Spaghetti meat and gaping in broilers: emerging myopathies challenging poultry meat quality and processing

Abstract

The global poultry industry has experienced rapid growth, driven by consumer demand for affordable, high-quality protein. However, this progress has been accompanied by the emergence of growth-related myopathies, notably Spaghetti Meat (SM) and the more recently described Gaping, which compromise meat quality, yield, and marketability. SM, first reported in 2015, is characterized by the detachment and weakening of Pectoralis major fibers, resulting in a stringy, mushy texture of the meat. Gaping, affecting the Pectoralis minor, presents as separations between fiber bundles and tendon tears. Incidence rates of SM vary widely, from 10% to over 36%, while gaping prevalence remains inconsistently reported. Detection traditionally relies on visual scoring, but novel approaches, such as bioelectrical impedance, automated imaging, and spectroscopy, are under investigation to enable early identification. Histological analyses reveal shared features of muscle degeneration, fibrosis, lipid infiltration, and altered collagen structure, with oxidative stress and impaired regeneration implicated in both myopathies. Multiple factors contribute, including genetics, muscle hypertrophy, nutrition, environment, and processing conditions. Notably, delayed chilling, aggressive defeathering, immersion water chilling, and mechanical breast filleting increase SM severity, while dietary antioxidants and trace minerals can sometimes mitigate occurrence. SM and gaping affect technological quality by reducing textural integrity, protein solubility, and water-holding capacity, although their impacts are less severe than wooden breast or white striping. Current mitigation strategies focus on selective breeding, optimized nutrition, and modified processing parameters, but the complexity of these myopathies underscores the need for integrated approaches. Future research should prioritize in short term, the non-genetic strategies, including improvements in processing, nutrition, and management, while in the long term, the genetic basis of this myopathy, particularly the coexistence of affected and unaffected birds within the same population, to support balanced breeding strategies that preserve meat quality and consumer acceptance.

Introduction

Poultry is among the most widely consumed meats worldwide, with consumption anticipated to rise further. By 2034, poultry meat is projected to account for 45% of the total protein intake derived from all meat sources (OECD/FAO, 2025). Poultry meat is also increasingly popular in developed countries because of its nutritional and dietary benefits. Consumers want transparent production processes, both ecologically and in terms of animal welfare, while keeping high standards (Vukasovic and Stanton, 2017). At the same time, because of poultry’s affordable production and lack of religious prohibitions, it is an essential source of protein in underdeveloped nations (Pica-Ciamarra and Otte, 2010). Over the past six decades, global poultry meat production and consumption have expanded dramatically, establishing poultry as the leading driver of growth in the global meat sector. As consumer demand increasingly favors boneless and ready-to-eat poultry products, the industry has adapted by producing larger birds that can be efficiently portioned and processed in centralized facilities (Barbut et al., 2024).

Recently, the poultry industry has encountered growth-related muscle anomalies predominantly affecting the Pectoralis major (breast fillet) and Pectoralis minor (tender) in fast-growing genotypes selected for their production efficiency (high growth rate and breast yield) (Velleman, 2015; Petracci et al., 2019). Myopathies have become more prevalent in modern poultry production due to the use of fast-growing broiler chickens, which have high breast meat yield. These myopathies are referred to as White Striping (WS), Wooden Breast (WB), Spaghetti Meat (SM), and Gaping. Their names are derived from the unique and unusual characteristics that each myopathy possesses. WS is defined by the presence of white striation, caused by fat deposition along the muscle fibers running parallel to them on the surface of the pectoralis major muscle, one of the earliest signs noticed by the industries across the world (Kuttappan et al., 2013a). WB shows a hardening in the Pectoralis major (Sihvo et al., 2014), and SM shows up as a breakdown of the muscle fiber bundles that make up the breast muscle, making it look mushy and not very tight in certain areas (Baldi et al., 2018). The economic effect of the aforementioned myopathies was estimated at 200 million USD in North America in 2015, and subsequently, with an increase in prevalence, exceeding 1 billion USD (Barbut, 2020).

Three myopathies are related to Pectoralis major, whereas gaping is the only abnormality present in bipinnate Pectoralis minor, showing separations between muscle fiber bundles and tears in the anteroventral and anterodorsal of the tendon. This newly reported defect exhibits fiber separation patterns that appear similar to SM and can affect the quality and marketability of chicken tenders (Soglia et al., 2019a). Numerous studies have examined WS and WB abnormalities, focusing on their etiology, molecular pathways, and impact on meat quality, nutritional, and technological properties. However, there is not as much known about SM or gaping, and compared to WS and WB, fewer studies have been reported since SM and gaping first appeared, as shown by Fig. 1.

Fig. 1.

Yearly published studies on white striping, woody breast, spaghetti meat, and gaping myopathies in broiler chicken Pectoralis major and Pectoralis minor. Data gathered from Scopus on December 31, 2025, from 2000 through 2025, using the terms “white striping,” “wooden breast,” or “woody breast,” “spaghetti meat,” or “friable,” or “stringy spongy,” and “gaping” or “feathering” in the title or summary of the papers.

Given the limited scientific knowledge on these emerging conditions, there is a clear need to consolidate current understanding. This review aims to summarize the known morphological, histological, and physiological characteristics of SM and Gaping, evaluate their incidence and detection methods, explore contributing factors, and discuss their impact on meat quality. By identifying knowledge gaps and potential mitigation strategies, this review article aims to provide a foundation for future research and industry practices.

Morphology and incidence level

A muscle problem that weakens the P. major muscle in fast-growing chickens was first discovered in 2015 and was called “Mushy Breast,” also known as the Spaghetti Meat defect (Bilgili, 2015; Baldi et al., 2021). The condition became known as “Spaghetti Meat”, “Spaghetti Breast”, “Friable”, or “Stringy-spongy”. As the name implies, the pectoral muscle fiber bundles become detached, giving the affected area a mushy, sparsely tight appearance that resembles spaghetti pasta. Since SM deficiency severity may vary, a three-score scale was developed by Sirri et al. (2016) as a criterion for categorization (Fig. 2). SM can be recognized either through manual palpation (soft, stringy structure) or visually (expanding superficial lacerations), depending on the severity level. Meats with a moderate presence of SM can be sold fresh, but those with severe cases are often degraded and used in the making of other processed products. While the SM defect is most commonly seen in the ventro-cranial region of the fillet, there is growing evidence that it can also affect the caudal region and, on rare occasions, the muscles in the legs (the adductor muscle) (Baldi et al., 2021). There has been some indication of SM deficiency in commercial turkeys' pectoral muscles, but there is currently no data available linking this to the same problem in broilers (Zampiga et al., 2020).

Fig. 2.

Different breast myopathies and their scoring (SM and Gaping) based on their severity. Adapted from Baldi et al. (2021) and Che et al. (2025).

Recent evidence indicates that the Pectoralis minor muscle of broilers is affected by a novel meat quality defect that phenotypically resembles SM. This condition, termed “gaping” was first characterized by Soglia et al. (2019a), who described the presence of tears or holes resulting from postmortem rigor development that disrupts the intramuscular connective tissue. In some publications, this defect is also referred to as “tender feathering” (Maynard et al., 2023; Barbut et al., 2024). Based on the extent of tissue disruption, gaping severity has been classified into three distinct categories. (Fig. 2).

There is a lack of data and inconsistency in the data regarding the incidence rates of SM and gaping conditions. This may be because different abattoirs use different criteria for classification, and due to other myopathies, they can occur in the same meat tissue at the same time. For example, SM can coexist with WB and, more often, with WS (Baldi et al., 2018; Bailey et al., 2020). Fig. 3 illustrates the co-occurrence of breast myopathies in 9,250 broilers sampled from 40 flocks over 2 years (Che et al., 2022). It is important to acknowledge that the pinching assessment is inherently subjective, as there is no standardized force applied to the muscle during evaluation. Moreover, in commercial settings, mechanical deboning of tenders applies additional and variable forces that may enhance or even induce muscle fiber separation. Although macroscopic alterations associated with WS and WB develop during the bird’s lifetime, SM is typically detected during processing, often after deboning (Greene et al., 2025).

Fig. 3.

Venn diagram illustrating the incidence of spaghetti meat (SM), woody breast (WB), and white striping (WS) in 9,250 fillets. A total of 7,876 fillets presented multiple myopathies together. Within 7,876 fillets, 3,038 fillets presented SM and WB and WS together. Out of 9,250 scored fillets, only 305 fillets did not show myopathies. Che et al. (2022) with permission.

A study conducted in Brazil evaluated 5,580 breast fillets and determined that SM affected around 10% of the fillets with an average body weight of 2.92 kg (Montagna et al., 2019), while another study in Italy evaluated 16,000 breast fillets and reported that SM affected about 21% of the samples (Baldi et al., 2020). Research by Che et al. (2022) on 9,250 Canadian broilers found that the prevalence of SM was 36.3%, which is quite high and can be alarming. It is well-established that the incidence rates of WS and WB markedly increase with elevated growth rates, breast yield, body weight, and slaughter age (Kuttappan et al., 2013a; Papah et al., 2018), but this pattern has not been conclusively validated for SM. However, Pascual et al. (2020) found that SM rates were much higher in females than in male birds (25.0% vs. 3.1%, respectively), supporting the results by Maynard et al. (2023). This was different from what was seen for WS and WB, where rates were higher in males, at the same slaughter age, regardless of slaughter weight (Trocino et al., 2015; Mudalal et al., 2015). The reasons behind the higher occurrence of SM in females so far remain unclear. Females have been shown to have higher fat deposition compared to males of the same weight (Leenstra, 1986). Additionally, female Japanese quail have been shown to have slower collagen maturation compared to males (Tavaniello et al., 2014). However, Brothers et al. (2019) did find that male broilers are more likely to demonstrate the WB condition than females due to the increased expression of genes associated with connective tissue proliferation.

Studies show conflicting evidence on whether increased broiler weight correlates with higher SM incidence, unlike more consistent findings for WB and WS (Baldi et al., 2021). The younger and smaller broilers had a greater proportion of SM incidences (21% moderate and 7% severe) compared to the older and larger roasters (15% moderate and 4.0% severe), according to a comparison of the SM occurrence in broilers and roasters recorded in two North American industrial facilities (Barbut et al., 2024) also supported by Che et al. (2022). Where broilers were heavier than 2.45 kg at slaughter, they had higher odds of having SM compared with those smaller than 2.30 kg. On the other hand, Maynard et al. (2023) found that big females expressed a significantly higher incidence of SM than the smaller females. But for males, it was the opposite, as Che et al. (2025) did not observe a significant difference in SM between heavy (4.1 kg) and light (2.7 kg) bird processing plants. The heavy bird plant had 22.8% moderate and 8.3% severe SM, while the light bird plant had 24.1% moderate and 8.2% severe. Similarly, Novoa et al. (2022) observed that bird body weight did not significantly influence SM incidence, further supporting the lack of a consistent association between carcass weight and SM development.

Reported prevalence rates of gaping, a poultry myopathy that has received considerably less attention than SM, WB, and WS, vary markedly among studies. Che et al. (2025) observed a high incidence in a population of 5,180 birds, with 53.7% classified as severe and 24.8% as moderate. The light bird plant (2.7 kg) had a lower prevalence of moderate gaping, while the heavy bird plant (4.1 kg) had a lower prevalence of severe gaping. In contrast, Soglia et al. (2019a) reported substantially lower prevalence rates in a larger cohort of 8,600 birds, identifying only 8.0% severe and 8.8% moderate cases. Although birds in both studies belonged to similar commercial genetic strains, differences in rearing duration to achieve target body weights may have contributed to the variation in gaping incidence. In the study by Che et al. (2025), birds were processed at 50 days of age for the 2.7 kg group and 58 days for the 4.1 kg group, whereas Soglia et al. (2019a) evaluated birds ranging from 42 to 54 days of age with an average body weight of 2.84 kg. Interestingly, like SM, gaping myopathy also showed higher prevalence in females than males (Maynard et al., 2023; Che et al., 2025). It should be noted that increased gaping in tenders may also result from the processing steps involved in defeathering, chilling, and deboning during primary processing. For instance, the use of tender pullers in automated deboning systems may intensify the gaping effect in P. minor muscles that are more susceptible (Barbut et al., 2024). However, Future studies using standardized processing conditions across male and female broilers, along with controlled variation in processing parameters, may help clarify the relative contribution of each factor.

Detection of SM and gaping

Other than visual inspection, which is commonly used, there are different methods available to identify SM and gaping conditions in the broiler breast fillets and tenders. The processing of SM is a significant operational challenge due to the increased demand for thin-sliced chicken breast by consumers, and can be found after slicing, resulting in processors losing more meat due to downgrading or waste. Because of this, the in-line detection method is becoming popular, and researchers are trying to find a reliable method to detect both myopathies (Baldi et al., 2021). SM detection in poultry reveals a diverse array of technological approaches, ranging from imaging and near-infrared spectroscopy to machine learning and bioelectrical impedance. Siddique et al. (2021) successfully detected SM using both bioelectrical impedance analysis (BIA) coupled with supervised machine learning algorithms. When BIA data were analyzed using support vector machines and backpropagation neural networks (BPNN), SM fillets were accurately classified, with the BPNN model achieving 100% accuracy in the training dataset and up to 75% accuracy in the testing dataset, although performance was affected by data complexity and overfitting due to limited sample size. Notably, BIA is time-consuming and requires direct electrode contact with each sample, a process currently incompatible with the high-throughput line speeds of modern processing plants. However, in-line near-infrared spectroscopy was able to differentiate SM from normal and WB fillets, with classification accuracies reaching approximately 78–86%, although separation of SM was more challenging because of overlapping spectral characteristics among quality classes (Wold and Løvland, 2020). Compared with BIA, VIS-NIR hyperspectral imaging offers greater commercial potential because it is non-contact, very rapid, and compatible with conveyor-belt inline sorting systems already used commercially for WB detection, although instrument standardization and SM-specific validation are still needed.

Campo et al. (2020) found that SM was associated with significantly higher yellowness (b*) and chroma values compared to unaffected breasts, attributing this to increased lipid accumulation and liposoluble pigment deposition, while the reflectance index R630–R580 successfully separated SM-affected samples from normal ones, achieving 94.7% classification accuracy using just two wavelengths (420 and 630 nm). In terms of texture, SM-affected raw meat showed significantly lower resistance to compression at both 80% and full compression rates, reflecting the characteristic disintegration of connective tissue in this myopathy. In contrast, Dayan et al. (2024a) used a deep‑learning automated image analysis tool to measure myofiber diameter and area from histology images of 14‑day‑old broilers. By combining these measurements into a single metric called the relative myofiber size value, they could clearly separate normal/moderate birds from those showing early severe myopathy. Birds with severe myopathy, including early SM‑like microscopic features, had smaller fibers, higher fiber density, more collagen, and poorer vascularization, allowing early detection before visible defects appear.

The complexity of myopathy co-occurrence and overlapping phenotypes challenges the robustness of current classification models. Early detection remains an outstanding need, with most effective tools currently applied postmortem. Furthermore, non-invasive in vivo techniques such as ultrasonography and infrared thermography have been validated for detecting other poultry myopathies, with biopsy used as a complementary tool for detailed morphological assessment (Castilho et al., 2021). However, comparable studies focusing specifically on SM and gaping remain limited, highlighting an important area for future research. Future advances should focus on scalable, cost-effective, and rapid technologies capable of real-time, in vivo assessment, alongside deeper integration of biological factors to tailor detection and mitigation strategies. To date, a summary of available detection methods with their technique and effectiveness for SM and Gaping myopathies is given in Table 1.

Table 1.

Different detection methods with their technique and effectiveness for SM and Gaping myopathies.

Method	Target Myopathy	Technique	Effectiveness	Limitations	Reference
Visual and Morphometric Scoring	SM and Gaping	Manual scoring (0-2) in Pectoralis major and Pectoralis minor	Detects severity	Subjective; time-consuming; high inter-observer variability	Sirri et al. (2016) Soglia et al. (2019a)
Bioelectrical Impedance Analysis	SM	Measures the resistance and reactance of breast fillets	Accurately separates normal from severe WB and SM fillets	Lower accuracy for SM (52-55% in cranial segment)	Morey et al. (2020) Siddique et al. (2021)
Automated Image Analysis	SM	Deep learning based automated image analysis	Detects early myopathy signs by analyzing myofiber size and collagen quantification	Not applicable at the slaughter line; limited sample size	Dayan et al. (2024a)
Color and Texture Analysis	SM	Color reflectance	Differentiate breast myopathies	Only moderate SM existed in the dataset (no severe SM). Texture alone cannot detect SM	Campo et al. (2020)
Near-Infrared Spectroscopy	SM	Near-infrared spectra	Better accuracy in detecting WB than SM	SM overlaps with both normal and WB, reducing classification accuracy	Wold & Løvland (2020); Muñoz-Lapeira et al. (2025)

Histological traits of spaghetti meat and gaping

While the histological features of gaping and SM do have certain similarities, there are also noticeable differences. Both defects are characterized by alterations in the structure of muscles, namely in the collagen and the overall connective tissue composition. Muscle fibers affected by SM are characterized by degeneration, necrosis, and loose separation of fibers, indicating a loss of structural integrity of the muscle (Wu et al., 2024). In comparison to normal and WB circumstances, the connective tissue, especially the perimysium, exhibits higher quantities but thinner, fewer, and shorter collagen fibers (Zhang et al., 2023). The mushy and stringy texture of SM is a result of this changed connective tissue architecture (Sanden et al., 2021).

Fat deposition is another interesting feature; in SM, intramuscular fat is more frequent. This explains the changing look and texture of the meat (Petracci et al., 2019). The presence of neutrophils and the increase in metabolic processes related to fat breakdown and inflammation show signs of cell damage in SM, which involves higher levels of oxidative stress and inflammation (Wu et al., 2024; Choi et al., 2024). Moreover, SM changes the expression of proteins, including desmin and vimentin, which are markers for muscle regeneration, suggesting impaired regenerative mechanisms (Soglia et al., 2020). Vimentin, mostly associated with mesenchymal cells, is frequently connected to fibroblastic infiltration and tissue remodeling, whereas desmin is a muscle-specific intermediate filament essential for preserving myofiber integrity (Soglia et al., 2022). These cellular and histological changes are linked to muscle hypertrophy and fast growth rates, which cause hypoxia, compromised blood supply, and muscle degeneration (Petracci et al., 2019; Kuttappan et al., 2013b). Mazzoni et al. (2020) also looked at how collagen type III and its predecessor, procollagen type III, are distributed in SM samples. Since they contribute to fibrillogenesis, modifications in their deposition may be a major contributing factor to the emergence of muscular disorders. The results showed altered immunoreactivity for procollagen type III in SM samples, suggesting impaired collagen synthesis. There is limited research on gaping myopathy compared to SM, WB, and WS, but work by Che et al. (2025) shows some interesting similarities and differences between SM and gaping. Both conditions involve similar pressures, like rapid growth and ischemia, which cause myofibers to break down and undergo necrosis. Gaping, on the other hand, shows very strong interstitial fibrosis and lipid infiltration (Che et al., 2025).

Factors contributing to the formation of SM and gaping

SM myopathy in broilers arises from a complex interplay of genetic, nutritional, and environmental factors, each contributing variably to its multifactorial etiology (Che et al., 2024). Potential factors contributing to the occurrence of SM are presented in Fig. 4.

Fig. 4.

Factors contributing to the development of spaghetti meat. Created with BioRender.

Genetic Factors. Genetic selection for faster growth and increased output of breast muscle has been a major factor in the production of broilers today. Compared with WB and WS, the heritability of SM is poorly characterized, with limited studies providing robust genetic parameter estimates. Most available heritability data focus on WB and WS, and although generally low genetic correlations with performance traits suggest potential for genetic selection, SM-specific information remains scarce. Bailey et al. (2020) reported a h² of 0.25, 0.07, and 0.04 for WS, WB, and SM, which is moderate to low. Whereas Lake et al. (2021) estimated at 0.5 for WB and WS, with a high genetic correlation between them (0.88). This shows that there is a small genetic factor for SM, but the most important factors are non-genetic, management, and environmental. These estimates vary widely across studies, largely due to differences in phenotyping scales and analytical approaches. Although the presence of a genetic component provides some opportunity for selection, the low heritability suggests that genetic improvement would be gradual. Evidence from WB demonstrates the feasibility of this approach, as selection in higher-generation broilers has achieved an annual relative reduction of approximately 9.2% in WB incidence (Bailey et al., 2020). Moreover, genetic correlations between myopathies and key performance traits, including breast yield and body weight, are generally low to moderate and vary across studies, suggesting a partially shared and complex genetic basis. This supports the use of balanced breeding strategies to reduce SM incidence while maintaining broiler performance (Barbut et al., 2024).

Additionally, SM is more common in some broiler strains than in others because of underlying genetic differences. Strains chosen for better breast yield and rapid development rates have a higher SM rate (Bailey et al., 2020). This hypothesis was confirmed by Maynard et al. (2023), observing high-yielding strains expressing a higher incidence of SM and gaping. Montagna et al. (2019) could not detect differences in the incidence of severe SM among different strains (Ross, Cobb, Hubbard). However, Ross broilers exhibited a significantly higher incidence of normal breast fillets, whereas Hubbard broilers showed a significantly greater incidence of moderate SM compared with the other strains evaluated.

A recent study Che et al. (2025) analyzed 19 different genes across four biological pathways: oxidative stress response, cellular signaling, muscle development and repair, and collagen and connective tissue organization and their role in development of gaping in broiler tenders. In particular, the downregulation of antioxidant and energy-regulating genes such as SOD3, PRKAA2, and LKB1 in severe gaping of heavy birds indicated reduced oxidative defense and compromised metabolic adaptation, potentially increasing muscle fiber fragility and separation. Additionally, altered expression of muscle development and repair genes, including MYOD1, MSTN, and IGF1, suggested an imbalance between muscle regeneration and growth inhibition during the progression of gaping. To our knowledge, this remains the only study that has investigated gene expression associated with gaping myopathy. Consequently, further research is warranted to elucidate genetic and transcriptomic differences underlying this condition.

It has been well established that a bird's sex influences different myopathies, though not enough data is available on gaping. The occurrence of WB is more common in males, while females have higher SM (Che et al., 2022; Novoa et al., 2022; Wu et al., 2024). Transcriptomic analyses indicate that males exhibit more pronounced transcriptional alterations associated with SM, suggesting the presence of sex-specific molecular mechanisms underlying this myopathy (Bošković Cabrol et al., 2025). It is not confirmed yet what the actual causes for these occurrences are, and it could be an important field for future study.

The combination of transcriptomic and genetic data reveals that SM and gaping myopathies share similar molecular pathways, specifically those related to extracellular matrix remodeling, immune responses, and inflammatory processes, indicating a common pathogenic foundation despite their unique macroscopic phenotypes (Che et al., 2024; Bošković Cabrol et al., 2025). Recent omics studies indicate that SM shares substantial molecular similarities with WB, particularly in pathways related to extracellular matrix remodeling, inflammation, and energy metabolism, as evidenced by overlapping differentially expressed genes and metabolites (Che et al., 2024; Wu et al., 2024; Choi et al., 2024). Transcriptomic analyses revealed no differentially expressed genes between SM and WB directly, suggesting common underlying physiological alterations, while metabolomic and proteomic data identified both shared and SM-specific metabolic disruptions, including altered NAD⁺/NADH balance and collagen-related processes (Sanden et al., 2021). Collectively, these findings support the concept that SM represents a distinct phenotype within a broader spectrum of breast muscle myopathies.

Most studies on genetic mutations have looked at WB and WS, but there is limited research on SM-specific mutations. This makes it difficult to understand the unique genetic basis for this myopathy (Che et al., 2024; Lake et al., 2021). The heritability of SM is inadequately quantified compared to WB and WS, as limited studies have provided reliable estimates of its genetic parameters. Consequently, extensive genetic parameter estimation studies are essential in commercial poultry populations, utilizing genomic relationship matrices to enhance the precision of heritability and genetic correlation estimates (Bailey et al., 2020; Bailey, 2023). Also, there are no validated early molecular biomarkers for predicting the onset of SM and gaping in live birds. Early detection is essential for reducing economic losses; however, the majority of presently available biomarkers are identified postmortem or at advanced stages of the condition (Kong et al., 2021).

Muscle Hypertrophy. Studies both in SM and gaping from recent years showed a common feature in the muscle. Breast muscles grow faster in modern broilers, which leads to muscle hypertrophy. Accelerated growth rates promote rapid muscle hypertrophy, resulting in a marked increase in muscle volume. As muscle fibers enlarge and myofiber density increases, particularly in the cranial region of the breast, the spatial distance between capillaries and muscle cells also expands, thereby limiting the efficient diffusion of oxygen and nutrients to the tissue (Soglia et al., 2021). The imbalance between supporting connective tissue and vascular supply results in hypoxia and oxidative stress, a common trait for most of the myopathies (Petracci et al., 2019). The possible time series of events, molecular pathways, and biological processes involved in the onset and triggering of the occurrence of WS, WB, and SM myopathies affecting broilers’ Pectoralis major muscle have been previously reviewed and are presented in Figure 5 (Soglia et al., 2021; Green et al., 2025). Gene expression analyses consistently highlight the importance of hypoxia and oxidative stress pathways, especially the activation of the hypoxia-inducible factor 1 (HIF-1) pathway, which alters energy metabolism and promotes fibrosis and lipidosis in the affected muscles (Alnahhas et al., 2023; Malila et al., 2019 and 2022). These results support existing theories that muscle fiber hypertrophy and inadequate vascularization play a role in the onset of myopathy. In terms of gaping, Che et al. (2025) found upregulated specific genes such as HIF-1, SOD1, SOD2, and SOD3 related to oxidative stress, which confirms the similarities between gaping and SM.

Fig. 5.

Schematic representation of the possible time series of events, molecular pathways, and biological processes involved in the onset and triggering of the occurrence of WS, WB, and SM myopathies affecting broilers’ Pectoralis major muscle (Soglia et al., 2021).

Nutrition. Nutritional mitigation strategies are well established for other myopathies, but remain poorly characterized for SM and gaping. Specific dietary treatments can affect the occurrence and severity of SM. Additives such as guanidinoacetate (GAA) help to lower the SM by increasing the water-holding capacity of the meat and lowering the oxidative stress (Dayan et al., 2024b). Also, organic trace elements, including zinc, copper, and selenium, have been demonstrated to increase antioxidant defenses and lower the frequency of SM (Kuttappan et al., 2021). A study conducted by Novoa et al. (2022) showed that feed particle size influences SM, suggesting coarser feed reduces the incidence of SM, which may be due to improved gut health and reduced oxidative stress.

Increasing the dietary digestible arginine-to-lysine (Arg:Lys) ratio was associated with a reduced incidence of SM in broiler breast meat. In particular, supplementation with approximately 30% additional arginine resulted in a significantly higher proportion of breasts without SM abnormalities compared with the control diet (Zampiga et al., 2019). These findings suggest that optimizing the Arg:Lys ratio may represent an effective nutritional strategy to mitigate SM development without negatively affecting growth performance or meat quality. A summary of the studies on the effect of nutritional supplementation on SM is shown in Table 2.

Table 2.

Effects of nutritional supplementation on SM in broiler breast fillet.

Nutritional Supplement	Dosage / Inclusion Level	Broiler Strain	Sex	Effect on Spaghetti Meat	Reference
Amino Acid Complex	50 mg/kg	Ross 308	Mixed-sex	Significantly reduced the incidence of SM	(Onbaşılar et al., 2025)
Guanidinoacetate	0.06%	Ross 308	Male	The risk of spaghetti meat myopathy was reduced	(Dayan et al., 2024b)
	0.06%	Ross 708	Male	No effects of supplementation	(Córdova-Noboa et al., 2018)
Sodium Butyrate	500 mg/kg	Ross 308	Male and Female	No significant difference in SM	(Pascual et al., 2020), (Huerta et al., 2024)
Chlorella vulgaris	0, 3, and 6%	Ross 308	Male and Female	No significant difference in SM	(Bošković Cabrol et al., 2024)
Dietary Fat	7.7% vs. 6.7% in the grower diet; 8.9% vs. 7.7% in the finisher diet	Ross 308	Male and Female	Reduction in dietary fat greatly reduces SM occurrence	(Huerta et al., 2024)
Chestnut wood and Grape pomace	0.2%	Ross 308	Male and Female	No significant difference in SM	(Pascual et al., 2022)
Tannic Acid	0.25, 0.5, 1, 2 g/kg	Cobb 500	Male	No significant difference in SM	(Choi et al., 2022)
Arginine-to-Lysine	20 and 30% higher levels of crystalline L-arginine (dig Arg:Lys ratio = 1.25, 1.25, 1.26, 1.27 and 1.35, 1.35, 1.36, 1.37, respectively)	Ross 308	Male	Increasing the Arg:Lys ratio reduces the SM	(Zampiga et al., 2019)
	Control: (Arg:Lys) of 1.06, 1.06, 1.07,1.08 Other Treatment: Arg:Lys ratio of 1.15, 1.25 and 1.35	Ross 308	Male	No significant difference	(Westreicher-Kristen et al., 2025)
Probiotic (Chlorohydroxyquinolines)	Half Dose: 5 × 108 cfu/chick Positive control: 1 × 109 cfu/chick Double Dose: 2 × 109 cfu/chick	Cobb Slow	Male	Reduce the incidence of SM feed Double Dose	(Almeida Paz et al., 2019)
Grape seed extract	1%, 0.2%, or 0.4%	Ross 308	Male and Female	No significant difference	(Huerta et al., 2022)
Frass (BSFLF)	5, 10, or 15%	Cobb MX × 500	Male	No significant difference	(Beckman et al., 2025)
Crude fiber concentrate	Diet 1: 0.4%, 0.8%, 0.8%, 0.2% Diet 2: 0.6%, 1.0%, 1.2%, 0.4%	Ross 308	Male	No significant difference	(Urban et al., 2025)
Herbal choline	450, 900, 1000 mg/kg	Cobb 500	Male	No significant difference	(Dias et al., 2023)

Environment. The low heritability of SM suggests that, while a genetic component exists, non-genetic factors, especially management practices and environmental conditions, can exert a more significant influence on its development (Naeem et al., 2026). There have been several studies available on SM and other myopathies, determining different effects of environmental factors. Che et al. (2022) found that an increased risk of SM has been related to higher ambient temperatures throughout the grow-out stage, as they exacerbate oxidative stress and muscle hypertrophy, appearing to compromise muscle integrity and increase myopathy prevalence. However, there is currently a lack of published data regarding the impact of this factor on gaping. There is also the effect of housing systems, where dark houses with poor ventilation showed a higher occurrence of SM than tunnel-ventilated systems (Montagna et al., 2019). Increased metabolic stress and oxidative damage are potentially responsible for these defects. However, Składanowska-Baryza et al. (2025) were unable to detect a difference in the incidence of SM based on the housing system (free-range vs. indoor). Yi et al. (2016) showed that high concentrations of atmospheric ammonia can alter gene expression, leading to lipid metabolism changes and increases in breast myopathies. Other environmental factors such as humidity and stocking density may also influence the development of SM and gaping. However, limited research is currently available regarding their specific effects, highlighting important areas for future investigation.

Processing. The low heritability of SM suggests the influence of some other non-genetic factors on the variation of breast muscle myopathy (Radaelli et al., 2017; Bailey et al., 2020). Certainly, the currently limited correlation between the occurrence of SM condition and birds' growth rate, age, and market weight suggest other "contributing factors" connected to processing activities (e.g., scalding, de-feathering, chilling, post-mortem deboning time and technology) that could influence the expression of SM condition or, at least, an increase of its apparent severity level (see list of factors in Table 3). Recent reports indicate that mechanical action applied to the breast fillets can impact the level and severity of SM. The muscle already having the underlying condition makes it susceptible to showing the SM problem (less collagen crosslinking and a different type of collagen than normal meat) before the birds arrive at the processing plant (Barbut et al., 2024). Factors such as agitation during water chilling and mechanical force exerted during primary processing, including defeathering, deskinning, and deboning, can potentially exacerbate the severity and visibility of this problem (Che et al., 2022). Technical research showed that delayed carcass chilling resulted in 50% more SM compared to their fast-cooled equivalents (Anton et al., 2019). The researchers attributed this to excessive lactic acid accumulation in the muscle when the carcass is still warm. This can enhance the activity of protease enzymes capable of speeding up the breakdown of connective tissue, and lead to enhanced softening of the meat. Che et al. (2022) also reported a greater SM in water chilling over air chilling due to the increased agitation in water chilling systems.

Table 3.

Effects of processing conditions on SM incidences in broiler breast fillet.

Step	Technique	Potential Effects*	Main Findings	Reference*
Feed Withdrawal	Time	(Too short and have more lactic acid in the muscles) (Too long may lead to metabolic stress, increased muscle proteolysis and reduction in glycogen reserves)	(Suggest no shorter than 4 hrs or longer than 12 hrs)	(Industry recommendation)
Chilling	Water vs air	Excessive massaging causes separation	Using an air chill is less aggressive	Che et al. (2022)
	Air	Slow air chilling prolongs postmortem warm conditions, increasing proteolytic degradation	Adequate chilling after processing to prevent loss of muscle integrity	Anton et al. (2019)
	Time to 4°C	(Reduce incidence if faster cooling)	(Suggest reaching within 2 hrs post-mortem)	(Industry recommendation)
Scalding	Water temperature	Higher scalding temperatures (>60°C) cause thermal softening of perimysial collagen	Optimize temperature and feather plucking time	Anton et al. (2019); Baldi et al. (2021)
			(52/53°C and adjust as needed for young broilers)	(Industry recommendation)
Feather Plucking	Too aggressive	Worsen an already compromised muscular structure	Making muscle fibers more susceptible to tearing during handling and filleting	Cahaner (2024); Anton et al. (2019); Baldi et al. (2021)
	Plucking fingers elasticity	(Stiff fingers as used for the leg area, can result in more apparent SM)	(Use softer fingers for the breast fillet area, which can be distinguished by color)	(Industry recommendation)
	Distance between drums	(Too narrow causes more force and SM)	(Individual flock weight and uniformity are very important for continuous adjustment)	(Industry recommendation)
De-skinning equipment	Automated high-speed skin removal from the breast area	(Unadjusted equipment can cause initial tearing of the muscle that will later expand)	(Adjust scoring blades, including pulling angle, force and speed. Flock size uniformity is also important)	(Industry recommendation)
Deboning equipment	Automated high-speed fillet removal from breast area	(Unadjusted machines will cause a ripping effect that will later expand)	(Adjust scoring blades, including depth and angle. As well as carrier orientation and movement. Flock size uniformity is also important. Some split broilers by size)	(Industry recommendation)
Use of Peroxyacetic acid	Higher level to reduce microbial contamination	Impact connective tissue and can cause degradation	Negatively impact meat quality at certain concentrations and exposure times (Avoid using the highest legal level for too long. More common levels are 50 to 200 ppm)	Che and Hall (2024); Bailey (2023) (Industry recommendation)
Further processing	Use of alginate	Alginate bind SM texture and reduces cooking loss	The texture of SM can be restored to near-normal meat levels by this cold-set gel	Wang et al. (2023)
	Use of dairy proteins	Mitigate the low water-holding capacity and texture deficiencies of SM	Dairy proteins improve the yield and texture of SM minced meat	Wang et al. (2022)
	Use of citrus fiber	Using SM in chicken frankfurters does not significantly affect color, TPA, fat, and protein concentration	Citrus fiber affected the frankfurter texture, pH, and fat and protein concentration	Belem (2022)

^⁎Material in parentheses is based on industry practice/ recommendations since such information is currently not available in the scientific literature. See discussion in Section 5.5.

Process steps such as scalding and plucking should be investigated further to see whether set parameters can variably worsen textural issues in fillets with mild muscle damage. There is some evidence that SM samples have weakened perimysial connective tissue (Baldi et al., 2018). This may imply that combining high scalding temperature with aggressive defeathering can further damage an already compromised muscular structure. Overall, an automated de-feathering process utilized rotating drums equipped with hundreds of rubber / plastic fingers that apply force that can affect the severity of the pre-existing SM condition. In fact, decreasing the number of defeathering machines, thereby reducing the overall force applied during the process, can lower both the occurrence and severity of SM (Cahaner, 2024). Fig. 6 shows data comparing two levels of the de-feathering intensity achieved by using 5 versus 7 de-feathering units in a commercial processing line, employing the Kosher process (i.e., without hot water scalding; process that requires more mechanical force to pluck the feathers than hot water scalding). The results show that 5 units produced lower levels and severity of SM compared to 7 units in both male and female broilers.

Fig. 6.

Comparing the effect of two levels of defeathering intensity on spaghetti meat occurrence. Data obtained from broilers not exposed to hot water scalding (Kosher process), using a 5-level gradual scale (0.0, 0.25, 0.50, 0.75 and 1.00; from none to severe SM), in 200 male broilers (BM=2.9 kg) and 200 female broilers (BM=2.5 kg) obtained from 5 farms, employing 5 or 7 defeathering units. Cahaner, 2024. With permission.

However, it should be indicated that reducing the number of plucking units comes with a cost of more pin/regular feathers at the end of the line, and this is unacceptable in many places, as it translates to costly downgrading. Another point is adjusting the distance between the picker’s drums (positioned on both sides of the moving birds). In this case, information in the scientific literature could not be located about the exact measurements, but different industry contacts, who were interviewed during the preparation of the manuscript, strongly advise to closely monitor the distance and adjust it on a daily basis (e.g., when switching from a light to heavy flock). Because there is little information in the scientific literature about the equipment used at the processing plant, such information/industry recommendations were included in Table 3. Parentheses were used to list these parameters with a specific note indicating that those are not peer-reviewed data but may be the starting points for future research. Another area associated with the defeathering operation is the elasticity/softness of the plucking fingers. Again, some industry contacts indicated that replacing the fingers directed towards the breast fillet area with softer fingers has helped their operation (note: from a practical point of view, this can add another layer of complexity to the maintenance crew, but using for example, different color fingers has been reported useful). Fig. 7 illustrates the presence of SM at various stages of processing within a commercial plant. Overall, the incidence increases as additional manipulation, cutting, and deboning steps are introduced. Some companies are now actively monitoring these stages with the goal of reducing prevalence at each point. Fig. 7 provides an overview of the processing steps that may exacerbate the issue and therefore serves as a useful starting point for evaluating and optimizing plant procedures. In addition, gaping conditions in the tenderloins (Pectoralis minor), which have similar phenotypic conditions to SM, should also be examined to evaluate the potential correlation between these two myopathies and their relation to processing steps.

Fig. 7.

Prevalence of normal breast fillets versus spaghetti meat (SM) after different processing stages at a commercial processing plant. Data were collected from 40 flocks (average BW 2.9 kg) over a 5-month period. At each point, 40 ± 5 fillets/flock were evaluated. Mild and severe SM scores were combined for statistical analysis. (Barbut; personal communication).

Peroxyacetic acid (PAA) is extensively utilized in North America and other parts of the world (not in the EU) as an oxidizing antimicrobial agent to reduce foodborne pathogens such as Salmonella in raw poultry (Mahamud et al., 2025; Manjankattil et al., 2025). In contrast to chlorine-based sanitizers, PAA decomposes into non-toxic byproducts, acetic acid, oxygen, and water, resulting in no harmful residues; hence, it is ecologically friendly and authorized by regulatory bodies such as the USDA and FDA. PAA is currently applied in the main poultry chiller at concentrations up to 220 ppm (Kataria et al., 2020), while concentrations as high as 2,000 ppm are permitted in the post-chill immersion tank in USA (Cano et al., 2021). Recent evidence suggests that PAA may influence muscle structural defects. Che et al. (2025) reported that the prevalence of severe gaping significantly increased from 71.0% to 81.3% following PAA treatment. However, studies conducted in Italy and Canada also found SM occurrences where PAA was not used. De Rezende et al. (2024) reported no significant change in texture or water holding capacity after PAA treatment (up to 1993.5 ppm for 32.05 s). However, Laranja et al. (2021) showed that chicken skin was affected by PAA exposure time (700 ppm for 15 s, 5 min, 30 min). Although it is effective against microbes, PAA shows a lower redness value because of the oxidizing property of antimicrobials, but no sensory attributes are affected (Chen et al., 2014; Moore et al., 2017). In Table 3, some industry recommendations about the use of PAA are listed. More research is warranted to explore how PAA treatment settings affect these significant quality features. Other processing steps, such as scalding, chilling, storage, and deboning time, may have an influence on SM and gaping occurrence that should also be taken into consideration.

As mentioned above, the poultry meat industry needs to use a multi-focused approach to reduce the incidence of myopathy-affected meat. While short-term mitigation strategies encompass on-farm interventions, such as regulating growth rates, and processing-level strategies, can also include the removal of severely affected SM and gaping fillets for conversion into further processed products. Long-term solutions, like genetic selection, on the other hand, would take longer to put into place and cannot immediately resolve the problems the industry is facing. Both short- and long-term mitigation strategies are summarized in Fig. 8.

Fig. 8.

Possible ways to mitigate SM and Gaping myopathies. Created with BioRender.

Effects on meat quality

Although some studies have been published on the effect of SM on meat quality, its influence on technical and functional characteristics remains less well characterized compared to WS and WB (Baldi et al., 2019; Pascual Guzmán et al., 2021; Wu et al., 2024). Multiple studies have demonstrated that SM leads to softer, mushy breast meat with reduced shear force and altered textural profiles, distinguishing it clearly from normal and other myopathies (Baldi et al., 2018; Wu et al., 2024; Pascual Guzmán et al., 2021). After cooking, SM fillets exhibit a softer texture, as determined by compression, Meullenet–Owen's razor shear, and Allo–Kramer tests (Baldi et al., 2019; Pascual Guzmán et al., 2021), likely due to a reduced degree of collagen cross-linking. However, data are currently lacking on whether Meullenet–Owen's razor shear or blunt Meullenet–Owen's razor shear instrumental methods can reliably differentiate SM and or gaping from normal in the raw breast fillets.

Yu et al. (2020) found no difference in measured water holding capacity in fresh, frozen, and cooked SM samples while Pascual Guzmán et al. (2021) found more drip and cooking losses in SM samples compared to normal samples. On the other hand, for gaping, Soglia et al. (2019a) found similar results on drip loss and cook loss compared to normal samples. This supports earlier results from time-domain nuclear magnetic resonance analysis, which revealed higher amounts of bound and extra-myofibrillar water in SM fillets. These water fractions were less strongly bound to the tissue, suggesting changed water distribution and mobility in the SM muscle and reduced water retention capacity (Baldi et al., 2021). Tasoniero et al. (2020) also found that SM fillets had a significant reduction in myofibrillar protein solubility, salt-induced water uptake, final yield, and emulsion stability compared to normal breast. Interestingly, even though SM shows these negative changes, studies by Baldi et al. (2018) and Soglia et al. (2019b) did not report any increase in protein carbonylation levels, suggesting that oxidative damage is probably not the main reason for the poor water-holding capacity in SM meat. This indicates that something else is causing these changes. Research suggests this is due to major alterations in the muscle structure itself. The muscle undergoes degeneration and necrosis, which leads to fewer functional myofibrils, which are the main components of the muscle that can hold water (Baldi et al., 2020; Tasoniero et al., 2017). As a result, the muscle loses its ability to retain its natural water. The lower water-holding capacity in SM fillets might be directly linked to the reduced protein solubility. This could be attributed to protein degradation, as shown by the higher myofibrillar fragmentation index in SM samples and the increased amount of free amino acids released during protein breakdown (Baldi et al., 2018). On the other hand, SM is quite similar to gaping, as Soglia et al. (2019a) showed longer NMR relaxation periods in the extra-myofibrillar water fraction in gaping muscles, indicating that water is less firmly bound and more readily released during processing. This suggests that there is more drip loss since the water in the gaping muscles varies. For the instrumental color, SM shows higher yellowness (Baldi et al., 2018; Tasoniero et al., 2020), whereas gaping has a higher lightness value (Soglia et al., 2019a), reflecting a reverse correlation between lightness and pH, where lower pH tends to be lighter in color. In SM, muscle fiber degradation reduces essential amino acids and alters the fatty acid profile, particularly affecting linoleic and arachidonic acid pathways linked to inflammation and oxidative stress (Wu et al., 2024). In addition, gaping negatively correlated with methionine, serine, and glutamine concentrations, suggesting their potential role in collagen synthesis and muscle integrity (Soglia et al., 2019a).

Researchers are still trying to fully understand and mitigate the occurrence of these myopathies, while processing-based approaches are still quite useful and effective, but result in some financial losses. According to Baldi et al. (2021), separating the affected outer surface and the unaffected inner core of the fillets can be a practical solution. In SM, the affected outer surface can be used as further processed products, while the addition of functional ingredients (e. g., starches, phosphates, hydrocolloids, see also Petracci et al. (2019)) can improve the meat quality, and the inner sections can be used as fresh retail. The use of dairy proteins and alginate gels to restore texture in SM meat has also shown promising technological applications (Wang et al., 2022 and 2023). Soglia et al. (2019a) suggested that freezing and thawing do not affect the SM meat quality traits, which can be used with more flexibility by the processing industry.

Conclusions

SM and gaping represent emerging and increasingly important challenges for the poultry industry, arising from complex interactions among genetic, physiological, nutritional, and environmental factors. Compared with more extensively studied myopathies such as WB and WS, the prevalence of SM and gaping appears to be increasing, underscoring the need for greater research attention. Future research priorities should focus on improving early and online detection technologies, advancing selective breeding strategies, and optimizing nutritional and processing interventions. Short-term mitigation approaches, including dietary modifications such as GAA supplementation and adjustments to the arginine-to-lysine ratio, show promise; however, these strategies may increase feed costs or alter nutritional density, potentially leading to performance losses that negatively affect production margins. Therefore, nutritional interventions must be carefully evaluated to ensure that they deliver consistent, biologically meaningful improvements without compromising efficiency or profitability.

In addition, slaughter and post-slaughter operations, including scalding, defeathering, chilling, post-mortem deboning time, PAA use, and processing technologies, should be optimized to minimize the incidence and severity of SM and gaping while ensuring high-quality products for consumers. Enhanced emphasis on online detection systems and value-added further processing may also help mitigate economic losses associated with these defects.

Over the long term, sustainable solutions will require targeted genetic improvement supported by deeper mechanistic and genomic studies. A coordinated approach integrating genetics, nutrition, processing, and detection technologies will be essential to effectively manage SM and gaping, safeguard poultry meat quality, maintain industry profitability, and preserve consumer acceptance.

CRediT authorship contribution statement

Zahidul Hasan Tushar: Writing – review & editing, Writing – original draft, Conceptualization. Shai Barbut: Writing – review & editing. Muhammad Naeem: Writing – review & editing. Rishav Kumar: Writing – review & editing. Rabin Raut: Writing – review & editing. Dianna V. Bourassa: Writing – review & editing, Supervision, Conceptualization.

Disclosures

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Bourassa reports financial support was provided by USDA Agricultural Research Service. Bourassa serves as a associate editor for Poultry Science. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.