Modern food processing developed from the needs of armies and travellers for large quantities of safe and portable food. It later provided convenience food for our modern lifestyles. In the 21st century crops may begin life in the laboratory and food may be preserved by irradiation
FOOD processing is not new. For as long as agriculture has existed, people have had to find ways of keeping crops to provide food between harvests. Processing grain, fish, meat and dairy products was, and still is, a way of using valuable and limited supplies of food in the most economical and efficient way.
Advertisement
People the world over have come up with the same simple techniques of preventing food from spoiling and keeping its nutritional value. In Scotland, crofters survived through winter by making oatcakes and porridge from their sacks of dried oats. In Tibet, the staple food is tsampa, made by roasting barley grains and milling it into a flour. The flour is then ready to eat and is mixed into buttertea, soups and stews or made into flat bread. People of the Andes learned more than 2000 years ago how to make chunõ, a dehydrated form of potato that can keep for four years. Drying and salting meat to prevent its decomposition is a global tradition.
Processing food also provides consumers with food that is safe, wholesome and attractive. Bread and beer are popular examples of how ancient food technologists tackled the problems of processing wheat and barley.
Yoghurt, which has become popular in Europe only in this century, has been a traditional food in the Indian subcontinent and the Middle East since AD 200.
Although most traditional techniques of processing food originated in the Middle East and Asia, many were later developed commercially in Europe. Modern food scientists still study such traditional crafts and try to improve their efficacy.
The commercial food processing we know today arose from different needs than those of small farmers. Armies of fighting men and travellers need large quantities of “convenience” food that they can carry easily and which would remain wholesome for longer than they would otherwise need at home.
In the late 18th century, when Europe began to industrialise, people moved from the land to towns and cities to work in the new factories. In these industrial towns, agricultural land was scarce or nonexistent. Instead of growing their own food, people bought it with the money they earned.
Institutional feeding in schools, hospitals and factories increased the demand for large quantities of readily usable food. Today, much of the developed world’s population lives in urban areas. For example, 85 per cent of Britons live in cities and towns.
In developing countries, too, opportunities for economic improvement are often concentrated in urban areas and have encouraged hordes of people to leave the countryside. This rapid migration presents the governments of such countries with additional problems of providing the infrastructure needed to guarantee food supplies for rapidly expanding cities.
Considering the wide variety of peoples and climates in the world, it is surprising that we are so conservative in what we eat. Most of our food comes from just 14 crops which we eat directly or feed to animals whose products we then eat.
Carbohydrates dominate our diet. They provide our main source of food energy. Most sugar is made from sugar cane from sub-tropical and tropical regions, or from sugar beet from those regions of the world with more temperate climates. Maize, made into glucose syrups, provides more and more of our alternative sweetening agents.
Wheat, rice and maize are the world’s three main cereal crops. Barley and oats are also important in the cooler parts of the world and sorghum in the hot, drier regions of Asia and Africa. Cereals so dominate our diet that, as well as being the major energy source, they are also our main source of protein.
In just 20 years, from 1963 to 1983, the world’s population increased from around 3300 million to 4700 million. Almost everywhere in the world people have managed to produce enough extra food to feed their increasing numbers by boosting the yields of cereals. Between 1963 and 1983 total cereal production rose from 600 million tonnes to 1500 million tonnes per year. This phenomenal increase was the result of improved varieties of cereal that gave higher yields, better control of crop pests and the use of considerably more fertiliser.
How food spoils
Chemical changes
ALL OUR food, whether plant or animal, contains enzymes that can change its chemical composition. Spoiled food – food that has “gone off” – is food whose chemical composition has changed in some way. Living things depend on enzymes – proteins that speed up the many chemical reactions that living cells need to survive. Enzymes are at work even in the meat in a butcher’s window or in the plastic-wrapped mushrooms on supermarket shelves. Left alone, the enzymes will continue to drive chemical reactions, the products of which may be unpalatable or even dangerous.
We share our food with many other organisms, but microorganisms are our most important rivals as far as spoilage of food is concerned. They are present in all foods and introduce their own enzymes to use the nutrients they need.
Microbes break down long-chain carbohydrates, particularly starch from plants or glycogen from animals to make shorter chain sugars and glucose which they need to produce energy. Some make toxins that can injure us, others may infect us and cause life-threatening illnesses.
Microorganisms may convert fats to smaller fatty acids. They may break down proteins to their amino acid building blocks, to amines, or to ammonia and sulphurous compounds. The build up of some of these volatile strong-smelling compounds signal to us that the food is spoiled.
Foods containing more than 100 million live bacteria per gram would generally be regarded as spoiled, unless the organisms are added deliberately, for example to ferment the food.
The way we process food is designed to slow down deterioration or spoilage caused by enzymes naturally present in food or those in the microorganisms that live off the food. Enzymes cannot work without water. They are very sensitive to changes in temperature and can also be damaged or destroyed by other chemicals. Our methods of preserving food rely on making the best of these “weak spots”.
The commonest thing we do to make food keep longer is to cook it. Heat destroys both the enzymes that are naturally present in food and those introduced by microorganisms.
Microorganisms can grow in temperatures between 4 and 63°C but grow best between 30 and 40°C. Hence, in the hotter, subtropical and tropical regions, which is where most developing countries are situated, microorganisms and enzymes are at their most active and food spoils quickest.
About 100 years ago, food scientists introduced less severe heat treatments than cooking, in particular, pasteurisation. They designed pasteurisation not to kill all microorganisms but to destroy the dangerous pathogenic microorganisms – those that cause disease.
In the US and Germany, batches of milk were first pasteurised by heating to 63°C for 30 minutes. As food scientists found a way of processing continuously, rather than in batches, the milk was heated to 72°C for just 15 seconds.
This achieves the same effect of ridding the milk of pathogenic organisms but the milk still contains other organisms that can spoil it. Refrigerating or chilling milk slows down the action of these organisms, preventing it from souring, clotting or spoiling for a longer period of time.
Packing milk in bottles or plastic containers not only makes it easier to transport but prevents it from being recontaminated by microorganisms present in the air.
Freezing similarly suppresses the deterioration of food. If freezing is fast enough, the ice crystals that form in the food are too small to cause much damage to the cell walls, and so the structure of the food does not suffer.
Enzymes work much more slowly in refrigerators and can be stopped completely by freezing. Although refrigeration or freezing damages some microorganisms, they may resume growth when placed in more favourable temperatures.
Even at the very low temperatures of -18°C, at which frozen foods are stored, food enzymes still remain active in “live” foods such as fruits and vegetables. To minimise their effects, many fruits and vegetables are blanched – dipped into hot water – or exposed briefly to steam in order to inactivate most of the enzymes prior to freezing.
A much older method of preserving food is to dry or dehydrate it. This method reduces the “water activity” of the food – the amount of water available in which enzymes can carry out their reactions.
Records of drying meat and fish date back to 4000 BC. People would lay food on the ground to dry in the sun, or place it over a hot fire to evaporate moisture. Evaporation from the outer layers of the food produces a hard “skin” which inhibits further loss of moisture. This is called “case hardening” and tends to make the food difficult to dry completely.
Modern food scientists overcome this by reducing the size of the food particles. The ratio of the surface area of the food particle to its volume increases and the product dehydrates more evenly.
Liquids or slurries dry most efficiently in a thin layer on the surface of a heated drum. Powdered milk and instant coffee are dried today by spraying them from a nozzle atomiser into an upward-moving column of heated, dry air in a cone-shaped spray drier.
More expensive products, such as high-quality instant coffee, must retain their delicate flavour after drying. Much of the allure of good coffee comes from more than 500 organic compounds, most of which are volatile – they vaporise readily – and give coffee its characteristic aroma and flavour. Food technologists tackle this problem by freezing a mass of coffee and subjecting it to a high vacuum. The ice in the frozen mass passes directly to water vapour without passing through the liquid water stage. The volatile compounds are not given the chance to vaporise and so the flavour of the coffee is conserved.
Chemical additives
Why bacon is pink
COMMON SALT, or sodium chloride, is probably the oldest known preservative. It was used to make the first cured meats. Salting, like dehydration, reduces the water activity of food.
The rock salt that people originally put on meat to preserve it was not pure sodium chloride but was “contaminated” with small amounts of sodium or potassium nitrates. Soaking meat in brine helps it to absorb the salt into its deepest layers. Bacteria that can withstand high concentrations of salt, such as species of Micrococcus and Staphylococcus, convert the nitrate in the rock salt to nitrite. This nitrite combines with haemoglobin in the blood of the meat to form nitrosohaemoglobin and other compounds, which give the meat a distinctive pink colour even when cooked.
The persistent, characteristic pinkness provided the food processors and consumers with a clear signal that the bacon, ham or continental sausage is properly cured. This early “natural” form of food preservation, together with mild pasteurisation, effectively prevents the growth of spores of Clostridium botulinum that may otherwise make the eating of mildly cooked and stored meat products a potentially life-threatening venture.
Food technologists today add known quantities of sodium chloride, nitrate and nitrite under controlled conditions of drying and heating to produce a wide range of good meat products.
Cheese makers have long added sodium nitrate to some Dutch cheeses to prevent undesirable changes of flavour resulting from the growth of species of Clostridium during the cheese maturation of over several months. During the maturation process, under controlled temperature and humidity, lactic acid bacteria in the cheese release particular enzymes that split proteins and produce the characteristic flavour of mature cheese. Recent work, led by the Institute of Food Research Reading Laboratory in Britain, aims to speed up maturation by controlling the rate at which the enzymes are released into the cheese curd.
Traditional methods of making vinegar or pickling fruit and vegetables rely on the lactic and acetic acids produced by bacteria that live off the food. Food technologists borrow from nature and speed up the preservation by adding such acids directly. This is called pickling.
Microorganisms in food are not always bad. Fermentation, for example, relies upon the presence of desirable microorganisms to produce bread, beer, wine and yoghurt. Microorganisms use components of food, often the carbohydrates and sugars, to make the energy they need. In the process, the sugar is converted to the lactic acid of yoghurt and cheese, or ethanol (alcohol) which helps to preserve beer and wine.
Alcohol and lactic acid are end products of fermentation, but along the way other compounds are formed. Many of these contribute the characteristic flavours we recognise in fermented foods. Examples are diacetyl, which gives yoghurt a particular flavour, or the propionate that characterises Swiss cheeses.
Just as chemicals can inhibit the growth of unwanted microorganisms and encourage the growth of others, altering the atmosphere surrounding food can have the same result. Fresh meat goes slimy and spoils because bacteria belonging to the genera Pseudomonas, Alcaligenes or Moraxella grow on it. These organisms can grow only if they have access to oxygen. Lactic acid bacteria grow better, however, if the amount of carbon dioxide in the atmosphere increases.
Food scientists take advantage of this fact by modifying the atmosphere surrounding the chilled meat, either in large cold stores or within retail packs. They reduce the oxygen content of the air and increase the amount of carbon dioxide. This tips the balance – encouraging the growth of the lactic acid bacteria and inhibiting the growth of chill-spoilage bacteria.
The same technique works for fresh fruit and vegetables, which still “live” after they are cut. Modified atmospheres alone are limited in their effects, but combined with chill temperatures, near 0°C, the shelf life of meat, fish, fruits and vegetables increases substantially.
Biotechnological future
Tastes of things to come
HOW WILL the food industry of the future compare with that of today? Biotechnology promises to play a central role in improving varieties of food crops. More food plants may start life in laboratory cultures before being planted out in the field.
As we learn more about fermented foods, and the enzymes responsible for their production, these enzymes may be selected, purified or genetically improved to produce the particular properties we require in food products.
Fewer chemical additives may be used, but will we have come to terms with the fact that all food, and life, is a mixture of chemicals. Will consumers appreciate that the word “natural” does not in itself mean wholesome or safe? Will we want to develop a wider range of food crops, or will our natural caution lead us to a more restricted choice in our diet?
The trend towards more information for consumers will probably continue, but will we examine more closely the logic of our requests? Tighter controls on the labelling of food may imply that newer methods of preservation, such as food irradiation, are more risky than older methods such as chemical additives. Is this really logical or are we just happier with what we have grown familiar with?
Wider travel and better methods of communication should provide the opportunity for a more interesting and varied diet in all parts of the world. We should aim to apply minimal methods of food preservation.
The culprits of food poisoning
SOME PEOPLE are intolerant of, or allergic to, certain foods. Allergies are usually specific to a paticular food ingredient; often the natural proteins in milk or eggs. High levels of histamine in spoiled fish, known as scombrotoxin, can also cause illness.
Food may also cause illness because it is contaminated with toxins or with disease-causing – pathogenic – organisms. Viruses, for example, may be carried by foods – such as shellfish from waters polluted by sewage – and may cause gastroenteritis or other diseases.
Bacteria, however, are most often the culprits in food poisoning. Some, such as Salmonella and Campylobacter, may infect the body. Small numbers of live bacterial cells multiply and cause illness over a period of days or weeks.
In contrast, bacteria such as Staphylococcus aureus and Clostridium botulinum, may grow in food and produce toxins. Even though there may be no live bacterial cells left in the food, the poisons may cause vomitting, diarrhoea and, in some severe cases, breathing difficulties.
Food irradiation
FOOD IRRADIATION is our first alternative to heat as a way of inactivating microorganisms or parasites in food. As such, it may prove invaluable in the future.
Short-wave ionising radiation from electrons, X- or gamma-rays passes through food carried on a conveyor belt. As it does so, it inactivates or destroys microorganisms without cooking the food.
Scientist have conducted much research on food irradiation in projects funded by the United Nations Food and Agriculture Organisation (FAO), the World 91ɫƬ Organization (WHO) and the International Atomic Energy Agency (IAEA). Their Joint Expert Committee on Food Irradiation (JECFI) has recommended limits for food irradiation doses. The international FA/WHO Codex Alimentarius Commission has also produced a recommended code of practice for regulating the irradiation of food.
The technique is already being used selectively in many parts of the world. US and Russian space missions have used irradiated food to prevent the possibility of food poisoning. Irradiated food has also been provided in Britain to intensive care patients whose immune systems have for medical reasons been suppressed.
Insects and parasites are particularly sensitive to low doses of irradiation. Inactivating pests in stored food, particularly cereals, would improve food supplies by diminishing the amount of food damaged by insects.
Medium doses of irradiation could “pasteurise” foods such as raw chicken, prawns and shellfish. This would inactivate or kill live Salmonella and Campylobacter. High doses would reduce bacteria and spores on spices where the safety of alternative chemical treatments is uncertain.
Fears have been voiced about the safety of irradiated food, in particular that such food would have unacceptably high levels of radioactivity. However, research has shown that irradiation does not significantly increase the natural radioactivity of food. Nor would the technique be used on all foods.
Advantages of food irradiation are that it could improve safety; reduce spoilage; increase the shelf-life of foods and favourably modify the texture of certain foods.
Disadvantages of food irradiation are that it reduces the levels of some vitamins, particularly vitamin C and thiamin. Low doses would not usually inactivate enzymes or viruses that cause food to spoil. Nor would irradiation normally render harmless any microbial toxins that are already present in food. The technique would not be used on very oily or fatty foods because it oxidises the fats and results in the food tasting bad.
Like all techniques in food processing, there are good and bad points about food irradiation. It must be left to researchers and regulatory bodies to ensure that when irradiation is used, it is used safely to produce healthy food.
Further reading
J. M. Jay, Modern Food Microbiology (Van Nostrand Reinhold, New York, 1986); G. Campbell-Platt, Fermented Foods of the World. A dictionary and guide (Butterworths, London, 1987); J. Hawthorn, Foundations of Food Science (W. H. Freeman, Oxford, 1981): D. S. Robinson, Food Biochemistry and Nutritional Value (Longman, Harlow, Essex, 1987).
