Summary

There are advantages to feeding some added fat in a diet, namely increased caloric content of diet (fat is roughly 2.5 times as much energy on a weight basis compared to other ingredients used), reduces feed intake (often times the animal eats to satisfy energy needs), increases weight gain, improves feed efficiency, improves feed quality, improves reproductive efficiency and reduces heat increment.

Depending on fat type used in a swine diet, the impact on carcass fat quality can be very different. Fat quality associated with meat products can be partitioned into three categories (physical, sensory, and chemical). Physical assessment of fat quality is evaluated through fat firmness, cohesiveness and color, while sensory assessment measures palatability. Because of the impact composition has on fat quality, determination of fatty acid composition of fat tissue is crucial. Varying melting points of different fatty acids affects fat firmness.

Therefore, fatty acid composition has an important effect on fat quality. In addition, unsaturated fatty acids are more prone to lipid oxidation, because the double bonds in their chemical structure are subject to free radical attack. The ability of unsaturated fatty acids to rapidly oxidize affects shelf life of meat.

Introduction

One of the strongest determinants of carcass fat quality in pigs is the level and composition of triacylglycerols in the diet. Because the utilization efficiency of dietary fat is 90% in pigs fed above maintenance (Freeman, 1983) and the transfer coefficient of dietary fat to carcass lipid is high, 31-40% depending on the specific fatty acid (Kloareg et al., 2007), the carcass lipid composition is a reflection of dietary fat.

Dietary lipids may have different effects or carcass lipid depending on the timing of feeding during growth and finishing phases. Stress, management, and environmental conditions play important roles in fat metabolism and thus carcass quality. These factors result in hormonal and physiological responses which stimulate changes in growth performance and lipid metabolism. Understanding and managing the factors that control carcass fat quality is a challenge for swine producers and provides opportunities to improve final carcass quality and profitability of pork production.

Dietary Fat Sources and Carcass Lipid Quality

Dietary triacylglycerol composition plays a major role in determining adipose tissue composition.

Monogastric animals directly incorporate dietary fatty acids into tissue lipid deposits (Azain, 2001; Wiseman, 2006) and, therefore, to manipulate carcass lipid quality, it is important to understand the effects of dietary triacylglycerol sources and characteristics.

Because unsaturated dietary fatty acids are minimally hydrogenated before deposition into swine adipose stores, carcass fatty acid profile closely mimics dietary fatty acid profile (Allen et al., 1976; Azain, 2001).

Dietary Fat

Dietary triacylglycerols primarily alter carcass lipid composition by changing level of saturation in the carcass fatty acid profile (Azain, 2001). Saturated fatty acids are fatty acids with no double bonds andare generally solids at room temperature. Mono-, di- and poly-unsaturated fatty acids have one or more double bonds. As the number of double bonds increases, so does the liquidity and level of unsaturation.

The ratio of saturated to unsaturated fatty acids is a way of describing the relative composition of a fatty acid profile (Azain, 2001).

Iodine value is a measure of double bonds and is another way of standardizing the characteristics of lipids into a composite number (Madsen et al., 1992; Azain, 2001). The level of saturation and iodine value of the feed lipid source will thus be reflected in the carcass fatty acid profile. Vegetable oils are typically high in linoleic acid, have an unsaturated to saturated fatty acid ratio of 12:1 (Wiseman, 2006) and an iodine value greater than 100 (Azain, 2001). Diets high in these unsaturated vegetable oils will result in oily, soft carcass fat (Azain, 2001). Conversely, tallow, which is high in palmitate and stearate, has a saturated to unsaturated fatty acid ratio of 1:1 (Wiseman, 2006), an iodine value of 40 or 45 (Azain, 2001) and will result in firmer carcass fat when fed in the diet.

Bacon Quality

The belly is one of the most valued primal cuts of the carcass, thus, the quality of bacon produced from the belly is linked to overall carcass value. The industry has shifted to genetically lean lines with decreased backfat and thus, the bellies of these pigs have also become thinner, leaner, and softer (Morgan et al., 1994; Gatlin et al., 2003). Thinner bellies are typically softer, produce fewer grade one slices, and have increased problems with processing, tissue separation, storage stability (Morgan et al., 1994; Gatlin et al., 2003). Providing dietary fat from a more saturated source has been shown to increase belly thickness and improve belly firmness (Gatlin et al., 2003). Also, feeding conjugated linoleic acid (CLA) has been shown to improve belly firmness in finish pigs (Gatlin et al., 2003; Weber et al., 2006).

Bacon is scored according to lean content and slice thickness to identify premium quality slices (Person et al., 2005). Premium slices have greater than 50% lean content and are wider than 1.9 cm at all points (Person et al., 2005). Accordingly, bacon slices are graded as either number one slices, number two slices or as ends and pieces (Person et al., 2005). Pork bellies that are classified below standard based on these characteristics represent a decrease in carcass value.

Carcass Lipid Quality

Acceptable quality standards for pork carcasses vary between processors and researchers according to IV, ratio of saturated to unsaturated fatty acids, belly firmness, and percent lean in bacon. High levels of unsaturated fatty acids result in rapid oxidation, which decreases shelf life (Wood, 2003). Furthermore, high levels of unsaturated fatty acids in the diets also produce bacon which is smeary, separates and causes processing difficulties (Pearson et al., 2005).

Increased IV (Madsen et al., 1992) and decreased saturated to unsaturated fatty acid ratios (Azain, 2001) indicate decrease in carcass quality due to decreased fat firmness. Many processors utilize IV as numerical evaluation of carcass quality and thus have goal IV values. An IV > 65, for some processors may be unacceptably high (Eggert et al., 2001), while an IV > 75 may be the threshold for other processors.

Conjugated Linoleic Acid

Conjugated linoleic acids (CLA) are a group of polyunsaturated fatty acids that are positional and geometric isomers of linoleic acid (C18:2). Because CLA is naturally produced during bacterial fermentation in the rumen of ruminant animals, the main sources of CLA in human nutrition are dairy products and ruminant meats (Wang and Jones, 2004; House et al., 2005). There are numerous isomers of CLA though the main isomers are cis-9, trans-11(c9t11)and trans-10, cis-12 (t10c12), shown in Figure 1.2.

Though the main isomer produced by ruminants is c9t11, commercially available products commonly contain equal proportions of c9t11 and t10c12 (Wang and Jones, 2004; House et al., 2005). Research in rodents, pigs, and humans has been conducted on the effects of CLA and has shown beneficial effects of CLA against obesity, cancer, atherosclerosis, and diabetes, some of which are isomer specific (Belury, 2002; Wang and Jones, 2004; House et al., 2005).

Many studies have shown CLA is able to reduce adipose tissue depots in rodents, pigs, and humans and that this effect is specific to the t10c12 isomer or a mixture containing greater than 50% t10c12 (Belury, 2002; Wang and Jones, 2004). Postweanling mice fed 1% CLA for 28-30 d and had a 50% reduction in total adipose tissue compared to control mice (Park et al., 2001). In pigs, CLA inclusion in feed has resulted in decreased backfat thickeness in grow-finish pigs (Tischendorf et al., 2002; Wiegand et al., 2002). Recent work in overweight or obese people given CLA for 12 weeks had reduced body fat mass but their body mass index remained unchanged (Blankson et al., 2000).

Another noted effect of CLA is the inhibition of cancer, specifically, mammary, prostate, skin, colon, and forestomach cancers (Belury, 2002). The anticarcinogenic effects of CLA have been mainly attributed to the c9t11 isomer (Wang and Jones, 2004). In studies transplanting mammary and prostate cancer cell lines into mice, feeding 1% CLA significantly reduced growth of the cancerous cells; however, some studies exaiming the same types of cancer have shown no effect with CLA feeding (Belury, 2002).

Another area of CLA research has shown that it is able to reduce atherosclerotic plaque formation (Belury, 2002). Inclusion of 0.5g/day in hypercholesterolemic diets fed to rabbits for 12 weeks resulted in significantly reduced serum triacylglycerols, low density lipogprotein (LDL) cholesterol levels and atherosclerotic plaque formation in the aortas (Lee et al., 1994). The reduction of plaque deposits by CLA was proposed to be due to changes in LDL oxidative susceptibility (Belury, 2002).

Effects of CLA on the onset of diabetes and insulin resistance are contradictory and complex.

Rats fed CLA have shown significantly reduced fasting glucose, insulinemica, triglyceridemia, free fatty acids, and leptinemia (Belury, 2002).

Butter enriched with c9t11 CLA failed to reduce glucose tolerance, lower adipose tissue or enhance glucose uptake leading to the conclusion that perhaps it is the t10c12 isomer which is responsible for the antidiabetogenic responses (Belury, 2002). Insulin tolerance testing on CLA-fed mice showed marked insulin resistance without changes to blood glucose concentrations after oral glucose tolerance testing (Tsuboyama-Kasaoka et al., 2000). Other studies have examined the reduction of plasma leptin by CLA and the concomitant changes in blood glucose level due to regulation by leptin (Wang and Jones, 2004). Feeding male mice high-fat diets with 1% CLA has resulted in reduced plasma leptin levels in one study (DeLany et al., 1999) while resulting in no change in plasma leptin or glucose levels in another (West et al., 2000).

Feeding CLA in Swine Production

The effects of including CLA in livestock diets have been examined in numerous studies to elucidate their effect on fat quality (Cox et al., 2004). Gilts fed 1% CLA for seven wk had firmer bellies, higher levels of saturated fatty acids, lower levels of unsaturated fatty acids and decreased IV when compared to control (Eggert et al., 2001). When CLA was included in a grow-finish diet at 0.75% inclusion rate, barrows feed CLA had improved feed efficiency, decreased backfat, and improved loin marbling and firmness when compared to controls (Wiegand et al., 2001). When CLA was fed to genetically lean gilts for eight weeks, an increase in average daily gain and the gain:feed ratio was observed (Weber et al., 2006).

The same study also noted an increase in saturated fatty acids, decrease in unsaturated fatty acids, and an increased level of saturation of the belly tissue (Weber et al., 2006). Several other studies have also shown that CLA feeding increases fatty acid saturation, and firmness in back fat and belly fat (Ostrowska et al., 1999; Aalhus and Dugan, 2001; Dugan et al., 2004).

Dried Distillers Grains with Solubles

Distillers dried grains with solubles (DDGS) are the by-product of the yeast fermentation of grains such as corn (Newland and Mahan, 1990). During fermentation, corn starch is converted into alcohol for fuel and the remaining grain components, protein, fat, fiber, minerals, and vitamins, are left in a highly concentrated form (Newland and Mahan, 1990).

The nutritional value of corn DDGS is variable and dependent of lysine content (Newland and Mahan, 1990; Cromwell et al., 1993). There are two processes by which ethanol can be extracted from corn, wet milling and dry grinding.

Dry grinding has become the more common production procedure, accounting for 70% of ethanol production (Rausch and Belyea, 2005). Dry grinding focuses on extracting the maximum value from the corn as ethanol while wet milling extracts other products such as oil and corn gluten meal during the process (Bothast and Schlicher, 2005; Rausch and Belyea, 2006). The dry grind process begins by grinding the corn and mixing it with water. The resulting mash is then heated with enzymes to convert the starches to sugars which can be fermented by yeast. The product contains particulates and solubles which are distilled and dehydrated, producing ethanol and wet distiller’s grains. The distiller’s grains are then dried in order to increase shelf life (Bothast and Schlicher, 2005; Rausch and Belyea, 2005).

The nutritional values of DDGS for pigs are influenced by the processing procedure and production plant equipment and techniques (Spiehs et al., 2002; Belyea et al., 2004). Regardless of processing advances, DDGS remain highly variable even within the same production site. Typical content ranges are: dry matter, 87-94%; crude protein, 24-31%; crude fat, 3-12%; ash, 3-6%; and lysine, 0.59-0.89% (Shurson et al., 2004). Two limiting factors for including DDGS in swine diets are the high level of unsaturation in the dietary fatty acid profile and the high fiber content (Newland and Mahan, 1990; Rausch and Belyea, 2005). These two factors have been shown to result in both decreased feed intake and greatly increase the unsaturated content of adipose tissue. In a trial utilizing 0, 10, 20, and 30% DDGS in growfinish diets, pigs fed 20 or 30% DDGS showed decreased growth performance and increased IV when compared to control fed pigs (Whitney et al., 2006).

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