Food Preservation

FOOD PRESERVATION. As food material in its natural state only keeps sound and edible for a comparatively short time, its preservation has engaged the attention of mankind from the very earliest times and has been practised from the dawn of civili zation. Pastoral man preserved milk in the form of butter or cheese, and grape juice in the form of wine ; the hunter soon learned to dry or salt his game or fish. The ever increasing spread of man over different parts of the world, into widely differing climates, and still more the increase in the population of towns and large cities has rendered the development of adequate methods of preservation a matter of vital necessity in the maintenance of the food supply of the world. For many centuries the art of preservation developed but slowly; it was purely empirical, dry ing, smoking or curing with salt being the principal methods. Rapid advance follows when the underlying cause of a matter is understood : so, when the biological causes of the decomposition of food were realised, appropriate methods were soon developed for dealing with the causative agents.

Very few substances decompose of their own accord : almost always the changes associated with decomposition are due to the activity of living organisms, usually bacteria, yeasts or moulds. In the case of fruit and other vegetable substances, there may also be the activity of ferments or enzymes. The very fact that a sub stance is available as food for man pre-supposes that it is decom posable by living agency, either microbes or enzymes. If bacteria are completely excluded foodstuffs will keep indefinitely, so that the problem to be solved is how to destroy all living organisms in the food and prevent their subsequent access. As an alternative we can so reduce the temperature that the activity of the bac teria or of any enzyme present is suspended. Yeasts decompose sugary substances into alcohol and carbon dioxide; some bacteria produce acetic, lactic or other acids ; others decompose the nitrog enous constituents of the food, giving rise to various gases which often have offensive smells. Moulds attack the carbohydrates, thereby forming various compounds. All these processes may be going on simultaneously if conditions are favourable. The pre liminary infection of the food may arise from the bacteria natur ally present therein or from contamination from outside, since the air, water and soil are all normally infested with an almost infinite variety of organisms.

Sterilization by Heat.

Of the various methods of preserving food to be considered, only one—sterilization by heat—aims at the complete destruction of all bacterial life. The others merely reduce the numbers of organisms or produce such conditions that their development or activity is prevented; such methods include refrigeration, drying, salting, smoking, the addition of sugar or pickling with vinegar, salt or nitre, or the addition of a chemical antiseptic or preservative. The efficacy of sterilization by heat was known for centuries before its theory was understood. Thor ough cooking renders food temporarily sterile, so by repeated cooking it can be made to keep indefinitely, if adequate means are taken to prevent re-infection. This fact is used by the housewife unwittingly when she covers the potted meat with a layer of hot fat, or the jam with paper soaked in brandy. Few bacteria can long survive a temperature above 8o° C (176° F) in the presence of moisture (those which can are termed thermophilic) with the exception of some spore-formers which need temperatures as high as 140° C (284° F) for their destruction. Sterilization proceeds more rapidly in an acid medium than in an alkaline one, and the fact is that, except for milk, all natural foods are slightly acid in reaction. In ordinary operations of cooking the temperature em ployed is above boo° C (the boiling point of water) so almost all bacteria are killed—but not their spores—with the important reservation that in the case of large masses the central portions may not actually reach Ioo° C (212° F). If conditions are favour able the surviving spores may develop into the vegetative form, hence it is needful either to repeat the cooking or to keep the food under such conditions that the spores do not develop. The pres ence or absence of air is an important factor since bacteria and their spores are of two classes, aerobes and anaerobes, which respectively grow in the presence or absence of air; of these the aerobes are much the larger class and among its members are those organisms which are most frequently the cause of spoilage.

The application of these principles to the canning of food dates back to the Napoleonic wars, although at that time they were little known. Nicholas Appert, in 181o, was the first to preserve meat; he had but indifferent success. Fish and fruit were pre served (1840) and in 1856 Borden patented the first successful process for the manufacture of condensed milk. Since the indus try has advanced and extended greatly, and is now being placed on a surer and more scientific basis ; cruder methods and those involving the use of chemicals suspected of being injurious to health have been replaced by processes beyond the reach of criti cism. Of prime importance in the canning of food is the vessel in which it is to be cooked, transported and sold. This must be unaffected by heat, or by contact with the food material in it ; it must be light, cheap and strong. Glass is sometimes employed, but the most usual material is tin-plate; this is thin sheet steel which has been cleaned and dipped into molten tin and subse quently rolled so as to leave a very thin layer of tin all over the surface of the iron. It is not quite an ideal substance as, quite apart from mechanical defects, such as pinholes or an imperfect surface, it is to some extent attacked by all acidic foods so that a tiny quantity of tin is communicated to the food. When this amount is only a fractional quantity, at the most not exceeding a grain or so per lb., it is not considered to be prejudicial to health, as tin is not very toxic, but as it may be avoided by the use of lacquered tins these are much to be preferred, especially for the more strongly acid foods such as tomatoes. Glass vessels are mainly used for potted meats, tongues and similar articles. There are two principal types of tin-can, the "hole-and-cap" and the "sanitary," the former is the older, into it the foodstuff is filled, then the lid is put on, leaving a small hole which is filled in with a dab of solder after the processing or sterilization. In the sani tary can the lid is put on in its entirety, usually before the cook ing, by means of a special "crimping" machine; a gasket is usually inserted to ensure that the can is air tight. This latter type is now the more common.

The process of sterilization generally consists in heating the tins or pots in trays in a large autoclave or pressure chamber, by means of superheated steam, to a temperature in the neighbour hood of Ito° C (248° F), but varying according to the nature of the foodstuff. The time also is variable, though it should be suffi cient to ensure that the central parts of the tins reach at least Ioo° C (212° F) ; it therefore depends upon the size and distribu tion of the cans in the oven. After this cooking the sealed tins are allowed to cool slowly in the oven with suitable adjustment of the pressure, or they are removed and conveyed through tanks of water until a temperature of about ioo° F is reached. It is inter esting to note that the development of the food canning industry has been to a large extent favoured by fortune; it was intended to sterilize the food in the tin but in fact this is rarely accomplished. What does happen is that most of the bacteria are killed and those which survive, and any spores, are under such conditions that they cannot develop owing to the lack of air or suitable condi tions of food. Almost all the air has been excluded during the process and the proteins of the food have been so changed as to become unsuitable for the organisms which developed in it before cooking. (See also PRESERVING AND BOTTLING.) Refrigeration.—This is really the only method which affords a means of preserving food in its raw state for a reasonably long period ; it is therefore the only process which is entirely unex ceptionable. Its introduction on a commercial scale has immensely influenced the food supply of the world and has affected all civi lized countries. It would be no exaggeration to say that no sci entific advance since the advent of railways and steamboats has so altered the economic life of Europe and America. Enormous quantities of food, meat, fish, fruit, butter, eggs, and vegetables are exported, imported and stored, so that the abundance of supply in one country is made to serve the scarcity of another. The use of cold in this way depends upon a plentiful supply of ice ; this is rendered possible and cheap by the ingenuity of the engineer (see article REFRIGERATING), and the use of this method is rapidly extending from the large-scale operations of the importer to the small scale of the household, which has its own refrigerator. The object in refrigeration is not to kill bacteria, since these micro scopic forms of life can resist a very low temperature without injury; it is to reduce or to arrest completely their activity. Most bacteria are rendered incapable of growth at the freezing point of water, so that at or below this temperature all biological change ceases and the only changes which may go on are physical , ones, depending on the evaporation of water and volatile flavouring substances, or certain chemical oxidation changes. Enzymes like wise are rendered inoperative by low temperatures. Refrigeration includes both freezing and chilling, not merely because it involves much lower temperatures but because of its effects on the food itself. On cooling foods, particularly nitrogenous ones such as meat, two kinds of change take place, some reversible, some ir reversible, so that the food may or may not return to its natural condition when it is allowed to resume a normal temperature. A point of great practical importance also is the rate of cooling, since it is found that if the cells and tissues of food are frozen suffi ciently rapidly the changes are usually reversible; therefore, to obtain the best results, both the rate of cooling and the tempera ture must be regulated according to the nature of the particular foodstuff. The foods most usually preserved by freezing are meat, fish, poultry, and butter; freezing of eggs is now also ex tensively carried on, some i,5oo,000 dozens of frozen eggs being used every year in the United States.

Two methods are in general use for the freezing of flesh foods, (i.) by cold air and (ii.) by cold brine. The former method is slower and occasions more loss of moisture but, as meat does not dry very rapidly, it is preferable; the air must be carefully filtered and purified to remove bacteria and moulds (which grow at low temperatures) or any contaminating odours. Fish suffers much from desiccation and usually has been treated by the cold brine process. This latter method has the advantage of being much more rapid, but the disadvantage that it communicates some salt to the tissue of the foodstuffs. The more rapid the freezing, the less the penetration of salt ; it is therefore the practice to use the lowest possible temperatures ; with ordinary brine this is about C F). Meat may be frozen by the air method, using a much lower temperature, not only because the time taken for the cooling of thick masses of flesh would otherwise be too long, but also to minimise the "drip." A temperature of —6o° C (-76° F) or even —7o° C (-94° F) is employed for the freez ing, and is followed by storage at —6° C (+2I ° F) or —1o° C (+14° F) in Europe, or often —20° C (-4° F) in America. A recent development has been the quick freezing of foods in the original package. This process was first used, about 1931, for fish, but has since been extended to practically all foods. Research has shown that the freezing of flesh has no detrimental effect on its vitamin content or its nutritive value.

Preservation by Drying.

Desiccation or drying is nature's method of preservation. It will be found that all natural sub stances which will remain unchanged for long periods of time contain but small amounts of moisture, not exceeding say io%. For example, cereal seeds of all kinds and nuts will keep almost indefinitely ; nature preserves them simply by removing water. The reason for this is that bacteria cannot grow on dry materials, so if any food is desiccated, or the water content reduced below 8% or io%, it is immune from attack—except for a few inno cent types of mould. Meat is not now dried in the more civilized countries, though in the wilder parts of South America and in the Balkans the method is still in use ; freezing is preferable. Fish is still cured by drying, with or without salting or smoking. Eggs, milk, fruit and vegetables also are dried in enormous quantities. Different kinds of grapes dried in the sun produce raisins, sultanas and currants; plums, apples, pears, peaches, apricots and other fruits are evaporated by exposure on wooden or galvanized iron trays in a warm atmosphere. Vegetables such as peas, carrots, and onions are widely prepared in the dry state and sold in packets, and will keep almost indefinitely. The flavour and colour of such vegetables is apt to be impaired, a fact which sometimes leads to the use of harmless blanching agents, such as sulphur.

Although the desiccation of meat in the ordinary way is not now practised, mention may be made under this heading of the manufacture of meat extract since the keeping property of this depends largely upon a low water content, usually about 15 or 18%. This process was discovered by Justus von Liebig and von Pettenkofer about a century ago. Lean meat, beef or mutton, is extracted with warm water, the fat is skimmed off and the extract heated to coagulate the albumen, which is then removed ; salt is added and the broth is evaporated in vacuo to the required degree of dryness. One pound of such extract is obtainable from about 25 lb. of lean meat. Yeast when similarly treated yields an extract almost indistinguishable in taste from beef extract; chem ically it is recognisable by the absence of certain specific proteins. A well-known preparation of yeast in this form is in common use and is a rich source of the vitamin B complex. Allied to the desic cation method of preserving is the addition of sugar. Although dilute sugar solutions are peculiarly liable to fermentation and other decomposition, a solution containing less than about 25% or 30% of water does not undergo bacterial change, particularly in the presence of a small amount of acids such as those from fruit. For this reason jam keeps well ; it is only liable to a growth of mould on the surface if this is not adequately protected from such infection.

Various sweetmeats, jellies and fruits are preserved in this way.

Salt is one of the oldest preservative substances and is used for the preservation of all kinds of fish or flesh; sometimes salt petre is added with it to give the meat a bright pink colour. The method of application is simply to sprinkle salt on the layers of meat or fish, or in the case of large joints it may be dissolved in water and forcibly injected into the tissue. Alternatively the flesh is soaked in strong brine.

Butter also is often salted, the amount added varying from I up to io%.

Salt is a poor preservative but quite a useful accessory in conjunction with drying methods, as it enables a larger per centage of moisture to be safely left in the foodstuff. It suffers from the disadvantage of rendering the food less palatable, though much of it can often be washed out before consumption. Smoking is a practice confined almost entirely to fish and ham ; its efficacy depends partly upon the drying which accompanies it and partly upon the bactericidal action of the chemical compounds, such as phenols and aldehydes, in the smoke. Other classes of food are preserved in media which inhibit bacterial growth; a notable instance is pickling in vinegar or in brandy. Vinegar is essentially the product of bacterial life ; the vinegar bacillus with the aid of oxygen from the air converts alcohol into acetic acid, but when the strength of the acid reaches about 6% the bacillus can no longer grow and other types of bacilli may be killed. For this reason vinegar, which is essentially a solution of acetic acid, inhibits the growth of bacteria in pickled vegetables or fruits or fish. The preservative qualities of these products are strictly limited and it is necessary to minimise risk of infection by ex posure to the air. About 3% of acetic acid is necessary to keep pickled vegetables and in making such it must be borne in mind that the vegetables themselves contain about 8o% or 85% of water so that the acid added will slowly be diluted to a corre sponding extent by the process of diffusion as the water passes out of, and the acid passes into, the tissue of the fruit. Strong alcohol acts in a similar way, for which reason wines of a suffi cient alcoholic strength keep indefinitely if evaporation is pre vented, and fruit may be preserved by immersion in spirits, as for example in cherry brandy.

The preservation of eggs is worthy of special mention. Several methods are in common use -on a large or small domestic scale. On the large scale, refrigeration is generally employed. Millions of eggs arrive in Europe from South Africa and other parts after a voyage of a month or six weeks in perfectly good condition by reason of storage in a cold chamber at or just above the freezing point of water. There is a critical temperature for eggs at about —6° C (+20 ° F) ; if in the chilling or storage the temperature falls below this point a non-reversible change takes place in the white or albumen, so that the egg cannot be restored to its normal transparent condition. Egg is sometimes preserved in the frozen state for manufacturing purposes, in which case it is cooled to a temperature approximating to —2o° C F), and becomes a solid mass which takes a long time to thaw and which, though fluid, never returns to its normal physical state. On a domestic scale eggs are usually preserved by the simple expedient of pre venting the access of air through the shells. Scores of devices for effecting this have been patented; common. methods include immersion in water-glass, or in bran or sawdust, coating with vaseline, or even wrapping in oiled paper. When eggs are im mersed in sodium silicate solution (water-glass) or milk of lime the shell becomes impregnated with the silicate or lime and is then impervious to the atmosphere. One minor defect in this very good method of preservation is that the eggs have a tendency to burst when boiled—this may be overcome by first pricking the shell with a needle. Also the yolk is apt to burst easily; this is because, when the egg is preserved, water tends to pass from the white to the yolk (which has normally a less percentage of water) and to distend the membrane surrounding the yolk.

Chemical Preservatives.

A very large number of chemical substances, having real or supposed preservative properties, have at different times been added to foodstuffs; many of these are dis cussed in the article on ADULTERATION. Some are considered harmless and even necessary in certain articles and are permitted to be added in most countries. Nevertheless it is generally agreed that chemical preservatives are not desirable and should be ex cluded wherever possible ; they are as a class self-condemned, for it is obvious that if a chemical inhibits bacterial or enzyme action it may, at least to a small extent, retard digestion and assimilation. The compounds, other than positively objectionable or toxic ones, best known as preservatives include boric acid and borax, sulphurous acid and its salts, salicylic acid and benzoic acid, and formaldehyde or formalin. The last-named substance is now recognized as being injurious and is not tolerated in most civilized states. Perhaps the most used compound is boric acid; at one time all margarine contained it, as did a large percentage of butters, and often cream, bacon and ham. Its use became the more general as the public palate turned against salted foods, and for years it held almost universal sway, as about one-half per cent of boric acid serves to keep most foods sweet for a reasonable period. Salicylic acid and its salts found favour in saccharine beverages, wines and preserves, as it is particularly useful in preventing the growth of moulds. Benzoic acid is in nature similar to salicylic and serves a similar purpose but has the ad vantage of being less active pharmacologically. Sulphurous acid, or sulphites, are specially useful in light wines where the alcoholic strength is insufficient to preserve them, and in beer and cider ; it is also used as an adjunct in the drying of fruits such as sultanas, currants, apples, apricots, etc.

Nearly all civilized countries have now legislated on the ques tion of preservatives in food; all agree in prohibiting formalin and fluorides and any other substance of such definitely toxic properties. Boric acid is tolerated in certain dairy products in Australia, New Zealand, Canada and Holland, but is prohibited in England, the United States and Sweden. Salicylic acid is very rarely permitted, and then only in very small declared amounts in certain countries; it is prohibited in Britain and America. Sulphurous acid is permitted in certain commodities in England up to small stated quantities; it is prohibited generally in meat stuffs but sometimes permitted in sausages and in wines, beer and dried fruits. Benzoic acid is allowed in stated proportions in cer tain foods in England, Australia and Denmark and is generally permitted in small amounts, properly labelled, in the United States Federal Law. Japan and a few States in the United States are quite intolerant of any preservative of any kind in any food. There can be no doubt that such a veto is the ideal to be aimed at, but its realization in many countries awaits the better development of refrigeration and other methods of food preservation. See also