In the present study, male Cobb 700 broiler chicks were obtained from Cobb-Vantress (Siloam Springs, AR). Feed and water were provided ad libitum, with all the treatments receiving a commercial feed depending on the phase. Diets were formulated to approximate the nutritional requirements of broiler chickens as recommended by the National Research Council (1994) and were adjusted to breeder recommendations (Cobb-Vantress Inc., Siloam Springs, AR). At 57 d, 206 birds were processed using a commercial in-line system at the University of Arkansas. All animal handling procedures complied with the Institutional Animal Care and Use Committee at the University of Arkansas, Fayetteville.

About 10 h before slaughter, feed was withdrawn, but the birds were given an ad libitum supply of water. A commercial-style processing in-line system was used where the birds were electrically stunned, manually slaughtered by severing the left carotid artery and jugular vein, bled out, soft scalded, and defeathered (Mehaffey et al., 2006). The carcasses were then manually eviscerated, prechilled at 12°C for 15 min followed by chilling for 90 min at 1°C in immersion chilling tanks. While prechilling and chilling, the carcasses were manually agitated frequently to prevent the thermal layer in the tank and to enhance the chilling efficiency. The carcasses were taken out of the tanks, packed in ice, and aged at 4°C until deboning at 3 h postmortem, common industry deboning times. Ready-to-cook weight of each carcass was measured before deboning. The Pectoralis major muscle was removed from each carcass by 6 trained people to avoid any alterations in fillet dimensions and other meat quality parameters due to the deboning procedures. The butterfly fillet from each bird was placed in common zip-sealable freezer bags and stored at 4°C until analysis.

Each butterfly fillet was halved into left and right. The left fillets were used for measuring pH and color, whereas texture analysis was conducted on the right fillets.

The data were analyzed using an analysis of variance with the WB categories as treatments: normal (score 0), mild (score 1), moderate (score 2), and severe (score 3). Least-squares means were separated with a t test when only 2 factors and Tukey’s honestly significant difference for more than 2 comparisons at a significance P < 0.05 using JMP^® Pro 14 (Cary, NC). The Pearson correlation was done using multivariate, and the scores were considered continuous. For the effect of freezing on CF, a mixed model (JMP Pro^® 14) was used to analyze repeated measures data. Nominal logistic regression was also conducted; in this case, scores were collapsed into 2 categories: Normal (scores 0 to 1) and Severe (1.5 to 3). The covariates or continuous variables (inputs) included the carcass/meat quality parameters such as ready-to-cook weight, fillet length, fillet width, keel length, thickness, caudal length, pH, L*, a*, b*, CF, cook loss, peak counts, and MORSE. The data were analyzed using nominal logistic regression procedure with JMP^® Pro 14. The model will test each one of the inputs and proceed adding the next most significant input until all the significant parameters are included in the model. The results from the analysis are reported mainly as the odds ratio (OR), 95% CI, and the respective P values. OR is the ratio of the probability of an event of interest (e.g., probability of Normal) to the probability that the event will not occur (e.g., probability of Severe), and OR is the ratios of 2 odds comparing 2 groups. The OR indicates the increased or decreased chance of a dependent category as a result of an increase in the continuous variable by 1 unit or with a categorical variable in comparison with a reference. OR > 1 indicates an increased chance, whereas OR < 1 denotes a decreased chance. When the OR is equal to 1, there is an equal chance for the category in question and the reference category (Kuttappan et al., 2013a). The estimated probability of occurrence of the 2 degrees of WB was determined for all the categorical variables.

Out of the 206 fillets evaluated, 45 (22%) were normal; 51 (25%) showed mild lesions of WB, 62 (30%) showed moderate lesions of WB, and 48 (23%) showed severe lesions of WB. In both the left and right sides of fillets on day of processing (day 0), CF significantly increased (P < 0.05) as WB score increased (Table 1), and CF was highly correlated to WB category (r = 0.77; Table 2). However, the right breast side had higher CF on normal, mild, and severe fillets compared to the left side within each WB category (P < 0.05; Table 1).

As the WB score increased on chilled or thawed breast fillets, CF also significantly increased (P < 0.05; Table 3). Thawed breast fillets for each WB score had a significantly lower CF compared with chilled fillets (P < 0.05) and resulted in CF reductions of 51% to 65% due to freezing/storage (Table 3).

A significant increase in thickness of the fillets was observed as the severity of the WB score increased. However, the fillet length was significantly shorter (P < 0.05) in fillets with a severe WB score. No significant differences were noted in the cranial width, caudal width fillet, or keel length among the fillets, regardless of the WB score (Table 4). Thickness was highly correlated to WB category (r = 0.67), whereas other dimensions had a low and/or nonsignificant correlation to WB category (Table 2).

Cook loss (percent) of fillets significantly increased as severity of WB increased (P < 0.05) (Table 5). No significant differences were observed on MORS force (P > 0.05); however, fillets with severe WB score had the highest (P < 0.05) MORS peak count, BMORS force, and BMORSE compared to moderate, mild, and normal fillets, which did not differ from each other (P > 0.05; Table 5). Severe and moderate fillets also had higher BMORS peak count compared to other categories, and normal fillets had the lowest BMORS peak count (P < 0.05). The pH of breast fillets increased as WB severity increased; severe WB had higher pH (P < 0.05) than normal fillets, while mild and moderate WB fillets were intermediate (Table 5). Moderate and severe fillets had higher L* and b* values (P < 0.05) than normal fillets, and mild fillets were intermediate. At processing, broilers with moderate and severe WB had higher live weight, carcass weight, and breast yield compared to broilers with normal or mild WB fillets (P < 0.05).

Significant and positive correlations were observed between WB score and CF, thickness, fillet length, breast yield, L*, b*, pH, cook loss, MORS peak count, as well as BMORS force, BMORSE, and BMORS peak count (Table 2). All other parameters had nonsignificant correlations.

The nominal logistic model obtained in the present study showed the main carcass and meat quality factors that were significantly (P < 0.05; Table 6) associated with severe WB. These factors include cranial thickness, L* value, fillet length, breast weight, and caudal width. The L* value (which indicates lightness) had the highest OR value influence on the occurrence of severe fillets (OR 1.30; 95% CI 1.07–1.59), followed by cranial thickness (OR 1.29; 95% CI 1.14–1.49), indicating that, as the L* value or cranial thickness of the fillets increases, there is a greater probability that it could have a severe degree of WB (Table 6).

Subjective scoring of fillets can be done based on a butterfly or single fillet (right or left) in research and in plants for sorting purposes. CF of the left side and right side of the breast fillet were significantly different, with the right side of the breast having higher CF than the left side (P < 0.05; Table 1). The reason that the right side was higher than the left side is unknown. However, the differences between each WB category were greater than the differences due to side (left vs. right). With either side, significant differences (P < 0.05) between WB categories were still present. The CF was also highly correlated to WB category, which supports the high correlations reported by Sun et al. (2018).

The reduction in CF between chilled (day of processing, day 0) and thawed (after freeze-thaw cycle) (CF chilled > CF thawed; Table 3) may be due to the freezing process and/or due to changes that occur simply because of aging. During freezing, ice crystals form in the meat and can lead to the loss of membrane strength and subsequent structural damage (Leygonie et al., 2012). Though freezing should halt most changes that normally occur in postmortem aging, the time required to freeze and then time to thaw would allow some aging to occur. Substantial changes in CF when fillets were stored multiple days (at 4°C) have been previously reported (Sun et al., 2018; Hasegawa et al., 2020). Sun et al. (2018) reported approximately 30% to 60% reduction from day 0 to days 3 or 4. Accounting for the time to freeze and time to thaw, the fillets in this study would be an equivalent of 3 to 4 days of aging (minus the frozen storage time). In the current study, 51% to 65% reductions in CF were observed, which would be similar to reductions observed by Sun et al. (2018) in a similar aging period. Bowker and Zhuang (2019) also reported that the freeze-thaw cycle resulted in a softening effect on fillets with varying degrees of WB severity. It is important to note that, although a softening effect of the raw fillets may be observed due to freezing and/or aging, the shear characteristics may not be impacted, as was observed with the BMORS results in this study and further supported by results from Bowker and Zhuang (2019). Further research is needed to determine the effect of freezing alone.

WB has negative implications for meat quality (Tijare et al., 2016). The pH was significantly different (P < 0.05) between normal and severe WB, and mild and moderate WB fillets were intermediate (similar to both normal and severe, P > 0.05; Table 5). This trend was also observed in previous studies (Dalle Zotte et al., 2017; Kuttappan et al., 2017; Xing et al., 2017; Cai et al., 2018), but no differences were reported in many other studies (Mudalal et al., 2015; Trocino et al., 2015; Soglia et al., 2016; Wold et al., 2017; Chen et al., 2018; Dalgaard et al., 2018). However, L* value in this study increased with WB scores, indicating that paleness increased as WB severity increased (Table 5), which is in agreement with previous research related to WB (Dalle Zotte et al., 2017; Wold et al., 2017; Cai et al., 2018). In this study, the OR was high for L* value, suggesting that as L* increases, the chances of WB increase as well (Table 6). Other factors may be involved in increasing L* values in breast fillets that are not related to WB. Owens et al. (2000) and Woelfel et al. (2002) reported that increased L* values in poultry meat were related to low pH and to pale, soft, and exudative meat, which has different characteristics than WB (Tijare et al., 2016). Similar to L* values, the b* values of fillets increased with WB severity, indicating that yellowness increased (Table 5), which is in agreement with previous research (Tasoniero et al., 2016; Kuttappan et al., 2017; Baldi et al., 2018). Negative effects were also noted with cook loss (percent), such that fillets that had severe WB scores lost more water during the cooking process (Table 5), which is in agreement with previous studies (Mudalal et al., 2015; Trocino et al., 2015; Soglia et al., 2016; Tasoniero et al., 2016; Tijare et al., 2016; Xing et al., 2017; Dalgaard et al., 2018). Overall, the occurrence of different degrees of WB were associated with changes in pH, color L* and b* values, cook loss, MORS peak count, BMORS force, BMORSE, and BMORS peak count (assessment of texture quality) (Table 5). These data confirm the negative impact of WB on poultry meat quality.

In recent years, myopathies have caused significant economic losses to the poultry industry due to lost yield and value as a result of consumer complaints and the negative impact on the quality of additional processed poultry meat products (Kuttappan et al., 2016). WS is a disorder characterized by the occurrence of white striations parallel to muscle fibers on breast, thigh, and tender muscles of broilers, whereas WB is characterized by having a distinct hardness of the muscle associated with histological, compositional, and quality changes (Sihvo et al., 2014; Tijare et al., 2016; Griffin et al., 2018; Zhuang and Bowker, 2018). These myopathies have also been moderately to highly correlated to larger broilers (live or carcass weight) and high breast yield (Kuttappan et al., 2012a, 2013b; Mudalal et al., 2015; Alnahhas et al., 2016; Chatterjee et al., 2016; Zambonelli et al., 2016; Dalle Zotte et al., 2017; Xing et al., 2017; Dalgaard et al., 2018). Results from the nominal logistic model indicate that the factors that are most predictive are related to fillet dimensions along with color (Table 6). Thickness (of breast) was highly correlated to WB scores (r = 0.67, P < 0.0001; Table 2) and had high OR, suggesting that, as thickness increases, the probability of WB increases. Previous research has shown that fillet thickness had a much greater impact on fillet weight compared with length and width of the fillet (Lubritz, 1997; Griffin et al., 2018), and generally, higher degrees of WS and WB are associated with heavier or thicker fillets (Kuttappan et al., 2012a, 2013b; Mudalal et al., 2015; Alnahhas et al., 2016; Kuttappan et al., 2017). Similarly, Griffin et al. (2018) reported that models using breast length, width, thickness, and yield and P. minor (tender) width and yield were most predictive of WB in broilers.