15

Meat and Meat Products

YOULING L. XIONG and WILLIAM BENJY MIKEL University of Kentucky, Lexington, Kentucky

I. INTRODUCTION II. EXPORT AND IMPORT III. CANNED MEATS IV. FROZEN MEATS V. COOKED REFRIGERATED MEATS VI. DRY-PRESERVED MEATS VII. CURED MEATS A. Ingredients and Functions B. Hams C. Bacon VIII. SAUSAGES A. Ingredients B. Fresh Sausages C. Cured Sausages D. Fermented Sausages IX. LUNCHEON MEATS X. PREPARED DINNER MEATS REFERENCES

I. INTRODUCTION Meat and meat products, referred to here as red meats or postmortem muscles from mammalian species (beef, veal, pork, and lamb/mutton), are an important component in the American diet. Despite the surge in poultry product consumption in the past two decades, red meats, with a current annual production totaling about 20 million metric tons (or 53 kg consumed per capita), remain to have a dominant market share in all muscle foods produced in the United States (American Meat Institute [AMI], 1999). Today, meat and meat

Copyright 2001 by Marcel Dekker, Inc. All Rights Reserved.

352

Xiong and Mikel

Figure 1 Examples of processed and prepared meat and meat-based products. (This is an illustrative example only. The use or illustration of a trade name does not mean that the authors and their affiliation endorse the product.)

Table 1 Meat and Meat Products by Variety

Meats Canned meats Processing/characteristic Retort to sterilize; fully cooked; cured or noncured; metal or plastic containers Cooked or raw; most microwavable; include home-meal-replacement items; breakfast items Low water activity; cured; refrigeration not required Cured with nitrite/nitrate, salt and adjuncts by injection or dry rub Fresh, cured, or fermented; comminuted or emulsified; spiced Prepared meals (HMR); pumped products; battered/breaded meats; precooked or raw; frozen or refrigerated Deli meats; lunchables; fully cooked and ready to consume; restructured meats Example Ham; pork luncheon meat; corned beef; beef stew; beef in chili sauce Breaded boneless pork cutlet; pork sausage; meat loaf; beef stew and steaks; deli pouch; meatball; Beef jerky; pastrami Hams; bacon; jowl; most deli meats Bratwurst; frankfurters; salami; pepperoni Steak with vegetables; barbecue smoked pork; seasoned pork roast Sliced ham; bologna, and salami; ham and cheese; head cheese

Frozen meats

Dry-preserved meats Cured meats

Sausages Dinner meats

Luncheon meats

Copyright 2001 by Marcel Dekker, Inc. All Rights Reserved.

Meat and Meat Products

353

products available in the marketplace are no longer limited to the traditional items; in the past few years, numerous new items of processed meats, particularly those in the form of convenience foods, have emerged. Many of the products are fully cooked and ready to consume, whereas others are portion-controlled and case-ready, to be cooked in a microwave or conveniently heated in an oven at home. These products are widely distributed at retail and deli markets, and in a typical large food store, over 100 meat items can be readily found. Figure 1 shows examples of meats that are sold in grocery stores. Despite the great variety, meat products can be divided into seven groups based on their product characteristics and the general processing procedures required (Table 1). Postmortem muscle that has gone through major physical or chemical alterations is generally considered processed meat. Thus, in a broad sense, meat processing may include protein extraction, chemical and enzymatic treatments, massaging or tumbling, curing, stuffing, canning, smoking, and other related preliminary preparations, such as meat particle size reduction and mixing of meat with various additives. It is noteworthy that simple handling of fresh meat in retail stores and in homes (e.g., cutting, grinding, and packaging) is generally excluded from the definition of meat processing. II. EXPORT AND IMPORT Today, food production and merchandise have become a globalized enterprise. This has allowed the United States to export or import greater amounts of fresh and processed meats. Despite the large U.S. trade deficit in textiles, electronics, and a number of other economic sectors, U.S. agriculture maintains its trade surplus with foreign countries. For examples, through July of fiscal 1999, U.S. agriculture exported $41.0 billion worth of commodities, in contrast to $31.6 billion imported goods during the same period (United States Department of Agriculture [USDA], 1999). Among the leading commodities that contribute to the trade surplus are meats and meat products, whose 1.7 million-ton exports amount to a $3.7 billion value, versus $2.5 billion for imports. The surplus, however, is expected to decrease as a result of the U.S. dollar strength and relatively low prices of domestic meat and meat products. Countries that lead in exporting fresh chilled and frozen meat and meat products to the United States include Canada (451 106 tons), Australia (209 106 tons), and New Zealand (183 106 tons), whereas those that contribute to most of the U.S. import tonnage of further processed meats are Denmark (50 106 tons, about 40% are canned pork), Argentina (37 106 tons, all are processed beef products), and Uruguay (23 106 tons, with 10% canned beef products) (USDA, 1998). Table 2 compares U.S. exports and imports of meats by product type, and the changes that occurred from 1990 to 1996. III. CANNED MEATS Canning is a thermal process that employs heat (steam) to sterilize the food material placed in a sealed container. Thus, canning produces shelf-stable meat products that can be conveniently consumed in outdoor activities or situations where refrigeration is not readily available. The annual production of canned meats (including poultry) has stayed at about 1 billion pounds in recent years (Pearson and Gillett, 1996). Pasta with meat, chili, and canned hams represent about 70% of total canned meat products produced in the United States. Vienna sausage, canned luncheon meat, and meat spread are some other major forms of canned meats. The internal temperature of canned meat products must reach

Copyright 2001 by Marcel Dekker, Inc. All Rights Reserved.

Copyright 2001 by Marcel Dekker, Inc. All Rights Reserved.

Table 2 United States Meat Exports and Imports by Type of Products (in thousand metric tons), 1990 and 1996
Beef and veal Fresh chilled, and frozen Lamb, mutton, and goat Fresh, chilled, and frozen Pork Variety meats fresh and chilled

Year

Canned

Prepared/preserved

Hams and bacon

Canned

Other, prepared

Sausages, all types

Mixed sausages

Other livestock meats

Total

Imports 1990 1996 699.3 640.7 57.6 53.4 10.9 13.6 19.1 33.1 233.5 183.6 Exports 1990 1996 339.9 596.9 7.8 14.6 2.5 2.5 66.8 267.4 10.1 25.6 1.0 5.3 4.5 15.8 14.2 92.5 226.6 469.3 70.6 26.2 743.8 1507.8 31.5 55.2 13.4 15.3 3.5 2.4 1.9 1.6 11.4 32.5 18.6 21.9 1100.7 1053.3

Source: USDA Agricultural statistics, 1998.

Meat and Meat Products

355

Figure 2 Pork ham processed (pasteurized) in a plastic can (left) and beef stew processed in
semirigid retortable plastic bowls.

121C to achieve commercial sterility. However, this severe heat treatment may adversely affect the product flavor, texture, and color. Therefore, an increased number of canned meats are now only pasteurized to an internal temperature between 65 and 75C. In a typical pasteurization process, canned meat is cooked in water at about 80C for several hours, or steam pasteurized. It is desirable to keep the temperature difference between the heating medium and meat to a minimum to minimize cooking losses and jelly formation. All canned pasteurized meats must be cured to comply with federal regulations. They must be labeled PerishableKeep Under Refrigeration. Thus, canned pasteurized meat products require refrigeration during distribution and storage. Meat canning is similar to fruit and vegetable canning in processing operations. Pretreated and prepared meat, which may contain various ingredients, is packed into cans varying in size and shape, e.g., pullman, round sanitary, pear-shaped, and oblong. Most cans used for meat are made of thin sheets of steel coated with a thin film of tin to prevent rusting. An enamel consisting of sulfur-resistant resins is formed on the surface of tin to prevent corrosion of metal due to interaction with sulfur compounds produced from meat. Plastic cans, such as semi-rigid retortable bowls and trays, have also been developed in recent years. Although the products are not exactly canned, they are processed in the same manner as metal-canned meats. Plastic can-like containers (e.g., D-shaped) made of nylon, surlyn, and other ethylene/vinylacetate copolymers are used for producing cookin hams that receive pasteurizing treatments (Fig. 2). Vacuum-sealed cans are either sterilized or pasteurized in retorts, and are subsequently cooled, labeled, and marketed. IV. FROZEN MEATS Frozen meat and meat products make up a major sector of the meat industry; with the increased consumer demand for convenient foods, production of this group of meats has con-

Copyright 2001 by Marcel Dekker, Inc. All Rights Reserved.

356

Xiong and Mikel

tinued to grow in recent years. Freezing as a means of preserving meat became known following the development of efficient mechanical refrigeration during the 19th century; however, the freezing technology was not fully commercialized in the meat industry until the 1960s, during which decade there was a rapid growth in home freezers and refrigerators. The availability of efficient freezing systems has greatly facilitated meat exportation as well as importation (Table 2). Domestically, it has enabled meat producers and processors to market various convenient, packaged, and prepared foods (e.g., the so-called home meal replacements that have entered the market in the past few years) at the retail level. The freezing process involves placing meats in a cold room, typically 18 to 20C. Water in the product exposed to the freezing temperatures will transform into ice. Due to the presence of various water-soluble and hydrophilic compounds, ice crystals actually do not form until the temperature of meat reaches a few degrees below 0C. Quality of frozen meat and meat products is influenced by many factors. The rate of freezing has a profound effect on meat tenderness and drip loss. In general, fast freezing is conducive to the formation of small ice crystals that are located intracellularly and cause little physical damage to meat components. On the other hand, a slow freezing process favors large ice crystals to form, extracelluarly, which results in disruption of muscle cells and causes exudation (Fennema, 1975). In almost all commercial meat plants, forced air circulation (blast freezing) is used to achieve rapid freezing. Blast freezing may operate on either a batch or a continuous basis and normally employs 20 to 40C cold air. Air with higher velocity allows a greater heat transfer coefficient, and hence, a more rapid temperature drop in meat. To avoid thaw rigor, beef and lamb must have gone through rigor mortis prior to freezing. To initiate early onset of rigor, thereby preventing thaw rigor, electrical stimulation can be employed. To prevent quality loss due to protein denaturation, cryoprotectants are often included in the formulation of meat products. Among them, Polydextrose, polyphosphate, and to a lesser extent, sorbitol, are used. Packaging is another important factor affecting the shelf-life of frozen meats. Large, wholesale meat cuts are often vacuum sealed to prevent lipid oxidation and formation of metmyoglobin. For retail meats, cooked or raw, films with low water permeability and adequate mechanical strength, e.g., Sarlyn, are required. To prevent the products from being exposed to light, thereby eliminating oxidation catalyzed by light-sensitive compounds, nontransparent outside cardboard-type packages are commonly used. There are various kinds of frozen, cooked meat products in todays food market. A quick survey has shown many specific items manufactured by different companies, e.g., entree items: breaded boneless pork cutlet; country fried beef steaks; meat loaf; beef pot roast; and corn dog (batter-dipped wieners on a stick); dinner items: gravy and sliced beef; tomato sauce and meat loaf; steak and gravy; noodles with beef stew; steak with mushroom gravy; meat loaf in gravy; deli pouches (cheese burgers inside); croissant pockets; stuffed sandwiches (beef, ham, pepperoni); and meatballs in sauce with pasta (Fig. 3). Most of these meats are packaged with side dish items, such as green beans, tomatoes, carrots, mashed potatoes, onions, macaroni and cheese, and others, and are microwavable. V. COOKED REFRIGERATED MEATS This group includes various kinds of ready-to-consume products, many of which are sandwich-type prepared meats and belong to the HMR category (discussed later). Side dish items are normally included in the same package and the product needs only to be warmed

Copyright 2001 by Marcel Dekker, Inc. All Rights Reserved.

Meat and Meat Products

357

Figure 3 Cooked frozen meat products: croissant pockets with ham, cheddar, and selected vegetables (left), and beef steak with peas and mashed potato (right). either with a microwave or conventional oven before serving. In some cases, flavor and freshness appearance may be a problem. For example, a precooked porkchop may develop warmed-over-flavor upon reheating, rendering the product unacceptable. To enhance product shelf-stability, antioxidants and specific flavor ingredients are often included. VI. DRY-PRESERVED MEATS Drying of meat is a very old process that was originally used to preserve meat, namely, to keep the meat at ambient temperature for an extended period of time. Historically, sundrying was the only drying method, but today hot air-drying is prevalent commercially. Alternatively, freeze-drying is done, but because of its prohibitively high costs, it is restricted to some specialty products that require rapid rehydration. The most well-known dried meats are beef jerkies and dried meat bits as a soup base. Dried meats prepared in the United States are cooked before drying. The usual process is to cut meat into slices and then cook, mince, or cut it into small cubes to provide a large surface-to-volume ratio, and dry it. Precooking is important not just because of food safety, but also because it facilitates the removal of a large amount of water from meat following protein aggregation. For beef jerkies, marination is a required extra pretreatment: meat slices are marinated (e.g., in teriyaki soy sauce, brown sugar, liquid smoke, black and red pepper, and other seasonings) before cooking and drying. During the drying process, meat undergoes extensive physical and chemical changes. Conventional air-drying involves the extraction of almost all of indigenous water in muscle. The process is fundamentally a diffusion of water to the meat surface, which is driven by the difference in water vapor partial pressure between the air and the meat surface. Both continuous belt oven and fluidized bed driers are used to dry meat. The removal of water from the muscle cells as well as between muscle fibers results in stronger hydrophobic interactions of the myofibrils, thereby hardening the texture of meat for desirable chewiness. VII. CURED MEATS

Curing of meat refers to treatment of fresh meat with salt, nitrite or nitrate compounds, and adjuncts for the purpose of preservation and obtaining desirable color and flavor. It is not

Copyright 2001 by Marcel Dekker, Inc. All Rights Reserved.

358

Xiong and Mikel

known where and when the use of curing first began, but some evidence suggests that around 3000 B.C. the Chinese began using salt that was contaminated with nitrate to preserve meat. Many cured meat products are available in the current food market. They include traditional pork products such as ham and bacon, and to a lesser extent, beef products such as corned beef. More recently, a number of new cured meats have been developed, with most belonging to the restructured and sliced deli meat category. This section will concentrate on ham and bacon because both not only are popular products in the cured meat category, but are also of a long tradition, have unique characteristics, and involve some special processing technologies. A. Ingredients and Functions 1. Curing Agents Salt in the form of NaCl is commonly used in cured meats. Occasionally, KCl is used to replace or partially substitute for NaCl to alleviate its potential adverse effect on health of the humans. The main functions of salt, other than providing flavor, are solubilizing proteins, dehydration, and altering osmotic pressure so as to inhibit bacterial growth and subsequent spoilage. Salt can be applied to meat directly in its dry crystal form, or dissolved in solution (pickle) before incorporation into meat. Although salt is an indispensable ingredient in cured products, the true curing agent is nitrite (NO2) or nitrate (NO3). Nitrate was originally approved for color fixation in cured meats, but now it has largely been replaced by nitrite. This is because nitrate is reduced to nitrite either by organisms or by reducing compounds before the actual curing process begins, and because direct application of nitrite can be more easily controlled. In the United States, the use of nitrate is now restricted to drycured products, such as country cured hams and dry sausages. Nitrite (or nitrate) is a multifunctional compound. It induces and stabilizes the pinkish color of lean meat, contributes to the characteristic flavor of cured meat, inhibits the growth of spoilage and pathogenic microorganisms (especially Clostridium botulinum), and retards development of oxidative rancidity. The level of nitrite or nitrate allowed in cured meats, both ingoing and residual, is strictly regulated by the USDA. For instance, the maximum level of ingoing NaNO2 in pumped bacon is 120 ppm, and the residual level (in the finished product) shall not exceed 40 ppm. Establishment of nitrite regulation is based on the concern that nitrite can form nitrosamines by reacting with secondary amines in cooked cured meat as well as in the intestines of the human body. In response to consumer concern, an increased number of meat processors are now reducing the nitrite level in cured products. A recent survey showed that residual nitrite levels in cured meats (ham, bacon, wiener, and bologna) produced in the United States average about 10 ppm, which represents only 10% of the recorded levels in cured meats two decades ago (Cassens, 1997). The pinkish red color characteristic of cooked cured meats results largely from the reaction of the heme group in myoglobin with nitric oxide forming the nitrosylmyoglobin pigment. Nitric oxide is derived from nitrite in the presence of reducing compounds such as erythorbic acid. Part of nitrite dissolved in water can form nitrous acid (HNO2). Under a reducing condition, nitrous acid decomposes to nitric oxide. When nitric oxide binds to the heme iron, it changes the electron distribution or resonance of the heme structure, thereby producing a pinkish color. Under heating conditions, nitrosylmyoglobin is converted to the more stable forms, i.e., nitrosylhemochromogen and dinitrosylhemochrome. The concerns with possible carcinogenic effect of residual nitrite present in cured meats have prompted researchers to search for alternative curing methods. One of the pos-

Copyright 2001 by Marcel Dekker, Inc. All Rights Reserved.

Meat and Meat Products

359

sible methods was the so-called nitrite-free curing (OBoyle et al., 1990). In this curing system, the three major functions of nitritecolor fixation, antimicrobial activity (antibotulism in particular), and inhibition of oxidative rancidityare conferred through the use of combined materials. Dinitrosylhemochrome is presynthesized by reacting nitric oxide gas with hemin extracted from beef red blood cells, an abundant animal by-product. Dinitrosylhemochrome is not very stable, so the pigment is microencapsulated with carbohydrate-based encapsulating agents to prevent it from direct contact with air. To reproduce the antioxidative activity of nitrite, antioxidants such as ascorbate, tripolyphosphate, and various phenolics (TBHQ, BHA, etc.) are added to the nitrite-free mixture. In addition, antimicrobial agents, such as sorbate, sodium hypophosphite, methyl fumarate, and sodium lactate, are also used. The mixture is blended with meat in sausage production. Frankfurters and wieners manufactured using the nitrite-free curing system cannot be distinguished from products made by the traditional nitrite-cured method. 2. Curing Adjuncts Reducing compounds are added in meat-curing mixtures to hasten color development via converting nitrite to nitric oxide, and ferric iron of the heme to ferrous iron. The most commonly used reducing agent is sodium ascorbate (vitamin C), or its isomer, erythorbate, which is less expensive and more stable. Muscle itself also contains endogenous reductants and enzymatic reducing activity, but the reducing power of these factors is relatively small. In addition to reducing metmyoglobin (Fe3) to myoglobin (Fe2), and nitrite to nitric oxide, ascorbate (or erythorbate) also serves as an antioxidant to stabilize both color and flavor, and to decrease the formation of nitrosamines. Another curing adjunct is phosphate, usually sodium pyrophosphate, tripolyphosphate, and hexametaphosphate, that can be used individually or in various combinations. In either case, the total addition cannot exceed 0.5% of the meat product weight as regulated by the USDA. Phosphates do not directly enter the curing reactions; they are added mainly to increase water-holding capacity of muscle, thereby reducing shrinkage of finished products. Phosphates are also effective antioxidants and can retard rancidity development. However, because nitrite and ascorbate are stronger antioxidants, the antioxidative effect by phosphates may not be significant in cured meats. Because phosphates are corrosive (they bind with metal ions), the equipment used must be made of stainless steel or plastic, or metal with a plastic coating. B. Hams Hams can be separated into two groups: bone-in hams (processed using the whole pork legs) and boneless hams (made from deboned pork meat). The latter type also belongs to the deli meat group and therefore will be further discussed later. A typical ham is cured with a mixture consisting of salt, sugar, sodium nitrite, sodium erythorbate, and sodium tripolyphosphate. Occasionally, corn syrup in lieu of sugar is utilized. Hams can be cured by two fundamental methods: dry curing and pickle (wet) curing. In dry curing, the curing ingredients are applied to fresh pork leg by rubbing without the addition of water. Thus, the curing ingredients, particularly salt, draw enough water from the meat due to the osmotic pressure gradient to form a brine, which serves to transport the curing ingredients into the meat through diffusion. This curing method is obviously labor-intensive and distribution of the curing ingredients is not efficient. Furthermore, dry-cured meats are often too salty and may be unacceptable to many consumers. Therefore, except for country ham, commercial

Copyright 2001 by Marcel Dekker, Inc. All Rights Reserved.

360

Xiong and Mikel

processing of hams is now accomplished mostly by pickle injection. Dry curing is also still used for fatty cuts, such as fat backs, clear plates, and jowls. 1. Dry Curing Country hams are prepared using dry curing methods. Country hams are not cooked but must be free of Trichinae and have a salt content in the finished product of at least 4%, and shrink a minimum of 18% during processing. The flavor and appearance of country hams distinctly differ from common pickle-cured hams. Country hams are salty, somewhat dry, and rather hard products with a rich flavor. They are primarily produced in the southern regions of the United States, including the states of Kentucky, Virginia, North Carolina, Tennessee, Georgia, and Missouri. Most commercial country ham processors rely on added nitrite or nitrate for curing. However, a small percentage of processors do not add nitrate or nitrite and still make highly flavored and well-colored country hams. Salt and seasoning may be contaminated with nitrate or nitrite and produce the color and flavor characteristic of country hams. The curing mixture is divided into three equal portions, which are applied by rubbing around the entire cut immediately, and 7 and 14 days later, respectively. It is important to make sure that the flesh surface is adequately rubbed with the cure mixture because the cure penetrates into the ham through the flesh surface, not the skin surface. For best product quality, country hams should be cured under refrigeration conditions with a temperature of 2.2 to 4.4C (3640F) and a relative humidity of 70% to 90%. Hams are stacked with overhauling two to three times during a 30- to 50-day curing period. The exact curing time depends on the size of ham (~2 days/pound). Following curing, country hams are held for approximately 20 more days for salt equalization. The equalization room should be maintained at 7.2 to 12.8C (45 to 55F) at a relative humidity of 75% to 90% and can be done in the curing room. The hams should then be hung by the shank using a string, or placed in stockinette and suspended from a rack, and allowed to age for 6 to 9 months but no longer than 12 months. The temperature of the aging room is relatively high (21 to 35C) and the humidity relative low (50% to 60%) to facilitate dehydration and development of firm texture and distinct flavors. It is generally believed that both cathepsins (which degrade proteins to small flavor-active peptides) and lipases (which generates volatile compounds by catalyzing lipid oxidation) are involved in the flavor development. Frequently, molds will grow on the surface of country hams during salt equalization and subsequent aging. These molds are generally unharmful and can be removed by wiping with a cloth dampened with edible oil. Finished country hams should have a moisture content of 50% to 60% and an average salt content of 4.5% to 5.5%. There are other types of dry-rubbed hams made in other countries that are more or less similar to country hams made in the United States. Examples are Chinese Jinhua ham, Italian prosciutto ham, German Black Forest ham, and Spanish serrano ham. 2. Pickle Curing This is the most widely used curing method for hams. The method differs from dry curing in that the curing mixtures are dissolved in water to form a brine or pickle. Furthermore, nitrite instead of nitrate is used in pickle curing. Among the different ways to incorporate curing solution into meat, stitch pumping is by far the most widely used. In a typical commercial operation, multiple needles are used to inject the curing solution into the ham flesh. The

Copyright 2001 by Marcel Dekker, Inc. All Rights Reserved.

Meat and Meat Products

361

pickle, in the amount of ~10% of the raw pork leg weight, is injected from different sides of the leg, allowing a uniform incorporation of the curing ingredients. This amount of added solution is usually lost by the time the ham is fully cooked. The amount of residual brine after cooking is regulated. If the finished ham does not weigh more than it did before cure injection, no label restrictions are imposed. However, if the finished ham is heavier than its fresh cut, i.e., the protein fat-free (PFF) value falls below 20.5, then it must be labeled cooked ham with natural juice (PFF 18.5) or cooked ham with water added (PFF 17.0) (USDA, 1984). C. Bacon Similar to ham processing, bacon can be cured by both dry rub and pickle curing. Since pork bellies are relatively thin, they do not require a long time to cure before cooking and smoking. A typical pork belly requires only 10 to 14 days to cure by dry rub, compared with hams, which require several months. Most commercially processed bacon is now cured through multi-needle stitch pumping. Finished bacon is usually partially cooked; the final internal temperature of the belly reaches only 52 to 56C. Before consumption, bacon slices must be fully cooked to ensure a complete destruction of possible pathogenic microorganisms and the parasite Trichinae. Recently, microwavable bacon has been developed. The process involves frying of cured belly for sufficient length to develop the characteristic flavor and texture of cooked bacon. The fully cooked bacon is packaged in a special pack designed to enhance microwave heating (5 to 10 seconds). Additional procedures may be necessary, including the use of binders to minimize moisture loss during heating and texturizing through rollers to prevent excessive distortion of the rashers. The product is rather stable, requiring no refrigeration until opened. There are several other kinds of bacon, e.g., jowl bacon, beef bacon, and Canadian bacon. Canadian bacon, made from pork loin muscle, differs sharply from conventional bacon, which is made from the pork bellies.

VIII. SAUSAGES Sausages are a unique type of comminuted meat products that are usually spiced or seasoned to obtain various flavor intensities and profiles. The development of sausages was initially driven primarily by economic factors, i.e., it utilizes low-quality meats such as trimmings, head and shoulder meat, and edible by-products. Convenience and variety are other important reasons why sausages are widely consumed in modern society. In the United States, about 4 billion kg of sausage products are produced annually, and the per capita consumption is estimated to be 15 kg per year. Based on the product characteristics and the specific processing method used, sausages can be classified into three major groups: fresh sausages, cured sausages, and fermented sausages. A comprehensive list of sausages produced in the world is provided by Roman et al. (1994). Technologically, sausage making consists of several common stepscomminution to reduce meat and fat particle size (grinding, mincing, chopping, or flaking), mixing with ingredients, stuffing into a specific casing, linking to obtain specific lengths, and finally, packaging.

Copyright 2001 by Marcel Dekker, Inc. All Rights Reserved.

362

Xiong and Mikel

A. Ingredients 1. Raw Meat A proper selection of meat ingredients is essential for the production of sausages of uniform quality. Raw meats used for sausages are generally low-valued materials, but they must be fresh, i.e., with very low microbial counts. These include cuts high in connective tissue or fat, tough meat from mature animals, carcass trimmings, mechanically separated meat, and edible animal by-products. The function of each selected raw meat ingredient may be unique. Meats used for binding should have a sufficiently high protein content and the proteins should be readily extracted and form gels during cooking. Skeletal muscle from cows, bulls, and sows is an excellent meat binder, whereas high-fat trimmings are generally poor binders. There is another group of meats that are included in sausage formulations to fill the void space in sausages. Filler meats have little or no binding ability, and they include offal meats (e.g., tripe, snouts), skin, and partially defatted beef and pork tissue. The rapidly increased use of poultry meat in the sausage industry is worth particular mentioning. Poultry meat has been blended into pork and beef sausages. The increased use of poultry meat in sausage production has resulted mainly from the relatively low cost for poultry meat (particularly turkey), and increased consumer demands for light meat, which is perceived as more healthy than red meat. 2. Salt and Nitrite Salt is the single most critical nonmeat ingredient. The main form of salt utilized in sausage production is sodium chloride. Its principal function is to solubilize and extract the myofibrillar proteins needed to form a bind during cooking. Of course, it also imparts flavor and has antimicrobial effects. Thus, salt is responsible for the textural characteristics and integrity of finished sausage products. Most commercial sausages contain 1.5% to 2.5% added salt. Phosphates at a level up to 0.5% in finished products are used to improve water-binding capacity of meat by increasing fiber swelling and solubilizing proteins. Phosphates may also help to stabilize flavor and color in finished product, presumably by sequestering transitional metal ions (Fe and Cu), thus reducing oxidation. Many sausage products are cured with nitrite. Sodium nitrite is commonly used, although in certain cases it may be substituted for by potassium nitrite. The maximum level of nitrite allowed in sausage is 156 ppm. The use of nitrate is more restricted; it can be used only in dry and semi-dry, fermented sausages. Nitrite is used in conjunction with the reducing agent ascorbate or erythorbate, and phosphates. 3. Water and Extenders Water, sometimes together with ice, is added in sausage making to help distribute nonmeat ingredients and increase the product yield. In 1988, the USDA implemented a new regulation to permit water addition to partially substitute for fat in cooked sausages, so long as the sum of fat (30% maximum) and added water in the final product does not exceed 40% of the product weight. As a result of this regulatory change, numerous new, low-fat sausage productsfor example, reduced-fat sausages (25% fat reduction over traditional products), low-fat or light sausages (contain 10% fat), extra lean sausages (contain 5% fat), and fat-free sausages (contain 1% fat)have been produced. Along with the addition of water, nonfat dry milk, whey protein concentrate, sodium caseinate, wheat gluten, cereal flours, tapioca dextrin, soy flour, soy protein concentrate, and more recently,

Copyright 2001 by Marcel Dekker, Inc. All Rights Reserved.

Meat and Meat Products

363

polysaccharide gums, at limited levels, are used as extenders or fillers in sausages. The terms extender and filler are often used interchangeably. Their main functions are to improve functional properties related to product texture and flavor, and to aid in meat particle binding and water retention. 4. Seasonings Unlike most other processed meats, sausages are seasoned products. Different spices and flavorings are added in sausages, and their use levels are primarily dictated by product identity standards and not by regulations. Spices are aromatic vegetable substances in whole, broken, or ground form. Spices may be added as natural spices or spice extracts. In the latter case, they must be labeled as flavoring. Flavorings refer to extractives that contain flavor constituents from fruits, vegetables, herbs, roots, meat, seafood, poultry, eggs, dairy products, and other food sources. Flavoring compounds can also be synthesized. A good example is monosodium glutamate, which is a potent flavor enhancer. Most flavorings are oil-based extracts. Because of their high flavor intensity, they can be more accurately applied in sausage to obtain desired flavor intensity than their natural counterparts (spices). Sugars in a variety of formssucrose, dextrose, corn syrup, and so onare most commonly used in sausages. Almost all sausage products contain sugar in one form or another. B. Fresh Sausages As the word fresh indicates, fresh sausages include a variety of uncooked sausages, such as breakfast sausage and sausage patties, whole hog sausage, bratwurst, Italian-style sausage, and Polish-style sausage, all of which can have a fat content up to 50% of the raw product weight. They are salted but not cured with nitrite, and are generally coarsely ground and not emulsified. Fresh smoked sausages such as Kielbasa also belong to this product group. Fresh sausages are manufactured by grinding meat through plates with holes ranging from 0.32 cm (1/8) to 0.95 cm (3/8) in diameter. Particle size reduction is achieved through extrusion and cutting in a screw auger operating in a horizontal chamber. The ground meat is mixed with salt, seasonings, and other ingredients by blending in a mixer or similar equipment. Stuffing is done by extrusion of the batter into casings through a smallopening tube called the horn. There are two major types of sausage casingsnatural or synthetic. Natural casings are small intestines from hog and sheep. The intestine is inverted and thoroughly washed in a dilute chlorine solution (0.5%) followed by water rinsing. Excess fat and connective tissue are removed by brushing with a soft brush. Natural casings are normally packed in saturated salt solution and stored in a cold room or a freezer. Natural casings are denatured upon cooking. Shrinkage of the casing allows it to firm up the sausage links. Synthetic casings are made of edible collagen materials or inedible cellulose. For fresh sausages, edible casings are always used. After stuffing, sausage is linked to make individual links of equal length. Because casings contribute a significant cost to sausage products, technology has been developed to manufacture sausages without the use of casings (Frye, 1996). In one method (co-extrusion), collagen solution is extruded around an endless rope of sausage batter, forming a thin collagen layer on the surface of the sausage. Subsequent treatment with a saturated salt solution causes dehydration of collagen and transforms it into a fibrous structure or coating. After heating and cooling, a thin invisible film is formed as the result of collagen cross-linking, thereby encasing the sausage. Another method (using sintered

Copyright 2001 by Marcel Dekker, Inc. All Rights Reserved.

364

Xiong and Mikel

mold) is based on the principle that myofibrillar proteins extracted to the surface of sausage can coagulate upon acidification. The meat batter is extruded into a porous mold. When a small amount of acetic acid is pumped through the mold, it lowers the pH and causes the proteins on the surface to coagulate, forming a natural skin. Fresh sausages are sold uncooked, and require either refrigeration or freezing. C. Cured Sausages Nitrite-cured sausages are mostly finely chopped and emulsified. The most popular products in this group are frankfurters and bologna, which account for about 25% and 20% of all sausages (including both red meat and poultry) consumed in the United States. Frankfurters are prepared in various forms, and those with a relatively large diameter are often referred to as wieners. Most frankfurters in the United States are a blend of beef and pork, with or without poultry, mildly seasoned with paprika and other spices, and smoked. The processing technology for bologna is similar to that for frankfurters; however, bologna is much larger in diameter (e.g., ~10 cm compared to less than 2.5 cm for frankfurters) for the purpose of sandwich preparation, and the spices used may be different. Many luncheon meats with distinctly different standards of identity actually also belong to the cured sausage category. In a typical cured sausage processing, the mixture of meat and ingredients is finely chopped and emulsified. Frequently, mixing and chopping are done simultaneously in a bowl chopper where a series of vertically positioned rotating knives cut through meat that is forced to move with the horizontally positioned rotating bowl. The chopping process creates sufficient shear to comminute meat and fat into fine particulates. Because myofibrillar proteins are extracted in the presence of added salt and phosphate during chopping, they will form coatings on the surface of the fat droplets and produce protein matrices surrounding the emulsified fat particles, thereby reducing the surface tension. Some sausage manufacturers use a separate emulsifier to produce fine emulsions after the meat is already chopped. However, with a good chopping machine, a separate emulsifier is often found unnecessary. The finely chopped meat system is completely stabilized during cooking, where three-dimensional gel structures are formed and fat particles are imbedded in the gel matrices. Most cured sausages are also smoked. One of the most critical factors in the production of emulsified meats is the temperature of meat batters during chopping. This temperature should be maintained low enough to prevent emulsion collapse, but not too low to keep fat soft. It varies depending on type of fat added or meat used. For most frankfurter emulsions, a chopping temperature of about 10 to 12C is desirable. Due to friction between the high-velocity rotating blades and meat particles, a 10C temperature rise in meat batters is not uncommon. To prevent heat rise during chopping, ice is used to replace water in the product formulation. Finely chopped sausages are stuffed mostly in synthetic casings. Therefore, after cooking, casings are removed by peeling prior to vacuum packaging. Natural casings described above are also used for certain finely chopped products, and they are not removed after cooking. D. Fermented Sausages The first use of fermentation in meat is lost in the mists of antiquity but may date back to the Babylonian culture around 1500 B.C. Fermented sausages can be divided into two main groups based on the processing procedure and product characteristics: dry and semi-dry. For both groups, lactic acid is produced; thus, meat is fermented. Dry and semi-dry

Copyright 2001 by Marcel Dekker, Inc. All Rights Reserved.

Meat and Meat Products

365

sausages, as their names imply, differ in moisture content, which averages between 30% to 40% and 40% to 50%, respectively. Of the different fermented meat products produced domestically or imported, pepperoni and salami are two of the most popular items, with pepperoni alone amounting to 180 million kg consumed annually (Hinkens et al., 1996). The most crucial processing step in sausage fermentation is to timely lower the pH of fresh meat (which averages about 5.6 to 5.8 post rigor) so as to curtail the growth of spoilage microorganisms The final pH of fermented sausages typically ranges from 4.8 to 5.2, depending on tanginess, firmness, and other product characteristics desired. Lactic acid bacteria, which produce lactic acid through glycolysis, can be introduced into meat either by chance inoculation (natural fermentation) or by inoculating a starter culture. In natural fermentation, lactic bacteria are inoculated by chance from the processing environment (e.g., processing equipment). Sometimes, a portion of already fermented meat from a previous batch is added to a new batch to start fermentation. This procedure, called backslopping, reduces the incubation time for the bacteria to reach a productive level. Natural fermentation has been used for centuries but it has many obvious disadvantages. The fermentation usually takes a long time (e.g., more than one week). The population and type of lactic acid bacteria in fresh meat are difficult to control. If the initial population of lactic bacteria is small and the meat pH cannot be rapidly lowered, spoilage microorganisms will predominate and the product will fail. Moreover, pathogens can grow well in meat when the pH is not sufficiently low, especially when they do not have to compete with lactic bacteria. Many lactic bacteria from chance inoculation are heterofermentative, i.e., in addition to producing lactic acid, they also produce acetic acid, alcohol, gas, etc. Because different bacteria species may be introduced each time, batch-to-batch variability in product flavor, acidity, and textural characteristics can be very high. Today, almost all commercial production of fermented sausages is done by using selected starter cultures. Starter cultures are available in two forms: frozen concentrate and lyophilized dry powder. Starter cultures available commercially are typically blends of two or more different microorganisms and sometimes different strains of the same microorganism. The most commonly used microorganisms are Lactobacillus, Pediococcus, Lactococcus (all three are homofermentative), and Micrococcus (used to reduce nitrate to nitrite). Specific examples are L. plantarum, P. acidilactici, and L. lactis sub sp. lactis. Fermented sausages are salted and cured, and both nitrate and nitrite can be used. Salt is needed to facilitate dehydration and impart flavor. Fermented sausages are usually heavily spiced, making the product particularly palatable. Organic acidulants, such as encapsulated glucono-delta-lactone and lactic acid, are sometimes mixed with fresh meat at the beginning of fermentation. They are used to quickly establish an acidic environment that will stimulate the growth of lactic acid bacteria and inhibit spoilage microorganisms. In fact, dry or semi-dry sausages can be produced by direct acidification with proper acidulants such as lactic acid and glucono-delta-lactone, a slow acid-releasing compound. Sausages prepared by direct acidification have a characteristic tangy flavor closely resembling that of fermented products, and they are most widely used in pepperoni production for pizza toppings. In order to produce lactic acid during fermentation, sugar must be present, which serves as substrate for glycolytic enzymes inside the bacterial cells. Simple sugars, such as sucrose and dextrose, are preferred because they can be readily transported through the bacterial cell wall. The amount of sugar added to dry or semi-dry sausages are typically in the range of 0.5% to 2.0%. The lower the desired pH, the more sugar will be needed. After ground meat is blended with all ingredients, including the starter culture, the mixture is stuffed and subsequently incubated in a closed chamber (sometimes a smoke-

Copyright 2001 by Marcel Dekker, Inc. All Rights Reserved.

366

Xiong and Mikel

house) to allow fermentation to take place. Temperature of the incubator is typically maintained at 21 to 24C with a 75% to 80% relative humidity for dry sausage, and 30 to 37C with a 75% to 80% relative humidity for semi-dry sausage. The fermentation time, however, is longer for dry sausage (1 to 3 days) than for semi-dry sausage (8 to 20 hours) (Terrell, 1977). For dry sausage production, the fermented meat is placed in a drying room to allow further dehydration and flavor development. As a general recommendation, the temperature of the drying room should be controlled to 7 to 13C and the relative humidity to 70% to 72%. The air of the drying room should be changed periodically to ensure air quality and prevent moisture buildup on the surface of sausage. The drying time varies considerably, depending on the size (diameter) and type of product. Most dry sausages are aged for somewhere between 10 days to 3 months. Dry sausages are not cooked, and most are not smoked. They do not require refrigeration after manufacture. The low moisture content (aw 0.91) and low pH conditions in the sausage are effective to preserve the product. Semi-dry sausages, however, are generally cooked to an internal temperature of at least 68C following fermentation. Semi-dry sausages have a relatively high moisture content (aw ~ 0.95) and hence, require refrigeration to prevent microbial spoilage. Most semi-dry sausages are smoked. IX. LUNCHEON MEATS This group encompasses a broad range of processed meats that are manufactured in the form of loaves or slices (Fig. 4). They have been introduced to the retail and convenience store or deli markets as consumer demand has grown for ready-to-eat or ready-to-heat products. Luncheon meats are fully cooked/pasteurized and require refrigeration for storage. Many of the loaves are pre-sliced and packaged for distribution to retail stores. Wholesale loaves, typically in the 3 to 5 kg range, are usually sliced in the deli at the time the customer purchases them. Because these meat products are sold in the deli section, they are sometimes referred to as deli meats. Luncheon meats, as the name indicates, are utilized primarily for lunch sandwich preparation. These products are normally restructured and may be cured or uncured with a variety of flavor profiles to meet the demand of ever-changing

Figure 4 Sliced deli or luncheon meats: restructured ham (left) and pickled loaf (right).

Copyright 2001 by Marcel Dekker, Inc. All Rights Reserved.

Meat and Meat Products

367

consumer makeup. Among the popular luncheon meat items manufactured in the United States are sliced ham (e.g., honey ham 98% fat free), bologna- and salami-type loaves, liver cheese (wrapped in a layer of pork fat), ham and cheese loaf, and head cheese (made of pork snout and tongue meat with added water, salt, gelatin, spices, flavorings, monosodium glutamate, dextrose, nitrite, etc.). It must be pointed out that the traditional luncheon meats, which are high-temperature processed and canned, have lost their true meaning, and should be distinguished from the deli-type meats that are consumed as convenient lunch items. There is a great variation in flavor and texture among luncheon meats, and the differences are derived from the specific processing procedures involved as well as the different formulations employed. For bologna-type luncheon meats, essentially the same meat ingredients as for sausages are used, and the same emulsion for making frankfurters can be used for luncheon meat loaves. However, luncheon meats often contain also other condiments to enhance palatability, e.g., pickles, pimentos, and olives. Luncheon meat loaves are stuffed into plastic or cellulose bags inside a stainless steel tube that has a 10 to 12 cm inner diameter. The meat batter is compressed into the tube, capped, and then cooked. Some luncheon meat loaves are prepared by stuffing inside a pan (similar to the bread pan). The open top surface is sometimes brushed with a thin layer of syrup to create a brown appearance after cooking. This type of luncheon meat, after slicing, can easily be used for sandwich preparation. A notable processing change that is made by some luncheon meat manufacturers in recent years is repasteurization of the fully cooked product after it is sliced. This is to assure that psychrotrophic pathogens, particularly Listeria monocytogenes, that could be introduced during slicing and handling are inactivated. Since reheating tends to cause additional water loss, it is important that a proper water-binding agent, such as polyphopshate and polysaccharide gums, be included in the product formulation. Another group of luncheon food product, referred to as Lunchables, have been developed in the past few years, led by Oscar Mayer Foods Division. This type of product is sealed in a small plastic package that consists of several compartments. Bite-size meat (e.g., ham, franks, and pepperoni) is placed in one compartment while cheese, cookies, crackers, desserts, or other nonmeat items are packed in other compartments (Fig. 5). This new marketing concept was developed to target children, including elementary school pupils, from busy families. It is nevertheless more of an ingenious marketing strategy than it is a true processing innovation.

Figure 5 Lunchables containing ham slices (left) or cooked beef patties (right).

Copyright 2001 by Marcel Dekker, Inc. All Rights Reserved.

368

Xiong and Mikel

X. PREPARED DINNER MEATS In the United States today, an estimated 50% of the food dollar is spent on foods prepared away from home. This is especially true for double working households that have little time to spend on meal preparation. According to the U.S. Department of Commerce, the percent of women working outside the home has increased from 42% in 1960 to 70% in 1995. These consumers are looking for options that allow their families to continue to enjoy the mealtime experience as a family without the hassle of extended preparation time. Options available may include eating out at restaurants, having takeout foods, or purchasing ready prepared meals to be reheated at home for the family meal. Taking advantage of this shift in consumer attitudes and lifestyle, many processors have developed new products as home meal replacements and component meals. In essence, the preparation time is shortened for the consumer by preparing the meal at the processing plant. Pre-prepared meat items have been available for years, including frozen dinners, sandwiches, pot pies, as mentioned previously. Today, frozen or refrigerated sandwiches are popular items in many institutional settings and can even be found in convenience vending machines. However, in many cases these items tended to lack consumer appeal, which prompted meat processors to develop new processing strategies. The first of the renewed generation of these products were home meal replacements that emerged a few years ago. This concept, similar to frozen dinners, provides all the components of the meal ready for the table. Everything, including the meat, vegetables, and other items, are included in the package (Fig. 6). The consumer only needs to cook the items or in some cases just reheat in a conventional oven or microwave. Although these products have filled a niche, consumers are still looking for other options with greater flexibility. This has led to the development of so-called component meals. These meals allow for an entree, and sometimes a side dish, to be prepared at the processing plant and packaged for refrigeration storage until reheated by the consumer. This allows even greater flexibility by the consumer to add variety to the meal at their own discretion. The sale of prepared refrigerated meals has gained wide popularity and is becoming a substantial market force with both the beef and pork industry. Another group of prepared products that are gaining sizable market share are battered and breaded meats. These types of products (long perfected by the poultry and seafood segments of the muscle foods processing industry) are becoming more commonplace in both the retail and the food service segments of the marketplace. Technologically, breading con-

Figure 6 Prepared meat items: home meal replacement (left) and injected pork roast (right).

Copyright 2001 by Marcel Dekker, Inc. All Rights Reserved.

Meat and Meat Products

369

sists of three major steps: predustingapplying a finely ground flour and seasoning mixture to the raw meat; batteringapplying a flour/seasoning batter of specific consistency to the predusted meat; and breadingapplying a flour mixture with a coarser crumb texture. Not all three steps may be required, depending on the specific products. Sometimes, the raw meat is marinated in water and phosphate solutions before predusting to pick up moisture for improved palatability and product yield. The breaded meat can be fried for a few seconds (raw) or up to a few minutes (fully cooked) to help the batter adhere to the meat, and the final product is either frozen or refrigerated before marketing. Breaded meats are of great interest to the meat industry partly due to profit margins available with these items induced by a lower input cost per unit. In addition, this process allows for creation of greater product variety. Whether the product is restructured or is made from whole muscle, the addition of up to 30% breading allows the processor to meet the consumer demand for convenient products while also targeting both the retail and food service industries. Many of these products are now being used or evaluated as finger foods or appetizers. Another type of product that has gained widespread acceptance is that of enhanced products. In essence, these are fresh whole muscle products, both beef and pork, that have been subjected to injection or marination to improve sensory acceptability (Fig. 6). These products normally are injected with a combination of water, salt, and phosphate with the addition of seasoning or flavoring if an ethnic flavor profile is desired. In most cases, the products have an addition of less than 10% added ingredients (which are labeled as x% solution containing ....). This process not only allows the processor to improve the economic viability of many lower value meats but also enhances consumer acceptance by reducing dissatisfaction with the product that may result from the use of less palatable raw materials or consumer abuse due to overcooking. REFERENCES
American Meat Institute (AMI). Meat and Poultry Facts. Washington, DC: American Meat Institute. 1999. Cassens, R.C. Residual nitrite in cured meat. Food Technol 51(2):5355, 1997. Fennema, O. Freezing preservation. In Principle of Food Science. Part II. Physical Principles of Food Preservation. M. Karel, O.R. Fennema, and D.B. Lund (Eds.), Ch. 6, pp. 173215, New York: Marcel Dekker, 1975. Frye, C.B. Manufacturing sausage without casings. Proc Recip Meat Conf 49:3948, 1996. Hinkens, J.C., Faith, N.G., Lorang, T.D., Bailey, P., Buege, D., Kaspar, C.W., and Luchansky, J.B. Validation of pepperoni processes for control of Escherichia coli O157:H7. J Food Prot 9:12601266, 1996. Pearson, A.M., and Gillett, T.A. Processed Meats. 3rd ed., New York: Chapman & Hall, 1996. Romans, J.R., Costello, W.J., Carlson, C.W., Greaser, M.L., and Jones, K.W. The Meat We Eat, 13th ed. Danville, IL: Interstate Publishers, 1994. OBoyle, A.R., Rubin, L.J., Diosady, L.L., Aladin-Kassam, N., Comer, F., and Brightwell, W. A nitrite-free curing system and its application to the production of wieners. Food Technol 44(5):88104, 1990. Terrell, R.N. Practical manufacturing technology for dry and semi-dry sausage. Proc Recip Meat Conf 30:3948, 1977. United States Department of Agriculture (USDA). Control of added substances and labeling requirements for cured pork products. Updating provisions. Fed Reg 49:1485614887, 1984. USDA. Agricultural Statistics. National Agricultural Statistics Service, Washington, DC, 1998. USDA. U.S. Agricultural Trade Update. Monthly Supplement to Foreign Agricultural Trade of the United States, Washington, DC, 1999.

Copyright 2001 by Marcel Dekker, Inc. All Rights Reserved.