Preserved Foods - Part 1
( Originally Published 1939 )
Products packed. All the common fruits and vegetables are now canned, and new ones are added as their technology is worked out. Among the meat products canned are beef, mutton, lamb, and pork, including hash, broth, soup, ragout, stew, corned beef and cabbage, Hungarian goulash, potted veal, veal loaf, sausage, and also poultry and its products. Numerous kinds of fish are canned, also roe, crab, and lobster meat, shrimp, oysters, and clam chowder. In the list are table syrups, preserves, spaghetti, and other food specialties. Milk has been canned for a long time, and lately cheese has been added to the list. Bread, puddings, and other pastry are included, also coffee, tea, and candy. Esty states that 275 different products have been investigated for packing in California alone.
Grading. A cannery endeavors to produce a line of goods which is uniform in quality, embracing appearance, flavor, soundness, wholesomeness, and nutritive value. This necessitates careful selection and grading of incoming raw material. It is physically impossible to pack an absolutely uniform grade from year to year because of variation in composition, texture, and other such quality factors caused by different weather conditions. The soil in different parts of the country also causes differences in composition and other qualities of crops. Tomatoes, for example, may be more "watery" one year as compared with those of the previous year, meaning that the texture or structure of the food does not have the firmness that usually obtains. In the fresh fruit this may not be so noticeable to the consumer but it may be very obvious in the canned product.
Sorting. The incoming stock is carefully examined for unsoundness. To a limited degree, the washing operations may remove some of the more advanced decomposed parts, such as those affected by soft rot, but, in any event, the food is always subjected to a final sorting by hand. This is especially necessary when the fruit is automatically manufactured in pulped or juiced products like tomato catsup, apple butter, or the fruit juices. When the food is to be canned whole, as apples or tomatoes, the trimming is usually a hand operation, so that unsound parts are discarded at the same time.
Washing the raw material. When the product is received at the plant, it carries a full assortment of dirt and microorganisms. Some-times these are difficult to remove, particularly when the dirt has become caked, as it may on tomatoes, or when the food entraps the dirt, as in lettuce or spinach. The effectiveness of the final heat processing is largely determined by the adequacy of this preliminary cleaning. The greater the number of microorganisms in a product, the higher or longer must be the processing treatment. It is not known why or how the microbes have such a protective influence, but the fact is well established. Effective washing removes many. Decayed portions and cracks in the tissue harbor enormous numbers, and these must be cut away. In fact, one of the important control items in the effective elimination of botulinus outbreaks from commercial canned foods is just this matter of starting with clean, sound raw material. As in other phases of public-health practice, prevention is an important factor.
In some of the leafy or bulky products, much of the sand, small gravel, leaves, pieces of stem, and other such heavier foreign material are shaken out by tumbling or shaking the product over screens. Washing may be started in soaking tanks to loosen the caked dirt. The most effective washing is done by conveying the products under water sprays or jets which, under pressures of several pounds up to 50 pounds or more per square inch (according to the structure of the raw material), impinge with sufficient force to penetrate cracks and cut the dirt. Different types of machines are designed to rotate or turn the product so that all parts become exposed to the cleansing treatment.
Peeling and trimming. Peeling and trimming vegetables and fruit for canning is often a hand operation, and is paid for on the production basis—so much for a basket. For some of the firm fruits and vegetables, equipment has been designed to facilitate peeling. A strong solution of caustic soda (sodium hydroxide) will attack plant tissue, and this property has been utilized in lye-peeling. The fruit, especially peaches, is dipped into or sprayed with the lye solution of 1-3 percent strength, according to the specific method, for about half a minute. This is a long enough exposure to attack and loosen the skin, but not to cause any damage to the product further than a few surface layers. It is then quickly sprayed with water to remove the adherent lye. Many tests have shown that the lye is completely re-moved, and that no harm is done to the fruit.
Peeling machines have been developed for paring, coring, or other-wise removing skin from firm products like apples, potatoes, beets, and other fruits or vegetables, but these must sometimes be further trimmed by hand to remove the skin from depressions and other parts skipped by the machines.
Blanching. After raw vegetables are cleaned, trimmed, and sorted, they are usually given a blanch, or treatment with hot water or steam, to improve their appearance, or to facilitate packing. The immediate purpose is different with many products. In some, such as peas, it removes a mucilaginous coating which may give a cloudy liquor in the can, or which may hold some dirt and foreign material. It wilts bulky greens, asparagus, and string beans to facilitate packing the right quantity in the can. It prevents discoloring of some products before they can be packed. It heats the product to assist the formation of a partial vacuum when the canned product is cooled. It helps to remove air from the tissue, and thereby further aids the keeping quality and appearance of the finished product.
A slight loss of soluble nutrients, and possibly of some of the less stable vitamins, is entailed. 'Ways and means to reduce these losses are now under investigation.
Cleaning of cans. Although tin cans are made on automatic machinery which involves no contamination from human handling, they are sometimes exposed to dust, insects, and other foreign material, incident to plant operations, transportation through streets, storage in warehouses, and handling in the plants. The introduction of the sanitary can makes it possible (though not always practiced) for the packer to clean the can of any such dirt.
Tin cans. Ordinary tin plate is made by rolling a mild steel into thin sheets which are cut into the desired size and then coated with a thin layer of tin. Actually, a "tin" can is about 98 percent iron. The tin does not form a perfectly continuous coating. Bare spots, of microscopic size, are left on the iron base. A gelatin ferricyanide mixture placed on tin plate will show blue spots wherever the ferricyanide comes in contact with exposed iron. Increasing the amount of tin does not cover all these spots, although they are reduced in degree. Small scratches and other such plant abuse increase the exposure of iron. To reduce this defect, protective enamels have been made to coat the cans. These are lacquers of various compositions, particularly the synthetic resins, which are baked onto the tin surfaces of the cans at high temperatures. Their chief purpose is to preserve the natural color and flavor characteristics of some foods, particularly those which lose their color in the ordinary can. However, with certain fruits, the formation of hydrogen springers and perforation occur to a greater extent in enamel cans than in plain cans.
The type of can formerly used exclusively but now mostly in the canning of meat is known as the solder-top, or hole-and-cap can. These cans are closed by placing the cap on the rim of the open top and sealing it with soldering iron. The restricted opening on the top of the can makes it difficult to clean; also, occasionally the can may be contaminated during the soldering procedure. Milk is canned in vent-hole cans by filling through the small vent-hole, which is only a fraction of an inch in diameter.
The open-top or sanitary can is now generally used. The top is entirely open so that anything in the can can be completely removed by inverting it. The top is sealed onto the can by rolling the edges together under such strong pressure that the sealing compound (a rubberlike or paper gasket) in the rim of the top forms an hermetically tight closure in the crimped hook of the rolled edges. The machines which perform this closing operation are called double-seamers because they utilize two seaming operations in the rolling or crimping together of the edges of the tops to the bodies of the cans.
The replacement of the former hole-and-cap (solder) type of cans by the sanitary type removes the possibility of contamination of the food contents by the lead and other metals in the solder, as well as any of the soldering flux. Some of the earlier sanitary cans were soldered down the vertical seam on the inside, but this exposure of the food to lead has been eliminated by soldering entirely on the outside of the can.
Filling the cans. Most fruits and some vegetables are placed in the cans by hand. Automatic fillers add brine to vegetables, or syrup to many fruits, to fill the cans to the desired level. Water is added for the cheaper grades. The brine may contain 1 to 3 percent salt, and it imparts a desirable seasoning. Sugar syrups not only give desirable seasoning but also help to preserve the color and to protect the more delicate fruit from disintegration.
Exhausting. The filled cans are conveyed through an exhaust box to raise their temperature and to expel air and other gases from the product. This box may consist of a hot-water bath through which the cans are conveyed, partially immersed, or a conveyor belt to carry the cans through a blanket of steam. This heating expands the contents so that, if the can is sealed while hot, there will be a partial vacuum or reduced pressure when the contents cool and contract. Commercial experience has indicated that, for products packed in brine without a mechanical vacuum, the average temperature of the contents of each can at the time of closure should be at least 130° F. to insure a satisfactory degree of reduced pressure in the finished can. This heat treatment has several advantages. In the first place, it brings the cans to a predetermined temperature so that they will be delivered to the processing retort at a uniform temperature which facilitates a dependable heat treatment. The removal of air prevents excessive oxidation of the product during the cooking process and subsequent storage. The expansion of the contents during the heating helps to prevent overfilling. Further, the partial vacuum relieves to some extent the great internal pressure when the cans are being processed and retards some of the chemical action in the can. This reduced pressure causes the ends of the cans to be drawn in (concave), and shows the consumer that the can is not spoiled by the usual form of gaseous fermentation which bulges the ends. Machinery also has been developed to impart any predetermined degree of vacuum by sealing the can in a vacuum chamber.
Processing. This is a term applied in commercial practice to the heating of the can after it is sealed to cook the contents and to prevent their spoilage by microorganisms. It is not necessarily sterilization. As a matter of fact, probably most of the cans are really sterilized in the strict sense of the word, in that all the microorganisms are killed. However, it has been shown that there are spores in many food products that are so resistant to heat that their destruction would entail a degree of cooking that would make the food unmerchantable. A great deal of excellent research has shown that canned food can be made perfectly safe from the standpoint of the health of the consumer as well as the keeping quality of the food if it is packed under such conditions that occasional heat-resistant (thermophilic) spores cannot proliferate. The detailed processing times for different products have been worked out in great detail and published by the Research Laboratories of the National Canners Association.2
The temperature to which a food is heated (process) and the length of time at which this is held (the processing time) are determined by the nature of the food itself, the types of organisms which are associated with its spoilage, the degree of acidity of the food, the amount of original infection, the initial temperature of the can when the process begins, the degree of heat penetration, the size of the can, and the rate and degree of cooling. The time-temperature relation for the death points of spores is a continuous function. This means that there is no one temperature within the range of those used in processing canned food that can be regarded as a critical temperature. The use of rotating cookers agitates the contents to some extent, facilitates the penetration of the heat, and reduces the degree of process necessary to effect adequate keeping.
In general practice, the cans are placed in circular iron crates which are lifted by hydraulic hoists and lowered into processing kettles. In some types of process cookers, the crates are wheeled into the steam chest through a door on vertical hinges. High-pressure steam is admitted to raise the temperature of the cans and maintain it at the desired processing point. The handling of a retort requires skill to insure absence of air pockets and uniform distribution of heat through-out the whole batch of cans.
After the cans are removed from the process kettles, they are immediately cooled in water. This improves the appearance of the food by giving a clear liquor and precluding stack burning (overheating in the stored cans on the fireless-cooker principle), preventing the germination of any surviving thermophilic microorganisms which might occur when the cans had reached their optimum temperature, and relieving the internal pressure on the cans.
Full details of the manufacturing operations, containers, times and temperatures of processing, and much other practical information are published by A. W. Bitting in his admirable treatise, Appertizing or The Art of Canning, published by The Trade Pressroom, San Francisco, Calif., 1937.
RELATION TO THE PUBLIC HEALTH
Composition. The proximate analysis of canned fruits and vegetables may be like that of the fresh products or may be materially different when these products are packed in syrups or brines. It is to be expected that there will be variations in the composition of a given canned product because of the differences in growing conditions, differences in the composition of the same variety, and changes in the canning practices. Generally, the more fancy grades of fruit are packed in the heavier syrups, whereas the cheaper or "pie" grades are packed in water. Juicy products like tomatoes are packed with nothing added and with the skins and cores removed, although sometimes salt is added to the juice. Dry products like spinach, peas, and corn are measured or weighed into the cans and then brine is added up to the proper level. Specialty products like spaghetti, and pork and beans, are covered with tomato or other sauces. Fish may be packed in sauce, oil, or brine. Meat and its products are usually packed dry. Representative proximate analyses, compiled from recently published and reliable data on the composition of canned foods by the American Can Company.
A compilation of the vitamin content of canned foods has also recently been made with complete literature references.' Some of these values for vitamin B are not differentiated from vitamin B2 (G), and those for vitamin C include many made by the rapid titration technic (which certainly needs further study). The more common products are listed in Table XLIII.
Nutritive value. Packing operations bring about certain changes in the composition of the food very similar to those which result from regular cooking. However, ordinary cooking is usually done in more or less open vessels, subject to free access of oxygen. Often the juices or liquors are discarded. On the other hand, canned foods are cooked in closed containers under more or less vacuum (or, at any rate, in the absence of most of the oxygen), and the juices are retained. There-fore, the only practical losses of nutrients in canned foods are those entailed in the momentary blanching and in the processing.
In general, the vitamins, flavor, taste, appearance, texture, and food value of fruits and vegetables are at their maximum when they are allowed to ripen naturally and not held long. Canning is usually done in the agricultural sections which produce the respective products. The fruits and vegetables are picked at the optimum time, hauled relatively short distances, and promptly canned. The vitamins are quite stable in canned foods when they are stored; for example, the vitamin A and C content of spinach is maintained after 3 years' storage.'
The canning industry has exerted a powerful effect on our dietary.''' It guides growers in the production of improved grades of food plants. It utilizes crops promptly after harvesting, thereby obviating the extensive handling and deteriorative changes which accompany early picking, delayed transportation, and storage before marketing. It practices the latest technology in handling and processing food, and preserves it under conditions which minimize its deterioration. It prepares food for transportation to great distances, and makes it available in districts which are locally unable to raise products of adequate nutritive value. It gives greater variety and improved palatability to diets, and makes fruits and vegetables staple articles of daily diet when they cannot be purchased fresh. Products for canning are not subject to some of the nutritive losses which often occur when the same kind of products are picked before ripening and then held for marketing as fresh food several days later.
The specifically nutrient values of fruits, vegetables, sea foods, meats, and milk are discussed in their respective chapters.
Minerals. In the practice of canning, the valuable mineral constituents of the foodstuffs are made available in unimpaired effectiveness. This is particularly important with such essential elements as calcium, iron, and iodine. For example, the population of great areas suffer from certain glandular deficiencies because of lack of iodine in the diet. Crops grown in some coastal regions are rich in iodine, and therefore are particularly valuable to supply this nutritive need.
In household cooking, cabbage has been found to lose an average of about half its calcium in the cook-water, and the losses from other leafy vegetables also are high. Although calcium loss in most of the common vegetables is not as much as this, it averages about 25 percent (actually 1 to 46 percent) in several of them. In canning, a mini-mum of cooking water is used and this remains in the can. The use of the entire contents assures the maximum conservation of these valuable nutrients. Moreover, Kohman, Eddy, and associates 8 show that rats on experimental diets of a wide variety of only canned foods assimilated calcium at least as well as those on raw or home-cooked foods. They attribute this to the more thorough cooking of canned food, whereby the vegetable tissue is broken down, with attendant increase in calcium availability.
Vitamins. Although the vitamins have been classified as a group of essential food factors which are characterized by the spectacular effectiveness of minute amounts to maintain the body in proper nutrition, they are quite different from one another in their physiological and chemical properties. Some are stable under the handling and processing treatments given the foodstuffs which contain them, whereas others are subject to the destructive effect of various factors.
Appreciable amounts of some of the water-soluble vitamins are lost in the cook-water. For example, Tressler reports that peas have lost about half of their vitamin C,9 and Halliday and Noble that a dozen common vegetables have been found to lose from 12 percent, in asparagus, to 80 percent, in white onions." Here, again, the use of limited amounts of water in the canning operation conserves these soluble constituents when all the liquid in the can is served with the food.
Vitamin A. Both vitamin A and its precursor carotene are relatively stable to heat but are sensitive to oxidation. Canned foods of both animal and vegetable origin have been found to retain their vitamin A to a high degree. Fellers's review of the literature shows that this vitamin is not diminished in tomatoes, carrots, runner beans, cabbage, turnip greens, collards, peas, grapefruit, prunes, spinach, asparagus, pineapple, and others. Likewise, vitamin A in canned sea foods is not destroyed, as shown by Moore and Moseley 12 for shrimp. Any means of avoiding oxidation and enzyme activity will aid in conserving the vitamin in food-preservation processes. Canning does both.
Vitamin B. Some of the constituents of this vitamin complex have been known to be more or less sensitive to heat, and therefore the relatively high process temperatures of non-acid (vegetable) foods would be expected to exert a much greater destructive effect than the lower process temperatures and the stabilizing effects of the acids of the fruit products. Moreover, the situation is complicated by the discovery of the multiple nature of this vitamin, thereby invalidating many of the earlier bioassays. Kohman states that vitamin B', tested as a single factor, is partially destroyed in leafy vegetables such as spinach and cabbage, but no appreciable amounts are lost in canned pineapples, peas, peaches, and tomatoes, although Jones and Nelson report 13 an appreciable loss on storage. Many reports of bioassays of this vitamin in various foods are not in good accord either as to content or as to the effects of the heating process. Canned fruits and vegetables contain appreciable amounts of this vitamin as shown by Fellers's review of the literature 11 and the compilation of the American Can Company 3 in their Table XVII.
Vitamin C. Vitamin C is the most unstable of all known vitamins when the foodstuffs are heated with exposure to oxygen or copper and iron (as they usually are to a greater or less extent in most food-packing plants). Many investigations have been made of the vitamin C content of canned fruits and vegetables.' Overwhelmingly, they show that the vitamin is usually destroyed to a greater or less degree but that often appreciable amounts remain. Fellers reports that the more common canned fruits (such as the citrus fruits, tomatoes, and certain apple varieties) are excellent anti-scorbutics whereas the non-acid vegetables have lost a large part of their originally high content. He states that peas, asparagus, Lima beans, and spinach lose from 60 to 90 percent of their vitamin C content, and that canned sieved (puréed) vegetable baby foods have the same or lower vitamin C potency than unstrained canned vegetables, but that sufficient amounts are retained to be of distinct anti-scorbutic value.
On storage, canned foods lose little if any vitamin C 4; spinach after 3 years and tomato juice in 5 months at room temperature lost none.
Most of the above work on vitamin C has been done with the indophenol titration method; certain apparent inconsistencies indicate that further work is required before the method can be used with certainty on cooked vegetables to
Vitamin D. Vitamin D has been claimed by Kohman and associates 15 to be present in substantial amounts in a mixture of canned sweet potatoes, spinach, peas, carrots, and roast beef by reason of the approximately equal ash content of the bones (tibia) of experimental animals on an otherwise rachitogenic diet with or without the addition of cod-liver oil. Such a finding arouses much surprise because it is generally believed that fruits and vegetables are quite devoid of vitamin D.
McCay questions the validity of the claims and the conclusiveness of the experimental data. Although some of his points are well taken, it does seem that the research supports the main thesis, namely, that a selected diet of canned foods protects experimental animals against the development of rickets when fed rachitogenic diets. Further work in this important field is awaited with interest.
This vitamin is known to be present in notable amounts in canned sea food. For example, Moore and Moseley 12 found an appreciable quantity in canned shrimp. There are small amounts in oysters and clams. Tolle and Nelson report " that canned salmon contains from 6 to 15 percent oil, which compares favorably with cod-liver oil as a source of vitamin D, and that there was (in 1931) more vitamin D in the canned salmon sold in this country than in the cod-liver oil used for both human and animal feeding.
Vitamin G (B2). Pellagra, recognized as a food deficiency disease, has been successfully treated by feeding patients with such canned foods as corned beef, salmon, haddock, green peas, collards, kale, turnip greens, and tomato juice, and furthermore, some help was furnished by spinach and soybeans. It is water soluble, and is largely found in the liquor. This vitamin is very widely distributed in limited amounts in fruits and vegetables, is present in canned chicken, and is fairly stable under the conditions of canning.
Strained foods. One of the newer developments in dietetics is the increasing use of canned strained fruits and vegetables in infant feeding 19 as a supplement to regular feeding; children so fed thrive and grow better than when kept on strictly milk diets. They exhibit no digestive disturbances from this food, and utilize it in a manner evidencing benefit, particularly on account of its mineral and vitamin content. The Council on Foods of the American Medical Association reports 2° that strained fruits and vegetables are valuable foods for infant feeding and for certain types of therapeutic diets, and also for teaching an acceptance of a variety of flavors and textures. The Council has published the composition of several such canned sieved foods, as tabulated in Table XLIV.
Ordinarily, the additional treatment given to these foods to purée them would decrease their vitamin content by reason of greater exposure to oxygen, metals, and heat. However, particular care is exercised in their manufacture to maintain these fugitive nutritive values and to impart dependable quality. Properly selected and matured raw materials are rapidly harvested and delivered, cleansed and trimmed, and then, after preliminary heat treatments in steam or a limited amount of water, are strained through fine-meshed screens in the liquid in which they are cooked. High-pressure steam is used to displace the air in the pressure cooker. Throughout the various processes the food is always in a closed system or protected from air by a blanket of steam upon the surface.
Epidemiology. Canned foods are the safest foods that come to our tables on account of the processing to which they have been subjected. .This is the conclusion reported by Rosenau 22 as the result of the study of 1750 cans of a wide variety of canned foods which were eaten without further cooking by a lunch club averaging 15 to 20 persons, over a period of 16 months during all seasons and including two summers. Although 12 percent of the cans were not sterile, no ill effects whatever could be discovered as the result of the experiment. More recently, canned fruits and vegetables were fed to 100 children, aged 6 months to 10 years, for 5 months, as a complementary food to the normal diet, with no unfavorable influence on convalescence and resistance to infection. Children aged 4.5 to 5.5 months absorbed nitrogen, calcium, magnesium, phosphoric acid, and ash' of canned vegetables to the same extent as from fresh vegetables.
The uninformed public for a long time was inclined to ascribe numerous and varied kinds of intestinal disturbances to the consumption of canned foods. As Fellers pointed out, canned foods have been convenient pegs on which to hang obscure or undiagnosed causes of illness. Often such illnesses were ascribed to "ptomaine" poisoning, and many alleged cases were reported in the news press. This Ied one of the commercial trade organizations to take the constructive step of collecting all such reports and seeking their verification. These selective records are available to students of food poisoning and, when added to all the miscellaneous and scattered reports of all food poisoning in the literature, make the number of cases, attributed by investigators to canned foods, disproportionately large. In England, Savage 25 reports 62 outbreaks of illness from canned meats, marine products, and fruits, out of a total of 203 reported outbreaks, constituting 30.5 percent, but inasmuch as no details are given concerning the correctness of the diagnosis, there is no warrant for assuming that these figures represent the percentage of actual cases of alleged illness from canned foods.
Intoxication by the Salmonella group. Growth of the Salmonella type of organisms on most foodstuffs has not been found to change their appearance appreciably. Toxins are irregularly produced and are occasionally thermostabile. The Salmonella group does not form spores, so that any cells in the food (never in fruits and vegetables, and rarely in meats) are likely to be killed even by a relatively light process. Illness from the action of these organisms would probably be caused by any of the heat-stabile toxins which may be present, al-though our information on the subject of the thermostability of the toxins of this group is contradictory, and uncertain at best.
Botulism. Meyer 26 lists 193 outbreaks of poisoning by Clostridium botulinum in the United States and Canada from 1899 to 1931. Of these, 131 were from home-canned food. The majority of the out-breaks were due to underheated or underprocessed foods, most of which were home canned.
In the greater number of outbreaks of botulism, the food was visibly spoiled. In Geiger's study of 38 outbreaks, the food and containers in 10 were stated to be normal in odor, taste, and appearance, and, in 14, abnormal. Investigators of the U. S. Bureau of Chemistry 27 reported that the food products of 618 containers in which toxin was present were distinctly offensive and that in practically every case the poisonous food could be organoleptically identified. Washing, icing, and flavoring could mask the telltale odor.
Cans of food containing botulism toxin usually are swelled, and the odors vary from only slightly abnormal to a decidedly obnoxious putrid odor. Sometimes a cheesy or butyric odor is encountered. (See the discussion of botulism on page 40).
The science and technology of botulism control have now been so effectively applied that there have been no cases of botulism from commercially packed canned food in this country since 1925, although the number of outbreaks from home-canned products maintains its general average of about a dozen or so every year, and there have been 2 recent outbreaks from imported products.
Tanner shows that there is a large amount of misinformation concerning home-canning, promulgated by agencies which should know better, both official and commercial. Non-acid vegetables and meats should be processed only under steam pressure. Reliable information is published in the U. S. Department of Agriculture Farmers' Bulletin 1762, issued September, 1936, but the extension services of only 10 states have adopted similar recommendations.
Microbiology. Although the general purpose of canning usually is to seal food in cans or glass jars hermetically, and to sterilize them by heat, it is known that under commercial conditions true sterilization is not always accomplished, even though the container may be sound and the food unspoiled. Absolute sterility is not possible in many instances without making the product unmerchantable by excessive processing. Survival of a relatively few spores is immaterial, provided conditions in the can preclude the possibility of their vegetation. Pathogenic organisms have such a slight' resistance to heat that they are destroyed in all sterilizing procedures used with canned foods.
Spoilage of non-acid canned vegetables is caused by four bacterial groups, all of which form spores highly resistant to heat. The spoil-age bacteria most frequently met are called "flat sour" bacteria be-cause they produce lactic acid without the formation of gas, and hence do not bulge the can. They are thermophiles (or heat-loving bacteria) with optimum growth at 50° to 60° C. (122° to 140° F.). The next in importance are the gas-forming thermophilic anaerobes, with optimum growth at the same temperatures as the foregoing, and with production of hydrogen and carbon dioxide which swell the cans. They grow in acid foods of pH 4.2 to 4.3. Another one is a thermophilic anaerobe which produces hydrogen sulphide at 45° to 55° C. (113° to 131° F.). The gas is highly soluble and also combines with the iron in the can so that there is no bulging or swelling of the can unless heated. The fourth group is not a thermophile but is a putrefactive anaerobe producing a gas which swells the can and emits a putrid odor.
The spoiling of acid canned foods in the range of pH 4.5 to 2.0 generally may be classed according to the following types:
Bacterial: acid tolerant, sometimes produce hard swells, frequently in tomatoes but also in numerous other acid products like pickles and condiments. Yeast: fruits and fruit products, tomatoes, and beverages.
Flat sours: seldom in acid products. Caused by understerilization or slow cooling.
Special media and other microbiological technic have been developed for isolating and studying these various types of spoilage as they occur in canned foods.
As a rule, thermophilic spoilage develops from conditions largely in the cannery itself. Spoiled raw materials, contaminated cannery equipment, unclean cans, and sugar with thermophilic bacteria all con-tribute to spoilage. Although the thermal death curves of microorganisms, the rates of heat penetration for different products, the times of processing under many conditions, and other related factors have all been worked out with mathematical precision, the most recent progress in spoilage control is based on prevention.
Spoilage. Normal cans present no signs of leaks around the rims or seams. The ends are slightly concave by reason of the partial vacuum which properly packed cans should have. Bulging of the ends may indicate that the pressure within the can is greater than it was when the can was sealed. Such a can is called a "swell." Bulging may be caused by generation of gas from microbic action, chemical action of contents on tin plate, insufficient exhausting and removal of oxygen, or overfilling. It may be caused only by the buckling of the can from faulty processing or freezing. Sometimes a can may be stored in a warm place or be shipped to high altitudes where the temperature and atmospheric pressure respectively may be sufficient to cause the ends to bulge, whereas under normal conditions they would remain drawn in. When a can is sharply tapped against a hard surface so that one end bulges and does not return unless pressed in, it is called a "flipper." If only one end bulges or springs, and when it is pressed in, the other end bulges, it is called a "springer." When both ends are bulged, the can is a "swell." Inasmuch as all swells must pass through the flipper and springer stages when the gas is just beginning to develop, then any can whose ends bulge may be the beginning of a swell.
The great majority of swells in canned foods are caused by fruit which attacks the can with the evolution of hydrogen gas. The number of swells of this kind enormously outnumbers those due to bacterial decomposition. Practically all the springers and swells in canned fruit occur a year or more after the product is packed, and are due to this hydrogen gas. It is only in the non-acid products that swells are likely to be due to bacterial decomposition, and the end-products are generally not toxic. Inasmuch as the consumer may not learn to distinguish between the spoilage of fruits and vegetables, it is advisable that no swell nor springer be used for food.
Some types of spoilage cannot be detected from the appearance of the unopened can. One type is called "flat sour," because microbic growth develops a certain amount of acidity which makes the product taste sour but does not form any gas. Such products may or may not have an abnormal appearance. No food that is abnormal in appearance or odor should be eaten or even tasted.
Effect of tin. Numerous tests have shown that, when most foods are canned, they dissolve some of the tin. In England, the maximum limit for tin in foods is 2 grains per pound (285 p.p.m.) .33 The amount varies with the acidity of the food itself, the temperature at which it is held, the length of storage, and other conditions. That other factors are involved in this property is evidenced by the large amounts of tin that are taken up by string beans and pumpkins, although neither of these vegetables is very acid. The extent of the content of tin is indicated by a few examples from the analyses of Bigelow.
Tin combines with the protein or some other constituent of the canned food to form an irreversible, insoluble compound. This renders the tin harmless. Flinn and Inouye 36 fed rats as much as 2 milligrams of tin a day, and found that 98.5 to 99 percent was excreted in the feces. They state that all metal salts ingested in the absence of food have deleterious effects, but in the amounts and form in which tin occurs in canned foods, it is quite harmless. Rosenau quotes Cushny that chronic poisoning from tin is unknown, and that animals exhibit no symptoms when subjected to prolonged treatment with larger quantities of tin than are contained in any preserved foods.
Probably the most extensive and clear-cut answer to the question as to the physiological effect of tin in food is provided in the work of Blanck, quoted by Tanner. He fed a group of guinea pigs for about 5 months with canned pumpkin containing the excessive amount of 777 parts of tin per million parts of food, analyzed their organs, and found no tin accumulation. Cats ate canned sardines containing 212 parts of tin per million for about 7 months with no storage in their organs. Then 4 persons ate canned pumpkin and asparagus containing tin in such large quantities that, during the feeding period of 6 days, they ingested 2278 to 2942 milligrams of tin. None experienced any illness or discomfort. Blanck concluded from this experimental work, involving the ingestion of far larger amounts of tin than any previously reported, and supported by the experimental evidence of other investigators, that the amount of tin as found ordinarily in canned foods is practically eliminated and does not produce any harmful effects.
Prolonged preservation. Properly canned food remains sound and nutritious for a surprisingly long time. [ (See editoral in Am. J. Pub. Health 29, 793 (1939).] In 1824 a ship, carrying many tins of canned food, was wrecked in Prince's Inlet. Forty-four years afterward, some of these tins were recovered and opened. Although they had been alternately frozen and thawed, the contents were found to be in perfect condition. In another incident, canned foods that had been buried in mud from a flood for sixteen years were recovered and found to be in good condition. Most striking of all are the records of a four pound can of roast veal from Parry's stores in his Arctic voyages of 1824 and 1826, and a two pound can of "carrots and gravy" which had been on an Arctic voyage in 1924. The can of veal contained hydrogen gas under great pressure, caused by the action of the acid soup on the metal. The meat was in excellent condition, the odor appetizing, the fat free of rancidity although completely hydrolyzed, and the taste good though insipid. Both flavin and vitamin D were found but vitamin B was absent. Bacteriological examination revealed aerobic spore-formers which grew at 131° F. (55° C.). The carrots and gravy were sterile but the can contained mostly hydrogen gas under high pressure. Carotene was found in concentration almost equal to that of fresh carrots.
These data confirm the experience of years that a tin of canned food is edible and nutritious as long as the can remains sound.
Food in the open can. Sometimes when canned acid foods are opened and part of the contents is left in the can, there is noticeable a metallic taste. This is not due to the small amounts of tin but rather to the iron that has been dissolved. The tin coating on most cans is very thin and does not cover the iron base completely. The food dissolves some iron, which can impart an appreciable metallic flavor.
It is widely believed that the contents of a partially emptied can should not be left in the can but be transferred to a glass, enamel, or earthenware container to preclude spoilage. However, spoilage is caused by microorganisms, and proceeds just as quickly in one kind of an open container as in another. There is nothing in the can, or in the tin on the can, that causes food to spoil any more quickly than if it were in contact with a glass or earthenware vessel.
Canning compounds. Several years ago, large amounts of canning compounds were sold as alleged helps for household canning. They were not used for commercial canning. These products were mixtures of various chemicals which possessed more or less preservative properties. Sometimes they were mixed with a high percentage of relatively inert material. They were added to the product that was being canned to insure its freedom from spoilage. These preservatives were more or less deleterious to the health of the consumer, but were particularly dangerous because their use decreased the care of the home canner to insure adequate heat sterilization. Such health hazards led to the prohibition of their sale.
Standards. The McNary-Mapes amendment to the U. S. Food and Drugs Act, constituting the fifth paragraph of Section 8, on "misbranding," was enacted July 8, 1930, and authorizes the Secretary of Agriculture to establish minimum quality grades of canned foods (except meat and meat food products, and canned milk) . These are only minimum standards below which the products are designated by law as being substandard. They cover the quality, condition, and fill of the container for each class of canned food, as will promote honesty and fair dealing. Substandard products may be sold when plainly labeled as such. The label must carry a rectangular box with solid border in which on the first line will appear the statement "Below U. S. Standard" and on the second line the statement "Good food—not high grade." Canned foods which fail to meet the standard of fill by having too much head space (exceeding 10 percent of the inside height of the container, or over 1/4 inch) or too much liquid must carry the statement "Slack fill" and "Contains excess added liquid," respectively. The Secretary issues rules and regulations for the enforcement of this provision and from time to time the standards for the respective canned products. These are revisions of or supplements to "Service and Regulatory Announcements, Food and Drug, No. 4." The fourth revision was issued in September, 1937.
Grades. In addition to the minimum standards, the Secretary of Agriculture is authorized by Congress in the annual appropriation acts to establish and promulgate standards for grades of canned fruits and vegetables. These are issued by the Bureau of Agricultural Economics, first as tentative grades, and then later as official ones. They define the several commercial grades, as, for example, Grade A (formerly known as fancy), Grade B (formerly known as extra standard or choice), and Grade C (known as standard), together with the methods for determining these grades.
Only the finest products may use the term Grade A. These fruits and vegetables are very carefully selected as to size, color, and maturity. In Grade B will be found vegetables that are more succulent than those in Grade C, and the fruit is better selected as to color, size, and maturity. This is the grade largely used for general household purposes. The products in Grade C are good wholesome food which may be just as nutritious as the above grades, but the raw materials may not be so carefully selected as to size, color, and maturity.
Different factors are involved in determining the grades for the several commodities. For example, the grade of peas is determined chiefly by considering five factors: clearness of liquor surrounding the peas, uniformity of size and color of peas, freedom from defects, tenderness and maturity, and flavor. The grade of fruits is usually based on color, uniformity of size, freedom from defects, nature of fruit, and flavor. The highest grades of vegetables are those that are young, tender, most succulent, and immature, whereas the highest grades of fruits are those that are fully mature but not overripe.
A canned food labeled with the grade designation must comply in fact. The dealer under whose name the goods are sold is responsible for compliance. If the product is below the grade claimed, it is liable to seizure for misbranding under the Federal Food, Drug, and Cosmetic Act. These grade designations on labels are not compulsory, but if they are used they must comply with the standards.
The requirements for the grades of many products have been published. Full descriptions may be obtained by addressing the Bureau of Agricultural Economics, U. S. Department of Agriculture, Washington, D. C.
It is physically impossible for a canner always to produce the same percentage of these grades in his pack from year to year. The composition of vegetables and fruits varies with the soil, climate, and sea-son, and these factors influence the grades. Sometimes grades are established on a regional basis. Such quality grades are based entirely on organoleptic considerations and do not involve food values or degrees of wholesomeness in the strict sense of their interpretation in measurable nutritive effects.
Types of illegal canned food. Canned foods have been condemned on account of bacterial spoilage from underprocessed and leaking cans. Cases have been based on the use of undeclared colors, the addition of water, and other ingredients. Numerous condemnations have resulted from the use of more or less decomposed stock, sometimes containing insect and other microbiological debris. Many seizures arise from mislabeling, the sale of substandard grades, or the marketing of slack-filled containers. Occasionally, spoiled canned food has been re-processed by venting the cans to relieve the pressure, soldering the vents, and sterilizing the contents.
Physical examination. A normal can is free from excessive rust, has good seams, does not exhibit signs of rough handling, and has concave or drawn-in ends. Excessive rust may hide perforations which may have allowed spoilage organisms to enter, or gas from fermentative spoilage to escape. The seams should be examined for freedom from leaks, and for good mechanical condition. The presence of many dents indicates rough handling which may have weakened the seams enough to allow the entrance of spoilage organisms.
Bulged ends indicate that pressure exists which was not there when the can was packed and sealed. This internal pressure is usually caused by gas. (See discussion on page 440.) Immersion of a flipper or springer in cold water or placing it in a cooler may restore such cans to normal appearance. This condition does not increase, and its nature can be indicated by holding the can for further observation. On the other hand, gas that develops from the action of micro-organisms continues to develop, and may eventually burst a can. Inasmuch as a swell must pass through the springer stage, it is inadvisable to use any food from a can that was in this condition until further tests have shown that the gas is not caused by bacterial spoilage.
The presence of two vents in a can does not necessarily indicate that it is a resealed can which has been vented to relieve gas from a can that had spoiled from fermentative bacterial action. When such reprocessing is done, the overcooked or spoiled nature of the contents indicates its history, to be confirmed by laboratory examination.
The loss of vacuum in canned or potted meat can often be indicated by palpation or feel of the can, described by Savage. When the vacuum is lost, the can has a springy feel, and gives off a tympanitic or ringing sound when sharply struck. The gas pressure in a can can be measured by puncturing the top with a special type of vacuum gauge. The condition of the seams is ascertained by filing through them to remove short segments, and examining the surfaces with a low-power magnifying glass.
The extent of the fill of a can is measured from the top of the double seam to the level of the contents.
The odor is noted, together with the degree of clearness or turbidity of the liquor or brine.
The contents are examined for worm infestation, mold, dirt, or other evidence of decomposition or filth.
Chemical examination. Preparation of the sample. The con-tents of the can are poured onto a No. 8 standard screen, and drained for 2 minutes. The percentage of liquid and dry parts is calculated. Both constituents are then pulped through a food chopper, well mixed, and kept in a glass container.
The moisture, total solids, and acidity are determined as described for fruits and vegetables (see page 421).
There is seldom need to examine a canned food for metals. Tin is not harmful in the amounts that are at all likely to be present.
Sometimes knowledge of the gases that are present in a bulged can is a help in ascertaining the type of spoilage. For example, an excess of hydrogen and a high metallic content indicate corrosion of the metal, whereas a high carbon dioxide content may indicate microbial action. The gases that are usually determined (if at all) are oxygen, nitrogen, carbon dioxide, and hydrogen.
Bacteriological examination. Treatment of container. After the can has been- examined for pinholes and other defects, the top is cleansed and sterilized. The liquids are removed with a pipette, and solids with a glass tube or sampling device with a discharging plunger.
Culturing. Linden and Cameron 43 use a pH value of 4.5 as the dividing line between the non-acid and the acid products, because spores do not ordinarily germinate below this figure. They recommend special media for each of the several common types of microbic contamination: dextrose tryptone agar to detect and isolate organisms which cause flat sours, liver broth to detect certain anaerobes, beef heart peptic digest broth to detect putrefactive anaerobes, and sulphite agar to detect thermophilic anaerobes which produce hydrogen sulphide gas.
Canned products in the acid range of pH 4.5 to 2.0 spoil by acid-tolerant bacteria and yeasts. Flat sours are seldom found in acid products. Special media are used for these organisms.
Esty and Stevenson, Esty,' Tanner, and Savage 39 give detailed procedures for examining canned foods. In brief, the cans are classified as flat sours, flippers, springers, soft swells, hard swells, and buckled. All available pertinent information about the pack is assembled. The seams and covers are examined for soundness, correct mechanical construction, and adjustment of seaming machines. Samples are incubated at 37° C. and 55° C. for specified times, and cooled. The contents are then bacteriologically examined, and re-moved from the cans. The cans are cleaned and examined for leaks and soundness of manufacture. Cultures are made after enrichment of the can contents. The cultures of non-acid products like vegetables, meat, fish, and milk are made on standard plain and dextrose nutrient broth, and are incubated aerobically and anaerobically at 37° C. and 55° C. for at least 1 week. Stained preparations from the cultures and also from the original cans are examined microscopically. Special media are used for the systematic examination or classification of the types found. The cultures of such acid products as tomatoes and fruits are made on the standard media and incubated aerobically at 37° C. The spoilage should then be reproduced by inoculating into sterile samples of the particular food from which it was isolated. These findings must be interpreted in the light of whatever collateral in-formation is available concerning the history of the samples and the condition of the original pack.
Microbiological examination. The detection of the incorporation of decayed or other spoiled fruit in pulp or puréed products has been facilitated by the development of a method for the direct microscopic identification of mold, yeasts, spores, and bacteria . Their presence in excessive amounts indicates that the product contains more or less spoiled and decomposed materials. Main reliance is placed on the determination of the number of mold hyphae (or filaments) in g sample mounted in a Howard cell. The data are recorded as the percentage of the fields which contain mold. A good product should not have mold in more than 25 percent of the fields.
Yeasts, spores, and bacteria are counted in a diluted sample, mounted on the disk of a Thoma blood counting cell.
Insects from canned spinach and turnip greens 46 are determined by floating the insect parts in a gasoline layer on a large volume of water in an Erlenmeyer flask, and collecting them on a filter disk in a Büchner funnel. The particles are then re-examined and counted.
Insects in canned blueberries, raspberries, and loganberries are determined by methods based on the above technic. (See page 420).
Biological test. In testing food suspected of having been incriminated in a botulism outbreak, the presence of toxin is determined by inoculating a sample into mice, guinea pigs, or rabbits. Type is determined by neutralization with specific antitoxins. Examination of stools is not very helpful.
In the examination procedure following food poisoning by the Salmonella group, every effort should be made to demonstrate the presence of filterable toxic substances in the incriminated food. The food itself and the excreta of the patients should be examined bacteriologically, and the blood sera for agglutinins.
Control procedure. Sterilization of food in closed containers is no substitute for initial soundness of the raw material and sanitation of all the food-handling operations. A sterilization time and temperature that is adequate for the regular run of material may be inadequate if the microbic load was greatly increased (such as might hap-pen if substantial amounts of decomposed stock were used or if sanitation of plant operations were neglected). In addition, sterilization is no substitute for cleanliness. The natural sense of decency that is deep-seated in most persons demands that a product be free from decomposed parts and that it be handled with due appreciation of the principles of good housekeeping.
The most important item in the control of canning operations is the examination of the canned foods themselves. An outline of this procedure is given on page 445. In public-health regulatory practice, it is not necessary to ascertain the cause of spoilage. If the canned food itself appears abnormal in any way, the officer is warranted in prohibiting its sale until its harmlessness has been proved. Inasmuch as a can undergoing fermentative spoilage must pass through the successive stages of flipper, springer, swell, and hard swell, and since the public has no means of ascertaining whether such abnormality is due to harmless hydrogen swells, harmless insufficiency of head space, uncertain effect of bacterial spoilage, or dangerous development of toxic products, it is advisable to detain it and hold it for careful examination. This should reveal whether the batch is safe and should be released, or whether it is actually or even possibly a health hazard and should be condemned.
Canning equipment must be kept sanitary, if for no other reason than to prevent the development of spoilage organisms. Even such an acid product as tomato juice has been found to be spoiled from the action of spore-forming bacteria. The juice extractor and pumps need particularly close attention. In pea-canning, the blanchers are potent sources of spoilage. Mere inspection of canning-house equipment is not sufficient to determine whether it is satisfactory from the contamination standpoint. The only safe method is to investigate actual operations.
In addition to the usual requirements for sanitary practices and a clean plant and equipment, the National Canners Association include in their sanitary code the following special provisions:
Containers shall not be filled by dipping.
Pails, pans, and other vessels used for food-packing shall not be used in clean-up operations.
Cans and covers must be kept protected.
Food products except cabbage must be washed before canning. Refuse must be removed daily.
Washers, scalders, blanchers, etc., must be emptied and cleaned at least once a day.
The State of Maryland has regulations for the operation of canneries, comprising the following principles:
Roof must be water-tight.
Floors shall be tight, smooth, and graded to drains.
Table tops shall be smooth, and drained.
Waste and overflow shall be drained away from all places where created. Litter and waste shall not be allowed to accumulate.
All machinery shall be scalded with steam or hot water at least daily. Every scalder or blancher shall be cleaned at least twice daily.
Only pure, wholesome water shall be used in washing equipment or food, or in making brine or syrup.
Overflow of brine, juice, or syrup may not be reused.
No cans may be filled by submergence.
Wash-rooms or stations must be provided with ample running water, sanitary towels, and plenty of soap.
Dressing rooms shall be provided for employees.
Employees must wear clean working clothes, and female employees must wear clean, washable caps to cover hair.
Employees must keep their finger nails clean, and wash hands before commencing work and after each absence.
Employees shall be free from communicable disease, and shall not spit on floor in any section where food is being prepared.
One toilet shall be provided for every 25 persons.
Toilets shall be provided with automatically closing doors, windows, effectively screened.
All living quarters shall be adequate and sanitary.
The State of California has created a special cannery inspection division authorized to promulgate rules for the detailed operations of canning, including the kinds of products that may be canned, the general packing equipment, the processing times, the licensing of the operators themselves, the keeping of complete canning records, the coding of each pack for identification, and the holding of all products by the packer until released by the inspector?' 26 Their control of botulism in commercial canning rests on the following provisions: raw . products must be sound and thoroughly cleaned, the packing equipment must be adequate and properly designed, the processing operations must be correct in temperature and time for each product treated, and the finished pack must be held until the inspector has examined the records and samples. In the last analysis, the adequacy of the processing treatment is the really dependable factor.
Extensive research has so advanced our knowledge of food bacteriology that the processing phase of this subject is now amenable to mathematical expression. Such preciseness elevates food packing from the status of an industrial art to that of a scientifically established technology.
1. J. R. EsTu, Am. J. Pub. Health, 25, 165 (1935).
2. National Canners Association Res. Lab. Bul. 26-L, 3rd revision, 1937; W. D. BIGELow and associates, ibid., 16-L, 1920; W. D. BIGELOW and J. R. EsTy, J. Infectious Diseases, 27, 602 (1920) ; C. O. BALL, Bul. Natl. Res. Council, Part 1,. No. 37, 1923; C. O. BALL, University of California Publications on Public Health, 1, 15 (1928).
3. Research Department, American Can Co., New York, 1939, Canned Food Reference Manual.
4. W. H. EDDY and associates, Ind. Eng. Chem., 21, 347 (1929).
5. E. F. KOHMAN, J. Am. Diet. Assoc., 6, 123 (1930).
6. E. F. KOHMAN, Ind. Eng. Chem., 24, 650 (1932).
7. R. JORDAN, J. Home Econ., 27, 376 (1935).
8. E. F. KOHMAN and associates, J. Nutrition, 14, 9 (1937).
9. F. FENTON, D. K. TRESSLER, and C. G. KING, ibid., 12, 285 (1936).
10. E. G. HALLIDAY and I. T. NOBLE, J. Home Econ., 28, 15 (1936).
11. C. R. FELLERS, Mass. Agr. Exp. Sta. Bul. 338, 1936.
12. M. C. MooRE and H. W. MOSELEY, Science, 78, 368 (1933).
13. D. B. JONES and E. M. NELSON, Am. J. Pub. Health, 20, 387 (1930).
14. C. R. FELLERS, ibid., 25, 1340 (1935).
15. E. F. KOHMAN and associates, Ind. Eng. Chem., 26, '758 (1934).
16. C. M. McCAY, ibid., 27, 235 (1935).
17. C. D. ToLLE and E. M. NELSON, ibid., 23, 1066 (1931).
18. G. A. WHEELER and W. H. SEBRELL, J. Am. Med. Assoc., 99, 95 (1932). See also Pub. Health Repts., 49, 754 (1934).
19. G. W. CALDWELL, J. Pediatrics, 1, 749 (1932).
20. Council on Foods, J. Am. Med. Assoc., 108, 1259 (1937).
21. F. HANNING, J. Am. Diet. Assoc., 9, 295 (1933).
22. M. J. RosENAU, Med. Clinics N. Amer., 3, 913 (1920) ; Committee Report, Am. J. Pub. Health, 18, 893 (1928).
23. E. NEHRING, Food, 7, 50 (1937); quoted from Chem. Abs., 32, 1799 (1938).
24. C. R. FELLERS, Am. J. Pub. Health, 17, 470 (1927).
25. W. G. SAVAGE, Brit. Med. J., II, 560 (1925).
26. K. F. MEYER, J. Preventive Med., 5, 261 (1931).
27. G. G. DEBoRD and associates, J. Am. Med. Assoc., 74, 1220 (1920).
28. I. C. HALL, Food Research, 1, 171 (1936).
29. F. W. TANNER, Am. J. Pub. Health, 25, 301 (1935).
30. F. W. TANNER, Food-borne Infections and Intoxications, Twin City Printing Co., Champaign, Ill., 1933.
31. E. J. CAMERON and H. M. Loomis, Am. J. Pub. Health, 20, 741 (1930) ; also, W. D. BIGELOw and E. J. CAMERON, Ind. Eng. Chem., 24, 655 (1932).
32. B. A. LINDEN, J. Assoc. O/cc. Agr. Chem., 19, 440 (1936).
33. W. B. ADAM and G. HORNER, J. Soc. Chem. Ind., 56, 329T (1937), quoted from Chem. Abs., 32, 257 (1938).
34. W. D. BIGELOW, Ind. Eng. Chem., 8, 813 (1916).
35. B. C. Goss, ibid., 9, 144 (1917).
36. F. B. FLINN and J. M. INOUYE, J. Am. Med. Assoc., 90, 1010 (1928).
37. M. J. RosENAU, Preventive Medicine and Hygiene, D. Appleton-Century Co., 6th ed., 1935.
38. Reference 30, p. 89.
39. W. G. SAVAGE, Canned Foods in Relation to Health, Cambridge University Press, 1923.
40. Methods of Analysis, Association of Official Agricultural Chemists, 4th ed., 1935.
41. C. R. FELLERS, J. Assoc. Ofjec. Agr. Chem., 19, 430 (1936); F. W. TANNER, ibid., 19, 431 (1936).
42. B. A. LINDEN, ibid., 19, 440 (1936).
43. E. J. CAMERON, ibid., 19, 433 (1936).
44. J. R. EsTY and A. E. STEVENSON, J. Infectious Diseases, 36, 486 (1925).
45. F. W. TANNER, The Microbiology of Foods, Twin City Printing Co., Champaign, Ill., 1932, p. 549.
46. J. D. WILDMAN, Typewritten instructions, Microanalytical Division, U. S. Food and Drug Administration, May 21, 1937.
47. J. C. GEIGER, Am. J. Pub. Health, 14, 302 (1924).
48. Sanitary Code of the National Canners Association, Washington, Bul. 93-A, 1923.
49. Regulations for Canneries, State Board of Health of Maryland, June 12, 1930.