Processed food
People seek to preserve food and to improve its quality using a variety of techniques such as:
- drying
- canning
- pickling
- adding chemical preservatives
- refrigeration
- freezing
- irradiation
The main aim of these processes is to allow foods to remain in good edible condition, without serious deterioration, for longer than would be possible if these preservation methods were not used.
The processes include:
- cooking
- adding substances to improve the taste or appearance of the food
- taking measures to make the foods more nutritious (eg. adding micronutrients or germinating grains)
- removing undesirable constituents, including toxins
Today food processing includes both traditional and some more industrial and modern techniques. Almost all aspects of food processing have some relevance to nutrition. In addition to those effects, milling and cooking break down cell walls so that nutrients are digested more easily. Fortification of foods with nutrients is an aspect of food processing directly aimed at reducing deficiency diseases.
Cooking
- Cooking techniques have changed much over the years in some societies and very little in others.
- Many people still cook over open fires and on traditional stoves, but in contrast now almost a majority of households in Western Europe and North America have a microwave oven in the kitchen, a relatively new invention.
- Cooking is practised by almost everybody, everywhere. Except for fruits and some vegetables, most groups of foods are generally cooked before being eaten.
- In many African and Asian countries even vegetables are seldom eaten uncooked, and there is little tradition of eating salads. The practice of cooking vegetables probably helps protect consumers from diseases spread by faecal contamination including parasitic, bacterial and viral infections of the gastro-intestinal tract.
- Most tropical fruits are eaten raw, but the exposed peel is not consumed so they do not present the same risk of infections. Bananas, mangoes, papayas and citrus fruits, for example, are not dangerous because their peel is not eaten.
- Cooking of food is a universal practice mainly because it improves the taste of food, makes inedible foods edible or makes foods more digestible. Cooking also kills organisms, including many disease-causing microorganisms in food.
- Cooking of high-starch foods including cereals (rice, wheat, maize, etc., which for most of humankind provide the bulk of the energy and even protein consumed) and also potatoes, yams and cassava makes these foods palatable and also more digestible.
- There is more to cooking than merely roasting, baking, grilling, or boiling of foods as gathered or harvested. It usually also involves mixing of foods or perhaps more commonly adding food items to the main food being cooked, which may alter the nutritional value of the main food but is usually intended to make the food, dish or meal taste better.
- For all its good points, cooking can have some negative nutritional effects. Frying foods at very high temperatures can destroy some vitamins and can produce undesirable components such as carcinogens in the food. Smoking of food can also produce such substances. Boiling some items in water that is then discarded can remove water-soluble vitamins.
Germination of grains
- There is intense interest now in the use of traditional germinating methods to produce malted foods.
- For many years people in the United Republic of Tanzania and other countries have allowed sorghum, millet and other cereals to germinate by soaking the grains in water for some hours, then keeping them damp for two or three days, and finally drying them, often by spreading them in the sun.
- The dried cereal grains are then pounded using a traditional large pestle and mortar. The resulting flour is stored, and small amounts are used mainly for brewing local beer (pombe).
- The dried germinated flour, known as kimea, is also used to thin and sour traditional porridges made from maize for child feeding.
Preservation of food
Physical methods
- Cooling or freezing greatly prolongs the time it takes for many foods to spoil or become inedible. In this sense it is a very important method of food preservation.
- Other methods are drying and smoking of foods. Removal of water prevents or reduces the ability of organisms to grow and multiply on or in many foods. Organisms thus inhibited include moulds and their toxic products, such as aflatoxin, as well as microorganisms that spoil the food and produce undesirable odours and taste. Dry cereals store better, and dried fish remains edible for relatively long periods. Some foods, such as milk, are dried in factories so that the preserved product can be easily marketed, transported and made available for consumption.
Chemical methods
- Food may be kept edible longer by the use of substances termed chemical preservatives. The most widely used in the home are salt (sodium chloride) and sugar, which most homemakers would not consider to be chemical preservatives. Foods with high levels of salt or sugar are less attacked by organisms and so are preserved. Industry also uses salt and sugar to preserve food.
- Among the widely used preservatives are sulphur dioxide and benzoates, which are mainly effective in controlling moulds and yeasts, respectively. Baked goods such as bread are often preserved using propionic acid, which inhibits the attack and growth of moulds and then prolongs the time before spoilage.
- Meats, particularly salted meats such as bacon and ham, are further preserved with sodium nitrite and sometimes sodium nitrate.
Sterilization
- Foods of almost all kinds are preserved by a process termed canning, although some are actually put in jars or bottles.
- In general the foods (vegetables, fruits, meat products and others) are sterilized by heating them to kill all living organisms and are sealed in a can or bottle while still hot.
- Sometimes salt and sugar are used as part of the process.
- Home canning or bottling of foods of animal origin, particularly meat or fish of any kind, can be risky. Highly resistant bacteria such as Clostridium botulinum can survive, produce toxins and cause very serious disease.
Microbiological methods
- Fermentation is used traditionally to preserve foods or to improve their palatability, as is the case with soy products in Indonesia.
- The process is also used commercially, for example in the manufacture of yoghurt or commercial alcoholic beverages.
- Fermentation using yeasts and other organisms which act on the carbohydrate in the food produces alcohol.
- Thus with any carbohydrate they have, they use some to make alcoholic beverages.
- The carbohydrate may be a common cereal such as wheat, rice, barley or sorghum, or it may be honey, used to make mead in ancient Britain and modern Africa; coconut sap to make coconut wine in Oceania; or cassava or plantain to make strong drinks called waragi and koinage in Uganda.
- Yeast also acts on sugars to produce carbon dioxide gas in food. This principle is used to make bread.
- In some foods non-disease-causing organisms are encouraged to multiply to sour the food. Souring results when the microorganisms produce acid from the carbohydrate. Souring foods to some extent prevents pathogenic or harmful organisms from multiplying in the food, which keeps it safer and makes it last longer. Common soured foods are dairy products such as sour milk and yoghurt; fermented soy products such as tempeh; and fermented cereal porridges, consumed in much of sub-Saharan Africa. In some cases souring enhances the nutrient content of the food.
- In many countries, including China, pickling is widely used to preserve vegetables and vegetable products.
Other methods
- A purely industrial method of food preservation is irradiation.
- In this process the food is exposed to radiation, usually gamma rays, which kills microorganisms and fungal spores.
- The food is then sealed and is safe until opened.
- Irradiation can also be used to prevent or delay sprouting of certain cereals, legumes or other seeds and so to increase their shelf-life.
Fortification
- Fortification has been defined as the addition of one or more nutrients to a food to improve its quality for the people who consume it, usually with the goal of reducing or controlling a nutrient deficiency. This strategy may be applicable in nations or communities where there is a problem or a risk of a deficiency of the nutrient or nutrients concerned.
- Food fortification is easier with one food, such as salt, and where there are very few manufacturers. Under other circumstances fortification is possible, could work and might have a major role in improving nutritional status and reducing the risk of deficiencies, even at the local level. In the past people have tried to find one ideal food to fortify with vitamin A or iron.
- Two kinds of fortification that have been highly effective in many countries are the addition of iodine to salt (iodization) and the addition of fluorine to water (fluoridation).
Micronutrients
- Food fortification offers an important strategy to help control, in particular, the three main micronutrient deficiencies, namely deficiencies of iodine, vitamin A and iron. In developing countries the greatest priority should be given to fortification with these nutrients. With iodine, fortification alone, in the form of salt iodization, is often the only strategy used.
- With vitamin A and iron, fortification should be used in combination with, not to the exclusion of, other interventions.
Macronutrients
Enrichment could involve the addition of:
- fat or oil to increase the energy density of a food
- amino acids to cereal products to improve protein quality
- protein, sugar or oil (as well as micronutrients) to a formulated food (eg. a manufactured weaning food, or to a food supplement such as corn (maize)/soybean/milk (CSM) for emergency feeding)
(Source: Foods composition tables, nutrition requirements and food balance sheets, FAO)
