There is nothing new about man’s need to preserve food for storage or convenience of preparation. Sundrying, salting, pickling and smoking are methods which have been used for centuries and are still used today. Recent advances in technology have made it possible to preserve food by decreasing the risk of microbiological spoilage with less loss of taste, texture and nutritive value. The demand for more processed food has arisen for two reasons, firstly the population of Britain can no longer be agriculturally self supporting (about fifty per cent of our food is imported), secondly less time is now devoted to the preparation of food because many married women take up employment instead of staying at home.
One of the oldest methods of preserving food is by drying but this causes considerable damage to the food, and is rarely used nowadays except for some fruits. More sophisticated methods of drying have been developed recently, such as puff drying for breakfast cereals and popcorn. Vacuum and freeze drying processes are used for products with delicate flavours and textures like coffee granules, eggs, prawns and vegetables. The process involves freezing, and evaporating the ice away, under reduced pressure. It is an expensive process but gives a product which is high in flavour and palatability. Milk may be dried on rollers or by spraying concentrated milk into hot air chambers.
Preserving dairy produce
Dairy produce is a nutritious food not only for man but also for microorganisms. Most milk sold nowadays is pasteurized, which involves heating milk to 71°C for 15 to 20 seconds, followed by rapid cooling. A little loss of thiamin and vitamin C occurs, but this is acceptable in view of the decreased risk of contamination from pathogenic organisms. Cream and yoghurt are also pasteurized. Pasteurization has meant that clean milk can be distributed to the majority of the population. This has made an enormous impact on our food habits and raised the nutritional quality of our diets. Homogenized milk is becoming popular.
This has been pasteurized, and forced through small jets which break the fat globules into smaller particles, which stay evenly distributed throughout the milk, instead of rising to the top. Sterilized milk is heated to 63C under pressure, bottled and heated again for 30 minutes. It has the advantage of keeping for several weeks even when opened, but has a caramalised flavour and is less nutritious. Ultraheat treatment of milk is the most recently invented method, where it is heated to 132°C for one second. It is packed in foil and polythenelined cartons, and keeps for several months until opened, when it is like fresh milk.
Canning is another method of preserving food by heating and the exclusion of air. First the food is cleaned, and fruits are peeled and blanched. It is then packed hot into cans made of steel, coated with a very thin layer of tin which is inert to corrosion, and hermetically sealed. The heating time and temperature depend on the contents but acid foods like fruit need less intense treatment as the acid medium itself inhibits microbe growth. Vegetable foods and larger sizes of can, need longer times and higher temperatures.
Losses of B vitamins and vitamin C can occur but fat soluble vitamins (A, D, E) are not severely affected. As the food is virtually cooked in the tin, the juices are usually at least as nutritious as the food. Texture and taste changes can be quite marked but we now accept certain tinned foods like peaches, peas and strawberries as types of food in their own right. Tinned foods keep for several years if the tins are not dented or damaged.
The freezing of food has made an enormous impact on our eating habits. Vegetables are washed and frozen within a few hours of picking, and can then be eaten at any time of year. Fruits which enjoy only a very short growing season can also be enjoyed at any time, and can make a significant contribution of nutrients that would otherwise be in short supply. Food needs to be frozen quickly, in order to cause minimum damage and nutrient loss, and although this is quite an expensive process it is highly effective in preserving food quality. Frozen foods often require less cooking than fresh varieties, so nutrient losses need not be any greater. There is virtually no waste with frozen foods which is a factor to consider in assessing value compared with fresh foods. Buying frozen foods in bulk can be a great economy, provided that you have tested the food quality before buying large quantities. Home freezing is a boon, especially if there is home produce to preserve.
Vacuum packing is an effective way of preserving meats, cheeses and other perishable foods. The shelf life of foods can also be improved by very small quantities of chemical preservatives like antioxidants, and the appearance of processed foods can be improved with bleaching or colouring agents. It is important to maintain an active pressure on manufacturers to keep these agents to a minimum, by not buying highlycoloured or heavily preserved foods. The additives used in foods must be described on the package but the value of these agents cannot be denied and their safety is strictly monitored by rigorous laboratory testing. Many of the traditional chemicals like salt, vinegar and possibly even sugar, would not have passed the current legal standards.