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Comparison of grain sources for swine diets and their effect on meat and fat quality traits¹

Department of Animal Science, Iowa State University, Ames 50011

	Abstract

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

 
A study was conducted to evaluate the effect of dietary grain sources on various compositional and quality characteristics of pork from pigs reared in a commercial environment. Pigs were fed 1 of 5 dietary treatments containing the following single or blended grain sources throughout most of the grow-finish period: 1) yellow corn, 2) white corn, 3) 1/3 yellow corn and 2/3 white corn, 4) 2/3 yellow corn and 1/3 white corn, and 5) barley. Pigs were from 2 sire genetic types, Duroc and Hampshire x Duroc, mated to PIC 1055 females. A total of 1,040 pigs were included in the study in a 2 x 2 x 5 factorial arrangement with 2 genetic types, 2 sexes (barrows and gilts), and 5 dietary treatments. Eight pigs were randomly selected from each pen of 26 (n = 320) for meat and fat quality evaluation. Pigs were 27.6 kg at the beginning of the experiment and were fed to 130.2 kg. All animals were held overnight at a commercial abattoir before slaughter. One whole, skin-on, boneless loin was collected from each carcass and held at –1°C in a cryovac-sealed bag at the Iowa State University Meat Laboratory. At 25 to 27 d postslaughter, loins were evaluated for meat and fat quality. Dietary treatment had no effect (P > 0.05) on 24-h pH, sensory tenderness, sensory chewiness, Instron tenderness, loin purge, or cook loss. At 25 to 27 d postslaughter, pigs fed diet 4 had a greater (P < 0.05) loin pH than pigs fed diet 1, and diets 2, 3, and 5 were not different from all treatment means. Pigs fed diet 4 had a greater (P < 0.05) Japanese color score than pigs fed diets 2, 3, and 5, and diet 1 was not different from all treatment means. Pigs fed diet 3 had a greater percentage of intramuscular fat than pigs fed diets 1 and 2, although diets 1, 4, and 5 and diets 1, 2, and 5 were not different (P > 0.05). No differences among dietary treatments were found for fat color values on a subjective basis. Pigs fed diet 5 had a more desirable objective fat color than pigs fed all white corn, and diets 1, 3, and 4 were not different (P > 0.05). Pigs fed diet 5 had greater levels of SFA and MUFA, and lower levels of unsaturated fatty acids and PUFA, in the subcutaneous fat than pigs fed all other diets. These results indicate that the energy sources evaluated in this study had little effect on eating quality of pork that was held for 25 to 27 d postslaughter.

Key Words: energy • grain source • pig • pork quality

	INTRODUCTION

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

 
The US pork export market has increased dramatically in both value and volume in the past decade. As export markets expand, so will requirements for processing, carcass fabrication, food safety, and sanitation. Pork produced in the United States is considered to be safe and reliable (Morgan et al., 1995). Export markets, such as the Asian market, have identified a need for improved consistency in US pork (Morgan et al., 1995; Vonada et al., 2000). With significant opportunities in the export market, suppliers of pork have attempted to offer products that differ from the average. Barley-fed pigs have been differentiated with a mild flavor and firmness advantage (Makise, 2002). Patience and de Lange (1996) indicated that the future of the pork industry will involve greater expectations, and the concept of quality will extend beyond the relative absence of fat.

Corn is considered to be one of the most economical energy feedstuffs available for swine production. Barley is a feed grain with greater fiber that provides approximately 89% of the ME content of corn (Jurgens, 1993). Barley lacks the carotene content that yellow corn possesses, and it has been proposed as a substitute for corn to produce pigs that will yield a greater meat and fat quality that is desired by certain export markets (Makise, 2002). White corn is an additional potential energy source that lacks the carotene content of yellow corn. The objective of this research was to determine the effect of dietary energy source (yellow corn, white corn, and barley) on various compositional and quality characteristics of pork from pigs reared in a commercial environment. A secondary objective was to evaluate the effects of sire genotype and sex on meat and fat quality characteristics.

	MATERIALS AND METHODS

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

 
The experimental protocols for this study were approved by the Iowa State University Institutional Animal Care and Use Committee. The experiment was a completely randomized design with 5 dietary treatments. A total of 40 pens, each containing 26 pigs (n = 1,040; initial BW = 27.6 kg), was used in the study. A total of 999 pigs completed the study and were included in the analysis in a 2 x 2 x 5 factorial arrangement with 2 genetic types, 2 sexes (barrows and gilts), and 5 dietary treatments. Pigs were from 2 sire genetic types represented equally across pens [Hampshire x Duroc (HD) and Duroc] and were mated to PIC 1055 females (PIC USA, Franklin, KY). Eight pigs were randomly selected from each pen of 26 (n = 320) for meat and fat quality evaluation. One carcass was lost in the plant; therefore, data are reported on 319 head.

Pigs were housed in a mechanically ventilated, curtain-sided finisher building with totally slotted floors on a farm owned by a commercial cooperator in Iowa. Each pen was equipped with a 5-space, single-sided, stainless steel self-feeder and nipple waterers, allowing ad libitum feed and water consumption. Pigs were allowed 0.67 m² of space per pig. Pigs were fed 1 of 5 dietary treatments containing the following single or blended grains throughout most of the grow-finish period: 1) yellow corn, 2) white corn, 3) 1/3 yellow corn and 2/3 white corn, 4) 2/3 yellow corn and 1/3 white corn, and 5) barley. The grains used were from a single, commercial source, and only one variety was used for each energy source. The yellow and white corn varieties used in the diets were considered equivalent in nutrient value.

The diets were formulated to meet or exceed NRC (1998) requirements and were fed in 4 phases according to BW: 27.6 to 49.3 kg, 49.3 to 67.2 kg, 67.2 to 103.3 kg, and 103.3 to 130.2 kg. The total lysine content of the diet was 1.12, 0.93, 0.71, and 0.65% during the 4 phases, respectively (Table 1). Choice white grease was added for dust control in the finishing barn and was limited to 1% in all treatments in the finisher phases (67 to 130 kg) of the feeding regimen (Table 1). Dietary energy was not equal across treatments in the finisher phases (67 to 130 kg) because the primary objective of this investigation was to evaluate the effect of grain source on quality traits. The composition of each diet was evaluated, and each diet met the calculated composition presented in Table 1.

At 25 to 27 d postslaughter, loins were removed from the cyrovac bags, and both the loin and the amount of purge in the bag were weighed to determine the percentage of purge during storage. Each loin was cut to provide one 2.54 cm thick boneless loin chop from the blade, center, and sirloin end of the loin for the evaluation of whole muscle pH using a glass pH electrode (model FC200B) attached to a pH meter (model HI 9025, Hanna Instruments, Inc., Woonsocket, RI). A Hunter Lab Miniscan XE (Hunter Associates Laboratory, Inc., Reston, VA) calibrated according to the manufacturer’s instructions was used to evaluate L* color values. Additionally, a visual Japanese color score (Nakai, 1991) was assigned to each chop.

Five additional 2.54 cm thick loin chops were removed from the center of the loin for fat-lean color scanning, 24-h purge, moisture, fat composition, fatty acid profile, and sensory and instrumental analyses. For the 24-h purge measurement, the chop was placed in a bag, and the bag and the amount of purge in the bag were weighed (Explorer Pro, model EP2102, Ohaus Corporation, Pine Brook, NJ) after being held for 24 h at 0 to –4°C. One boneless chop was cryovac-sealed and stored for fatty acid profiling of the subcutaneous fat layer (Park and Goins, 1994).

One boneless chop from each loin was trimmed of fat, sealed in a bag, and stored for chemical analysis of lipid percentage using the method of Bligh and Dyer (1959). One boneless chop from each loin was used for evaluation of fat color by subjective and objective measurements. The subjective measurement utilized 4 Japanese fat color blocks with grades of 1 through 4 (1 = pure white to 4 = yellow discoloration) to assign a fat color grade to the subcutaneous fat of each loin. The objective measurement utilized the fat blocks and boneless chops with subcutaneous fat and were scanned on an HP 7400c (Hewlett-Packard Company, Palo Alto, CA) flatbed scanner. The uncompressed files were imported into an image analysis program to evaluate yellow channels (Ringkob, 2003). The yellow channel values were then converted to fit the same scale of the subjective measurements of 1 through 4.

Two boneless chops from each loin were broiled simultaneously to 71°C in a 210°C Amana Distinctions Electric Range (model RS-1, Amana Appliances, Amana, IA). Individual chop temperatures were monitored using Chromega/Alomega thermocouples (0.02 diameter, 1.8 m length) attached to an Omega digital thermometer (model MDSS 41-TC, Omega Engineering Inc., Stamford, CT). Cooking loss was determined by weighing the chops before and after cooking.

Sensory analysis was completed by a highly trained, professional sensory panel with a minimum of 3 members. During training, the panelists were served broiled loin samples displaying various intensities of juiciness, tenderness, chewiness, pork flavor, and off flavors. For this study, the broiled loin chops designated for sensory analysis were cut such that 3 or more 1.3 cm³-cubes were removed from the center of the chop. Each panelist was presented with one warm cube for evaluation of the degree of juiciness, tenderness, chewiness, pork flavor, and off flavor using a 10-point category scale. The scale was anchored on the left end of the scale with a term representing a low degree of juiciness, tenderness, chewiness, flavor, and off-flavor intensity. On the right end of the scale was a term representing a high degree of each characteristic (AMSA, 1995). Deionized distilled water at room temperature and unsalted crackers were served to cleanse the palate between samples.

The second broiled chop was evaluated for instrumental texture using a circular, 5-pointed star probe attached to an Instron Universal Testing Machine (model 4502, Instron Corporation, Canton, MA). A 10-kN load cell was used with a crosshead speed of 200 mm/min. The star probe attachment was used to determine the amount of force needed to puncture and compress the chop to 80% of the sample height. Each chop was punctured 3x, and the mean of 3 measurements was used for statistical analysis.

Statistical Analysis
Data were analyzed using the GLM procedure of SAS (SAS Inst., Inc., Cary, NC). Sex, diet, genetic type, and cut day (day of sensory analysis; range = 25 to 27 d postmortem) were fixed effects in the model for all sensory traits, Instron tenderness, and Japanese color score. Sex, diet, genetic type, and off date (1 of 2 slaughter groups) were fixed effects in the model for both pH measurements (24 h and 25 to 27 d postmortem), purge percentage, cook loss percentage, and Hunter values. Sex, diet, and genetic type were fixed effects in the model for intramuscular fat (IMF). All 2-way and 3-way interactions were tested and found to be nonsignificant (P > 0.05); thus, they were deleted from the final model. Individual loin sample was considered the experimental unit; the average value was used when multiple measurements were taken.

	RESULTS AND DISCUSSION

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

 
Least squares means for meat quality and sensory traits by dietary treatment are presented in Table 2. Sex and sire genetic type means are presented in Table 3. Traits were analyzed at 25 to 27 d postslaughter with the exception of pH, which was also evaluated 24 h postslaughter.

At 25 to 27 d postslaughter, loins from pigs fed the 2/3 yellow corn and 1/3 white corn diet (diet 4) had a greater (P < 0.05) pH than loins from pigs fed the diet containing yellow corn (diet 1). Sire genetic type values for pH were lower at 25 to 27 d when compared to 24 h postslaughter, but the differences between sire genetic types were nearly the same at both measurement times. No differences were found between sexes for pH at the 10th rib location for loins evaluated at either 24 h or 25 to 27 d postslaughter.

IMF.
A significant difference in IMF percentage between pigs fed the white corn diet and the 2 blended corn diets (diets 3 and 4) was observed. Differences were also significant between diets 1 and 3. These differences, although significant, are small and are assumed to be random. Intramuscular fat percentage of the loins from pigs fed the barley diet was intermediate and not significantly different from pigs fed all other diets. McConnell et al. (1975) did not find differences in marbling of loins when comparing pigs fed barley and pigs fed corn diets. Fent et al. (2003) also did not report any differences in IMF in pigs fed either white or yellow corn diets.

No differences (P > 0.05) in IMF were observed between sire genetic types. However, the NPPC (1995) reported Duroc-sired pigs had a greater IMF than did HD-sired pigs. Barrows had a significantly greater IMF than gilts. Greater IMF values for barrows vs. gilts is supported by the NPPC (1995) and Ball et al. (1996).

Instron Tenderness.
No significant treatment effects were observed for Instron tenderness. Fent et al. (2003) reported that loins from pigs fed yellow corn diets were greater (P < 0.08) in shear force when compared with loins from pigs fed white corn. Barrows in the current study were observed to have lower (P < 0.05) Instron tenderness values than gilts, but there were no differences among genetic types. This finding is supported by the NPPC research (1995), which compared sex as well as pigs sired by both Nebraska specific pathogen-free Duroc and Hampshire x Duroc boars.

Sensory Traits
Tenderness.
Dietary treatment had no effect (P < 0.05) on sensory panel tenderness values. Similar results by McConnell et al. (1975) showed no differences in tenderness values of loins from pigs fed either yellow corn or barley diets. Results in the current study disagree with those of Fent et al. (2003), who reported that loins from pigs fed white corn diets had a tendency (P < 0.07) to be more tender than loins of pigs fed yellow corn diets. The difference between sexes for Instron tenderness was also detected by the sensory panel; barrows had a more tender (P < 0.05) product than gilts. Differences in tenderness were also reported by the NPPC (1995) when barrows and gilts from several different purebred and crossbred lines were evaluated. The sensory panel in the current study did not detect a difference between genetic types for tenderness characteristics. This result agrees with those of the NPPC (1995) in which comparisons of HD and Duroc sires were made.

Juiciness and Chewiness.
Pigs fed the 1/3 yellow corn, 2/3 white corn diet (diet 3) had greater (P < 0.05) juiciness values than pigs fed the yellow corn diet (diet 1); however, dietary treatment had no effect (P > 0.05) on sensory panel chewiness values. Juiciness values for pigs fed the all white corn diet (diet 2) were not different from those fed the all yellow corn diet (diet 1); therefore, this difference in juiciness for pigs fed diet 3 is difficult to explain and may be due to random chance. Barrows were less (P < 0.05) chewy than gilts, and no differences between sexes were detected for juiciness. Sire genetic type had no effect (P < 0.05) on sensory panel juiciness or chewiness. No differences were detected by the NPPC (1995) for chewiness or juiciness between sexes (barrow, gilt) or sire lines (HD, Duroc).

Flavor and Off Flavor.
Loins from pigs fed diets containing either white corn (diet 2) or barley (diet 5) had less (P < 0.05) flavor than loins from pigs fed the 2/3 yellow, 1/3 white corn diet (diet 4). Skelley et al. (1975), in agreement with this research, did not detect differences in flavor of loins at 1 mo postslaughter from pigs fed either yellow corn or barley diets. Fent et al. (2003) reported no difference in flavor of loins from pigs fed either white or yellow corn diets. Loins from pigs fed the white corn diet (diet 2) had greater (P < 0.05) off-flavor values than loins from pigs fed the 2/3 yellow, 1/3 white corn diet (diet 4).

Loins did not differ (P > 0.05) in off-flavor values between barrows and gilts. However, loins from barrows had more (P < 0.05) flavor than loins from gilts. Research conducted by the NPPC (1995) agrees with this difference in flavor of loins from barrows and gilts. Duroc-sired pigs had loins with more flavor and less off flavor than pigs sired by HD boars. This sire line difference is in agreement with results from the National Terminal Sire Evaluation (NPPC, 1995).

Water-Holding Characteristics
Loin Purge and Cook Loss.
Loin purge is an important characteristic in products that are destined for export, as importers of pork do not like to see loins that leak excess moisture when taken from vacuum-packaged bags (Morgan et al., 1995). Dietary treatment, sex, and genetic type had no effect (P < 0.05) on purge or cooking loss of loins held for 25 to 27 d postslaughter. Ball et al. (1996) also reported no differences in moisture loss of loins from different sexes or genetics using purebred Hampshire and Duroc pigs. Research conducted by the NPPC (1995) differs with these results, as they found that HD-sired pigs had more cook loss than pigs sired by Duroc boars.

Color Evaluation
Hunter L*.
Loins from pigs fed the 2/3 yellow, 1/3 white corn diet (diet 4) had darker (P < 0.05) meat than loins from the barley-fed pigs (diet 5). McConnell et al. (1975), using a subjective measurement, found no differences in color of loins from pigs fed either yellow corn or barley diets. Research conducted by Fent et al. (2003) agrees with the current data; those researchers found that feeding pigs white and yellow corn diets had no effect on color evaluation of loins. Sex had no effect on Hunter L* values of loins used for sensory analysis. Duroc-sired pigs had significantly darker loins than pigs sired by HD boars. Data from the National Terminal Sire Evaluation agree with our findings that loins from barrows and gilts do not differ in color (NPPC, 1995).

Japanese Color Score.
Pigs fed the 2/3 yellow, 1/3 white corn diet (diet 4) had a greater (P < 0.05) Japanese color score than pigs fed the white corn diet (diet 2); the 1/3 yellow, 2/3 white corn diet (diet 3); and the barley diet (diet 5). Although results show significance (P < 0.05), the differences may not have an economic value that can be realized. Barrows had a greater Japanese color score than gilts. Research conducted by Ball et al. (1996) and the NPPC (1995) disagrees, as they reported that barrows and gilts do not differ in color values. Pigs sired by Duroc boars had a greater Japanese color score than HD-sired pigs, which is in agreement with the NPPC (1995). The study of Ball et al. (1996) also agrees with our research, reporting that purebred Duroc pigs had greater Japanese color scores than did purebred Hampshire pigs.

Fat Quality
Fatty Acid Profiles.
Research to date has predominantly shown that manipulation of fatty acid profiles can be done easily with the use of dietary fat sources (Miller et al., 1990; Wiseman and Agunbiade, 1998; Averette Gatlin et al., 2002; Bee et al., 2002). Fatty acids used for the formation of triglycerides of phospholipids are synthesized de novo from dietary carbohydrate or are contributed directly from dietary fat. The contribution of each of these 2 sources depends on the amount of fat in the diet. In a literature review by Schinckel et al. (2002), a typical corn and soybean meal diet that is low in fat (3 to 4%) has been estimated to contribute 80% of deposited triglycerides derived from the corn or barley in the diet.

Least squares means for fatty acid profiles and fat color by dietary treatment in this study are presented in Table 4. Backfat of pigs fed the barley diet had a significantly lower iodine value, lower PUFA, and an overall lower unsaturated fatty acid concentration and had greater SFA and MUFA than backfat from pigs fed corn diets. This significantly lower iodine value is correlated with a more desirable fat firmness for the pigs fed the barley-based diet. Research by McConnell et al. (1975) did not find differences in iodine value in subcutaneous fat of pigs fed barley and corn diets. Skelley et al. (1975) utilized diets of corn with soybean meal and barley with roasted soybeans and did not find a difference in overall saturation and unsaturation. Skelley et al. (1975) did find that barley-fed pigs had lower MUFA and greater PUFA than pigs fed corn diets. The differences found in the data of Skelley et al. (1975) suggest that changes in pork fat fatty acid profiles are correlated to the amount of fat in the diet and the fatty acid profile of the fat. No significant differences were found for the main effects of genetic type and sex (data not shown).

Results from this study show that pigs fed a barley-based diet do not produce meat with superior eating quality, but do produce carcasses with a harder fat. Carcasses with highly saturated fat are more desirable to the processor for ease of handling and cutability. The health food sector has proven that meat products high in saturated fats increase the risk of heart disease. Although PUFA are too soft for meat processors and SFA are unhealthy for consumers, an increase in MUFA or fatty acids with one double bond (palmitoleic, oleic, and eicosenoic) may offer the best compromise for these 2 sectors. Subjective fat color scores were not different among treatments, and the objective measurement did detect a whiter fat color for pigs fed the barley diet. Pigs with a brighter white color of fat are desired more for export markets. Although the barley-fed pigs did have an improved objective fat color score over the corn-fed pigs, all treatments had a highly desirable fat color score that would be acceptable for most markets.

	IMPLICATIONS

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

 
Results of this study suggest that loins from pigs fed diets containing barley or white corn as the primary energy source do not have an advantage in meat quality over loins from pigs fed yellow corn diets. Barley-fed pigs had an improved objective fat color score when compared with pigs fed the corn diets, but all dietary treatments resulted in highly desirable fat color scores that would be acceptable for most markets. Consequently, there is little reason for producers to change grain sources to attempt to improve meat and fat quality.

	Footnotes

 
¹ This journal paper of the Iowa Agric. and Home Econ. Exp. Stn. (Ames, IA), Project No. 3614, was supported by Hatch Act and State of Iowa funds.

² Corresponding author: tjbaas{at}iastate.edu

Received for publication February 11, 2005. Accepted for publication November 29, 2005.

	LITERATURE CITED

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

 


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