The Effect Of Diet On Measures Of Pork Quality - Considerations For Meat Quality Through Good Nutrition

Maintaining and increasing the demand for Manitoba pork, in both domestic and international markets, will depend heavily on such factors as the assurances of food safety, environmental sustainability, animal welfare, and the final quality of the product. Programs, such as CQA™ , have been designed and, when implemented, will go a long way to assure consumers of the safety of our pork products. However, while food safety is of critical importance and provides a strong measure of the quality of pork, this paper seeks to specifically address the impact that feeding practices have on pork quality. To do this, we must first consider the question: How do we measure quality?


DEFINITION OF PORK QUALITY

The definition of pork quality is truly in the eyes of the beholder. Defining the quality of pork must be considered from the perspective of producers, packers, retailers, and consumers. While at times, the interests of one particular group may conflict with those of another, for the most part, the factors used to define the quality of pork are complimentary amongst the major interests groups, and can be broadly classified into three areas:

1) Indexing Parameters
2) Sensory Parameters
3) Nutritional Parameters

Indexing parameters are those factors related to the lean yield of carcasses, carcass fat content, and the cross-sectional surface area of the loin muscle. Sensory parameters encompass those factors that affect the aroma, flavour, appearance and taste of pig meat. Finally, those dietary factors that affect the nutrient density and profile of pork are referred to as nutritional parameters. Diet components can have influence on more than one of the above mentioned parameters, at times in an antagonistic fashion. The goal in feeding pigs for meat quality is finding a balance between the optimization of producer returns, the needs of the packing and retailing industry, and the demands of consumers. This paper will address the effects that feeding programs and specific diet ingredients have on the perceived quality of pork.


FEEDING FOR INDEX

Hog producer returns are calculated as a function of the market price for hogs and the index value of the hog carcass. Index values are derived from equations used to estimate lean yield and the weight of the carcass, with bonuses paid for meeting certain weight and loin eye area standards. While market prices do not fall under the control of individual producers, the control that the producer can exert on carcass index values is substantial. In order to approach maximal rates of lean tissue deposition, pigs must consume optimal amounts of a diet formulated to meet all of their known requirements. The committees responsible for the most recent "Nutrient Requirements of Swine, 10th edition" (NRC, 1998), chose to utilize modeling techniques for the expression of the pig’s requirements for protein/amino acids, energy, vitamins and minerals. This approach is founded on the basis that, for any stage of development of the pig, the optimal amount of nutrients required can be predicted using a mathematical approach. Judicious use of these recommendations can help producers to maximize carcass index values. In summary, considerations for the feeding of pigs to maximize carcass index include:

1) feeding the optimal levels of nutrients - pigs eat to satisfy their energy requirements. Pig feeds should reflect this fact such that diets are formulated to contain the desired nutrient densities relative to the pig’s stage of growth (ie: protein:energy ratio). NRC recommends the formulation of pig diets to account for the true ileal digestibilities (measure of bioavailability) of the amino acids contained in the protein sources, in order to meet the biological demand for the maintenance and growth of tissues, including skeletal muscle (lean meat). Crystalline amino acid sources can be used to correct for deficiencies of individual amino acids in cereal grains, thereby reducing the need to increase the protein/nitrogen content of the diet. This will allow for optimal rates of lean growth, while helping to reduce overall nitrogen excretion. Other factors that should be considered include the feeding of pigs to their genetic potential (different patterns and rates of lean tissue deposition), split-sex feeding (account for differences between barrows and gilts), and phase-feeding (feeding several diets during the grower-finisher phase to better meet nutrient requirements). These issues have been addressed in detail in the NRC (1998) publication and in papers published in previous volumes of the Manitoba Swine Seminar series (Shurson, 1995; Baidoo, 1996). The reader is referred to these publications for more details.

2) the use of feed additives with carcass modifying properties - several feed additives have been promoted for their reputed beneficial carcass modifying properties. Antimicrobial or antibiotic agents have been included in swine feeds for several years, at subtherapeutic levels, and have been shown to improve feed efficiency and growth performance. The exact mode of action is not clear, but may be due to an improvement in nutrient utilization caused by alterations in the microbial population. Probiotics, or compounds that promote the growth of specific classes of beneficial microorganisms within the gastrointestinal tract, were also said to improve growth and feed efficiency, presumably via a similar mechanism, but have failed to yield consistent improvements in productivity, especially when supplements of microorganisms were provided. The supplementation of pig diets with enzyme cocktails may improve feed efficiency and pig performance by initiating the breakdown of complex carbohydrates, proteins and phytate, thereby enhancing the bioavailability of nutrients. Enzyme cocktails may prove to be extremely valuable in the quest to reduce the nitrogen and phosphorus content of manure.

Beta-agonists (ie: ractopamine, clenbuterol, etc...) are well characterized nutrient repartitioning agents that lead to enhance lean muscle deposition and decreased fat deposition. They can be extremely effective, however the need to allow for a withdrawal period, in order to reduce the risk of residues of these agents remaining in the pork, results in compensatory fat gains, that virtually remove any benefits in terms of carcass modification. These agents are not licensed for use in livestock feeds, however there is evidence of abuse of these agents. Other agents being investigated for their potential to enhance lean muscle deposition in pigs include chromium picolinate, conjugated linoleic acid, carnitine and betaine, however these agents have failed to yield consistent results (NRC, 1998; Shurson, 1995).

Careful attention to pig diets will continue to play a crucial role in terms of obtaining optimal carcass quality, as measured by index values. The decision to use feed additives as carcass modifying agents must be made with consideration towards recent research results, and the production goals of the unit.


FEEDING FOR THE SENSES

Consumer acceptance of pork products (both domestic and international markets) is heavily influence by the appearance of the product. A survey of 1115 shoppers from a total of 15 supermarkets in Calgary and Edmonton produced the preference data reported by Jeremiah (1994). In general, consumers were at least 2.5 times more likely to decline purchasing fresh pork loin chops if they exhibited PSE (pale, soft and exudative) characteristic (colour score 2) versus those that didn’t. In fact, consumers tended to prefer pork chops that were classified as moderately DFD (dark, firm and dry; colour score 4). However, Jeremiah (1984) found a much weaker relationship between the acceptance of the cooked product and whether or not the pork was PSE, normal or DFD, but normal to DFD pork did tend to rate as slightly more satisfactory. The sensory eating qualities of pork may be irrelevant, however, if consumers don’t buy the product. Reducing the occurrence of PSE pork has and should continue to receive a strong commitment from all parties involved in the production cycle. Major advances, such as the ability to specifically identify and cull those animals carrying the halothane-sensitive (malignant hyperthermia) gene, a genetic contributor to Porcine Stress Syndrome (PSS), as well as an improved understanding of ways to minimize stress to pigs, during transport and holding prior to slaughter, have the potential to significantly reduce the occurrence of PSE pork. A complementary approach to the above measures may involve the use of dietary means to reduce pre-slaughter stress to pigs. Increasing the dietary concentration of the amino acid tryptophan has been shown to lead to increases in the level of serotonin in the brain. Serotonin is a neurotransmitter that can elicit a sedative like effect on animals, potentially leading to a reduction in stress. Pigs consuming diets containing 5 g tryptophan/kg for 5 days prior to slaughter exhibited significantly higher brain serotonin concentrations and tended to produce pork that had a lower incidence of PSE (Adeola and Ball, 1992). Henry et al. (1996) found that supplemental dietary tryptophan (0.16 vs. 0.12 %) resulted in higher muscle pH values both at 45 min and at 24 hr. Since the development of PSE pork is characterized by a fast and sustained drop in post-slaughter muscle pH, leading to a reduction in water holding capacity and increased drip loss, the latter data provide evidence for a protective effect of tryptophan against the development of PSE pork. An alternate approach to reducing the occurrence of PSE pork through dietary manipulation is that of D’Souza et al (1998). This Australian group fed Large white X Landrace boars (live weight approx. 77 kg) either a standard finisher ration or the same ration supplemented with magnesium aspartate to provide an additional 1 g of elemental magnesium per kg diet. Pigs were fed the diets (95% ad libitum) for 5 days prior to shipment to the abattoir. The authors reported the muscles isolated from the pigs receiving supplemental magnesium aspartate had higher 45 minute and 24 h pH values, lower drip loss values, a darker colour, with no muscles exhibiting PSE characteristics, as opposed to pigs fed control diets. When the pigs were exposed to greater levels of handling stress, the protective effect of dietary magnesium aspartate was even more dramatic. Magnesium supplementation is thought to reduce pre-slaughter stress through a reduction in the release of stress hormones, including cortisol and the catecholamines, however a definitive mechanism has not been established. Further work in this area is needed to evaluate the potential benefits of increased dietary magnesium levels in pig diets on reducing the incidence of PSE pork, including an examination of the effects of diet composition, sex of the animal, pig genetics and stress susceptibility, pre-slaughter handling protocols, and the optimal level and duration of magnesium supplementation.

It is clear that the colour of fresh pork products will have a strong effect on its marketability, with consumers shunning products that appear pale and greyish. Dietary and handling techniques designed to reduce pig stress will undoubtedly reduce the occurrence of PSE pork. However, is there an opportunity to use dietary approaches to maintain a darker, pinkish-red colour for fresh pork products during retail storage? Numerous studies have documented that substantial increases in the muscle/meat content of the anti-oxidant vitamin E can be readily achieved by increasing its content in pig diets (Jensen et al., 1998). In those studies that examined the effect of enhanced muscle vitamin E status, some, but not all, of the studies documented greater colour stability of fresh pork products. The putative mechanism whereby increased dietary and, in turn, muscle vitamin E may maintain colour stability is by slowing the oxidative process of myoglobin to the brownish metmyoglobin. Lack of general agreement between studies may be a function of the total anti-oxidant status of the diet, nature of the lipid fed (saturated vs. unsaturated), and the level of vitamin E used (generally 100-200 mg a-tocopherol acetate/kg diet). Meats with higher concentrations of myoglobin (ie: beef) tend to exhibit greater colour stability when the animals are fed supra-nutritional levels of vitamin E.

Supra-nutritional levels of vitamin E in the diet of pigs have also been evaluated for their potential to prevent the oxidation of unsaturated fatty acids. The current desire to enhance the omega-3 fatty acid, particularly a-linolenic acid, content of pork by increasing the content of these fatty acids in the pig feed, can lead to a problem with rancidity, as the unsaturated fatty acids become oxidized. Sensory analysis data have provided evidence of a higher detection rate of off-flavours in processed pork products derived from pigs fed diets containing canola (low-erucic acid rapeseed) oil, due to the presence of oxidation products of linolenic acid (Wood and Enser, 1997). Feeding higher levels of vitamin E in conjunction with the sources of unsaturated fatty acids can enchance the oxidative stability of the fatty acids (Wood and Enser, 1997), and therefore permit the modification of the lipid profile of pork products to one that better suits the demands of the consumers. The consumer desire for increased omega-3 fatty acids in pork products will be addressed in the ensuing sub-section. However, it should be remembered that, since pigs tend to deposit fat in the form that it was consumed, feeding a higher proportion of unsaturates to pigs may produce an end-product that consumers prefer, but it will in turn produce carcasses with soft-fat, a condition deemed unfavourable by packers. This is an example where the interests of one particular group conflict with those of another.

Other means of manipulating the sensory attributes of pork products through dietary means include:

1) Alterations in intramuscular fat content: As discussed in the preceding section, reducing the carcass fat content of market hogs can be realized when pigs are fed to their genetic potential, by feeding diets containing the optimal protein:energy ratio (NRC, 1998). Advances in our understanding of nutrition, as well as the major strides made by breeding companies to produce high-lean yield pigs have led to major reductions in the fat content of pig carcasses. However, have we reached the limit of leanness that will still allow us to market a palatable commodity? Pressure to reduce backfat levels has also resulted in the reduction of intramuscular fat depots. Higher levels of intramuscular fat in pork are associated with better scores from consumers and trained sensory analysis panelists for tenderness, juiciness, and overall palatability (Castell et al., 1994). Feeding programs designed to maximize lean growth should also make considerations for the sensory qualities of the final product.

2) Manipulation of skatole (boar taint) levels: The presence of off-odours in the meat from intact male pigs has limited their use as meat animals in North America, despite the documented advantages in terms of production efficiency. The off-odours predominantly arise from two compounds: androstenone, a male sex hormone derivative, and skatole, an indole derived from the microbial breakdown of the amino acid tryptophan in the hind gut. While skatole is found in higher concentrations in the tissues of boars, it can also be found in lean meat arising from barrows and gilts. Skatole concentrations can be influenced by the dietary regimen, as well the housing conditions and the season. Animals that are heavily soiled with feces, especially during warmer months, tend to have higher blood and backfat levels of skatole. Since this compound is produced by microbial activity in the hindgut, there is the possibility that dietary additives could be used to reduce its production. Studies in Denmark have demonstrated that the use of antibiotic feed additives can reduce skatole levels (ie: Hansen et al., 1997). Alternatively, the use of probiotics, (ie: those products that promote the growth of non-skatole producing organisms) may be beneficial in reducing the levels of skatole in pig tissues, but this remains to be rigorously examined.


FEEDING FOR NUTRITIONAL VALUE

Consumer concern regarding their intakes of dietary cholesterol and saturated fats has led to substantial reductions in the Canadian per capita disappearance of red meats, especially beef, since the 1980's. In 1997, Canadians consumed, on average, 31 kg of beef, down from the historic high of 50 kg in 1976. Per capita pork consumption was 25 kg in 1997, virtually identical to that in 1976, but down from the historic high of 32 kg in 1980 (CANSIM; Statistics Canada, 1998). One reason why pork may have faired better than beef includes the fact that today’s pork is much leaner, due primarily to a shorter selection interval for lean traits, and improvements in feeding and management strategies. We have addressed the issue of pork leanness in preceding sections, in relation to the index value of hog carcasses and the sensory attributes that the fat component of pork impart. It is, however, important to re-emphasize that a primary driving force in reducing the fat content of today’s pigs is the consumers demand for a high quality, nutritional product. While overall fat content has played an important role in the definition of the nutritional quality of pork, other factors are being or should be emphasized.

From a nutritional standpoint, pork is an excellent source of high quality protein and available iron. Pork is a good source of many of the B vitamins, and is one of the richest dietary sources of thiamin. Today’s consumers are becoming increasingly aware of the importance of achieving optimal intakes of nutrients, in order to maintain good health and to help combat the onset of several diseases, most notably cardiovascular disease and cancer. The recent identification of a new risk factor for cardiovascular disease, homocysteine, has led to this compound receiving considerable media exposure and consumer interest. Increased levels of homocysteine in the serum are associated with a greater risk for the development of cardiovascular diseases and peripheral vascular diseases (Refsum et al., 1998). This compound, which is produced normally in the body, can become elevated for a number of reasons, including an inadequate intake of the B vitamins folic acid, B12 (cobalamin), and B6 (pyridoxine), which act as co-factors in the removal of homocysteine. Animal products, including pork, provide the main dietary sources of vitamin B12, since plant-based products do not normally contain this compound. Therefore, promoting the nutritional quality of pork, relative to its content of B vitamins, could aid in bolstering domestic per capita consumption, especially if steps are taken to ensure the maintenance and/or improvement of the vitamin profile. There has been some discussion/consideration in the industry on removing vitamins and minerals from pig diets during the finishing phase. While this would result in some savings to producers, through reduced feed costs (a pressing issue during the current hog price crisis: fall 98/winter 99), it would undoubtedly diminish the nutritional quality and nutrient density of pork. Initial investigations at the Prairie Swine Centre have shown that the removal of the vitamin and mineral premix from finisher rations for the final 35 days prior to marketing had no effect on performance or index values, but did lead to reduced muscle thiamin contents (Prairie Swine Center, Research Briefs, 1998). Any perception by consumers that our product has been nutritionally "downgraded" could negatively impact efforts to increase domestic consumption of pork products. In fact, it may serve the long term interest of this industry to investigate means to efficiently augment the vitamin content of pork products. A recent study by Leonhardt et al. (1996) demonstrated that the inclusion of sodium ascorbate (vitamin C) in pig diets resulted in a greater retention of riboflavin and, to a lesser extent, thiamin in pig muscle following cooking, due presumably to the antioxidant role of vitamin C. While the absolute changes may appear small, they do point to the potential for improving the nutritional quality of pork via dietary means.

Additional research efforts to improve the nutritional quality of pork products have included:

1) Increasing omega-3 fatty acid content: Increased awareness by consumers of the link between an increased intake of omega-3 fatty acids, including alpha-linoleic acid, and a reduction in the risk for cardiovascular disease, has led to an increased demand for products with a higher content of these unsaturated fatty acids. As discussed above, it is possible for nonruminants, including pigs, to deposit unsaturated fats in the form that they were presented in the diet. Therefore, research has focused on the increased incorporation of flax and canola in pig diets, as these oilseeds contain a higher percentage of alpha -linolenic acid. Leskanich et al (1997) fed diets containing 2% rapeseed oil and 1% fish oil to pigs during the finishing phase, and significantly increased the omega-3 content of pig fat, without significant effects on the sensory attributes of the meat. These diets contained a minimum of 100 mg/kg diet of -tocopherol, as an anti-oxidant. The ideal incorporation ratio of omega-3 fatty acids into pig diets will require future studies designed to determine the optimal ratio of unsaturated fat source to vitamin E that will produce pork products with the desired omega-3 content, without undue increases in carcass fat softness or oxidative rancidity.

2) Vitamin E: As discussed above, an increased content of vitamin E (alpha-tocopherol) may reduce the oxidation of unsaturated fatty acids, such as alpha-linolenic acid, and may help to maintain the colour stability of fresh pork products during retail display. Vitamin E itself has been investigated for its potential to reduce the risk for cardiovascular disease, through its action as an antioxidant in combating peroxide radicals in response to oxidative stress, potentially minimizing the oxidation of LDL cholesterol (a key step in the generation of atherosclerotic plaques). Many studies have documented the ability of pig tissue stores of vitamin E to be increased by increasing its dietary concentration (see Jensen et al, 1998 for review). Because of its role as an antioxidant, further increases in the content of vitamin E ( alpha-tocopherol) in pig diets, especially during the finishing phase, may be warranted.


SUMMARY

Pig diets can significantly influence the quality of pork, as defined by those factors affecting the index values of hog carcasses, the sensory attributes of pork products, and the nutritional value or nutrient density of pork. By paying careful attention to the feed composition and feeding programs, as well as addressing new research advances and the desires of consumers, the quality of pork products can be maintained, and even enhanced. This would undoubtedly lead to an increased demand for these products in domestic and international markets.


REFERENCES

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