1. Introduction
The use of agro-industry by-products as cattle feed could be effective in reducing both the competition between food and feed, and the environmental cost of beef production [
1]. In the Pacific Northwest region, grape pomace (GP) is one such by-product that has appeal as cattle feed due to its ease of availability and lower cost relative to traditional feed ingredients including corn grain. Besides providing essential nutrients including fiber and fat, grape pomace also contains polyphenolic compounds such as proanthocyanidins (condensed tannins; CT) that include catechin, epicatechin, and procyanidins, which could potentially enhance beef quality. However, to the best of our knowledge, the impact of feeding grape pomace to finishing cattle on product quality and composition remains to be fully evaluated.
Meat attributes including marbling, color, and appearance are key determinants of consumer acceptability of beef. However, the high degree of marbling that is currently desired as it enhances the eating experience, also increases the probability of the occurrence of lipoperoxidation, which compromises sensory properties [
2]. Therefore, limiting or delaying lipid oxidation is vital as it enhances quality by reducing sensory spoilage, including meat discoloration and the development of rancid flavors and odors [
3]. Because of their antioxidant properties, increased intestinal absorption and transfer of polyphenolic compounds into meat when GP is fed to finishing cattle could potentially enhance oxidative stability of beef products [
4]. In studies with finishing lambs [
5,
6], feeding up to 2.5% (DM basis) grape seed extract either reduced or prevented the accumulation of volatile lipo-oxidation compounds such as heptanal, 2-nonenal, 2-propanone and dimethylsulfide in
longissimus muscle, suggestive of increased lipid stability. Moreover, dietary inclusion of 2.5% (DM basis) grape seed extract in finishing lamb diets reduced lipid oxidation [−15.5%; mg malondialdehyde (MDA)/kg] in
longissimus muscle over 7 d of simulated retail display [
7]. However, the effects of feeding grape pomace to finishing cattle on the sensory properties including lipid oxidation and color stability of beef remain to be fully evaluated.
In addition to sensory quality, healthiness of beef is increasingly an important attribute that shapes consumer acceptability [
8]. Although beef is a good source of various nutrients including iron and zinc, there are concerns about the saturated fatty acid (SFA), including palmitic (16:0) acid it contains, whose consumption in high amounts has been linked to increased risk for chronic diseases, including cancer and cardiovascular disease (CVD; [
9]). Therefore, there is continued interest in the use of dietary strategies to reduce the concentration of the undesirable SFA and/or increase the accumulation of the desirable polyunsaturated fatty acids (PUFA) including linoleic (18:2
n-6) and α-linolenic (18:3
n-3) acids in meat. When fed to ruminants, polyphenolic compounds such as CT can reduce microbial biohydrogenation of dietary PUFA [
10,
11]. This in turn increases the ruminal accumulation and subsequent transfer into meat of the desirable dietary PUFA (e.g., 18:2
n-6, 18:3
n-3) and biohydrogenation intermediates (e.g., 18:2
c9
t11) at the expense of the undesirable fatty acids (e.g., 18:0 and 18:2
t10
c12). Feeding grape pomace to dairy ewes and cows increased the milk content of 18:2
n-6, 18:1
t-11, total PUFA, and 18:2
c9
t11, and also reduced the content of the undesirable SFA [
12,
13,
14]. However, there is limited information on the impact of feeding grape pomace in finishing cattle on the FA profile of beef. Therefore, the objective of the current study was to evaluate the effect of feeding a high amount of grape pomace to finishing cattle on meat quality (e.g., color, and shelf life) and the FA composition of beef. We hypothesized that dietary inclusion of a high amount of grape pomace during the finishing period increases the sensory quality of beef due to the transfer of polyphenolic compounds into meat, and enhances the health value of beef lipids due to the increased accumulation of desirable FA at the expense of the undesirable FA in meat.
4. Discussion
A key goal when formulating the experimental diets was for them to be iso-lipidic and iso-nitrogenous. This was achieved as the lipid content was 7.85 and 7.12% (DM basis), whereas the CP content was 15.5 and 16.0%, for the CON and HGP diets, respectively. However, energy supply was likely lower for the HGP than CON diet as reflected by the 63% decrease in dietary net energy of gain following the replacement of all starch sources including high-moisture corn and bakery waste with grape pomace. Grape pomace contains a high concentration of NDF and polyphenolics; therefore, this accounts for the NDF and polyphenolics content of the HGP diet being over 1-fold greater than the CON diet. There is an inverse relationship between dietary NDF content and dry matter intake [
23]. Polyphenolics can also suppress nutrient digestibility [
24]. Therefore, the greater NDF and polyphenolic content of the HGP compared to CON diet possibly compromised nutrient supply and, thus, growth performance in the present study. This is supported by the lower HCW, backfat thickness, and yield grade for HGP than CON steers. [
25] also reported a decrease in metabolizable energy intake and dry matter digestibility that compromised BW gain and feed efficiency in steers fed a diet containing 30% (DM basis) ensiled grape marc. On the other hand, feeding up to 10% GP (DM basis) did not compromise growth performance and carcass traits in lambs [
26,
27]. The lower dietary inclusion level of GP (10 vs. 30 to 58% of diet DM) possibly accounts for the discrepancy between studies. This is supported by [
28] who recently recommended a maximum inclusion level of 12% of diet DM after observing quadratic responses in DMI, ADG, and HCW when feeding increasing amounts of grape pomace in lamb diets.
The oxidation of lipids by reactive oxygen species (ROS) is considered the major non-microbial cause of meat deterioration [
29]. Therefore, the use of antioxidants to enhance oxidative stability of beef products, particularly through feeding strategies as it leads to cellular incorporation, is of great importance today. Meat content of unsaturated FA (UFA) is one of the major determinants of oxidative stability of beef products [
30]. Because the PUFA content of LL and SM steaks was 49 to 75% greater, we also anticipated greater oxidative susceptibility for HGP compared to CON steers. However, feeding the HGP diet resulted in a desirable decrease in the MDA content of both LL and SM steaks on d 4, 6, and 8 of simulated retail display. Besides FA composition, other intrinsic factors, including the meat antioxidant content, influence lipid oxidation [
29]. For instance, [
27] reported a polyphenolic compound-induced decrease in the LL muscle MDA and ROS content, and increase in the activity of several antioxidative enzymes, including glutathione peroxidase, superoxide dismutase and total antioxidant capacity in lambs fed grape pomace. Similarly, [
7] observed a decrease in the
longissimus MDA and ROS content in lambs fed grape seed extract. Although it is unlikely that there is direct absorption of the polyphenolic compounds along the gut given their high molecular weight, the end-products of their degradation, which have a low molecular weight, could end up absorbed and transferred into tissue [
31]. This is supported by the detection of different phenolic compounds including epicathechin and unknown phenolics in plasma of sheep dosed with a polyphenolic extract derived from several sources including grape peel and seed [
32]. [
33] also reported a 31% increase in muscle phenolic concentration following the addition of quebracho tannins to finishing lamb diets. Although not measured in the present study, feeding the HGP compared to the CON diet possibly increased the muscle phenolic content, thereby leading to the improvement in the antioxidative status. Therefore, this supports the feeding of grape pomace as a viable strategy to improve the shelf life of beef products, while also addressing consumer concerns about the use of synthetic antioxidants.
In addition to rancidity, a change in meat color due to oxidation is a major cause of consumer rejection of beef products at the point of purchase [
34,
35]. In the present study, overall lightness of LL steaks during simulated retail display was greater for HGP than CON steers. Redness, which is strongly correlated to myoglobin state, was also sustained over time suggesting a slower rate of myoglobin oxidation for HGP than CON steers possibly because of the antioxidant contribution of grape pomace. [
33] also reported greater color stability over a 7-d aerobic storage period for
longissimus muscle from quebracho tannin-fed compared to control lambs. On the other hand, feeding up to 20% grape pomace (DM basis) in lamb diets did not affect color attributes (L*, a* and b*) of
longissimus muscle 24 h post-harvest [
27,
28]. [
36] also did not observe changes in the lightness, redness and yellowness of
longissimus muscle (3 d post-harvest) from Friesian calves feed supplemental grape pomace (10% of diet DM). The lower dietary grape pomace inclusion level (up to 20%) could account for the lack of an impact compared to the present study. It is also important to note that measurement of antioxidant status and color oxidation was 1 to 3 d postmortem, which could have been before benefits of any antioxidant effect are manifested. There was no change in overall lightness or an improvement in color stability for SM steaks from steers fed a high amount of grape pomace in the present study. Although reasons for this are not clear, the water holding capacity (WHC) of SM steaks was greater for CON that HGP steers; therefore, we had anticipated a darker color that is associated with a high WHC [
35]. Browning, discoloration, and uniformity of both LL and SM steaks increased over time as expected. However, there was greater browning and discoloration of LL and SM steaks from HGP than CON steers, which was not consistent with objective color attributes. Besides differences in approach (visual vs. instrument appraisal) possibly accounting for the discrepancies, color evaluation is complex, with numerous factors including animal-to-animal variation, and inter- and intra-muscular effects influencing color perception [
35].
Besides lipids, ROS also oxidize myofibril proteins, leading to a reduction in the WHC of beef products [
29]. Therefore, the meat transfer of compounds from grape pomace including gallic acid and catechin could limit fluid loss by limiting structural changes in myofibril proteins including cross-linking and fragmentation, which compromise functionality. Although it did not result in changes in cook loss, fluid loss for LL steaks from HGP steers was 27.4% lower than from CON steers in the present study. [
26] also reported an improvement in the WHC for
longissimus muscle from lambs fed GP (up to 10% of diet DM). Therefore, although not evaluated in the present study, feeding GP could have also enhanced juiciness of LL steaks. As for SM steaks, there was an undesirable increase in both fluid and cook loss (40.7 and 32.4%, respectively) for HGP that CON steers, and the reasons for this are not clear. However, there is an inverse relationship between the rate and extent of meat pH decline and WHC, with muscle type being one of the factors that influences post-harvest pH changes [
37,
38]. Because it typically cools at a slower rate than
longissimus muscle, SM muscle is more prone to low pH [
37]. Meat pH was measured in the present study; however, it was not reported as some of the recorded data was misplaced. Therefore, it is not possible to deduce whether changes in muscle WHC were linked to pH. However, other factors, including sarcomere length, meat cooling temperature and rate, and heating rate and end point center temperatures during cooking, also influence fluid and cook losses [
39,
40].
Tenderness is one of the key palatability attributes that greatly influences the overall consumer beef eating experience [
41]. Despite the additional 27 d needed to finish the HGP compared to CON steers, there was no diet effect on WBSF for both LL (3.31 vs. 3.62 kg for CON vs. HGP steers) and SM steaks (4.39 vs. 4.74 kg). In addition, the numerical differences between treatments for both muscle types were < 0.5 kg, the threshold for perceptible differences in tenderness by the average consumer when consuming meat at home [
42]. [
26] also did not observe changes in WBSF for
longissimus muscle following the addition of GP (5 and 10% of diet DM) in ewe diets. On the other hand, [
27] observed a decrease in WBSF (LL muscle) when ram diets contained GP (5 and 10% of diet DM), and this was attributed to a proanthocyanidin-induced decrease in collagen deposition, as protein expression of TGF-β, a promoter of collagen synthesis, was suppressed. However, muscle collagen content and profibrogenic cytokine expression were not quantified in the present study, and the study by [
26].
Longissimus steaks from the present study would qualify to use the USDA AMS labeling claim “very tender” (<3.9 kg; [
43]). However, the WBSF values for SM steaks were >4.4 kg, the threshold for use of the USDA AMS labeling claim “tender” across treatments. This was expected as tenderness is typically greater for LL than SM muscle [
44].
Because of the reported associations between dietary fat consumption and health outcomes including CVD and cancer, consumers are increasingly taking into consideration the fat and FA composition of meat products when making purchasing decisions [
45,
46]. Therefore, current efforts to enhance the health value of beef lipids have focused on feeding strategies that reduce the meat content of SFA (e.g., 16:0) and specific
trans-FA (TFA; e.g., 18:1
t-10), and increase the content of specific CLA (e.g., 18:2
c9
t11) and TFA (e.g., 18:1
t-11) isomers and n-3 and n-6 PUFA (e.g., 18:3n-3 and 18:2
n-6) [
45,
46]. Although the capacity is limited, the dietary content of lipids, FA, and polyphenolic compounds are key factors that can be manipulated to enhance the FA profile of beef [
24]. In the present study, diets were formulated to be isolipidic. However, the 18:2
n-6 content for the HGP diet was 81% greater than for the CON diet. This resulted in from the contribution of GP, which is rich in 18:2
n-6 [
14]. A proportion of dietary PUFAs escapes ruminal biohydrogenation and ends up deposited in various organs and tissue. Polyphenolic compounds also limit biohydrogenation and increase ruminal escape of PUFA as they reduce the growth and activity of microbes including members of the genus
Butyrivibrio [
24]. In the current study, we observed a 58 to 80% increase in the 18:2
n-6 content of LL and SM muscle for HGP compared to CON steers. Although ruminal concentration was not measured, this suggests limited biohydrogenation of 18:2
n-6 possibly due to the greater polyphenolic content of the HGP than CON diet. Others [
14,
36] also reported a 25 to 49% increase in the 18:2
n-6 content of milk and LL muscle following the addition of 10 to 27% grape pomace (DM basis) in Friesian-Holstein cow and Friesian calf diets, respectively. This enrichment of 18:2
n-6 in muscle from finishing cattle fed grape pomace could be beneficial from a human health standpoint, as dietary replacement of SFA or total carbohydrate with 18:2
n-6 lowers the risk of CVD [
46].
Biohydrogenation of PUFA, mainly 18:2
n-6 and 18:3
n-3, results in production of monounsaturated
cis- and
trans-FA (18:1
c and 18:1
t) and CLA isomers. The muscle content of 18:1
c9, which is the most common MUFA in the human diet, did not differ across dietary treatments in the present study. Similarly, dietary inclusion of either grape seed (11% of diet; as-fed) or grape pomace (up to 10% of diet DM) in ewe or calf diets did not result in changes in the 18:1
c9 concentration of ruminal fluid [
47], milk [
13] or
longissimus muscle [
36]. The total 18:1
t content of LL and SM muscle was greater for steers fed the CON than HGP diet in the present study. This was unexpected, as polyphenolic compounds have been reported to result in an increase in the muscle content of total 18:1
t at the expense of 18:0 [
24]. The reasons for this are not clear; however, the impact of polyphenolic compounds on 18:1
t FA is isomer specific and is influenced by other factors including dietary forage:concentrate ratio. For instance, feeding supplemental quebracho powder (4% dietary tannins; DM basis) to sheep offered either a high forage or high concentrate diet had no effect on the 18:1
t-11 content of
longissimus muscle [
48]. However, the
longissimus 18:1
t-10 content was greater for sheep fed the high concentrate diet with the tannin supplement compared to without, whereas, there was no supplement effect on the high forage diet [
48]. 18:1-
t FA intake also influences health outcomes in an isomer specific manner; for instance, increased 18:1
t-11 but not 18:1
t-10 intake has been associated with enhance cardiovascular health [
49]. Because the peaks for all isomers were not resolved in the present study, it is not possible to fully deduce the potential implications of feeding the HGP diet on muscle accumulation and, ultimately, health outcomes.
The 18:0 content of LL muscle tended to be greater for HGP than CON steers, whereas there was no diet effect for SM muscle in the current study. [
47] also observed an increase in the 18:0 concentration of ruminal fluid following the addition of grape seed to ewe diets. However, [
36] did not observe changes in the 18:0 content of LL muscle from calves fed grape pomace. This lack of the anticipated decrease in ruminal fluid and muscle 18:0 content could be due to the 18:2
n-6 contribution of grape seed that increases dietary PUFA content and, thus substrate for biohydrogenation. In the present study, there was also no diet effect on total SFA and MUFA; however, feeding the HGP than CON diet led to an increase in total PUFA in both LL and SM muscle. Besides 18:2
n-6, this increase in total PUFA was in part due to the greater contribution of 18:2
c9
t11, which makes up >80% of total CLA in ruminant muscle [
50]. Others [
14,
47] also reported an increase in the 18:2
c9
t11 content of ruminal fluid and milk after adding grape pomace to diets. This is suggestive of increased intestinal absorption of 18:2
c9
t11 and possibly
de novo synthesis from 18:1
t11 in tissue, a process regulated by Δ
9—desaturase [
51]. Estimated Δ
9—desaturase 18 activity, which did not differ across dietary treatments for SM muscle, was greater for CON than HGP steer in LL muscle, and this was not consistent with the noted change in 18:2
c9
t11 content as dietary polyphenolic content increased. This contrasts with [
52] who observed increased muscle Δ
9—desaturase protein expression and total 18:1
t and 18:2
n-6 content in lambs fed tannins. However, it is not possible to determine the absolute contribution of
de novo synthesis and intestinal absorption to muscle 18:2
c9
t11 content. In addition, estimation of Δ
9—desaturase 18 activity using the ratio of product to substrate might not be as accurate as direct measurement of muscle protein expression, thereby possibly accounting for the discrepancies between studies. On the other hand, the milk and meat content of 18:2
c9
t11, 18:1
t-10 and 18:1
t-11 were reported [
13,
36] not to change when diets contained up to 10% grape pomace. This possibly was because of the low dietary inclusion level. The muscle content of 18:2
t10
c12, which makes up 5% of total CLA in ruminant muscle [
50], was lower for HGP than CON steers in the present study. Therefore, the divergent responses in muscle 18:2
c9
t11 and 18:2
t10
c12 content suggests polyphenolic-induced changes in ruminal biohydrogenation pathways that could have led to increased accumulation of 18:1
t-11 at the expense of 18:1
t-10 in muscle for HGP steers [
53]. Total muscle CLA was also greater for HGP than CON, and this could be beneficial from a human nutrition standpoint as increased intake of CLA has been suggested to improve energy and bone metabolism, immune and inflammation status, and inhibit mutagenesis, among other factors [
45]. Besides CLA, feeding the HGP compared to CON diet also resulted increased accumulation of long chain PUFA including 20:3
n-6, 20:4
n-6, and 22:5
n-3 in LL and SM muscle. Estimated elongase activity was also greater in LL muscle for steers fed the HGP than CON diet. [
13] also reported an increase in the milk content of 20:3
n-6 and 20:4
n-6 following the addition of grape pomace to dairy ewe diets. This increase is desirable as long chain PUFA like 22:5
n-3 have anti-atherosclerotic and cardio-protective properties, and enhance brain, neural and visual development and function [
45]. Therefore, given the noted changes in the muscle content of several bioactive FA including 18:2
c9
t11, total CLA and PUFA, feeding grape pomace to finishing cattle could be an effective strategy to enhance the health value of beef lipids. However, it is worthwhile noting that potential benefits ultimately are dependent on FA intake, which is not only a function of muscle fatty acid composition as reported, but also muscle fat content and total muscle/meat consumption.