Humans are omnivorous and have consumed both animals and plants as foods throughout recorded history. However, before animals, birds, and fish can provide meat, eggs, or milk, their own physiological requirements for energy and synthesis must be satisfied. These requirements are met largely through the consumption of plant materials, which, if consumed directly by humans, could support a greater population than can the animal products derived from them. This is true with respect to total available calories, protein, and other nutrients needed to sustain life. In most cases, the amount of animal products consumed by a society is positively correlated with the affluence of the society.
Most human societies have preferred animal foods and have been willing to expend the greater effort generally required to satisfy this appetite when possible. In agriculturally advanced societies, it is possible to convert gain into meat (live weight basis) at rates of about 2 kg/kg chicken, 4kg/kg pork, and 8 kg/kg beef, although grass and forage crops also go into the feeding of beef. These conversion rations are in part responsible for the relative prices of food.
Foods from animal products (including fish, which is discussed in Chapter 15) represent concentrated sources of many of the nutrients required by humans. This is to be expected since our tissues and body fluids are similar to their counterparts in other animals with respect to the elements and compounds they contain. Although it is probably true that we could supply all our nutritional needs directly from plant sources, this would require the consumption of a sizable number of plant types and a sophisticated knowledge of nutrition if no animal products such as dairy foods and eggs were included in the diet. This would be especially so with respect to meeting the requirements for essential amino acids, vitamins, and minerals. Moreover, farm animals convert large quantities plant roughage materials unsuited for human consumption into human food. For example, humans cannot digest cellulose and, therefore, can derive no energy directly from it. However, ruminants such as cows can digest cellulose and turn it into foods (i.e., milk) which are useful to humans.
MEAT AND MEAT PRODUCTS
Meat and meat products generally are understood to include the skeletal muscles of animals; also included are the glands and organs of these animals (tongue, liver, heart, kidneys, brain, and so on). In a broader sense, meat also includes the flesh of poultry and fish, but these are generally considered separate from the red meats of terrestrial animals. In the United States, the principal sources of meat are cattle (beef), calves (veal), hogs (hams, pork, and bacon), sheep (mutton), and young sheep (lamb). Other societies consume different animals, including dogs, kangaroos, reindeer, and reptiles.
Meat products also include many by-products from animal slaughter: animal intestine for sausage casing; fat which is rendered into tallow and lard; hides and wool; and hormones used by the food, pharmaceutical, and other industries. For this reason, the major meat processing companies are seldom in a single business but usually produce a wide variety of products.
Government Surveillance
Essential to the meat industry are two kinds of government surveillance: grading and meat inspection. It is important to understand that meat and poultry inspection in the United States is a mandatory program for all products in interstate shipment as well as products used within the states. Inspection is primarily concerned with health and safety matters. Grading is voluntary and is undertaken to inform consumers about the quality of meats and poultry.
Grading
The need for grading is clear. Like all natural products, meat is very heterogeneous and varies. Animal carcasses are of all sizes, from many breeds, of varying ages, and have been fed on many different kinds of feeds. These factors result in cuts of meat varying in yield, tenderness, flavor, cookout losses, and general overall quality. A system of grading is essential to ensure that the wholesale buyer and ultimately the retail customer get what they pay for.
Quality grades are based on subjective evaluations of three main factors: carcass maturity, degree of fat marbling, and muscle firmness. Color is also considered. Maturity relates to tenderness; younger carcasses typically are more tender than those from older animals. Marbling is the deposition of fa within the lean muscle. Such intramuscular fat increases tenderness and palatability. In addition, lean meat should have a certain degree of firmness. Overly soft muscle is downgraded. Meat which is excessively dark when cut indicates possible stress on the animal prior to slaughter. Beef grades, in order of decreasing quality, are Prime, Choice, Select, Standard, Commercial, Utility, Cutter, and Canner. The grades, however, have very little relationship to the nutritional value of the cuts, except where one may wish to limit fat intake.
Of course, grades cannot be assigned until the animal has been slaughtered. A recently developed technique that may influence future grading and the purchase price paid for animals involves the use of ultrasonic energy to reveal the gross structure of meat prior to animal slaughter. Meat, fat, and bone reflect ultrasonic energy differently. By radiating such energy over the body of a live animal and recording the reflected energy pattern, it is possible to develop an X-ray-like cross-sectional view of portions of the animal carcass. In this way purchases of live meat-yielding animals can be made more efficient in terms of intended end use than is now possible.
Meat Inspection for Wholesomeness
Federal employees inspect all meat going into interstate commerce, in accordance with the Federal Meat Inspection Act of 1906, to ensure a clean, wholesome, disease free meat supply that is without adulteration. Unlike USDA grading practices, which are optional, inspection for wholesomeness is mandatory and is administered by USDA’s Food Safety and inspection Service (FSIS).
If animals are diseased, the meat can carry a wide variety of organisms pathogenic to humans. These may include organisms capable of causing tuberculosis, brucellosis, anthrax, trichinosis and salmonellosis. There are some 70 such diseases that animals can transmit to man. For this reason, inspections are made by veterinarians or persons under their supervision at places of animal slaughter and at meat processing facilities. A federal law enacted in 1967 requires that all states adopt and enforce meat inspection practices at least comparable in thoroughness to the federal meat inspection laws without regard to where the meat will be shipped.
Slaughtering and Related Practices
There has been a law in the United States since 1958 that all animals coming under federal purchase must be rendered insensible to pain before being hoisted by their hind legs and bled. This practice has since been widely adopted. An exists in slaughtering according to religious ritual.
One common humane method of rendering an animal insensible in by striking it on the head with an air-or gunpowder-driven blunt or penetrating device. This has largely replaced stunning with a sledge hammer. Another method employs electric shock, and a third uses a tunnel filled with carbon dioxide through which the animal passes. Each of these various methods can differently affect blood hormone levels, muscle chemistry, and meat properties.
After stunning, hoisting, and bleeding, a modern slaughterhouse in an efficient continuous disassembly line. Virtually every component of the animal body is utilized, including the hide, viscera, blood, and carcass. The skinned, washed, and eviscerated carcass is then moved by monorail into a chill room, where the deepest part of the meat is chilled to 2C in about 36 h. this prevents rapid bacterial spoilage.
The practice of resting animals before slaughter can help delay bacterial spoilage of meat. Animal stored glycogen in their muscles as a source of reverse energy. After an animal is killed, this glycogen is converted under the anaerobic conditions in the muscles into lactic acid, which lowers the Ph. and act as a mild preservative. But if animals are excited or exercised before slaughter, the glycogen is largely consumed and there is very little left to be converted to lactic acid in the postmortem tissues. Such meat can spoil more quickly. Additional research has shown that ante mortem stress also can affect other carcass characteristics such as the defect of dark-cutting beef, and pale, soft, watery pork.
Structure and Composition of Meat
The gross structure of a cut of meat can be seen in Fig. 14.1. The dark areas are principal muscles and the shite areas are fat; however, microscopic observation is required to see the fine structure of the muscles. Figure 14.2 is a diagram of a longitudinal section of lean muscle showing that the muscle is composed of bundles of hair like muscle fibers. These protein muscle fibers and held together by proteinaceous connective tissue which merges to from a tendon which, in turn, connects the muscle to a bone. The muscle fibers themselves are elongated cells that contain many smaller highly oriented fibrils.
Figure 14.1
A major protein of muscle fiber is myosin. The connective tissue contains two proteins called collagen and elastin. Collagen on heating in the presence of moisture dissolves and yields gelatin.
Figure 14.2
Elastin is tougher and is a constituent of the ligaments. A cooked chicken leg nicely reveals the bundles of muscle fibers, the connective tissue between the bundles of muscle fibers, and the gelatinous substance in the connective tissue which is dissolved collagen.
When an animal is well fed, fa penetrates between the muscle fiber bundles; this is called fat marbling and makes muscle more tender. In addition, thinner muscle fibers are more tender than thicker muscle fibers and are more common in young animals. On cooking, muscle fibers contracted and may become together, but cooking also melts the fat, and dissolves the collagen into soluble gelatin, so the overall effect is increased tenderness.
The compositions of meat cuts will vary with the relative amounts of fat and lean, but a typical cut of beef may contain 60% water, 21% fat, 18% protein, and 1.0% ash. Compositions of the meats of other food animals, poultry, fish, and some milk products are given in Table 14.1 for comparison.
Ageing of Meat
Within a few hours after an animal is killed, rigor mortis sets in with a contraction of muscle fibers and an increasing toughness of the meat. This is correlated with the less of glycogen and disappearance of ATP from the muscles of newly killed animals. If the meat is held cool, rigor mortis subsides in about 2 days, the muscles become soft again, and there is a progressive tenderization of the meat over the next several weeks. The tenderization is believed to be due principally to natural proteolytic enzymes in the meat, which slowly break down the connective tissue between the muscle fibers as well as the muscle fibers themselves. The typical time course of meat tenderness during ageing dis shown in Fig. 14.3.
Figure 14.4 compares photomicrographs of raw beef freshly slaughtered and the same beef after cold storage for 6 days. The freshly slaughtered beef is characterized by compactness of muscle fibers; the separation between muscle fibers and breaks in the fibers of the stored beef is evident.
Ageing or ripening of meat generally is done at 2 C by hanging the carcass in a cold room anywhere from 1 to 4 weeks. The best flavor and the tenderness develop in about 2 -4 weeks. Humidity must be controlled and the meat may be covered with wrappings to minimize drying and weight loss. As pointed out in a previous chapter, ageing processes have been developed using higher temperatures for shorter times such as 20C for 48 h. Tenderness results, but bacterial slime also develops quickly on the meat at this high temperature. In commercial practice, ultraviolet light may be used to keep down bacterial surface growth during quick ageing at high temperature.
Because of the costs involved with ageing, not all beef is deliberately aged for increased tenderness and flavor before being shipped by meat packing companies. Further, some beef that is used for sausage manufacture may not be aged or even cooled following slaughter. So-called “hot beef” that has not yet passed through complete rigor mortis has superior water holding characteristics compared with cold stored beef, a desirable property in the making of sausage meat emulsions. The superior water holding capacity of hot beef can be retained also for later use if the beef is rapidly frozen before rigor has had time to subside.
Artificial Tenderizing
Cold room storage results in ageing, or ripening, of the meat with tenderizing from the meat’s natural enzymes. There are several artificial means of tenderizing meat to various degrees.
Meat may be tenderized by mechanical means. During cold room storage the carcass can be hung in a manner to stretch the muscles and thereby encourage elongated, thinner muscle fibers. Further tenderizing of meat cuts can be obtained by pounding, cutting, or separating and breaking meat fibers with ultrasonic vibrations.
Meat may be tenderized somewhat by the use of low levels of salt, which solubilizes meat proteins. Salt is hygroscopic. Therefore, if salt is placed within the meat (e.g., ground hamburger), it holds water within the mass; if it is placed on the surface of the meat, it draws moisture out of the mass to the surface. Phosphate salts may be even more effective than common table salt in tenderizing meat, and either may be blended into ground meat or diffused into the flesh of fish, poultry, or meats to help retain juices and minimize bleeding or drip losses.
Another artificial tenderizing method involves the addition of proteolytic enzymes to the meat, such as bromelain from pineapple, ficin from figs, trypsin from pancreas, or papain from papaya. The native practice in tropical countries of wrapping meat in papaya leaves before cooking results in this kind of tenderization. Enzymes may be applied to meat surfaces, but penetration is slow; injection into the meat or into the bloodstream of the living animal before slaughter is more effective for large cuts. If this is done, then cold rom ageing time is markedly reduced. Tenderizing enzymes function before cooking and during the cooking operation until the meat temperature reaches about 82C; then they become heat inactivated.
Electrical stimulation of carcasses following slaughter is the newest commercial method of mea tenderization, although tenderizing effects on poultry killed electrically were noted by such early observers as Benjamin Franklin. Electrical tenderization involves application of sufficient voltage to cause rapid muscle contractions. There associated with changes in levels of glycogen, ATP, lactic acid, Ph. and enzyme activity. Through mechanisms not yet well understood. Impulses of about 100 -600 V over 1-2 min, given within about 45 min of slaughter, not only increase beef tenderness but are reported to improve lean meat color, texture, flavor, and to accelerate subsequent ageing. Both manual and automatic continuous electrical stimulation equipment have come into commercial use within the past decade.
Curing of Meat
Whereas ageing or ripening by cold room storage and tenderizing by artificial methods have as their prime objective increased tenderness, the curing of meat is a different process and has additional objectives. Curing refers to modifications of the meat that affect preservation, flavor, color, and tenderness due to added curing ingredients. Proper ageing still leaves the meat recognizable as a fresh cut, but curing ingredients. Proper ageing still leaves the meat recognizable as a fresh cut, but curing is designed to grossly alter the nature of the meat and produce distinct products such as smoked and salted bacon, ham, corned beef, and highly flavored sausages including bologna and frankfurters.
Originally, curing treatments were practiced as a means preserving meat before the days of refrigeration, and curing goes back at least to 1500 B.C. In less developed areas without modern preservation facilities, the prime objective of curing is still preservation. But where more effective preservation methods are available, the prime purpose of curing is to produce unique flavored meat products; a secondary purpose is to preserve the red color of meat after cooking. Thus, cured corned beef when cooked remains red, whereas beef that is not cured turns brown on cooking. Similarly, cured ham retains its red color through cooking, but uncured pork becomes brown.
The principal ingredients used for curing or pickling meat are:
1. Sodium chloride, which is a milk preservative and adds flavor.
2. Sodium nitrate and/or sodium nitrite, which help cured meats develop their unique flavor, act as preservatives and have anti botulinum activity, and fix the red color of cured meats.
3. Sugar, which helps stabilize color and also adds flavor; and
4. Spices, mainly for flavor.
Sodium nitrate and nitrite have received considerable attention with respect to their safety in this application.
These ingredients are available in commercial mixtures or may be formulated by the meat processor and may be applied to meat in dry form by rubbing on surfaces or mixing directly into ground meats. In the case of hams or corned beef, they may be applied as a wet cure or pickle, by soaking in vats. When the meat cut is large and penetration of the cure is slow, the cure may be pumped directly into the meat via an artery, as is common with large hams; or the cure may be injected with multiple needles into slabs of bacon bellies, as shown in Fig. 14.5.
Meat Pigments and Color Changes
An understanding of meat pigments and the changes they undergo is important in meat processing. These changes are chemically complex, and only a few general principles are discussed here and outlined in Fig. 14.6. The chief muscle pigment is a protein called myoglobin. The physiological function of myoglobin is to store oxygen in the five animal’s muscle. It has a purplish color when not carrying an oxygen molecule, but when exposed to oxygen, it becomes ox myoglobin, which has a bright cherry-red color. Thus, when fresh meat is first cut, it is purple in color, but its surface quickly becomes bright red upon exposure to air. Large cuts may be bright red on the surface but more purplish in the interior due to less oxygen within. The desirable bright red of ox myoglobin is not entirely stable; on prolonged exposure to air and excessive oxidation it can shift to meet myoglobin, which has a brown color.
Figure; 14.5
When fresh meat is cooked, these protein pigments are denatured and also produce a brown color. Steak cooked to a rare condition has less of the ox myoglobin denatured and is more red. Well-done meat is more denatured and is more brown. Meats cured with nitrites and red and remain red through cooking. Nitrate combines with myoglobin to produce nitric oxide myoglobin, which is pink, incurred meats. Nitric oxide myoglobin on cooking is converted to nitrosohemochrome, which is pink or red as in cooked ham and bacon and quite stable.
These pigments shift, some of which are reversible, are affected by oxygen, acidity of the meat, and exposure to light; and the combination determines which pigments will dominate. Within the normal pigment shifts, the color of meat does not indicate wholesomeness or nutritional value; however, red color is often used by consumers to judge the freshness of meat. For this reason, packaging films are designed to protect meat color, largely by controlling diffusion of oxygen.
In the case of fresh meat cuts, films are used that allow oxygen to penetrate and keep myoglobin in the bright red Sox myoglobin form. However, cured meats are affected differently by oxygen –the pink nitric oxide myoglobin can be oxidized to the brown met myoglobin. Thus, cured meats are generally vacuum packed to exclude air and wrapped in films which are a high barrier to oxygen. Fresh or cured meats also can develop brown, yellow, and green discolorations from bacterial growth.
Smoking of Meats
Following curing, processed meats are often smoked. Smoking also was originally employed as a mild preservative, but today smoking is used mostly for its flavor contribution. At one time, smoking was done primarily in large smokehouses by hanging the meat over burning hardwood logs or wood chips; hickory smoke was preferred for flavor. If a smoke room is used, it should be at about 57 C to give the meat an internal temperature of about 52C; smoking may take from 18 to 24 h. this is satisfactory in the case of pork products if the meat is cooked before smoking or will be cooked afterward. If, however, the meat is to be a ready-to-eat product without additional heat, then smoking must bring pork products to an internal temperature of 58 C or higher to ensure destruction of the trichinosis parasite; this procedure is required by the federal meat inspection laws. In Fig. 14.7, hams are being removed from a typical cabinet-type smokehouse.
Today there are several ways to generate smoke remotely and then circulate it into a smoke room or smoke tunnel (Fig 14.8). In addition, smoke can be generated in a special device without fire by high-speed frictional contract with the wood. The smoke can be given an electric charge and electrostatically deposited onto the meat surface.
Figure; 14.7
There also are solutions of the flavor chemicals (i.e., liquid smoke) which have been extracted from smoke and are applied without the direct use of smoke.
Sausages and Table-Ready Meats
Cured meats especially, and uncured meats to a lesser extent, find their way into enormous quantities of sausage products. There are over 200 kinds of sausage products sold in the United States, the most popular of which are frankfurters. Most have their origins in countries outside the Unites States.
Classification of sausage types is confusing but generally takes into account whether the ground meat is fresh or cured and whether the sausage is cooked or uncooked, smoked or unsmoked, dried or not, and fermented. Examples would be frankfurters –which generally are mildly cured, cooked, and smoked; fresh pork sausage –which is not cooked, smoked, or cured in manufacture; and Italian salami –which is cured, fermented, and dried.
Figure; 14.8
Many sausages are prepared within a casing. Natural casings are made from cleaned animal intestines; the different sizes being used for different types of sausages. But natural casings are expensive and no uniform and so artificial casings are more important. These casings are extruded tubes of regenerated collagen, cellulosic materials, or plastic films. The casings hold the ground meat together and prevent excessive moisture and fat losses during cooking and smoking operations. Large sausages such as bologna may have the casing removed after cooking and smoking, and then be sliced and packaged. Such products are known as table-ready meats.
Frankfurter Manufacture
The most important sausage product in the United States is frankfurters, and except for size; tis production method is the same as for bologna. Generally, franks are made from finely ground cured beef, which is referred to as a frankfurter meat emulsion. The emulsion is pumped into great lengths of artificial casing which is automatically twisted every 6 in, to form links (Fig. 14.9). Following this procedure, the links are cooked by passing through hot water or steam and then hung for smoking, or smoking may precede the final cook.
“Skinless” franks are made in casings which are designed to be removed after cooking and smoking. Then after smoking, the casing is mechanically peeled from the now congealed form, which had its shape set within the casing during cooking.
These mechanical operations, which today dominate the frankfurter industry, are somewhat cumbersome, and so new continuous frankfurter processes have been developed. One is the Tender Frank Process of Swift and Company in which no casings are used. In a continuous fashion the meat emulsion is injected into frank-shaped molds where it is coagulated with electronically generated heat. The ranks are then conveyed through a tunnel in which they are smoked and then cooled, and from which they emerge for packaging. Another process that recently has been commercialized in Europe utilizes co extrusion (Fig. 14.10) to continuously form casing from a premixed collagen dough as meat emulsion simultaneously is extruded into the casing.
Freezing of Meat
Meat may be frozen and held in frozen storage for months in the case of pork and fatty meats, and for years in the case of beef. Storage time for pork and fatty meats is limited by gradual development of oxidized fat flavors. As with other frozen foods, to assure quality demand that the meat be quick frozen and then not thawed and refrozen to avoid excessive bleeding and drip when the product is finally thawed and cooked.
Figure 14.9 and 14.10
Few cured meats or sausages are commercially frozen since salt in their formulation increases the rate of fat oxidation and development of rancid flavors. Further, frozen storage tends to alter the flavor of the seasoning spices used in many sausage products.
Properly packaged fresh meat cuts store well in frozen condition but are not popular in supermarkets because customers like to see and feel frost-free meats and associate quality with unfrozen bright red cuts. They then take the meat home and often store it in a freezer. Restaurant operators use a great deal of frozen meat, which the customer does not see, and the military buys great quantities, some of which is imported from Australia and New Zealand.
Freezing temperatures can be used to destroy the trichinosis parasite in pork products. As was stated earlier, smoking or cooking to a uniform internal temperature of 58c ensures destruction of the larvae of this organism. The USDA recommends that previously unheated pork products should be cooked by consumers to a uniform internal temperature of 77 C, followed by a short dwell time. This procedure provides a margin of safety, especially with fast cooking methods which may not provide uniform heating and the lethality of prolonged temperature and time indicated in Table 14.2 also destroys the trichinosis parasite and is another recommended treatment by the USDA to render pork products safe. Concern over treatments for the safety of pork products is due in part to the fact that normal meat inspection practices are unable to detect the presence of trichina organisms with certainty.
Storage of Fresh Meat
Meat cutting in supermarkets is labor consuming, and as the cost of labor continues to increase, ways to reduce these costs are sought. Deboned and trimmed meat, in the unfrozen state, offers cost savings in labor and transportation and requires less energy for storage then does frozen meat. This has resulted in a shift to more centralized meat cutting at packing plants. These central processing lines reduce the size of the carcass to smaller sections called primal and sub primal cuts which may represent one-fourth to one-eighth of a whole carcass. The cuts are vacuum packaged in high barrier bags and paperboard boxes for shipment to retailers, where they are further reduced to retail sized cuts. They are then repackaged in the familiar tray and overwrap packages.
Beef and pork may be vacuum packed in a high brier film and stored at 0C (unfrozen) up to about 3 weeks. Such meat, acquires the purplish color of myoglobin at its cut surfaces. When the film is removed; the myoglobin reoxygenates to ox myoglobin and the meat reddens. Such meat is further cut into retail portions and wrapped in an oxygen-permeable film for consumer purchase. It also has been proposed that consumer cuts be centrally prepared and wrapped in oxygen-impermeable overwrap. When the latter is removed, the individual units would redden and be ready for direct sale.
Most recently, new technologies have been developed which are designed to allow centralized processing in the retail cuts, thus eliminating entirely the need for further processing in the retail store. The most successful technology is called Modified Atmosphere Packaging (MAP). MAP utilizes sealed high barrier packages in which the air has been replaced with a mixture of gases which will reduce the rate of deterioration of the meat. Most often these gases include 10-50% carbon dioxide which inhibits the growth of many microorganisms which cause spoilage of refrigerated meats. For fresh red meats the gas mixture often contains 20-50% oxygen so that the myoglobin will be in the oxygenated cherry-red form. The meats must be sealed in high barrier films which will keep the air out and prevent the modified atmosphere from escaping.
Cooking of Meat
Cooking can make meat more or less tender than the original raw cut. When meat is cooked, there are three tenderizing influences: fat melts and contributes to tenderness; connective collagen dissolves in the hot liquids and becomes soft gelatin; and muscle fibers separate and the tissue becomes more tender. There also are two toughening influences: overheating can cause the muscle fibers to contract and the meat to shrink and become tougher; and moisture evaporates and the dried out tissue becomes tougher. Generally, lower cooking temperatures for a longer period of time to any given degree of doneness. But this also depends on the meat cut, which can vary considerably, as seen for beef in Fig. 14.11. Depending on the cooking methods, the relationship between cooking temperature and tenderness can become quite complicated, especially with the newer methods of microwave, dielectric, and infrared heating, and much research remains to be done.
Figure 14.11
The nutritional value of cooked meat generally remains very high. Normal cooking procedures do little to change the high value of the meat proteins, and minerals are heat resistant. Some minerals re lost in meat drippings, but on the other hand, cooking dissolves some calcium from bone and so enriches the meat in this mineral. The B vitamins are heat sensitive and so cooking to the rare condition. Even to the well-done stage, most meats retain about 70% of the B vitamins present in the uncooked meat Advances have been made in the production of beef and pork of lower fat content via improved breeding and feeding of livestock in response to the growing demand for lower-fat products.
POULTRY
In the United States, the principal types of poultry are chickens, turkeys, ducks, and geese, and the amounts consumed are in that order. In other parts of the world, different types of fowl are consumed, such as emu and ostriches. Poultry is raised for meat and for eggs. We will first consider poultry for meat and confine discussion to chicken since much of the same technology applies to the other types of birds.
Production Considerations
In the past, most chicken meat came from egg-laying birds whose ability to lay eggs had declined, but, today, to satisfy the demand for meat, special genetic strains are produced that exhibit rapid growth, disease resistance, and good meat qualities such as tenderness and flavor. Chicken breeds include types with white feathers and types with brown and black feathers. The white-feathered types are favored for meat broilers because of the absence of dark pinfeathers.
Broiler farms are often quite large and raise several million birds per year. Chicks are highly susceptible to many diseases and so broiler producers must practice rigid husbandry with respect to bird housing, temperature and humidity control, sanitation, and feeding practices.
A remarkable achievement of breeders, poultry nutritionists, and feed manufacturers is that, with advanced technology, it is common to raise a 2.3-kg (5-lb) broiler in just 6 weeks with a feed conversion of 1.8 kg of feed per kg of bird. In other words, a 2.3-kg broiler is raised from a chick on just about 4.1 kg of feed. This is one reason why chicken may be purchased at a lower price on an edible weight basis than beef, which has a less efficient feed conversion ratio.
The market classification of poultry is generally based on age and live weight. Going from smaller and younger to large and older birds, the following designations are given to chickens: broiler or fryer, roaster, capon, stag, stewing chicken, and old rooster. The tenderness of the flash generally decreases in the same order. Broilers or fryers are generally preferred for processing into fresh or frozen chicken where tenderness is essential. Processors of canned chicken or chicken soup are able to use older and tougher birds because the sterilization heat of canning usually tenderizes the meat.
Within these marketing classes there also are U.S grade standards for quality of individuals birds based on feathering, shape, fleshing, fat, and freedom from defects. There are three quality grades: A Quality or No.1, B Quality or No.2, and C Quality or No.3. Birds are purchased by the processor from the grower depending on the type of products to be manufactured and competitive price.
Processing Plant Operation
Plants for dressing poultry vary in size up to the largest that a process more than 10,000 birds per hour. These modern plants are efficient continuous-line facilities in which the birds are moved from operation to operation via monorail. Live birds are shackled, electrically stunned, bled, scalded to facilitate feather removal. Plucked of feathers, eviscerated, government inspected, washed and chilled, dried, packaged, and frozen if production calls for freezing. The operations can be partially mechanized and highly efficient in large plants, since the birds purchased are remarkably uniform with respect to size, shape, weight, and other characteristics. To ensure high quality uniform birds, large processors generally contract with growers in advance and set up rigid specifications on the kind of bird they want.
Slaughter and Bleeding
Birds generally are not fed for 12 h before slaughter to ensure that their crops are empty, which makes for cleaner operations. Bleeding time depends on efficiency of the cut, type of bird, and whether the bird was electrically stunned or not before cutting. Bleeding may take anywhere from 1 to 3 min depending on these factors. But bleeding must be quite complete in order to produce the desirable white or yellow skin color in the final dressed bird.
Scalding
After bleeding, the birds are conveyed through a scalding tank. Scalding loosens the feathers and makes for easier plucking and unfeather removal. The higher the temperature, the shorter the time required, but careful time and temperature control is very important because at higher temperatures there is a greater danger of removal of portion skin in the DE feathering machines. Scalding may be done at 60C in about 45 secs, or more safely with less chance of skin removal at 52 C in about 2 min. optimum conditions must be established for the kind of bird being dressed.
DE feathering
DE feathering is commonly done mechanically by a device that has many rotating rubber fingers. This removes all but a few pinfeathers, which may be removed by dipping the bird in melted wax, chilling to harden the wax, and then peeling.
Eviscerating
Evisceration is generally done in a separate cool room (Fig. 14.12). Evisceration includes inspection of the viscera by a veterinarian or someone under such supervision.
Figure; 14.12
The lungs and other organs that are difficult to dislodge may be removed by suction tubes. Birds passing inspection are thoroughly washed.
Chilling
The washed birds are rapidly chilled from about 32 to 2 C to prevent bacterial spoilage and to preserve quality. Chilling is done with ice slush, and the birds absorb a small amount of moisture from the slush, which makes them more succulent after packaging. But the maximum allowable water pickup is fixed by low. After chilling, birds are drained of excess moisture and are sized and graded for quality.
A few years ago, the majority of broilers were distributed as whole carcasses which were further cut by consumers. Now consumers prefer precut carcasses or, in many cases, they prefer to buy only selected portions of the carcass, such as breasts or thighs. Thus, partially automated processing lines which reduce whole carcasses into smaller portion have become common. Whole carcass sales now represent only about one-fifth of fresh poultry sales.
Packaging
The graded poultry or poultry parts may now be packaged as fresh poultry in boxes surrounded by crushed ice. If so, birds must be kept below 4C and moved to retail channels rapidly, since shelf life may be only a few days. Shelf life depends on the bacterial load. Should this b about 10,000 organisms per square centimeter of surface, which is not uncommon, odor and slime will develop even at 4C in about 6 days.
In order to overcome the drawbacks of shipping the added weight of ice and problems of its disposal, the poultry industry has rapidly moved to other technologies which eliminate the need for ice. For example, in the deep-chilling process, parts are sealed in consumer packages and then sent through a low-temperature blast tunnel and frozen to a depth of a few millimeters. This frozen crust quickly equilibrates with the rest of the carcass, which is then maintained at a temperature of -1 to -2C which is above the freezing point of poultry meat. Modified atmosphere packaging (MAP) as discussed above for red meats is also used to ship prepackaged poultry products. Both of these technologies with controlled refrigeration can produce a product which has a shelf life of 18-21 days’ post slaughter, which is sufficient time for distribution in most cases.
To prolong storage life, poultry can also be individually wrapped in low-moisture, low-oxygen transmission films or bags and frozen. When this is done, the bags are made to fit snugly, and the birds are vacuum packed in the bags to remove most of the air since the fat of chicken is highly susceptible to oxidation. The bags are made of plastic films which shrink when heated. Individual packages are passed thorough a hot air tunnel to tighten the bag around the carcass. This is particularly popular for turkey, for which the consumption is seasonal.
Government Inspection
In the United States all poultry sold in interstate commerce must be inspected for wholesomeness and, therefore, must be processed in plants having full-time government inspection service. Poultry is inspected live prior to slaughter, during evisceration, and during or after packaging. This inspection to protect the public health is mandatory. Inspection for quality grading by USDA representatives is optional, as in the case of meat. Poultry is graded as A or B grade. The two types of marks that may appear on poultry and represent these different forms of inspection are shown in Fig. 14.13.
Tenderness, Flavor, and Color
The color of the flesh of poultry and other birds is commonly described as dark or light. Dark muscle contains more myoglobin and is used by the bird for sustained activity. Light muscle has less myoglobin. Ducks or other birds which fly distances have dark breast muscle, whereas birds which do not fly distance, such as chickens and turkeys, have light colored breast muscle. The color of the skin of most birds reflect the amount of the plants pigments consumed in the diet. Chickens raised on corn or other grains have very light colored skin. However, if highly pigmented plant material such as yellow flower petals are mixed into the diet, the skin will take on a yellow color. This does not directly affect the quality of the meat but, in some areas is taken as a measure of quality.
In general, the same factors favoring tenderness in red meat do so in poultry as well. Thus, meat from young birds is more tender than that from older ones, as is meat with less connective tissue (breast meat versus thigh meat) and more fat. In addition, birds grown in confined quarters without exercise yield more tender meat than do birds grown on open range.
Like meat and fish, poultry enters into a state of rigor mortis soon after being killed. Rigor mortis is associated with a conversion of glycogen to lactic acid, which has a mild preservative effect on the flesh, and with a contraction of the muscles and a stiffening of the tissues. Rigor mortis naturally subsides in poultry with a relaxation of the muscles after about 10 h or less. If poultry is cooked or frozen while the meat is in a state of rigor mortis, the meat may be excessively tough; this is avoided in good processing schedules.
Figure 14.13
Very fresh high quality chicken meat has little flavor. In fact, scientific tests have shown that meat which has aged for a few days is preferred over meat that is only 1-day old. Chicken meat flavor also is affected by the feed received during growing. Excessive amounts of fish meal can give poultry a fishy flavor.
Contributing during cooking, has been the development of the self-basting bird intended for roasting. This type of product is presently more common with turkey but can also be produced with chicken. To prepare self-basting birds, the manufacturer injects basting liquids at several points under the skin of the bird before packaging and freeing. The basting liquid generally contains vegetable oils, water, salt, emulsifiers, and artificial flavor and color. During roasting, the basting liquid moistens the skin and flesh and contributes succulence.
Nutritive Value
The composition of the edible parts of chicken depends on the cut and the method of cooking. Roasted white meat without the skin contains about 64% water, 32% protein, and 3.5% fat. Roasted dark meat without the skin contains about 65% water, 28% protein, and 6% fat. The skin is higher in fat. Chicken flesh contains more proteins, less fat, and less cholesterol than red meat. The protein is of excellent quality and contains all of the essential amino acids needed by humans. The fat is more unsaturated than the fat of red meat and this can provide further nutritional advantages. Like other animal tissue, poultry flesh is a good source of B vitamins and minerals. Because of its high protein-to-fat ration, chicken is a favored food of people who want to restrict fat intake and patients with vascular sclerotic tendencies.
Poultry Meat Products
the per capita consumption of poultry in the United States has increased over the last decade not only because people are eating more poultry directly, but also because the poultry industry has provided a large variety of products made from poultry meat. In many cases, these products are similar to products that might conventionally be made from red meats, including poultry based frankfurters, hams, sausages, bologna, salami, pastrami, ham, and other lunchmeats. Many of the newer products utilize mechanically separated poultry meat which is further ground to a fine emulsion. The emulsion may be cured, seasoned, smoked, and processed. Such products closely resemble their red meat counterparts, usually are lower in cost, may offer nutritional benefits, and must be appropriately labeled.
EGGS
Special strains of chickens are bred for large-scale egg production. Today, on the average, a hen often lays more than 260 eggs per year; in the United States about 70 billion eggs are produced each year. About 90% of these are consumed directly in the form of shell eggs. The remainder, broken out of the shell, is frozen, dried, or otherwise processed for use in the bakery, confectionery, and noodle industries, although there are also minor nonfood uses.
Egg Formation and Structure
Egg production is an integral part of the reproductive cycle in poultry (Fig. 14.14). Yolks containing the female germ cell are formed in the ovaries. These yolks drop into the mouth of the oviduct and then slowly pass down the oviduct. They are covered with layers of egg white from albumen-secreting cells, then with membranous tissue from other protein-secreting cells, and finally with calcium and other minerals from mineral-secreting cells near the bottom of the oviduct. This results in the egg shell. This process occurs whether the egg is fertilized or not. If fertilization is to take place, the sperm must travel up the oviduct and reach the yolk before the albumen and shell are deposited.
This sequence of events helps to explain several defects possible in shell eggs for human food: fertilized egg yolks produce embryos; ruptures in the ovary or oviduct can produce blood spots and sometimes meat specks; diseases of the ovary or oviduct can produce eggs infected with bacterial or parasites inside a sound shell. However, the contents of eggs from a healthy bird in unbroken shells are sterile when freshly laid.
The structure of the egg is diagrammed in Fig. 14.15. The central yolk is surrounded by a membrane called the vitelline membrane. Immediately beyond this is another membranous layer known as the chalaziferous layer. The yolk is connected to thick or firm albumen by two extensions of the chalaziferous layer called chalazas. The yolk further is surrounded by layers of thin and thick albumen, and outside of these by another layer of thin albumen. This is surrounded by the sell, which has two inside shell membranes and an outer protective layer known as cuticle or “bloom.”
The shell is porous and allows gases to pass in and out of the egg; these gases are used by the developing embryo in the case of a fertilized egg. On ageing, as air enters through the shell, the air cell at the blunt end between the shell membrane and the shell enlarges; a large air cell is an indication of storage and less fresh quality. When eggs are washed, the cuticle or bloom on the outside is removed, exposing the open pores of the egg shell. Under these conditions, bacteria can more easily enter the egg contents through the shell pores.
Composition
Eggs contain about two parts white to one-part yolk by weight. The whole mixed egg contains about 65% water, 12% protein, and 11% fat. But the compositions of the white and yolk differ considerably. Virtually all of the fat is in the yolk, and when eggs are separated into white and yolk, we want to keep it this way since small amounts of fat adversely affect the shipping property of egg white. The 12% solids of egg white are virtually all protein (Table 14.3). The yolk is rich in fat-soluble vitamins A, D, E, and K and in phospholipids including the emulsifier lecithin. Nutritionally, eggs are a good source of fat, protein, vitamins, and minerals, especially iron.
Figure 14.14
Eggs also contain about 240 mg of cholesterol which is all contained in the yolk. For this reason, people who must restrict their cholesterol intake usually consumer fewer whole eggs. This concern has led to a decline in the per capita consumption of eggs in recent years.
Quality Factors
Eggs may range in size from Peewee to Jumbo classifications, but quality grades are independent of size. The most common method of grading eggs is by candling, in which the egg is held up to a light source.
Candling will reveal many defects –a cracked shell, a fertilized yolk, a blood spot, an enlarged air cell, firmness of white which becomes thinner on ageing, and position of yolk which tends to drift off-center when the egg becomes stale. The extent of staleness also can be seen in broken-out eggs (see Fig. 6.13). Fresh eggs have a thick and high yolk rather than a flat yolk and a larger amount of thick white relative to runny thin white than do stale eggs, which spread out over a larger area than a fresh egg. Egg quality grades are based largely on these measures of freshness, since fresher eggs taste better, are easier to separate into whites and yolks for manufacturing purposes, and perform better in whipping and baking applications.
The shell color of eggs depends on the reed of chicken, but the yolk color depends largely on feed. Feeds high in carotenoids produce darker yolks, which are favored in some markets and in food manufacture to give a golden color to baked goods and products like needles and mayonnaise. Despite common belief, shell color does not influence nutritional or other quality parameters.
Egg Storage
Because an abundance of eggs is produced in the spring of the year, eggs must be stored for use at other times. Storage is best at a temperature slightly above the freezing point of the egg. A temperature of -1C in warehouses is deal; to minimize moisture loss from eggs, the relative humidity may be as high as 80%. In proper cold storage, Grade A quality can be maintained for as long as 6 months. After laying, eggs lose carbon dioxide through the porous shell, making the eggs more alkaline. The loss of carbon dioxide is associated with the staling process also, and some lengthening of storage stability can be achieved by storing eggs under carbon dioxide to minimize carbon dioxide loss.
More common, however, is the practice of spraying eggs to be stored with a light mineral oil. This closes the pores of the egg shell and retards both carbon dioxide and moisture loss. In another method of prolonging storage life, known as thermos stabilization, eggs are dipped in hot water or hot oil for a brief period to coagulate a thin layer of albumen around the inside of the shell and thus further seal it. The heat also kills some of the surface bacteria.
Bacterial Infection and Pasteurization
As stated earlier, the contents freshly laid eggs generally are sterile. However, the shell surface contains many bacteria, especially if the shell is soiled with chicken droppings. Even if the shell is not cracked, bacterial can enter through the natural shell pores. When eggs are washed, the shell cuticle is easily removed. If washing is not complete and the eggs are not dried, bacteria are especially likely to pass via the water through the shell. If the eggs are washed with warm water, increased temperature can make gases within the shell expand escape through the pores. Then when the egg cools, a reduced pressure can result within the shell. This tends to draw bacterial and moisture from a wet shell into the egg through the pores. Cracked shells are obviously worse.
A particular group of bacterial belonging to the genus Salmonella are pathogenic to humans and commonly found in the poultry digestive tract. It is difficult to keep Salmonella organisms out of egg products and Salmonella-infected eggs have caused numerous outbreaks of disease. Because of the prevalence of Salmonella infections, food laws of the United States and several other countries require that all commercial eggs broken out of the shell for manufacturing use be pasteurized. This is a relatively new law in the United States, having been extended to egg white in 1966. Pasteurization of whole egg and egg yolk had been practiced for many years, but this was not the case with egg white.
Egg white is very sensitive to heat, being easily coagulated very near efficient pasteurization temperatures. For this reason, an effective pasteurization treatment with minimal damage to the egg white was slow in being developed. The current pasteurization conditions for egg white or whole egg in the United States involve heating to the range of 60-62C and holding for periods of 3.5-4.0 min. Egg white also may be pasteurized at lower temperature of 52-53C when combined with hydrogen peroxide. In one method the liquid whites are heated to this temperature and held for 1.5 min. hydrogen peroxide at a level of 0.075-0.10% is metered into the egg white which is held at 52-53C for 2 min more. They hydrogen peroxide is then broken down to water and oxygen by the addition of the enzyme catalase. Pasteurization processes may vary, but the treated eggs must be Salmonella-negative and meet other bacteriological standards.
More recently, concern has been over the occurrence of Salmonella enteritis infection from diseased flocks. This human pathogen can enter eggs during their formation in the chicken before being laid. Contaminated eggs which have been eaten without sufficient cooking have been responsible for human disease including a number of deaths.
Freezing Eggs
Large quantities of eggs for use in food manufacturing are preserved by freezing. This is not done in the shell but rather with the liquid contents of the egg, which may be frozen as the whole egg or separated into yolk and white or various mixtures of yolk and white for special food uses.
Freezing plants generally are combined with egg-breading facilities. The egg-breaking section of the plant receives eggs, may wash and dry them, and then break the egg contents from the shell. This used to be a hand operation but is now highly automated with egg-breaking machines. Where one operator can break and separate 60-90 dozen eggs per hour by hand, an operator-inspector can break and separate 600 dozen eggs per hour with these automatic machines. The operator-inspector also performs the very important function of rejecting spoiled eggs, one of which can ruin substantial amounts of good product. The common bacterial that are found in bad eggs generally fluoresce under ultraviolet light. This property has been used to help identify eggs to be rejected.
The whole or separated eggs are mixed for uniformity, screened to remove chalazae, membranes, or its of shell, pasteurized, and placed in 13.6-kg (30-lb) cans or other suitable containers for freezing. Freezing generally is done in a sharp freezer room with circulating air at -30C. freezing may take from about 48-72 h.
Egg white and whole egg may be frozen as such, but egg yolk may not be frozen without additives since by itself it becomes gummy and thick, a condition known as gelation. Gelation of egg yolk on freezing is prevented by the addition of 10% sugar or salt or the addition of 5% glycerin. Sugar yolk is the product that goes to bakers, confectioners, and other users than can tolerate sugar in their end products, whereas mayonnaise manufacturers may use salt yolk. These ingredients may be dissolved in the yolk during mixing and prior to screening.
Drying Eggs
The whites, yolks, or whole eggs after pasteurization may be dried by any of several methods, including spray drying, tray drying, foam drying, or freeze-drying. Egg white contains traces of glucose. Whichever dehydration method is used, on drying or during subsequent storage at temperatures mush above freezing, the glucose combines with egg proteins and the Millard browning reaction occurs. This discolors the dried egg white. It has been found possible to prevent this browning reaction by removing glucose through fermentation by yeasts or with commercial enzymes. This is known as DE sugaring and is a step practiced prior to the drying of all egg white.
Egg Substitutes
The high level of cholesterol (about 240 mg per yolk) in egg yolk has caused many consumers to cut down on their consumption of eggs. Different approaches to reduce the cholesterol level have involved physically separating the yolk into high-and low-cholesterol fractions, decreasing the amount of yolk relative to albumen in the egg blend, and replacing yolk with a vegetable-oil-based yolk analog. One supplier formulates the “yolk” from corn oil, milk solids, emulsifiers, appropriate vitamins, and other additives, blends it with albumen, and then pasteurizes and freezes the product, which is sold for home, restaurant, and institutional use. Natural egg yolk, in addition to fat and protein. Contains lecithin, which contributes to its emulsifying properties and the fat-soluble vitamins plus other nutrients. The importance of each of these constituents must not be overlooked in formulating an egg substitute.
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