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National Research Council (US) Board on Agriculture and Renewable Resources. Fat Content and Composition of Animal Products: Proceedings of a Symposium Washington, D.C. December 12-13, 1974. Washington (DC): National Academies Press (US); 1976.

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Fat Content and Composition of Animal Products: Proceedings of a Symposium Washington, D.C. December 12-13, 1974.

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Eating Quality of Meat Animal Products and Their Fat Content

G. C. Smith and Z. L. Carpenter

Historical Aspect

The belief that fat deposition enhances the value of meat is not of recent origin, having been suggested or implied in both the Old and New Testaments of the Holy Bible. Adam's second son apparently believed that fat cuts were desirable, since the Book of Genesis (chapter 4, verse 4) records that "Abel brought the fatty cuts of meat from his best lambs and presented them to the Lord" (The Living Bible, 1971). In the account of the reaction of the father to the prodigal son (Luke, chapter 15, verse 23), the slaves were ordered to "kill the calf we have in the fattening pen" for a celebration feast (The Living Bible, 1971).

Included among those who perpetuated the belief that fatness was positively associated with the palatability of cooked meat were several of the more famous animal husbandmen of the eighteenth and twentieth centuries. In 1756 Robert Bakewell set about to improve the Leicestershire sheep of England. According to Ensminger (1960), "Bakewell gradually transformed the large, heavy-boned, and heavy-framed sheep, that had little or no propensity to fatten quickly, to a shorter-legged, blocky form with finer bone and quick-fattening propensities." Hall (1910) associated marbling with tenderness and postulated that "increased tenderness results from a decrease in the elasticity of the connective tissue due to the deposition of fat therein." Henry and Morrison (1916) explained that ''a fat animal has fat deposited between the bundles of muscle fibers thus separating them, and the lean from such an animal is more tender than the lean from an animal which has not been fattened." Bull (1916), in his discussion of the reason for fattening market animals, stated that "the main object in fattening is to improve the flavor, tenderness, and quality of lean meat by the deposition of fat between the muscular fibers."

Armsby (1908) said "experience has shown that the tenderness and palatability of the lean meat are notably greater when it is accompanied by considerable fat." Armsby (1917) also stated that "fattening of animals as a commercial process is a practice based on experience which has shown that tenderness and palatability of the meat are increased thereby, so that the consumer is willing to pay a higher price." According to Helser (1929), "a well-marbled piece of meat is usually more tender and juicy than meat deficient in fatness." Morrison (1937) said that "lean meat from a well-fattened animal is better flavored and more juicy and tender than meat deficient in fatness. Storage of fat, which forms the so-called 'marbling' of meat, adds to its tenderness, juiciness, and flavor." Since none of these husbandmen presented substantiating data, their statements were probably assumptions, personal opinions, or conjecture.

Some early research was conducted. Gardner and Adams (1926) studied consumer habits and preferences with regard to beef purchases. They concluded (in contrast to Armsby's opinion in 1917) that "consumers are not always willing to pay a proportionately higher price for a highly finished carcass" and (in contrast to Bull's opinion in 1916) that "either fatness is not related to the quality of meat or the American people know very little about quality, otherwise the Prime grade would constitute a much greater percentage of the total number of carcasses sold on the market." Willman (1937) studied consumer demand for lamb and reported that "Prime lamb carries excess fat not desired by the consumer and is in less demand than either Choice or Good lamb in the eastern markets." Hammond (1932) found practically no relationship between marbling and tenderness in lamb, yet concluded that "no doubt such a correlation [a positive relationship between marbling and tenderness] does exist with animals of different degrees of fatness." This statement indicates a decided reluctance on Hammond's part to disagree with an idea that had become so firmly entrenched in the minds of most animal husbandmen.

Lowe (1932) stated that "the deposition of fat, either intramuscularly, intrafasicularly or intracellularly, tends to lessen the toughness of meat." She referred to data collected by Nelson et al. (1930) that documented an 18% to 30% decrease in shear-force values for samples from fat animals in relation to the force required to shear samples from thinly finished animals. Cover et al. (1944) noted that "a theory widely held for some time is that the fatter the animal the tenderer its meat will be, but conclusive proof of this theory is lacking." Cover et al. (1956) also said that "it is doubtful that fatness by itself is responsible for a marked increase in tenderness and juiciness. It is disconcerting that something which has appeared so obvious to so many for so long should be so extraordinarily difficult to prove in the laboratory."

In summary, most of the early statements associating fatness with palatability were largely unsupported by experimental fact. Statements to the contrary, even those supported by research data, were usually disregarded. Cover et al. (1958) relate that in the early 1930's it was thought that the rib and loin cuts could be relied upon for tenderness if they came from fat animals of beef breeding and from high grading carcasses; if such cuts were not tender after cooking then the belief was that "a poor cook had spoiled good meat." It is the purpose of the present review to briefly survey the literature regarding the palatability attributes of meat and the relationship of fat content to the eating quality of cooked muscle. Regardless of the nutritional excellence and adaptability of meat as an item in the diet, meat will be consumed in adequate and increasing quantities only if it appeals to and is accepted by the consumer on the basis of its palatability characteristics (Weir, 1960).

Eating Quality

The ultimate goal of the meat industry is to place a product on the consumer's table that will result in a high degree of eating satisfaction and that will be available at a reasonable cost. Wismer-Pederson (1958) observed that the demand for a meat product depends upon its quality, thus the market for fresh meat will become more and more discriminative with regard to quality attributes. Investigations of quality in meat are complicated from the outset by the lack of a clear definition for the term "quality" (Joubert, 1956). Pearson (1968) suggested that quality is a combination of the attributes—flavor, juiciness, texture, tenderness, and appearance—that contribute to the eatability or the desirability of the product. Kauffman (1959) noted that the quality factors of pork, lamb, and beef are related in terms of tenderness, flavor, juiciness, and color. The consumer relates to quality in terms of the tenderness, juiciness, and flavor of the cooked product (Bray, 1966). Meat palatability depends upon such qualities as color, odor, flavor, juiciness, tenderness, and texture (Weir, 1960).

Pork-quality research conducted at the University of Wisconsin (1963) revealed that the optimal indicators of cooked-pork palatability are marbling, color, firmness, and physical structure. Bray (1966), Rust and Topel (1969), and Skelley and Handlin (1971) agree that marbling, color, and firmness are the best visual indicators of quality in pork. Quality at the retail level can probably never be described exactly, since it depends on the palatability preferences of consumers; and not all consumers agree regarding palatability attributes (Smith, 1968). In general, however, a given consumer's acceptance of a cooked meat is determined by his singular or combined responses to the flavor, juiciness, and tenderness of that product (Jeremiah et al., 1970).

Flavor

Flavor is a complex sensation involving odor, taste, texture, temperature, and pH (Lawrie, 1966). Of these, odor or aroma is most important, because without odor, one or the other of the four primary taste sensations (bitter, sweet, sour, and salty) will predominate. When the effect of odor or aroma sensations is reduced or removed, meat flavors are extremely difficult to distinguish. Crocker (1948) reported that differences in meat flavor are primarily the product of differences in odor. Under ideal circumstances, response to odor is about 10,000 times more sensitive than that to taste (Lawrie, 1966). Thus, while ethyl mercaptan can be detected in air at a concentration of 3 × 10-9 percent, the sensation of bitterness, which is the most acute taste, is detectable from strychnine at a concentration in water of 4 × 10-5 percent (Lawrie, 1966). Aroma condensates from cooked meat have been shown to contain ammonia, amines, indoles, hydrogen sulfide, and short-chain aliphatic acids; but the relationship of these compounds to specific cooked meat aroma decriptions ("animal," "brothy," "metallic," "sour," ''sweet,'' "nose-filling," and "fatty") has not been established (Weir, 1960).

Meat flavor, like aroma, is very difficult to evaluate and describe. Communication among researchers regarding flavor is effected by use of such description terminology as "bouquet," "serum," "brothiness," "mouthfulness," "aftertaste," "mouth-coating," "animal," "metallic," "astringent," "sweet," "sour," "flat," "bland," "chickenlike," and "liver-like" (Bratzler, 1971). Early writers (e.g., Ziegler, 1962, and the National Live Stock and Meat Board, 1950) attributed the distinctive flavor of meat to the presence and quantity of nitrogenous extractives such as creatine, creatinine, purines, and pyrimidines. Caul (1957) attributed beef flavor to a combination of cooked Mood salts, products of pyrolysis, and saliva.

Studies of volatile compounds from cooked meat have suggested that ring compounds (e.g., oxazoline and trithiolan), hydrocarbons, aldehydes, ketones, alcohols, acids, esters, ethers, lactones, aromatics, sulfur-containing compounds (e.g., mercaptans and sulfides) and nitrogen-containing compounds (e.g., amines and ammonia) are of importance in creating the characteristic flavor of meat (Hornstein, 1971). As a result of a series of investigations of beef, pork, lamb, and whale meat, Hornstein et al. (1960, 1963) and Hornstein and Crowe (1960, 1963) concluded that an identical basic meaty aroma is associated with the lean portion of these meats and that species differences in flavor reside in the fat. A recent report by Pearson (1974) citing research conducted by Wenham and associates in New Zealand tends to confirm the previous findings. Hornstein (1971) has concluded that a nonenzymatic, browning-type reaction between reducing sugars and amino acids is largely responsible for development of characteristic meat flavor and that the similarity in amino acid and carbohydrate composition of beef, pork, and lamb may account for the similarity in flavor of the lean meat from these species.

Nevertheless, there are differences in flavor among species (Table 1). Beef flavor is characterized as mouth-filling, serumlike, and with good bouquet; veal is sweet, sour, or fiat; pork flavor is bland, sweet, and chickenlike; and lamb has a livery, predominately animal-like flavor and a greasy mouth-coating effect and aftertaste (Weir, 1960). There is considerable variability among human subjects in intensity and quality of response to a given flavor or odor stimulus, with some individuals preferring meat that is essentially bland and others desiring meat that is very intense. It is interesting to note that members of an uninformed panel (Table 1) like the flavor of lamb, despite the fact that many consumers, when questioned, express disdain for such product. Wasserman and Talley (1968) confirmed that perceived differences in flavor and aroma between meats from different species were largely a result of changes in components in the fatty portions of the sample.

TABLE 1. Flavor Ratings for Rib or Loin Samples from Five Meat Animal Species.

TABLE 1

Flavor Ratings for Rib or Loin Samples from Five Meat Animal Species.

Fat may affect flavor in two ways (Hornstein, 1971): (a) fatty acids, on oxidation, can produce carbonyl compounds that are potent flavor contributors, and (b) fat may act as a storage depot for odoriferous compounds that are released on heating. The volatiles released from fat or produced from triglyceride or phospholipid fractions may be responsible for the characteristic differences that are associated with the flavors of beef, pork, and lamb (Hornstein, 1971). Hornstein and Crowe (1960, 1963) reported that octanal, undecanal, hepta-2,4-dienal, and nona-2,4-dienal are derived from heated pork fat, but not from beef fat, while few 2,4-dienals are generated by heating lamb fat.

Smith and Carpenter (1970) reported that hard, white subcutaneous fat was associated with high flavor and aroma ratings for lamb cuts, while soft or brownish-colored external fat was associated with undesirable flavor and aroma scores (Table 2). Hofstrand and Jacobson (1960) had previously suggested a relationship between depot fats and the aroma of lamb and mutton broths. Kauffman et al. (1964c) reported that pork carcasses with subcutaneous fat depots containing higher quantities of moisture and unsaturated fatty acids produced cooked pork that had less-favorable palatability characteristics. Depot fats serve either as the source of flavor and aroma precursors or as the storage medium for odoriferous compounds that are volatilized and released from fat during cooking.

TABLE 2. Relationship of Subcutaneous Fat Cover Characteristics and USDA Quality Indicators to Flavor and Aroma Scores.

TABLE 2

Relationship of Subcutaneous Fat Cover Characteristics and USDA Quality Indicators to Flavor and Aroma Scores.

Although the basic meaty flavor is nonlipid in origin, some quantity of fat is undoubtedly necessary to make beef, for example, taste rich, full, and "beefy" and to assure that flavors are species-specific. As animals increase in age, flavor precursors or odoriferous compounds may be concentrated in the fat depots and intense flavors or odors may result. In the latter case, increased deposition of fat could serve to dilute these precursors or compounds and to make the flavor or aroma less pronounced. The role of fat as a flavor or aroma diluent deserves greater study.

The amount of fat, on or in the animal and meat, that is necessary to fulfill the appropriate flavor and aroma functions is not presently known. The latter conclusion obtains, despite the endeavors of a large number of investigators to determine the fat content necessary to establish the optimum quality level with respect to tenderness, juiciness, and flavor factors (Simone et al., 1958). Numerous researchers have attempted to relate fatness to flavor desirability and/or intensity in cooked meat. Results of some of these studies are presented in Table 3. In these and subsequent tables of the same kind, we have attempted to categorize relationships from "very low" to "high," knowing full well that other readers, searching the same literature, may well have interpreted these data in another manner and could well have assigned a different rank to the associations described. Nevertheless, these data suggest that fatness has a low relationship to flavor desirability in lamb and a low-to-moderate relationship to desirability of flavor in both pork and beef.

TABLE 3. Research Reports Relating Level of Fatness, USDA Quality Grade, USDA Marbling Score, and Intramuscular Fat Content to Flavor of Lamb, Pork, and Beef.

TABLE 3

Research Reports Relating Level of Fatness, USDA Quality Grade, USDA Marbling Score, and Intramuscular Fat Content to Flavor of Lamb, Pork, and Beef.

Juiciness

Lawrie (1966) reported that differences in pH, water-holding capacity, fatness, and firmness were directly related to juiciness scores for cooked meats. Kauffman et al. (1964a) and Carpenter et al. (1965d) generally supported such relationships for pork muscle; Smith and Carpenter (1970) reported that differences in moisture, fat, and pH were related to juiciness in lamb muscle; Berry (1972) found that differ ences in fat, moisture, and water-holding capacity were associated with the observed variability in beef juiciness.

In that it affects the appearance of the meat before cooking, its behavior during cooking, and juiciness on mastication, the water-holding capacity of meat is an attribute of obvious importance (Lawrie, 1966). Diminution of water-holding capacity is manifested by exudation of fluid known as "weep" or "purge" in uncooked meat that has not been frozen, as "drip'' in thawed (previously frozen) uncooked meat and as "shrink" or "cooking loss'' in cooked meat, where it is derived from both aqueous and fatty sources (Lawrie, 1966). When muscle fibers are cut perpendicular to their longitudinal axis they vary in exudation—from a complete absence of exudate to an extremely large quantity of exudating juice (Briskey and Kauffman, 1971). The presence of surface juice is the result of changes in the water-holding capacities of muscle proteins and is closely associated with pH—a low pH is extremely detrimental to water-binding if storage temperatures are above 20º C (Briskey and Kauffman, 1971). Firmness in meat is associated with a rigid structure, high juice retention, and limited losses of fluid during processing or cooking; however, the presence of intramuscular fat deposits can increase apparent firmness without actually influencing fluid retention (Carpenter, 1962).

Weir (1960) reported that juiciness is comprised of the combined effects of initial fluid release and the sustained juiciness resulting from the stimulating effect of fat on salivary flow. Descriptions of differences in juiciness among samples of cooked meat (Bratzler, 1971) are related in terms of (a) initial fluid release (the impression of wetness perceived during the first chews, produced by the rapid release of meat fluids); and (b) sustained juiciness (the sensation of juiciness perceived during continued chewing, created by the release of serum and due, in part, to the stimulating effect of fat on salivary flow). Initial fluid release from meat is undoubtedly affected by degree of doneness and method of cooking, while sustained juiciness is related to intramuscular fat content (Pearson, 1966).

One of the most important factors influencing juiciness of meat (especially initial fluid release) is the cooking procedure. Methods of cookery which result in the greatest retention of meat fluid (water or lipid) and hence in the lowest cooking losses are associated with enhanced juiciness of the final product (Smith, 1972). Beef cooked "rare" is juicier than beef cooked "well-done"; and pork, lamb, and veal, which are ordinarily cooked "well-done," are less juicy than beef (Weir, 1960). Cooking losses from good-quality meat tend to be lower than those from poor-quality meat (Saffle and Bratzler, 1957). Although high-quality meats lose more fat during cooking (which is expected because of their greater fat content), they lose less moisture, possibly because some structural change (caused by the presence of marbling) enhances the water-holding capacity (Saffle and Bratzler, 1957). Some of the shrinkage loss during cooking is due to the loss of fluid fat, since high temperatures will melt fat, and some is due to the method, time, and temperature of cooking, since the high temperatures involved will cause protein denaturation, considerable lowering of water-holding capacity, and subsequent loss of fluid or vaporous moisture (Lawrie, 1966). An increase in the degree of shrinkage during cooking is directly correlated with a loss of juiciness upon consumption.

Since sustained juiciness during chewing leaves a more lasting impression than does the initial release of fluid, it is quite understandable that most studies of factors affecting meat juiciness have shown a closer correlation between juiciness scores and fat content of the meat than between juiciness scores and amount of press fluid (as a measure of water-holding capacity) from the meat (Bratzler, 1971). Tenderness and juiciness are closely related; the more tender the meat, the more quickly the juices are released by chewing and the more juicy the meat appears. For tough meat, however, the juiciness is greater and more uniform if the release of fluid and fat is slow (Weir, 1960). Since marbling or intramuscular fat would increase the sensation of sustained juiciness in less-tender meat, its association with juiciness is apparent.

Bray (1964) reported that beef that is practically devoid of marbling is less palatable than beef with some marbling. Those fats that are present in and around the muscle fiber serve to lubricate the fibers and so make for a juicier cooked product (Carpenter, 1962). A moderate quantity of marbling is adequate to lubricate the muscle fibers and thus provide for a juicy and flavorful cooked product (Briskey and Kauffman, 1971). Too little marbling may be responsible for a dry, flavorless product, whereas excess marbling fails to contribute proportionate improvement to eating satisfaction. If marbling enhances juiciness by serving as a lubricant around muscle bundles, then it is important that marbling be uniformly and finely dispersed throughout the muscle (Briskey and Kauffman, 1971).

A number of researchers have related fatness to the juiciness of cooked meat. Results of some of these studies are presented in Table 4. The consensus from these data suggests that fatness has a moderate relationship to juiciness in lamb, a moderate-to-high relationship to juiciness in pork, and a low-to-moderate relationship to juiciness in beef.

TABLE 4. Research Reports Relating Level of Fatness, USDA Quality Grade, USDA Marbling Score and Intramuscular Fat Content to Juiciness of Lamb, Pork, and Beef.

TABLE 4

Research Reports Relating Level of Fatness, USDA Quality Grade, USDA Marbling Score and Intramuscular Fat Content to Juiciness of Lamb, Pork, and Beef.

Tenderness

Consumer studies have shown that tenderness is the most important palatability factor in acceptance of beef and probably of other meats, although variations in tenderness of pork, lamb, veal, and poultry are not great (Bratzler, 1971). Smith (1972) concluded that tenderness is the most important single attribute contributing to beef palatability. The lack of variability among samples of lamb and pork in relation to the wide range in tenderness inherent in beef was once attributed largely to differences in age at slaughter. However, steadily declining ages at slaughter and increased finishing of beef cattle on high-concentrate rations have not completely alleviated the observed variability in tenderness.

Of all the attributes of eating quality, texture and tenderness are presently rated most important by the average consumer and appear to be sought at the expense of flavor or color, notwithstanding the fact that both of the former terms are most difficult to define (Lawrie, 1966). Cover and Hostetler (1960) described tenderness differences in terms of (a) amount and firmness of connective tissue; (b) crumbliness of muscle fibers; and (c) softness to teeth, tongue, and cheek. Weir (1960) emphasized that the overall impression of tenderness consists of at least three components: (a) the initial ease of penetration of the meat by the teeth, (b) the ease with which the meat breaks into fragments—friability or mealiness, and (c) the amount of residue remaining after chewing. Friability may well reflect muscle-fiber resistance to breakage perpendicular to its axis, while the amount of residue is thought to reflect the amount of collagen or connective tissue present in the meat (Bratzler, 1971).

Differences in tenderness among muscles or samples of meat occur as a result of the collective effects of numerous traits that can be broadly classified (Smith et al., 1973b) as follows: (a) actomyosin effects—contractile state of actomyosin and/or integrity of the Z line; (b) background effects—amounts of connective tissue and/or chemical state of collagen; and (c) bulk density or lubrication effects —amount, distribution, and chemical or physical state of intramuscular fat and moisture. Research by Berry et al. (1974b) indicated that smaller-diameter muscle fibers, longer sarcomeres, shorter muscle-fiber fragments following homogenization, lower percentages of wavy fibers (actomyosin effects); decreased collagen content, increased percentages of soluble collagen, lower myofibril fragmentation scores (background effects); and increased percentages of fat, decreased percentages of moisture, and smaller quantities of expressible juice (bulk density or lubrication effects) were associated with increases in the tenderness of the beef longissimus muscle.

According to Lawrie (1966), the degree of tenderness can be related to three categories of protein in muscle—those of the myofibril (actin, myosin, tropomyosin), of the connective tissue (collagen, elastin, reticulin, mucopolysaccharides of the matrix), and of the sarcoplasm (sarcoplasmic proteins, sarcoplasmic reticulum). The importance of their relative contribution depends on circumstances. The importance of state of contraction of the myofibrillar proteins in determining tenderness has been emphasized by several researchers. Hostetler et al. (1970), Orts et al. (1971), and Smith et al. (1971) demonstrated that changes in the manner of carcass suspension and/or in postmortem chilling rate increased the tenderness of beef by increasing the length of the sarcomeres in muscle and/or by disrupting the integrity of the Z lines in muscle fibers. Marsh and Leet (1966) and Quarrier et al. (1972) reported similar findings regarding postmortem chilling rate, manner of carcass suspension, and/or tenderness of lamb carcasses. All five of these studies were designed to identify procedures that would counteract the effects of cold shock in shortening sarcomeres and muscle fibers (cold-shortening) and thereby decreasing the tenderness of the cooked product.

Cross et al. (1973) concluded that, although connective tissue is a contributing factor to the toughness of meat, total concentrations of connective tissue components (collagen and elastin) were not closely related to sensory panel ratings for tenderness. Percent-soluble collagen, a measure of the susceptibility of collagen to heat, was significantly related to the contribution of connective tissue to toughness, but elastin concentration was not (Cross et al., 1973). Cooking generally makes connective tissue more tender by converting collagen to gelatin, but it coagulates and tends to toughen the protein of the myofibril (Lawrie, 1966). Both of these effects depends on time and temperature of cookery, the former being more important for the softening of collagen and the latter more critical for myofibrillar toughening (Weir, 1960). For muscles with large amounts of connective tissue, the toughening of muscle fibers is less important than the softening of collagen; thus cooking methods combining a long heating period and a moist atmosphere are chosen. For muscles with only small amounts of connective tissue, cooking methods involving dry heat for a short time are used to minimize the toughening effect on the muscle fibers (Bratzler, 1971).

That the tenderness of beef increases with fatness has been believed widely for a long time; however, this relationship is not as direct as was formerly believed, and other factors may have as great or greater importance in determining tenderness or toughness (Cover et al., 1958). A number of investigators have related fatness to tenderness in cooked meat. Results of some of these studies are presented in Table 5. As has been previously suggested by Simone et al. (1958) and Wellington and Stouffer (1959), experimental evidence supporting the value of fat for improving tenderness has been somewhat conflicting and subject to a high degree of variability among the findings. Cover et al. (1958) concluded that a few of the factors determining tenderness may be included among those used in determining carcass grade; however, carcass grades are designed to classify things other than tenderness and exact tenderness classification should not be expected.

TABLE 5. Research Reports Relating Level of Fatness, USDA Quality Grade, USDA Marbling Score and Intramuscular Fat Content to Tenderness of Lamb, Pork, and Beef.

TABLE 5

Research Reports Relating Level of Fatness, USDA Quality Grade, USDA Marbling Score and Intramuscular Fat Content to Tenderness of Lamb, Pork, and Beef.

Little is presently known of the exact mechanism by which fat deposition influences the ultimate tenderness of cooked meat. Whatever is the effect of fatness on tenderness, the relationship is presently believed to be most aptly related in terms of the deposition of intramuscular fat or marbling. Jeremiah et al. (1970) concluded that if finish contributes to the eating satisfaction of a meat product, then some measure of finish deposited within the muscle should serve as an indication of its relative palatability. Marbling is the subjective measure of intramuscular fat included or alluded to in USDA quality-grading standards. Many research workers have utilized solvent extractable fat as an absolute measure of fatness within the muscle.

There are at least four theories regarding the effect of intramuscular fat or marbling on the tenderness of muscle.

1.

Bite theory This theory suggests that within a given bite-size portion of cooked meat, the occurrence of marbling decreases the mass per unit of volume, lowering the bulk density (Smith et al., 1973b) of the chunk of meat by replacing protein with lipid. Since fat is much less resistant to shear force than coagulated protein, the decrease in bulk density is accompanied by an increase in real or apparent tenderness. Henry and Morrison (1916) suggested that marbling is deposited between the bundles of muscle fibers, thus separating them and making the lean more tender. Cover and Hostetler (1960) described a theory in which fat deposited as marbling within muscle cells was supposed to distend them and make them more tender. Lawrie (1966) states that intramuscular fat (marbling) tends to dilute the connective tissue of elements in muscle in which it is deposited, thereby explaining the greater tenderness of meat from well-fed, good-quality animals. Jeremiah et al. (1970) concluded that marbling may serve only as a dilution factor, so that within a given area of muscle, fewer muscle fibers must be severed in the chewing or shearing process.

2.

Strain Theory According to this theory, as marbling is deposited in the perivascular cells inside the walls of the perimysium or endomysium, the connective tissue walls on either side of the deposit are thinned, thereby decreasing their effective width, thickness, and strength. Hall (1910) suggested that the increase in tenderness associated with the deposition of marbling results from a decrease in the elasticity of the connective tissue. Lowe (1932) stated that the deposition of fat, either intramuscularly, intrafasicularly, or intracellularly, tends to lessen the toughness of meat. According to Cover and Hostetler (1960), marbling has been regarded as an important indicator of tenderness, because fat is supposed to spread apart the strands of connective tissue between the muscle fibers and between the muscle bundles, making the meat more tender. Carpenter (1962) concluded that it is reasonable to assume that marbling, which has infiltrated connective tissue, aids in the ultimate breakdown of collagen upon cooking. Fat deposits may spread the connective tissue fibrils apart, thereby providing a looser structure that aids in the heat penetration and, consequently, the solubilization of these connective tissue strands (Carpenter, 1962).

3.

Lubrication theory This theory suggests that intramuscular fats, present in and around the muscle fibers, serve to "lubricate" the fibers and fibrils and so make for a more tender and juicier product that confounds the sensation of tenderness alone (Carpenter, 1962). Tenderness is intimately associated with juiciness. Meat that is tender releases its juices and fat more readily upon chewing, and meat that is juicy remains moist even during long periods of sustained mastication and thus "seems" more tender. Correspondingly, tough meat may be perceived as being more tender than it actually is if fluid and fat release is slow and sustained. Marbling that is uniformly dispersed through the muscle will enhance juiciness by lubricating maximal numbers of muscle fibers (Briskey and Kauffman, 1971). According to Jeremiah et al. (1970), intramuscular fat is solubilized when muscle is heated and becomes a part of the meat juices that serve to lubricate the meat during the chewing process; in this manner marbling contributes directly to juiciness and indirectly to tenderness.

4.

Insurance theory This theory suggests that the presence of higher levels of marbling allows the use of high-temperature, dry-heat methods of cookery and/or the attainment of advanced degrees of final doneness without adversely affecting the ultimate palatability of the cooked meat. As such, marbling would provide some insurance that meat cooked too rapidly, too extensively, or by the wrong method of cookery would still be palatable. Since fatty tissue does not conduct heat as rapidly as lean tissue, it is probable that marbled meat can endure higher external cooking temperatures without becoming overcooked internally; thus, consumers who prefer meat cooked "well-done" would probably be more nearly satisfied with the outcome if highly marbled meat was used (Briskey and Kauffman, 1971). Cover and Hostetler (1960) investigated but could not completely confirm the theory that carcass grade and marbling level were good bases for deciding between moist-and dry-heat methods of cooking. This theory suggests that dry-heat cookery is suitable only for naturally tender cuts of beef (those with no tough connective tissue and with high levels of marbling and thus from high-graded carcasses) like the rib and loin from Prime, Choice, and Good carcasses and the top round, rump, and blade chuck from Prime and Choice carcasses. Avoidance of the high temperatures associated with dry-heat cookery was believed necessary to prevent adverse toughening in the "less-tender" cuts like the rib and loin from carcasses below the Good grade; the top round, rump, and blade chuck from carcasses below the Choice grade, and the bottom round from carcasses of all grades (Cover and Hostetler, 1960). Briskey and Kauffman (1971) are of the opinion that meat with higher levels of marbling can be prepared by use of more severe methods of cookery with greater assurance that ultimate eating quality will not be adversely affected.

Irrespective of the exact mechanism by which marbling or intramuscular fat exerts an influence on tenderness, a number of researchers (cited specifically by Cover and Hostetler, 1960; Blumer, 1963; and Jeremiah et al., 1970) have concluded that marbling is not an infallible indicator of tenderness. Numerous investigators have related marbling or intramuscular fat content to the tenderness of lamb, pork, or beef. Results of some of these studies are presented in Table 5. These data suggest that fatness has a moderate relationship to tenderness in pork and a low-to-moderate relationship to tenderness in both lamb and beef.

The most recent theory regarding the mechanism by which increased fatness enhances the tenderness of meat animal carcasses is that of Smith et al. (1974c). Smith and his associates, working with goat carcasses of widely varying sizes and degrees of finish, noted that cabrito carcasses from very small and very lean animals sustained extensive shortening of the myofibrils when subjected to the cold temperatures encountered during postmortem chilling. Smith (1968) had reported that increased quantities of kidney knob and pelvic fat in lamb carcasses were highly associated with the tenderness of loin chops. Smith et al. (1969a) determined that cores of cooked beef longissimus muscle from the most peripheral and most lateral locations in the cross-section (those nearest the outside surfaces of the carcass) were the least tender. Smith et al. (1971) suggested that a decrease in the rate of postmortem chilling was associated with a marked improvement in the tenderness of beef carcasses. The latter studies led these researchers to hypothesize that the real effect of fatness (subcutaneous, kidney knob and pelvic, and intramuscular fat) in increasing the tenderness of meat animal carcasses was a result of the ability of fat to insulate muscle fibers against "cold-shock" during postmortem chilling. According to their original hypothesis (Smith et al., 1974c), "since fat animals are usually more tender than lean lambs of the same maturity, subcutaneous fat may insulate muscle against cold-shock early in the postmortem chilling period and thereby increase tenderness by preventing cold shortening."

Results of the first experiment designed to test the latter hypothesis are presented in Table 6. Carcasses with a thick subcutaneous fat covering chilled more slowly, had muscles with slightly longer sarcomeres, and produced more tender chops than did thinly finished carcasses. In a series of subsequent experiments with both lamb and beef carcasses, the original hypothesis has been modified to include the possibility that the naturally occurring proteolytic enzymes in muscle remain active for longer periods of time postmortem when chilling is delayed and thereby increase the ultimate tenderness of cooked meat (Dutson et al., 1974, unpublished data, Texas Agricultural Experiment Station). It seems quite likely, at the present time, that increased quantities of subcutaneous fat and/or marbling insulate the muscles or muscle fibers during postmortem chilling, decrease the rate of temperature decline, enhance the activity of proteolytic enzymes in muscle, and lessen the extent of muscle-fiber shortening, thereby increasing the tenderness of cooked muscle.

TABLE 6. Temperature, Tenderness, and Sarcomere Length Data for Lamb Carcasses in Three Subcutaneous Fat Thickness Groups.

TABLE 6

Temperature, Tenderness, and Sarcomere Length Data for Lamb Carcasses in Three Subcutaneous Fat Thickness Groups.

Fatness and Eating Quality in Other Meat Products

The effect of fatness on palatability has been studied using several other kinds or classes of meat and meat products. Results of some of these studies are included in Table 7. Cole et al. (1960) studied the preferences of consumers for ground beef that contained 45%, 35%, 25%, and 15% fat and reported that members of the laboratory panel preferred patties with 45% fat while members of the consumer panel preferred patties with 25%-35% fat. Carpenter et al. (1972) compared all-beef patties with 20% or 25% fat and reported flavor, juiciness, texture, and overall satisfaction scores (8=extremely desirable) of 4.7 and 6.9, 4.1 and 5.7, 4.6 and 6.3, and 4.7 and 6.1, respectively. Smith et al. (1974b) reported that patties comprised of ground beef and textured vegetable protein containing 30% fat were more desirable in all palatability characteristics than counterpart samples containing 20% fat. Juhn (1972) observed that consumer panelists preferred frankfurters containing 35% fat to those containing 25% fat.

TABLE 7. Studies of Fat Content in Other Meat Products.

TABLE 7

Studies of Fat Content in Other Meat Products.

Smith et al. (1974b) reported that increases in fat deposition (thickness and firmness of flank; marbling in the ribeye; streaking in the upper flank, lower flank, diaphragm muscle, and intercostal muscle) were associated with increases in flavor intensity but not with increases in juiciness, tenderness, or overall satisfaction of goat meat. Carpenter (1962) found that the tenderness of bacon increased as the amount of marbling in the lean increased, but that differences in marbling were not associated with the variability in flavor or juiciness ratings. Smith and Carpenter (1974) reported that overall satisfaction ratings for cooked bacon decreased by only 1 unit on an 8-point scale, even though the fat percentage of the slices increased from 55% to 85%. The data in Table 7 suggest that even when the fat content of certain meat products is already high (e.g., 15% or 20% in ground beef, 25% in frankfurters, and 55% in bacon slices), increased fatness either increases palatability (ground beef and frankfurters) or does not decidedly decrease eating quality (bacon).

Eating Quality and Proximate Composition of Pork Loins

A number of researchers have attempted to identify, quantitatively, the amount of fat needed to ensure satisfactory eating quality in cooked beef (Simone et al., 1958). Similar research has been conducted using pork loins (Smith and Carpenter, 1974), and data from this study are presented in Table 8. Among samples from 403 pork loins, the highest ratings for overall satisfaction were assigned to loin chops that averaged 9.1% crude fat in the longissimus muscle. From these data, Davis (1974) determined that the minimum fatness level necessary to achieve acceptable palatability and cooking shrinkage could be obtained by requiting that wholesale pork loins exhibit "traces" of marbling in the blade face, "slight" marbling in the tenth fib face, or ''slight-minus" marbling in the sirloin face. Present evidence suggests that the longissimus muscle of the pork loin will be acceptable in palatability if it contains 3.5% to 4.5% intramuscular fat (Davis, 1974). Use of a specific level of fatness (defined in terms of marbling score or intramuscular fat content) as a standard, over which the product is considered "acceptable" and under which the product is termed "unacceptable," would seem to be appropriate for use in quality assessments or for grade identification and may be more reasonable than attempting to reflect degrees of acceptability in response to changes in degrees of fatness.

TABLE 8. Eating Quality, Cooking Losses, and Composition of Pork Longissimus Muscles.

TABLE 8

Eating Quality, Cooking Losses, and Composition of Pork Longissimus Muscles.

It has recently been proposed (Consumer Protection Report, 1973) that "the present beef grading system should be changed along the lines of reducing the amount of internal fat of the retail grades, which would probably help reduce the incidence of heart disease in this country without significantly lowering the quality of the meat." The same report quoted Dr. Jean Mayer, resident nutritionist at Harvard University, as saying, "I would like to see the meat grading system based more on protein than fat content." Davis (1974) tested these latter hypotheses using pork loins and reported the following:

1.

Selection of pork loins to maximize crude protein content would identify and segment cuts which have (a) "small" or lower scores for marbling, (b) soft, extremely open muscle structure, and (c) less than 4.6% crude fat (WTB) in the longissimus muscle.

2.

Cooked product from loins selected to contain maximum quantities of protein would (a) be tougher, (b) have greater cooking shrinkage, and (c) be less desirable in eating quality than would loins with lesser quantities of protein.

Conclusions

The belief that fat deposition enhances the eating quality of meat has its origin in antiquity, was perpetuated through the centuries by animal husbandmen, and has been seriously studied only in the past half-century. Eating quality or palatability is determined by the consumer's singular or combined responses to the flavor, juiciness, and tenderness of cooked meat.

The basic flavor and aroma of meat arises from a browning-type reaction between reducing sugars and amino acids and from the presence of a number of volatile compounds, e.g., ring compounds, carbonyl compounds, sulfur-containing compounds, and nitrogen-containing compounds. Fat may affect flavor directly, by oxidation of fatty acids to carbonyl compounds, or indirectly, by acting as a storage depot for odoriferous compounds, and is probably responsible for the characteristic difference in flavor associated with meat from certain species. Research included in the present review of literature suggests that fatness has a low association with flavor desirability in lamb and a low-to-moderate relationship to flavor desirability in both pork and beef.

Juiciness of cooked meat is determined by the moisture content, pH, water-holding capacity, and firmness of the raw product and the residual fluid (moisture or fat) remaining in the final product, which is dependent upon the conditions of temperature—time imposed on the cut during cookery. Fat may affect juiciness by enhancing the water-holding capacity of meat, by lubricating the muscle fibers during cooking, by increasing the tenderness of meat and thus the "apparent" sensation of juiciness, or by stimulating salivary flow during mastication. Research considered in this review of literature indicates that fatness has a moderate relationship to juiciness in lamb, a moderate-to-high association with juiciness in pork, and a low-to-moderate relationship to juiciness in beef.

Differences in tenderness among meat samples can be characterized as resulting from actomyosin effects, background effects, and bulk-density or lubrication effects. State of contraction of the muscle fibers following rigor mortis and susceptibility of collagen to thermal degradation are of paramount importance in determining tenderness, as is the method and extent of cooking. Little is known of the mechanism by which fat deposition influences the ultimate tenderness of cooked meat. Present theories include mechanisms by which marbling decreases bulk density, strains connective tissue, lubricates muscle fibers during cooking, or provides some assurance (insurance) that meat cooked improperly will still be reasonably palatable and mechanisms by which external or intramuscular fat prevents cold-shortening or stimulates enzymatic proteolysis via changes in postmortem chilling rate. Research included in the present review of literature suggests that fatness has a moderate association with tenderness in pork and a low-to-moderate relationship to tenderness in both lamb and beef.

In certain meat products that have characteristically high percentages of fat (e.g., 15% or 20% in ground beef, 25% in frankfurters, and 55% in bacon slices), additional increments of fat (5%-30%) either increase palatability (ground beef and frankfurters) or do not decidedly decrease eating quality (bacon). Such data suggest that minimal fatness levels necessary to assure "acceptable" palatability to the majority of consumers will differ among kinds and classes of meat products (e.g., muscle meats may be "acceptable" with 3%-5% intramuscular fat while ground or comminuted meat products may be "acceptable" with a fat content of 20%-25% ). Use of a specific level of fatness (defined in terms of marbling score, intramuscular fat percentage, or chemical fat content) as a standard, over which the meat product is considered "acceptable" in palatability and under which the product is identified as "unacceptable," would seem to be appropriate for use in quality assessments or for grading purposes and may be more reasonable than attempting to reflect degrees of acceptability in response to changes in degrees of fatness. Although some consumers may expect USDA grades to identify meat products according to their nutritional adequacy or excellence, research cited here reveals that selection of pork loins to maximize crude protein content would identify and segment cuts that are less than satisfactory in appearance and inferior in eating quality.

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