Skip to content Meat Science and Nutrition. Filaments of muscle tissue. A type of connective tissue in meat that dissolves when cooked with moisture and yields gelatin. Muscle fibres composed of bundles of thick and thin filaments arranged in a repeating pattern. One unit of a bundle of muscle fibres, also called the "little muscle. The thicker filaments of muscle fibre protein that contract muscles. The thinner filaments of muscle fibres that help regulate muscle contraction.
A chemical component of actin that assists in regulating muscle contraction movement. The degree to which a substance can be dissolved in water. An increase in connective tissue that occurs as animals age. In a live animal, these protein filaments make muscles contract and relax.
Both actions require enormous amounts of energy, which they get from the energy-carrying molecule ATP adenosine triphosphate. The most efficient generation of ATP requires oxygen, which muscles get from circulating blood.
Individual protein molecules in raw meat are wound-up in coils, which are formed and held together by bonds. When meat is heated, the bonds break and the protein molecule unwinds. Heat also shrinks the muscle fibers both in diameter and in length as water is squeezed out and the protein molecules recombine, or coagulate.
Regardless of the species, the most important factor that determines muscle fiber composition is muscle type, likely in relation to its specific physiological function.
For a given muscle, the fiber composition varies depending on the species. The composition of muscle fibers is also influenced by breed, gender, age, physical activity, environmental temperature, and feeding practices. As in mammals, the muscle fibers of birds can be classified based on their contractile and metabolic activities. However, additional classes, for example, the multitonic innervated slow fibers of types IIIa and IIIb, which are specific to avian muscles, have been described [ 15 ].
In birds, it is difficult to match an isoform of MyHC with a fiber type due to the simultaneous presence of adult and developmental types of MyHC in mature fibers. Fish also exhibit different types of muscle fiber characterized by their contractile and metabolic properties. However, in contrast to mammals or birds, an anatomical separation between the two main fiber types can be observed in fish.
For example, in trout, fast fibers similar to mammalian IIB fibers are found in the center in a cross-sectional body area, and slow fibers similar to the mammalian type I are found at the periphery along a longitudinal line under the skin [ 16 ].
In addition to these two main fiber types, minor types, such as the intermediate type e. The two main types of white and red fiber have been associated with the expression of fast and slow MyHC, respectively [ 17 ].
However, it can be difficult to systematically match a MyHC isoform with a fiber type due to the simultaneous presence of several MyHCs within the same fiber in fish, particularly in the small muscle fibers.
The connective tissue that surrounds muscle fibers and fiber bundles is a loose connective tissue. It consists of cells and an extracellular matrix ECM that primarily consists of a composite network of collagen fibers wrapped in a matrix of proteoglycans PGs [ 4 , 18 , 19 ]. This paper focuses on the molecules that have been demonstrated or suspected to play a role in the determination of meat sensory quality.
The collagens are a family of fibrous proteins. Regardless of the collagen type, the basic structural unit of collagen tropocollagen is a helical structure that consists of three polypeptide chains coiled around one another to form a spiral.
These fibrils are stabilized by intramolecular bonds disulphide or hydrogen bridges or intermolecular bonds including pyridinoline and deoxypyridinoline , known as cross-links CLs. Various types of collagen are found in skeletal muscle. Fibrillar collagens I and III are the major ones that appear in mammals [ 19 ]. In fish, collagen types I and V predominate [ 20 ]. The other main components of connective tissue are the PGs [ 21 ]. PGs form large complexes by binding to other PGs and to fibrous proteins such as collagen.
They bind cations e. The proportion and the degree of intramuscular collagen cross-linking depend on muscle type, species, genotype, age, sex, and level of physical exercise [ 23 ]. In poultry, the collagen represents 0. PGs represent a small proportion of the muscle dry weight 0. In mammals, reserve fat is located in several external and internal anatomical locations such as around and within the muscle for the intermuscular and intramuscular IMF fats.
In this paper, we focus essentially on IMF because intermuscular fat is trimmed during cutting and thus has less impact on pork and beef meat. In fish, fat are located subcutaneously and within the perimysium and myosepta, and mainly the latter contribute to flesh quality and is considered in this paper.
IMF mostly consists of structural lipids, phospholipids, and storage lipids the triglycerides. Between muscle types, the phospholipid content is relatively constant i. The IMF content strongly depends on the size and number of intramuscular adipocytes. In pigs [ 32 , 33 ] and cattle [ 30 , 34 ], the interindividual variation in IMF content of a given muscle between animals of similar genetic background has been associated with variation in the number of intramuscular adipocytes.
In contrast, variation in the IMF content of a given muscle between animals of the same genetic origin and subjected to different dietary energy intakes has been demonstrated to be associated with variation in adipocyte size [ 33 ]. In fish, the increase in myosepta width is likely related to an increase in the number and size of adipocytes [ 35 ]. The IMF content varies according to anatomical muscle origin, age, breed, genotype, diet, and the rearing conditions of livestock [ 30 , 36 — 39 ].
For example, Chinese and American pigs e. In cattle, the IMF content of Longissimus muscle varies from 0. In French cattle breeds, it has been demonstrated that selection on muscle mass has been associated with a decrease in IMF and collagen contents.
Studies based on comparisons between muscle types indicate that IMF content is typically positively correlated with the percentage of oxidative fibers and negatively with the glycolytic fibers [ 45 ]. Although oxidative fibers, particularly slow fibers, exhibit a higher intramyocellular lipid content than fast glycolytic fibers do [ 46 ] and although the IMF content has often been found to be higher in oxidative than in glycolytic pig muscles i.
In extreme cases, the IMF content can be three times higher in the white glycolytic than in the red oxidative part of the Semitendinosus muscle in the pig [ 34 ] Figure 6. A negative correlation between IMF content and the oxidative metabolism was also found in the pig Longissimus muscle in a functional genomic approach [ 48 ].
However, positive genetic and phenotypic correlations were observed between IMF content and muscle fiber CSA in pig Longissimus muscle [ 49 ].
In fish, in which white and red muscles are anatomically separated, it is assumed that red muscles exhibit more elevated fat content than white muscles due to higher numbers of fat cells in the perimysium and higher numbers of lipid droplets within muscle fibers. No systematic relationship between the biochemical characteristics of the connective tissue and muscle fiber type has been found in meat-producing animals.
In contrast, in fish, collagen content is higher in red than in white muscles [ 52 ]. The longest storage periods are used for beef one to two weeks for carcasses to one month for meat pieces stored under vacuum to facilitate a natural tenderizing aging process. The reduction of muscle fiber CSA observed during the refrigeration results from a lateral shrinkage of myofibrils whose amplitude depends on the slaughter stress of animals and of the stunning technology Figure 7 [ 53 ].
The aging phase is characterized by various ultrastructural changes and results in the fragmentation of muscle fibers. The action of different proteolytic systems results in characteristic myofibrillar ruptures along the Z lines Figure 7. Mitochondria are deformed and their membranes altered [ 18 , 54 ]. As a consequence of the degradation of costameres, that is, the junction of cytoskeletal proteins to the sarcolemma, the sarcolemma separates from peripheral myofibrils [ 55 ].
According to Ouali et al. Other proteolytic systems e. This degradation facilitates the solubilization of collagen during cooking, thus improving the tenderness of cooked meat. An indirect effect of PGs on the tenderness of cooked meat has also been suggested. In fact, during aging, reduction of the perimysium resistance is associated with decreasing amounts of PGs along with an increase in collagen solubility due to the increased activity of certain enzymes.
One hypothesis is that PGs may be degraded spontaneously or enzymatically during maturation and no longer protect collagen from enzymatic attacks [ 21 ]. In fish, flesh tenderization is associated with a gradual breakdown of the endomysium [ 58 ] and a detachment of the fibers from one another due to the rupture of ties with the endomysium and with the myosepta [ 59 ].
Soft-flesh fish demonstrate more endomysium collagen, PGs breakdown [ 60 ]. Fish myofibrils exhibit weak ultrastructural changes of the actomyosin complex, unlike bovine muscle [ 61 ]. Thus, in sea bream Sparus aurata , I and Z bands are only partially degraded after 12 days of refrigerated storage [ 62 ].
Among the various components of meat quality, the technological, nutritional, and sensory dimensions are considered. The nutritional quality component is primarily determined by the chemical composition of muscle tissue at slaughter, whereas the technological and sensorial components result from complex interactions among the chemical composition and metabolic properties of the muscle at slaughter and pm biochemical changes that lead to its conversion into meat [ 56 , 63 ].
The structure and muscle composition, the kinetics of pm changes, and the additional meat use and processing methods that are applied e. Therefore, the hierarchy between the most desired qualitative components varies between species. Prominent examples include tenderness in cattle, firmness in fish flesh, and water-holding capacity in pigs and chickens.
After slaughter, depending on the species and the markets, the carcasses are stored in a cold room and then cut into pieces or muscles. During storage, the internal structure of muscles changes. The muscle fibers shrink laterally while expelling intracellular water to extracellular spaces, whose size increases. Subsequently, this water is expelled at the cut ends of muscles [ 53 ]. Regarding processing into cooked products, the technological quality is related to the water-holding capacity of meat, that is, its ability to retain its intrinsic water.
The water-holding capacity is strongly influenced by the rate and extent of decrease in the pm pH. A high rate combined with a high muscle temperature e. A large extent of pH decrease i.
Measuring pH within one hour after slaughter and then on the following day to assess the rate and extent of pH decline, the determination of color and water loss during cold storage are the main indicators of the technological quality of meat.
Muscle fiber composition influences the technological quality of meat, such as the water-holding capacity, which depends on the evolution of muscle pm pH kinetics and temperature. The pm pH decrease generally occurs faster in glycolytic muscles than in oxidative ones [ 66 ] although this relationship is not systematic.
In addition, stimulation of muscle glycolytic metabolism in the hour following slaughter increases the rate of pH decrease, which when combined with a high muscle temperature may result in protein denaturation and pale, soft, and exudative PSE syndrome in white muscles, particularly in pigs and chickens. In Large White pig Longissimus muscle, the increase in rate and extent of pm pH decrease are associated with a paler color and higher luminance and exudation [ 49 , 67 ].
In pigs, two major genes that substantially influence the kinetics of pm pH decrease and water-holding capacity have been identified.
Mutation in the RYR1 gene also known as the halothane gene , which encodes a ryanodine receptor that is part of the calcium release channel of the sarcoplasmic reticulum, is responsible for a rapid decrease in pm pH and the development of PSE meat [ 68 ].
Interestingly, the Longissimus muscle of mutated PRKAG3 pigs contains more oxidative fibers [ 47 ] and a lower buffering capacity [ 70 ] which contributes to the low ultimate pH in addition to the greater lactate production from glycogen. A recent proteomic study in cattle revealed some correlations between metabolic, antioxidant and proteolytic enzymes with pH decline. These data allow a better understanding of the early pm biological mechanisms involved in pH decline [ 71 ].
Meat and flesh are an important source of proteins, essential amino acids AAs , essential fatty acids FAs , minerals, and vitamins A, E, and B , which determine nutritional quality. The AA profile is relatively constant between muscles or between species [ 72 ]. However, collagen-rich muscles have a lower nutritional value because of their high glycine content, a nonessential AA [ 19 ].
Compared with white muscles, red muscles have larger myoglobin content and thereby provide higher amounts of heme iron, which is easily assimilated by the body. Although IMF constitutes a small fraction of muscle mass, it is involved in human FA intake because the content and nature i. Dietary strategies have been intensively studied and optimized to decrease saturated fatty acid intakes and increase cis-monounsaturated and polyunsaturated fatty acids or other bioactive lipids in animal-derived products for human consumption [ 30 , 73 ].
In addition, because n-3 fatty acids with more than 20 carbons are primarily incorporated into phospholipids rather than into triglycerides, it is possible to enrich meat content in these polyunsaturated fatty acids without increasing IMF. For example, regarding bioactive lipids, the peculiarity of meat from ruminants is the presence of fatty acids that directly or indirectly result from ruminal biohydrogenation and that are proposed to be bioactive fatty acids, such as rumenic acid, which is the main natural isomer of the conjugated linoleic acids [ 30 ] and known to prevent certain forms of cancer in animal models.
However, during pm aging and meat storage, lipids undergo alterations e. These alterations may impair the sensory e. The composition of muscle fibers influences meat color via the amount and the chemical state of myoglobin. The high myoglobin content of type I and type IIA fibers results in a positive relationship between the proportion of these fibers and red color intensity.
In deep muscles and meat stored under vacuum, myoglobin is in a reduced state and exhibits purple red color. When exposed to oxygen, myoglobin is oxygenated into oxymyoglobin, which gives the meat an attractive bright red color. During meat storage, myoglobin can be oxidized into metmyoglobin, which produces a brown, unattractive color that is negatively perceived by consumers [ 75 , 76 ].
Many ante- and pm factors, such as animal species, sex, age, the anatomical location and physiological function of muscles, physical activity, the kinetics of pm pH decrease, the carcass chilling rate, and meat packaging, influence the concentration and chemical state of pigments and consequently meat color [ 77 ]. Muscles from cattle, sheep, horses, and migratory birds e.
In contrast, a high proportion of glycolytic fibers results in the production of white meat, as found in chickens and pigs. Double-muscled cattle mutation in the myostatin gene present muscles with a high proportion of fast glycolytic fibers and consequently pale meat [ 3 ]. Meat color also depends on diet. In fish, only the superficial lateral red muscle, which is rich in myoglobin, exhibits intense generally brown color, whereas the white muscle is rather translucent.
In the case of salmonids, the orange-red color of the flesh is due to the presence of food-supplied carotenoid pigments, such as astaxanthin, in the muscle fibers. Differences in lipid levels can result in variations in the thickness of myosepta i. Consumer perception of the red muscle, which oxidizes quickly pm to brown and then to black, is generally negative, and this red muscle is occasionally removed for premium products e. In addition to color, the quantity and distribution of marbling within a muscle slice affect appearance and thus can affect the acceptance of meat and meat products by consumers cf.
Section 6. Tenderness and its variability are the most important sensory characteristic for beef consumers. Beef meat has a much higher basic toughness determined by the proportion, distribution, and nature of the intramuscular connective tissue and lower pm tenderization process than those of pork or poultry [ 63 ].
Thus, the pm aging duration is essential for beef tenderness [ 79 ]. In pigs and poultry, the pm acidification kinetics of muscles, which is faster than in cattle [ 79 ], strongly influences the texture i.
In cattle, the relationships between fiber characteristics and tenderness are complex and vary according to muscle, sex, age, and breed [ 80 ]. For example, among bulls, Longissimus thoracis tenderness is often associated with a decrease in fiber CSA and an increase in oxidative metabolism, whereas in the Vastus lateralis and semitendinosus muscles, the higher that the glycolytic activity is, the tenderer the meat is [ 81 ].
However, a negative correlation between the intensity of the oxidative metabolism and tenderness has also been observed in the Longissimus muscle of cattle [ 82 ]. Using biomarkers of beef tenderness Picard et al. On the contrary, in breeds whose muscle metabolism is more oxidative, such as Aberdeen Angus, the most glycolytic Longissimus thoracis are the tenderest.
This is in accordance with the fact that in breeds that exhibit oxidative muscles, such as Angus or dairy breeds, rib steaks with low red color intensity are tenderer. In contrast, among the main French beef breeds that exhibit more glycolytic muscles, the reddest the muscle is, the tenderer the meat is [ 83 ].
However, for other authors, the improvement in meat tenderness associated with the increase in the type I fiber proportion is explained by the higher protein turnover and associated proteolytic activity in the oxidative fibers [ 85 ]. Among bulls, except for rib steak, meat tenderness does not seem to be associated with fiber CSA but with the metabolic properties of muscle fibers. In pigs, a functional genomic study has reported a negative impact of the abundance of fast fibers and of high glycolytic metabolism on meat tenderness [ 48 ].
This study also demonstrates that reduced expressions of protein synthesis genes e. A negative relationship between average fast glycolytic fiber CSA and tenderness has been demonstrated in pigs [ 86 ]. Therefore, a strategy aimed at increasing the total number of fibers combined with moderate fiber CSA and an increase in the percentage of slow-twitch oxidative fibers could be a promising means to increase muscle quantity while preserving the sensory quality of pork [ 6 ].
In contrast, in chickens, an increase in fiber CSA in the Pectoralis muscle is associated with a decrease in muscle glycogen content, higher ultimate pH and water-holding capacity, and improved tenderness [ 87 ]. However, contradictory data for chickens also report negative effects of fiber CSA on meat water-holding capacity and tenderness [ 88 ].
In fish, comparisons between species have observed a negative correlation between the mean diameter of muscle fibers and flesh firmness. However, this relationship seems more controversial within species: similar results have been found for smoked Atlantic salmon and the raw flesh of brown and rainbow trout, whereas other studies did not demonstrate a relationship between fiber size and the texture of salmon or cod flesh.
Altogether, as in pigs, it appears that hyperplasic rather than hypertrophic muscle growth is better for the quality of fish products. Connective tissue influences meat tenderness by its composition and structure [ 4 ], particularly in cattle, whereby collagen is generally considered to be the major determinant of the shear force.
However, there are substantial differences between raw and cooked meat. The shear force of raw meat is highly correlated with its collagen content [ 21 , 89 ]. In cooked meat, the level of correlation between the content, thermal solubility, or cross-linking level of collagen and meat shear force is unclear and varies according to muscle type and cooking conditions [ 90 , 91 ].
During heating, the collagen fibers shrink and pressurize muscle fibers with a magnitude that depends on the degree of collagen cross-linking and the organization of the endomysium and the perimysium. First, the heart stops beating and circulating blood around the body. In meat processing plants, the blood is removed as part of the harvest process. Blood is responsible for bringing oxygen to the muscles and for bringing waste products away from the muscle.
Even though the animal has died, the muscle is still living and breaking down nutrients for energy. Because the oxygen is gone, the way the muscle breaks down energy changes, and it begins to produce a waste product called lactic acid this is the same acid that is produced when you work out too hard and your muscles cramp. In a living animal, the lactic acid would be sent in the blood to the liver.
In the conversion of muscle to meat, there is no blood, so the lactic acid builds up in the muscle.
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