**4. Discussion**

The implication of Cr in reducing glucose levels in heat-stressed broilers was well documented [20–22]. However, the literature is limited in studies regarding the effect of combinations of Cr with other antioxidant compounds. In this study, dietary supplementation with CrPic + Zn had a lowering effect in glucose level compared with CrPic + vitamin C. Nevertheless, there is evidence confirming that dietary Zn can decrease glucose level in heat-stressed broilers [23]. Contradictory results were reported by Abuajamieh et al. [24], showing that dietary organic zinc supplementation (50% and 100% of the Zn level from basal diet) in HS chicks increased blood glucose and additionally decreased blood calcium. Conversely, Saleh et al. [25] showed that Zn methionine supplementation (25, 50 and 100 mg/kg) in heat-stressed broiler diet significantly decreased plasma triacylglycerol, total cholesterol concentrations as compared to the control group. AST and glucose parameters did not record any difference. In this study, dietary supplementation with CrPic + VC or CrPic + CWS did not have any effect on serum biochemical parameters tested. In contrast, some researchers reported that combination of Cr with vitamin C has synergistic action and decreased glucose and cholesterol [10], while data using CrPic + CWS were not found. Perai et al. [26] showed that under stressful condition caused by transport, broiler chickens fed diet enriched in Cr + vit. C had a lower triglyceride level and a higher glucose level than before transport.

The dietary supplementation with CrPic + CWS determined an increase in crude protein concentration in breast meat compared to those fed C diet; otherwise, the other groups had a similar protein content with group C. Untea et al. [27] observed the positive influence of dietary chromium supplements (200, 400 μg/kg) on crude protein concentrations of breast meat in a study on broiler chickens raised under normal temperature conditions. Nevertheless, it is interesting to note that CrPic + VC and CrPic + CWS caused a significant reduction in the concentration of crude fat in chicken breast compared to group C. This reduction in crude fat is a good achievement because a high content of fat led to lipid oxidation, a key factor that negatively affects meat color and texture. Additionally, some researchers [28,29] have reported higher protein and lower fat content in breast meat, when chickens fed diets supplemented with 200 and 400 ppb Cr3+. The probable explanation for fat reducing effect could be the inhibitory potential on lipogenic activity in chick adipose tissue. Toghyani et al. [30] showed that under heat stress, broilers fed a diet enriched in Cr (500, 1000, 1500 ppb Cr nicotinate) recorded increases in crude protein of breast meat.

In this study, it was observed that the use of dietary VC, Zn, and creeping wood sorrel changed the meat color. The redness value (a\*) of the thigh collected from E1 group was significantly lower than from E3 group. Therefore, the combination of CrPic + CWS had a favorable effect on thigh meat color compared to that of CrPic + VC. Nevertheless, the redness parameter was higher in E1, E2, and E3 groups compared to the C group. Definitely, those results showed that CrPic in combination with VC, Zn, and CWS improved the meat color. Moreover, the breast from E3 group had the highest concentration of crude protein and the highest redness. Myoglobin is the main heme protein in muscle tissue; perhaps an increase in myoglobin concentration has led to an increase in the redness of chicken breast color. According to Sałek et al. [31], Zn binds myoglobin and increase its oxygenation, which maintain the meat color. Consumers associate the increase in the redness parameter with a better quality of the meat [32]. Meat color is influenced by animal diet [33], its heme components concentration consisting of myoglobin, haemoglobin, cytochrome C [34,35], their oxidation-reduction state, chemical reactions, etc. [36]. The explanation of increasing the meat redness might be the antioxidant activity of supplements, being well-known that vit. C and Zn are involved in the redox reactions, delaying the meat oxidations processes. On the other hand, creeping wood sorrel contain phenols, but also large amounts of vitamin C, which could contribute to maintain the pigment stability of the meat. The luminosity parameter was significantly lower in the breast meat collected from groups that included CrPic + VC, CrPic + Zn, CrPic + CWS in the diet than in the C group. The lightness of the breast ranged from 46.93–51.38; redness from 0.61–1.49 and yellowness from

12.13–14.40. The breast meat of chickens fed E1, E3 diets recorded a significant decrease in the yellowness parameter than those fed the C diet. Notably, the lowest value of b\* parameter was recorded in the breast samples collected from E3 diet. Similar results were reported by Huang et al. [9] who studied the effect of three sources of Cr added in the diet of broiler chickens (Cr propionate, CrPro; Cr picolinate, CrPic; Cr chloride, CrCl3) and two concentrations of added Cr (0.4, or 2.0 mg of Cr/kg) on meat quality of broilers raised under heat stress. The authors reported that broilers supplemented Cr had decreased b\* values of meat color in breast muscle. Peña et al. [37] showed no differences on breast color when included ascorbic acid (250, 500, and 1000 g/ton) + citric flavonoids in the diet of heat-stressed chickens.

Results from our study revealed that the dietary supplementation with combinations of CrPic with VC, Zn, and CWS had a beneficial effect on the mechanical strength of the muscle fiber. From a textural point of view, Tudoreanu [38] has found that meat firmness depends mainly on the structure of myofibrin and connective tissue, while Astruc [39] has shown that it depends on the amount of fat and collagen. The texture profile analysis (TPA) of thigh samples resulting from the application of the double compression test shows that the parameters of hardness, elasticity, cohesiveness, and gumminess did not register significant (*p* > 0.05) differences between groups. The resilience of chicken thigh was significantly lower in the group that included CrPic+ Zn (E2), CrPic + CWS (E3) in the diet compared to group C and E1. The lowest resilience value was obtained in group E2 diet was supplemented with CrPic + Zn. This observation could be related with the increase in EE in E2 than in C. The pH values of thigh meat were lower in E2 and E3 compared to C group. Normally, postmortem, the muscle undergoes various reactions, through which it is transformed into meat. For example, muscle glycogen is denatured by glycolysis, forming lactic acid. This reaction results in an increase in the acidity of meat. Heat stress can affect the pH of meat, increasing it, as a consequence of glycogen consumption in reserves and thus the production of lactic acid is reduced [40]. According to Listrat et al. [33], in poultry, the texture is strongly affected by the postmortem acidification kinetics of muscle.

The breast meat obtained from groups fed combination of CrPic + Zn and CrPic + CWS had a lower hardness (*p* < 0.05) compared to that from groups C and E1. This indicates that the caused an increase in the tenderness of the chicken breast compared to the control diet and the combination of CrPic + VC. Chromium, in combination with Zn or CWS may lead to a decrease in the mechanical strength of muscle fiber, having an effect of increasing its firmness. The breast meat in group E1 had a significantly lower elasticity (99.42%) compared to that of group C (99.74%). This result suggests that dietary supplementation with CrPic + VC leads to a significant decrease in the ability of the chicken breast meat to return to its original shape after compression. Nevertheless, the lowest resilience value of breast meat (2.16 adm) was recorded in E2 group, which included CrPic + Zn in the diet. This achievement could be correlated with the low hardness of the chicken breast meat in E2. The inclusion of CrPic + VC in the chicken diet led to an increase in the pH of breast meat compared to C group. pH is considered one of the crucial variables determining meat quality [41]. According to Berri et al. [42] the normal pH values of chicken meat at 15 min postmortem are around 6.2 to 6.5, whereas normal ultimate pH values are around 5.8 [43].

The results of PCA revealed that the assessed meat characteristics were clustered in function of similarity. The first component, PC1 underline a close association between EE, cohesiveness and a\* parameters, variables which are negatively correlated with CP, Ash, L\* and b\*. The second component, PC2 distinguished between DM and a\* parameters and between textural parameters, hardness and springiness, this fact being expectable. Additionally, an opposite relation can be observed between cohesiveness and springiness, meat springiness showing a direct relation with sensorial parameters, consistency, juiciness, and tenderness.

Sensory attributes of meat such as tenderness, juiciness, muscle appearance, are among the most critical attributes in consumer preferences [44]. In this regard, several studies conducted on consumers have shown that these attributes could be influenced by supplementation of broiler diets. For example, Velasco and Williams [45] showed that chicken meat quality could be improved by adding natural antioxidants. Some selected panelists have indicated an improvement in the flavor of thigh meat, reduced the metallic taste and the overall aftertaste as consequence of supplementing chicken diets with faba bean compared to a soy-based diet [46]. However, Suliman et al. [47] evaluated the sensory attributes of meat obtained from broilers fed diet supplemented with 1, 2, 3, 4, 5, and 6% clove seeds. The authors showed that the sensory attributes were not significantly different between the treatment groups except tenderness. However, in our study, according to panelists, the experimental diets had no influence on sensory attributes tested compared to the conventional diet.

Taken together, the achievement such as improvement of meat color could be probably attributed to the antioxidant potential of the tested supplements being involved in the redox reactions. However, the improvement of breast meat tenderness observed in this study as consequence of supplementing the broiler chicken diet with antioxidants is an important result as it strongly influences consumer satisfaction. Nevertheless, the influence of dietary antioxidants on the regulatory mechanisms that define metabolic and physiological changes in muscle tissue is complex, poorly understood, and further studies are needed to investigate it.
