Mexican oregano oil from Lippia berlandieri Schauer was used in the study. A total of 300 1-d-old Ross-308 broiler chicks (49.40 ± 1.30 grams) were randomly assigned to five treatments with six replicates (pens) and 10 chicks for each. The experiment considered the production phases of grow-out (starter, 1–2 wk.; grower, 3–4 wk.; finisher, 5–6 wk) to establish the treatments: OEO₀, control diet without OEO (0 mg/kg); OEO₁₂, diet with 100 mg/kg of OEO in weeks 1 and 2; OEO₃₄, diet with 100 mg/kg of OEO in weeks 3 and 4; and OEO₅₆, diet with 100 mg/kg of OEO in weeks 5 and 6; and OEO₁₆, diet with 100 mg/kg of OEO from weeks 1 to 6. The broiler chicks were allocated in iron pens (1.0 × 1.20 × 0.75 m) with a feeder, waterer, and fresh wood shavings. Feed and water were offered ad libitum. Oregano oil was extracted by steam distillation (Natural Solutions S.M.I., Ciudad Jimenez, Chihuahua, Mexico) from leaves of L. berlandieri Schauer and was composed of 76.16% carvacrol, 10.07% thymol, 2.01% α-pinene, and 9.05% other compounds (Clarus 600 and MS SQ8 Perkin Elmer Inc., Waltham, MA; Silva-Vázquez et al., 2017). The diets were formulated according to Ross-308 Broiler Nutrition Specifications (Aviagen®, 2022) and the nutrient compositions are presented in Table 1.

On day 42, bird slaughter was according to the Official Mexican Standard NOM-033-SAG/ZOO-2014 (2014) and Hernández-Coronado et al. (2019). A total of six chickens per replicate per treatment were randomly selected (n = 36 per treatment). After the slaughter and evisceration processes, slaughter weight (SW; weight was recorded before slaughter), hot carcass weight, and cold carcass weight were recorded to estimate the hot carcass yield (HCY; weight was recorded after slaughter; 36 °C) and cold carcass yield (CCY; weight was recorded after cold storage; 4 °C) according to Sánchez-Zamora et al. (2019). The carcasses were stored at 4 °C for 12 h. The breast was then separated from the carcass, and its weight was recorded to estimate the breast yield (BY) and to prepare them for quality analyses.

Physicochemical analysis of breast meat was performed on both sides (measured in each side of the breast). For pH, water holding capacity (WHC), and color, 12 samples per treatments were used [n = 12; 2 breasts per replicate (pen) per treatment]. The pH was measured with a potentiometer (Orion® 3-star Thermo Fisher Scientific, PA, USA) using a puncture electrode introduced directly into the samples.

The WHC was carried out in accordance with the modifications established by Méndez-Zamora et al. (2015) and Cázares-Gallegos et al. (2019). Breast meat samples were weighed (0.30 ± 0.01 g) and placed between two pieces of filter paper, next between acrylic-plastic plates (12 × 12 cm), applying a force of 4 kg for 20 min. The initial (Wi) and final weight (Wf) of samples were recorded to estimate the WHC:

Color was measured directly on the surface on both sides of the breast samples. A colorimeter (CR-400, Konica Minolta®, Tokyo, Japan) was used to determine the lightness (L*), redness (a*), yellowness (b*), saturation index (Chroma), and tonality (Hue angle). Cooking loss (CL) was determinate in 12 breasts (n = 12; 2 breasts per replicate per treatment) according to Sánchez-Zamora et al. (2019). The breasts were deboned then packed in vacuum bags and cooked in hot water (75 ± 0.1 °C) for 90 min. Afterwards, samples were pre-cooled at 20 °C for 30 min. Next, the samples were stored at 4 °C for 12 h. Finally, the weight of cooked breast was recorded. The CL was estimated:

The texture analysis was carried out with the cooked breasts used to estimate the CL according to Sánchez-Zamora et al. (2019). The analysis was done for each side of the breast (n = 12; 2 breasts per replicate per treatment). Shear force (SF; N) and texture profile analysis (TPA) were obtained with a texturometer (TA.XT. Plus, Stable Micro Systems®, Surrey, United Kingdom). In the SF test, a Warner Bratzler knife was attached to the texturometer. The sample dimensions were 1 cm wide and high and 3 cm long, cut parallel to the muscle fibers. The test speeds were pre-test 1 mm/s, during test 2 mm/s, and post-test 5 mm/s, and 15 mm high. In the TPA test, cylindrical samples were used (1.8 cm in diameter and 1.5 cm in height); samples were cut perpendicularly to the muscle fibers. A 75-mm diameter of cylindrical piston was used to compress the sample in two cycles, 50% of the sample height. The test speeds were pre-test 1 mm/s, test 2 mm/s, and post-test 5 mm/s, obtaining force-time deformation curves. The variables measured were hardness (N), adhesiveness (g/s), cohesiveness (dimensionless), springiness (mm), gumminess (g), chewiness (g mm), and resilience (dimensionless) (Cázares-Gallegos et al., 2019; Hernández-Coronado et al., 2019).

An affective sensory test for attributes (Hernández-Coronado et al., 2019) was conducted to measure the satisfaction level of 30 semi-trained consumers. One breast per replicate per treatment (n = 6 breasts) was used for evaluation of cooked breast meat (75 °C ± 0.1 °C; 90 min). Each consumer received four cubes (1.5 cm) per treatment placed in cups coded with three random numbers. The attributes evaluated were odor, taste, juiciness, softness, and overall acceptability. A five-point hedonic scale was used for evaluation, where 5 = liked very much and 1 = disliked very much.

The general linear model (GLM) used to analyze the meat yield and quality was as follows:

where: y_ij = response variables, μ = general mean; T_i = effect of the ith treatment (OEO₀, OEO₁₂, OEO₃₄, OEO₅₄, OEO₁₆) and ε_ij = random error. A significance level of 0.05 was used to find significant differences between treatments (p < 0.05). When H₀ was rejected, the means were compared with the Tukey test (α = 0.05). The sensory analysis was carried out with the non-parametric Friedman test, considering the consumer as a block effect in the analysis; when H₀ was rejected (p < 0.05), the mean comparisons were carried out according to Nemenyi test (Núñez-Colín, 2018). Orthogonal polynomial contrasts (OPC) were performed to evaluate the trend effects (linear, quadratic and cubic) of treatment on the response variables. Minitab® (2013) 17.1.0 statistical software was used for data analyses.

Table 2 shows the carcass traits variables at 42 d for broilers supplemented with oregano oil at 100 mg/kg of diet over the prescribed production phases. The HCY showed differences (p < 0.05) between treatments with OEO₁₆ having the highest yield and OEO₃₄ the lowest. The SW, CCY, and BM variables did not show differences (p > 0.05). The analysis of orthogonal polynomial contrasts (OPC) for meat yield variables was not significant (p > 0.05).

The effect of 100 mg/kg of dietary OEO on physicochemical properties is shown in Table 3. The pH and CL were different (p < 0.05) between the experimental groups. The pH of breast meat was highest in OEO₁₆ and lowest for OEO₀. Cooking loss was highest in OEO₁₂, while OEO₅₆ showed improvement. The WHC did not show differences (p > 0.05) between the experimental groups. The OPC showed significant effects for pH and CL (p < 0.05). For pH, linear, quadratic and cubic components were significant, indicating that the response pattern ranged from linear to cubic. For CL, significant quadratic and cubic effects were observed.

The color analysis of breast meat from chickens supplemented with OEO showed differences (p < 0.05) for yellowness (b*), saturation (Chroma), and tonality (Hue angle) (Table 4). b* was highest for OEO₀ and lowest for OEO₁₆, while saturation was highest for OEO₀ and lowest for OEO₃₄. In relation to tonality, OEO₀ was highest and OEO₁₆ was lowest. Lightness (L*) and redness (a*) were not different (p > 0.05) between the treatments. The OPC showed no significant effects (p > 0.05) for the color variables.

Table 5 presents the texture analysis data for breast samples from broilers supplemented with OEO at 100 mg/kg of diet during production phases. The SF and hardness (H) were different (p < 0.05) between the experimental groups. OEO₀ was the treatment with the lowest SF compared to the those supplemented with 100 mg/kg of OEO. OEO₁₂ exhibited the highest hardness, while OEO₃₄ showed the lowest. The variables adhesiveness, springiness, cohesiveness, gumminess, chewiness, and resilience did not differ significantly (p > 0.05) between treatments. Likewise, these variables were not significant (p > 0.05) in the contrast analysis.

The sensory attributes of chicken breasts meat are presented in Table 6. Juiciness and softness differed significantly (p < 0.05) among treatments. OEO₀, OEO₃₄, OEO₅₆, and OEO₁₆ were scored as the juiciest, whereas OEO₁₂ was least preferred. In the meat softness attribute, OEO₅₆ received the highest scores, while OEO₁₂ was rated lowest.

According to results obtained in meat yield, similar results for HCY were obtained by Cázares-Gallegos et al. (2019). Those authors found differences when using Mexican oregano oil at 200 mg/kg in broiler chicken diets. Meat yield in the current study was higher than that obtained by Cázares-Gallegos et al. (2019). Forte et al. (2018) obtained higher BY with the use of 200 mg/kg in the diet using an aqueous extract of oregano. Better processing yield in meat can be attributed to the antioxidant activity of plant extracts (Arif et al., 2022). This assertion can be attributed to OEO₁₆ with improved CCY, while OEO₁₂ and OEO₅₆ showed improved BY. Possibly, OEO may increase the number of muscle myofibril (via cell division) when this natural additive is administrated during weeks 1 to 6.

The pH and CL were different among treatments. The pH results are somewhat similar to those found by Méndez-Zamora et al. (2015) with pH 5.8 to 6.05. He et al. (2023) indicated that a pH lower than 6.0 affects the quality, texture, and flavor of meat. Breast meat pH values were higher in treatments where broilers were supplemented with OEO, indicating improved meat quality (pH > 6.0). This effect may be attributed to regulation of ante-mortem physiological processes related to stress or natural metabolic reactions, which can influence the rate of pH decline while the muscle is still warm (36 °C) (Hernández-Coronado et al., 2019). Conversely, Hernández-Coronado et al. (2019) did not find difference for CL in chicken breasts meat supplemented with oregano oil at 400 mg/L, although those authors reported relatively high pH values (p = 0.0605) in meat from chicks supplementing with OEO. However, their results differed from those of the current study possibly because they supplemented OEO in water and in this study OEO was presented in the diet. OEO may enhance metabolic pathways involved in amino acid synthesis, thereby improving molecular interactions between water and protein, and reducing water loss (lower CL) during heat treatment.

Regarding color, Al-Hijazeen (2021) found differences for L*, a*, and b* compared to the current study, being that the treatment supplemented with oregano oil 100 mg/kg exhibited the least yellowness. Chang-Song et al. (2017) did not obtain difference in color variables with oregano powder at 150 mg/kg. Color can be influenced by the manner in which OEO is supplemented, depending on diet and environmental conditions (Teixeira et al., 2013). The results of the current study indicated that supplementing the diet with 100 mg/kg of OEO throughout the six-week production period (OEO₁₆) reduced b* values in broiler chicken breast meat. With respect to the orthogonal polynomial contrast, the results indicate that pH and CL of breast meat exhibited complex variation, which may be associated with curvature responses or inflection points when OEO levels are increased across broiler production phases.

The shear force and hardness of breast meat from broiler chickens supplemented with oregano oil were affected. Compared with the control treatment (without supplementation of OEO), Salama et al. (2023) found lower SF in broilers supplemented with OEO at 300 and 600 mg/kg. Furthermore, Hernández-Coronado et al. (2019) did not obtain differences for hardness in chicken breast meat when supplementing OEO at 400 mg/kg in the drinking water. The findings of the current study with 100 mg/kg in diet showed that meat was softer (decreased hardness) when OEO was supplemented during weeks 3 and 4 (grower phase). The results for texture were similar to those of Herrera-Balandrano et al. (2020), who did not obtain an effect of oregano oil on adhesiveness, cohesiveness, gumminess, chewiness, and resilience of marinated chicken breasts. The findings of the current study indicated that OEO supplementation at different stages affects the shear force and hardness of breast meat.

In the current study, sensory parameters of breast meat had effect on juiciness and softness. Forte et al. (2018) found differences in flavor, softness, and global acceptability when broilers were supplemented with 200 mg/kg of European OEO in the diet. Likewise, Cázares-Gallegos et al. (2019) found improved consumer preference for breast meat from chickens supplemented with OEO. Hong et al. (2012) also obtained differences in preference for meat softness in meat form broilers supplemented with essential oils containing carvacrol. Al-Rawashdeh et al. (2022) found that oregano oil affected odor and global acceptability of chicken meat supplemented with natural additives. The effect observed in the current study may indicate that OEO supplementation during different periods affects the juiciness and softness of breast meat.