Hygiene and Environmental Health Module: 10. Food Protection and Preservation Methods

Study Session 10  Food Protection and Preservation Methods

Introduction

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.

Learning Outcomes for Study Session 10

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)

10.1  Food protection, storage and preparation

10.1.1  General principles of food protection

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:

• any water that is not known to be safe, including overhead leaks and drips
• dirty hands
• coughing and sneezing
• dust and soot
• flies, rodents and other vermin
• insecticides and other chemicals
• unclean utensils and work surfaces
• cigarette smoke.

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:

• To prevent food contamination and spread of disease.
• To ensure the good health of people eating the food.
• To protect the health of the food handler.

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:

• All water used in food preparation should be wholesome.
• All dishes, glasses and utensils must be kept clean by regular washing in clean water, and clean utensils should be kept covered.
• All surfaces that come into contact with food should be meticulously clean (Figure 10.1).
• Food storage, preparation and serving areas should be free of pets, rats, mice and insects.
• Food should be covered, and kept separate from chemicals and poisons (which should be clearly labelled).
• Cloths that come into contact with dishes and utensils, and that are used to cover food, need to be changed daily and boiled before use.

Figure 10.1  Food preparation surfaces and equipment should be kept very clean to avoid contamination.

10.1.2  Precautions for food storage

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.

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 10^ºC. Frozen foods should be stored in a freezer below -18^ºC.

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 60^ºC. 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.

10.1.3  Precautions for food preparation

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 70^oC. 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.

Figure 10.2  Vegetables must be clean before cooking. (Photo: Janet Haresnape)

10.2  Food preservation

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:

• Prevention of contamination of food from damaging agents.
• Delay or prevention of growth of microorganisms in the food.
• Delay of enzymic spoilage, i.e. self-decomposition of the food by naturally occurring enzymes within it.

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.

10.2.1  Prevention of contamination (aseptic technique)

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.

10.2.2  Removal or reduction of microorganisms

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.

10.2.3  The use of high temperature

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.

10.2.4  The use of low temperature

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

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).

Figure 10.3  Using a refrigerator for keeping easily spoiled food items.

10.2.5  Drying

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.

Figure 10.4  Quanta: an example of food preserved by drying. (Photo: Pam Furniss)

10.2.6  Fermentation and pickling

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.

10.2.7  Chemical preservation

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.

10.2.8  Other methods of food preservation

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.

Summary of Study Session 10

In Study Session 10, you have learned that:

1. The aim of food protection is to protect food from all possible sources of contamination at all stages, including storage and preparation.
2. It is essential that all food handlers are aware of the need for good personal hygiene to protect the food from contamination and prevent disease.
3. Food must be stored correctly, in an appropriate space, at the correct temperature and avoiding contact with any source of contamination.
4. Food preservation methods are used to keep foods safe for extended periods of time.
5. Recommended methods for safe food preservation are aimed at preventing contamination, reducing microbial numbers, preventing microbial growth and delaying self-decomposition.
6. There are many different methods of food preservation that can be used for different foods.

Self-Assessment Questions (SAQs) for Study Session 10

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.