Preserved Foods - Part 4
( Originally Published 1939 )
OTHER METHODS OF PRESERVATION
The preservation of food by the common methods of smoking, salting, sugaring, curing, pickling, fermenting and, to a less extent, spicing, are dependent on the chemical or physical action of one or more constituents.
These substances may act directly by bactericidal or fungicidal action, or indirectly by creating an unfavorable environment for microbic growth. In the first case, some of these preserving factors may poison microorganisms. It may be surprising to think of such commonly eaten foods as smoked or salted products, for example, as corntaining ingredients which are poisonous. Poison, in this sense, is to be understood as relative. Ordinary table salt is poisonous in large doses, whereas a powerful drug like strychnine is beneficial in small doses.
In the second place, some of these preserving substances may be considered bacteriostatic. This means that their presence prevents the microorganisms from multiplying without actually killing them and holds their metabolic processes in abeyance. This consideration applies also to the chemical preservatives which may be used in such small concentrations that their effects are not germicidal at all. As Rosenau points out, such substances as sugar, salt, vinegar, and the pyroligneous products in wood smoke are not commonly called chemical preservatives but "natural" ones, although the mode of action of both classes is just the same. Another effect on the environment to make it impossible for microbic growth is abstraction of available water. Microbic cells, and also enzymes, to a greater or less extent, can function only when a certain amount of water is available to enter into their metabolism or reactions. A given food may be so high in its concentration of soluble products, like sugar, that its osmotic property is as great as or even greater than that of the microbic cell contents, thereby precluding the possibility of penetration of water from the medium (the food) through the cell walls into the microorganism.
Other preservative effects are the influence of the substance on the pH of the medium as, for example, pickling. Pathogenic bacteria do not usually grow in media with a pH below 4. High concentrations of salt may exercise a plasmolytic effect. Partial drying, curing, and smoking may cumulatively be preservative but ineffective in their individual application. Microorganisms differ in their susceptibility to these bacteriostatic or preservative treatments, and a combination which is useful to retard the growth of some is useless for others. For example, Pederson and Breed 1 showed that 0.1 percent of acetic acid, or 5.0 percent of salt, or 0.2 percent of sodium benzoate was required to stop the growth of several types of spoilage bacteria inoculated into catsup. Sugar itself was only partly effective in strength of 35 percent. However, a combination of 15 percent sugar and 3.5 percent salt stopped all growth except one yeast, but combinations of sugar or salt with acid did not appreciably lower the amount of acid required. Hucker and Haynes also showed 2 that a medium containing 30 percent sucrose, 10 percent sodium chloride, and 0.1 percent acetic acid greatly reduced the number of toxin-producing staphylococci whereas, when used alone, sugar in 50 percent strength and salt in 6 percent strength were only mildly inhibitive in 24 hours.
Smoking. The preservation of food by smoking involves a combination of several preserving substances and processes. The product, usually meat or fish, is first given a curing or salting treatment, and is then hung in hot smoke. The smoking evaporates some of the moisture, but particularly introduces several chemical preservatives into the food. The smoke itself is made by the burning of various organic products under a restricted supply of oxygen, although woods of special grades are usually employed. The destructive distillation of the wood forms a number of germicidal products such as acetic acid, acetone, methyl (wood) alcohol, formaldehyde, and the phenol-creosote mixtures. Although the use of these products individually is prohibited in foodstuffs generally, their presence in smoked products is allowed because of the general acceptance of smoke préservation.
The physiological effects of eating smoked food have not been studied, but no harmful results have been reported. However, there is complete loss of vitâmin C.3 Inasmuch as vitamin A is largely destroyed in the drying of vegetables and fruits, it would be expected that smoke products would not contain much of this product.
Salting (or curing). Food may be cured by treatment with ordinary salt (sodium chloride), or by mixtures with saltpeter (sodium nitrate). Salt is a true chemical preservative, as Rector points out, because it makes the food unsuitable for the growth of spoilage organisms. It is germicidal only in those concentrations which may make the food unpalatable, because some organisms are salt-resistant, and some even require a concentration of 25 percent salt to grow. However, most microbic growth ceases at 5 to 8 percent salt concentration. For example, its preservative action in butter is due to its concentration of about 13-16 percent in the droplets of water: about 2 to 2.5 percent salt is added to the butter, and this is taken up by the approximately 15 percent water. At the salt concentrations of ordinary salting and curing, the organisms are mostly prevented from multiplying and greater concentrations cause them to die off. One of the effects of salt is to draw some of the water out of the tissue, producing a partial dehydration. In the curing of meats, the common practice is to use a solution of ordinary salt, sodium nitrate and nitrite, sugar, and sometimes spices at temperatures at or slightly above 32° F. Salting is applicable particularly for meat, nuts, fish, butter, cheese and eggs.
A salt strength of 10.5 percent is reported by Tanner to be effective to inhibit the growth of Cl. botulinum, although some strains may survive this amount. Acid-pickled foods should have 2 percent acidity as acetic or citric acid with a pH not above 4.0. Curing is reported to kill some of the pathogenic organisms usually found in meat and fish. Inasmuch as smoke penetrates only superficially, and brine may not uniformly reach all parts, neither of these methods of preservation can be depended on to make infected or infested meat safe. The curing process extracts some of the nutrients which are lost in the discarded pickle.
Sugaring. Ordinary spoilage bacteria do not usually grow in sugar solutions of 50 percent strength, but yeasts and molds can cause spoil-age up to the saturation point. Strong sugar concentrations will pre-vent growth but cannot be expected to kill microorganisms. Usually, sugared products like jellies, jams, marmalades, preserves, and syrups are heated in the manufacturing operations. This kills the microorganisms that are present, and the high sugar concentration keeps the water from being available for invading microbial growth. Sugared fruits also are preserved by this principle of the abstraction of water. Invert sugar is stated by Rector to be a better preservative because its saturated solution contains a lower percentage content of water than that of cane sugar.
The common use of the word "preserves" to designate a fruit cooked with sugar attests the popular recognition of the role of sugar in preserving fruit from spoilage. In the older practice, the fruit and sugar were used in equal amounts, pound for pound as it was called, but the ratio has been changed to 45 parts of fruit to 55 of sugar. The sugar may be sucrose (cane or beet sugar) or dextrose, without distinction or label declaration.' Sucrose from cane sugar and from sugar beets is the same chemically, but differences in the refining methods, especially the crystallization, make the products distinguish-able in certain plant operations. Dextrose is a sweet constituent of the mixture of carbohydrates in the product known as "glucose" or "corn syrup," made from the hydrolysis of corn starch. It has about two-thirds the sweetening power of sucrose. This product formerly contained impurities, such as arsenic from the commercial grades of the acid used in the manufacturing. This caused the well-known Manchester outbreak of arsenical poisoning due to the consumption of beer, prepared from glucose contaminated through the arsenic in the sulphuric acid. Now hydrochloric acid is used almost exclusively. Such happenings gave commercial glucose a bad name, and this was accentuated by its substitution for sugar (sucrose) in the manufacture of cheap preserves and jellies, sometimes called "compound" products. Improvements in the process of manufacture have enabled the producers to market corn sugar (called cerelose) as a fairly pure, crystalline product. Several good grades of the liquid product are made for various special uses in the general food, confection, and baking industries. Fellers describes its method of manufacture, properties, nutritive value, and sanitary wholesomeness in Am. J. Pub. Health 29, 135 (1939).
There has been a question among consumers in the past as to the healthfulness of glucose. A committee of the National Academy of Science has reported that the manufacturing processes are unobjectionable, that the product is uncontaminated with injurious substances, and that it has no harmful effect upon the system.
Preserves and jams differ only in the physical condition of the fruit in the finished product. It is present in more or less whole pieces in preserves, whereas it is macerated or disintegrated in jams. In their manufacture, the cleaned, trimmed, and otherwise prepared fruit is cooked in steam-jacketed kettles until the desired consistency is reached. Sometimes this is done in vacuum kettles to retain more of the flavor and color. When large batches are made, the quality usually is not so good as that of small batches. This is largely the reason why American-made preserves, produced in large batches, are not so attractive and palatable as some of the foreign-made products in smaller batches, "custom-made" as it were. The larger the batch, the longer is the period of heating for each pound of finished product. This subjects the preserves to the volatilization of the desirable aromatic constituents, and to the discoloration from oxidation and metallic contamination. Products from large batches taste flatter than those from small ones.
Jellies. Fruits vary widely in their jelly-making power. This is a property of the pectins, which are carbohydrates, extracted by hydrolysis from the insoluble protopectins in the fruit tissues. 'When their water solutions, containing about 0.3 percent titratable acid as citric (or a pH of about 3.2-3.5) and sugar, are concentrated by cooking and then cooled, the pectins solidify to form a jellylike mass. The stiffness depends upon the amount of pectin present, the acidity, the amount of sugar, the duration of the heating, and the absence of agitation while the jelly is cooling and setting.
Jellies are made by cooking the sliced or chopped fruit with water, sometimes acidulated, until it is soft but still holds its shape. Generally, better jellies are made from fruit which has not reached full ripeness but whose flavor is well developed. The batch is then run onto cloth-covered racks and frames in an hydraulic press, and the juice is expressed as in cider and grape-juice production. The juice is clarified by settling, filtration, centrifugation, or straining through cloth. It is then brought to boil in a steam-jacketed kettle and the calculated amount of sugar is added. The boiling is continued until enough water has been removed to increase the percentage of solids to the point where experience has found the concentration to give the desired texture of finished jelly.
Preserves and jellies are poured hot into glass tumblers or jars. Some of the cheaper grades are poured into wooden or tin pails. Bacteria will not grow on them because of the high concentration of solids and the acid reaction. Molds will grow unless the surface is protected. The fancy grades are sealed under a vacuum and pasteurized; the next lower grades are coated with a thin layer of paraffin; and the cheapest grades are covered with circles of waxed paper.
Marmalades are preserves or jellies containing thinly sliced skin.
Jellies and preserves have had a comparatively clean bill of health.' No epidemiology connected with them has been reported.
Fermentation. The preservation of food by fermentation depends essentially on the conversion of soluble sugars to various acids by the action of microorganisms, and the inability of spoilage and pathogenic organisms to grow in the resultant acid medium. Lactic and acetic are the predominant acids formed. Large industries are based on these fermentations. Those on lactic acid are butter, cheese, the fermented milks and cream, sauerkraut, and pickles. Vinegar is the main product of acetic fermentation.
The fermentation of dairy products has been discussed in earlier chapters.
Sauerkraut is made by the lactic fermentation of properly salt-cured cabbage. The cabbage is cored, shredded, and packed in wooden tanks with about 2.5 percent salt. The salt withdraws water from the cabbage by osmotic effects, and forms brine which inhibits the growth of undesirable microorganisms but favors the growth of lactic acid fermenters. Different types of organisms predominate during the, process. The final result is a content of about 1.8 percent lactic acid.
Pickles are made somewhat similarly. Slightly underripe cucumbers are submerged in a brine of 10 percent initial strength. This is increased gradually up to about 15 percent. During the fermentation of the extracted sugars to lactic acid (which may require several weeks), the brine prevents the growth of undesirable organisms but does not prevent the growth of a surface film or mycoderma of yeast-like forms. This attacks the lactic acid unless removed. The pickles are soaked in water to remove the salt, and then treated with acetic vinegar.
Vinegar is made by the acetic fermentation of alcoholic solutions from fruit juices or other natural sugar or hydrolyzed starch solutions. In commercial practice, tanks with false, perforated bottoms are filled with wood shavings or other material with a large amount of surface. The alcoholic solution, introduced at the top, trickles downward over the packing and against an upward current of air. The resultant aerobic fermentation converts the alcohol to acetic acid. The official standards call for at least 4 percent acidity. The vinegar is then filtered, bottled, and often pasteurized to produce a clear product which keeps well.
Catsup. Tomato catsup (or ketchup, as it is also called) is concentrated tomato pulp and juice, preserved with sugar, vinegar, and spices. The tomatoes, freed from decayed portions, are softened by steaming or cooking, and then strained. through cyclone machines. These force the juice and soft flesh through a horizontal, cylindrical, perforated screen while the skins, cores, and seeds are forced out through a gate at the far end. The mixed juice and flesh, called tomato pulp, is pumped into steam-jacketed kettles or into wooden vats with steam coils, and boiled down to a concentration of about 2 : 1. Then sugar, vinegar, spice, and a small amount of salt are added, and the cooking is continued until the ingredients are well mixed and the consistency has reached the desired degree. The batch is again strained through a finishing machine, very similar to the cyclone except that it has a smaller mesh screen, to remove any fine particles of spice or other foreign products and to impart a smooth texture to the finished product. The catsup is then poured into heated bottles, and pasteurized to insure keeping.
Health aspects. In the salting process given to cabbage and cucumbers, an appreciable part of their soluble constituents is withdrawn and lost in the juice. The vitamin C in the original cabbage and cucumbers seems to be markedly decreased in the finished product, and possibly also vitamin A, but vitamin B is reported to be practically unaffected. Putrefactive and pathogenic microorganisms do not grow in acid solutions. Neither lactic acid nor acetic acid has been reported as incriminated in illness from the consumption of food containing them. Lactic acid, indeed, is widely prevalent in spoiled food, but the unpalatability of such food is due to the unwanted presence of the acid. For example, lactic acid in sour milk is repulsive whereas in buttermilk it is desirable. The lactic acid itself is not harmful to the consumer but its presence indicates that microorganisms have been active.
Sodium benzoate. Much controversy has raged over the advisability of the use of sodium benzoate as a food preservative. This compound was studied by Wiley and his associates, who reported that, as a result of tests of its effects on a "poison squad," it created "a very serious disturbance of the metabolic functions, attended with injury to digestion and health." The question was referred by President Theodore Roosevelt to a Referee Board of Consulting Scientific Experts, who reported that the consumption of less than 0.5 gram per day, mixed with the food, was "without deleterious or poisonous action and is not deleterious to health," and that use in large doses up to 4 grams per day, mixed with the food, was not found to exert "any deleterious effect on the general health nor to act as a poison in the general sense of the term," although "in some directions there were some modifications in certain physiological processes." Small amounts of benzoic acid are present in prunes and cranberries. Folin points out that its presence in natural foods has never been connected with any injurious action. Kohman and Sanborn showed 10 that quinic is the dominant acid in these fruits to the extent of 1 percent, and Quick showed that it is excreted (like benzoic acid) as hippuric acid by reaction with glycocoll in the liver. Inasmuch as benzoic acid can be harmlessly eliminated by a special mechanism in the body, it is not to be expected that it could exert any harmful effect, at least in the small amounts that are used in sodium benzoate preservation.
Common commercial practice uses 0.1 percent in foods. It is not germicidal in neutral solutions but must be in an acid solution to be effective. The "federal government rescinded the earlier prohibitions against its use as a preservative of food, provided that when used its presence and concentration are stated on the label. Some states limit the maximum amount to 0.1 percent, whereas others follow the federal practice.
Sulphites. Sulphurous acid occurs in foodstuffs quite extensively by reason of its use in the drying of fruits. Some products are practically unmerchantable without it. Wiley reported' that its use was inadvisable because its ingestion produced serious disturbances, malaise, or positive suffering, together with more obscure symptoms. Some investigators believe that it interferes with the action of enzymes and therefore may influence digestion. Free sulphurous acid is irritating. In the body, the sulphites are converted into the harmless sulphates and excreted without trace of any harmful effects.
This chemical is allowed only in dried foods (in this country), and its presence must be declared on the label. Sometimes it is used fraudulently by unscrupulous retail meat dealers to restore the color and destroy the odor of meat that is just beginning to spoil. Sodium nitrite is similarly used.
Salicylates. Wiley showed 8 that salicylic acid is one of the milder chemical preservatives but that it is harmful when used as a food preservative. It is a stronger antiseptic than the boron compounds. It was used mostly in jams, fruit juices, syrups, wine, and other fruit products. In the United States it has been replaced largely by benzoic acid.
Boron compounds. Boracic acid and borax have been extensively used in the past in butter, cream, margarine, liquid eggs, sausage, and marine foods. It was studied by Wiley and condemned because of its alleged harmful effects. The prohibition of its use by the British Ministry of Health in 1928 precipitated a heated and long-drawn-out controversy which brought forth much expression of opinion and a few facts. Some limitation as to the amount that could safely be used was necessary. There was evidence that the ingestion of small amounts of boric acid over a considerable period was harmful. In defending the prohibition, Neville Chamberlain, Minister of Health, stated that boron compounds are not excreted as rapidly as they are absorbed and hence have a cumulative effect, and that before he would cancel the order, the proponents of its use would have to prove its necessity; that many enlightened nations prohibit its use; and that some traders had found that they really did not need it after all. It is not allowed in the United States, although Canada, France, and others permit it in butter, condensed milk, and cream under restrictions. It should be looked for in imports of liquid eggs from China.
Formaldehyde. This is one of the most harmful of the common preservatives. It is not only a powerful germicide but is also an effective deodorant. It is very irritating to the mucous membranes and is a strong protoplasmic poison. It delays the digestion of protein and starch, and the curdling of milk. It reacts with proteins and hardens tissue, even in solutions of 1 : 5000. Its use is universally prohibited by all nations, and the activities of food-control officials in this country have practically eliminated it here.
Fluorides. These compounds are extensively used as antiseptics, preservatives, insecticides, and vermicides. They are highly toxic to nearly all the lower forms of life. Bacterial growth is completely arrested at 1 : 200 strength. The fluorides mottle the teeth and form white crystalline depositions in the bones. They are definitely poisonous to man, and are not used as added ingredients to food, although traces are present in bones, teeth, milk, and eggs.
Other preservatives. Hydrogen peroxide is one of the most harm-less and also least effective of the common preservatives, because it undergoes decomposition into oxygen and water, thereby losing its preservative property. Formerly it was used to some extent in bottled beverages and milk. Carbon dioxide inhibits bacterial growth when under high pressure; it is used only in the bottled beverage industry. Some of the spices, especially cinnamon and cloves, have an appreciable preservative value.
Added preservatives. As Folin has pointed out,' formaldehyde, salicylic acid, boric acid, and sulphurous acid are not actually produced in the body, and there is no special physiological process avail-able for rendering them harmless. We really do not know to what extent they are actually harmful. Bigelow states that the past studies on the physiological effects of preservatives are inconclusive because only about 1 percent of the human life span was covered whereas these effects should be studied over a much longer time. We do not know the degree of harm, if any, that might come from the ingestion of small amounts over an extended period.
The addition of chemical preservatives to food sometimes makes it easier to use inferior or even partly spoiled raw materials. Natural fermentation is interfered with. Their use makes sanitation less necessary, and therefore they tend to cover up uncleanliness.
It is quite significant that, after the use of these various chemical preservatives was ruled out, the food industry did not find itself crippled nor was the public deprived of any necessary source of food. In fact, the industry has not only operated satisfactorily but has produced food on a higher plane of sanitary and nutritive quality than ever before.
Standards. The U. S. Food and Drug Administration has adopted standards 13 for preserves, jams, and jellies, tomato catsup, pickles, and sauerkraut, summarized as follows:
Preserves and jam are made by cooking to a suitable consistency 45 pounds of fruit to each 55 pounds of sugar or of sugar and dextrose. A product in which the fruit is whole or in relatively large pieces is customarily designated a "preserve" rather than a "jam."
Jelly is the semi-solid jellylike product made by cooking to a suitable consistency a water extract of fruit with sugar or sugar and dextrose.
Citrus fruit marmalade is the jellylike product made by cooking the properly prepared peel and juice with sugar or sugar and dextrose. It contains pieces of the fruit peel imbedded in the jelly.
Apple butter. The semisolid product made by cooking to a suit-able consistency not less than 5 parts of strained apples to each 2 pounds of sugar and/or dextrose, with or without added apple juice, spices and/or salt.
Tomato catchup. The concentrated product made from the pulp and juice of ripe tomatoes (exclusive of skins, seeds, and cores), a vinegar, salt, spice, and other seasoning, sugar and/or dextrose.
Pickles. Immature cucumbers, salted, and preserved in vinegar, with or without spices.
Sauerkraut. The product obtained by the fermentation, chiefly lactic, of properly prepared and shredded cabbage, containing 2-3 percent salt. Upon completion of fermentation, it contains not less than 1.5 percent acid as lactic, and when canned or repacked, 1 percent.
Vinegar, cider vinegar, apple vinegar. The product made by the alcoholic and subsequent acetous fermentations of the juice of apples. It contains in 100 milliliters at 20° C. not less than 4 grams of acetic acid.
Spirit vinegar, distilled vinegar, grain vinegar. The product made by the acetous fermentation of dilute distilled alcohol. It contains not less than 4 grams of acetic acid.
Types of adulteration. Preserves are sometimes found. with a smaller proportion of fruit than the definition requires. They may contain artificial fruit essences, added colors, or even saccharin or dulcin to sweeten when only glucose is used, chemical preservatives, fruit that is different from the label declaration, apple stock of lower grade or' quality than that declared, added coagulants or acid to jellies, starch or gelatin or agar-agar as adulterants, possibly traces of arsenic in compounds. Examination should be made for moldy and other such decomposed stock, and for evidences of insect infestation.
Chemical examination. Arsenic determinations are made on 2-5 grams of the sample (without the charring or destruction of the organic matter) by the Marsh test or the Sanger-Black-Gutzeit test.
Artificial colors are determined by extraction from the prepared solutions and identification by specific reactions with various reagents, sometimes after dying wool or silk.
Decayed, moldy, or fermented products in tomato catsup are detected by direct microscopic examination with the Howard technic (see page 448).
The presence and amount of insect debris and worm infestation in apple butter are determined by suspending three 100-gram samples in 1 liter of water each, adding gasoline, drawing off the lower layers of water and pulp, then transferring the gasoline layer containing the insect parts to a filter disk on a Büchner funnel If more than 2 insects are found, the product is considered to be infested with an excessive amount.
Benzoic acid is determined by extracting the prepared solution with chloroform, and then titrating the alcoholic solution with standard alkali.
Sulphites are determined by boiling the food for 14 1/2 hours in a flask with hydrogen peroxide, under a slow current of carbon dioxide, and then titrating with 0.1 N sodium hydroxide, using bromophenol blue as indicator, or determining the oxidized product gravimetrically as barium sulphate.
Salicylic acid in a specially prepared solution is treated with a 0.5 percent ferric chloride solution (or 2 percent ferric alum), so that the color can be matched against a known amount of standard salicylic acid solution.
Boron as boric acid and borates is determined by preparing a free boric acid solution which is treated with mannitol and a few drops of phenolphthalein, and titrated with standard sodium hydroxide until a permanent pink remains. By means of a conversion factor, the number of grams of boric acid is calculated from the titration.
Formaldehyde can be identified in milk by treating 15 milliliters with 1 milliliter of 1 percent phenylhydrazine hydrochloride solution and a few drops of ferric chloride. A red color, changing after some time to yellow, indicates the presence of formaldehyde.
Fluorides can be found by treating the specially ashed residue with a few drops of sulphuric acid and covering it with a glass plate. The characteristic etching reveals the presence of compounds of fluorine.
Regulatory procedure. In almost every community of average size there are one or more pickling and food-preserving establishments. Some of these operate in makeshift manner, in unclean premises, with insanitary equipment, and under indifferent conditions of personal hygiene. Both large and small firms are guilty. Pickle works, pre-serve and jelly plants, and tomato catsup factories are difficult to keep clean, and are particularly likely to dull the sensibilities of the management to the demands of good plant housekeeping. They need continual reminders of their responsibility to the public. The fact that disease has not been traced to the handling of food under such conditions of manufacture and that pathogenic microorganisms will not ordinarily grow on pickles and preserves is no excuse for handling these foods under grossly unclean conditions. Massive doses of viable pathogenic organisms may create or find locally favorable conditions for their maintenance, viability, and even growth.
The control of the use of chemical preservatives must be based on the laboratory examination of samples. Their presence should always be declared on the labels. A simple test is to incubate the suspected sample alongside of a control known to contain no preservative. If the control spoils significantly sooner than the unknown, then it is in order to give the unknown a careful laboratory examination.
1. C. S. PEDERSON and R. S. BREED, N. Y. Agr. Exp. Sta. Bul. 538, 1926.
2. G. J. HUCKER and W. C. HAYNES, Am. J. Pub. Health, 27, 590 (1937).
3. W. D. BIOELOW, J. Assoc. Offic. Agr. Chem., 18, 21 (1935).
4. F. W. TANNER, Microbiology of Foods, Twin City Printing Co., Champaign, Ill., 1932.
5. Report of the Chief of the Food and Drug Administration, 1931.
6. M. J. RosENAU, Preventive Medicine and Hygiene, 6th ed., D. Appleton-Century Co., New York, 1935.
7. C. R. FELLERS, Mass. Agr. Exp. Sta. Bul. 338, 1936.
8. H. W. WILEY and associates, U. S. Dept. Agr., Bur. Chem. Bul. 84, Parts 1-5, 1906-1908.
9. O. FoLIN, Preservatives in Foods, Harvard University Press, 1914.
10. E. F. KOHMAN and N. H. SANBORN, Ind. Eng. Chem., 23, 126 (1931).
11. A. J. QuicK, J. Biol. Chem., 92, 65 (1931).
12. U. S. Dept. Agr. Food Inspection Decision 104, issued March, 1909.
13. "Service and Regulatory Announcements," Food and Drug, 2, fifth revision, November, 1936.
14. A. E. LEACH and A. L. WINTON, Food Inspection and Analysis, 4th ed., John Wiley & Sons, New York, 1931.
15. Methods of Analysis, Association of Official Agricultural Chemists, 4th ed., 1935. (a) p. 432; (b) p. 439; (c) p. 430; (d) p. 436; (e) p. 436.
16. W. S. GREENE, A Method for the Recovery of Insects from Apple Butter, Mimeographed release, Microanalytical Laboratory, Food & Drug Administration, January, 1933.