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Heat treatment, in view of later knowledge, is seen to have other effects than to destroy or lower the vitality of microorganisms initially present there are the more obvious changes of flavour and of consistency brought about by the partial cooking, but there are also the possible lowering of the vitamin potency and the still more subtle changes in the salts which may, after heat treatment, be rendered less available than in the raw product. The importance of these considerations cannot be too much stressed when it is remembered that heat treatment is, generally speaking, an inherent stage in the process of canning. It is the heat treatment which preserves the goods, the sealing of the can being merely a means of prevent recontamination. The chemist, no less than the physiologist, has been much concerned with the changes in foods caused by heat treatment as a method of preservation, and, as a result of his investigation, there is now a better understanding of the changes which take place, with a consequent improvement in the methods of processing. For a number of years, however, this country, in common with many others, has relied, in so far as its supplies of meat are concerned, on products preserved by cold, and the freezing of beef, the chilling of mutton, have made available to us the cattle of the Argentine and the sheep of New Zealand. Initially the processes employed were crude, the postmortem changes were imperfectly understood, conditions of storage, before, during and after shipment, were haphazard, and the methods of defrosting far from scientific. How far the methods have advanced, and to what extent the scientist has been concerned in the elucidation of the many problems, will be realised from the reports of the Food Investigation Board. It is not suggested that all the advance is due to the work of the Low Temperature Station a Cambridgemuch has been done in other countriesbut the investigations carried out by the scientists a this station have been fundamental. Food producers in America were the first to realise the importance of the latest development in freezing, the advent of the Quick Freezing Processes marking a distinct advance in technique. When cellular tissue is normally frozen and subsequently defrosted, rupture of the cells may have occurred and the structure of the substance consequently partially broken down. When, however, the tissue is quickly brought down to a very low temperature, it is found that in many cases this breakdown in tissue does not take place. These principles have been applied to commercial installations, and fish, meat, fruit and vegetables so treated show on defrosting remarkably little change in character. Preservation by desiccation is a method employed for certain materials with great success. Sundrying of fruits sultanas and dates, to quote but two and the sundrying of cereal products such as macaroni is still practised. An important industry concerned with the drying of milk has developed in most milkproducing countries, whilst dried eggs and dried eggalbumin form important items of commerce. It is obvious that the object of concentrating such substances as fruit juices, milk and vegetables and animal liquid extracts is ideally to reduce the water content and obtain a product which, when the water is ultimately restored, gives a solution or material having the original taste, aroma and food value. The effect of heat is often, however, to change these characteristics, and although by the use of a vacuum the temperature to which the substance is submitted is lowered, changes still take place, and much of the aroma depending on volatile constituents is lost. To a very great extent this has been overcome by a method of desiccation which is essentially partial freezing, a method which has not yet received much publicity as it has only lately emerged from the experimental stage. The practical application of this principle is due to Dr. G. A. Krause, of Munich, who has invented and designed a dual process of concentration. In this process the liquid is first concentrated by freezing out water as ice, which is removed by mechanical separation in a centrifuge. By ingenious mechanical and regenerative devices this process has been made extremely efficient, the losses being only 12 per cent. of the original juice, although the efficiency is not maintained when the solidscontent of the product has been raised to 4050 per cent. This liquid is then further concentrated by evaporation at a low temperature, about 1015 C. The differential evaporation of water as compared with the aromatic flavour constituents occurs because the removal of water as vapour at this temperature depends solely on the rate of diffusion of the molecules into the gas space. As water has a small molecule compared with the large molecules of the esters, ethers and alcohols of the flavouring substances, it escapes more readily the conditions of evaporation as given in the patent are all designed to aid this escape. A reduction in pressure may be used to speed up the process without interfering with the differential diffusion, and the provision of an atmosphere of small molecules e.g., hydrogen also has the same effect. A large surface for the evaporation is made by spreading the liquid as a thin continuously renewed film. The condenser is situated very near the evaporating liquid to remove the water molecules quickly a distance of 3 cm. is the maximum diffusion path. The atmosphere may be circulated or disturbed to hasten the diffusion and, most ingenious of all, it may be blown towards the evaporating liquid when, if a velocity is used just greater than that of the heavy molecules leaving a liquid surface, the loss of flavour may be entirely eliminated while the rate of water evaporation is only reduced by 10 per cent. By these means a concentrate containing as much as 65 per cent. solids and capable of storage without deterioration at ordinary temperatures may be prepared, and 80 per cent. of the original vitamins retained. The use of refrigeration in the preservation of food has necessitated the use of refrigerated transport to complete the links between producer, manufacturer, retailer and customer. The variety of commodities and the different conditions they need create varying demands on the methods of insulating and refrigerating transport vehicles. The British railways have 4,000 refrigerated railway vans, and such vans, containing perishable produce, came regularly to England from Austria and Italy by way of the train ferries. These vans are designed for fairly high temperatures, 3540 F., and long hauls, and use ice as a refrigerant. At the other end of the scale is the road vehicle, which may have a temperature as low as 0 F., but is only on its journey about 12 hours. It is in these road vehicles that the greatest advances have been made, for conditions in England do not justify the railways in expenditure on elaborate equipment. The early road vehicles were insulated boxes on a lorry chassis and were refrigerated by ice and salt, which was messy and caused bad corrosion of the chassis. The introduction of an eutectic solution, virtually a mixture of a freezing salt and water in a definite proportion, which was frozen as a whole in a sealed tank, was made some few years ago. This removed the messiness, conserved the salt and produced greater efficiency and a more stable temperature.
British Food Journal – Emerald Publishing
Published: Nov 1, 1941
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