J. Anim Sci. 2008. 86:923-935. doi:10.2527/jas.2007-0244
© 2008 American Society of Animal Science
Impact of beef cattle diets containing corn or sorghum distillers grains on beef color, fatty acid profiles, and sensory attributes1,2
R. K. Gill*,
D. L. VanOverbeke
,3,
B. Depenbusch
,
J. S. Drouillard and
A. DiCostanzo*
* Department of Animal Science, University of Minnesota, St. Paul 55108-6111;
and
Department of Animal Science, Oklahoma State University, Stillwater 74078; and and
Department of Animal Sciences and Industry, Kansas State University, Manhattan 66506
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Abstract
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Strip loins from 236 carcasses from crossbred yearling steers were collected on each of 2 slaughter dates (slaughter 1 or 2) to determine the effects of feeding corn or sorghum distillers grains (DG) on beef color, fatty acid profiles, lipid oxidation, tenderness, and sensory attributes. Dietary treatments consisted of a steam-flaked corn (SFC) diet without (control) or with 15% (DM basis) corn dry or wet DG (CDDG and CWDG) or sorghum dry or wet DG (SDDG and SWDG) and alfalfa hay (R). Additional treatments included SDDG or SWDG with no alfalfa hay (NR). In slaughter 2, steaks from steers fed SFC had lesser L*, but greater a* (P < 0.05) values than those from steers fed DG. When comparing sorghum and corn DG steaks, the same color differences were detected. Steaks from steers fed sorghum DG had lower L*, but greater a* (P < 0.05) values than those from steers fed corn DG. Also, L* values in steaks from steers fed SWDG with R were greater (P < 0.05) than those from steers fed SWDG with NR. In slaughter 1, feeding DG increased (P < 0.05) steak n-6 fatty acid concentrations compared with SFC. In both slaughter groups, feeding dry DG increased (P < 0.05) steak linoleic acid concentrations compared with wet DG. In slaughter 2, feeding corn DG diets increased (P < 0.05) linoleic acid concentrations of steaks compared with sorghum DG diets. In addition, increased (P < 0.05) concentrations of 
-linolenic acid in steaks resulted from feeding SDDG or SWDG with R compared with those sorghum treatments with NR. In each slaughter group, feeding DG increased (P < 0.05) the n-6:n-3 ratio of steaks compared with SFC, and feeding corn DG increased (P < 0.05) this ratio compared with sorghum DG. Furthermore, steaks from steers fed corn DG had greater (P < 0.05) concentrations of trans-vaccenic acid than those from steers fed sorghum DG. In slaughter 1, the CLA isomer 18:2, trans-10, cis-12 was greater (P < 0.05) in steaks from DG diets. On d 1 of retail display, steaks from steers fed SDDG with R in slaughter 2 had greater (P < 0.05) thiobarbituric acid reactive substances values than those from steers fed SDDG with NR. Feeding DG at 15% of the dietary DM did not affect sensory attributes or Warner-Bratzler shear force values of steaks. Feeding DG from either corn or sorghum as either a wet or dry by-product had no effect on beef sensory attributes.
Key Words: beef • distillers grain • meat quality • sensory trait
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INTRODUCTION
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Cattle feeding studies have demonstrated that performance is not compromised when conventional feeds such as corn or sorghum are replaced with moderate inclusion levels (20 to 40%) of dry corn milling by-products (Lodge et al., 1997
). A study conducted by Al-Suwaiegh et al. (2002) documented that steers fed either corn or sorghum wet distillers grains (DG), fed at 30% of the ration DM, had increased efficiency of gain and increased HCW, fat thickness, and USDA yield grade compared with those steers fed dry-rolled corn. Performance data on the use of DG in feedlot diets is an ongoing research effort, but limited data exist on the impact of feeding DG on meat quality characteristics and sensory traits. Distillers grains contain tocopherols (a known antioxdative agent; Gordon et al., 2002) and the yellow pigment xanthophyll (Roberson et al., 2005), which may enhance or preserve certain color attributes. Previous work with DG (Roeber et al., 2005) or condensed distillers solubles (a component of DG; Dahlen et al., 2005) revealed that feeding steers either product led to steaks that were more red (greater a* values) during retail display compared with those from steers fed the corn-based control diet. Steaks from steers fed wheat- (Shand et al., 1998) or corn-based (Roeber et al., 2005) DG were similar in sensory traits and shear force values than from those steers fed brewers grains, barley, or corn. Quality attributes related to visual appearance and eating experience of fresh beef impact consumers’ purchasing decisions (Killinger et al., 2004).
Therefore, the following experiment was conducted to determine the impact of feeding corn or sorghum DG on beef quality, fatty acid profiles, lipid-oxidation, and sensory attributes. This experiment is a companion installment to the evaluation of corn and sorghum DG on performance of feedlot cattle, conducted by Kansas State University (Depenbusch et al., 2005).
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MATERIALS AND METHODS
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Animal care, feeding, and management procedures were approved by the Kansas State University Animal Care and Use Committee.
Dietary Treatments
Two hundred ninety-nine crossbred yearling steers were fed at Kansas State University to evaluate animal performance in response to feeding corn dry or wet distillers grains (CDDG and CWDG) or sorghum dry or wet distillers grains (SDDG and SWDG) with solubles. The treatments evaluated (7 pens/treatment with 5, 6, or 7 steers/pen) consisted of steam-flaked corn (SFC; control) or SFC with a 15% inclusion of CDDG, CWDG, SDDG, or SWDG with 6% afalfa hay (R) on a DM basis or SFC with a 15% inclusion of SDDG or SWDG with no alfalfa hay (NR; Table 1) on a DM basis.
Strip Loin Collection and Sample Preparation
Before slaughter, 4, 5, or 6 steers were randomly selected from each pen containing 5, 6, or 7 steers/pen, respectively, by using a random number generator. Four steers that were not randomly selected via the random number generator were used to replace steers that were picked in cases where carcasses were dark cutters, held for regrading, subjected to excess trimming, or those that graded USDA Prime or Standard. Strip loins were fabricated according to North American Meat Processors specifications (NAMP, 1997). Strip loins (NAMP, #180) were collected from 236 of the 299 carcasses during 2 slaughter dates (slaughter 1 conducted on December 8, 2004, n = 138; and slaughter 2 conducted on January 4, 2005, n = 98) resulting from 2 blocks formed by BW at the beginning of the feedlot performance portion of the study. Slaughtering cattle in 2 blocks to control BW variation led to different transit conditions from the feedlot to the abattoir (182 km). Weather on December 8, 2004, was clear and dry with a high temperature of 13° C, whereas weather on January 4, 2005, was overcast with a freezing rain-snow mix and a high temperature of – 1° C. Strip loins were individually identified using carcass identification tags cross-referenced to animal identification tags during slaughter. Tagged strip loins were followed through fabrication and vacuum-packaged at a commercial abattoir (Tyson Inc., Emporia, KS).
Strip loins were maintained at 2° C during transport to the University of Minnesota Meat Lab where they were unloaded, inspected for vacuum seal, repackaged if necessary, and aged for 11 d postmortem at 2° C before further evaluation of quality and sensory attributes, lipid content analysis, and lipid oxidation. After aging, the cranial end of the strip loin was faced, and two 100-g samples from each strip face were obtained, vacuum-packaged, and frozen at – 20° C for subsequent fatty acid profiling (Sukhija and Palmquist, 1988) and predisplay thiobarbituric acid reactive substance (TBAR) analysis (Witte et al., 1970), which were performed at Kansas State University. The cranial 2.54-cm-thick steak was cut and prepared for simulated retail display. The remaining portion of the strip loin was vacuum-packaged and frozen at – 20° C for subsequent shear force and taste panel analysis. At the end of retail display, a 100-g sample was taken from each steak for postdisplay TBAR analysis in the same manner as described previously. Three, 2.54-cm thick, frozen steaks were cut from the cranial end of each strip loin, vacuum-packaged, and placed back in the freezer (– 20° C) until further analysis. The most cranial steak of the 3 steaks cut from the frozen strip loin was used for Warner-Bratzler shear force (WBSF) analysis, and the remaining 2 steaks were used for consumer panel sensory evaluation.
Simulated Retail Display
Each steak was placed on a Styrofoam tray, overw-rapped with a polyvinyl chloride film, and then placed on a table in a cooler maintained at 4° C ± 1° C. Lighting conditions were in accordance with those of Hunt et al. (1991). To simulate retail display, steaks were exposed to continuous 807 to 1,614 lx of deluxe, warm-white, fluorescent lighting (bulb type F32T8/TL741, Phillips Inc., Somerset, NJ). Beginning at 0 h under display conditions, each steak was objectively evaluated. At 6 h of retail display, the steaks were objectively and subjectively evaluated for color attributes every 12 h for 7 d. Steaks were randomly rotated every 24 h to be exposed to all possible light angles and intensities.
Objective Color Evaluation.
The color of each steak was measured using a HunterLab Miniscan spectrophotometer equipped with a 6-mm aperture (HunterLab Associates Inc., Reston, VA) to determine color coordinate values for L* (brightness, 0 = black and 100 = white), a* (redness/greenness, positive values = red and negative values = green), and b* (yellowness/blueness, positive values = yellow and negative values = blue) following the procedures of the Commission Internationale de I’Eclairage (CIE, 1976). Readings for each of the L*, a*, b* values were taken at 3 spots on the surface of the steak exposed to the light; readings were averaged for each steak at each time of evaluation.
Subjective Color Evaluation.
A 4-person, trained panel consisting of University of Minnesota personnel including experienced meat scientists subjectively evaluated steak color under retail display. Evaluators assigned scores to each steak for muscle color, overall color, and surface discoloration at each evaluation time. Muscle color (oxygenated pigment) was characterized on an 8-point scale (1 = extremely dark red; 8 = extremely bright cherry red) as outlined by Hunt et al. (1991). Scores for overall color (1 = extremely undesirable; 8 = extremely desirable) and surface discoloration [1 = complete (76 to 100%) discoloration; 8 = no (0%) discoloration] were also assigned according to the procedures of Hunt et al. (1991). Subjective color evaluation was terminated when at least 80% of the steaks received a mean overall appearance score of 3 (moderately undesirable) or lower.
Tenderness Determination
Tenderness was measured on a steak from each strip loin using the WBSF instrument (G-R Elec. Mfg. Co., Manhattan, KS). Steaks were thawed for 24 h at 2° C, then cooked using electric clamshell-type grills (model GGR88DK, Salton Inc., Lake Forest, IL) to a final internal temperature of 70° C, as measured by a thermocouple (Type T thermocouple, Omega Engineering, Stanford, OH). After reaching the desired internal temperature, each steak was cooled to room temperature. When the steaks reached room temperature, six to ten 1.27-cm cores were removed from each steak parallel to the muscle fiber by using an automated coring device (Craftsman 9-in drill press, model 137.219090, Sears, Roebuck and Co., Hoffman Estates, IL). A single, peak shear force measurement was obtained for each core. Peak shear force measurements from the cores collected from each steak were averaged to determine the average shear force value for the steak.
Palatability Determination
Procedures utilizing human subjects for consumer panel evaluation of sensory attributes were approved by the University of Minnesota Institutional Review Board. Ninety-six consumers (24 consumers during each of 4 sessions) were recruited by the University of Minnesota’s Food Science and Nutrition Sensory Center. Sessions were arranged so that each dietary treatment and slaughter date were represented in each session. Steaks were thawed for 24 h at 2° C and cooked in the same manner as described for WBSF. When steaks were removed from the grill, cubes of approximately 1.3 cm x 1.3 cm x 2.5 cm were cut and served to the panelists for evaluation. Panelists were asked to taste a total of 16 samples, representing 2 steaks from each treatment (9 or 10 samples from slaughter 1; 7 or 6 samples from slaughter 2), with a break after evaluating 8 samples. Within a session, each steak was evaluated by at least 6 panelists. Demographic information was also collected from each panelist to characterize the panel population.
Panelists were asked to evaluate tenderness, juiciness, and flavor of the steaks. Distilled water and unsalted crackers were used by the panelists to cleanse their palates between samples. Samples were evaluated using a labeled affective magnitude scale. A mark was placed anywhere on the scale that appropriately described the panelist’s liking of tenderness, flavor, and juiciness (0 = greatest imaginable dislike, 120 = greatest imaginable like). Additionally, panelists were asked if they were satisfied with the overall eating quality of the steak sample (yes or no). When unpleased with a sample, panelists were further asked to indicate which sensory attribute(s) they were unpleased with (i.e., tenderness, juiciness, flavor, or other).
Statistical Analysis
Statistical analysis for retail display, fatty acid profiles, TBAR concentrations, WBSF, and palatability were performed using PROC MIXED (SAS Inst. Inc., Cary, NC). Restricted maximum likelihood was used to estimate the variance components, and the Kenward-Rogers method was used to compute degrees of freedom as the experiment contained missing data points (Searle et al., 1992). For those variables having a significant (P < 0.05) random effect of pen within treatment, pen was used as the experimental unit. Individual data points were used as the experimental unit for those variables in which the random effect of pen within treatment was not significant (P > 0.05). For all dependent variables, the model included the fixed effect of treatment and the block effect of slaughter date. With the exception of shear force and taste panel data, slaughter date was significant (P < 0.05), so the data are presented by slaughter date; shear force and taste panel data were pooled across slaughter date. In addition, for repeated-measures data (retail display, subjective and objective), the model included the fixed effects of time of determination and the interaction between dietary treatment and time. For all retail display variables, each variable analyzed was subjected to 6 covariance structures: compound symmetric, autoregressive of order one [AR(1)], heterogeneous first-order autoregressive [ARH(1)], toeplitz, heterogeneous toeplitz, and ante-dependance of order one [ANTE(1)]. The covariance structure that yielded the smaller Akaike and Schwarz’s Bayesian criterion based on their – 2 res log-likelihood for retail display variables was considered to provide the best fit. In slaughter 1 objective retail display data, ANTE(1) yielded the best fit for muscle color and overall color data and AR(1) yielded the best fit for surface discoloration data. For slaughter 2, ANTE(1) provided the best fit for muscle color and overall color data and ARH(1) provided the best fit for surface discoloration data. For objective retail display data, only one covariate structure was used due to the unequal time spacing in these data sets. Thus, L*, a*, and b* data were analyzed using the covariate structure termed spatial power [SP(POW)]. Differences among treatment means were determined using nonorthogonal contrasts (Steel et al., 1997). Contrasts of interest included: 1) SFC vs. DG; 2) CDG vs. SDG; 3) wet vs. dry DG; and 4) R vs. NR. When the main contrasts were significant (P < 0.05), the following subset contrasts were analyzed: 1) CWDG vs. CDDG; 2) SWDG vs. SDDG; 3) SDDG R vs. SDDG NR; and 4) SWDG R vs. SWDG NR.
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RESULTS AND DISCUSSION
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Performance and carcass characteristics of steers from this experiment were collected, analyzed, and reported by Kansas State University personnel (Depenbusch et al., 2005).
Retail Display
Subjective Evaluation.
No treatment or treatment x time differences (P > 0.05) in visual color scores during simulated retail display were detected in steaks from either slaughter date (Table 2); however, visual color scores for all dietary treatments decreased (P < 0.05) during display time for both slaughter dates (tabular data not shown). Similar results have been reported by both Dahlen et al. (2005) and Nelson et al. (2000) and were expected given that the process of lipid oxidation during time on display results in meat color deterioration.
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Table 2. Least squares means and main contrasts for visual color evaluation1 of strip loin steaks, from steers fed 15% (DM basis) corn or sorghum distillers grains diets, at 138 h of retail display by slaughter date
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Objective Evaluation.
Color coordinate values of L* (lightness), a* (redness), and b* (yellowness) data for slaughter 1 and 2 are presented in Table 3. There was no treatment x hour interaction (P > 0.05) in either slaughter date; however, all color coordinate values gradually decreased (P < 0.001) over the display period (tabular data not shown) in both slaughter 1 and 2. This gradual decrease in objective color evaluation was expected due to meat color deterioration caused by lipid oxidation. In agreement with the results from both slaughter dates, Nelson et al. (2000) observed that L*, a*, and b* values decreased as time on retail display progressed.
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Table 3. Least squares means and main contrasts for instrumental1 color analyses of strip loin steaks, from steers fed 15% (DM basis) corn or sorghum distillers grains diets, under retail display by slaughter date
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Although color differences were not visually detectable, L*, a*, and b* color differences were detected on both slaughter dates (Table 3
). In slaughter 1, steers fed corn DG had steaks with greater (P = 0.014) a* values (were more red) than those from steers fed sorghum DG diets. The opposite was true when looking at the contrast between the same diet comparison in slaughter 2; feeding corn DG diets produced steaks with lower (P < 0.05) a* values compared with sorghum DG diets. Such diverse results in a* values from slaughter date to slaughter date may be more attributable to weather conditions as discussed above, rather than to true treatment differences. Previous research has documented that preslaughter stress factors such as weather and transportation time can affect muscle color in fresh beef by depleting muscle glycogen stores (Lister, 1988; Scanga et al., 1998).
Other significant L*, a*, and b* value differences between the analyzed contrasts were solely detected in slaughter 2. In this group, steaks from the SFC diet had lower (P = 0.023) L* (were darker), greater (P = 0.013) a* (were more red), and greater (P = 0.04) b* values (were more yellow) across the period of retail display than steaks from diets containing DG. Earlier research conducted by Boles et al. (2004) found that steers fed corn-based diets tended to have steaks that were darker (lower L* values; P = 0.08) than those steers fed diets that were formulated with various barley varieties. In contrast, Nelson et al. (2000) detected no L*, a*, or b* value differences due to dietary treatment when steers were fed either corn- or barley-based diets with varying concentrations of potato by-product inclusion.
Comparisons of DG diets in slaughter 2 revealed that steers fed diets containing corn DG produced steaks with greater (P < 0.001) L*, lower (P = 0.036) a*, and lower (P = 0.017) b* values than those of steers fed sorghum DG. Values for b* were greater (P = 0.038) in steaks from steers fed wet DG than those from steers fed dry DG. Specifically, SWDG diets produced steaks with greater (P = 0.025) b* values (SWDG R = 13.06, SWDG NR = 12.96) than those from steers fed SDDG diets (SDDG R = 12.91, SSDG NR = 12.60). Likewise, Roeber et al. (2005) found greater (P < 0.05) b* values in steaks from steers fed CWDG at 12.5 or 25% of the diet DM than those in steaks from steers fed the control or 25% CDDG.
Feeding roughage with sorghum DG diets affected color with respect to L* values. Steaks from steers fed sorghum DG diets with R had greater L* values (P < 0.001) than those from steers fed sorghum DG with NR. Further, steaks from steers fed SWDG with R (35.25) had greater (P < 0.001) L* values than steaks from steers fed SWDG with NR (33.76). The addition of alfalfa forage to the SDDG diet may have influenced this variation in L* color difference. For example, fat color of lamb from lambs finished on high-forage diets had greater L* values (P < 0.001) than meat from those finished on concentrate or whole barley diets (Caneque et al., 2003).
The color of fresh beef is visually appraised at the retail counter and is an important selection criterion for consumers. A visual evaluation score of 3 (moderately unacceptable) is typically reflective of the time at which the price of fresh meat is discounted at the retail counter. In addition, visual appearance of fresh beef can be moderately to highly correlated (r = 0.42 to 0.86) to CIE a* values (Zerby et al., 1999). The use of DG in the current experiment produced steaks that were brighter (greater L* values), but less red (lower a* values). In contrast, at 138 h of retail display, Roeber et al. (2005) found a quadratic response in a* values in steaks from steers fed corn DG. Red color (a* values) of steaks was enhanced when steers were fed diets that contained corn DG at 10 or 25% of the diet DM compared with steaks from steers fed corn grain-based diets; however, red color was negatively affected when corn DG were included at 20 or 40% of the diet DM (Roeber et al., 2005).
Fatty Acid Composition
Overall, the proportions of total fatty acids in fresh steaks from either slaughter group were not affected by dietary treatment (slaughter 1 P = 0.650; slaughter 2 P = 0.783; Tables 4 through 7). However, when fatty acids were grouped according to their structure and function, differences were noted on both slaughter dates.
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Table 4. Least squares means and main contrasts for proportions of saturated and monosaturated fatty acids in raw strip loin steak samples from steers fed 15% (DM basis) corn or sorghum distillers grains diets (harvest 1, n = 138 steaks)
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Table 5. Least squares means and main contrasts for proportions of saturated and monosaturated fatty acids in raw strip loin steak samples from steers fed 15% (DM basis) corn or sorghum distillers grains diets (harvest 2, n = 98 steaks)
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Table 6. Least squares means and main contrasts for proportions of CLA, total fatty acids (% of sample), and total fatty acids concentrations found in raw strip loin steaks from steers fed 15% (DM basis) corn or sorghum distillers grains diets (harvest 1, n = 138 steaks)
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Table 7. Least squares means and main contrasts for proportions of CLA, total fatty acids (% of sample), and total fatty acids concentrations found in raw strip loin steaks from steers fed 15% (DM basis) corn or sorghum distillers grains diets (harvest 2, n = 98 steaks)
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Saturated Fatty Acids.
There were no differences (P > 0.05) in total SFA concentrations among treatment groups for either slaughter date; however, differences in individual SFA were detected in slaughter 1. Greater proportions of margaric (P = 0.014; 17:0) and stearic (P = 0.012; 18:0) acid were found in steaks from steers fed DG than from those from steers fed the SFC diet (Table 4). Increased concentrations of margaric and stearic acid are not a major concern because they do not aid in increasing human plasma cholesterol concentrations (Baghurst, 2004). Even though saturated fat in beef contains approximately 40% stearic acid (Baghurst, 2004), it is lauric (12:0), myristic (14:0), and palmitic (16:0) acids that are the primary SFA responsible for increasing plasma low-density lipoprotein and total cholesterol concentrations in the body (Hegsted et al., 1965). There were no individual SFA differences observed in slaughter 2 (Table 5).
n-6 and n-3.
Type of diet influenced PUFA content. In slaughter 1, steaks from steers fed diets containing DG had greater n-6 fatty acid concentrations (P < 0.001), in the form of linoleic acid, than steaks from steers fed the SFC diet (Table 6). Schingoethe et al. (1999) determined that a 31.2% inclusion of CWDG to a corn silage-based diet provided an additional 2 percentage units of fatty acids to the diet, due mainly to increases in 18:1 and 18:2 PUFA. In slaughter 2, feeding corn DG treatments led to steaks that had greater concentrations (P = 0.015) of n-6 fatty acids as well as greater (P = 0.014) concentrations of linoleic acid than steaks from sorghum DG treatments (Table 7). These combined factors are likely the reason for increased (P = 0.020) concentrations of PUFA in steaks from corn DG than those from sorghum DG. These differences resulted mainly from differences in PUFA concentrations of the n-6 and n-3 fatty acid series. Fatty acid concentration of the lipid fraction in DG may account for this observation. The fatty acid composition of corn oil is abundant in 18:2n-6 (linoleic acid), which belongs to the n-6 fatty acid series (Porsgaard and Hoy, 2000). In previous research, Brandt et al. (1992) found linoleic acid concentrations in steaks of corn-fed steers to be greater than those in steaks of sorghum-fed steers. The observation by Brandt et al. (1992) was a direct result of greater (P < 0.05) concentrations of linoleic acid found in steam-flaked corn than in steam-flaked sorghum diets. In slaughter 1, steaks from steers fed wet DG had greater (P = 0.015) proportions of n-3 fatty acids compared with those steaks from steers fed dry DG diets. In both slaughter 1 and 2, steaks from steers fed wet DG had lesser (P = 0.002 slaughter 1; P = 0.015 slaughter 2) proportions of n-6 fatty acids compared with steers fed dry DG diets. These findings may be attributed to the fact that within wet DG treatments, steaks from steers fed CWDG in slaughter 1 had lesser (P < 0.05) concentrations of linoleic acid than those from steers fed CDDG, and for both slaughter dates the same was true for SWDG. Steaks from steers fed SWDG had lesser (P < 0.05) linoleic acid content than those from steers fed SDDG. In slaughter 2, steers fed SDDG had steaks that were greater (P < 0.05) in 18:3n-3 (-linolenic acid) compared with those fed SWDG. Additionally, in slaughter 2, steaks from steers fed R diets had greater (P = 0.021) concentrations of -linolenic acid than those from steers fed NR diets. Lack of alfalfa hay in the NR diet was likely the cause for reduced -linolenic acid concentrations in steaks from this treatment group. French et al. (2000) reported that forages contain high concentrations of -linolenic acid, and it has also been documented that muscle tissue from steers fed grass-based diets typically have greater n-3 fatty acid concentrations compared with steers fed high-concentrate diets (Enser et al., 1998).
The n-6 and n-3 fatty acids are associated with having beneficial effects on human health. Linoleic acid is widely recognized as the primary n-6 fatty acid, and -linolenic acid is the major n-3 fatty acid found in feed ingredients of cattle diets. Linoleic acid is needed to synthesize proinflammatory eicosanoids (Akoh and Min, 2002), and increasing linoleic acid content in human diets has been found to lower blood cholesterol concentration and reduce the risk of coronary heart disease (Zock and Katan, 1998). Although n-6 fatty acids have positive health benefits, human diets today are high in n-6 fatty acids and are consequently not sufficient in n-3 fatty acids. The latter are of importance because they moderate inflammation by competing with n-6 fatty acids (MacRae et al., 2005). It is believed that in order for these dietary fats to be beneficial to human health, diets need to contain a proper ratio of n-6 to n-3 (Simopoulos, 1991). Kris-Etherton et al. (2000) reported that the average n-6:n-3 ratio consumed in human diets is around 10.6:1, which is still much greater than the recommended ratio of 2.3:1. Ratios of n-6 to n-3 calculated using LS means in this study (Tables 6 and 7) indicated that steaks from steers fed DG diets had significantly greater (P < 0.05) n-6:n-3 ratios compared with those from steers fed the control diet.
Contrasts comparing DG diets demonstrated that steaks from steers fed corn DG had greater n-6:n-3 ratios (P < 0.05) than those from steers fed sorghum DG, and steaks from steers fed dry DG had greater n-6:n-3 ratios (P < 0.05) than those from steers fed wet DG. Even though the n-6:n-3 ratio improved with some dietary treatments, the n-6:n-3 ratio for all diets was well over the recommended values [from 7.53:1 ± 0.51 (SFC) to 7.26:1 ± 0.51 (SWDG)]. High n-6:n-3 ratios found in this study were largely the result of feeding high-concentrate diets. Beef muscle in steers fed a concentrate diet had greater concentrations of n-6 fatty acids, whereas beef muscle in steers fed a grass diet had greater n-3 fatty acids (Enser et al., 1998; Muir et al., 1998). In a more recent study, Noci et al. (2005) documented that steers fed increasing concentrations of grass had a linear decrease (P < 0.001) in the n-6:n-3 ratio of muscle fat from 2.00 to 1.32 due to the greater concentrations of -linolenic in grass compared with that of concentrate. Realistically, improving the n-6:n-3 ratio in feedlot cattle is difficult given that concentrates are naturally high in n-6 fatty acids.
Conjugate Linoleic Acid.
Trans-vaccenic acid (18:1, trans-11) is a common intermediate produced from bio-hydrogenation of linoleic acid. Following biohydrogenation, trans-vaccenic acid is either reduced to stearic acid or is used for CLA synthesis. Conjugated linoleic acid synthesis by trans-vaccenic acid occurs through either incomplete biohydrogenation or it is used as a substrate by 
9-desaturase to produce the CLA isomer 18:2 cis-9, trans-11 (Griinari et al., 2000), which is the most abundant isomer in meat and milk products (Mosley et al., 2002). Trans-vaccenic acid is also an intermediate of -linolenic acid, which is used to synthesize 18:2 cis-9, trans-11 via endogenous synthesis and not through incomplete biohydrogention (Lock and Garnsworthy, 2002). For both slaughter dates in the current study, trans-vaccenic acid was more prevalent (P < 0.05) in steaks from steers fed corn DG than in those of steers fed sorghum DG. The increase of trans-vaccenic acid (Tables 4 and 5) in steaks from steers fed corn DG could be because steaks from steers fed corn DG diets had greater concentrations (P < 0.05) of linoleic acid compared with steaks from steers fed sorghum DG diets (Tables 6 and 7). Total concentrations of CLA in slaughter 1 (Table 6) were greater (P < 0.05) in steaks from steers fed DG diets than those from steers fed the SFC diet. In addition, of the 4 CLA isomers analyzed in this study (Tables 6 and 7), steaks from steers fed DG in slaughter 1 had increased concentrations (P < 0.05) of 18:2 trans-10, cis-12. Although this is not the main linoleic isomer found in meat, this particular CLA has potential for beneficial health impacts because it has been shown to hinder obesity by inhibiting lipogenesis (McGuire and McGuire, 2000).
Thiobarbituric Acid Reactive Substances
Dietary treatment did not have any effect on lipid oxidation as indicated by the TBAR concentrations in steaks from slaughter 1 (Table 8). In slaughter 2, greater lipid oxidation (P < 0.05) occurred predisplay in steaks from steers fed SDDG with R than those from steers fed SDDG with NR. When comparing dry and wet DG, CDDG had steaks with greater amounts (P < 0.05) of lipid oxidation compared with those from steers fed CWDG diets. Oxidation of lipids is one of the primary culprits of quality deterioration in meat (Gray et al., 1996). Lipid oxidation affects fatty acids, particularly PUFA (Johns et al., 1989). Rhee et al. (1988) found that TBAR concentrations in frozen ground pork increased when muscle linolenic acid concentrations were elevated. In both slaughter dates, it was observed that steaks from dry DG treatments had greater concentrations (P = 0.007 slaughter 1; P = 0.015 slaughter 2) of PUFA than steaks from wet DG treatments. Thus, increased PUFA concentration in steaks from steers fed dry DG may have subjected them to greater lipid oxidation.
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Table 8. Least squares means and main contrasts of thiobarbituric acid reactive substances (TBAR)1 measured pre- and postretail color display in strip loin steaks from steers fed 15% (DM basis) corn or sorghum distillers grains diets, by slaughter date
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Taste Panel Evaluation
Demographic analysis of panelist responses indicated that they all consumed beef at least once on a weekly basis. Age of consumers was skewed to the younger age bracket (20 to 29 yr old) and the majority of consumers had a household size of 1 or 2 occupants. These demographics are likely due to the fact that 44.8% of consumers indicated they were students. Yearly individual income was skewed to the lower (under $20,000) and upper (over $60,000) limits, and the majority of the panelists were Caucasian (81.3%) and female (62.5%).
Although no differences were found among treatments with respect to juiciness or flavor (Table 9), a difference was identified in tenderness when comparing steaks from corn vs. sorghum diets. Steaks from corn DG diets were preferred over steaks from sorghum DG diets (P = 0.033, Table 9). Experiments conducted with different types of DG by-products have revealed similar results. Experiments in Canada indicated that steaks from steers fed wheat-based distillers grains were not different in flavor, off-flavor intensity, or amount of connective tissue compared with steaks from steers fed wet brewers grains or barley (Shand et al., 1998). Dahlen et al. (2005) compared steaks from steers fed a combination of condensed distillers solubles (a dry milling co-product that includes soluble protein and fat) and barley by-product with those fed wet corn gluten feed and reported that neither flavor, juiciness, connective tissue, nor off-flavor intensity was influenced by treatment. Also, Brandt et al. (1992) found no differences in flavor, juiciness, off-flavor, myofibrillar tenderness, connective tissue amount, or overall tenderness when comparing steam-flaked corn to steam-flaked sorghum.
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Table 9. Pooled least squares means and main contrasts for sensory characteristics and shear force values of strip loin steaks from steers fed 15% (DM basis) corn or sorghum distillers grains diets
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In the current study, 33.62% of panelists indicated that they were unpleased with the eating quality of the steak samples (Table 10). Overall, panelists were more unpleased (P < 0.05) with steaks from steers fed DG (34.75%) than with steaks from steers fed the control treatment (25.65%). Panelists’ reasons for being unpleased with tenderness, flavor, or other traits were not (P > 0.05) related to dietary treatment (Table 10).
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Table 10. Percentages and main contrasts for panelists that were unpleased with strip loin steak samples, from steers fed 15% (DM basis) corn or sorghum distillers grains diets, and reasons for being displeased
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Shear Force Evaluation
Tenderness differences identified from taste panel results were not supported by WBSF, because no differences were identified among treatments (Table 9). Similar WBSF results were documented by Roeber et al. (2005) and Koger et al. (2004) when evaluating varying inclusions of WDG or DDG. Brandt et al. (1992) also failed to detect differences in WBSF values of steaks from steers fed steam-flaked corn or steam-flaked sorghum. The results of the WBSF in the current study were confirmed by the consumer panel, which indicated that no treatments were found to have objectionable tenderness. Reports for consumer threshold for acceptability in tenderness measured by WBSF vary greatly. Shackelford et al. (1991) documented that the US consumer threshold for "slightly tender" in retail food service ranged between 3.9 and 4.6 kg of shear force. In other research, some consumers have found beef with WBSF values of 2.3 kg to be unacceptable whereas other consumers have found beef with WBSF values of 4.5 kg to be acceptable (Savell et al., 1987). In the current study, the average WBSF across all dietary treatments was 3.15 ± 0.13 kg, which falls below US consumer threshold levels as documented by Shackelford et al. (1991).
Feeding corn or sorghum DG as either a wet or dry product at 15% of the diet DM had no effect on beef sensory attributes. Effects of feeding DG either from corn or sorghum on color coordinates were dependent on slaughter date, the implications of which may be well beyond the control of feedyard or abattoir managers. Feeding DG from sorghum enhanced the ratio of n-6:n-3 fatty acids, but not sufficiently to reach ratios recommended for beneficial effects on human health; the upper limits of this enhancement may necessitate further testing with additional DG inclusion, DG co-product refinement, or both.
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Footnotes
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1 Funding for this project was provided by the University of Minnesota Graduate College Grant in Aid Program. 
2 Strip loins for this project were obtained from cattle fed at Kansas State University.
3 Corresponding author: deb.vanoverbeke{at}okstate.edu
Received for publication April 27, 2007.
Accepted for publication January 4, 2008.
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Abstract
INTRODUCTION
