Impact of Stunning Methods on Chicken Meat Quality: A Review

Electrical stunning

Electrical stunning (ES) is a method frequently used in the poultry industry to ensure animal welfare and improve meat quality before slaughter. In this procedure, an electric current is passed through the bird’s brain and body, causing unconsciousness, which helps to reduce pain and stress during slaughter. This procedure is regulated by international standards by the Humane Methods of Slaughter Act (1958), enforced by European Council Regulation, EC 1099/2009, and the guidelines of the World Organization for Animal Health, OIE (2013). However, methods of ES vary significantly from country to country in terms of voltage, frequency, and duration, which in turn affects the quality of the meat, including tenderness, color, and shelf life.

In most Islamic countries, ES is mainly used in large poultry processing plants to meet the demand of the local and international markets. Most Asian abattoirs use low-voltage stunning, typically between 50 and 200 V, as this meets halal requirements (Riaz et al., 2021; Sazili et al., 2023). However, there are concerns when lower voltages are used, as the birds may not be fully stunned, allowing them to regain consciousness before slaughter. This can have a negative impact on animal welfare and meat quality. In contrast, in countries such as the United States and the European Union, higher voltages (up to 400 volts) are generally used to ensure more effective stunning. However, this can sometimes lead to minor tissue damage or blood splashing (Goksoy et al., 1999). In addition, technological advances are paving the way for new approaches to ES. Research from China and the Netherlands has explored the use of pulse-modulated currents to optimize stunning results while minimizing the negative effects on meat texture and appearance (Riggs et al., 2023). These innovations could benefit Asia and other developing countries by allowing them to meet different market needs without compromising animal welfare standards

Pulsed modulated current, one of the more recent developments in ES methods, is used in chicken processing to improve meat quality and animal welfare. In this technique, birds are rendered unconscious before being killed through the application of pulsed direct current (DC) at specific frequencies and voltages (Huang et al., 2017; Girasole et al., 2015). For example, Siqueira et al. (2017) used a constant voltage of 70 V and a current of 100 mA to study the effects of pulsed DC shocks with frequencies of 300 Hz and 650 Hz. According to their findings, higher frequencies, especially 650 Hz, could cause unconsciousness with less stress and better meat tenderness. Another experiment by Ali et al. (2007) and Girasole et al. (2015) demonstrated that using a frequency of 750 Hz and a current of 200 mA reduced wing flapping by 35% and improved meat tenderness by 22%.

Gas stunning/CAS

Gas stunning (GS), also known as CAS, has become a more advanced and humane method in the poultry processing industry, particularly as a viable alternative to ES. This method uses controlled atmospheric systems to render the bird unconscious before slaughter, with a focus on improving animal welfare while ensuring meat quality. In this process, the birds are exposed to gases such as carbon dioxide (CO₂), argon (Ar), or a combination of these gases in carefully regulated concentrations. These gases create conditions that lead to hypoxia (low oxygen levels) or hypercapnia (high carbon dioxide levels) and induce unconsciousness in a more gradual, seemingly less stressful way than other methods (Fuseini et al., 2023). CAS is increasingly being considered by the poultry industry due to the growing focus on halal compliance and animal welfare and the increasing consumer demand for high-quality poultry products (Li et al., 2022; Raj, 2004). McKeegan et al. (2007) reported that CAS systems are increasingly favored in commercial poultry plants because they offer better control over gas concentration and exposure time, leading to more consistent stunning outcomes compared to older GS systems. These systems typically employ a multi-phase process involving different gas combinations and are fully automated and monitored through computerized controls to ensure unconsciousness and maintain insensibility.

A common approach involves initially exposing the bird to 30–40% CO₂ to induce unconsciousness, followed by increasing the concentration to 80–90% to maintain insensibility and minimize distress. (R et al., 2024). On the other hand, inert gases like nitrogen or argon, used at concentrations above 90%, create anoxic conditions with minimal distress but require longer exposure times and are more costly (Riggs et al., 2023; Gerritzen et al., 2013; Raj et al., 2006). Due to their large size and stress sensitivity, birds require careful handling during CAS. Ylä-Ajos et al. (2012) studied the effects of 4 CAS gas mixtures on chicken meat quality: 60% CO₂ in air, 98% nitrogen, a 76% nitrogen–24% CO₂ mix, and a biphasic method with sequential gas exposure. The bird stunned with nitrogen exhibited the fastest drop in muscle pH postmortem; however, after 4 h, no significant differences in final pH were observed across the treatments.

Behavioral observations are essential to assess how CAS affects animal welfare. Some studies have found that birds exposed to CAS show fewer signs of stress, such as erratic movements or vocal expressions of stress, than birds exposed to ES (Sazili et al., 2023). In addition, the duration of unconsciousness induced by CAS is often longer than with ES, ensuring a more reliable state of insensibility during the slaughter process (Benson et al., 2012). However, despite its advantages, the introduction of CAS is also associated with challenges. The high initial costs associated with installing CAS equipment and ensuring halal compliance can be a barrier for smaller poultry producers.

Mechanical stunning

Mechanical stunning (MS) method involves using a captive bolt stunner or a similar device to deliver a powerful blow to the animal’s head. This method aims to render the animal unconscious by immediately interrupting brain activity through blunt force trauma. The physical impact leads to a rapid increase in intracranial pressure and thus to immediate loss of consciousness. The correct application ensures humane treatment and minimizes stress before slaughter (Sinclair et al., 2023). Shock stunning is associated with a lower incidence of bruising and bleeding compared to ES. It ensures a uniform pH drop and optimal water-holding capacity, resulting in improved meat quality characteristics (Riggs et al., 2023). Although MS is a recognized technique for rendering chickens unconscious before slaughter, its use in commercial environments is limited due to animal welfare and practical concerns (Lambooij et al., 2007).

The impact of stunning methods on the quality of chicken meat is an important area of research, and several studies highlight the need for further investigation in key areas. Comparative studies on different stunning methods are crucial for a better understanding of their impact on animal welfare and meat quality. For example, Zulkifli et al. (2019) emphasized the importance of further investigating how the different stunning methods, ES, CAS, and MS, affect the release of stress hormones, and thus meat quality, including the occurrence of PSE meat. In addition, the long-term effects of stunning are often neglected in chicken processing studies. These long-term effects can strongly influence factors such as tenderness and shelf life. Furthermore, there is a lack of standardization of methods to evaluate the effectiveness of stunning. Three stunning techniques are shown in Table 1, each with special benefits and drawbacks that emphasize the importance they are to improving chicken handling procedures.

According to a study by Rigg et al. (2023) comparing the effects of CAS and ES on the quality of broiler meat, CAS birds presented higher post-stunning blood glucose levels and more noticeable wing lesions compared to ES birds, an indication of increased acute stress and possible handling problems. However, key meat quality traits (color, pH, 24 h drip loss) showed minimal difference between methods. Similarly, Xu et al. (2011) had explained that high-voltage, high-frequency ES can lower pH, increase glycolytic potential, and improve tenderness, but may also raise the risk of PSE meat. Moreover, Banach et al. (2024) reported that although stunning doesn’t directly affect microbial load, ES increases muscle exudation, promoting bacterial growth due to stress-induced PSE meat. In contrast, CAS minimizes stress and exudate, reducing microbial risks.

In another study, McKeeganet (2007) investigated different gas mixtures in CAS and found that a biphasic hypercapnic hyperoxygenation approach resulted in less wing breakage and bleeding than a single-phase hypercapnic anoxic mixture, indicating better results in animal welfare and meat quality. Likewise, CAS with CO₂ + O₂ offered notable welfare and processing advantages over an Ar + CO₂ methodology.

In terms of mechanical techniques, the study showed that the broilers stunned with MS had fewer fractures and haemorrhages than those stunned with ES. The MS method helps to maintain the integrity of muscle tissue and reduce physical stress by eliminating the shackling before stunning and the cramping effects of electricity. It is very rarely used in chicken processing plants (Lambooij et al., 2007).

Overall, the compared evaluation of the 3 stunning methods shows that CAS offers clear advantages over ES and MS in terms of animal welfare and meat quality. While ES remains widely used due to its cost-effectiveness and operational efficiency, the associated animal welfare and meat quality concerns cannot be overlooked. While MS is effective in certain situations, it is not sufficiently scalable for modern chicken processing. Therefore, despite higher costs, CAS is proving to be the most humane and quality-preserving method, in line with evolving industry standards and consumer expectations.

Stress factors

The release of stress hormones in chickens serves as a key indicator for assessing the impact of stunning procedures on both animal welfare and meat quality (Zulkifli et al., 2019; Scanes, 2016). Stress-induced hormonal changes, particularly the release of corticosterone, adrenaline, and noradrenaline, play a central role in shaping postmortem muscle metabolism, thereby influencing critical meat quality traits such as tenderness and the development of Dark, Firm, and Dry (DFD) meat and leading to unfavorable traits such as PSE meat (Dadgar et al., 2012; Terlouw, 2015). Different types of stress hormones and their effects on poultry welfare and meat quality are illustrated in Figure 1.

Figure 1.

Impact of stress and hormonal changes on chicken welfare and meat quality (Nawaz et al., 2021).

When broilers are subjected to handling, restraint, or stunning, the hypothalamic–pituitary–adrenal (HPA) axis is activated, increasing corticosterone levels (Kannan et al., 1997). Simultaneously, the sympathetic–adrenal–medullary system releases adrenaline and noradrenaline, which increases heart rate and muscularity and accelerates muscle glycogen depletion (Zhang et al., 2009). Glycogen is the principal substrate for postmortem lactic acid development; its depletion limits pH decline, resulting in a greater eventual pH (>6.0) and predisposing meat to a DFD condition (Chauhan et al., 2019). DFD meat, although retaining water, is darker in color, harder in texture, of lower shelf life, and lower consumer acceptability (Ismail et al., 2018; Molette et al., 2003). High corticosterone also inhibits proteolytic enzymes such as calpains and caspases, limiting tenderization and increasing toughness. Increased chronic stress may also improve oxidative damage in muscle, further affecting texture and palatability (Steinbacher et al., 2020).

Several studies have shown that preslaughter treatment, catching, restraint duration, stunning duration, equipment parameters, and environmental conditions significantly influence the levels of stress hormones (Pegan et al., 2019; Ismail et al., 2018; Zhang et al., 2019). Transport is also a powerful stressor; Gou et al. (2021) observed serum corticosterone in broilers increased from 63 μg/L baseline to 130.2 μg/L following just a 1-h road transport. On the contrary, Li et al. (2022) noted that birds from a stunned flock had lower cortisol levels than non-stunned slaughter, reflecting some mitigation of stress responses.

Zulkifli et al. (2019) observed a notable rise in plasma corticosterone after ES (50 V, 400 Hz), from 5.9 ng/mL before stunning to 14.14 ng/mL post-slaughter, indicating that normal slaughter procedures can cause acute stress as well. Cortisol, the universal biomarker, has been employed extensively to compare ES and CAS. Overall, CAS tends to produce smaller cortisol spikes, whereas ES often induces a more abrupt and intense stress response due to its immediate action (Raj, 1998; Zulkifli et al., 2019). Zhang et al. (2009) and Bedanova et al. (2007) also recorded higher plasma corticosterone and glucose levels in ES birds as compared to CAS-treated birds.

Gas composition in CAS also impacts stress level. Ali et al. (2007) reported that biphasic CO₂-based CAS reduced corticosterone levels by up to 40% compared with single-phase systems. Likewise, Pinto et al. (2016) reported that ES broilers showed significantly greater blood glucose (337.65 mg/dL) and lactate (7.8 mmol/L) than those stunned with CO₂ (315.7 mg/dL; 5.4 mmol/L) or control birds (305.95 mg/dL; 5.4 mmol/L), indicating greater physiological stress in the ES group.

Considered as a whole, the data show that all stunning methods do induce some degree of acute stress, but with varying magnitude and physiological impact. CAS, particularly in optimized biphasic use, tends to lower the endocrine stress reaction, with benefits to welfare and meat quality. ES, while favored for economy in use, may elevate the level of stress hormones to a greater extent, resulting in greater PSE meat and lower tenderness. MS, though less studied in hormonal terms, may offer welfare advantages by avoiding pre-stun shackling, but remains limited in commercial application.

Blood splash

Blood splashes, which are characterized by the presence of visible blood in muscle tissue, significantly impair both the aesthetic and sensory quality of chicken meat (Kadim et al., 2020). The occurrence of blood spatter is largely influenced by the effectiveness of stunning techniques, which differ in their ability to reduce stress and improve blood drainage (Sazili et al., 2023; Aghwan et al., 2016). Commonly used stunning methods include electric water bath stunning, GS, and CAS, with each method having its own impact on meat quality. In many countries, stunning practices are guided by halal requirements, with low-voltage ES often preferred (Hayat et al., 2024). However, studies show that inadequate stunning can lead to incomplete bleeding, increasing the likelihood of blood spatter, which has a negative impact on the appearance and texture of the meat (Ali et al., 2007). Comparisons between ES and non-ES have shown that stunning generally improves blood flow but requires precise adjustment of voltage and duration to prevent muscle bleeding (Aghwan et al., 2016). CAS is used worldwide to reduce stress and improve carcass quality by minimizing capillary rupture through gradual unconsciousness (Huang et al., 2017). In contrast, non-stunning and some electrical procedures can cause severe convulsions that increase vascular pressure and capillary damage (McKeegan et al., 2007; Riggs et al., 2023). ES can have different effects depending on factors such as voltage, frequency, and bird resistance (Riggs et al., 2023). However, it still results in significantly higher haemorrhagic scores in both breast and thigh muscles compared to MS, as shown in Figure 2A. In some Muslim-majority countries, acceptance is limited due to religious guidelines (Ishak et al., 2023).

Figure 2.

Effects of different stunning and killing methods on haemorrhagic scores and bleeding efficiency in broiler chickens. (A) Haemorrhagic scores of breast and thigh muscles in broilers subjected to electrical (whole-body, shackled) and mechanical (captive-bolt, cone) stunning methods. Source: Raj et al. (1997). ^a–cComparison of haemorrhagic score between electrical and mechanical stunnings within the same score of muscle (breast or thigh) at P < 0.05. source: Alam et al., 2024. (B) Weight loss due to bleeding (%) following gas stunning, gas killing, electrical stunning, and electrical killing procedures. Adapted from Nicolau et al. (2015). Means in a bar of weight loss due to bleeding (%) with different letters (^a–c) among different stunning methods differ significantly (P < 0.05) from each other.

Studies in Europe show that CAS significantly reduces the incidence of blood spatter compared to water bath stunning, with rates falling from 18% to 5% (Riggs et al., 2023; Raj, 2006). The percentage of blood loss (by weight) during slaughter under 4 different stunning/killing methods is shown in Figure 2B. Gas killing resulted in the highest blood loss, followed by GS, suggesting better vascular relaxation and cardiac activity that promote more complete exsanguination. In contrast, electrical killing resulted in the lowest bleeding percentage, possibly due to early cardiac arrest, which may restrict blood flow during bleeding and thereby compromise meat hygiene and shelf life (Nicolau et al., 2015).

Briefly, ES and CAS both have their own strengths, but GS is less blood splashing and yields more carcass quality, which renders it preferable for humane poultry slaughtering. Best practice in bleeding ideally within 30 s after stunning is essential to reduce blood splashes and product quality (Sazili et al., 2023). Enhancing meat quality requires understanding the interdependence of stunning efficacy, bleeding technique, and operator skill, supported by calibrated equipment and scientifically validated protocols.

Muscle contraction

The quality of chicken meat is highly influenced by stunning methods at the time of slaughter, and among the key issues is the occurrence of PSE meat. This condition is directly linked with stress and muscle contractions at stunning, and one must therefore be aware of the physiological responses involved (Prioriello et al., 2020). Stunning-induced stress leads to the release of stress hormones such as cortisol, leading to depletion of muscle glycogen and postmortem pH changes. Rapid pH decline with reduced water-holding capacity is a feature of PSE meat (Huang et al., 2014).

ES induces a well-characterized sequence of muscle contractions, tonic spasms, followed by clonic spasms, ending in flaccidity driven by massive Ca²⁺ release and ATP depletion, which accelerate rigor mortis. High-voltage ES increases the incidence of PSE meat, as seen in Asia, whereas EAS reduces muscle damage by minimizing physical handling (Linares et al., 2007). GS using CO₂ or inert gases produces less intense contractions and lower stress indicators (corticosterone, protease activity, sarcoplasmic Ca²⁺). CO₂ stunning, on the other hand, may activate glycolysis, resulting in faster pH decline and a greater risk of PSE-like characteristics. Inert gases such as argon or nitrogen reduce stress even more and preserve myofibrillar integrity to a greater extent, but must be precisely controlled to produce rapid unconsciousness (Govindaiah et al., 2023). MS provides immediate insensibility with minimal contractions, but is impractical on a large commercial poultry meat processing scale. Ultimately, the nature of the stunning method employed determines patterns of contraction, pH fall rate, and preservation of meat quality.

In addition to stunning techniques, factors such as bird genetics and preslaughter handling play significant roles in the development of PSE meat. Fast-growing broilers are particularly vulnerable to PSE due to their metabolic characteristics (Weng et al., 2012). Moreover, slow-growing broilers, on the other hand, have a more oxidative muscle profile with a greater proportion of type I and IIA fibers, resulting in slower glycolysis and a more gradual pH drop after postmortem. This metabolic form is often less susceptible to PSE and helps retain protein structure, which improves the meat’s color, texture, and water-holding ability while slowing postmortem alterations (Ismail et al., 2017).

Overall, the method of stunning affects the pattern, intensity, and duration of muscle contraction, which has an impact on postmortem biochemical changes, rate of pH fall, and final meat quality traits such as color, water-holding capacity, and tenderness.

Bleeding efficiency

Efficient breeding in chicken processing is crucial for ensuring high meat quality and adhering to humane treatment standards. Proper bleeding helps avoid blood splashes, a defect that negatively affects the visual appeal and quality of the meat, while also improving the color and flavor (Jackson, 2024). Research highlights that the time gap between stunning and bleeding plays a significant role delay can cause increased stress in birds, leading to worsened rigor mortis, which in turn reduces tenderness and overall quality (Abeyesinghe et al., 2007).

The amount of blood that remains in the muscle tissue is directly influenced by the degree of bleeding, and this in turn affects the protein structure and oxidative reactions of the chicken meat. Darker meat is caused by a large amount of blood residue in the muscle tissue due to insufficient bleeding (Hafiz et al., 2015; Hakim et al., 2020). Proper bleeding promotes the removal of pigments that carry oxygen and gives the chicken meat a lighter, more uniform color (Qamar, 2019). Ikusika et al. (2020) found that inadequate bleeding in broilers resulted in higher shear force values, indicating tougher meat. This was attributed to disturbed postmortem processes such as pH drop and reduced proteolytic enzyme activity. The study also showed a strong positive correlation between stunning methods and bleeding efficiency. Nevertheless, the residual blood provides the bacteria with nutrients, increasing spoilage, affecting texture, and shortening shelf life.

Various stunning methods, such as ES water-bathing and CAS, have been studied for their effect on bleeding efficiency and meat quality. CAS, for example, generally results in less blood loss compared to ES, which can affect meat color but helps maintain tenderness (Raj, 2004). A study from Shukry et al. (2021) found that optimizing ES voltages between 50 and 63 volts AC significantly improves carcass appearance and reduces defects in chickens, consistent with international findings on poultry stunning. This balance ensures effective stunning while maintaining high meat quality and animal welfare standards.

The best practices for bleeding emphasize severing the neck immediately after stunning, ideally within 10 to 15 s, to ensure maximum blood drainage and minimize the amount of residual blood (EFSA, 2020). In the United States, following these protocols results in up to 90% blood removal efficiency, outperforming regions with less strict regulations (Nielsen et al., 2019). The Asian poultry industry has seen improvements in yield and quality by implementing automated grading, carcass, and cutting systems, in line with global standards (Nyalala et al., 2021). Ongoing efforts to refine bleeding protocols include advancements in processing equipment and lines designed to reduce delays and improve efficiency. The integration of humane practices, such as minimizing preslaughter stress, further ensures ethical treatment and superior meat quality (Njoga et al., 2023). As consumer demand for high-quality meat continues to rise, the industry needs to adhere to these optimized protocols, ensuring both competitiveness in the market and the welfare of the animals.

pH

An important quality factor that is influenced by stunning techniques is the pH value of the meat, as different stunning methods can alter muscle metabolism, stress responses, and the rate of postmortem acidification. Stunning affects both the final pH (measured at the end of rigor mortis) and the initial pH (immediately after slaughter), which is shown in Figure 4. The figure demonstrates how stunning and preslaughter stress affect the decrease in pH of the meat after slaughter. Under normal conditions, the pH gradually decreases from 7.0 to a final value of 5.5–5.8 within 6 h (Obanor, 2002; Gardzielewska et al., 2003). In PSE meat, excessive stress leads to a rapid drop in pH below 5.4 within the first hour, resulting in poor meat quality. DFD meat, on the other hand, is the result of prolonged stress leading to a slower drop in pH, which remains above pH 6.2 due to depleted glycogen reserves (Adzitey et al., 2011). These differences emphasize the importance of proper handling to maintain meat quality (Santos et al., 2019). The mechanism behind the pH changes in meat is primarily driven by postmortem metabolic processes. After slaughter, muscle tissue is no longer supplied with oxygen, forcing a transition from aerobic to anaerobic metabolism. This transition leads to the accumulation of lactic acid through glycolysis and the release of hydrogen ions (H⁺) from ATP hydrolysis. Together, these factors cause a gradual drop in muscle pH (Schreurs et al., 2000; England et al., 2014). The initial pH of ES or MS can stimulate the muscles, which accelerates the breakdown of glycogen and the formation of lactic acid. This rapid acidification could cause the initial pH to drop (Riggs et al., 2023; Huang et al., 2014). In addition, glycogen stores may be depleted before slaughter by stressful stunning techniques (such as poorly performed ES), increasing the final pH (>5.8) (Schreurs, 2000; Savenije et al., 2002; da Silva-Buzanello et al., 2018).

Figure 4.

Effect of time post-mortem on pH for several meat quality types. Post-mortem pH decline patterns in chicken meat categorized by quality types: DFD (dark, firm, dry) meat shows a slow pH decline, normal meat shows a moderate pH decline, while PSE (pale, soft, exudative) meat exhibits a rapid pH decline. pH was measured at intervals up to 15 minutes post-slaughter and at 24 h. ^a–c Comparison of meat pH among DFD, normal, and PSE meats within the same time point at P ≤ 0.01. The error bar indicates standard deviation. Data were retrieved from Gardzielewska et al. (2003).

DFD meat is undesirable as it has a shorter shelf life and is perceived negatively by consumers (Jiang et al., 2017; Huang et al., 2017). DFD is characterized by its dark, firm, and dry texture, which results from physiological stress and exhaustion in animals before slaughter, leading to high pH levels due to insufficient glycogen reserves. This stress causes microstructural changes like sarcomere shortening and protein degradation, which negatively impact the meat’s texture and water-holding capacity. Additionally, DFD meat exhibits amino acids, organic acids, and nucleotides that alter metabolite profiles, contributing to its undesirable characteristics (Wang et al., 2020). Studies have shown that CAS, especially with CO₂, minimizes stress and provides optimal pH levels that are conducive to meat quality. Although less common than PSE meat, DFD meat still occurs in broiler chickens (Grandin, 2020). Prevalence varies depending on handling and processing conditions, with studies typically reporting a range of 0.5% to 3% in commercially processed broilers. Petracci et al. (2015), for example, observed DFD-like characteristics in approximately 1–2% of chicken carcasses. Recent evidence suggests that the incidence of DFD-like meat in chicken can drop to less than 1% under optimal handling and transportation conditions (Langer et al., 2010). These figures underline the importance of minimizing preslaughter stress to reduce the incidence of DFD meat and maintain meat quality.

According to available evidence, CAS, especially using inert gases, is most effective in minimizing stress reactions and maintaining the optimal pH range, thereby reducing PSE meat occurrence and improving tenderness. ES, especially high voltage, causes a rapid pH drop, greater muscle contraction, and higher stress markers. Low atmospheric pressure stunning (LAPS) and head-only ES yield equivalent meat quality outcomes, even though their performance is greatly dependent on precise control of such factors as voltage, frequency, and exposure time.

Color

Stunning mainly affects myoglobin chemistry and muscle oxygenation, which influences meat color, a primary determinant of consumer acceptability. The values L* (lightness), a* (redness), and b* (yellowing) are used to quantitatively assess the color. The color of the meat is usually lighter and more enticing when stunning techniques are used to reduce tension and promote adequate bleeding (Figure 5). Postmortem pH decline is a key determinant of meat quality, influencing water-holding capacity, color, and tenderness. The rate and extent of this decline are strongly affected by preslaughter stress and stunning method.

Figure 5.

This figure illustrates the impact of various stunning methods, such as mechanical, electrical, CO₂ gas, argon gas, and non-stunning, on the color attributes of broiler breast meat. The color parameters measured include lightness (L*), redness (a*), and yellowness (b*). Values are presented as means. Means in a line of respective meat color with different letters (^a,b) among different stunning methods differ significantly (P < 0.05) from each other. The error bar indicates standard deviation. Data were retrieved from Alam et al. (2024).

ES, when properly calibrated, produces consistent color profiles by enhancing the rate of bleeding (Linares et al., 2007). GS, particularly using inert gases like argon, reduces preslaughter stress and oxidative reactions of myoglobin, preserving redness (a*) (Xu et al., 2018; McKeegan et al., 2007; Li et al., 2022). Argon stunning has also been shown to preserve color quality longer than CO₂ stunning, which is often accompanied by higher lightness (L*) but lower redness (a*). For instance, Gerritzen et al. (2013) reported that birds slowly exposed to 10–30% CO₂ produced lighter, less red meat than ES birds. Similarly, Xu et al. (2018) found that GS in 40% (G40%) and 79% (G79%) CO₂ increased L* at 1- and 9-d postmortem (P = 0.03, P < 0.01), while decreasing a* and overall color quality. Higher percentages of CO₂ (G79%) led to the poorest color stability during storage at 4°C, which demonstrates that prolonged exposure to CO₂ exacerbates discoloration.

All these color changes are directly linked with postmortem pH decline. Lightness (L*) is negatively correlated with pH in poultry meat; reduced pH reduces water-holding capacity, allowing more reflection of light (Allen et al., 1998; Fletcher et al., 2000). The more rapid pH decline observed in ES aligns with increased muscle contraction and ATP consumption, which accelerate anaerobic glycolysis. In contrast, CAS’s slower decline suggests reduced metabolic disturbance. Post-stun CAS-created increases in L* can be due to elevated circulating glucose levels that activate glycolysis, reduce pH, and create lighter-colored meat (Fletcher et al., 2000). Kang and Sams (1999) also reported that 40–60% CO₂ stunning resulted in L* being greater between 1.25 and 24 h postmortem, while Battula et al. (2008) reported that vacuum-stunned birds were lower in L* when compared with ES-treated birds at 0.75 and 4 h postmortem.

Religious slaughter without stunning (e.g., Halal) is another area of interest. Salwani et al. (2016) noted that birds that were not stunned had higher redness (a*) than those subjected to 40% CO₂ GS, showing that stun-to-kill procedures can reduce muscle pigmentation. On the other hand, poor MS may cause bruising or imperfect bleeding, producing darker meat with reduced lightness and redness. Overall, MS produces the darkest meat (lowest L* and a* values), ES significantly improves color through better bleeding, and GS outcomes are gas dependent. Argon GS produces the optimal color and water retention, indicating reduced oxidative stress and higher-quality meat, whereas CO₂ stunning has a proclivity to improve lightness but reduce redness (Alam et al., 2024). Overall, GS methods generally maintain a more stable ultimate pH compared to ES, indicating lower preslaughter stress and better preservation of muscle glycogen reserves

Water-holding capacity

Water-holding capacity (WHC) is the ability of the meat to retain its inherent water during storage, processing, and cooking conditions and is a key predictor of juiciness, tenderness, and yield. Stunning practices significantly affect WHC by altering the integrity of muscle protein, sarcomere structure, and cell membrane permeability. Drip loss, thawing loss, and cooking loss are the most employed indices for WHC, with larger values generally suggesting poorer WHC (Warner, 2023). Low WHC not only reduces consumer acceptability but also leads to increased fluid loss during storage, thawing, and cooking.

As Table 2 shows, ES has lower drip loss (2.58–4.68 %) compared to other processes and a cook loss of 20.09–27.1%. No stunning has the same drip loss range (2.55–4.11%) but slightly higher cook loss (27.12–29.48%). CAS has the highest drip loss (4.84–9.55%) but the lowest cook loss (13.81–21.72%). Thawing loss measurements are less commonly reported by studies, and therefore, comparison is not straightforward. Sirri et al. (2017) reported that high current flows (150 mA/bird) as per EU recommendations resulted in lower drip loss compared to low current flows (90 mA/bird), showing a beneficial impact on WHC. Furthermore, low-voltage stunning (15 V) and high-voltage stunning (100 V) both showed better WHC compared to mid-voltage stunning (50 V) and no stunning, with lower drip loss and pressing loss (Huang et al., 2017).

Sudden postmortem pH drop in some ES applications leads to protein denaturation and sarcomere shortening, both of which decrease WHC and render meat susceptible to PSE characteristics (Cam et al., 2021). The phenomenon is due to hyperstimulation of muscle metabolism, which results in higher glycolysis and heat production. Denaturation of protein and disruption of fiber structure reduce the binding sites for water, as reported by Bowker et al. (2015). To the same effect, myopathies that disrupt muscle fiber structure further weaken WHC through a reduction in protein-water interactions.

On the other hand, GS using nitrogen tends to have fewer drip and cooking losses, signifying higher WHC. This is likely due to reduced oxidative stress and enhanced cell protein maintenance because hypoxia caused by nitrogen lowers levels of ROS production and maintains cell membrane stability (Barbut, 2024; Song et al., 2023). In general, while ES can yield the same WHC when it is competitive under standardized conditions, particularly at higher currents and specific frequencies, tends to improve the WHC of chicken meat. GS, especially with nitrogen, tends to better maintain protein function and WHC by avoiding oxidative and structural damage. No stunning results in the poorest WHC outcomes.

Tenderness

Tenderness is one of the most important sensory properties of meat and is strongly influenced by stunning due to its effects on muscle fiber integrity and enzymatic activity. Razor blades are often used in combination with the Warner-Bratzler shear force method to assess chicken meat tenderness (Cavitt et al., 2005). Less stressful stunning methods, such as CAS, generally result in lower shear force readings, indicating more tender meat (Alam et al., 2024; Huff-Lonergan et al., 2010). Muscle fiber integrity is a critical factor, as mechanical shocks can damage muscle fibers, resulting in inconsistent pain. Improper ES can also lead to over-contraction, which impairs tenderness (Contreras and Beraquet, 2001; Papinaho et al., 1995). Another component is proteolysis, where stunning affects the activation of proteolytic enzymes called calpains, which are essential for postmortem tenderness (Zhao et al., 2018; Lee et al., 2008). It has been suggested that CAS is associated with optimal proteolytic activity (Kaur et al., 2021)

ES is a widespread procedure in the poultry industry; however, it may enhance preslaughter stress, which could have adverse effects on meat tenderness. Additional voltage use (e.g., 80 V) was reported to reduce tenderness significantly compared with non-stunned controls (Xu et al., 2025). In contrast, GS normally show the lower shear force values, representing greater tenderness than non-stunned slaughter chickens (Salwani et al., 2016). When directly compared, CAS produces more tender meat than ES, primarily because it can reduce preslaughter stress (Salwani et al., 2016; da Silva-Buzanello et al., 2018). Nevertheless, ES is still a workable alternative when optimized appropriately with proper attention toward minimizing stress and preventing tissue damage.

Lipid oxidation

The nutritional value, shelf life, and flavor are impaired by the oxidation of lipids. The oxidative stability of lipids in meat is affected by stunning techniques. By increasing oxidative stress, stressful stunning procedures promote lipid oxidation, ultimately leading to poor taste and rancidity (Zhang et al., 2009). Stressful ES increases malondialdehyde (MDA) levels, a marker of lipid oxidation insight important new information about how stunning methods affect meat quality, especially in relation to lipid oxidation (Huang et al., 2017). The findings demonstrated that oxidative stress in muscle tissues was significantly increased by severe electrical shock, which resulted in a noticeable rise in MDA levels (Huang et al., 2014).

According to Hayat et al. (2024), ES significantly influenced lipid oxidation in broiler breast meat. The highest MDA content (2.84 ± 0.11 mg/kg) was recorded at 30 V after 7 d of ageing. In contrast, birds stunned at 10.5 V showed lower MDA levels (2.09 ± 0.08 mg/kg). This indicates that higher voltage stunning increases lipid peroxidation and oxidative stress.

In contrast, CAS, particularly using CO₂, appears to better preserve lipid stability by reducing oxygen exposure in muscle tissues and lowering oxidative stress. The study by Xu et al. (2011) highlights the impact of different CAS methods on meat lipid oxidation. It demonstrates that CO₂ stunning significantly reduces lipid oxidation compared to ES, likely due to reduced oxygen exposure in muscle tissues. The MDA levels (μmol/100g) in Pectoralis major (breast) and Musculus iliotibularis (thigh) muscles of broiler chickens subjected to different stunning methods are shown in Figure 6. As shown in the figure, ES at 50V induced the highest oxidative stress, particularly in breast muscle, while higher voltage (65V) or CAS (especially G60%) resulted in lower MDA levels, suggesting these may be more favorable for meat oxidative stability.

Figure 6.

Malondialdehyde (MDA) levels (μmol/100g) in Pectoralis major (breast) and Musculus iliotibularis (thigh) muscles of broiler chickens subjected to different stunning methods. Treatments include control (no stunning), gas stunning with 40% CO₂ (G40%) and 60% CO₂ (G60%), and electrical stunning at 35 V (E35 V), 50 V (E50 V), and 65 V (E65 V). MDA is a biomarker of lipid peroxidation and oxidative stress. Means in a bar of pectoralis major muscles with different letters (^a,b) among different stunning methods differ significantly (P < 0.05) from each other. The error bar indicates standard error. (Source: Xu et al., 2011).

The effects of different CO₂ concentrations on lipid peroxidation in broiler muscle were investigated in a study by Xu et al. (2018). They found that lower levels of reactive thiobarbituric acid substances (TBARS), a marker for lipid oxidation, were measured in broilers stunned with 79% CO₂ than in those stunned with 40% CO₂. Consequently, the lipid stability of chicken meat during storage can be improved by increasing the CO₂ concentration during CAS. In contrast to ES, CAS reduces postmortem oxygen exposure and stress, thereby reducing lipid oxidation. The use of CO₂ and inert gases to maintain lipid stability has significant advantages (Xu et al., 2011).

Overall, CAS, especially with higher CO₂ concentrations or inert gases, offers clear advantages in maintaining lipid stability compared to ES. Given the limited research on this topic, further studies are needed to better understand how different stunning methods influence lipid oxidation. Investigating these effects could provide valuable insights into optimizing chicken processing techniques to improve shelf life and consumer acceptance.

Sensory quality

The most important sensory characteristics for chicken meat are usually flavor and juiciness, followed by texture. Investigating the interaction between these characteristics and stunning procedures could help to clarify their impact on the overall meat quality for consumer satisfaction. The flavor of chicken meat is influenced by various compounds like aldehydes, ketones, alcohols, acids, esters, hydrocarbons, furans, and sulfur compounds. These are the products of Maillard reactions, lipid oxidation, and other biochemistry (Xu et al., 2024). The stress caused by stunning can lead to the oxidation of proteins and lipids, which can result in poor flavor. Favorable flavor characteristics have been associated with CAS (Cam et al., 2021). Juiciness perception is directly positively influenced by WHC (Warner, 2023). A higher WHC typically results in improved sensory attributes, including increased tenderness and juiciness, which are key determinants of consumer satisfaction (Petracci et al., 2017; Petracci and Cavani, 2011). Juiciness is enhanced by stunning methods that reduce protein denaturation, such as CAS, which helps maintain moisture content, which is critical for a juicy eating experience (da Silva-Buzanello et al., 2018; Riggs et al., 2023). Although intramuscular fat (IMF) can contribute to the juiciness of chicken, its influence is minimal as muscles naturally contain very little fat. Therefore, it is believed that intramuscular fat is not the most important factor in the juiciness of chicken meat. However, the specific role of the IMF in chicken meat juiciness is debated. Some studies suggest that chicken breast meat, which is lean with low IMF content, tends to be less juicy and flavorful (Sun et al., 2019). Another important aspect of chicken meat quality is texture, especially softness. The measurement of shear force corresponds well with the consumer’s assessment of tenderness.

More uniform muscle texture can be ensured through the correct application of CAS or ES. However, tenderness is often secondary to flavor and juiciness, especially in chicken meat, which is generally more tender than red meat. In addition, only a limited number of studies have evaluated the methods for assessing meat tenderness. According to Xiong et al. (2006), a significant negative correlation (r = −0.76) was found between sensory tenderness and the Warner-Bratzler & razor blade shear methods. Similarly, Qiao et al. (2001) demonstrated a significant correlation between Warner-Bratzler shear scores and sensory scores. They also found that lighter-colored fillets with higher pH were generally more tender, a result that was confirmed by both sensory ratings and reduced shear force. Both ES and LAPS systems can be used without detrimental effects on consumer acceptability of chicken breast fillets. However, meat from birds stunned with LAPS and deboned at 4 h postmortem had higher overall acceptability ratings compared to other treatments (Schilling et al., 2012)

Shelf life extension

The longer shelf life has been attributed to a decrease in microbial growth and lipid oxidation, 2 processes that hasten spoiling. It produced meat that stayed fresher for longer, probably because it was less stressful and had the ability to lower postmortem metabolic activity (Nychas et al., 2008). After undergoing ES, the shelf life was found to be 7 d, which is consistent with normal manufacturing settings (Novoa et al., 2019; Small et al., 2012). However, meat treated with inert CAS had a longer shelf life of 10–12 d, which was an improvement of 3–5 d over ES (Jessup et al., 2025). The meat’s freshness, flavor, and safety are preserved for extended periods of time due to the stunning process’s decreased oxygen exposure and stress.

Both ES and CAS methods could be employed efficiently in poultry processing with practically no difference in terms of consumer acceptance. Nevertheless, benefits through reduction of stress and enhancement of some of the meat quality parameters to enhance shelf life could be found with GS. Vacuum packaging and modified atmosphere packaging under optimal storage conditions can provide a longer shelf life of chicken meat. In addition to providing consumers with higher-quality items, this longer shelf life helps producers financially by lowering waste and spoilage.