) needs to be longer.
Cooking can have some disadvantages. It can damage the food’s appearance, texture and flavour, and may also destroy some important vitamins. Nevertheless, the advantages of cooking outweigh the disadvantages because it inhibits spoilage and possible disease transmission.
Pasteurisation is named after its inventor, Louis Pasteur, a French chemist.
Pasteurisation is a process of heat treatment of milk, beer and some other beverages. It requires sufficient holding time to assure the thermal destruction of pathogens and organisms responsible for spoilage, without altering the nutritional value. It involves heating the food to a specific temperature for a specific time and then cooling rapidly.
Pasteurisation kills most but not all of the microorganisms present. It is a very useful method when more rigorous heat treatment could harm the quality of the product, as in the case of milk, and when the aim is to kill only the pathogens that are not very heat-resistant.
The temperature applied and the holding time of pasteurisation vary with the equipment available and the type of food product. In milk pasteurisation, the time-temperature combination is selected on the basis of the thermal death time of the most resistant pathogens (TB bacilli) that may be present in raw milk, and the maximum temperature and time at which the taste, palatability and nutritive value of milk are maintained. Normally milk is pasteurised at 62.8°C for at least 30 minutes or at 71.7°C for at least 15 seconds, or, if using ultra-high temperature (UHT), at 135°C for 1–2 seconds. UHT milk is sterilised, meaning all forms of life are destroyed. This extends its storage time but does affect the taste.
Blanching is a mild pre-cooking operation which can reduce the bacterial load on vegetables by 90%. It means the application of boiling water or steam for a short time. It wilts some bulky vegetables and prevents discolouring of others. It cleans peas of the moist and sticky material around them. Blanching vegetables prior to canning, freezing or drying helps to remove soil, insects and microorganisms, and destroys or slows the action of enzymes. It sets the green colour and generally facilitates dicing, peeling and packing.
Canning is one of the most widely used modern methods of processing and preserving food. It involves the careful preparation of food packed into a sealed tin, glass or plastic container which is subjected to defined high temperatures (above 100ºC) for an appropriate period of time, and then cooled. Following the thermal (heat) processing, the sealed container must be cooled immediately to a temperature of about 38ºC to prevent unnecessary adverse effects of heat on the texture, flavour or colour of the food.
The canning method involves the following steps: sterilising the food to be canned, packing it in sterile, air-tight stainless metal, glass or plastic containers, and then hermetically sealing (i.e. with a complete, airtight seal) the containers to prevent contamination during handling and storage. In the heat process, all vegetative bacteria are destroyed and spores cannot grow. Any can that is damaged or swollen should not be used. A swollen, bulging can indicates that gas is being produced on the inside and demonstrates there is microbial activity in the food, so it would not be safe to eat.
Unlike high temperature, cold is not an effective means of destroying pathogenic bacteria, viruses and toxins in foods, but it can retard their multiplication and metabolic activities.
No food or food product is rendered free from microorganisms by low temperature (by freezing or refrigeration). This explains the generally accepted danger of refreezing any kind of thawed foods. Certain parasites, such as Taenia cysts in beef and all stages of Trichinella spiralis, can be completely destroyed by storage of infected food at -18ºC for periods of 20 to 30 days, depending upon the rate of cold penetration. The most important prerequisite for successful preservation by cold is that the food must be clean to start with.
Chilling involves reducing food temperatures, but only to approximately -1ºC. Refrigerators for cold storage/chilling are normally used at 0ºC to +8ºC for preservation of a wide variety of food products (see Figure 10.3).
Freezing of food, when carried out properly, is one of the best methods of preserving foodstuffs in as nearly natural a state as possible. Freezing preserves the storage life of foods by slowing down enzyme reactions and the growth of microorganisms. A low storage temperature of at least -12°C is important if prolonged storage life is desired without losing flavour. Needless to say, freezing foods to preserve them is only possible with a freezer and reliable power supply.
Vegetables with a high moisture content do not freeze well because cellulose (in plant cell walls) tends to be broken down by enzymes regardless of the rate of freezing, making the vegetables soft. Therefore, for such food items, blanching to destroy enzyme activity is required prior to freezing.
This is a dehydration process by which the water/moisture content of the food is removed or decreased. Pathogenic and other bacteria cannot multiply in the absence of water. Most tend to die in foods that have been dehydrated to a moisture content of 10–20% of weight. Drying, however, may not kill spores. Drying also achieves food preservation by inactivating enzymes.
Drying or evaporation methods have been applied to nearly every kind of watery food, including milk. Although the loss in vitamins and nutritional value is usually minor, some foods change physically and chemically, and are sometimes altered in natural colour and flavour. Other dried products do not compare favourably with their fresh counterparts due to difficulties in reconstitution, i.e. adding water to return the food to its original form. One traditional form of dried food is quanta (Figure 10.4). Quanta is made from sliced meat which is hung in the air to dry.
Not all microorganisms are bad. Certain microorganisms are necessary in the preparation and preservation of many foods and beverages. Essentially, fermentation (a controlled microbial action) is a process of anaerobic or partially anaerobic oxidation of carbohydrates that produces acids and alcohol. It is one of the oldest methods of food preservation. In fermentation, food preservation is achieved by the presence of acid or alcohol, which creates unfavourable environmental conditions for decomposing and other undesirable bacteria.
Foods commonly processed and preserved by fermentation methods are milk and milk products, beef, vinegar, drinks like beer and wine, and pickled fruits and vegetables. Pickling is the process of preserving food by anaerobic fermentation either in brine (salt solution) or in an acid solution, usually vinegar. The concentrations of the pickling agents and the time needed for pickling are determined by the type of food. Fermented and/or pickled food products are semi-perishable and must be protected from moulds, which are able to attack the acids and permit the invasion of spoilage organisms.
It has been customary to classify chemicals incorporated into food for preservation purposes as ‘intentional additives’. Additives used at food industry level include vitamins, mould inhibitors, bactericides, emulsifiers, minerals, food colouring, synthetic flavours and sweeteners. Chemicals that get into food accidentally are referred to as ‘unintentional additives’. They include the unavoidable residues of agricultural chemicals, pesticides or antibiotics.
There are several traditional methods of food preservation used at the household level that can be classed as chemical methods. Substances such a sugar, salt, vinegar, spices and wood-smoke are generally regarded as safe and natural preservatives. Salting, sugaring and smoking are all methods of curing foods. Curing is a general term that covers all these types of food preservation.
Salting is the addition of salt (sodium chloride or NaCl) to food for the purpose of preservation. The growth of microorganisms is inhibited by the salt, which has the effect of drawing water out of the bacterial cells so they become dehydrated and die. In this manner, salt, in combination with other measures, acts as a preservative in many foods such as butter, cabbage, cheese, cucumber, meat and fish. It also gives a desired flavour to the food. Salting can be done by rubbing adequate quantities of dry salt into foods, or by immersion, where the food item is soaked in a concentrated salt solution (i.e. brine). For effective preservation, the concentration of the brine solution has to be maintained above 18%. This is approximately one cupful of salt to five cups of water.
Sugaring refers to the action of sugar in food preservation. It is similar to the action of salt in that it depends on the removal of water. In concentrations of at least 65%, sugar solution is widely used as a sweetening and preserving agent. However, care is needed because at low concentrations, sugar solution can support the growth of microorganisms. It has been found that microorganisms rarely survive in solutions above 20–25% sugar concentration.
Smoking is one of the oldest methods used to improve the quality of food and is commonly used to preserve meat and fish. The smoking process involves exposing food to smoke from burning or smouldering wood or other plant material. It partially preserves the food by surface drying, i.e. removing moisture from the surface of the food, but it is not a reliable method of preservation unless combined with some other method such as salting or drying.
Spices also have some uses in food preservation because they tend to inhibit the growth of staphylococci and other bacteria. However, they have a very limited application because they often get contaminated themselves by a number of bacteria.
There are some other methods of food preservation that are used in the food industry and require special equipment, for example, irradiation and vacuum packing. Irradiation is the process of exposing food to ionising radiation in order to destroy microorganisms. Vacuum packing depends on the removal of oxygen from food packaging to prevent the growth of aerobic bacteria that will decompose the food.
In Study Session 10, you have learned that:
Now that you have completed this Study Session, you can assess how well you have achieved its Learning Outcomes by answering these questions. Write your answers in your Study Diary and discuss them with your Tutor at the next Study Support Meeting. You can check your answers with the Notes on the Self-Assessment Questions at the end of this Module.
Define food protection and food preservation.
Food protection is the set of methods used to prevent food from being contaminated.
Food preservation is the process of treating the food so that it can be kept unspoiled for a long time.
Explain why it is important to keep work surfaces clean in food preparation areas.
Work surfaces need to be kept free of dirt and scraps of food, and out of reach of pets, rodents and insects. This is so that possible causes of microbial contamination are removed and food can be placed on the surfaces without danger to its quality.
Describe the best way to prepare (a) fruit and (b) meat for immediate eating.
Fruit should be washed thoroughly in safe water.
Meat should be cooked at a temperature greater than 70ºC, for as long as it takes to cook the inside properly – this depends on the exact temperature and the size of the meat pieces. It should be eaten as soon as it is cooked, or kept at above 60ºC if there is a delay before it is eaten.
Emebet is preparing a meal, but has to stop to chase a dog out of the house. What should she do when she returns to the food preparation?
After any interruption in the food preparation process, Emebet should wash her hands, even if she has not touched the dog. She is likely to have touched other contaminated surfaces; microorganisms could be transferred on to the food by her hands if she does not wash them.
Your community does not have a refrigerator to keep foods like meat safely. What traditional meat preservation method would you recommend for the community in order to keep meat safe for many days?
If meat cannot be kept chilled it could be smoked over a fire, or salted by rubbing in dry salt or by soaking the meat in brine. These methods will keep the meat safe for several days.
Which of the following statements is false? In each case, say why it is incorrect.
A Salting and sugaring are chemical methods of food preservation that rely on soaking food in a weak solution of salt or sugar.
B Pasteurisation of milk kills all microorganisms by rapidly heating the milk until it boils and then allowing it to cool slowly.
C Canned foods should not be eaten if the can is swollen and bulging.
D Fermentation is a method of food preservation that relies on keeping food in an alkaline environment.
A is false. For salting and sugaring to be effective food preservation techniques, a concentrated solution must be used and maintained.
B is false. Pasteurisation is a controlled process in which milk is heated to a specific temperature, not to boiling, for a specific time and then cooled quickly. It will kill most microorganisms but cannot be guaranteed to kill all.
C is true. Bulging or swollen cans indicate that gas is being produced inside by microbial activity.
D is false. Preservation by fermentation relies on the presence of acid or alcohol.