A study has been made of the onset of rigor mortis in the longissimus dorsi muscle of the ox, thus extending similar observations previously made on rabbit muscle. The changes in pH and adenosine triphosphate (ATP) content accompanying the physical manifestations of rigor are described. As in rabbit muscle, the development of inextensibility accompanies the dephosphorylation of ATP, and, provided the ultimate pH is low, the fall in pH runs parallel to these changes. The rate of decrease in pH is almost identical with that in rabbit muscle at physiological temperature, but differs appreciably from it at lower temperatures. The results are discussed in relation to the time required for the completion of the onset of rigor mortis in commercial chillers.

http://onlinelibrary.wiley.com/doi/10.1002/jsfa.2740050202/abstract

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Sales of meat are influenced by the appeal of meat to consumers, that is described as

“quality”. Many factors determine the quality in meat. It includes requirements of food safety

and animal welfare. It also includes the sensory appeal of meat such as palatability and

perceived healthiness, especially in relation to the amount and type of fat and other fatty

components. Meat quality describes how much meat is attractive to consumers. Meat must

look good to consumers before satisfying their palate when they decide to buy it. Once the

meat is bought, cooked, and served, the aroma, tenderness, juiciness, and flavor must meet the

expectations (Aberle et al., 2001). The aroma and juiciness can be improved using spices and

cooking method. However, the tenderness and flavor depend on textural characteristics,

composition of meat, and many other factors.

2. MEAT TENDERNESS

2.1. Meat Protein and Muscle Fibers

Meat tenderness is the most difficultly predicted trait, but it is very important to meat

quality and consumer acceptance. Tenderness is based on ease of chewing that is contributed

by many factors. Among them, the fibrous nature of muscle contributes to chewing resistance

(Gerrard and Grant, 2003). The fact that many myofibrils are arranged in register across the

muscle fibers leads to more strength for muscle and decreases muscle tenderness. The unit of

skeletal muscle is the muscle fiber. Among many components of muscle fiber, protein is the

most important one. Muscle proteins are categorized as sarcoplasmic, myofibrillar, and stromal

proteins based on their solubility. Sarcoplasmic proteins are extracted in aqueous solution with

low ionic strength (0.15 or less). Myofibrillar proteins are extracted by salt solutions and

require higher ionic strength, called salt-soluble proteins. Stromal proteins include proteins of

connective tissues, which are very fibrous and insoluble (Aberle et al., 2001). Of insoluble

proteins, collagen is composed about 0.5 proportions, elastin is about 0.03, and the remaining

0.47 is a mixture of various proteins such as reticulin. Both type and amount of connective

tissues affect meat tenderness.

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The connective tissues, which are present in all muscles in the epimysial, perimysial, and

endomysial components surrounding the muscle fibers, lead to strength of muscle but now

make it tough. The strength of connective tissues is derived from its collagen fibers. Hence,

meat tenderness is obviously influenced by collagens of the muscle. The resilience depends on

the elastic fibers in the intercellular matrix. The collagen fibers are straight, inextensible, non-

branching, white in color, and different in diameter, from 16 nm in fetal tissues to about 250

nm in some tendons from adult animals. The connective tissues can be divided into two main

categories: loose and dense (regular or irregular) connective tissues based on density and

organization of fiber bundles. The dense connective tissues that are very common in tendons

are much tougher than the loose ones. Additionally, the five types of collagen in different

tissues are categorized by their elemental polypeptide chains (Lawrence and Fowler, 2002).

Although the high ratio of type III collagen may result in an increase in toughness, the extent

and type of cross-linking between different types of collagen are of greater importance. The

increase in cross-linking associated an expansion of dense connective tissues and the

insolubility of collagen increase with age; therefore affect muscle tenderness. Thus, when

considering collagen content, its types, and cross-linking, younger animals will produce more

tender meats than will older animals (Gerrard and Grant, 2003).

Another considerable muscular factor is muscle types. The ratio of slow-twitch oxidative

(type I) and fast-twitch glycolytic (type II) muscle fibers also evidently influence meat

tenderness. The ratio varies among individual animals of the same breed, breeds, and crosses.

Beef tenderness is positively related to type I muscle and negatively related to the others.

These differences are linked to higher ratio of protein turnover in tender muscle and higher

level of calpain (Lawrence and Fowler, 2002), which plays an important role in protein

degradation, so in the meat tenderness.

2.2. Postmortem Conditions

The contractile system of muscles is another factor that contributes to meat tenderness. The

contraction makes the Z discs closer to each other and increases density of filaments, therefore

reduces chewing ability. After slaughter, but before the rigor mortis, it is still able for filaments

to slide over each other. However, after rigor mortis, the two types of filaments become fixed.

Because of some reason by which the rigor mortis comes right at the time of contractile state,

the meat will be very tough. It results from not enough ATP to operate the calcium pump, so

the calcium concentration in sarcoplasm begins to rise, and muscle relaxation fails to start.

Additionally, the stored oxygen supply is reduced, and the anaerobic metabolism starts to

maintain the homeostasis. This process needs energy from ATP. All of those start the major

energy reserve through glycogen catabolism because the reaction using creatine phosphate in

postmortem muscle lasts shortly. Glycogen must first be degraded by glycogen phosphorylase

to form glucose-1 phosphate. The whole process produces three ATP with glycogen as starting

material. Due to lack of oxygen, enzyme lactate dehydrogenase catalyzes the reduction of

pyruvate to lactic acid by NADH (Scanes, 2003).

This process also regenerates NAD+ for glycolysis step in ATP formation. However, the

built-up lactic acid inactivates the enzymes relating to glycolysis and slows down the process of

ATP regeneration. That is why there is still not enough ATP for muscle relaxation, and rigor

mortis is maintained. On the other hand, lactic acid makes the pH gradually fall and the rigor

mortis is rapidly developed (Aberle et al., 2001), which decreases meat tenderness. The

proteolysis is also one of factors affecting meat tenderness. Meat cuts from carcasses at 24 h

postmortem are less tender than cuts taken for several days although they were the same sub–

primal cuts taken from the same carcasses. The reason for this phenomenon is the action of

various enzymes that are active postmortem and start to hydrolyze contractile proteins.

Resulted from this destruction, meats become more tender. Of the protease system, calpain,

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which organizes the protein turnover in living muscle tissues, attacks proteins that hold

myofibrils together or locate in Z-lines. Hence, meat is tender when cooked.

2.3.Genetic and Growth Effects

Meat tenderness depend on breeds within a species, types within a breed, and gender.

Within a breed, the low proportion of collagen and high solubility of collagen per se with small

degree of polymerization of constitute chains is probably the reason why some muscles are

more tender than the others. There is a special case of callipyge gene in sheep that

biochemically produces large buttocks consisting of muscle with very little fat and high

proportion of type IIB muscle fibers (Scanes, 2003). This gene is expressed only if it is on

chromosome 18 inherited from the father and is not on the chromosome from the mother,

which causes lower tenderness. This fact may be also caused by an increase in calpastatin

activity, which decreases protein degradation postmortem (Lawrence and Fowler, 2002),

because calpastatin is a specific endogenous inhibitor of calpain.

Back to postmortem contraction of muscle, regardless of stress caused by transportation,

handling, etc., pigs genetically predisposed to stress usually carry at least one of the following

genes: halothane (stress) gene or Rn (Napole) gene. Halothane gene causes a rapid decline in

pH, whereas Rn gene causes low ultimate pH. Elimination of these two genes from the swine

population should reduce the frequency of PSE pork. However, these stress-related genes are

prevalent in swine because both are linked to improvement of carcass leanness. The other

important factor affecting meat tenderness is muscle growth performance of animals. The bulls

achieving high growth rates produced lean meat with significantly lower shear force value for

m. biceps femoris muscle (Sinclair et al., 1998). There is an evidence that speed of growth in

the later stages of the production cycle before slaughter can essentially affect quality attributes

(e.g. drip loss) as well as tenderness (Lawrence and Fowler, 2002). The bull produced more

tender muscle after a compensatory growth (Hornick et al., 1998). Compensatory growth also

improves meat tenderness in pigs (Kristense et al., 2004; Blanchard et al., 1999a). Reasons for

that effect were related to the fast growth rate inducing the deposition of “younger” muscle in

which the connective tissues are less structured. Moreover, the length of compensatory growth

influenced level of in vivo protein turnover and postmortem proteolysis (Therkildsen et al.

2002). In addition to compensatory growth, double muscling is another growth phenomenon

on cattle that can make the muscle more tendered with less total collagen (up to 40% or less on

dry matter). The majority of difference in collagen content is in perimysial connective tissue.

The alterations in the collagen maturation may also change toughness. The concentration of a

collagen cross-link, histidinohydroxymero-desmosine, is lower in semitendinosus muscle of

double muscling cattle, compared to normal animals (Ngapo et al., 2004). Because of

differences in muscle fiber types, connective tissues, etc., muscles that are located in various

locations are also different in toughness. These differences were also indicated in a study on

Warner–Bratzler shear-force evaluations of forty bovine muscles (Belew et al., 2003).

3. FAT CONTENT AND MEAT FLAVOR

3.1. Fat and Meat Flavor

Flavor is the most important factor in our food choices (Pearson and Gillett, 1999). “Fat is

the source of flavor in meat and is particularly important to characteristics of species flavor

variations”. The different flavors among beef, pork, chicken, turkey, and lamb come from fatty

components. Fatty tissues give them specific flavor profiles. Fat acts as one of precursors of

flavor by combining with amino acids from proteins and other components when heated. When

fat melts, it releases flavors and gives sudden burst of these flavors. Fat consists of a glycerol

molecule linked by ester bonds to three fatty acids and is termed as triacylglycerol or

triglyceride. The main component, which is responsible for properties and functions of fats, is

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fatty acid. Fatty acids differ from each other in length of their hydrocarbon chains and

presence or absence of double bonds. Fatty acids that lack double bonds are described as

saturated and those having double bonds are unsaturated. Nutritional treatments can be used to

manipulate the fatty acid content of muscle to improve nutritional balance, such as an increase

in the ratio between polyunsaturated fatty acid (PUFA) and saturated fatty acid. Increasing

PUFA levels may also change flavor because of their greater susceptibility to oxidative

breakdown and the generation of abnormal volatile compounds during cooking. To increase

PUFA is to supplement vegetable oil to animal diet. There is much research on

supplementation of vegetable oils to obtain the same type of fatty acids in animal fat (Fiego et

al., 2005; Eder et al., 2004). The results showed that animals would get what they ate,

especially for monogastric animals. Ruminants are different due to biohydrogenation of fatty

acids after their breakdown in the rumen. The biohydrogenation is caused by ruminal

microbial population, which probably plays the most important role in ruminant digestion.

3.2. Importance of Marbling

Lipid content of muscle varies from 1.5% to 13%. Although most of lipids are present in

adipose tissues, some called marbling and found intracellularly in muscle fibers has the most

important roles in meat quality. Marbling is intramuscular fat that is deposited within muscle

in loose network of perimysial connective tissues, between the muscle bundles. Marbling has

much stronger and more predictable effects on juiciness and flavor than tenderness. This

intramuscular fat was thought to accumulate in the finishing phase of growth, but recent

research has suggested that marbling increases linearly overtime.

Marbling cells develop within muscles that help the animal stand, the so called postural

muscles. This muscle is destined to become strip loin cuts of meat. Fat is a lately developing

tissue in cattle because fat growth accelerates as animals approach maturity after formation of

muscle and bone. While muscle growth slows down and bone growth ceases as an animal ages,

fat growth continues in a well-fed animal. However, some marbling is also present even in the

unborn calf. The total amount of marbling in a muscle results from increase in number of

marbling cells as well as size of cells. Cells are distributed among several groups of fat cells.

When several of these groups merge, it looks like a seam. That is why marbling sometimes is

called seam fat. Number of cells in any particular group determines appearance of marbling

and increases with animal age. Most marbling leaves meat during cooking process because of

melting. However, some is certainly left behind, tends to lubricate meat, contributes to mouth

feeling, and does have positive addition to the whole flavor of meat. Practically, marbling

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score has great effect on meat tenderness and cooking quality. Because marbling melts over

meat surface, moisture of meat, and therefore meat juiciness, is maintained during cooking.

Moreover, melting marbling leaves a porous structure in meat and helps improve chewing

ability. In beef, marbling mostly determines total beef quality. Marbling is usually considered

as “cooking insurance” of meat.

3.3.Genetic Effects

Genetic make-up of animals is one of the most important factors in determining marbling

(Scanes, 2003). Breeds like Angus are well known for their marbling potential. Genetic traits

may also determine how marbling is distributed though it is very difficult to be controlled. The

good plan of nutrition during finishing stage contributes to higher marbling scores, certainly, in

animals genetically able to marble. Having more cells rather than bigger cells in early stages of

life is almost the key effect on the potential to deposit fat later, therefore, the accumulation of

marbling fat.

3.4. Fatty Tissues

The lipid of fatty tissues is important to the development of flavor in meat, which

undergoes cooking process and releases flavoring volatiles. These volatiles increase with age

in all animals (Lawrence and Fowler, 2002). Therefore, the older animal will produces greater

flavor. High concentration of linoleic acid in the lipid of fatty tissues can have a definite effect

on flavor. If level of linoleic acid in lipid of sheep and cattle adipose tissues is high, it will

produce oily, sweet or bland tastes during cooking. Conversely, high concentration of oleic

acid in lipid will improve the flavor. Additionally, temperature control in storing and in packing

pork is also important if high concentration of linoleic acid is present in the lipids. This is

because linoleic acid is much unsaturated and easily combines with atmospheric oxygen to give

rise to oxidative rancidity and makes the products unpalatable. Moreover, the fatty tissues are

also relating to level of abnormal odor of animal. Two compounds that are responsible for

increase of boar odor are skatole and androstenone. Both of them are found in fatty tissues.

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