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Introduction
Brazil is the largest beef exporter (USDA, 2018). Corn and sorghum grains are the main starch sources used for Brazilian steers in feedlots (Oliveira and Millen, 2014). However, both cereals have a strong protein matrix around the starch granules, limiting ruminal and intestinal digestion. This problem is aggravated in Brazil, where most corn hybrids have a hard endosperm, rich in protein matrix (Costa et al., 2014). Grain rehydration and subsequent ensiling can be used to increase starch digestibility by reducing the protein matrix integrity, providing greater access for microbes and intestinal enzymes to the starch granules (Arcari et al., 2016).
High digestibility of starch is a prerequisite for increasing glucose uptake, either indirectly, through gluconeogenesis -which uses propionate as the main substrate- or directly through glucose uptake in the small intestine (Rowe et al., 1999). Glucose is used in muscle for glycogen synthesis, which plays a key role in the decline of post-mortem muscle pH when converted to lactic acid and hydrogen protons (Volpi-Lagreca and Duckett, 2017). A muscular drop in pH is of great importance for meat quality traits, such as color, water retention capacity and tenderness (Ferguson and Gerrard, 2014). Moreover, glucose is the preferred substrate for the synthesis of intramuscular fat (Smith and Crouse, 1984; Rhoades et al., 2007; Smith et al., 2009).
Although it is well known that rehydration of cereal grains has positive effects on animal performance, to the best of our knowledge no studies have evaluated its effects on meat quality. Thus, our hypothesis was that corn and sorghum rehydration could affect beef quality. Therefore, the aim of this study was to evaluate the effects of two cereals and rehydration on meat quality of Nellore steers in a feedlot.
Materials and methods
Ethical considerations
All animal procedures were approved by the Animal Care and Use Committee of Universi dade Federal de Viçosa (CEUAP), Brazil, pro tocol number 29/2017.
Animals and treatments
Twenty-four non-castrated Nellore steers with 270 ± 53 kg initial body weight and 7 months of age were used. All animals were identified, weighed and dewormed (Ivermectin 1%, Merial, Paulínia, SP, Brazil; 1 ml per 50 kg body weight) at the beginning of the trial. The treatments were cereal grain type (corn or sorghum) and grain processing method (dry or rehydrated and ensiled), which were offered in the concentrate.
Grain rehydration and ensiling
Corn and sorghum grains were rehydrated sixty days before the beginning of the experiment. A total of 6000 kg of each grain were ground in a hammer mill through a 3-mm screen. Then, samples were collected to determine dry matter content of the ground material. Subsequently, water was added to the ground material until 35% moisture was reached, using a concrete mixer for homogenization. The ground and rehydrated grains were ensiled in horizontal concrete silos at an approximate density of 1000 kg/m³. Silos were covered with plastic canvas, and a 10 cm sand layer was placed on the canvas.
Diets and chemical analysis
The diets were isoproteic and formulated according to the BR-CORTE for 1.2 kg gain per day (Valadares Filho et al., 2010). The diets were composed of 28.44% corn silage and 71.56% concentrate (Table 1). The samples of corn silage and concentrates were analyzed for dry matter (INCT-CAG-003/1 method), ash (INCT-CA M-001/1 method), crude protein (INCT-CA N-001/1 method), ether extract (INCT-CA G-005/1 method), neutral detergent fiber (INCT-CA F-002/1 method) and non-fiber carbohydrates according to Detmann et al. (2012).
Animal management and slaughter
After 28 days of adaptation, the steers were confined for 140 days in individual covered stalls with concrete floor, water supply and mangers for feeding. The animals were fed twice a day, at 0700 and 1500 h, allowing no more than 10% orts. Silage and concentrate were mixed directly in the manger. Cattle slaughtering was carried out according to animal welfare regulations (MAPA, 2000).
Carcass temperature and pH measurements
After skinning and evisceration, the carcasses were stored in a cold room at 4 °C for 24 h. Temperature and pH were measured in the Longissimus lumborum muscle every 2 h using a potentiometer (SevenGoTM, Mettler Toledo- Schwerzenbach, Switzerland).
Collection of meat samples
Meat quality was determined in fresh and aged meat (7 days). After the cooling period, L. lumborum muscle samples were collected. Then, two one-inch thick steaks were vacuum-packed, one being immediately frozen (fresh meat) and the other aged for seven days at 4 °C and then frozen at -20 °C (aged meat).
Meat color measurement
Color of subcutaneous fat on L. lumborum muscle was determined immediately after carcass cooling. For color analyses, steak samples were thawed at 4 °C for 16 h. Before analysis, steaks were removed from the packages and exposed to oxygen for 30 min. Color readings were obtained with a Hunter MiniScan EZ spectrophotometer (4500L, Hunter Associates Laboratory, Inc., Reston, VA, USA) using illuminant D65, a 31.8 mm port size and a 10° standard observer. The L* (lightness), a* (redness) and b* (yellowness) values were calculated according to the CIELab scale and represented the means of five spectrophotometer readings at specific points on the sample surface.
Thawing, weight loss, and shear force
Thawing loss was calculated as the weight difference of steaks before and after thawing for 16 h at 4 °C. Cooking loss was the weight difference before and after cooking in a water bath at 80 °C for 60 min. Total loss was the weight difference between frozen and cooked steaks (AMSA, 2015).
Shear force measurements were performed on the same steaks used to estimate cooking losses. Five cylindrical samples 1.27 cm in diameter were collected from each steak, parallel to the orientation of the muscle fibers. The samples were sheared perpendicular to muscle fiber orientation using a V-shaped cutting blade at a 60° angle, at 1.016-mm thickness and at a fixed rate of 20 cm/min, coupled with a Warner- Bratzler machine (GR Electrical Manufacturing Company, Manhattan, KS, USA).
Sarcomere length
The sarcomere length was estimated according to the laser diffraction technique (Cross et al., 1981). Six thin filaments were removed from the thawed steak samples parallel to the orientation of the muscle fibers, which were placed separately on a glass slide. One drop of sucrose solution (0.2 M sucrose and 0.1 M NaHPO buffer at pH 7) at 4°C was placed on each filament. Then, the slides were placed in a holder where the laser (632.8 nm) was focused on the filaments using a helium-neon laser (Model 05-LHR-021, MelleGriot, Carlsbad, CA, USA). The diffraction bands were taken at 12 cm below the holder. Six diffraction bands were obtained for each sample and the mean value was used.
analysis
Data were analyzed using the Statistic Analysis System, version 9.1 (SAS Institute, Cary, NC, USA, 2003). The following statistical model was used: Y = μ + α + β + αβ + e, where μ = mean, α = effect of grain type, β = effect of processing, αβ = interaction between grain type and processing, and e = random error. The experiment was conducted in a 2×2 factorial design and animals were randomly divided into 4 groups, 6 animals in each group. The first factor was cereal grain type in concentrate (corn or sorghum), and the second one was grain processing (dry or rehydrated and ensiled). The Tukey’s test was used to compare means between treatments. Statistical differences were considered at p<0.05.
Results
There was no effect (p>0.05) of grain type, processing or interaction between these factors on the carcass pH during the 24 hours of cooling (Table 2).
Treatments did not affect (p>0.05) meat color parameters (L*, a* and b*) in both fresh or aged steaks (Table 3). However, offered corn grain diets had a higher (p=0.03) intensity of yellow (b*) in subcutaneous fat compared to those containing sorghum grain, regardless of processing type (Table 3).
There were no differences (p>0.05) between treatments for shear force, thawing, cooking and total losses in fresh or aged steaks (Table 4). Sarcomere length was higher (p=0.01) in the aged meat of steers fed sorghum, regardless of the processing method (Table 4).
There were no effects (p>0.05) of cereal grain type, their processing nor interaction between them on the chemical composition of beef (Table 5).
Table 2 Average pH of Longissimus lumborum muscle evaluated at different postmortem times in carcasses of steers fed corn (dry or rehydrated) or sorghum (dry or rehydrated) grain.
SEM = standard error of the mean.
p-value: G = effect of grain type; P = effect of grain processing; G x P = grain type by processing interaction.
Table 3 Average values of lightness (L*), redness (a*) and yellowness (*b) in the fresh or aged steaks and in the subcutaneous fat of steers fed corn (dry or rehydrated) or sorghum (dry or rehydrated) grain.
SEM = standard error of the mean.
p-value: G = effect of grain type; P = effect of grain processing; G x P = grain type by processing interaction.
Table 4 Average values of shear force, sarcomere length, thawing, cooking and total losses in fresh or aged steaks of steers fed corn (dry or rehydrated) or sorghum (dry or rehydrated) grain.
SEM = standard error of the mean.
p-value: G = effect of grain type; P = effect of grain processing; G x P = grain type by processing interaction.
Table 5 Average values of chemical composition of fresh steaks from steers fed corn (dry or rehydrated) or sorghum (dry or rehydrated) grain.
SEM = standard error of the mean.
p-value: G = effect of grain type; P = effect of grain processing; G x P = grain type and processing interaction.
The final pH values, after 24 hours of cooling, were within the ideal range (5.5 to 5.8) for beef (Ferguson and Gerrard, 2014). Although diets with rehydrated grains provide high energy density, the lack of effect of grain processing on the rate of muscle pH decline could suggest that diets did not influence the synthesis and storage of muscle glycogen. The lack of effect of dietary energy density on pH drop has already been reported in ruminant carcasses (Immonen et al., 2000; Lowe et al., 2002; Apaoblaza et al., 2017). This suggests that although dietary energy density has a strong impact on muscle glycogen synthesis (Pethick and Rowe, 1996; Frylinck et al., 2013), the relationship between muscle glycogen concentration and postmortem pH drop is not always linear since it is also dependent on the activity of enzymes involved in glycogenolysis (Apaoblaza et al., 2015). Similar to our study, the lamb carcass pH also did not differ between animals fed diets containing dry or ensiled high-moisture corn grain (Oliveira et al., 2015).
The lack of treatment effect on meat color parameters could be related to similar values of ultimate pH of carcasses in all experimental diets, which were within the ideal pH range (≤ 5.8) to obtain a normal beef color (Mahmood et al., 2017). Oliveira et al. (2015) also did not observe differences in L*, a* and b* parameters in meat of lambs fed dry or ensiled high-moisture corn.
The higher intensity of b* in the subcutaneous fat of steers fed corn diets could be due to a greater accumulation of carotenoids in these animals (Moloney et al., 2008; Rossi et al., 2016). Carotene and xanthophyll level in corn is 2.0 and 20.1 ppm, while it is 0.3 and 18.0 ppm in sorghum, respectively. These carotenoids are related to yellow pigmentation in products of animal origin (Álvarez et al., 2015). Yellow color in subcutaneous fat is an undesirable characteristic in some southern European markets, which prefer whiter fat due to an association of yellow fat to low meat quality from older animals (Dunne et al., 2006). Therefore, the use of certain feeds with low capacity for pigmentation of subcutaneous fat, such as sorghum, could be an alternative to meet the demands of these stricter markets.
The tenderness values found in all treatments are within the range considered as tender meat (below 4.6 kgf; Shackelford et al., 1991) and have been reported in other studies under similar production system, genetic group and age (Igarasi et al., 2008; Rubiano et al., 2009).
The values found among all treatments analyzed for sarcomere length are within the natural variation (1.3-2.1 μm) found in tender meat (Starkey et al., 2016). Post-mortem sarcomere length interferes with meat tenderness, because a shorter sarcomere results in increased muscle fiber diameter and reduction in the area of action of proteolytic enzymes (Ertbjerg and Puolanne, 2017). Sarcomere length was higher in aged meat for 7 days of steers fed sorghum grain, regardless of the processing method. Proteolysis is the major structural change that occurs during meat aging, and can be related to increases in sarcomere length (Takahashi, 1999). Thus, differences in the activity of proteolytic enzymes such as calpain and cathepsins could be related to differences in sarcomere length between treatments.
The lack of grain type effect on thawing, cooking and total losses have already been reported by Igarasi et al. (2008), who evaluated the meat quality of steers fed wet corn or sorghum grains. Kazama et al. (2008) also did not observe effect of different types of energy concentrates on thawing and cooking losses in beef. Huck et al. (1998) and Gorocica-Buenfil et al. (2007) also did not observe differences in meat quality characteristics of cattle fed dry or high-moisture corn grain.
Moisture and protein in meat are relatively constant (approximately 75% moisture and 19 to 25% protein), with little influence of diet (Passini et al., 2002; Lage et al., 2012; Carvalho et al., 2014). The fat content found in the Longissimus muscle is correlated with intramuscular fat deposition, thus, fat content is a reliable marker of marbling (Bindon, 2004). The fat content found in all treatments was above the minimum 3% recommended to obtain the perception of meat flavor (Baghurst, 2004).
Glucose is the main substrate used in growing intramuscular adipocytes (Smith et al., 2009; Hocquete et al., 2010). Rehydration and ensiling of grains increase the digestibility of starch in the total digestive tract (Arcari et al., 2016), indirectly providing more substrate (propionate) through gluconeogenesis or directly (glucose) for the synthesis of intramuscular fat. Thus, the lack of grain processing effect on meat fat content was not expected, and this could be due to the low capacity of intramuscular fat deposition in non-castrated males (Bong et al., 2012; Baik et al., 2014), as well as the absence of genetic predisposition of Nellore steers for deposition of intramuscular fat (Teixeira et al., 2017).
The inability to increase gluconeogenic precursors to promote changes in fat content in the L. dorsi muscle of non-castrated Nellore steers has already been shown in studies in which intramuscular fat deposition was not affected by inclusion of crude glycerin (glyconeogenic precursor) in diet (Lage et al., 2014; San Vito et al., 2015).
In conclusion, neither the processing method of the grain nor its interaction with cereal type affected meat quality characteristics of Nellore steers. Cereal grain only affected subcutaneous fat color, which was more yellow in steers fed corn, and sarcomere length of aged beef, which was higher in steers fed sorghum.
