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FOOD PRESERVATION
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See also:FOOD PRESERVATION . The preservation of food material beyond the See also:short See also:term during which it naturally keeps See also:sound and eatable has engaged human thought from the earliest See also:dawn of See also:civilization. See also:Necessity compelled See also:man to See also:store the plenitude of one See also:season or See also:place against the need of another. The See also:hunter dried, smoked and salted See also:meat and See also:fish, See also:pastoral man preserved See also:milk in the See also:form of See also:cheese and See also:butter, or fermented See also:grape-juice into See also:wine. With the separation of See also:country from See also:town, the development of manufacturing nation as distinct from agricultural and food-producing See also:people, the spreading of civilized man from torrid to See also:arctic zones, the needs of travellers on See also:land and See also:sea and of armies on the See also: All decomposition of food is due to the development within the food of living organisms. Under conditions under which living organisms cannot enter or cannot develop food keeps undecomposed for an indefinite length of See also:time. The problem of food preservation resolves itself, therefore, into that of keeping out or killing off all living things that might feed upon and thus alter the food, and as these organisms mainly belong to the See also:family of moulds, yeasts and bacteria, modern food preservation is strictly a subject for the bacteriologist. The changes which food undergoes on keeping are easily intelligible when once their biological origin is recognized. Yeasts cause the decomposition of saccharine substances into See also:alcohol and See also:carbon dioxide, acetic and lactic ferments produce from See also:sugar or from alcohol the organic acids causing the souring of food, moulde as a See also:rule cause oxidation and See also:complete destruction of organic See also:matter, nitrogenous or saccharine, while most bacteria See also:act mainly upon the nitrogenous constituents, producingalbumoses and peptones and breaking up the complex albumen-See also:molecule into numerous smaller molecules often allied to alkaloids, generally with the production of evil-smelling gases. These processes may go on simultaneously, but more frequently take place successively in the decomposition of food, one set of organisms taking up the See also:work of destruction as the conditions become favourable to its development and unfavourable to its predecessor. The organisms may come from the air, the See also:soil or from See also:animal See also:sources. The air teems with organisms which See also:settle and may develop when brought upon a favourable nidus; the organic matter of the soil largely consists of fungoid life; while the intestinal See also:canal and other mucous membranes of all animals See also:harbour bacteria, sarcinae and other organisms in countless millions. Whenever, therefore, food material is ex-posed to the air, or touched by the soil or by animals or man, it becomes infected with living cells, which by their development See also:lead to its decomposition and destruction. Fungoid organisms may be killed by See also:heat or by chemicals; or their development may be arrested by See also:cold, removal of water, or by the presence of agents inhibiting their growth though not destroying their life. All successful processes of food preservation depend upon one or other of these circumstances. Preservation by Heat.—At the boiling-point of water all living cells perish, but some spores of bacteria may survive for about three See also:hours. Few adult bacteria can live beyond 75° C. (167° F.) in the presence of water, though dry heat only kills with certainty at 140° C. (284° F.). Destruction of life takes place more rapidly in solutions showing an See also:acid than a feebly alkaline reaction; hence acid See also:fruit is more easily preserved than milk, which, when quite fresh, is alkaline. By cooking, therefore, food becomes temporarily sterile, until a fresh See also:crop of organisms finds See also:access from the air. By repeated cooking all food can be in-definitely preserved. One of the most important functions of See also:cookery is sterilization. Civilized man unwittingly revolts against the See also:consumption of non-sterile food, and the use of certain fungus-infected material is an See also:inheritance from barbarous ages: few materials of animal origin are eaten raw, and in vegetables some sort of sterilizing See also:process is attempted by washing (of salads) or removal of the See also:outer skin (of fruits). All preparation of food for the table, cooking being the most important, tends towards preservation, but is effectual only for a few hours or days at most, unless See also:special means are adopted to prevent reinfection. The housewife covering the jam with a thin See also:paper soaked in See also:brandy, or the potted meat with a thin layer of See also:lard, attempts unconsciously to See also:bar the road to bacteria and other See also:minute organisms. To preserve food in a permanent manner and on a commercial See also:scale it has to be cooked in a receptacle which must be sufficiently strong for transport, cheap, light and unattacked by the material in contact with it. None of the receptacles at See also:present in use quite fulfils the whole of these conditions: See also:glass and See also:china are heavy and fragile, and their See also:carriage is expensive; tinned See also:iron, so-called See also:tin-See also:plate, is rarely quite unaffected by food materials, but owing to its strength, tenacity and cheapness, it is used on an ever-increasing scale. The See also:sheet iron, which formerly was made of soft wrought iron, now generally consists of See also:steel containing but very little carbon; it is cleaned by See also:immersion in acid and covered with a very thin layer of pure tin, all excess of tin being removed by hot rollers and brushes. The layer of tin, which formerly constituted from 3 to 5% of the See also:total See also:weight of the plate, has, owing to the increased See also:price of tin and the improvement in machinery, gradually become so thin that its weight is only from I to 3%. Not rarely, therefore, the tin-See also:surface is imperfect, perforated or See also:pin-holed. Tin itself is slightly attacked by all acid juices of See also:vegetable or animal substances. With the exceptson of milk, all human food is slightly acid, and consequently all food that has been preserved in tin canisters contains variable traces of dissolved tin. Happily, salts of tin have but little physiological See also:action. Nevertheless, the employment of tin-plate for very acid materials, like tomatoes, peaches, &c., is very objectionable. The process of preservation in canisters is carried out as follows: The canister, which has been made either by the use of See also:solder or by folding machinery only, is packed with the material to be preserved, and a little water having been added to fill the interstices the lid is secured by soldering or folding, generally the former. Sterilization is effected by placing the tins in pressure chamhers, which are heated by See also:steam to 12o° C. or more. The tins are exposed to that temperature for such time as experience has shown to be necessary to heat the contents throughout to at least roo° C. The temperature is then allowed to fall slowly to below the boiling-point of water, when the tins can be taken out of the pressure chamber, or they are placed in pans filled with water or a See also:solution of See also:calcium chloride and are therein heated till thoroughly cooked. Sometimes a small See also:aperture is pierced through the lid, to allow of the See also:escape of the expanding air, such holes before cooling closed by means of a drop of solder. This process, which was originally introduced by See also:Francois See also:Appert See also:early in the 19th See also:century, is employed on an enormous scale, especially in See also:America. The use of lacquered tins, having the inner surface of the tin covered with a heat-resisting See also:varnish, is gradually extending. Imperfect sterilization shows itself in many cases by See also:gas development within the tin, which causes the ends to become See also:convex and drummy. More frequently than not the contents of the larger tins, containing meat or other animal products, are not absolutely sterile, but the conditions are mostly such that the organisms which have survived the cooking process cannot develop. When they can develop without formation of gas dangerous products of decomposition may be produced without showing themselves to See also:taste or See also:smell. Numerous cases of so-called ptomaine poisoning have thus occurred; these are more frequently associated with preserved fish and See also:lobster than with meats, although no class of preserved animal food is See also:free from liability of ptomaine formation. The formation of poisonous substances has never been traced to preserved fruit or other material poor in See also:nitrogen. The mode of preserving food in china or glass is quite similar, but the losses by breakage are not inconsiderable. Food which has been preserved in tins is sometimes transferred to glass and re-sterilized, the feeling against " tinned " food caused by the " See also:Chicago scandals " not having entirely subsided. Were it not for the facts that sterilization is rarely quite perfect, and that the food attacks the tin, the contents of tin canisters ought to keep for an indefinite length of time. Under existing circumstances, however, there is a distinct limit to the See also:age of soundness of canned food. Preservation by Chemicals.—Salt is the See also:oldest chemical preservative and, either alone or in See also:conjunction with See also:saltpetre and with See also:wood-See also:smoke, has been used for many centuries, mainly as a meat preservative. It is used either dry in layers strewn on the surface of the meat or fish to be preserved, or in the form of brine in which the meat is submerged or which is injected into the carcasses. The preserving See also:power of See also:salt is but moderate. It has the See also:great See also:advantage that in See also:ordinary doses it is non-injurious, that an excess at once betrays itself in the taste, and that it can be readily removed by soaking in water. When aided by wood-smoke, which depends for its preservative power upon traces of See also:creosote and formaldehyde, it is, however, quite efficient. The addition of saltpetre is principally for the purpose of giving to the meat a See also:bright See also:pink tint. The strongly saline taste of pickled meat or salted butter appears gradually to have become repugnant to a large See also:part of mankind, and other preservatives have come into use, possessing greater bactericidal power and less taste. The serious objection attaching to them is discussed in the See also:article See also:ADULTERATION. At the present time the use of See also:borax or boracic acid is almost universal in See also:England. Meat which has been exposed to the vapours of formaldehyde, and has thus been superficially sterilized, is also coming into See also:commerce in increasing quantities. Formaldehyde in itself is distinctly poisonous, and has the See also:property of combining with albuminoids and rendering them completely insoluble in the See also:digestive secretions. Salicylic and benzoic acids are not in-frequently used to stop See also:fermentation of saccharine beverages or deterioration of so-called " potted meats," which are supposedto last fresh and sweet on the consumer's table for a considerable length of time. Sulphurous acid and sulphites are chiefly used in the preservation of thin See also:ales, wine and fruit, and See also:sodium fluoride has been found in butter. The whole of these substances possess decided and injurious physiological properties. Alcohol now rarely forms a preservative of food material, its employment being confined to small fruit. The use of sugar as a preservative depends upon the fact that, although in a dilute solution it is highly prone to fermentation and other decomposition, it possesses bactericidal properties when in the form of a concentrated See also:syrup. A sugar solution containing 30% of water or less does not undergo any biological change; in the presence of organic acids, like those contained in fruit, growth of organisms is inhibited when the percentage of water is somewhat greater. Upon this fact depends the use of sugar in the manufacture of jams, marmalades and jellies. Moulds may grow on the surface of such saccharine preparations, but the interior remains unaffected and unaltered. Preservation by Drying.—Food materials in which the percent-age of moisture is small (not exceeding about 8%) are but little liable to bacterial growths, at most to the attacks of See also:innocent Penicillium. Nature preserves the germs in seeds and nuts, which are laden with otherwise decomposable food material, by the simple expedient of water removal. The life of cereal grains and many seeds appears to be unlimited. By the removal of water the most perishable materials, like meat or eggs, can be rendered unchangeable, except so far as the inevitable oxidation of the fatty substances contained in them is concerned and which is See also:independent of life-action. The drying of meat, upon which a See also:generation ago inventors bestowed a great See also:deal of See also:attention, has become almost obsolete, excepting for comparatively small articles or animals, like ox See also:tongues or tails and fish. It has been superseded even among less civilized communities by the spread of canned food. Fruit, however, is very largely preserved in the dried state. Grapes are See also:sun-dried and thus form currants, raisins and sultanas, the last variety being often bleached by the addition of sulphites. Plums, apples and See also:pears are artificially dried in ovens on wooden battens or on See also:wire See also:sieves; from the latter they are See also:apt to become contaminated with notable quantities of See also:zinc. Excellent preparations of dried vegetables, including potatoes, carrots, onions, See also:French beans and cabbages, are also manufactured. The utilization of meat in the form of meat See also:extract belongs to some extent to this class of preserved foods. Its origin is due to J. von See also:Liebig and Max von See also:Pettenkofer, and See also:dates from the See also:middle of the 19th century. The soluble material is extracted mainly from See also:beef, in See also:Australia to some extent from mutton, by means of warm water; the albumen is coagulated by heat and removed, and the broths thus obtained are evaporated in vacua until the extract contains no more than about 2o% of water. One See also:pound of extract is obtained from about 25 lb of lean beef. Preservation by Refrigeration.—At or below the freezing-point of water fungoid organisms are incapable of growth and multiplication. Although it has been asserted that many of them perish when kept for some time in the frozen See also:condition, it is certain that the vast See also:majority of bacteria and their germs remain merely dormant. Even so highly organized structures as cereal seeds do not suffer in vitality on being kept for a considerable' length of time at the far lower temperature of liquid air. Biological change is, therefore, arrested at freezing-point, and as long as that temperature is maintained food material remains unaltered, except for See also:physical changes depending upon the evaporation of water and of volatile flavouring matters, or chemical alterations due to oxidation. Refrigeration, therefore, affords the means of keeping for a reasonably long time, and without the addition of any preservative substance, food in a raw condition. It is the only process of preservation which from a sanitary point of view is entirely unobjectionable as ordinarily and properly employed. Its introduction on a commercial scale has more powerfully affected the economic conditions of England and, to a less degree, of the See also:United States than any other scientific advance since the See also:establishment of See also:railways and steamboats. Enormous quantities of frozen carcasses, butter, fruit, vegetables and fish are introduced in the fresh condition into Great See also:Britain and stored until required. Extreme fluctuations of See also:supply or of price have become almost impossible, and the abundance of Australian and New See also:Zealand ranches, and of See also:West See also:Indian orchards, has been made readily accessible to the See also:British consumer. For See also:household purposes cooling in See also:ice-chests or ice-See also:chambers suffices to pre-serve food on a comparatively small scale. The ice used for the purpose comes, to a small extent, from natural sources, stored from the See also:winter or imported from See also:northern countries; a far larger quantity is artificially produced by the methods described in the article on See also:REFRIGERATING, which also contains an See also:account of the means by which See also:low temperatures are produced for See also:industrial purposes of importation and storage. Fleets of steamships fitted with refrigerating machinery and insulated cold-rooms are employed in carrying the food materials, which are deposited in cold-stores at docks, warehouses, markets and hotels. The first See also:cargo of frozen meat was shipped in See also:July 1873 from See also:Melbourne, but arrived in See also:October in an unsatisfactory state. In 1875–1876 sound frozen meat came from America. The first cargo of frozen meat was successfully brought to the United See also:Kingdom in 188o from Australia in the " Strathleven," fitted with a See also:Bell-Coleman air See also:machine. The temperature in the cold-storage rooms is generally kept near 340 F., whilst in the chilling chambers a somewhat lower, and in the freezing See also:room or chambers a much lower temperature (between o° and to° F.) is maintained. The carcasses to be frozen should be cooled slowly at first to ensure even freezing throughout and to prevent damage by the unequal expansion of the outer layer of ice. The carcasses when freezing must be hung separated from each other, but for storage or transportation they are packed tightly together. Fish such as See also:salmon is washed, thoroughly cleansed, and frozen on trays. Butter should be cooled as rapidly as possible to about to° F.; its See also:composition as regards proportion of volatile fatty-acids, &c., remains absolutely unaltered for years. Cheese should only be cold-stored when nearly ripe and should not be frozen. Eggs must be carefully selected, each one being inspected by See also:candle-light. They are placed in cases holding about three See also:hundred, which are taken first to a room in which they are slowly cooled to about 330 F., and are then kept in store just below freezing-point. Particular attention must be paid to the relative humidity of the air in See also:egg stores. Fruit should be quite fresh; grapes may be chilled to 26° F., while lemons cannot safely be kept at a lower temperature than 36°. The time during which soft fruit can be kept even in cold-store is limited, and does not exceed about six See also:weeks. In the early days of the chilled-meat trade considerable See also:prejudice existed against stored meat. While in many cases the flavour of fresh meat is rather See also:superior, the food value is in no way altered by cold-storage.' Preservation by Pickling other than Salt.—For the preservation of vegetables, See also:vinegar or other solution of acetic acid is used to a limited extent. Eggs are submerged in See also:lime-water or a dilute solution of sodium silicate (soluble glass). During the storage of eggs the more aqueous See also: Bacterial and other pollution at the time of milking arises from the animal, the stable, the milker and the vessels. In animals suffering from See also:tuberculosis and other bacterial affections the milk may be infected within the udder. Milk as it reaches the consumer rarely contains less than 50,000 bacteria and often many millions per cubic centimetre. In fresh country cream too millions per cubic centimetre are not unusual. These bacteria are of many kinds, some of them spore-bearing. The spores are more difficult to kill than the adult organism. The first step towards preservation is the removal of the dirt unavoidably present, to the particles of which a considerable proportion of the bacteria adhere. Filtration through cloths or, better, the passing of the milk through centrifugals effects that removal. Subsequent treatment is preferably preceded by a breaking-up of the larger See also:fat-globules by the See also:projection of a See also:jet of the milk under high pressures against a steel or See also:agate plate, a process known as homogenizing. From homogenized milk the cream separates slowly, and does not form the coherent layer thrown upbyordinary milk. See also:Heating is then effected either after bottling or by passing the milk continuously through pipes in which it is heated to from 16o° to 170' F. By a repetition of the heating process on two or more succeeding days, complete sterilization may be effected, although a single treatment is sufficient to render the milk stable for a few days. Many forms of pasteurizing apparatus for milk are in use. Since the See also:general introduction of pasteurization of the skim-milk used in See also:Denmark for the feeding of calves and pigs, tuberculosis in these animals has practically disappeared. On the See also:continent of See also:Europe the use of sterilized milk is now very genera'.. In England it has found little favour in households, but is making rapid progress on See also:board See also:ship. Milk which has been condensed has for many years found a most extensive See also:sale. The first efforts to condense and thus preserve milk date from 1835, when an See also:English patent was granted to See also:Newton. In 1849 C. N. Horsford prepared condensed milk with the addition of lactose. Commercially successful milk condensation began in 1856. The milk is heated to about 18o° F. and filled into large See also:copper vacuum pans, after having been mixed with from to to 12 parts of sugar per too parts of milk. Evaporation takes place in the pans at about 122° F., and is carried on till the milk is boiled down to such concentration that too parts of the condensed milk, including the sugar, contain the solids of 300 parts of milk. Sweetened condensed milk, although rarely quite sterile, keeps indefinitely, and is invariably brought into commerce in tin canisters. The preparation of sweetened condensed milk forms one of the most important branches of manufacture in See also:Switzerland and is steadily increasing in England. Although milk can quite well be preserved in the form of condensed unsweetened milk, which dietetically possesses immense advantages over the sweetened milk in which the See also:balance between carbohydrates and albuminoids is very unfavourable, such unsweetened milk has found little or no favour. Milk See also:powder is manufactured under various See also:patents, the most successful of which depends upon the addition of sodium bicarbonate and the subsequent rapid evaporation of the milk on steam-heated revolving iron cylinders. Milk powder made from skim-milk keeps well for considerable periods, but full-cream milk develops rancid or tallowy flavours by the oxidation of the finely divided butter-fat. It is largely employed in the preparation of so-called milk chocolates. (0. Additional information and CommentsThere are no comments yet for this article.
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