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J. Anim Sci. 2007. 85:2682-2690. doi:10.2527/jas.2007-0174
© 2007 American Society of Animal Science
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ANIMAL PRODUCTS

Interactive effect of ractopamine and dietary fat source on quality characteristics of fresh pork bellies1

J. K. Apple*,2, C. V. Maxwell*, J. T. Sawyer*, B. R. Kutz*, L. K. Rakes*, M. E. Davis*,3, Z. B. Johnson*, S. N. Carr{dagger} and T. A. Armstrong{dagger}

* Department of Animal Science, University of Arkansas, Fayetteville, 72701; and {dagger} Elanco Animal Health, a Division of Eli Lilly and Company, Greenfield, IN 46140


   Abstract
Top
 Abstract
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 
Crossbred pigs (n = 216) were used to test the interaction, if any, of ractopamine (RAC) and dietary fat source on the characteristics of fresh pork bellies. Pigs were blocked by BW (77.6 ± 6.5 kg) and allotted randomly to pens (6 pigs/pen). After receiving a common diet devoid of RAC for 2 wk, pens within blocks were assigned randomly to 1 of 4 treatments arranged in a 2 x 2 factorial design, with 5% fat (beef tallow vs. soybean oil) and RAC (0 vs. 10 mg/kg). At the conclusion of the 35-d feeding period, pigs were slaughtered at a commercial pork packing plant (average BW of 108.8 ± 0.6 kg), and fresh bellies were captured during carcass fabrication. Neither RAC (P = 0.362) nor fat source (P = 0.247) affected belly thickness. Subjective (bar-suspension) or objective (compression test) measures of belly firmness were not (P ≥ 0.148) affected by the inclusion of RAC in the diet; however, bellies from pigs fed soybean oil (SBO) were softer than those from pigs fed beef tallow (BT), as indicated by perpendicular (P ≤ 0.005) and parallel (P < 0.001) suspensions. Moreover, bellies from BT-fed pigs required more (P = 0.096) force to compress 50% of their thickness than bellies from SBO-fed pigs (52.29 vs. 43.51 kg). Color (L*, a*, and b* values) of the belly lean and fat was not (P ≥ 0.131) affected by RAC, and lean color was similar (P ≥ 0.262) between fat sources; however, belly fat from BT-fed pigs was lighter (P = 0.030) and redder (P = 0.013) in color than belly fat from SBO-fed pigs. Bellies of SBO-fed pigs had greater (P < 0.001) proportions of PUFA and lower (P < 0.001) proportions of SFA and MUFA than belly fat from pigs fed BT. Regardless of the RAC inclusion level, PUFA:SFA and iodine values were lower in belly fat from pigs fed BT than SBO; however, within SBO-fed pigs, PUFA:SFA and iodine values were further increased by feeding RAC (RAC x fat source, P < 0.001). As expected, dietary fat source altered the fatty acid composition of fresh pork bellies, which subsequently impacted fresh belly firmness. Interestingly, including RAC in swine finishing diets exacerbated the effect of feeding SBO on pork fat polyunsaturation.

Key Words: belly • color • fatty acid composition • firmness • pork • ractopamine


   INTRODUCTION
Top
 Abstract
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 
Soft pork fat and bellies are an economic concern for today’s pork processors, resulting in carcass handling and fabrication difficulties, reduced bacon yields, unattractive products, reduced shelf life, and discrimination by importers of US pork (Morgan et al., 1994Go). The increase in the incidence of soft fat in the United States has resulted from the adoption of leaner genetics and increased use of polyunsaturated fat sources (i.e., poultry fat, restaurant grease, etc.) as more cost-effective energy sources. More importantly, it is apparent that pork fat becomes softer as carcasses become leaner (Wood et al., 1989Go; Sather et al., 1995Go).

Dietary inclusion of ractopamine hydrochloride (RAC; Elanco Animal Health, Greenfield, IN) in swine finishing diets has repeatedly been shown to improve growth rate and carcass lean meat yields without detrimental effects on LM quality. Even though Jeremiah et al. (1994aGo, b)Go and Stites et al. (1991)Go demonstrated that RAC did not affect fresh belly thickness or cooking properties and palatability of bacon, some pork processors are concerned about belly quality because feeding RAC has been shown to decrease the quantity of saturated fatty acids (Engeseth et al., 1992Go; Perkins et al., 1992Go) and increase polyunsaturation of pork fat (Carr et al., 2005bGo; Xi et al., 2005Go; Weber et al., 2006Go).

Therefore, the objective of the current study was to determine the interactions, if any, of RAC and dietary fat source on quality characteristics of fresh pork bellies.


   MATERIALS AND METHODS
Top
 Abstract
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 LITERATURE CITED
 
Animals and Diets

Animal care and experimental protocols were approved by the University of Arkansas Interdepartmental Animal Care and Use Committee (protocol No. 04014) before initiation of this experiment.

Crossbred barrows and gilts (n = 216), from the mating of line 348 sows to EB boars (Monsanto Choice Genetics, St. Louis, MO), were blocked by BW (77.6 ± 6.5 kg) into 9 blocks (24 pigs/block) and allotted randomly to pens of 6 pigs within blocks (4 gilts and 2 barrows/pen in blocks 1, 3, and 6; 3 gilts and 3 barrows/pen in blocks 2, 4, and 5; and 2 gilts and 4 barrows/pen in blocks 7, 8, and 9). After a 2-wk adjustment period, during which all pigs were fed a common corn-soybean meal early-finishing diet devoid of RAC but containing 0.65% of an animal-vegetable fat blend (0.68% lysine and 3.34 Mcal/kg of ME; Table 1Go), pens within blocks were assigned randomly to 1 of 4 dietary treatments arranged in a 2 x 2 factorial design, with 2 RAC levels (0 or 10 mg/kg) and 5% dietary fat from 2 sources (beef tallow or soybean oil). Soybean oil (SBO) and beef tallow (BT) diets contained 3.59 and 3.55 Mcal of ME/kg, respectively; however, lysine levels were adjusted to maintain the lysine:energy ratio (3.1 g lysine/Mcal of ME) constant for the 2 fat sources (Table 1Go). Proportions of SFA, MUFA, and PUFA were 15.90, 24.44, and 58.95%, respectively, in the SBO source and 47.22, 44.88, and 4.05%, respectively, in the BT source (Table 2Go). Moreover, linoleic (18:2n6) and linolenic (18:3n3) acid levels were greater, and the proportion of palmitic (16:0) and stearic (18:0) acids was lower, in the SBO than the BT diet, resulting in PUFA:SFA and iodine value (IV) of 3.74 and 127.44, respectively, for SBO diets and 0.72 and 73.47, respectively, for the BT diet (Table 2Go). All diets met or exceeded the NRC (1998)Go requirements for 79.5- to 109.1-kg pigs. Additionally, pigs were housed in a curtain-sided building with slatted floors, and each pen was equipped with a single-opening feeder and a nipple waterer, which allowed ad libitum access to the diets and to water throughout the trial.


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  		Table 1. Composition (% as-fed) of pre-experimental and experimental diets
 

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  		Table 2. Fatty acid composition (% as-fed) of fat sources and experimental diets
 
At completion of the finishing period, pigs with an average BW of 108.8 kg were transported approximately 760 km to a commercial pork packing plant (Sara Lee Corp., West Point, MS), and slaughtered according to industry-accepted procedures. After a standard, 24-h spray-chilling period at 1°C, individually identified carcasses were fabricated, and fresh pork bellies (n = 162) were collected, vacuum-packaged, boxed, and transported under refrigeration to the University of Arkansas Red Meat Research Abattoir for quality data collection.

Belly Quality Data Collection

Subjective belly firmness was measured using the bar-suspension (flop) method of Thiel-Cooper et al. (2001)Go, by measuring the distance between belly ends when the length of the belly was suspended perpendicular (skin-side down and skin-side up) and parallel (skin-side up) to a 1.9-cm diam. bar. Additionally, color (L*, a*, and b* values) of the lean (rectus abdominus) and belly fat was measured from a mean of 4 random readings made with a Hunter MiniScan XE (Hunter Associate Laboratories, Reston, VA) using illuminant C and a 10° standard observer. Then, approximately 250 g of fat along the navel edge of each belly was removed and stored in Whirl-Pak bags (NASCO, Fort Atkinson, WI) at –20°C for fatty acid analysis.

Each belly was subsequently separated into 2 equal length portions, and two 5.1-cm diam. cores were removed from the posterior portion of the caudal (flank) half of each belly to objectively measure belly firmness. The thickness of each core was measured at 4 locations with calipers, and the average thickness of both cores was used to determine belly thickness. Subsequently, belly cores were compressed to 50% of their specific average thickness with an Instron testing machine (Instron Corp., Canton, MA) equipped with upper and lower flat plates, a 400-kg load cell, and a crosshead speed of 100 mm/min.

Fatty Acid Analysis

Duplicate samples of belly fat were weighed, placed in 30-mL beakers, and reweighed. Beakers were then placed into vacuum-flasks attached to the manifold of a Labconco freeze-dryer (Model 4.5, Labconco Corp., Kansas City, MO) with a temperature setting of –50°C and a vacuum of less than 10 mm of Hg. Samples were freeze-dried for 60 h, the beakers were reweighed, and the difference between the initial and dried beaker weights was used to calculate the percentage moisture.

Duplicate 30-mg, freeze-dried fat samples, as well as samples of fat sources and pulverized diets, were subjected to direct transesterification by incubation in 2.0 mL of 0.2 M methanolic KOH in 16 x 125-mm screw-cap tubes at 50°C for 30 min with vortex-mixing 2 to 3 times per minute until the tissue was dissolved (Murrieta et al., 2003Go). Tubes were allowed to cool to room temperature, and 1 mL of saturated NaCl was added to each tube. Then, 2 mL of hexane containing 0.5 mg/mL of an internal standard (methyl 13:0) were added to tubes, and the tubes were vortexed and subsequently centrifuged for 5 min at 1,100 x g to separate the phases.

Fatty acid methyl esters were transferred to GLC vials that contained a 1.0-mm bed of anhydrous sodium sulfate. Separation of fatty acid methyl esters was achieved by GLC [Model 5890 Series II GC with automatic sample injector (HP-7673) and HP-3365 software, Hewlett-Packard, Avondale, PA] equipped with a 100-m capillary column (0.25-mm internal diameter; Model 2560 Fused Silica Capillary, Supelco Inc., Bellefonte, PA) and He as the carrier gas (0.5 mL/min). Oven temperature was maintained at 175°C for 35 min, ramped at 5°C/min to 215°C, and then ramped at 10°C/min to 235°C, whereas injector and detector temperatures were maintained at 250°C. Identification of peaks was accomplished using purified standards obtained from Nu-Chek Prep (Elysian, MN) and Matreya (Pleasant Gap, PA). The PUFA:SFA ratio was calculated using the formula: {[18:2n6] + [18:3n3]} ÷ {[12:0] + [14:0] + [16:0] + [18:0]}; whereas iodine value was calculated according to the equation of AOCS (1998)Go: {0.95 x [16:1]} + {0.86 x [18:1]} + {1.732 x [18:2]} + {2.616 x [18:3]} + {0.785 x [20:1]}, where the brackets in the equations indicate the concentration.

Statistical Analysis

Data were analyzed as a randomized complete block design, with treatments arranged in a 2 x 2 factorial design, and pen as the experimental unit. Analysis of variance was done using the GLM procedure (SAS Inst. Inc., Cary, NC), with RAC level (0 vs. 10 mg/kg), dietary fat source (BT vs. SBO), and the RAC x fat source interaction included in the model as main effects. Least-squares means were computed and separated statistically using pairwise t-tests (PDIFF option) when a significant F-test (P < 0.05) was observed.


   RESULTS
Top
 Abstract
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 LITERATURE CITED
 
There were no (P ≥ 0.167) RAC x fat source interactions for any fresh belly quality characteristic measured, and neither dietary inclusion of RAC (P = 0.362) nor fat source (P = 0.247) altered belly thickness (Table 3Go). When subjective belly firmness was measured skin-side down, fresh bellies from RAC-fed pigs were softer (P = 0.005); however, firmness measures did not differ between pigs fed 0 or 10 mg/kg RAC when suspended skin-side up (P = 0.279) or parallel (P = 0.148) to the bar. Moreover, belly compression values were similar (P = 0.451) between pigs fed 0 or 10 mg/kg of RAC. Conversely, bellies from pigs fed BT were firmer than those from SBO-fed pigs, as evidenced by greater (P ≤ 0.005) belly-suspension measurements and approximately 8.8 kg more (P = 0.096) force to compress belly cores to 50% of their thickness.


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  		Table 3. Effects of ractopamine and dietary fat source on fresh pork belly quality measurements
 
Neither RAC inclusion (P ≥ 0.131) nor dietary fat source (P ≥ 0.262) affected L*, a*, and b* values for the rectus abdominus (Table 3Go). Furthermore, including 10 mg/kg of RAC in swine finishing diets did not (P ≥ 0.419) alter objective color measurements of belly fat. On the other hand, belly fat from pigs fed BT was lighter (higher L* values; P = 0.030) and redder (higher a* values; P = 0.013) than belly fat from SBO-fed pigs.

The main effects of RAC and dietary fat sources on the fatty acid composition of belly fat samples are presented in Table 4Go, whereas interactions are displayed in Table 5Go. Feeding RAC did not (P ≥ 0.187) affect weight percentages of the saturated fatty acids capric (10:0), lauric (12:0), myristic (14:0), pentadecanoic (15:0), margaric (17:0), and arachidic (20:0) acids (Table 4Go). Even though dietary fat source did not alter the proportions of 10:0 (P = 0.960) or 20:0 (P = 0.495), belly fat from BT-fed pigs had greater (P ≤ 0.001) weight percentages of 12:0, 14:0, 15:0, and 17:0 than belly fat from SBO-fed pigs. Interestingly, belly fat from pigs fed BT and 10 mg/kg of RAC had greater (P < 0.05) proportions of palmitic (16:0) than fat from pigs fed SBO and 0 mg/kg of RAC, whereas pigs fed SBO and 10 mg/kg of RAC had the lowest (P < 0.05) proportions of 16:0 than all other treatment combinations (RAC x fat source, P = 0.027; Table 5Go). Feeding pigs 5% SBO and 10 mg/kg of RAC reduced (P < 0.05) the proportion of stearic (18:0) acid compared with SBO-pigs fed 0 mg/kg of RAC and BT-fed pigs (RAC x fat source, P = 0.060). Moreover, total belly fat SFA percentages were highest (P < 0.05) in BT-fed pigs, regardless of RAC, and within SBO-fed pigs, total SFA percentages were greater (P < 0.05) in those fed 0 than 10 mg/kg of RAC (RAC x fat source, P = 0.028).


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  		Table 4. Effects of ractopamine and dietary fat source on the fatty acid composition of belly fat samples from finishing swine
 

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  		Table 5. Interactions of ractopamine and dietary fat source on the weight proportions of fatty acids and fatty acid totals of belly fat samples
 
With the exception of RAC x fat source interactions for myristoleic acid (14:1; P = 0.036), where RAC reduced 14:1 proportions within BT-fed pigs only, and total 18:1 trans fatty acids (P = 0.069), where proportions were greater in BT-fed than SBO-fed pigs, there were no (P ≥ 0.153) interactions on MUFA (Table 5Go). Feeding swine diets with 10 mg/kg of RAC did not (P ≥ 0.243) alter the proportion of all MUFA or the individual percentages of palmitelaidic (16:1t), palmitoleic (16:1c), heptadecenoic (17:1t), oleic (18:1c9), vaccenic (18:1c11), or gadoleic (20:1) acids in belly fat samples (Table 4Go). Conversely, belly fat from BT-fed pigs had greater (P ≤ 0.043) proportions of 16:1t, 16:1c, 17:1t, 18:1c9, 18:1c11, and 20:1, leading to an elevated (P < 0.001) level of total MUFA.

As expected, feeding diets formulated with SBO resulted in a 35.2% increase (P < 0.001) in the proportion of PUFA in belly fat, whereas total PUFA percentages tended to increase (P = 0.065) approximately 3.7% in response to including RAC in the finishing diet (Table 4Go). Even though weight percentages of eicosadienoic (20:2), eicosatrienoic (20:3n3), arachidonic (20:4n6), and docosapentaenoic (22:5) acids were increased (P ≤ 0.006) by feeding SBO, belly fat samples from SBO-fed pigs had 33.7 and 93.2% greater (P < 0.001) proportions of linoleic (18:2n6) and {alpha}-linolenic (18:3n3), respectively, than belly fat from BT-fed pigs. The increase in total PUFA by RAC could be attributed to increased weight percentages of 18:2n6 (P = 0.069) and 18:3n3 (P = 0.056); otherwise, RAC had almost no effect (P ≥ 0.110) on PUFA proportions. Including RAC in the finishing diet reduced (P < 0.05) the proportion of CLA in belly fat samples from BT-fed pigs, but CLA percentages were greater (P < 0.05) in BT-fed than SBO-fed pigs (RAC x fat source, P = 0.022; Table 5Go). Additionally, when RAC was included in the diet, belly fat from pigs fed SBO had higher (P < 0.05) percentages of dihomo-{gamma}-linolenic acid (20:3n6) than pigs fed BT, as well as pigs fed SBO and 0 mg/kg of RAC (RAC x dietary fat, P = 0.019).

Ractopamine did not alter the percentage of other (unidentified) fatty acids (P = 0.932), but the proportion of other fatty acids was elevated (P < 0.001) in belly fat from BT-fed compared with SBO-fed pigs (Table 4Go). There was a tendency for a greater (P = 0.062) percentage of n-3, and a lower (P = 0.070) percentage of n-6, fatty acids in RAC-fed pigs, whereas feeding SBO increased (P < 0.001) the proportions of both n-3 and n-6 fatty acids. Moreover, SBO-fed pigs, regardless of RAC, had lower (P < 0.05) n-6:n-3 ratios than BT-fed pigs, and within BT-fed pigs, n-6:n-3 ratio was lower (P < 0.05) in pigs fed 10 vs. 0 mg/kg of RAC (RAC x dietary fat, P = 0.034; Table 5Go). Additionally, the sum of all trans fatty acids was higher in belly fat samples from BT-fed pigs than SBO-fed pigs, regardless of RAC inclusion (RAC x dietary fat, P = 0.069). There were RAC x fat source interactions for belly fat PUFA:SFA ratio (P = 0.042) and iodine value (P = 0.038), indicating that, even though PUFA:SFA ratios and IV were less (P < 0.05) in BT- than SBO-fed pigs, RAC had no (P > 0.05) impact on these values within pigs fed BT, but increased (P < 0.05) both PUFA:SFA ratios and IV in SBO-fed pigs.


   DISCUSSION
Top
 Abstract
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
LITERATURE CITED
 
Very little information is available concerning the effects of RAC on fresh belly quality. Yet, Stites et al. (1991)Go and Uttaro et al. (1993)Go failed to detect a difference in fresh belly thickness among pigs fed 0, 5, 10, or 20 mg/kg of RAC. Moreover, Carr et al. (2005a)Go reported that feeding diets including 10 or 20 mg/kg of RAC had no effect on bar-suspension belly firmness measurements; however, in another experiment, feeding 10 mg/kg of RAC produced substantially softer bellies, as evidenced by a 5.5-cm reduction in the distance between belly ends when suspended across the bar (Carr et al., 2005bGo). The conflict between studies (Carr et al., 2005aGo,bGo) was notable in the current study, where bellies from RAC-fed pigs were deemed softer when suspended skin-side down, but other bar-suspension measurements and compression values did not differ between RAC treatments.

It is generally accepted that fat and bellies become softer as pigs become leaner (Wood et al., 1989Go; Wood and Enser, 1982Go). In addition, as fat becomes softer in response to reductions in carcass fatness, there is a tendency for greater fat-to-fat and fat-to-muscle separation (Wood, 1984Go). Sather et al. (1995)Go reported that increased leanness was linearly antagonistic to fat hardness, and this was supported by the work of Dransfield and Kempster (1988)Go and Sather et al. (1991)Go, who demonstrated that leaner boars produced substantially softer fat than the fatter barrows and gilts. There are 2 possible reasons that fat from leaner pigs is softer. First, fat and moisture contents are inversely related, and research has shown backfat and bellies from leaner pigs have less fat and more moisture than their fatter counterparts (Enser et al., 1984Go; Wood et al., 1986Go). Secondly, as pork carcasses become leaner, there is convincing evidence that the proportion of linoleic acid in pork fat increases (Wood et al., 1989Go, 1996Go; Lo Fiego et al., 2005Go). Whittington et al. (1986)Go reported that linoleic acid concentrations in backfat were inversely related to belly firmness, whereas Prescott and Wood (1988)Go exhibited a negative, curvilinear relationship between linoleic acid content and mechanical fat firmness measures.

Inclusion of RAC in swine finishing diets has been repeatedly shown to produce leaner (Aalhus et al., 1990Go; Carr et al., 2005bGo), heavier muscled (Aalhus et al., 1990Go; Xiao et al., 1999Go; Schinckel et al., 2003Go) carcasses, with elevated moisture contents (Dunshea et al., 1993Go; Uttaro et al., 1993Go). Furthermore, Aalhus et al. (1990)Go found that fat hardness was decreased in RAC-fed pigs, and they attributed the diminishing fat hardness to reductions in fat accumulation among RAC-fed pigs. Lending support, Lonergan et al. (1992)Go showed that daily injections of porcine somatotropin substantially reduced backfat depth, leading to increased belly moisture and production of thinner, softer bacon. Uttaro et al. (1993)Go reported similar belly thicknesses between RAC- and control-fed pigs; however, thickness of the visible lean portion was increased by feeding RAC. And, although total SFA and MUFA percentages were not affected by 10 mg/kg of RAC in this study, there was an increase in the proportion of PUFA, in particular linoleic acid, in fresh bellies, which is consistent to the RAC-induced changes in PUFA composition of pork backfat (Carr et al., 2005bGo). Thus, the minor discrepancy in the impact of dietary RAC on the bar-suspension method measures of belly firmness may be explained by changes in the moisture content associated with increased carcass leanness, increases in belly fat linoleic acid content, or both.

It is not surprising that feeding diets formulated with a more polyunsaturated fat source would produce thinner, softer bellies (Table 3Go). Gatlin et al. (2003)Go demonstrated that belly thickness measured on the dorsal and ventral edges decreased linearly as the IV of the diet increased. Both Mazhar et al. (1990)Go and Shackelford et al. (1990)Go reported that feeding ground canola, canola oil, safflower oil, and sunflower oil reduced subjective belly firmness scores compared with feeding animal fat, whereas bar-suspension measures indicated that bellies from pigs fed choice white grease or high-oil corn were considerably softer than those from pigs fed a conventional corn-soybean meal diet (Rentfrow et al., 2003Go). Furthermore, Rogers and Etzler (2000)Go reported that compression values were less for bacon from pigs fed fish meal or oil and restaurant grease, and even though Engel et al. (2001)Go reported similar compression values for bellies of pigs fed choice white grease or poultry fat, bellies from pigs fed 2% choice white grease had greater fat separation than bellies from pigs fed 2% poultry fat.

As early as 1926, research indicated that soft pork fat was caused by feeding diets with high proportions of PUFA, especially linoleic acid (Ellis and Isbell, 1926Go). Since then, research has established that the final fatty acid profile of pork carcass fat is the reflection of the dietary fatty acid composition (see reviews by Madsen et al., 1992Go; Wood et al., 2003Go; De Smet et al., 2004Go). In the current study, feeding diets formulated with 5% SBO reduced the proportions of total SFA and MUFA in fresh bellies by 5.4 and 7.6%, respectively, and increased belly fat PUFA percentages by 35.2% compared with bellies from pigs fed 5% BT. As previously mentioned, soft bellies have low proportions of palmitic and, in particular, stearic acids, and very high proportions of linoleic acid (Enser et al., 1984Go), and in the current study, palmitic, stearic, and linoleic acids were altered –4.2, –6.1, and +31.7%, respectively, by feeding SBO. Moreover, Shackelford et al. (1990)Go reported that the percentages of SFA were decreased, and the percentages of PUFA were decreased, in fresh bellies by feeding safflower, sunflower, and canola oils compared with tallow, whereas Mazhar et al. (1990)Go reported that feeding ground canola substantially increased the percentages of linoleic and linolenic acids in pork belly fat. Even though feeding 4% BT increased the proportions of myristic, palmitoleic, and stearic acids compared with feeding 4% yellow grease (Gatlin et al., 2002Go) or 10 to 20% full-fat soybeans (Leszczynski et al., 1992Go), the more unsaturated dietary fat sources caused robust increases in the proportions of linoleic and linolenic acids in belly fat (Leszczynski et al., 1992Go; Gatlin et al., 2002Go). Additionally, Gatlin et al. (2003)Go demonstrated that palmitic and stearic fat percentages decreased linearly, and linoleic acid percentages increased linearly, as the dietary IV increased.

Although not affected by fat source or dietary RAC inclusion, the proportion of trans fatty acids in the belly fat samples was higher than previously reported in belly fat samples of pigs fed 5% choice white grease (Rentfrow et al., 2003Go) or corn-soybean meal diets formulated without supplemental dietary fat (Gatlin et al., 2002Go; Rentfrow et al., 2003Go). Furthermore, levels of trans fatty acids in belly fat of BT-fed pigs closely mirrored the proportions of trans fatty acids reported in the belly fat of pigs fed 4% yellow grease or BT (Gatlin et al., 2002Go); however, belly fat samples from SBO-fed pigs had considerably higher percentages of trans fatty acids (1.28%; Table 4Go) than the pigs could have actually consumed (0.10%; Table 2Go). The reason for the discrepancy may be attributed to the partially hydrogenated animal-vegetable fat source fed to pigs prior to the initiation of this study. Several studies have demonstrated that feeding finishing diets formulated with hydrogenated (Fontanillas et al., 1997Go; Gatlin et al., 2003Go) or partially hydrogenated fats (Gläser et al., 2000Go) dramatically increased the levels of trans fatty acids in pork fat samples. Even though only 0.65% (as-fed basis) of the animal-vegetable blend was included in the 2-wk pretrial diet, Fontanillas et al. (1998)Go reported that feeding pigs a diet supplemented with hydrogenated animal fat increased the percentage of all trans fatty acids in backfat samples 4.2-fold (from 0.85 to 3.57%) in just 17 d.

The effect of RAC on fatty acid composition of subcutaneous fat and belly fat is not as well established as the impact of dietary fats and oils. Perkins et al. (1992)Go failed to detect differences in the fatty acid composition of pork backfat or neutral lipids of the LM; however, they noted that the proportion of linoleic acid in the polar fraction of LM lipids increased linearly as RAC dosage increased from 0 to 20 mg/kg. Moreover, feeding 10 (Carr et al., 2005bGo; Xi et al., 2005Go; Weber et al., 2006Go) or 20 mg/kg of RAC (Engeseth et al., 1992Go) to finishing swine has been shown to increase the percentage of linoleic acid in pork backfat, but had no effect on the fatty acid composition of belly fat (Xi et al., 2005Go).

In the current study, the main effect of RAC on the fatty acid profile of pork belly fat was the relatively minor increases in the proportion of linoleic and {alpha}-linoleic acids (Table 4Go). And, in agreement with the results of Weber et al. (2006)Go, results of this study indicate that including 10 mg/kg of RAC in diets formulated with BT had little to no impact on total SFA percentages, especially the proportions of palmitic and stearic acids, PUFA:SFA ratio, or IV of belly fat (Table 5Go). However, feeding RAC exacerbated the effect of dietary SBO on belly fatty acid composition, as evidenced by lowest proportions of SFA and the highest PUFA:SFA ratio and IV, suggesting that RAC interacts differently with different dietary fat sources. Interestingly, it has been shown that feeding finishing pigs 20 mg/kg of RAC caused robust reductions in de novo lipogenesis in fatty tissues (Mills et al., 1990Go); thus, if RAC reduced de novo lipogenesis in belly fat equally, then it is plausible that the fatty acid composition of the belly fat was an even greater reflection of absorbed fatty acids from the dietary fat source (Table 2Go).

Lean and fat color were not affected by the inclusion of 10 mg/kg of RAC (Table 3Go); however, the fat of bellies from BT-fed pigs was lighter and redder than belly fat from SBO-fed pigs. Interestingly, Mazhar et al. (1990)Go reported that belly fat from pigs fed ground canola was more yellow than that from pigs fed a diet devoid of added fat. Engel et al. (2001)Go found that formulating diets with 2, 4, and 6% of choice white grease or poultry fat failed to alter belly lean and fat L*, a*, and b* values, but belly lean and fat L* and lean a* values tended to increase and b* values for belly lean and fat tended to decrease when diets were formulated with catfish meal, catfish oil, or restaurant grease (Rogers and Etzler, 2000Go). Color and appearance are routinely measured as a quality indicator of the final product and may be affected by the degree of pigmentation (carotenoid or chlorophyll pigments), improper storage, or improper blending, bleaching, ineffective filtration, oxidation, and heating (O’Brien, 2004Go). Additionally, Lawson (1995)Go indicated that the color of oils and fats can impact the color of the final product. Thus, the minor differences in belly fat color were probably a reflection of the subtle quality differences between dietary fat sources.

As expected, formulating finishing diets with soybean meal instead of beef tallow resulted in increased polyun-saturation of pork belly fat, subsequently producing soft bellies. Yet, results of this study implied that including 10 mg/kg of RAC in diets of finishing swine had little to no effect on fresh belly quality. Furthermore, RAC had minimal effects on the fatty acid composition of fresh bellies, especially those from pigs fed beef tallow; however, results indicated that 10 mg/kg of RAC may have exacerbated the effect of feeding soybean oil on pork fat polyunsaturation.


   Footnotes
 
1 The authors wish to express their appreciation to Elanco Animal Health, a division of Eli Lilly and Company, for financial support of this experiment, and Tyson Foods Inc. (Springdale, AR) for donation of beef tallow. Additionally, the authors gratefully acknowledge the assistance of K. Richardson and employees at Sara Lee (West Point, MS) with pig slaughter, carcass fabrication, and subprimal cut procurement; A. Hays for animal care and performance data collection; and J. Stephenson for assistance with belly data collection and belly processing. Back

3 Current address: Agtech Products Inc., Waukesha, WI 53186. Back

2 Corresponding author: japple{at}uark.edu

Received for publication March 17, 2007. Accepted for publication June 5, 2007.


   LITERATURE CITED
Top
 Abstract
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 


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