An Overview On The Meat Quality Biology Essay


Meat quality is a term used to explain the overall meat characteristics such as physical, biochemical, nutritional morphological, microbial, sensory, cooking, and hygienic characteristics[1] (Ingr, 1989). Sensory characteristics like appearance or color, flavor, odor or smell, texture, juiciness, wateriness, firmness and tenderness are among the most significant and perceptible meat features. These sensorial features influence the final decision of consumers for purchasing meat and meat containing products [2](Cross et al., 1986). Furthermore, quantifiable characteristics of meat such as water holding capacity (WHC), shear force (SF), drip loss(DL), cook loss(CL), pH, shelf life etc are vital for the manufacturing of value-added meat products[3] (Allen et al., 1998). Raw meat used in meat products is necessary to have excellent functional properties that will ensure a final product of excellent quality and profitability. However, the poultry grading system used worldwide is based on visual features such as conformation, presence or absence of carcass defects, bruises, missing parts, and skin tears without taking into account the functional properties of meat[4] (Barbut, 1996). Therefore, this grading system has not been beneficial for the further processing industry that is for the most part interested in the functional properties of meat.

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2.1. Meat Quality

2.1.1. Color

In poultry as well as in other species, color variations in meat have received considerable attention from researchers because of their direct influence on consumer acceptance and high correlation with the functional characteristics of meat. Poultry is the only species known to have muscles with marked differences in color, and the meat has been classified as either white or dark. These marked differences are largely due to its muscle biochemistry and histology. Fresh raw breast meat is expected to have a pale pink color, while raw thigh and leg meat are expected to be dark red. Discoloration may occur in the entire muscle or only in a portion of a muscle due to bruising or broken blood vessels [5] (Froning, 1995).

2.1.2 Chicken breast meat

The breast muscle of chicken is more susceptible than the thigh and leg muscles to variations in color because (i) it comprises a high proportion of the carcass and (ii) its inherent light color makes any changes in color more visible. Meat color is important because consumers relate it with freshness and overall quality. Thus, it exerts a major influence on their decision to buy the product. Variation in color between fillets displayed in a retail package is very noticeable and leading to the rejection of an entire package.

For that reason, processors have been forced to sort the fillets in a package by color to increase product uniformity and increase consumer acceptability.

Meat color varies according to the concentrations of these two pigments i.e. myoglobin and hemoglobin content. The pigment chemical state, or the way that light is reflected off the meat. The principal heme pigments found in poultry meat are myoglobin, hemoglobin, and cytochrome c [5](Froning, 1995). Myoglobin is the principal heme pigment in poultry meat contributing largely to color definition. However, myoglobin concentration in poultry meats is significantly lower than in comparable muscles in other species [6,7,8] (Froning et al., 1968; Fleming et al., 1991; Millar et al., 1994).

It has been estimated that in a well bled bird, 20 to 30% of the hemoglobin is still present in the carcass, which has a profound effect on meat color[5] (Froning, 1995). Breast and thigh muscles of males have been observed to have higher myoglobin content than those of females at comparable ages. The myoglobin concentration has been reported to be 0.15 and 0.50 mg/g of tissue in the broiler and turkey breast muscle, respectively[6]. It is reported that the total pigment, myoglobin, and iron concentrations were significantly lower in pale breast meat compared to normal breast meat.[9] Chicken breast muscles had a small capacity to form oxymyoglobin (bloom) when exposed to air and have higher oxygen consumption rates which encourage the formation of metmyoglobin at the surface of the meat as compared to pork and beef meat. [8]

The color of meat is not only dependent on the concentration and chemical state of heme pigments, it is also determined by muscle structure. The amount of light reflected from meat is affected by the scattering of light due to differences in refractive index at the boundaries between light reflecting particles. Light scattering from meat has been associated with protein denaturation and changes at the inter-myofibrillar periphery that may involve the packaging of the myofibers. Light scattering from a muscle surface is directly proportional to the extent of protein denaturation. At a pH ≥6.0, protein denaturation is minimal, light scattering is low and the muscle remains translucent. However, at pH ≤6.0 protein denaturation is high, light scattering increases, and the muscle becomes very opaque. Changes in light scattering affect meat lightness (L*) in a fashion inverse to that caused by heme pigment concentration, having a minimal effect on meat redness (a*) and yellowness (b*).

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2.1.3. Water Holding Capacity

Water holding capacity (WHC) is among the most important functional properties of raw meat. It is the water binding potential, expressible juice and free drip to categorize the WHC of meat samples.[11] Free drip refers to the amount of water that is lost by the meat without the use of force other than capillary forces (gravity). About 88 to 95% of the water in the muscle is held intracellularly within the space between actin and myosin filaments. However, only 5 to 12% of water in the muscle is located between the myofibrils[12,13] (Ranken, 1976; Offer and Knight, 1988). Factors such as pH, sarcomere length, ionic strength, osmotic pressure, and development of rigor mortis influence the WHC by altering the cellular and extracellular components (Northcutt et al., 1994; Offer and Knight, 1988).[13,14] Tenderness, juiciness, firmness, and appearance of meat improve as the content of water in the muscle increases, leading to an improvement in quality and economical value. Lactic acid production and the resultant decline in pH after death results in protein denaturation, loss of protein solubility and in an overall reduction of reactive groups available for water binding on muscle proteins[15] (Wismer-Perdersen, 1986).

Nutritional management has become an integral part of poultry production. The increasing demand for white meat and the continuous improvements in the genetic potential of commercial lines has resulted in important changes in nutritional management of broilers. During recent years, it has become a common practice to grow broilers under high protein diets in an attempt to maximize growth and production of breast meat. In addition, companies are now growing males separately from females, and beyond 8 wks of age to obtain large quantities of breast meat. However, while the economic benefits of these changes are obvious in terms of meat production, the impact of such changes on meat quality is unknown. These factors could be important contributors to the incidence of pale soft exudative condition in broilers since rapid and extensive lean muscle growth seems to be associated with a decrease in the resultant muscle quality[16] (Solomon et al., 1998).

2.2.1. Sodium chloride (NaCl)

Salt (sodium chloride) is used to improve texture, enhance the flavor, and extend the shelf life of meat and meat products. Salt is commonly found in meat products at a 2% inclusion level 17(Offer and Trinick 1983). It was found that lowering salt levels to less than 1.3% in frankfurters resulted in incomplete protein extraction and allowed water to escape[18]. Protein extraction with the use of salt is important in the meat industry to obtain desirable textural properties. Salt changes the ionic strength and allows the proteins to be exposed within a meat batter. The hydrophilic ends of the protein bind to water, whereas the hydrophobic ends of the protein bind with fat stabilizing an emulsion. The concept of fat being encapsulated by solubilized proteins is important for emulsion stability in meat batter matrices, where the solubilized proteins swell up with free water [19] (Lan et al. 1993). Bind strength, water-holding capacity, break strength, and cook loss are influenced by the amount of salt-soluble proteins present and affect the overall texture of a product. The positive effects of texture and WHC of processed meat are attributed to myofibrillar proteins within the matrix of a meat batter. Myofibrillar proteins, which include myosin and actin, are two of the major proteins that are extracted from muscle tissue by salt. Binding strength of a meat product is increased with salt soluble proteins [21,22,](Swift and Ellis 1956; Mandigo et al. 1972, Acton 1972, Rhee et al. 1983).

In addition to bind strength, WHC and CL affect texture. If WHC is reduced and CL is increased in meat products, an undesirable texture is created. Meat is dry and overall palatability is reduced. Gelabert et al. (2003) demonstrated that The water-holding capacity increased with salt inclusion, while cook loss is reduced in meat when salt is added (Huffman et al. 1981)[25,26].

In addition to textural properties, salt is also used as a flavor precursor in meat products and for extending shelf life. The amount of water activity within a meat product impacts microbial growth. Lower water activity extends the shelf life by reducing microbial growth.

2.2.2. Concerns Surrounding Salt Use in the Meat Industry

Although salt has functionality purposes in the meat industry, it can also be detrimental to meat products by accelerating lipid oxidation, which is undesirable in food products and leads to oxidative rancidity. Furthermore, accumulation of lipid peroxides in the diet has been linked with certain human diseases such as atherosclerosis [27] (Kanazawa and Ashida 1998). Lipid oxidation (LO) is an auto-catalytic reaction involving free radical formation. Lipid oxidation consists of three stages that include initiation, propagation, and termination within the phospholipid bilayer of the muscle tissue. Initiation of the process occurs when a methylene hydrogen atom is removed from the double bond on the unsaturated fatty acid. Free radicals are generated from the unsaturated fatty acids as a result. The fatty acid free radical connects with a molecule of oxygen to create a peroxyradical during propagation [28] (Damodaran et al. 2008). Hydroperoxides are formed during the primary change of lipid oxidation [29] (Coxon 1987). Termination of free radical formation occurs when there is a combination of two radicals to form a nonradical species [28](Damodaran et al. 2008). Secondary products such as aldehydes, ketones, and alcohols result when primary products are broken down with accelerated oxidation. These secondary products are responsible for the production of off favors and off odors. [30](Ahn et al. 1993b). Meat products containing a higher degree of polyunsaturated fatty acids are more susceptible to lipid oxidation than products containing saturated fatty acids [31](Dawson and Gartner 1983) because free radical formation increases with the degree of unsaturation. Exposure to oxygen, grinding during processing, and transition metals such as iron and copper enable the primary radicals to form and accelerate oxidation[32,33] (Asgar et al. 1988; Kanner and Rosenthal 1992). Light and increased temperatures can accelerate the process as well, as cooked meat is known to oxidize faster than raw meat [24] (Rhee et al. 1996).

2.2.3. TBARS (Thiobarbituric acid reactive substances) and sensory characteristics

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Lipid oxidation is evaluated to assess food quality and is associated with sensory characteristics such as off flavor that are produced from the decomposition of hydroperoxides. There are various assays to measure lipid oxidation. The peroxide value determination method is used to quantify hydroperoxides. Secondary products can be measured by the TBA test [34, 35] (Tarladgis et al. 1960; Witte et al. 1970) or by hexanal values [36] (Shahidi and Pegg 1994). The TBA Test is the most common method used to measure lipid oxidation and is also referred to as the TBARS (thiobarbituric acid reactive substances) method. This assay measures the pink (red) chromophore that is formed by the reaction of 2-Thiobarbituric Acid (TBA) with secondary products, such as malondialdehyde (MDA), by using spectrophotometry [37] (Sørensen and Jørgensen 1996). TBARS values are reported as milligrams of malondialdehyde equivalents per killigram of tissue or samples and have been correlated with off flavor scores [38] (Nolan et al. 1989).

2.2.4. Salt as a pro-oxidant

Salt at varying levels has been proven to be a pro-oxidant in meat products. It is found that salt acted as a pro-oxidant at a 1% inclusion level in pork patties.[39] In another study, sodium chloride at a 2% inclusion level was more pro-oxidant compared with potassium chloride (2%) in turkey patties, even when the levels of copper and iron were held constant.[40] There are many postulations as to how sodium chloride acts as a pro-oxidant. It is found that NaCl acts as a pro-oxidant by displacing the iron ions with sodium in the heme pigments of the muscle tissue[33], whereas others recognize the Cl- ion acting upon the lipid as the source[41] (Ellis et al. 1968) .

The metal impurities, particularly iron, within salt are also thought to cause lipid oxidation[42,43,44] (Chang and Watts 1950; Denisov and Emanuel 1960; Salih 1986b). It is discovered that iron was more pro-oxidant than copper and cobalt in fish, turkey, chicken, pork, beef, and lamb [45]. It is found that when different salt varieties at a 2% inclusion level were used with added metal contaminants that included copper, iron, and magnesium in turkey breast and thigh meat mixtures, the combination of NaCl with copper and iron was the most pro-oxidant.[46]

2.2.5. Lipid oxidation and storage

Even though salt acts as a prooxidant, storage, species, and muscle type influences TBARS values as well. As storage time is increased, TBARS values will increase. Huffman et al.

(1981) evaluated restructured pork chops constructed from hams and Boston butts under frozen storage (-15 ° C) for 0 days and 30 days. At 0 days of storage the TBARS values were 0.18 mg MDA/ kg sample, whereas the TBARS values at 30 days were 0.26 mg MDA/kg sample [47]. In another study, ground beef samples were stored for 30 days and 60 days in frozen storage at -20 °C. The TBARS values for 30 days after frozen storage were reported as 2.46 mg MDA/kg tissue and 2.58 mg MDA/kg at 60 days [24].

In addition to storage time, species affects the rate of oxidation. Different species contain different levels of polyunsaturated fatty acids. It is found that species containing more polyunsaturated fatty acids have higher TBARS values. Fish oxidized quicker than turkey, chicken, pork, beef, and lamb, where lamb was the least oxidized [45]. Within species, muscle type impacts oxidation. Turkey breast meat oxidizes in a slower manner than thigh meat, as indicated by higher TBARS values in thigh meat [48,49] (Salih 1986a; Botsoglou et al. 2003). This is attributed to the fact that breast meat contains less fat than thigh meat [46] (Salih et al.1989).

Packaging and the use of antioxidants can delay the onset of the oxidation reaction when salt is included in processed products. The type of packaging used for a product is dependent upon how quickly the product will be used. Vacuum packaging, modified atmosphere packaging with the use of nitrogen or carbon dioxide gases, and polyvinylchloride overwrap are often used to minimize lipid oxidation in meat products. Overwrapping and modified atmosphere packaging are used for products undergoing retail display, whereas vacuum packaging is used for meat

Products that are going to be stored for extended periods of time. In addition to packaging, antioxidants are used in the food industry to interrupt the free radical mechanism involved in lipid oxidation before the process is catalyzed. Some antioxidants used in the meat industry include alpha-tocopherol (α-tocopherol), herbal extracts and oils. It is found that TBARS values of beef with salt (2%) and added α - tocopherol were higher than the unsalted control groups after 2 days of storage, yet were significantly lower than the TBARS values of beef with only salt (2%)[50]. This implies that the salt was pro-oxidant, but the α -tocopherol slowed the process of lipid oxidation when salt was included. Although, α-tocopherol serves as an antioxidant when salt is used in food products. It is reported that TBARS values and off flavor scores were most improved when grape seed extract was used in combination with NaCl in beef and pork patties compared to oregano or rosemary[51].

Antioxidants can also be added to the animal's diet to delay the onset of rancidity in the meat products that will be obtained from them. It is found that turkeys fed with α-tocopherol acetate and oregano oil resulted in less LO compared to turkeys that were not provided with antioxidants. They determined the combination of oregano oil and α-tocopherol provided the best protection against oxidation [49]. Another study is in agreement with these results, as it was determined that incorporating α-tocopherol in turkey diets lowered TBARS values in raw and cooked samples that had been stored in refrigeration and during frozen storage. Adding higher levels of α-tocopherol can further prevent oxidative rancidity from occurring in turkeys [52].

2.3.1. Selenium

Selenium (Se) is a trace element that plays a key role in the antioxidant defense system. It is the integral part of at least 25 selenoproteins and via their actions protects the organism from harmful actions of free radicals (Pappas et al., 2008). Inplants, Se occurs as part of an organic compound predominantly selenomethionine (Schrauzer, 2003). Selenium is added to the diet of animals either as an inorganic salt (sodium selenite or sodium selenate) or as an organoSe compound more often in the form of Se yeast (Navarro-AlarconandCabrera-Vique, 2008). Selenium from Se-yeast is more thoroughly absorbed and more efficiently metabolized than the inorganic salts, which are poorly absorbed (Schrauzer, 2000, 2003). Absorbed selenomethionine can be incorporated into tissue proteins in place of methionine or can be metabolized in liver yielding hydrogen selenide (H2Se) which is further used for synthesis of specific selenoproteins (Schrauzer, 2003; Pappas et al., 2008; Behne et al., 2009). Unlike metals that interact with proteins in form of cofactors, Se becomes co translationally incorporated into the polypeptide chains of selenoproteins as part of the amino acid selenocysteine (Pappas et al., 2008).

Selenium (Se) is involved in cellular antioxidant defense mechanisms by the activity of glutathione peroxidase (GSH-Px), which is a Se dependent enzyme, that catalyses the reduction of hydrogen peroxide and organic peroxides to water and the corresponding stable alcohol and thus inhibiting the formation of free radicals.[53] Se had a sparing effect on Vit E and increased its content of meat and egg yolk in chickens.[54] Another recent study has reported that the activity of α-tocopherol is improved by the addition of Se in the diets, thus resulted in better quality of meat.[55] In the past, meat producers relied on Vit E to reduce LO and increase meat shelf life. Now, it is clear that efficient utilization of Vit E is dependent upon the Se based antioxidant enzymes in the body, and an adequate Se intake is required to ensure the best utilization of this exclusive vitamin.[56]

2.3.2. History of Selenium

Se is a natural trace element in the environment that is intermediate between those of metals and non-metals. It is an essential nutrient for all animals and humans. One of the most important features of Se is the very narrow margin between nutritionally optimal and potentially toxic dietary exposures for vertebrate animals [57](Wilber 1980).

Selenium exists in different environmental compartments that are atmospheric, marine, and terrestrial in nature. Heterogeneity in its distribution results in movement of Se among those compartments [58]. (Nriagu 1989). Parent materials having the highest selenium concentrations are black shales (around 600 mg/kg dry) and phosphate rocks (1-300 mg/kg dry); both of which can potentially give rise to seleniferous soils and food chain Se toxicity. Se can become mobilized and concentrated by weathering and evaporation in the process of soil formation and alluvial fan deposition in arid and semiarid climates[59] (Presser 1994), and through leaching of irrigated agricultural soils and remobilization in irrigation water [60,61].(Presser and Ohlendorf, 1987; Seiler et al . 1999). Natural selenium content in the crust is ranges from 0.1 to 2.0 mg /kg, and an average of 0.2 mg / kg [8]. When the Se content of the soil in any region is less than 0.5 mg /Kg or the Se content in plants is less than 0.2 ng/Kg, the region called Se deficient area in the Se mineral. Around 24 countries in the world are Se deficient including some areas of United states of America, New Zealand and United Kingdom. [9, 10,11].

[8] Levesque M. Some aspects of selenium relationships in eastern Canadian soils and plants[J]. Canadian Journal of Soil Science, 1974, 54: 205-214.

[9] Ramírez-Bribiesca J, Tórtora J, Hernandez L, et al. Main causes of mortalities in dairy goat kids from the Mexican plateau[J]. Small Ruminant Research, 2001a, 41: 77-80.

[10] Ramírez-Bribiesca J, Tórtora J, Huerta M, et al. Diagnosis of selenium status in grazing dairy goats on the Mexican plateau[J]. Small Ruminant Research, 2001b, 41: 81-85.

[11]. NRC, 1983. Selenium in Nutrition. Subcommittee on Selenium, Committee on Animal Nutrition, Board of Agriculture, National Research Council, Washington, DC, 174 pp.

The Swedish chemist, Jons Jakob Berzelius discovered Se in 1817 in the flue dust of iron pyrite burners [62,63](Levander, 1986; Sunde, 1997). Selenium was named Se after the Greek term, selene, for moon. Since its discovery, Se has had an interesting history. Marco Polo probably had the first recorded observation of Se toxicity in 1295 when he described a disease he called "hoof rot"[62] (Spallholz, 1994). In the 1930's, several researchers identified Se toxicity to be a direct cause of alkali disease and blind staggers [63,64,65,66](Franke, 1934a,b; Franke and Potter, 1935; Moxon, 1937), and then it is Nelson et al. (1943) classified Se as a carcinogen. Early interest in Se was primarily related to its properties as a toxic element [67] (McDowell, 1992). In the 1930s, Se was recognized as the toxic element responsible for lameness and death of livestock grazing specific plant species in the Wyoming and North and South Dakotas [68](Franke, 1934). Similar symptoms were recognized earlier in 1860 by Madison (cited by NRC, 1983), who noted several mineral toxicity signs, including hair loss and lameness in Calvary horses at Fort Randall [69]. It was not until 1842 that Se was first proved toxic but was not associated with general livestock poisoning until 1931 when it was named "alkali disease" [70](Moxon and Rhian, 1943). In 1928, Dr. Kurt W. Franke was the first to observe the signs of poisoning in farm animals and took many samples from regions where poisonings were rarer. In the first half of the twentieth century, "blind staggers" was noted in several papers (Moxon and Rhian, 1943)[70]. Selenium toxicity was confirmed in 1933 to occur in livestock animals that consumed plants of the genus Astragalus, Xylorrhiza, Oonopsis, and Stanleya, which were selenium accumulator plants (Moxon and Rhian, 1943)[70].

However, in 1957, Schwarz and Foltz identified Se to be one of three compounds that prevented liver necrosis (vitamin E and cystine were the others), thus establishing Se as a nutritionally essential trace mineral [71]. Klaus Schwarz, while investigating the cause of dietary liver necrosis in rats, determined Se to be an essential nutrient for both animals and humans [72](Spallholz et al., 1990). The essential nature of selenium was demonstrated in the 1950s. Klaus Schwarz in Germany produced liver necrosis in rats fed a brewer's yeast-based diet. Schwarz moved to the United States and continued his experiments. However, he could only produce liver necrosis when brewer's yeast from the United States was replaced with torula yeast. Se was identified as the factor in U.S. brewer's yeast that prevented liver necrosis (Schwarz and Foltz, 1957)[71].

Among other species, Se deficiency is associated with a number of different symptoms. In rats, Se deficiency is associated with sparse hair coat, poor growth, poor sperm motility and cataracts (see Levander et al., 1995 for review). In ruminants, Se deficiency can cause white muscle disease (a nutritional muscular dystrophy) (Muth et al., 1958), dystrophic tongue, heart failure and retained placenta. Se deficiency is associated with reduced serum selenium, increased AST activity and white muscle disease in horses. Deficiencies of Se and vitamin E can cause sudden death of young, rapidly growing pigs and a deficiency of selenium in growing chickens causes exudative diathesis (a weeping edema of the skin with easy bruising) (see The Merck Veterinary Manual, Eighth Edition [2003] for review).

Furthermore, Rotruck et al. (1973) it is indicated that Se was essential for the proper function of the glutathione peroxidase enzyme, further establishing Se as a nutritionally essential [73]. In 2000, the approval of organic selenium for supplementation in poultry diets is the recent advancement by the FDA (Anonymous, 2000)[74].

2.3.3. Selenium history in China

In1970s, it was first time reported in China that Se can effectively prevent endemic Keshan disease [7]. In 1988, the Chinese Nutrition Society recommended Se as an essential nutrient trace mineral in human nutrition. In addition, Some parts of the China are rich in Se content such as Enshi, Hubei, Shaanxi, Ziyang and having the Se content of 7.1 ~ 27.4 mg /Kg [11], about 72% of the China having different levels of Se. The Yunnan-Guizhou Plateau, Qinghai, Tibet to the eastern most coastal areas, including Heilongjiang, Jilin, Liaoning, Beijing, Shandong, Inner Mongolia , Gansu, Sichuan, Yunnan, Tibet, Zhejiang and some other provinces having low Se contents while Keshan County in Heilongjiang province and Liangshan are Se deficient zones in the China [12, 13]. An example of the importance of selenium in the human diet, Keshan disease, an endemic juvenile onset cardiomyopathy, has been traced to the low soil selenium content in a region of China, leading to low Se intake and association with low body Se content (Keshan disease research group, 1979a). A study of Se supplementation followed these discoveries which showed that it helped to prevent the disease (Keshan disease research group, 1979b).Another interesting feature of Keshan disease is that it cannot be explained by Se deficiency alone. The cardiomyopathy has been associated with the virus Coxsackie B4, which mutates under conditions of low Se to become pathogenic (Levander and Beck, 1997).

7. 中国环境监测总站. 中国土壤元素背景值[M]. åŒ-京: 中国环境科学出版社, 1990: 127-130.

[11] 彭西. æ-¥ç²®ç¡’对雏鸡免ç-«åŠŸèƒ½å½±å“çš„机理研究[D]. 雅安: 四川农业大学, 2009.

[12] 侯少范, 王五一. 我国土壤中结合态硒的含量和分布规律[J]. 地理研究, 1990, 9(4): 17-24.

[13] 易秀. 生态环境中的硒及其地æ-¹ç-…[J]. 西安工程学院学报, 2000, 22(4): 69-72.

Jaan ais pora ko yaha adjust kurna ha. Ais ke waja sa number b dobara dana para gha.sorry. ais k refrence Chinese ha. Ma na separate dia ha. Aap jubh yah pa ao to plz reference ma add kur lana aur number new da dana.thanks.Jazak Allah

2.3.4. Chemistry of Selenium

Selenium, atomic number 34 on the periodic table of elements, is a member of Group VIA along with oxygen, sulfur, tellurium, and polonium [75] (Sunde, 1997), and it is classified as a metalloid element, indicating that it has both metallic and nonmetallic properties. Its natural atomic weight is 78.96, and Se has four common oxidative states: selenide (-2), Se (0), selenite or selenious acid (+4), and selenate or selenic acid (+6). Selenium has six naturally occurring stable isotopes with the 78 Se and 80Se forms accounting for over 73% of the total isotopes.

Because of their positions on the periodic table of elements, Se and sulfur, atomic number 16, have similar chemical properties, such as similar ionic and covalent bond lengths, and they also have similar electronegativites. These similarities can make it very difficult to chemically distinguish Se from sulfur compounds. However, these two elements can be easily distinguished under physiological conditions because H2Se is a much stronger acid than is H2S, and at physiological pH, selenocysteine is predominately deprotonated while cysteine is mostly protonated. Furthermore, selenious and selenic acids have much higher reduction potentials than do the sulfurous and sulfuric acids. These differences have significant implications in the biochemistry of Se. In general, selenate (Se6+) has a high solubility and is the most mobile in water. Selenite (Se4+) is soluble in water but its strong affinity to be adsorbed to soil particles greatly reduces its mobility. Elemental selenium (Se0) exists in a crystalline form and is usually incorporated in soil particles. In most surface waters, selenate and selenite are the most common chemical forms. Selenite is the most bioavailable of the dissolved phase inorganic species (Maider et al. 1993; Skorupa 1998). Selenate is readily taken up by plants and thereby enters the food chain (pers. comm., D. Lemly).

2.3.5. Bioaccumulation

In laboratory studies of the mussel Mytilus edulis, dissolved selenite (+4) is the most bioavailable form of inorganic selenium taken up from solution (Wang et al. 1996).However, Luoma et al. (1992) showed that the uptake rate of dissolved selenite explained less than 5% of the tissue concentrations of selenium accumulated by the clam Macoma balthica at concentrations typical of the San Francisco Bay-Delta.

2.3.6. Selenium Distribution In Tissues

Selenium can be found in all cells and tissues of the body, but the concentration of Se will depend on the chemical form and amount of Se provided by the diet.

Generally, in plant, Se content ranges from 0.05-2.0 mg /Kg (dry matter basis), and in animal, it is ranges from 20- 25 μg /Kg body weight. It is commonly believed that the Se content in food is the reflection of Se content level in the soil. Therefore, the Se level of the soil, plants, animals and humans are closely associated with each other [15].

[15] Yang G. Keshan disease: an endemic selenium-related deficiency disease[C]. In: Chandra, RK (Ed), Trace Elements in Nutrition of Children. Raven Press,New York, 1985: 273-288

Selenium content in plants varies tremendously according to its concentration in soil, which is related to the geographic zone. Around the world, there are many selenium-poor regions in Australia and Asia, mainly. The daily intake of selenium depends on its concentration in food, the amount of food consumed, the chemical form of the element, and its bioavailability, since, it should be kept in mind that the absorption, tissue distribution, and body retention of selenium depends on the chemical species of the element present in food.[18] Meat and fish appear to make rather stable contribution of Se intake, generally in the range 40-50% of the total Se ingested. Therefore, the interest of measurements of total selenium and related species in meat and fish samples is interesting due to two facts: their high consumption and the Se accumulation capability by some animals [25].

According to Behne and Wolters (1983) and Behne and Hofer-Bosse (1984), In using rats supplemented with 0.30 ppm Se, the highest concentration of Se is in the kidneys, followed in descending order, by the testes, liver, adrenals, erythrocytes, plasma, spleen, pancreas, lungs, heart, thymus, gastrointestinal tract, skeleton, brain, and muscle[76,77]. Sunde (1997) Further, when it was calculated total amounts of Se based on the data of previous studies[76,77] Behne and Wolters (1983) and Behne and Hofer-Bosse (1984) and reported that the largest total amount of Se was in muscle followed by the liver, plasma, erythrocytes, and kidneys[75]. Schroeder et al. (1970) Another study has also been reported a similar distribution in samples taken from autopsies of North Americans [78].

Scott and Thompson (1971) it is also reported that Se levels of the blood, muscle, liver, kidneys, and skin increased linearly in chicks fed up to 0.30 ppm Se from an inorganic Se source. They also have reported that increasing Se to 0.80 ppm only resulted in higher levels of Se in the liver and kidneys with no significant increase in blood or muscle Se concentration. In contrast, muscle and blood Se levels were increased more using an organic Se source up to 0.67 ppm than an inorganic source at the same dietary levels [79]. (Scott and Thompson, 1971). Similarly, Latshaw (1975) It is also found that increased muscle, liver, and egg Se levels when laying hens has been fed an organic versus inorganic Se [80]. Arnold et al. (1973) The Se concentration of chicken feathers has been increased as dietary levels of Se increased from 0.30 to 8 ppm [81].

2.3.7. Inorganic and Organic sources of Selenium

Se exists in two chemical forms, organic and inorganic. Inorganic Se can be found in different minerals in the form of selenite, selenate and selenide as well as in the metallic form. Organic Se can be found in forages, grains, and oilseed meals, bonded to different amino acids including methionine and cysteine (Surai, 2002).

Plant foods are the major dietary sources of selenium in most countries throughout the world. Se enters the food chain through plants. Selenomethionine (SeMet) and selenocysteine are the most common organic Se sources in foods. SeMet is incorporated into general proteins by the same codon as that to methionine, thus it is feasible to enrich the meat of animals with Se when excessive SeMet is given to animals.(A. Dokoupilová 2007).

2.3.8. Selenium bioavailability, absorption and Metabolism

In a comparative study between ruminant and monogastric animals, Wright and Bell (1966) reported that monogastrics absorbed more than 2.5 times as much Se as ruminants, being attributable to microbial action in the rumen.

Furthermore, many results from feeding of different chemical forms of dietary Se to animals showed that organic Se was more bioavailable than inorganic Se, resulting in increased Se contents in milk (Ortman and Pehrson, 1999), tissues (Lawler et al., 2004; Lee et al.,2006) and eggs (Payne et al., 2005).

Se from organic sources is more efficiently incorporated into tissue than inorganic sources of Se (Ehlig et al., 1967; van Ryssen et al., 1989).

Unlike inorganic Se, organic Se may be a useful Se supplement because organic Se can be nonspecifically incorporated into body proteins (McConnell and Hoffman, 1972), which may serve as a Se storage capacity. However, the use of Se-Y in animal feeds is less favorable, as it is relatively expensive. In contrast with organic Se, inorganic Se, although it is inexpensive, is much less effective in the Se transfer to animal products. In particular, inorganic Se fed to ruminants has extremely low effectiveness because most of the inorganic Se is reduced to insoluble selenide in the rumen, which cannot be absorbed in the lower intestinal tract (Butler and Peterson, 1961; Hidiroglou et al., 1968), resulting in the excretion to feces.Organic Se accumulates in tissues such as the liver, brain, and muscle [Surai, P. F. 2002]

The metabolism of Se is dependent on its chemical form and on the amount ingested. However, the location in the gastrointestinal tract (GIT) where Se is absorbed, regardless of Se source, seems to be consistent. There are few investigations in Wright and Bell (1966), using sheep, pigs, and Whanger et al. (1976), using rats, agree that the majority of dietary Se is absorbed in the duodenum[82,83]. Whanger et al. (1976) It has also been reported that some of Se absorption is in the jejunum and ileum, but practically none from the stomach [83]. However, there does seem to be differences in type of absorption depending on source. Combs and Combs (1986) It is indicated that inorganic sources of Se, such as sodium selenite (SS) or selenates, are passively absorbed, while organic sources, such as SY or selenomethionine (SM), are actively absorbed via amino acid transport mechanisms[84].

A kinetic model of the flux of Se in human metabolism has been proposed by Patterson et al. [85, 86, 87](1989), Swanson et al. (1991), and Patterson and Zech (1992) and is shown above in Figure 2.1. In this model, there are several differences in the flux and metabolism of inorganic and organic Se. Inorganic forms of Se are absorbed from the GIT at a lower rate than organic forms, resulting in higher excretion of inorganic Se in the feces. After absorption, approximately 76% of the absorbed inorganic Se moves quickly (peak of 3 h) into a fast turnover (t½of 20 min) plasma pool, while the remainder of the absorbed inorganic Se has a delayed appearance (peak of 10 h) in a much slower turnover (t1/2of 3 h) secondary plasma pool. Regardless of the plasma pool in which the inorganic Se is present, approximately 90% of the inorganic Se in each pool moves into the liver. After the liver, some of the inorganic Se returns to the gastrointestinal tract via the bile, while the rest moves into another slow turnover (t1/2of 12 h) plasma pool. From this plasma pool, practically all of the inorganic Se moves into fast turnover tissues with over 70% of it being recycled in these tissues. The remaining inorganic Se in the tissues fluxes into a fourth plasma pool (peak of > 50 h and t 1/2of 6.6 d), where very little of it is returned to the liver and the rest is excreted via the urine.

There are several distinct differences in the flux of organic Se. First, over 95% of the organic Se is absorbed from the GIT. Once absorbed, the liver clears more than 50% of the organic Se immediately, which is similar to the fate of an amino acid. The remainder of the absorbed organic Se fluxes through two plasma pools before moving into the liver. Similar to inorganic Se, there is some recycling through the GIT via bile. After leaving the liver, organic Se enters a third plasma pool where one-half of the Se then will go to fast turnover tissues and the other half of the Se will go into slow turnover tissues, such as muscle. The organic Se from both tissue types will then enter the fourth plasma pool where almost all of it is recycled back to the liver, whereas almost all of the inorganic Se is excreted into the urine from the fourth plasma pool.

Figure 2.1. Kinetic model of selenium metabolism. Adapted from Sunde (1997)[78] based on the model of Patterson et al . (1989)[85] and Swanson et al. (1991)[86].

Inorganic Se sources are metabolized in the following manner as summarized by [78]Sunde (1997). Axley and Stadtman (1989) reported that selenate first is converted to selenite[87]. Then, selenite is nonenzymically reduced to elemental Se by glutathione forming seleno-diglutathione (GS-Se-SG); Ganther, 1966)[88]. In the absence of oxygen, seleno-diglutathione is further reduced to selenide (HSe-) by glutathione reductase [89](Hsieh and Ganther, 1975). At this point, selenide can have several different fates. It can be methylated to form methaneselenol (CH3SeH), which then can form dimethylselenide or trimethylselenonium ion (CH3)xSeH[90](Hsieh and Ganther, 1977). Selenide also can bind to the Se-binding proteins, or it can be a substrate for selenophosphate synthetase for the tRNA-mediated synthesis of selenoproteins [78](Sunde, 1997). This last step converts inorganic Se into the organic forms of Se that are found in mammalian tissues.

Organic Se is metabolized differently than inorganic Se [78](Sunde, 1997). Dietary selenomethionine can be readily incorporated into protein ([Se] Met) as selenomethionine because it is esterified to methioninyl-tRNA only slightly less efficiently as Met [91,92](Hoffman et al., 1970; McConnell and Hoffman, 1972). Selenomethionine can be metabolized to Se-adenosyl methionine (SeAM), and then to Se-adenosyl homocysteine(SeAH) [93] Markham et al., 1980). The SeAH is readily converted to selenocysteine via cystathionine β-synthase and cystathionine γ-lyase.

Selenocysteine then can be incorporated into proteins or degraded, releasing selenite, or it can be degraded by selenocysteine lyase, releasing elemental Se (Se-), which can be reduced to selenide [95](Esaki et al.,1982). Another potential fate for selenomethionine is to be transaminated to methaneselenol [96](Steele and Benevenga, 1979), and then methaneselenol can be transformed to selenide via S-methyltransferase [78](Sunde, 1997). At this point, selenide would be metabolized as discussed above.

The currently proposed metabolic pathways for Se are shown below in Figure 2.2, and the pathways indicate that because of its reduction potentials, Se tends to be reduced when metabolized. Figure2.2.The proposed metabolic pathways of selenium. Adapted from Sunde (1997)[78].