Effect of pre-slaughter restraining method on blood loss
The fundamental objective of the meat processing industry is to increase blood loss during slaughter, as better blood loss can improve meat quality during storage. Blood loss in broiler chicks subjected to various pre-slaughter restraint procedures is presented in Fig. 1. The restraint method significantly (p < 0.05) affected blood loss as restraining the birds using the cone restraint resulted in higher blood loss than restraining the birds using a shackle restraint. The average blood loss during shackle restrain was 3.025%, while the loss following cone restrain was 3.682%. The consumption of blood is forbidden. Therefore, religious and modern slaughter requires that animals slaughtered for food be properly bled before consumption (Nakyinsige et al., 2014).
Numerous research on restraining measures on blood loss has produced contradictory findings. Our findings are congruent with Ismail et al. (2019), who discovered that the pre-slaughter restraint method substantially affects broiler chicken blood loss. According to the author (Ismail et al., 2019), birds with cone restraints lost more blood (3.26 percent) than birds with shackles (2.97 percent ). In contrast, Lambooij et al. (1999) examined the effect of two post-slaughter restraining devices, the shackle, and the cone, on bleeding efficiency in broiler chickens. Blood loss relative to body weight was significantly higher in shackled birds than cone birds. They utilised light broiler chicken (about 1.5 kg live body weight) with a bleed-out rate of around 2.68 and 2.46 percent after stunning while chained or restrained, followed by neck cutting. Although hanging may promote rapid bleeding by prompting birds in a shackle line to straighten up and flap their wings, it is established that wing-flapping occurs only for a few seconds immediately following shackling. Despite this, many birds continue to flap their wings if they are unexpectedly exposed to sunlight, startled, or receive electric shocks in the bath (Ismail et al., 2019).
Effect of pre-slaughter restraining method on meat quality
Muscle pH
The amount and rate of glycogen breakdown and lactate release in the muscle pre- and post-slaughter of fowl chicken are influenced by events that occur before slaughter handling. Table 1 presents the average pH of the pectoralis major muscle of hens subjected to various restraint methods. Although restraining methods did not affect ultimate pH (p > 0.05), the shackle group exhibited lower muscular pH readings than the cone group. The lower value of final pH in shackled birds could be due to struggle or free wing flapping during shackling, which raises lactate concentration in breast muscle, reducing muscle pH. Ismail et al. (2019) and Lambooij et al. (1999) discovered comparable muscle pH in shackle- and cone-restrained broiler chicks. Regardless of treatment, the pH levels steadily rose over time. The accumulation and proteolytic cleavage of metabolites due to bacterial action on meat may account for the increase in pH during preservation (Kim et al., 2019). Similar findings were made while chilling chicken breast meat (Kim et al., 2020).
Table 1
Muscle pH, water holding capacity and shear force as affected by pre-slaughter restraint method in broiler chickens of broilers
Parameter | Storage (day) | Pre-slaughter restraint method | P value |
Shackle | Cone | RM | RM×S |
pH (unit) | 1 | 5.514 ± 0.04y | 5.657 ± 0.05y | 0.8894 | 0.5495 |
5 | 5.808 ± 0.08x | 5.821 ± 0.02x | 0.5508 | |
P value | < .0001 | < .0001 | | |
Drip loss (%) | 1 | 2.958 ± 0.15ay | 2.267 ± 0.13by | 0.0039 | 0.3713 |
5 | 3.713 ± 0.22x | 3.419 ± 0.36x | 0.0891 | |
P value | < .0001 | < .0001 | | |
Cooking loss (%) | 1 | 29.879 ± 0.53ay | 27.46 ± 0.43bx | 0.0025 | 0.1996 |
5 | 26.449 ± 0.71y | 24.33 ± 0.77y | 0.0599 | |
P value | < .0001 | < .0001 | | |
Shear force (kg) | 1 | 1.133 ± 0.02ax | 1.084 ± 0.01bx | 0.0492 | 0.1121 |
5 | 1.078 ± 0.01ay | 1.034 ± 0.01ay | 0.0055 | |
P value | 0.0012 | 0.0024 | | |
a,b Means within the same row with different letters are significantly different at p < 0.05. |
x,y Means within the same column with different superscripts are significantly different at p < 0.05. |
Values are mean ± standard error. |
Water holding capacity
Table 1 depicts the water holding capacity in drip and cooking losses of chickens subjected to different pre-slaughter restraint methods. On the first postmortem day, there was a significant (p < 0.05) increase in drip and cooking loss among birds subjected to shackle restraint compared to cone restraint. Increased water retention in broiler chicks may be attributable to shackling-induced lactate production in the muscles. Additionally, a reduction in meat pH results in protein denaturation and a decrease in the meat's water-holding capacity (Ismail et al., 2016; Huang et al., 2014). These results were compared with Ismail et al. (2019) and (2016), who observed that pre-slaughter shackling caused significantly greater drip and cooking loss than birds held in a cone.
After 7 days of aging, the restraining method did not influence drip and cooking loss (p > 0.05). Water holding capacity is the ability of meat to keep both inherent and added water when subjected to external forces such as cutting, heating, grinding, and pressing (WHC). It is a vital quality parameter in the chicken meat market since it affects palatability and economic attributes (Kaboshio et al., 2020). Regardless of the treatment group, water holding capacity increased (p < 0.05) over time. This conclusion may be attributable to the aging-related degradation of myofibrillar proteins and collagens, which reduces the ability of myofibrillar proteins to hold water (Soglia et al., 2018).
Shear force
From a sensory perspective, tenderness is the most important meat quality characteristic determining customer acceptance of meat ( Li et al., 2021). The principal effects of restraint method and age on the shear force of broiler chicken meat (Table 1). Shear force values of pre-slaughter shackle restraints were significantly greater than those of cone restraints (1.133 kg vs. 1.133 kg at d 1 and 1.078 kg vs. 1.034 kg at d 7). This could be attributable to the higher water loss of meat from the shackle group during cooking. Ismail et al. (2019) gave a similar explanation, who attributed the higher force of meat from the shackled chickens to the coned chickens to significantly higher cooking loss.
Moreover, wing restraint treatment stretches the breast muscles and prevents contraction, resulting in longer sarcomeres, thus improving the tenderness of the meat. The values of shear force reduced significantly with aging. The reduction in shear force values could be due to rigor resolution caused by the enzymatic breakdown of collagen holding muscle fibers under refrigerated storage (Oliveira et al., 2021). It has been reported that the final meat tenderness depends on postmortem changes during aging, which affect the contractile system of the muscle or myofibrils (Shi et al., 2021; Bhat et al., 2018).
Color values
The meat's color is a useful indicator of its freshness at the time of purchase and is one of the most influential factors in determining customer acceptability (Wang et al., 2021; Hughes et al., 2020). Table 2 displays the color parameters of chicken pectoralis major muscle treated with various pre-slaughter restraint techniques. Restraining methods affected meat lightness, redness, and yellowness.
Table 2
Color characteristics as affected by pre-slaughter restraint method in broiler chickens
Parameter | Storage (day) | Pre-slaughter restraint method | P value |
Shackle | Cone | RM | RM×S |
Lightness (L*) | 1 | 62.55 ± 0.74ay | 60.09 ± 1.26by | 0.0117 | 0.5283 |
5 | 63.71 ± 1.06x | 60.97 ± 0.91x | 0.1662 | |
P value | 0.0039 | 0. 0443 | | |
Redness (a*) | 1 | 6.102 ± 0.15ax | 5.51 ± 0.18bx | 0.0261 | 0.8915 |
5 | 5.855 ± 0.20y | 5.47 ± 5.47y | 0.2295 | |
P value | 0.0211 | 0. 0224 | | |
Yellowness (b*) | 1 | 8.698 ± 0.28ax | 7.074 ± 0.22bx | 0.0003 | 0. 7431 |
5 | 8.151 ± 0.24y | 7.821 ± 0.29y | 0.0882 | |
P value | 0.0307 | 0.0111 | | |
Chroma (C*) | 1 | 10.36 ± 0.12x | 9.910 ± 0.28x | 0.0670 | 0.6586 |
5 | 9.986 ± 0.35y | 9.088 ± 0.20y | 0.3943 | |
| 0.0022 | 0.0273 | | |
Hue (h*) | 1 | 48.83 ± 0.62y | 47.68 ± 0.80y | 0.2698 | 0.9661 |
5 | 49.94 ± 1.08x | 48.89 ± 1.05x | 0.4990 | |
P value | < .0001 | < .0001 | | |
a,b Means within the same row with different letters are significantly different at p < 0.05. |
x,y Means within the same column with different superscripts are significantly different at p < 0.05. |
Values are mean ± standard error. |
At 1 day postmortem, meat from birds confined using the shackle method had significantly (p < 0.05) higher lightness (L*) and lower redness (a*) and yellowness (b*) values than meat from birds confined using the cone method. This can be attributable to differences in pH values across various restraint methods. Salwani et al. (2016) found that the color of breast meat is frequently related to postmortem muscle pH change. According to the author (Salwani et al., 2016), breast muscles with a lower pH appear less red than those with a higher pH in broiler chickens. Saláková et al. (2009) observed a significant positive correlation between redness and yellowness. Mir et al. (2017) posited that the redness and yellowness of meat are linked, where meat with a higher a* tends to have higher levels of b*.
The restraint method did not affect (p > 0.05) on the parameter of color, which consists of lightness, redness, and yellowness on 5 d postmortem. In agreement with the present findings, Ismail et al. (2016) also found a significant effect of the restraint method (shackle and cone) on the color coordinates (L*, a*, and b*) of broiler chickens. Although C* (Chroma) and h* (hue) values up to day 5 did not differ significantly between restraint methods, meat from birds restrained in the cone had lower chroma and hue color tone than meat from the birds restrained using the shackle method. Irrespective of the restraint method, lightness values increased (p < 0.05), while the redness and yellowness values decreased (P < 0.05) with increasing postmortem aging. A vital decrease in color characteristics could be due to myoglobin oxidation during aging, as it is the major hem protein responsible for the color of meat (AMSA, 2012).
Meat Lipid-protein oxidation
Lipid oxidation is the major non-microbial cause of spoilage in meat and meat products, primarily under pro-oxidative conditions such as storage (Sabow and Majeed, 2019). Malondialdehyde (MDA) is one of the essential aldehydes produced during the secondary lipid oxidation of polyunsaturated fatty acids. It is considered the major marker of lipid oxidation. Thus, the thiobarbituric acid reactive substances (TBARS) test for malondialdehyde determination is the most commonly used method for assessing lipid oxidation in muscle because of its sensitivity and relatively simple procedure (Domínguez et al., 2019).
Table 3 shows the TBARS value of muscle from broiler chickens subjected to different pre-slaughter restraining methods. At 1 and 5 d postmortems, pre-slaughter shackle restraint resulted in significantly higher lipid oxidation value than cone restrain. These values were consistent with the results for residual blood, which increases the concentration of home proteins (mainly hemoglobin) in meat. Hemoglobin is an influential promoter of lipid oxidation (Sabow et al., 2016).
Table 3
Lipid-protein oxidation of meat as affected by pre-slaughter restraint method in broiler chickens
Parameter | Storage (day) | Pre-slaughter restraint method | P value |
Shackle | Cone | RM | RM×S |
Lipid oxidation (mg MDA/kg meat) | 1 | 0.669 ± 0.01ay | 0.424 ± 0.01by | 0.0214 | 0.330 |
5 | 2.475 ± 0.01ax | 1.931 ± 0.01bx | < .0001 | |
P value | < .0001 | < .0001 | | |
Free thiol (nmole/mg protein) | 1 | 48.24 ± 0.19x | 49.74 ± 0.29x | 0.198 | 0.302 |
5 | 41.37 ± 0.38by | 46.01 ± 0.73ay | < .0001 | |
P value | < .0001 | < .0001 | | |
Carbonyl ( nmole/mg protein) | 1 | 2.320 ± 0.01y | 2.210 ± 0.007y | 0.622 | 0.347 |
5 | 2.976 ± 0.01ax | 2.523 ± 0.015bx | 0.011 | |
P value | < .0001 | < .0001 | | |
a,b Means within the same row with different letters are significantly different at p < 0.05. |
x,y Means within the same column with different superscripts are significantly different at p < 0.05. |
Values are mean ± standard error. |
Additionally, pre-slaughter conditions may cause stress and physical damage, greatly influencing lipid oxidation in meat and meat products during the early post-slaughter period (Bostami et al., 2021). In general, lipid oxidation increased significantly over storage in both groups. However, no group (shackle or cone) had an MDA value that reached detectable concentration for humans, as established by Abdulla et al. (2016). Lipid oxidation changes lead to off-odors, off-taste, discoloration, protein degradation, toxic compound accumulation, and shelf life decline, affecting consumers' health (Sabow et al., 2016).
The oxidation of protein is one of the most novel challenges in meat quality evaluation throughout processing and storage because muscle tissue has a large concentration of proteins, which have an important role in meat quality in terms of sensory, nutritional, and physicochemical aspects (Falowo et al., 2014). The principal oxidative changes of protein occur on the amino acid side chains and include the production of carbonyl groups, thiol oxidation, and aromatic hydroxylation (Morzel et al., 2006).
As a result, the quantity of thiols and carbonyl groups in meat is commonly used to assess protein oxidation (Guyon et al., 2016). Table 3 shows the results of the influence of pre-slaughter restraint measures on protein oxidation in terms of thiol and carbonyl levels in chicken. The pre-slaughter restricting approach did not influence protein oxidation values in broiler chicken flesh on day 1 postmortem.
Nonetheless, at 5 d postmortem, meat from birds restrained using the shackle method was significantly (p < 0.05) higher in the thiol and carbonyl content than meat from birds restrained using the cone. This observation coincides with the results obtained for lipid oxidation which showed that birds restrained using a shackle had greater TBARS value than those restrained with a cone. According to Falowo et al. (2014), protein oxidation occurs due to the interaction between proteins, especially the nitrogen or sulfur centers of reactive amino acid residues of protein and lipid hydro-peroxide or secondary lipid oxidation products such as aldehydes.
It was also observed that the onset of lipid oxidation in meat and meat products seems to occur faster than the oxidative degradation of myofibrillar proteins. Thus it is more likely that lipid-derived radicals and hydro-peroxides promote protein oxidation (Thanatsang et al., 2020; Domínguez et al., 2019). The free thiol content decreased, and carbonyl content increased (p < 0.05) over storage regardless of the pre-slaughter restraining method. These observations are consistent with those of Ferreira et al. (2018) and Smet et al. (2008), who showed that free thiol and carbonyl groups significantly decreased and increased as meat ages.
Microbiological quality
Figure 3 shows the effect of the pre-slaughter restraining method on microbial levels of breast meat from broilers during the first five days postmortem. At d 1, microbial counts were not significantly different for the two pre-slaughter restraining methods. However, at d 5 postmortem, more significant growth of total bacterial counts was indicated by meat samples obtained from the birds restrained using the shackle method than the meat samples obtained from birds restrained in the cone (p < 0.05). The higher bacterial growth exhibited by broiler chickens subjected to shackle restraint can be attributable to the low blood loss.
According to Bourbab and Idaomar (2012), residual blood in the carcass after bleeding is one of the most significant factors affecting contamination. It is a perfect medium for microorganisms' growth due to its high nutritive value. Some authors have also demonstrated that meat from broiler chickens that have suffered stress during the process of slaughtering is likely to have a shorter shelf-life due to spoilage since the glycogen levels in the muscles are not enough to develop the maximum level of lactic acid and an ideal pH (Iannetti et al., 2020; Petracci et al., 2010).
In general, increased growth of microorganisms with storage time was detected in meat samples from both pre-slaughter restraining groups and meat samples from the shackled chickens with the highest counts of total bacteria. However, the levels of total bacteria counts in both pre-slaughter restraining groups were within the acceptable limits. Rouger et al. (2017) reported that spoilage of poultry meat occurs when total bacterial counts reach 6–7 log cfu/g.