- SMOOTH MUSCLE
There are three kinds of muscles:
- Smooth or involuntary muscles
- Striated or voluntary muscles
- Cardiac Muscles
Smooth muscle is found in the walls of the intestines, the urogenital system, and the blood vessels, etc. It is called involuntary muscle because it works automatically, without any conscious effort by the person or animal. A good example is the peristalsis, the muscular contractions and relaxations which move food down the oesophagus, and later along the small and large intestines. This type of muscle is made up of elongated, spindle-shaped cells with the nucleus placed in the middle of the cell.
Figure 1: Smooth Muscle
- 2. STRIATED MUSCLE
These muscles are made up of bundles of long fibres with many nuclei. The cells appear to be striped, and the nuclei are placed near the surface of the cell. Each individual muscle cell is separate from its neighbour, like a bundle of rods, and each cell is controlled and activated by a branch from a motor neuron in the nervous system. Each muscle cell is covered by a sheath called the sarcolemma, which connects the cells to tendons and gives the muscle fibres elasticity. The muscle fibres, or individual cells, are arranged in bundles. The fibres are called myofibrils and are surrounded by fibrous connective tissue.
Figure 2: Striated Muscle Cell
Figure 3: Entire Muscle
Figure 4: Section of Entire Muscle Showing Muscle Fibres
Figure 6: Muscle Cell Showing Sarcolemma and Myofibrils
The banding that can be seen under a powerful electron microscope, on each myofibril, consists of very fine filaments of two proteins, actin and myosin, arranged as shown in the diagram below. During contraction of the muscle, it is believed that the actin filaments slide together so that neither the actin nor myosin filaments actually change their lengths.
Figure 7: Resting Muscle
Figure 8: Partly Contracted Muscle
Figure 9: Fully Contracted Muscle
The contraction of muscles is a positive action and requires energy to be carried out. When you pick up a brick, the muscles of the arm and fingers contract as you grip the brick, and the arm muscles contract as you lift it up. You can drop the brick by relaxing the arm and finger muscles, but throwing the brick requires more contraction of the muscles and a greater supply of energy. This energy is supplied as a result of a number of very complicated chemical reactions which take place inside the muscles. One example is the breakdown of a substance called A.T.P., or adenosine Tri-phosphatase. The protein filaments of myosin act as an enzyme to break down A.T.P. into A.D.P. (Adenosine Di- phosphatase) and phosphoric acid. This reaction or breakdown releases a great deal of energy which is used by the muscle fibres to cause contraction. As long as A.T.P. is being broken down the muscle remains active, but when this reaction stops, the muscle becomes relaxed. Another reaction is the breakdown of glycogen into lactic acid with the production of energy.
- 3. CARDIAC MUSCLE
This type of muscle is striated or striped and is involuntary and differs from striated voluntary muscle in the following:
- The nuclei are placed in the centre of the cells;
- The fibres branch out to form networks;
- Between the cells there are membranes called intercalated discs and;
- Cardiac muscle is not subject to fatigue; in other words, it does not get tired in the way that striated voluntary muscles get tired and become less effective.
Figure 10: Cardiac Muscles
| Subcutaneous: Situated or applied under the skin. |
STRUCTURE OF MEAT
Meat is made up of muscular, connective and fatty tissues, bone and skin. On average 50 – 55% of the animal’s live mass remains on the carcass.
Average Composition of Beef Animal:
| Carcass: | Bone | 8% | Other: | Blood | 3% |
| Fat | 10% | Offal | 16% | ||
| Meat | 37% | Hide | 6% | ||
| 55% | Waste | 20% | |||
| 45% |
Bone
Bone makes up 8% of the live body (or 15 to 20% of the carcass). Bone is an early maturing tissue so the proportion of bone in the body decreases with age, but the dressing out percentage increases as muscle and fat are deposited.
Bone contributes to meat quality by enhancing flavour on cooking.
Connective Tissue
Is closely associated with muscle and is found interspersed within muscle and around muscle sheaths. The white collagenous tissue (collagen) can be broken down by cooking but the yellow elastic tissue (elastin) of ligaments and tendons is not affected by heat.
Fat
Fat ranges from 10 – 50% of the carcass. Fat is an important factor in palatability, prevents drying of meat and is a source of A, D, E and K vitamins.
Fat is first deposited around the kidneys and intestines. It is seen on the finished animal as subcutaneous fat which is finally deposited with muscle bundles as marbling increases tenderness.
Muscle
Meat is post-mortem muscle cells and makes up to 40% of the live body or 65% of the carcass.
MEAT QUALITY
Meat quality is a broad term that can be broken down into numerous factors: Appearance, palatability (tenderness, juiciness and flavour) and nutritive value.
Tenderness
Collagen and elastin increase in content with age and work, and result in tougher cuts. Muscle bundle size increases with age up to four years and causes coarseness of meat. Tenderness is more than 60% heritable and this may be associated with the family trait of docility.
Rigor mortis becomes evident six hours after death and reaches a maximum stiffness by 24 hours. Anaerobic breakdown of muscle glycogen causes contraction of the muscle fibres which produces lactic acid. Then the actin and myosin filaments fuse. To prevent the toughening of meat by shrinkage, the muscles are stretched by hanging the carcass during the process of rigor mortis. Immediate freezing does not prevent rigor mortis – it merely delays it till the carcass is thawed.
Excitement and stress affect tenderness by depleting the animal’s muscle glycogen content. To ensure that animals are in a state of physiological balance at slaughter, they should be fed and watered for 12 hours before slaughter to restore the level of muscle glycogen.
Meat is tenderised by the natural process of autolysis or self-digestion due to certain enzymes present in the muscles. The muscle proteins become denatured and the bonds keeping the molecules together are broken down. These actions are favoured by slightly acidic pH and by elevated temperatures, but temperatures above 5°C permit the multiplication of ever-present bacteria. Aging of carcasses up to 40 days increases tenderness but longer periods have an undesirable effect on palatability. Meat tenderisers are preparations of these enzymes.
Juiciness
Depends on water-holding capacity of meat and the amount of fat embedded in the meat.
Flavour
Certain chemicals (Amino acids and Fatty Acids) impart the characteristic flavour to meat. Age and muscle activity improve flavour due to the larger amounts of derivatives and aromatics (myo-globin and connective tissue). Meat with a low pH has better flavour to which muscle glycogen also contributes. Autolysis improves flavour. Heating releases flavour but excess can drive away aromatic substances.
Palatability
Palatability is a combination of tenderness, juiciness and flavour. Other factors contributing are breed, sex, chronological age, finish and cooking.
Appearance
Appearance is enhanced by colour, fat and bloom.
- 5. Cuts and Joints of Meat
Once the animal has been slaughtered, skinned, and the head, hooves and offal have been removed, what is left is the carcass. This is halved, quartered and sold to the butcher, who cuts it up into smaller joints and sells it to the public.
The following are the retail cuts, or joints, of beef, mutton or lamb, and pork.
Figure 11: Retail Cuts from the Beef Forequarter
- Bolo
- Shin
- Hump
- Neck
- Prime Rib
- Chuck or Thick Rib
- Flat Rib
- Brisket
- Plate
Figure 12: Retail Cuts from the Beef Hindquarter
- Short Rib
- Wing Rib
- Sirloin
- Thin Flank
- Rump
- Aitchbone
- Silverside
- Thick Flank
- Shin
Two cuts not shown are the Topside and the Fillet. The Topside is situated along the inside of the leg, and the Fillet is next to the backbone. It can be removed or left as part of the Sirloin and Rump.
| Figure 13: Retail Cuts of Mutton or Lamb |
1 Neck
2 Thick Rib
3 Breast
4 Flank
5 Rib
6 Loin
7 Chump Chop
8 Leg
9 Raised Shoulder (not shown)
Figure 14: Retail Cuts of Pork
1 Shank and Trotter
2 Thick Rib
3 Breast
4 Rib
5 Belly
6 Loin
7 Chump Chop
8 Leg, fillet
9 Leg, shank
Bacon is produced from the ribs, and ham from the legs.
