( Originally Published 1939 )
Production. The great bulk of the supply of eggs comes from general farm flocks in the north central states, although many are produced on special poultry farms. The production is highest during the spring and early summer, and then gradually declines to a low point at the end of the year. The quality of the spring eggs is usually much better than that of summer eggs, because the hot weather brings about marked deterioration while the eggs are held for shipment. Sometimes, cold-storage spring eggs have better quality in the late summer than the fresher-produced summer eggs.
When eggs are produced in large numbers at a given point, they can be shipped directly to cold-storage warehouses with small loss, but this is not generally possible. Most of the eggs are collected in small lots. The farmer may take a few to the country store and exchange them for some necessities or sell them for cash. The storekeeper holds them until he has enough to make a shipment. Sometimes traveling hucksters go through the country to buy up the farmers' production. They may sell to larger dealers, and these in turn to others, so that the eggs may pass through several hands before they reach the large storage houses. There is much delay in sorting out the bad eggs and in giving the sound stock adequate protection from deterioration. Usually, 3 weeks or longer may elapse between the time the egg is laid and the time that it reaches the consumer.
Candling. The quality of eggs is commercially determined by the process of candling. When an egg is held in front of a bright light, the contents are revealed as varying degrees of shadow and color. When an egg is candled, it is grasped by the small end, and while held between the thumb and the tips of the first two fingers, it is given a few quick turns on its long axis, while it is held in front of a small bright light. This twisting motion enables the candler to observe the size of the air cell at the large end of the egg, the degree of stiffness of the contents, the freedom of motion of the yolk, and other physical indications of quality. The U. S. Bureau of Agricultural Economics will send, to any one who requests it, a colored chart illustrating the appearance of the different types or grades of eggs before the candle.
In normal fresh eggs, the yolk is centrally and freely suspended in the white which entirely fills the space between the yolk and the shell. The germ spot, located on the surface of the yolk, is small and irregular in shape in an infertile egg, and round and larger in a fertile egg. On aging, moisture evaporates through the shell so that the white shrinks from the large end, leaving the so-called air cell. As the age increases, the white becomes more watery, allowing greater motility to the yolk. When an egg has been heated or is stale, the germ spot develops, the air cell enlarges, blood veins or blood rings form, and the yolk assumes a deeper color. Foreign material appears as dark-colored particles in the white. Cracks in the shells may be invisible to the unaided eye but apparent before the candle. Candling also indicates the different degrees of deterioration, as well as the several types of spoilage.
Inasmuch as spoiled eggs may damage neighboring sound ones, shipments are usually candled before they are placed in cold storage, not only as an economy measure to save storage charges on unsalable eggs but also to reduce losses during storage.
Cleaning eggs. On account of the higher prices obtained for clean eggs, several methods are used for cleaning the shells. They may be treated with dilute sulphuric acid, wiped with a wet cloth or brush, washed with soap, or even sand-blasted. These treatments do re-move much or all of the stain, but they also introduce other factors which usually increase the rate of spoilage or deterioration. In spite of the common belief that washing hastens the deterioration of eggs, Bryant and Sharp showed 1 that this is not necessarily true if the eggs are handled properly after washing. The deterioration of washed eggs is caused by bacterial infection of the egg from the dirt that was on the shell.
Cold storage. Eggs begin to move into cold storage in March, reach their peak of holdings in September, and then decline. Shell eggs are never carried through the second season.
Care must be exercised in operating a cold-storage warehouse to preserve the shell stock in proper condition. If the eggs are allowed to freeze, they will expand and break the shells. If the air is too dry, the eggs will dry out with equivalent loss of weight and, according to some workers, a deterioration of flavor. It is generally agreed that a temperature range of 28° to 31° F. (-2.2° to -0.6° C.) is the best.
The optimum humidity will be determined by local conditions like climate, temperature, location, altitude, and other such factors. In general, a humidity which is just below the point at which mold would grow is the most desirable. A humidity range of about 85-93 per-cent seems to have been most generally observed. Circulation of the air is important if high humidities are carried.
Cases of eggs in cold storage are stacked with aisles between to allow ventilation and to prevent the formation of air pockets. Some warehouses circulate ozone through the plant to sweeten foul air. Cleanliness and sanitation are essential to keep down spoilage and foreign odors. Between seasons, the properly conducted warehouse is thoroughly cleaned by scrubbing the floors and walls, and then spraying them with a sanitary dressing of cold-water paint, hygroscopic salt, or whitewash. All litter is removed, and the premises are placed in a good sanitary condition.
Eggs kept in cold storage for several months may develop a storage flavor, more noticeable in the yolk, especially when soft-boiled or poached. This seems to come mostly from odors outside the egg, such as the packing cases or substances derived from the air of the storage room.
Oil-dipping process. Inasmuch as a large part of the deterioration of eggs in storage is caused by absorption of odors and loss of moisture and carbon dioxide through the shell, it has been found beneficial to close the pores by dipping the eggs for a few seconds into a bath of odorless, tasteless mineral oil, heated to 130° F. (54.5° C.) or higher to impart fluidity. When the oil cools, it seals the pores of the shell and prevents the loss of natural or the entrance of foreign products. It is not a substitute for refrigeration but is a valuable accessory treatment. Benjamin Franklin is said to have dipped eggs in oil for long sailing voyages. This treatment retards deterioration but should not be used fraudulently for marketing stored eggs as fresh eggs.
Carbon dioxide treatment. Pursuant to Sharp's discovery 4 that the loss of carbon dioxide from the egg entails a deterioration in quality, some warehousemen have charged the atmosphere of the storage rooms with about 1 percent of carbon dioxide, the actual percentage varying with the temperature of storage. This concentration is enough to retard the loss of the gas from the egg but is not enough to be harmful to the workmen who have to enter the storage rooms. This treatment can be accorded the eggs, without removing them from their cases, at an expense of about a cent per case of eggs. Its effect is more potent at higher temperature where refrigeration is not operative. The walls of the warehouse must be specially treated to prevent excessive diffusion of the gas, or successive charges of the gas are necessary to replace the losses. The beneficial effects on the improved quality of the eggs are shown by the odor, the condition of the thick white, the better standing up of the yolk, the viscosity of the yolk, and the taste and color of the white. Moran states 5 that the optimum atmospheric conditions of storage for the best egg quality are 2½ percent carbon dioxide and 85 percent moisture.
Vacuum-carbon dioxide-oil treatment. Swenson has worked out a possible improvement combining the advantages of the oil and carbon dioxide treatment.° He draws out a portion of the air in the egg with a vacuum, then coats the shell thinly with an oil, and releases the vacuum in carbon dioxide gas. The restoration of the normal air pressure carries the oil into the shell pores and seals them. As a large part of the oil is drawn to the inner surface of the shell, the eggs have a less oily appearance than eggs oiled by merely dipping. In an 11-month storage test, the unoiled eggs lost 7.71 percent of their weight, the open dipped 1.6 percent, and the vacuum dipped 0.1 percent. None of the unoiled was in the top two grades whereas 30 percent of the open dipped and 47 percent of the vacuum dipped were. This process is covered by U. S. Patent 1,888,415, and is available for free use by the public. Inasmuch as the closing of the pores seems to be the important factor, it would seem that releasing the vacuum in carbon dioxide gas would not offer much advantage over releasing it in ordinary air.
Frozen eggs. Shipments of shell eggs to the packing houses always contain some eggs that are too weak to ship, have cracked shells, are over- or undersized, or off-color, or dirty, or otherwise unmarketable, but are nevertheless edible. Such eggs may be salvaged by freezing. Frozen eggs may be grouped into 3 general classes: frozen whole or "mixed" eggs, frozen yolks, and frozen whites.
These eggs are first carefully candled to detect spoiled eggs. Each egg is then broken into a small cup and examined by the operator for normal appearance, smell and sometimes taste. If it is off in quality, it is discarded and sold to tanners, and the equipment is cleaned and scalded. The good yolks and whites may be kept separate or mixed in churns. The mixture is run into 30-pound tins and immediately placed in a sharp freezer where it is held for 72 hours at -12° to -15° F. (—24.5° to -26° C.). The can is then removed to a freezer at about 0° to -5° F. (—18° to -20.5° C.) to prevent bursting. Only about 8 minutes intervenes between breaking the fresh eggand placing it in the freezer. When the eggs are to be used, they are best thawed by placing the can in cold running water. The freezing of natural egg white does not affect its whipping property. Frozen eggs may have as good quality as regular storage eggs in the shell; they possess greater uniformity, and will keep in good condition for a year or more?
Inasmuch as freezing of yolks precipitates or gels the lecithoproteins, the substance which gives emulsifying properties to the yolk, it is common practice to mix 10-20 percent of glycerin or 5-10 percent of sugar or salt with them to lower their freezing points. This permits their storage at lower temperatures than would otherwise be possible. It also prevents formation of gummy, lumpy particles, and facilitates smooth thawing. If yolk mixed with these substances is actually frozen, undesirable denaturation of the protein occurs.
Egg whites are used especially in the baking and confectionery industries for candies, meringues, and cakes. Plain and salted yolks are used in the mayonnaise and noodle industries. Whole eggs, sugar yolk, and glycerol yolk find outlets in the baking, ice-cream, and confectionery trades. Several specialty egg products are packed with other products to definite specifications of color, composition, and other special properties.
Dried eggs. The same care is necessary in the discarding of spoiled eggs and in sanitary handling for drying as for freezing. A very large part of the dried egg used in this country is imported from China. The literature reports that sanitary conditions in some of the Chinese plants are very poor, but many are operated on as high a sanitary plane as those over here. Blomberg reports the Chinese and American methods as follows:
In China, egg white is dried by the pan process. The albumen is strained into a 700-pound wooden cask with a spigot about 3 inches above the bottom. It is allowed to ferment at about 70° F. for 36-60 hours. This fermentation is necessary to impart, when reconstituted, the whipping properties necessary to make a stable froth which is the basis for its value to the baking and confectionery industries. A scum rises and a sediment settles out. When action seems complete by cessation of bubbling, the clear liquor is drawn off into buckets, and 2 ounces of aqua ammonia and 3 ounces of alcohol are added per 100 pounds. The liquid white is thinner and not as sticky as it was before fermentation. It dries to a flaky consistency, and is not tough like unfermented dried albumen. The liquid is poured into round trays of zinc or aluminum, about 12 inches in diameter and 11/4 inches deep, heated to 120° F. (40° C.) for the first 18 hours, and then to 140° F. (60° C.) for 20 hours. Whole egg and yolk are dried similarly. Tray-dried egg white requires less alteration before drying than spray-dried, probably due to the changes which take place during the slower drying in the trays.
In this country liquid whole egg and yolk are churned to secure a smooth mixture, which is spray-dried by being pumped under pressures of about 1500 to 3000 pounds per square inch into a high-ceilinged, conical chamber kept at a temperature of 160° to 220° F. The moisture quickly evaporates, and the dry particles fall to the floor as a fine powder. Albumen is fermented to secure uniformity of consistency, and Swenson has accelerated this treatment by adding citric acid and acidified pepsin solution. The fermented liquid is poured onto (usually) aluminum pans or trays, and stacked on shelves in a heated cabinet where it dries in 48 to 72 hours.
Spray-dried yolk is preferred for ice-cream manufacture. The spray-drying of egg white seems to destroy its whipping properties when reconstituted with water, although some improvements in the process have overcome this difficulty.
RELATION TO THE PUBLIC HEALTH
Composition. Mitchell analyzed many fresh and stored eggs," and later made a study of the composition of the whole egg, the whites, and the yolks, obtained from eggs which were broken out in commercial plants under working conditions. The composition of such eggs is shown in Table XXIV.
Differences in composition of fresh and storage eggs are mostly due to the changes produced by osmotic action through the membranes of the egg, and the evaporation of the water through the shell.
Hepburn and Katz 14 studied the composition of eggs and tissues of ducks. These birds were secured from the vicinity of Philadelphia where Pennington and her associates secured hennery eggs for their studies. The results are presented in Table XXV.
In general, eggs have a little lower percentage of protein than meat, about the same percentage of fat, a relatively high mineral content, very little carbohydrate, and a high content of the vitamins, approximating that of milk. Most of the fat is in the yolk, associated with lecithin and designated as a phospholipid.
Egg white is mechanically separable into an outer layer of fluid white, an inner layer of firm white, a further inner layer of fluid white surrounding the yolk, and a small amount of firm white attached to the yolk. It is this intermediate layer of firm white that "stands up" in the surrounding fluid white when the egg is broken out onto a flat surface. Small amounts of glucose, mannose, and galactose are associated with the several proteins.
The breed of fowl and its food exert an appreciable effect on the flavor, composition, and nutritive property of the eggs. The nature of the fatty acids may be affected, and the vitamin D and iodine con-tents can be increased by the incorporation of these substances in the hen's food. On the other hand, undesirable effects may obtain, as, for example, the production of "pink whites" when the hen eats cottonseed meal or cheeseweed.
Nutritive value. Eggs, like milk, are outstanding among food-stuffs by reason of their sufficiency for the complete nutritive requirements of young animals during very rapid growth. Neither one is a perfect food, in that other food is necessary for normal growth and well-being, but both of them are singularly rich in the quality of their constituents. In egg white, the chief protein is ovalbumin, accompanied by conalbumin, ovomucin, and ovomucoid. In yolk, the chief protein is ovovitellin, together with some livetin. These proteins are adequate for growth, reproduction, and lactations and valuable for bringing to full nutritive efficiency proteins of other foods which when fed by themselves are not so efficient, such as the cereal proteins. Phosphorus is readily available in the phospholipids of the yolk. Egg yolk is a particularly rich source of iron in assimilable form.
Eggs are like meats, fish, and cereals in that their digested residues are acid-forming. One pound of eggs furnishes about 850 calories as compared with 700 to 1000 calories from 1 pound of meat. In general, eggs are not so nutritive as milk but are better than any other single food.
Rose and Borgeson fed 60 children of nursery-school age for 21 months on a simple diet of fruits and vegetables, cereals, and milk, one group receiving an egg daily and the other having no such addition but an equivalent number of calories from the other foods. In general health status, the egg-fed children were slightly superior, particularly in the hemoglobin values of the blood. There were no constipating or other digestive disturbances.
No difficulty in the utilization of raw yolk has ever been reported. Egg white digests somewhat better if cooked, and particularly if finely beaten; 20 it undergoes little digestion in the stomach, and contains a tryptic anti-enzyme which cooking destroys.
Vitamins. The yolk contains vitamins A and D but not in very large amount, although their content can be influenced by feeding. The yolk. of the average hen contains about 350 units of vitamin A.18 In comparison, a daily consumption of 2 ounces of butter would furnish 1700 units; 3 ounces of carrots, 2800 units; and 4 ounces of spinach, 5000 units. Although a darker-colored yolk usually contains more vitamin A precursors than a lighter one, the yellow coloring matter is not carotene but xanthophyll which does not have the properties of vitamin A
Cold storage does not seem to diminish the vitamin A content. Jones and his associates report 22 that some July firsts were stored in the shell at 29° to 32° F. for 2 months, and then their contents were thoroughly mixed and stored in a frozen condition at -0° F. to +10°. After 6½ years, there was no gas in lactose broth at any dilution and no objectionable odor. When stored for 9 years in the frozen condition, they were as effective as fresh eggs in curing xerophthalmia.
The vitamin D content of an egg yolk, as determined at the New Jersey Experiment Station, was about 1/75 of that of a good grade of medicinal cod-liver oil, equivalent to 1/20 teaspoonful of the oil. Its amount varies greatly with the season, and can be increased by incorporating a rich supply in the hen's food or by irradiating the hen with ultra-violet light. There is more of this vitamin in eggs than in any other foodstuff. It is not impaired by the cold storage or boiling of the egg.
The content of vitamin B1 (thiamin) of eggs compares favorably with that of milk and cereals. It is present mostly in the yolk, and averages about 15 to 20 international units per yolk. There is a slight loss of anti-neuritic power when the egg is boiled 7 minutes.
Eggs do not contain any appreciable supply of vitamin C.
Eggs and milk, on the dry basis, contain more vitamin G (riboflavin) than wheat germ. Chick and her associates 25 report that fresh egg white contains vitamin B2 (G) but none of the anti-neuritic B1. An average egg may be expected to yield 65 to 70 Sherman-Bourquin units. Dried whole egg contains about 1.80 mg. of riboflavin or 450 Sherman-Bourquin units per 100 grams.
Allergy. Allergic eczemas from the consumption of eggs have frequently been seen in pediatric practice. Smyth, Bain, and Stallings 26 found that 160 cases taken from children's allergy clinics were distributed as follows: 68 were due to egg white, 44 to wheat, 32 to milk, 12 to orange, 11 to oats, 9 to potatoes, and the remainder scattered. Sensitivity to milk and wheat occurs about as often as to egg, and this reaction to eggs can be alleviated or entirely cured or outgrown.
Epidemiology. There is a potential danger of infection to humans by bacteria carried by the eggs of diseased hens. Emmel isolated Salmonella aertrycke, S. schottmülleri, S. enteritidis, S. suipestifer, S. typhimurium, S. pullorum, and Eberthella typhi from 13 out of 18 birds affected with enteritis associated with coccidiosis. These isolated microorganisms constituted from 8.7 to 61.4 percent of the total microbial flora. Although these particular data were obtained from hens known to be diseased, it is thought that such birds may constitute a dangerous reservoir of typhoid, paratyphoid, and food-poisoning types of bacteria.
As a rule, hens' eggs are sterile, but Rettger and associates 29 point out that under certain conditions they may constitute a health hazard. A hen which is a carrier of Salmonella pullorum may infect eggs through her ovary. This microorganism is toxic for young rabbits, kittens, and guinea pigs by mouth, but no cases are on record of its poisoning humans. When this organism is present in fresh eggs, the number is so very small that no danger is believed to exist for the most infirm person or the smallest infant, but inasmuch as the organism multiplies very rapidly in infected egg yolks, it is necessary to keep the eggs cool. Many flocks are heavily infected. The authors state that since the ailments caused by infected eggs would not make themselves felt presumably until several days after their ingestion, little suspicion might fall on an egg. Ilzhôfer and Miiller, measuring heat penetration by thermoelectric methods, found that Salmonella enteritidis was killed when eggs were boiled 8 to 10 minutes and allowed to cool slowly, whereas these organisms were not killed if the eggs were cooled rapidly, or if boiled for only 5 minutes, or if fried. Inasmuch as widespread infection of flocks with Salmonella pullorum is comparatively recent, the attendant health hazard to man is relatively new.
Numerous investigators have studied the possibility of transmission of tuberculosis by the consumption of eggs from tuberculous hens. (See discussion of avian tuberculosis on page 298). Mohler and Washburn .31 showed that guinea pigs developed the disease when they were inoculated with the white of an egg from a tuberculous flock. Fitch and Lubbehusen 32 quote the work of several investigators who showed that chicks may be infected from eggs which had been artificially inoculated with avian tubercle bacilli, and that Higgins had found tubercle bacilli in 5 of 15 eggs from a flock in which tuberculosis had been diagnosed. Other workers are reported to have found tuberculous eggs from infected fowl. However, Fitch and Lubbehusen state that Fitch, Lubbehusen, and Dikmans found that, out of a total of 876 eggs from 43 tuberculous hens, tubercle bacilli were found only in a composite sample of 9 eggs—less than 1 percent. In another series (32) comprising 2000 eggs from 88 naturally infected hens, 697 chicks were hatched, of which 487 died. No tuberculosis was found in any. Eggs that were artificially inoculated with avian tubercle bacilli were reduced in hatchability, and chicks hatched from such eggs died within a week or two.
From artificially infected hens, Raebiger secured 12 eggs, in all of which he found tubercle organisms. Later he examined 52 additional eggs, laid within 2 months after the inoculation of the hens, and found 19 to be positive to these organisms, although there is some doubt as to their identification!
Tanner quotes Klimmer's summary of the literature that, out of 1033 eggs from tuberculous hens, 5.7 percent contained tubercle bacilli. Lichtenstein .36 tested 525 market eggs of various grades, 216 eggs from hens of a flock infected with tuberculosis, and 38 eggs artificially inoculated with organisms of chicken tuberculosis. She found living virulent tubercle bacilli in 2 instances in the market eggs and in 1 in-stance in the eggs from the infected flock. From the artificially inoculated eggs, she could still culture tubercle organisms 167 days after the inoculation.
The unreliability of the culture media and the uncertainty as to the bacteriological technic for determining the presence of tuberculosis germs in eggs weaken the significance of such studies.
In general, it seems that some eggs from naturally tuberculous hens may contain tuberculosis organisms, but the incidence must be very small or the hatchability low, because so few tuberculous chicks are hatched. Artificially infected eggs do not hatch well, and their chicks survive only a few weeks. In the light of these facts, it appears that the danger of contracting tuberculosis from hens' eggs infected with fowl tuberculosis is not large. Moreover, there is no direct evidence that the avian type of tubercle bacillus is pathogenic to man, although Jordan states that German investigators have attributed some cases of liver and kidney affection to the avian type, and that this organism has been found on several occasions in tuberculous persons. This whole field of the possible public-health relation-ship of avian tuberculosis to man needs thorough investigation.
With regard to the destruction of the germs of tuberculosis in eggs by cooking, the work of different investigators is not in perfect accord. The boiling of an egg for 6 minutes is said to kill all these germs, whereas boiling for 8 to 10 minutes is reported by other workers to be ineffective. When an egg is soft-boiled, viable organisms may be found between the firm and soft portions. Several investigators report that in a hard-boiled egg all organisms are killed. In the light of the work of Ilzhöfer and Milller, it seems that the bactericidal effectiveness of boiling for 8 to 10 minutes is just on the borderline, be-cause viability is destroyed by slow cooling whereas rapid cooling is ineffective.
Frozen eggs. European investigators report that imported Chinese frozen eggs may be highly infected, and that microbic growth is very rapid when the product is thawed. Verge and Grasset identified Sarcina alba, Staphylococcus albus and aureus, Escherichia coli, a liquefying bacillus similar to the last, enterococcus of Thiercelin, a species of Corynebacterium, an organism similar to Corynebacterium pseudotuberculosis, Proteus vulgaris, Alcaligenes fecalis, and bacilli of the paratyphoid group. Tanner reviewed the microbiological literature up to 1931, but no investigation in this country is recorded since 1914.
Redfield made an extensive study of frozen eggs to secure data by which to determine whether such eggs were wholesome, and devised an ingenious formula to express this condition (see page 322).
Duck eggs. The German press has carried reports of serious illness from eating duck eggs. Investigation by the German ministry of health revealed that all the cases presented dysenteric symptoms. An examination of 1500 eggs showed positive cultures in the yolks of 7 eggs, indicating infection through the blood stream. Hens' eggs are not considered to be so dangerous because dysenteric disease in chickens may be caused by Salmonella gallinarum, which is non-pathogenic to man. People are cautioned to eat duck eggs only after thorough cooking, and never raw as in mayonnaise dressing.
Scott reports a fatal case of aertrycke septicemia after 13 days of gastroenteritis which began 6 hours after eating a fried duck egg from a flock whose eggs were found to be similarly infected. In one flock, S. aertrycke were found in the spleen, ovary, and intestinal con-tents, and in an egg that was removed from the oviduct. In another flock, 18 out of 46 gave serologic evidence of aertrycke infection, and at least laid aertrycke-infected eggs. He cites strong circumstantial evidence which points to duck eggs as causing 7 such outbreaks in the last 3 years. He thinks that duck eggs are particularly prone to carry this infection because of the filthy habits of the bird, and emphasizes the importance of suspecting an egg in solitary cases of food poisoning.
Invasion of organisms through the shell. The development of a musty odor in eggs has been traced to a specific organism, Achromobacter perolens. This microbe will not grow at the temperature of the fowl's body but does grow at much lower temperatures. This indicates that the infection must occur after the egg was laid, and it may occur in cold storage.
Molds frequently develop on eggs and penetrate through the pores into the interior. Sometimes these so-called mold spots can be seen inside the egg by means of the candling treatment. Tanner 43 has re-viewed the work of numerous investigators on the various molds found in eggs.
Bacterial infection may possibly occur before but certainly after the formation of the shell. The extent to which the former obtains is not as clearly attested as bacterial invasion through the shell and accompanying membranes, particularly after the egg is laid. A fresh egg is covered with a thin membrane (of protein and salts) which imparts the dull, velvety characteristic appearance associated with a fresh egg. It has generally been thought that washing removes the protective film, opening the pores of the shell to the passage of invading organisms, but Bryant and Sharp have shown that this ex-planation is not entirely true. It is the soiling of the shells, especially with fecal matter, and the storage of eggs in damp places, rather than washing or otherwise cleaning the shells, which facilitate microbic invasion. Under experimental conditions such pathogenic microorganisms as those of typhoid and paratyphoid fevers, cholera, enteritis, and botulism have penetrated the treated shell, but if the shells of eggs are kept clean and dry, there is little danger of bacterial invasion.
Deterioration. Eggs kept in storage begin to deteriorate in quality. These changes may be as follows: (1) loss of water from the egg by evaporation producing a loss in weight, (2) the change of the thick jellylike white which surrounds the yolk of the egg to a fluid condition, commonly called "watery whites," (3) the passage of the water from the white to the yolks, producing a thinning effect, (4) the weakening of the yolk membrane, causing the yolk to flatten out when the egg is broken, (5) the development of more or less undesirable flavors, and (6) the decomposition of the proteins and fat.
As Sharp pointed out, the determining factors in these changes are the action of microorganisms, the original quality of the egg, the temperatures of storage, entry of undesirable flavors and odors, loss of water, and change in the pH of the egg contents.
Loss of water. On account of the porosity of the shell, the egg loses moisture in storage. This is variable according to the temperature and other conditions but may amount to about 3.5 percent in 6 months. This drying out causes the egg contents to shrink and thereby enlarge the air cell. The size and rigidity of this cell is one of the measures of the quality of an egg as determined by candling.
Thinning of white and yolk. The pH (or hydrogen-ion concentration) of the whites of newly laid eggs has been shown by Sharp to be 7.6, and that of yolk about 6.0. As soon as the egg is laid, it begins to lose carbon dioxide, causing a rise in the pH value. Sharp uses the stiffness of the yolk to measure the "interior quality" of the egg 45 The ratio of the height of the yolk to the width is called the yolk index. For 59 fresh eggs, this index varies from 0.442 to 0.361. When the index falls to 0.25, the yolk usually breaks when the egg is opened. This flattening of the egg yolk is caused by the passage of the water into it from the white. When this condition has reduced the total solids of the yolk from the fresh-egg average of about 52 percent down to about 46 to 47 percent, the yolks break from the weakening of the membrane produced by the increase in the size and the change from a spherical to a flattened form. The chance in the pH can be overcome and the rate of liquefaction can be retarded by preventing the escape of carbon dioxide.
The extent of the deteriorative changes which cause the weakening of the membranes around the yolk and the decrease in the proportion of the thick white to the thin white is roughly a function of the length of the storage period and the methods of handling the eggs. Balls and Swenson state 46 that the thinning of the thick white of eggs in storage is caused by a tryptic proteinase, and that eggs possess in themselves a proteolytic system which produces changes in their structure and composition, whether or not invading microorganisms are present. The proteinase in the thick white may be largely inactivated by the inhibiting substance in the thin white when the two portions are mixed. However, Sharp does not find that this sterile deterioration is explained on the tryptic basis.
Flavors. Eggs take on a "cold-storage taste" after about the seventh month in the warehouse. This may come from the other stored commodities in the environment. Sharp and associates have shown 3 that the different egg-packing materials vary in their property to transmit flavors to eggs during storage, spruce fillers imparting least and strawboard fillers most flavor. Eggs of relatively high pH value, irrespective of the type of packing material used, developed more undesirable flavor during storage than eggs in which the pH value was low. Some workers in this field think that low humidity in the storage room with consequent drying out of the egg contributes to the development of some off-flavor.
Decomposition of the protein and fat. Pursuant to the work of Chapin and Powick, who showed that when eggs deteriorate there is a progressive increase in the ratio of inorganic to total phosphorus, Pine showed that the increase in the phosphoric acid alone would not be a complete measure of decomposition because the phospholipids decompose to liberate glycerophosphoric acid as well as inorganic phosphoric acid. The vitellin yields inorganic phosphoric acid, and the phospholipids liberate glycerophosphoric and inorganic phosphoric acids. These observations were confirmed by the striking relation he found between this "acid-soluble" phosphoric acid and progressive deterioration or spoilage in eggs.
The ether extract of the yolk decreases, the ammoniacal nitrogen may increase, and the acid-soluble phosphorus may increase in eggs which are undergoing deterioration.
Lythgoe studied the deterioration of shell eggs in storage and worked out analytical data to indicate the age of eggs. He found that when they are aged, there is a reduction in their dextrose content, and an increase in the acidity of the fat and in the production of ammonia, mostly in the yolk. For broken-out eggs, the fat determination is essential for the correct interpretation of an analysis.
Calloway states that there is no known method for the determination of the decomposition of dried eggs after manufacture.
Factors contributing to deterioration. A large amount of research has shown that the action of microorganisms contributes materially to the deterioration of eggs, There is some indication that these microbes gain entrance before the shell is formed; it is well known that they can penetrate the shell, especially when it is wet and dirty. The egg contents are a rich medium for the growth of many kinds of micro-organisms. If the eggs are not kept cold, these germs will grow, and their metabolic processes will lower the quality of the egg.
The work of several investigators, especially Rettger has shown that freshly laid eggs are probably sterile, although some of the earlier work does not indicate this so clearly as that by later improved technic. It is surprising to observe that normal, clean, fresh eggs may be incubated for several weeks without bacterial spoilage. The original sterility of the egg, the protection against microbial invasion by the impervious shell, and the bactericidal property of the white all contribute to maintain the egg free from bacterial decomposition. However, when the eggs are mishandled, the work of Pennington and her associates showed 50, 51 that there is a great increase in the microbic content and also in the percentage of ammoniacal nitrogen. For example, they showed that the percentage of the samples opened commercially which contained more than a million organisms per gram.
Pennington and her associates 51 have shown that deterioration in the quality of eggs is accompanied by changes in their composition, microbic content, flavor, and appearance by candling. Likewise, Stiles and Bates 52 showed that eggs which were designated as "undersized," "cracks," "dirties," and "weak eggs," and also frozen second grades, contained a much smaller number of bacteria than frozen "light spots," "heavy spots," "blood rings," and "rots"; and in both groups, when dried, the counts were much higher. Tanner gives a good review of the work in this field.
The growth of microorganisms produces enzymes which decompose the constituents of the eggs along the following lines .9 The proteins break down to form loosely bound ammonia, and indol and skatol. The carbohydrates decompose, as measured by the decrease in the copper-reducing substances. The fat is split, as indicated by the increase in the acidity of the ether extract. Lecithin is broken down, as revealed by the increase in the acid-soluble phosphoric acid and also by the possible increase in the acidity of the ether extract. It is probable that this decomposition is also caused by enzymes in the eggs because it occurs in storage eggs which are free from bacteria. At the beginning of storage, ammoniacal nitrogen may amount to about 0.0011-0.0015 percent on the wet basis, increasing to 0.0023-0.0028 in 7 1/2 months, and to 0.0030-0.0033 in 7 1/2 to 11 months.
Deterioration is more rapid during the first few months, and may be about equal for clean, dirty, and washed eggs, although, in general, dirty and improperly washed eggs spoil more quickly in storage than clean eggs. Spring eggs are usually in better condition for storage than summer eggs, probably because of the deteriorating effect of the heat before the eggs are chilled.
These deteriorative changes in structure and composition are accompanied by a lowering of the palatability. Laboratory methods have been devised 49 to indicate whether eggs are fresh or have been treated by the more common methods of processing and preservation.
Preservation. All the deteriorative changes discussed above can be greatly retarded and held below the levels of organoleptic perception, or even substantially entirely prevented, by selecting eggs that are clean to start with, protected from becoming heated, low in content of bacteria and other microorganisms, cold stored at proper temperature and humidity, protected from exposure to undesirable odors, kept in an atmosphere with a controlled content of carbon dioxide, or oil dipped to close the pores of the shells against loss of water and escape of carbon dioxide. Pennington has shown that the flavor of properly stored eggs may actually be higher than that of commercial fresh eggs.
Waterglass method. The sterility of the contents of the normal, fresh egg, the protective effect of the shell, and the bactericidal property of the white explain why the household method of preserving eggs in waterglass is so effective. Rettger states ' that eggs in good fresh condition can be kept in waterglass for many months without showing the least sign of decomposition. They retain their normal odor and palatability, and can be prepared for the table in any of the usual ways. The waterglass itself has antiseptic properties, and furthermore seals the pores. Even very low storage temperature is not necessary. There is no evidence that fertilized eggs spoil or undergo natural bacterial decomposition more readily than the unfertilized eggs.
Standards. The conditions under which eggs are produced, handled, processed, and stored have resulted in the marketing of eggs under such a wide range of organoleptic (taste and flavor) quality that it has be-come necessary to establish grades to facilitate their orderly marketing. This enables the producer to receive compensation commensurate with the quality of his product, and also protects the consumer from being sold a grade below the quality expected. Canada was the first country to formulate egg standards systematically, and practice in this country follows closely the Canadian standards. Egg quality standards are listed in 18 countries; they are fairly uniform in items selected as basis for grading, such as the depth of the air cell and the visibility of the yolk. The development of the art of candling, sup-ported by the accumulated knowledge of the chemistry and bacteriology of eggs, has enabled the U. S. Department of Agriculture to draw up standards for shell eggs. These are summarized in Table XXVII.
There is considerable agitation to express quality of eggs on the basis of what it should be when broken out instead of how it looks before the candle. A committee of the Institute of Poultry Industry is preparing a recommendation favoring this more logical method.
Types of deterioration and adulteration. Shell eggs may be spoiled by heating or incubation, by infection with microorganisms through cracked or porous shells, by mold infections from leaky eggs or other sources, and by deterioration from holding for long periods in storage. Attempts are sometimes made to sell incubator-reject eggs. Frozen eggs may contain some spoiled eggs, or may have deteriorated before freezing, and may contain undeclared preservatives or artificial coloring.
Physical tests. The most widely used physical test for the examination of eggs is the candling procedure (see page 000). The size of the air cell is usually interpreted as indicating the age of the egg, but this evidence is vitiated by storage and processing practices which minimize evaporation of moisture.
Sharp showed that the interior quality of eggs can be determined by the measurement of the yolk index, namely, the ratio of its height to its width (see page 316).
A sensitive method for detecting flavor defects is to heat an egg in bubbling hot water for 3 minutes, stand the blunt end up, quickly slice off enough shell to expose the top of the yolk, and with a small spoon remove enough yolk to taste.
Chemical tests. Sampling. Liquid eggs are sampled by with-drawing about 300 grams in a long-handle dipper, and sealing the sample in a jar kept in a cool place. Frozen eggs are sampled by boring holes extending close to the bottom, and then warming on a bath below 50° and mixing well before analysis. Dried eggs are sampled by scooping small quantities from a depth of about 6 inches below the surface to make a composite sample of about 500 grams, and mixing by passing through a domestic flour sieve to break up the lumps.
Total solids. After most of the moisture is driven off by heating the sample of liquid eggs on a steam bath, drying is completed by heating in a vacuum oven at 98° to 100°-C. for 5 hours, transferring to a freshly charged desiccator, cooling, and weighing.
Ammoniacal nitrogen (for liquid eggs). A sample of 25 grams is transferred to an aeration cylinder, certain reagents added, the train aerated through 10 milliliters of 0.02 N sulphuric acid, and the excess of acid titrated back with 0.02 N sodium hydroxide. See also Thomas and Van Hauwaert.
Acidity of ether extract. Dried eggs. The ether extract of the sample is dissolved in 50 milliliters of benzene and titrated with 0.05 N sodium ethylate solution and phenolphthalein. The results are ex-pressed as the number of milliliters of N/20 solution of sodium ethylate per gram of ether extract.
Liquid eggs. A sample of about 5 grams is extracted with anhydrous ether and analyzed as above.
Determination of decomposition. Redfield detected decomposition 39 in eggs by determining the ammoniacal nitrogen, the ether ex-tract, the acidity, the reducing sugars, the solids not fat, and other factors. He gave a numerical value to each of these determinations, so that, when their summation gave a figure less than 0, he considered the product to be edible whereas a figure above 0 classed it as un-wholesome.
The work of Pennington and her associates (see page 318) showed that the determination of ammoniacal nitrogen measures deterioration or spoilage. This method is not applicable to dried eggs because the ammonia is lost during drying.
The content of acid-soluble phosphoric acid" measures decomposition by reason of the property of dilute hydrochloric acid to extract phosphoric acid (P2O5) which is liberated by decomposition of vitellin and the phospholipids. The phosphoric acid is determined by the official gravimetric method. Calloway states that this method is applicable to the liquid as well as the dried eggs, and is indicative of the condition of the liquid eggs before drying, so far as the decomposition of the lecithin and related substances is concerned.
Detection of washed eggs. Sharp's method of recognizing washed eggs is based on the observation that the material on the surface of the shell contains soluble potassium and chlorine compounds and also albumen which can be leached and determined by microchemical tests with cobalt nitrite, silver nitrate, and Millon's reagent, respectively. Eggs washed with soap solutions can be easily recognized by their feel.
Detection of oil-treated eggs. Oiled eggs were detected by Sharp simply by dipping the end of the egg in ethyl ether for a second. An oily ring is seen at the edge of the ether-dipped part.
Detection of abrased eggs. Scraped eggs are detected by immersing them in an aqueous solution of rosaniline hydrochloride which colors the unscraped portion of the shells pink or purplish red. Sanded eggs are indicated by the potassium and the chlorine tests of the shell extract, and also by staining, because most of the protein on the surface of the shell is removed.
Analytical methods for the determination of glycerol, sugar, and salt in egg yolks are reported by Guthmann and Terre.
Bacteriological tests. Shell eggs. The bacterial contamination of shell eggs can be simply detected by streaking a loop of material from the yolk onto an agar plate. When this is incubated, it shows the bacteria to be present either in large numbers or not at all.
Frozen egg yolks. The sample is thawed by warming it at 37° C. on a water bath, diluting, and plating it on standard nutrient agar, like milk, and incubating at 20° C. for five days. Esch. coli is determined by dilution in lactose broth tubes, and partially identifying it on E.M.B. plates.
Powdered egg. A 1-gram sample is transferred to a 99-milliliter water blank, and plated on standard nutrient agar in Petri dishes as described for milk.
Eggs should be handled under strictly sanitary conditions. They are highly perishable, are readily attacked by microorganisms, and are so delicately flavored that their quality is impaired by relatively slight factors. Insanitation need not be one of these.
Any endeavor to control production conditions is almost hopeless because of the very great number of farms, the irregularity of their egg supply, and the more or less incidental interest of the average farmer in egg production. However, the growth of the great egg-production ranches affords an opportunity for emphasis to be directed to starting with healthy flocks, clean nests, clean water, good food, sanitary equipment, adequate and cool storage, frequent shipment, and quality control from flock to consumer.
Candling, like any other work, can be performed more effectively when the operators are provided with clean, well-lighted, and convenient facilities for examining the quality of the eggs.
The egg-breaking room should be well-lighted, and all the equipment should be easily disassembled, cleaned, and sterilized. Table tops and belts should be made of stainless steel. Egg cups should not be made of glass because of breaking or chipping. Chairs or stools should be metal or enameled. Walls and floors should be made of non-absorbent material, and kept clean. The operators should wear hair-nets or caps, and preferably uniforms also. Their fingers and hands should be inspected daily. No operator with a communicable disease or a skin infection should handle any of the egg products. Every operator should be medically examined before employment. Adequate toilet and lavatory facilities should be provided, with ample and convenient means for washing and drying the hands, and signs should be prominently posted to encourage this practice. Locker rooms, sanitary plumbing, and adequate lighting, heating, and ventilating contribute to quality of product and health of employees. Bacteriological examination should be made of the equipment to insure proper cleaning and freedom from accumulations of microorganisms or putrefactive products.
Cold-storage warehouses should likewise be kept sanitary. Every year, between storage seasons, the premises should be thoroughly cleaned.
The frozen and dried egg products themselves should be examined physically for organoleptic quality, and chemically and bacteriologically for freedom from content of unwholesome eggs, as well as for preservatives, added color, or other foreign ingredients.
1. R. L. BRYANT and P. F. SHARP, J. Agr. Research, 48, 67 (1934).
2. M. K. JENKINS and M. E. PENNINGTON, U. S. Dept. Agr. Bul. 775, 1919; superseded by T. W. HEITZ, U. S. Dept. Agr. Circular 73, 1929.
3. P. F. SHARP, G. F. STEWART, and J. C. HUTTON, Cornell University Agr. Exp. Sta. Memoir 189, 1936.
4. P. F. SHARP, Science, 69, 278 (1929) ; and C. K. PowELL, Ind. Eng. Chem., 23, 196 (1931).
5. T. MORAN, Food Research, 3, 149 (1938).
6. L. H. JAMES and T. L. SWENSON, Year Book of Agriculture, 1932, p. 184.
7. L. D. OvsoN, Food Industries, 5, 502 (1933).
8. P. F. SHARP, Food Research, 2, 477 (1937).
9. C. G. BLOMBERG, Food Industries, 4, 100 (1932).
10. W. D. TERMOHLEN and associates, Agr. Adjustment Adm., U. S. Dept. Agr., 1938.
11. L. C. MITCHELL, J. Assoc. 0ffic. Agr. Chemists, 15, 310 (1932).
12. L. C. MITCHELL and associates, ibid., 16, 247 (1933).
13. L. C. MITCHELL, ibid., 17, 506 (1934).
14. J. S. HEPBURN and A. B. KATZ, J. Franklin Inst., 203, 835 (1927).
15. M. K. JENKINS and associates, Ice and Refrigeration, 58, 140 (1920).
16. M. SORENSEN, quoted from P. E. HOwE and H. W. TITUS, U. S. Egg & Poultry Mag., 43, 534 (1937).
17. M. S. ROSE and E. M. VAHLTEICH, J. Am. Dietet. Assoc., 14, 593 (1938).
18. W. C. RussELL, N. J. Agr. Exp. Sta. Circular 332, 1934.
19. M. S. RosE and G. M. BORGESON, Child Nutrition on a Low-Priced Diet, Bureau of Publications, Teachers College, Columbia University, New York, 1935, from Reference 17.
20. M. S. ROSE and G. MACLEOD, J. Biol. Chem., 50, 83 (1922) ; 58, 369 (1923-4), from Reference 17.
21. Reference 17, p. 604.
22. D. B. JONES, J. C. MURPHY, and O. MCELLER, Am. J. Physiol., 71, 265 (1925).
23. S. M. HAUGE and C. W. CARRICK, J. Biol. Chem., 64, 111 (1935).
24. A. F. MORGAN and M. J. HUNT, Cereal Chem., 12, 411 (1935).
25. H. CHICK and associates, Biochem. J., 24, 1748 (1930).
26. F. S. SMYT= and associates, J. Am. Med. Assoc., 97, 1291 (1931).
27. M. W. EMMEL, Florida Agr. Exp. Sta. But. 293, 1936, quoted from Year Book, American Public Health Association 1937-1938, p. 64.
28. "Committee Report," L. H. James, Chairman, Year Book, American Public Health Association, 1937-1938, p. 56.
29. L. F. RETTGER and associates, J. Exptl. Med., 23, 475 (1916).
30. H. ILZHbFER and C. MULLER, Arch. Hyg. Bakteriol., 114, 341 (1935), quoted from Nutrition Abs. Rev., 5, 757 (1935-6).
31. J. R. MOHLER and H. J. WASHBURN, 25th Ann. Rept. Bur. Animal Industry, 1908, p. 165.
32. C. P. FITCH and R. E. LUBBEHUSEN, J. Am. Vet. Med. Assoc., 72, N. S. 25, 636 (1927); ibid., 66, 43 (1924).
33. H. RAEBIGER, Beit. Klinik Tuberculose, Berlin, 71, 159 (1929), quoted from J. Am. Med. Assôc., 92, 1898 (1929).
34. W. H. FELDMAN, Avian Tuberculosis Infections, Williams and Wilkins Co., Baltimore, Md., 1939.
35. F. W. TANNER, Food-borne Infections and Intoxications, Twin City Printing Co., Champaign, Ill., 1933, p. 288.
36. S. LICHTENSTEIN, Z. Tuberculose, 64, 239 (1932), quoted from J. Am. Med. Assoc., 99, 353 (1932).
37. E. 0. JORDAN, Food Poisoning and Food-borne Infections, University of Chicago Press, Chicago, Ill., 2d ed., 1931, p. 124.
3S. J. VERGE and E. GRASSET, Rev. Hyg. Med. Prev., 50, 748 (1928), quoted from Chem. Abs., 23, 448 (1929).
39. H. W. REDFIELD, U. S. Dept. Agr. Bul. 846, 1920.
40. Anon., J. Am. Med. Assoc., 106, 719 (1936).
41. W. M. SCOTT, ibid., 99, 1274 (1932).
42. A. W. TURNER, Austral. J. Exptl. Biol. Med. Sci., 4, 57 (1927). See also M. P. SPANSwICK, Am. J. Pub. Health, 20, 73 (1930).
43. F. W. TANNER, The Microbiology of Foods, Twin City Printing Co., Champaign, Ill., 1932, p. 501.
44. L. F. RETTGER, Conn. (Storrs) Agr. Exp. Sta. Bul. 75, 1913.
45. P. F. SHARP and C. K. POWELL, Ind. Eng. Chem., 22, 908 (1930).
46. A. K. BALLS and T. L. SWENSON, ibid., 26, 570 (1934).
47. L. PINE, J. Assoc. Offic. Agr. Chemists, 8, 57 (1924).
48. H. C. LYTHGOE, Ind. Eng. Chem., 19, 922 (1927).
49. J. CALLOWAY, JR., J. Assoc. Offic. Agr. Chemists, 19, 201 (1936).
50. M. E. PENNINGTON, J. Biol. Chem., 7, 109 (1910) ; M. E. PENNINGTON and associates, U. S. Dept. Agr. Professional Paper 51, 1914.
51. M. E. PENNINGTON, U. S. Egg and Poultry Mag., Sept. 28, 1932.
52. G. W. STILES and C. BATES, U. S. Dept. Agr. Bur. Chem. Bul. 148.
53. "Foreign and Domestic Egg Standards," Foreign Crops and Markets (Bur. Agr. Economics) 22, 775 (1931); also, Handbook of Official United States Standards for Individual Eggs, U. S. Bur. Agr. Economics, Issued June, 1935.
54. Methods of Analysis, Assoc. Offic. Agr. Chemists, Washington, D. C., 4th ed., 1935, p. 297.
55. A. W. THOMAS and M. VAN HAUWAERT, Ind. Eng. Chem., Anal. Ed., 6, 338 (1934).
56. P. F. SHARP, Ind. Eng. Chem., 24, 941 (1932).
57. W. S. GUTHMANN and W. L. TERRE, Ind. Eng. Chem., Anal. Ed., 8, 377 (1936).