Selenium nutrition and poultry meat quality

Dietary minerals are vital for maintaining life. They are involved in a myriad of metabolic and physiological processes critical to human and animal health and general well-being.

With advances in mineral nutrition and detection capabilities, the importance of consuming adequate amounts of macro- (Ca, P, Na, Cl, Mg, K and S) and microminerals (Co, Cu, F, I, Fe, Mn, Mo, Se and Zn) has been emphasized.

Furthermore, the function and deficiency/toxicity syndromes of most minerals have been well established. This review will focus on a select group of microminerals, highlighting their role in meat quality.

The essentiality of minerals for optimizing livestock and poultry performance has been well established.

Often the initial symptoms of mineral imbalances, subclinical inadequacies, or severe deficiencies are depressed growth, body weight gain, feed utilization efficiency, and reproductive efficiency.

The prioritization of nutrient use in animals primarily involves homeostatic maintenance of body systems and health, with growth and reproduction of secondary importance. Thus, adequate dietary supplementation of minerals is critical for balancing maintenance and performance requirements for high-producing animals.

In the literature, the effect of mineral supplementation on growth and meat quality focuses primarily on inorganic sources; however, when appropriate and available, the response to organic minerals will be highlighted.


Selenium and meat quality

Trace elements can significantly impact animal performance and immunity. Furthermore, dietary trace elements, particularly selenium, may induce physiological changes in muscle tissue, which can alter meat quality in livestock and poultry. The conversion of muscle to meat involves dynamic physiological and biochemical processes with a concomitant change in muscle tissue properties.

Meat quality can affect palatability, nutrient contributions, and consumer acceptance. Meat quality attributes include color, flavor, aroma, texture (tenderness), juiciness (water-protein interactive states), microbial growth and dietary nutrient contributions. A variety of inherent and exogenous factors during meat production, including genetics, age, sex, diet, live animal management and pre-/ post-slaughter handling, can alter product quality.

In this discussion, the interaction between diet and meat quality will be explored. Beyond that, the role of selenium in muscle cell antioxidant protection and changes in cell water dynamics will be discussed.

The primary function of selenium in animal systems is as a component of glutathione peroxidase (GSH-Px), a plasma and cytosolic enzyme responsible for destroying potentially damaging hydrogen peroxide and organic hydroperoxides in animal tissues (Burk, 1989; Omaye and Tappel, 1974; Rotruck et al., 1973). Beyond its role as a component of GSH-Px, selenium is also involved in several enzyme systems regulating energy metabolism, spermatozoa function, prostaglandin synthesis and essential fatty acid metabolism, purine and pyrimidine base synthesis, and animal immunity (McDowell, 1992).

A discussion of selenium would be incomplete without mentioning the interrelationship between selenium and vitamin E. Vitamin E, a fat-soluble vitamin found in cellular and organelle membranes, is the first line of defense against cellular damage resulting from membrane phospholipid peroxidation.

Selenium, as a component of GSH-Px, acts as a second line of defense against cellular peroxide damage due to inability of vitamin E to destroy all metabolic peroxides. For the most part, vitamin E and selenium are mutually replaceable (except at low levels) and each acts as a sparing mechanism for the other (McDowell, 1992; Combs, 1981).

Highest selenium concentrations are found in kidney, liver, spleen, and pancreatic tissues. Several factors influence tissue storage and retention of selenium, including animal selenium status and supplemental selenium source. Cantor et al. (1982) and Osman and Latshaw (1976) showed that selenium concentrations in the gizzard, breast muscle, and pancreas were increased when turkey poults and chicks were fed selenomethionine instead of sodium selenite. Research at Auburn University showed increased Pectoralis major (breast fillet) selenium levels in response to an organic selenium source (Sel-Plex™, Alltech Inc.) compared to an inorganic source or no supplemental selenium (Downs et al., 2000) (Figure 1). Furthermore, selenium is absorbed and retained more efficiently in Se-deficient animals (McDowell, 1992).


Selenium and muscle tissue water dynamics

Water in muscle tissue is found in three associated states: constitutional, interfacial, and extracellular (Honikel and Hamm, 1994). Constitutional water (0.1% of total tissue water) is bound within muscle protein molecules (intramyofibrillar), interfacial water (5 to 10% of total tissue water) is bound at muscle protein surfaces (intermyofibrillar), and extracellular water (90 to 95% of total tissue water) is in extracellular spaces. The protein-water binding strengths form the basis of muscle tissue water loss.

Among the three water states in muscle tissue, protein-water binding energies are greatest for constitutional and least for extracellular water. In other words, extracellular water is lost (or expressed) much more easily than interfacial and constitutional bound water.

Excessive muscle tissue water loss is unappealing to meat processors (an economic loss) and consumers (product appearance and consistency).

A variety of quantitative measures, including waterholding capacity, expressible moisture, drip loss, cooking loss, capillary volumetry, and imbibition methods, have been used to assess differences in muscle tissue water properties. According to Honikel and Hamm (1994), three major factors contribute to the dynamics of muscle tissue water loss, including (1) water state in meat, (2) compartmentalization within cellular and subcellular structures, and (3) postmortem tissue changes.

Dietary factors also contribute to maintenance of meat quality and prevention of excessive tissue water loss. As indicated by meat’s perishable nature, postmortem oxidative stress on muscle tissue is high and contributes to meat quality losses. Likewise, oxidant stress within a living system is high, yet specific intracellular antioxidant processes prevent excessive tissue damage (Combs, 1981; Combs et al., 1975).

Vitamin E, as an intramembrane antioxidant, has been widely evaluated for its potential to reduce membrane lipid peroxidation and increase meat stability (shelf life) (Higgins et al., 1998; Baker, 1997; Wu and Squires, 1997; Combs and Regenstein, 1980). Excessive membrane oxidation leads to hydroxyl radical and hydroperoxide formation, which damages muscle cells resulting in excess tissue water loss (‘leaky membranes’) and decreased meat shelf life (e.g., oxidative rancidity).

Figure 1. Effect of selenium source on selenium level in broiler breast fillet tissue (From Downs et al., 2000). Control feed had no added selenium, while each of the other treatments had 0.30 ppm added selenium.

Selenium, as a component of intracellular GSHPx, acts with vitamin E for reduction of cellular oxidative stress. Selenium, then, may also contribute to maintaining meat quality. Several reports have suggested a link between antioxidant (i.e., vitamin E and selenium) intake and increased or maintained meat quality parameters for pork, poultry, and fish (Nielsen and Rasmussen, 1979; Mahan et al., 1999; Higgins et al., 1998; Combs and Regenstein, 1980; de Lyons, 1998; Edens, 1997; Mahan, 1996; Bartov, 1977). Vitamin E supplementation (as α-tocopheryl acetate at 600 ppm) of turkey diets increased the oxidative stability [measured by the thiobarbituric acid (TBA) reaction method] of processed, packaged raw turkey patties (Higgins et al., 1998).

Likewise, Combs and Regenstein (1980) showed dietary supplementation of vitamin E (or ethoxyquin) reduced peroxidative damage of dark chicken meat.

This study further demonstrated reduced oxidative damage of dark meat through dietary selenium supplementation. Interestingly, the impact of antioxidant supplementation was much less pronounced in white chicken meat due to lower fat content and less lipid peroxidation. When growingfinishing swine were fed sodium selenite to supply between 0.03 and 0.06 ppm Se, meat quality was unaffected, suggesting selenium supplementation above that required for optimum protection from tissue oxidative stress may be unnecessary (Nielsen and Rasmussen, 1979).

Beyond the role of selenium in protection from cellular damage and meat quality maintenance, dietary selenium source seems to affect meat quality, with particular reference to excessive muscle tissue water loss. Mahan et al. (1999) and Edens (1997) reported a reduction in pork and poultry meat quality (increased drip loss), respectively, when supplemental dietary sodium selenite was replaced with organic selenium in the form of Se-enriched yeast (Sel-PlexTM). When added to supply between 0.05 and 0.30 ppm Se, this form of selenium reduced drip loss of pork loin 13.7% when compared with sodium selenite.

Similarly, drip loss of chicken breast fillets (Pectoralis major) was reduced approximately 17% when organic selenium replaced sodium selenite in broiler diets to supply between 0.1 and 0.3 ppm Se. Downs et al. (2000) showed low drip loss in breast muscle fillets of broilers fed Sel-PlexTM Se-enriched yeast or no added selenium supplementation as compared to supplementation with an inorganic source (Figure 2). A more readily available source of selenium (e.g., Se-enriched yeast) may maximize the activity of cellular Sedependent GSH-Px, thereby reducing peroxide or free radical associated muscle cell rupture, which decreases membrane integrity and increases cell fluid losses (i.e., drip loss).

Figure 2. Broiler breast fillet drip loss in response to selenium level and form (from Downs et al., 2000).

Similar meat quality responses to dietary selenium source were noted in Atlantic salmon (de Lyons, 1998). Flesh quality, including color, pigment content, and texture, was improved over a control (containing selenium coming from fish meal) when Sel-Plex™ Se-enriched yeast was fed to supply 0.25 ppm Se. Likewise, a 6.9% reduction in flesh oil content was observed with the organic selenium treatment.

Tissue GSH-Px activity increased 101% when fish were fed Se-enriched yeast, which may explain much of the impact of organic selenium on fish flesh quality. Further, Baker (1997) observed a reduction in fillet post-thaw drip loss when African catfish were supplemented with α-tocopheryl acetate (100 mg/kg).

An interesting physiological phenomenon observed in swine and poultry is the condition of pale, soft, and exudative (PSE) meat. This condition, caused by a variety of factors including genetics (e.g., stress susceptibility), nutrition, management, and preslaughter animal handling, produces functional meat properties similar (yet to a greater extent) to those observed with oxidative stress associated drip loss (Lee and Choi, 1999; Ferket and Foegeding, 1994; Mihailovic et al., 1984).

Beyond the lack of consumer acceptance of PSE meat, it is also associated with increased drip loss, lowered processing yields, increased cooking losses, and reduced juiciness (Lee and Choi, 1999). The cellular mechanism of PSE meat parallels oxidative tissue damage associated with normal drip loss.

However, various factors produce excessive peroxide or free radical tissue damage, which reduces muscle cell integrity and functional characteristics with a concomitant increase in water loss. Dietary factors may influence PSE occurrence (Ferket and Foegeding, 1994; Mihailovic et al., 1984). For instance, vitamin E and Se deficiencies exacerbate the incidence of PSE meat. Feeding high levels of vitamin E (200 IU/kg) and adequate Se (along with Zn, Mn, copper and riboflavin) reduced PSE turkey meat. The importance of nutrition in modulating the PSE condition lies in optimizing vitamin E-membrane antioxidant activity and enzyme systems (i.e., Se-dependent GSH-Px, superoxide dismutase and glutathione reductase) designed to reduce peroxide, superoxide, or hydroxide radical cellular damage.


Other selenium effects

Beyond the role of selenium in the quality of meat produced, limited research has focused on the association of selenium and skin health. In a study evaluating skin collagen content of Japanese quail, increased dietary levels of selenium, up to 2.0 ppm as sodium selenite, increased collagen content to 40% and GSH-Px activity in the skin (Babu et al., 1986). Also, with low dietary selenium levels, skin collagen cross-linking was reduced with a predominance of the monomeric form of collagen.

When male mallard ducks were fed toxic levels of selenium as L-selenomethionine, the incidence of alopecia (areas of missing feathers) and nail epithelial necrosis was increased (O’Toole and Raisbeck, 1997). Even with limited data, the interaction between skin health and selenium seems evident. This is of importance with processed poultry as some portion of poultry products are marketed with the skin intact. Therefore, skin quality and appearance may affect product grade and/or sensory characteristics.


Other minerals

Beyond their metabolic roles as cofactors in enzyme systems and importance in cellular function, insufficient interest has been shown regarding the impact of other minerals on meat quality.

Microminerals are involved in other physiological processes besides redox reactions that can ultimately affect meat quality characteristics. However, drip loss of pork Longissimus thoracis and Biceps femoris muscles was reduced by supplementing control diets with magnesium aspartate (1300 to 2300 mg magnesium/kg diet) (D’Souza et al., 1998) and magnesium Bioplex™ (D’Souza et al., 2000).

This response is due, however, to the tranquilizing (stress reduction) effect of magnesium and not antioxidation. Another interesting study shows improved carcass quality (quality grade) of beef steers when dietary zinc was supplied as organic zinc methionine versus Zn oxide (Greene et al., 1988). Again, however, this response is due to alteration in carcass fat deposition and lipid metabolism, not a change in the functional property of the muscle tissue.

References

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