LITERATURE REVIEW

Meat is animal flesh that is eaten as food. The advent of civilization allowed the domestication of animals such as chickens, sheep, pigs and cattle, and eventually their use in meat production on an industrial scale (Womack, 2010). The meat industry is concerned with turning an animal carcass into many different end-products.These end-products are derived from all parts of the animal (muscle, bone, fat, cartilage, skin, fluids and glands) and are produced through a range of physical, chemical and biological processes.

Domestication of animals for meat

Paleontological evidence suggests that meat constituted a substantial proportion of the diet of even the earliest humans. The domestication of animals dates back to 10,000 BC (Lawrie and Ledward, 2006), allowing the systematic production of meat and the breeding of animals with a view to improving meat production. The breeding of beef cattle optimized for meat production as opposed to animal best suited for draught or dairy purposes began in the mid 18th century (Mark and William, 2001).

Growth and Development of Meat Animals

Agricultural science has identified several factors bearing on the growth and development of meat in animals. These factors include;

Genetic factors

Several economically important traits in meat animals are heritable to some degree and can thus be selected for by breeding. In cattle, certain growth features are controlled by recessive genes, one of such trait is dwarfism, another is the doppelender or „double muscling‟ condition, which causes muscle hypertrophy and thereby increases the animal's commercial value.

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Genetic analysis continues to reveal the genetic mechanisms that control numerous aspects of the endocrine system and through it, meat growth and quality (Alan, 2006).

Genetic engineering techniques can shorten breeding programmes significantly because they allow for the identification and isolation of genes coding for desired traits, and for the re-incorporation of these genes into the animal genome (Lawrie, 2003). To enable such manipulation, research aimed at mapping the entire genome of sheep, cattle and pigs is still on going. Lawrie (2003) reported a developed recombinant bacterium which improves the digestion of grass in the rumen of cattle and also genetically alter some specific features of muscle fibres.

2.2:2 Environmental Factors

Heat regulation in livestock is of great economic significance, because mammals attempt to maintain a constant optimal body temperature. Low temperatures tend to prolong animal development and high temperatures tend to retard it (Lawrie, 2003). Depending on their size, body shape and insulation through tissue and fur, some animals have a relatively narrow zone of temperature tolerance and others (e.g. cattle) a broad one.

2.2.3 Nutrition

The quality and quantity of usable meat depends on the animal's plane of nutrition. Scientists disagree about how exactly the plane of nutrition influences carcass composition (Schurgers and Vermeer, 2000). High quality protein feed is expensive. Hence, several techniques are employed to ensure maximum utilization of protein (Forrest et al, 2001). Environmental factors influence the availability of crucial nutrients or micronutrients a lack or excess of which can cause great ailments (Waxman et al., 2007). In Australia for instance, where the soil contains limited phosphate, cattle are being fed additional phosphate to increase the efficiency of beef production (Ronald and Fereidoon, 2004). Plant toxins are also a risk to grazing

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animals for instance, sodium fluoracetate, found in some African and ralian plants, kills by disrupting the cellular metabolism (Waxman et al., 2007) and some pesticide residues present a particular hazard due to their tendency to bio-accumulate in meat, potentially poisoning consumers.

2.2:4 Human Interventions

Animal producers may seek to improve the fertility of female animals through the administration of gonadotrophic or ovulation-inducing hormones. Artificial insemination is now routinely used to produce animals of the best possible genetic quality and the efficiency of this method is improved through the administration of hormones that synchronize the ovulation cycles within groups of females. Growth hormones particularly anabolic agents such as steroids, are used in some countries to accelerate muscle growth in animals. This practice has given rise to the beef hormone controversy, an international trade dispute. It may also decrease the tenderness of meat and have other effects on the composition of the muscle flesh. Where castration is used to improve control over male animals, its side effects are also counteracted by the administration of hormones (Amanda, 2013).

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Meat Composition

Meat is composed of: Water, protein, fat and other water-soluble organic material (Table 2.1).

Table 2.1

Nutritive Composition of Beef

Substance

Composition

Proportion %

Water

Hydrogen, oxygen

73

Protein

Amino acids

22

Fat

Phospholipids,

Cholesterol

3.9

and Fat Soluble Vitamins

Other Soluble Organics

Vitamins,Carbohydrates

1.1

Source: Rendle and Keeley, (1998).

Muscle proteins are either soluble in water (sarcoplasmic proteins, about 11.5 % of total muscle mass) or in concentrated salt solutions (myofibrillar proteins, about 5.5 % of mass). There are several hundred sarcoplasmic proteins that are involved in the glycolytic pathway in the conversion of stored energy into muscle power.The two most abundant myofibrillar proteins; myosin and actin are responsible for the muscle's overall structure. The remaining protein mass consists of connective tissue (collagen and elastin) as well as organelle tissue (Moon, 2006).

Fat in meat can be either adipose tissue used by the animal to store energy and consisting of true fats (esters of glycerol with fatty acids), or intramuscular fat which contains considerable quantities of phospholipids and of unsaponifiable constituents such as cholesterol.The fat portion includes some fat-soluble substances, including some vitamins. Meat is an important source of amino acids (the building blocks of proteins), minerals, vitamins and energy (Moon, 2006).

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Biochemical Changes: From Muscle to Meat

The most significant change occurring after slaughtering an animal is that blood circulation stops, the resulting effect is that oxygen is no longer sent to the animal cells. This means that reactions begin to take place under anaerobic conditions. One of the major consequences of this is that the pH decreases because in the absence of oxygen, glucose is converted to lactic acid rather than CO2 and H2O. Changes in key properties of the meat are outlined below (Pearson, 2012).

In life: Glycogen + O2 →CO2 + H2O + ATP (energy)

But in death: Glycogen

Acidity in Meat

pH is a measure of acidity or alkalinity. In beef, pH can be 5.4 to 7.0. The pH level of meat can affect the shelf life of the meat, its colour, its tenderness, and the eating quality. With the accumulation of lactic acid the pH falls from 7 to 5.5, and at the same time the energy rich ATP reserve is depleted. The drop in pH is a desirable feature as a low pH slows down growth of micro-organisms and enhances flavour, juiciness and colour of the meat to give an attractive saleable product (Pearson, 2012). The more glycogen in the muscles, the more lactic acid formed post slaughter. Meat from stressed animals hava a high pH, causing it to be Dark in colour, Firm in texture and Dry in taste (known as DFD meat).

The eating quality of meat is a function of the animal production and carcass processing that has been applied. While live animal factors (animal age, nutrition, breed, pre-slaughter stress) often have a marked effect on beef eating quality, on many cases the treatment of the carcass on the slaughter floor and in the chiller can have a far greater effect.

The combination of a very rapid fall in pH and slow cooling of the carcass can lead to heat or rigor shortening, whereas a slow fall in pH and rapid cooling can lead to cold shortening

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(Pearson, 2012). Heat shortening is undesirable in terms of eating quality as it causes dryness and slight to moderate toughness. Cold shortening is also undesirable as it can cause moderate to severe toughness. A less obvious but important penalty of both cold and heat shortening is a reduction in the ability of the meat to age. Thus, the aim is to achieve an optimal rate of pH fall and/or the mechanical prevention of shortening.

Recent studies have confirmed that, if the pH drops very rapidly while the muscle temperature is high, there may be considerable fluid loss from the muscle. This in turn may lead to lack of juiciness and therefore inferior eating quality (Pearson, 2012). The same combination may also contribute to the development of two toned meat, a condition in which there are undesirable gradations in meat colour within a cut, with the deep meat tissue being paler than the normal red meat closer to the surface.

As a guide, according to Pearson (2012), if the pH is at or below 6.0 while the temperature is at, or above 35°C, then there may be heat shortening. If at any time the pH is at or above 6.0 when the temperature is at or below 12°C, then it is likely that there may be cold shortening.

Meat Quality Parameters

2.6:1 Tenderness

Tenderness is one of or the most discussed features in meat. It is a real challenge for the scientific community and for the meat industry to achieve products with standardized and guaranteed tenderness, since these characteristics are exactly what consumers want in a meat product (Koohmaraie, 1995). It is recognized that processes leading to meat tenderization are mainly enzymatic in nature and involve proteolytic systems. There are two concepts based on enzymatic biochemistry used in explaining meat tenderness; Firstly, some researchers postulated that meat tenderization is affected mainly by calpain (a calcium-activated proteases consisting of at least three proteases; μ-calpain, m-calpain and skeletal muscle-specific calpain)

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which is responsible for myofibril protein degradation (Koohmaraie and Geesink, 2006). Secondly, others suggested a hypothesis that tenderization is a multi-enzymatic process corresponding to apoptosis, where calpain is an important enzyme (Koohmaraie et al., 2002). Variation exists among muscles and differences are mainly due to the amount of connective tissue in the various cuts and amount of connective tissue present is due to the function of the muscle.

Meat Colour

The first impression consumers have of any meat product is its color and thus color is of utmost importance. The color of meat may vary from the deep purplish-red of freshly cut beef to the light gray of faded cured pork (Rockland and Beuchat, 1997). Fortunately, the color of meat can be controlled if the many factors that influence it are understood. Fresh and cured meat color both depend on myoglobin, they are considerably different from each other in terms of how they are formed and their overall stability. Myoglobin is a water-soluble protein that stores oxygen for aerobic metabolism in the muscle. It consists of a protein portion and a nonprotein porphyrin ring with a central iron atom. The iron atom is an important player in meat color.

The defining factors of meat color are the oxidation (chemical) state of the iron and which compounds (oxygen, water or nitric oxide) are attached to the iron portion of the molecule (Kropf et al., 2004). Because muscles differ greatly in activity, their oxygen demand varies. Consequently different myoglobin concentrations are found in the various muscles of the animal. Also, as the animal gets older there is more myoglobin. A greater myoglobin concentration yields a more intense color. Muscle pigment concentration also differs among animal species. For example, beef has considerably more myoglobin than pork or lamb, thus giving it a more intense color.

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Immediately after cutting meat color is quite dark, beef would be a deep purplish-red. As oxygen from the air comes into contact with the exposed meat surfaces it is absorbed and binds to the iron. The surface of the meat blooms as myoglobin is oxygenated. This pigment, called oxymyoglobin, gives beef its bright cherry red color. It is the color consumers‟ associate with freshness (Kropf et al., 2004).

2.6:3 Cooked Meat Pigment

During the cooking process, myoglobin is denatured. All of the pigment is not affected at the same time or to the same extent and this is why you get reddish color at different end point temperatures when heat is applied. The cooked pigment is denatured metmyoglobin, it is brown and is easily recognized in cooked meat products. Certain meat conditions can result in protection of the myoglobin in meat (Hunt et.al., 1999).

The ultimate pH of meat or meat products will affect how the meat color changes during smoking. If the meat has a high pH, it will be cooked to higher end-point temperatures to get the same visual degree of doneness as one with normal pH. Frequently, complains of this hard to cook defect are associated with a high pH of the meat or meat product (Torngren, 2003).

2.6:4 Effect of Meat pH

The rate and extent that muscle pH declines postmortem are both variable and have a great impact on the color of meat and meat products. The normal pH decline in muscles is from approximately 7.0-7.2 down to near pH 5.5-5.7 over about 24 hrs (Sorheim et al., 1999). With this pH decline, whole tissue is the characteristic color of the species. If the pH declines to the normal pH of 5.5-5.7 within 45 min or less, the muscle will appear very pale and soft (PSE). A very low ultimate pH (<5.4) will also result in a paler color (Sorheim et al., 1999). If the pH does not drop much at post mortem, the meat will be dark with a dull, dry surface (DFD). As the ultimate pH increases, the meat gradually becomes darker. This darkening of color

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becomes noticeable when the muscle pH exceeds 5.7.Summary of meat quality parameters are shown in the table below;

Table 2.2

Parameters of Meat Quality

Parameters

Acceptable

Unacceptable

Apperance

Meat colour

Red/pink

Brown, Grey green

Fat colour

White

Yellow

Texture

Firm

Soft, mushy, dry

Palatability

Tenderness

Tender

Mushy, tough

Flavor

Typical of specie

Boar taint, rancid, acid taste

Juiciness

Moist

Lack of flavor

Source: Laird (2006)

Meat Smoking

Composition of Smoke

Hardwoods are made up of cellulose, hemicellulose and lignin. Cellulose and hemicellulose are the basic structural material of the wood cells while lignin acts as a kind of cell-bonding glue. Some softwoods especially pines and firs hold significant quantities of resin which produces harsh-tasting soot when burned, these woods are not often used for smoking (Hui et al., 2001).

Cellulose and hemicellulose are aggregate sugar molecules. When burnt, they effectively caramelize producing carbonyls which provide most of the color components, sweet, flowery and fruity aromas. Lignin, a highly complex arrangement of interlocked phenolic molecules, also produce a number of distinctive aromatic elements when burnt, including smoky, spicy, and pungent compounds such as guaiacol, phenol,syringol, and sweeter scents.Guaiacol is the phenolic compound mostly responsible for the smokey taste, while Syringol is the primary

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contributor to smokey aroma (Hui et al., 2001). Wood also contains small quantities of proteins, which contribute to roasted flavors. Many of the odor compounds in wood smoke especially the phenolic compounds, are unstable, dissipating after a few weeks or months.

A number of wood smoke compounds act as preservatives. Phenol and other phenolic compounds in wood smoke are both antioxidants (which slows rancidity of animal fats) and antimicrobial (which slow bacterial growth). Other antimicrobials in wood smoke include Formaldehyde, Acetic Acid and other organic acids which give wood smoke a low pH of 2.5, some of these compounds are toxic to people as well, and may have health effects if found in high quantities (Kjallstrand and Petersson, 2001).

Different species of trees have different ratios of components hence, impart different flavor to food. Another important factor is the temperature at which the wood burns. High temperature fire causes the flavor molecules to break down further into unpleasant or flavorless compounds (Fabech and Larsen, 1992). Woods that contain high lignin tend to burn hot, to keep them smoldering requires restricted oxygen supplies or a high moisture content (Arnim and Yetti, 2012). When smoking using wood chips or chunks, the combustion temperature is often raised by soaking the pieces in water before placing them on a fire.

The chemical aspects of meat smoking have been reviewed (Toth and Potthast, 1984) and the chemical composition of smoke condensation has been assessed in many studies, including some investigations by Guillen and Ibargoitia (1998 and 1999, respectively) where Methoxyphenols were identified as major components and of great importance for the smoke flavour and for the preserving antioxidant effect of the smoke.

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Plant Materials Used For Smoking

In Nigeria, reports on wood quality of various species of trees used as fuel energy for smoking meat are limited. Alien tree species that has encroached the North West region of Nigeria is

Eucalyptus Camaldutensis (Turare), and has good potentials as a wood fuel though it has not been reported as a source of smoke for meat smoking in Nigeria. It also has edible component and can ease pressure on A. raddiana and I. Doka which are the current tree of choice for meat smoking in this area (Oduor et al., 2008).

Coconut (Cocos nucifera) plantations provide numerous products such as coconut meat, milk and shells. Coconut shells can be used for cooking, heating and as the heating base for smoking meat and fish. The use of coconut shells for fish smoking was tried by Benjakul and Aroonrueng (1999), results from this study showed that fish smoked using coconut shell had good sensory qualities.

The main interest in using Neem (Azadirachta indica) in meat smoking is because of activity of its components as a deterrent for both insect activity and mould. It has multiple pesticidal and medicinal properties. Smoke from its‟ leaves are used as insect repellant. About 135 different compounds are found in every part of the tree and it also has antimicrobial effects (Battacharyya et al., 2007). Neem‟s various part reportedly have antihelmintic, antiseptic, antisyphilitic, astringent, demulcent, diuretic, emmenagogic, emollient and purgative actions and have also been used to treat boils, eye diseases, exzema, headaches, hepatitis, leprosy, rheumatism, scrofula, and ulcers (Ketkar, 1986). No study has been reported for the use of Neem tree for meat smoking in Nigeria and for control of insect infestation during storage of smoked meat.

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Table 2.3 Concentrations (mg m-3) of selected components of smoke from E. Camaldutensis (Turare), A.raddiana, A. indica (Neem) and C. nucifera (coconut shell).

Smoke Components

T1

T2

T3

T4

Phenols

2,6-D2,6-

16.8

16.1

24.1

14.3

Dimethoxyphenols

2-Methoxyphenols

5.3

5.3

2.6

39

Phenol

0.9

0.2

1.7

Anhydrosugars

1, 6-Anhydroglucose

6.0

Form'aldehydes

0.4

2-Furaldehyde

2.0

2.5

2.1

5-Methyl-2-

0.3

0.3

0.2

furaldehyde

Furans

2-Methylfuran

1.6

1.5

1.3

2,5-Dimethylfuran

0.3

Bensofuran

0.4

0.4

0.4

Hydrocarbons

Benzene

2.2

2.2

1.9

3.8

Methylbenzene

1.5

1.4

3.2

1.9

Styrene

1.3

0.2

0.2

1.0

Source: Kjailstrand and Petersons (200l).

T1= Smoke from Acacia,

T2= Smoke from Eucalyptus

T3= Smoke from Neem,

T4= Smoke from coconut shell

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Smoke House

The upright drum smoker (also referred to as an Ugly Drum Smoker or UDS) is exactly what its name suggests. An upright steel drum modified for the purpose of hot smoking. There are many ways to accomplish this, but the basics include the use of a complete steel drum, a basket to hold charcoal near the bottom and cooking rack near the top, all covered by a vented lid of some sort. They have been built using many different sizes of steel drums, but the most popular size is the common 55 gallon drum (Wikipedia, 2013a). The temperatures used for smoking are controlled by limiting the amount of air intake at the bottom of the drum and allowing a similar amount of exhaust out of vents in the lid. UDSs are very efficient with fuel consumption and flexible in their abilities to produce proper smoking conditions.

Smoking chambers with freshly generated smoke, according to the investigated procedure, offers several advantages. The hardwood produces smoke with a high proportion of 2,6-dimethoxyphenols, which have stronger protective antioxidant activity than 2- Methoxyphenols from softwood (KjaIlstrand and Petersson, 2001). The smoldering bed of wood particles in the smoke generator helps in keeping a constantly low combustion temperature, this results in a high proportion of the particularly anti-oxidative 2,6-methoxyphenols with an alkenyl side-chain in the smoke while it also keeps the concentrations of hazardous polycyclic aromatic compounds at a very low level. A drawback is the high concentration of benzene in the curing atmosphere, but benzene appears to be much less absorbed than the major methoxyphenols which are condensed on smoke particles (Kjällstrand and Petersson, 2010). The regulated internal temperatures for smoke cook beef is 1600F to ensure less risk of benzene and other carcinogenic component of the smoke (USDA-FSIS, 1999).

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Effect of Smoking on Meat

Smoking is the process of flavoring, cooking or preserving food by exposing it to the smoke from burning or smoldering plant materials most often wood. Meats and fish are the most commononly smoked foods (McGee, 2004).

Hot smoking exposes the foods to smoke and heat in a controlled environment. Like cold smoking, the item is hung first to develop a pellicle and then smoked. Although foods that have been hot smoked are often reheated or cooked, they are typically safe to eat without further cooking. Hot smoking occurs within the range of 52 to 80 °C (Myrvold, 2011). Within this temperature range, meat is fully cooked, moist and flavorful. If the smoker is allowed to get hotter than 85 °C, the meat will shrink excessively. Smoking at high temperatures also reduces yield, as both moisture and fat are cooked away. It can equally cause a toughening of meat fibres due to heat coagulation and shrinkage of the myofibrillar proteins and connective tissues. Initial toughening is due to protein denaturation which occurs when the meat reaches 50-80oC, this is followed by some tenderization which occurs as collagen hydrolyses to gelatin at temperatures greater than 75oC (Inneke, 2011). However, prolonged heating can increase the tenderness due to the conversion of collagen to gelatine by heating.

Preservation technique used on beef is vital as some of the nutrients can be damaged. Bangladesh (2009) compared different processing methods in relation to nutrient quality and observed the quality decreasing trend was lower in smoked cooked, flat size meat sample. The author observed that rural smoking method of meat drying could be a useful technique of meat preservation.

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Health Risks of Smoked Meat

Evidence suggests that smoked foods may contain carcinogens (Fritz and Soos, 1991). The smoking process contaminates food with Polycyclic Aromatic Hydrocarbons (PAHs) and nitrosamines, which are known carcinogens. So, in theory consuming smoked food increases the risk of gastrointestinal cancer. Some studies have found a positive statistical correlation between intestinal tract cancer and the frequent intake of smoked foods (Fritz and Soos, 1991). The United States National Cancer Institute emphasizes that population studies have not established a definitive link between cooked meats and cancer in humans, but suggests individuals reduce their exposure to PAHs.

Suya

Due to the chemical composition and biological characteristics of meat, they are highly perishable which provide excellent environment for growth of many hazardous microorganisms that can cause infection in humans and spoilage of meat and economic loss. Hence, the need for effective, cheap and simple preservative techniques. One of such simple preservative technique is intermediate moisture food processing. Obanu et al. (1981), observed that Intermediate Moisture Meat, are shelf stable under the tropical climate without refrigeration and may be eaten directly with or without rehydration. Suya is one of such intermediate moisture product that is easy to prepare and highly relished (Omojola et al., 2004).

Suya is a traditional meat product that is commonly produced by the Hausas in West Africa from mostly Beef. There are three types of Suya namely, Tsire, Kilishi and Balangu. Of the three types, Tsire, which is boneless meat pieces that are staked on slender wooden sticks and cooked by roasting, using a glowing fire from charcoal, is the most popular with consumers in Nigeria (Igene and Mohammed, 1993). In big cities and small towns, Suya vendors have

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become very prominent with their grill stands becoming very busy from about midday until late at night (Inyang et al., 2005).

Meat processing generally enables the processor to convert low priced meat cuts into high priced processed products (FAO, 1995). Traditionally, most tsire suya producers use expensive cuts of meat (for examples the Longissimus dorsi muscle), resulting into high prices of the products beyond the reach of the common Man.The prime cuts, apart from resulting in products with high prices might not be necessarily better than cuts from less choice parts of the carcass in terms of product yield and eating qualities.

Characterization of Connective Tissue of Bovine Skeletal Muscles

Commercially important skeletal muscles of bovine carcass are grouped as (a) neck muscles,

(b) shoulder muscles, (c) rib cage muscles, (d) loin muscles, (e) hind limb muscles, (f) sirloin muscles and (g) flank muscles (Swatland, 1994). The muscles of the hind limb which is of importance to this study include; Gracilis, Sartorius, Quadriceps Femoris, Vastus medialis, Vastus lateralis, Vastus intermedius, Rectus femoris, Semi-tendinosus, Semi-membranosus, Adductor, Pectineus, Biceps femoris and Gastrocnemius.

Different methods of carcass fabrication are reported, including the Conventional Approach, the Innovative cut technique (Pfeiffer et al., 2005) and Hot boning (Seideman and Cross, 1995). Conventional carcass fabrication includes separation of fore and hindquarters followed by separation of rib, chuck and brisket (and so on) by saw cutting. Meats from the cuts are then separated after trimming off fat. Briefly, the Innovative cut fabrication process includes in sequence, separation of fore and hindquarters of carcasses, trimming off all major connective tissues and fat and finally separation of individual muscles of each quarter from their points of attachment (Pfeiffer et al., 2005). In hot boning, lean meat and fat are removed from the bones prior to chilling (Seideman and Cross, 1995).

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2.8:2 Cuts of Beef

Cuts of beef are first divided into primal cuts, pieces of meat initially separated from the carcass during butchering. These are basic sections from which steaks and other subdivisions are cut. Since the animal's legs and neck muscles do the most work they are the toughest. The meat becomes tenderer as distance from hoof and horn increases. Different countries and cuisine have different cuts and names, and sometimes use the same name for a different cut e.g., the cut described as „Brisket‟ in the US is from a significantly different part of the carcass than British „Brisket‟ (Wikipedia, 2013b).

The following is a list of the American primal cuts and cuts derived from them; Chuck, Rib, Brisket, Foreshank or Shank and plate (all found in the forequarter). The hindquarter cuts include; the Loin, Round and Flank. The Round contains lean and is moderately tough having lower fat (less marbling) cuts, which require moist or rare cooking. Some representative cuts of the round are; Top round (Semimembranosus), Bottom round (Biceps femoris), Eye of round (Semitendinosus) and Sirloin tip (Rectus Femoris).

Meat Spoilage

The spoilage of meat occurs if the meat is untreated in a matter of hours or days and results in the meat becoming unappetizing, poisonous or infectious (Lawrie, 1990). Spoilage is caused by the practically unavoidable infection and subsequent decomposition of meat by bacteria and fungi, which are borne by the animal itself, by the people handling the meat and by their implements. Meat can be kept edible for a much longer time though not indefinitely if proper hygiene is observed during production and processing and if appropriate food preservation and food storage procedures are applied (Lawrie and Ledward, 2006)

The organisms spoiling meat may infect the animal either while still alive (endogenous disease) or may contaminate the meat after its slaughter (exogenous disease).There are numerous diseases that humans may contract from endogenously infected meat, such as

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anthrax, bovine tuberculosis, brucellosis, salmonellosis, listeriosis, trichinosis or taeniasis (Ledward and lawrie, 2006).

Infected meat however, should be eliminated through systematic meat inspection. Consumers will more often encounter meat exogenously spoiled by bacteria or fungi after the death of the animal. The large intestine of animals contains some 3.3×1013 viable bacteria, which may infect the flesh after death if the carcass is improperly dressed (Troller and Christian, 1990) Contamination can also occur at the slaughterhouse through the use of improperly cleaned slaughter or dressing implements. After slaughter, care must be taken not to infect the meat through contact with any of the various sources of infection in the abattoir, notably the hides and soil adhering to the carcass, water used for washing and cleaning, the dressing implements and the slaughter house personnels.

Bacteria genera commonly infecting meat while it is being cut, processed, packaged, transported, sold and handled include; Salmonella spp., Shigella spp., E.coli, B.proteus, Streptococci etc. These bacteria are all commonly carried by humans, infectious bacteria from the soil include; Cl. Botulinum. Among the molds commonly infecting meat are Penicillium spp., Mucor, Cladosporium, Alterneria, Sporotrichium and Thaminidium etc. (Lawrie and Ledward, 2006)

As these microorganisms colonize a piece of meat they begin to break it down leaving behind toxins that can cause enteritis or food poisoning, potentially lethal in the rare case of botulism (Rajasekhara et al., 2000). The microorganisms do not survive a thorough cooking of the meat, but several of their toxins and microbial spores do. Meat spoilage by micro-organisms can manifest itself as follows:

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Table 2.4

Symptoms of Microrganism found in meat

Oxygen

Microbial agent

Symptoms

Surface slime

Discolouration

Present

Aerobic bacteria

Gas production

Change in odor

Fat decomposition

Surface slime

Present

Yeasts

Discoloration

Change in odor and taste

Fat decomposition

Sticky and "whiskery" surface

Present

Molds

Discoloration

Change in odor

Fat decomposition

Putrefaction and foul odors

Absent

Anaerobic bacteria

Gas production

Souring

Source: Lawrie (2003).

Contaminants of Meat

Bacteria Contaminants of Meat

During slaughtering process, there is contamination of the sterile tissue with intestinal flora such as gram-negative organisms which includes Escherichia coli as well as contaminants such as Pseudomonas specie and gram-positive lactic acid bacteria and Staphylococci species associated with humans, animals and their environment. Meat spoilage is usually associated with gram-negative proteolitic bacteria which literally decompose the protein with production of offensive odour (Haman, 1977).

The addition of salt and drying of fresh meat have been an effective means to control the meat microflora and thus preserve the tissue for later consumption. The curing salt (sodium chloride or sodium nitrate or sodium nitrite) and subsequent proper handling method, favours the growth of gram-positive bacteria, primarily Staphylococcus aureus while inhibiting the

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proliferation of gram-negative bacteria (Boles et al., 2000). Ogunbanwo et al., (2004) reported that smoking has a preservative effect on the Suya meat

As part of the normal flora of the human intestinal tract, E. coli plays a crucial role in food digestion by producing vitamin K from undigested material in the large intestine. Most strains of E coli are harmless commensal members of the intestinal flora of mammals in which some strains adhere to the intestinal mucosa while others are only temporary transient in the lumen of the colon. While some strains of E. coli live as commensals, many are opportunistic pathogens of humans and other animals (Levine, 1994). E. coli is the major cause of Neonatal septicemia, Neonatal meningititis and urinary tract infections in humans and of a variety of invasive diseases in mammals. It is also a leading cause of diarrhoeal diseases in humans and other mammals (Neidhardt, 1995). E. coli can respond to environmental factors such as Chemicals, pH, temperature and osmolarity in a number of ways, since it is a single celled organism (Todar, 2002).

Salmonella is a genus of rod-shaped, Gram-negative, non-spore forming predominantly motile enterobacteria with diameters around 0.8 to 1.5µm, lengths from 2 to 5µm and peritrichous flaglla, (flagella that are all around the cell body). There are only two specie of salmonella;

Salmonella bongori and enterica. Salmonella is found worldwide in both cold blood and in the environment. They cause illnesses such as typhoid fever, paratyphoid fever, and food poisoning (Ryan and Ray, 2004).

2.9.1.2 Yeasts and moulds

Fungi are slow growers in comparison to bacteria and so are often out-competed seldom responsible for the spoilage of fresh proteinaceous material.Yeasts and moulds are more resistant to low temperature, low pH, lower aw values and the presence of preservatives than bacteria (Taniwaki et al., 2001).

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Generally speaking however, it appears that fungal metabolism is best suited to substrates high in carbohydrates, whereas bacteria are more likely to spoil proteinaceous foods with the exception of Lactobacilli. If meat surface dries out, it will inhibit bacteria and permits mould growth. Chill foods of low pH inhibit bacteria but may allow mould/yeasts to develop (Taniwaki et al., 2001).

2.9:2 Effect of Temperature on meat spoilage

The influence of temperature in food preservation and spoilage has two separate facets: Temperatures during processing and Temperature during storage (Beuchat and Toledo, 1994). As noted above heat-resistant fungal spores may survive pasteurising processes. Apart from a few important species, little information exists on the heat resistance of fungi. Low pH and preservatives increase the effect of heat (Beuchat, 1981) and also hinder resuscitation of damaged cells (Beuchat and Jones, 1995).

2.9.3 Effect of Water Activity on meat spoilage

Water availability in foods is most readily measured as water activity. Water activity (aw) is a physicochemical concept, introduced to microbiologists by Scott (1977), who showed that aw effectively quantified the relationship between moisture in foods and the ability of microorganisms to grow on them.

Water activity is defined as a ratio: aw ¼ p=po

Where p= is the partial pressure of water vapour in the test material and

Po= is the saturation vapour pressure of pure water under the same conditions. Water activity is numerically equal to equilibrium relative humidity (ERH) expressed as a decimal (Duckworth and Ormerod, 1997). In many practical situations, aw is the dominant environmental factor governing food stability or spoilage. Knowledge of fungal water relations will then enable prediction both of the shelf life of foods and of potential spoilage

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fungi. For example; Ascomycetous fungi and conidial fungi of ascomycetous origin comprise most of the organisms capable of growth below 0.9 aw (Taniwaki et al., 2001).

2.9.4 Effect of gas tension on meat spoilage

Food spoilage moulds like filamentous fungi, have an absolute requirement for oxygen. The concentration of oxygen dissolved in a substrate has a much greater influence on fungal growth than atmospheric oxygen tension (Miller and Golding, 1989). For example,

Penicillium expansum grows virtually in 2.1% oxygen over its entire temperature range and many other common food spoilage fungi are inhibited only slightly when grown in nitrogen atmospheres containing approximately 1.0% oxygen (Taniwaki et al., 2001).Most food spoilage moulds appear to be sensitive to high levels of carbon dioxide, although there are notable exceptions. When maintained in an atmosphere of 80% carbon dioxide and 4.2% oxygen, Penicillium roqueforti still grew at 30% of the rate in air (Miller and Golding, 1989), provided that the temperature was above 280C. In 40% CO2 and 1% oxygen, Penicillium roqueforti grew at almost 90% of the rate in air (Taniwaki et al., 2001). Xeromyces bisporus has been reported to grow in similar levels of carbon dioxide (Dallyn and Everton, 1969).

At least some species of Mucor, Rhizopus and Fusarium are able to grow and ferment in bottled liquid products and sometimes cause fermentative spoilage. Growth under these conditions may be yeast-like.

It is evident from the above discussion that the growth of fungi in a particular food is governed largely by a series of physical and chemical parameters and definition of these can assist greatly in assessing the food‟s stability. The situation in practice is made more complex by the fact that such factors frequently do not act independently, but synergistically. If two or more of the factors outlined above act simultaneously, the food may be safer than expected. This has been described by Leistner and Ro‟del (1996) as the „Hurdle Concept‟. This concept

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has been evaluated carefully for some commodities such as fermented sausages and is now widely exploited in the production of shelf stable bakery goods and acid sauces.