1. Introduction
Meat is an important source of protein for humans. Of all livestock, chicken is deemed as a healthy option, with high content of protein and low content of fat, which is prevalent in different areas. The Chinese population consumes chicken in large quantities every year. Especially aged chicken, which consumers believe would perform better in nutrition and flavor. This purchase propensity is also manifested in gender. The previous studies in our team have reported that age and sex affect the meat quality of chicken [
1,
2].
The content and quality of protein are critical factors to determine the nutritional value of meat. Previous results have shown that protein quality is usually assessed by analyzing the composition and content of amino acids. The amino acid score (AAS) recommended by the WHO/FAO/UNU committee [
3] is a widely used method for protein quality evaluation. The protein quality was evaluated through comparing the AAS of the tested sample against WHO/FAO/UNU standard AAS. Therefore, the content and proportion of amino acids become the crucial indicators to appraise the flavor and nutritional value of meat [
4]. In animals, an imbalanced proportion of amino acid will affect the utilization of amino acids and the absorption of other nutrients [
5,
6], and even lead to a series of health problems [
7]. Chicken is an accessible protein source with a balanced composition of essential amino acids (EAAs). It contains nine EAAs for humans, especially a high level of lysine [
8,
9]. The composition and content of amino acids fluctuated dynamically. There are many factors that affect the composition and concentration of amino acids in meat, such as breed, age, diet, sex, and similar [
10,
11,
12,
13]. However, whether and how these factors affect the content of free amino acids and total amino acids (TAAs) is obscure.
The free amino acids in cells can participate in the formation of flavor substances through the Maillard reaction [
14]. For instance, cysteine and glycine are important ingredients involved in the Maillard reaction to form meat flavor [
15]. The flavor of meat is mainly formed by a variety of flavor precursors, which are composed of flavor nucleotides, free amino acids, soluble sugar, peptides, organic acids, salts and others [
16]. Free amino acids are amongst the central flavor substances of meat, with sour, sweet, bitter, salty, monosodium L-glutamate-like (MSG-like) and other tastes. Glutamic acid (Glu) and aspartic acid (ASP) have a mixture of sour and MSG-like tastes. Serine, glycine, alanine, etc. show a sweet taste. Histidine, valine, arginine, etc. taste bitter [
17]. Subsequent studies demonstrated that amino acid mixtures, or combinations of amino acids and other substances, contributed more to the umami taste of meat than individual amino acids. As revealed in an earlier study, free Glu and Y-IMP with some sour and salty ingredients were better able to imitate the flavor of bouillon [
18].
Guangyuan grey chicken is a slow-growing indigenous breed, which has an average weight about 0.62 kg at 10 weeks of age. This breed is a famous indigenous breed for its grey feathers and skin. It is popular among local consumers based on the tenderness and succulence of the meat. Nonetheless, limited research has been done on this breed, resulting in indistinct data about its characteristics. Herein, to better understand the effect of the aforementioned three factors on the composition and content of amino acid in meat, we determined the composition and content of deproteinized free amino acids and total amino acids in the pectoral and thigh muscle of Guangyuan grey chickens of different ages and genders by liquid chromatography–tandem mass spectrometry (LC-MS/MS) and an amino acid analyzer, respectively. The results provide novel insights as to the nutritional value and flavor of Guangyuan grey chicken in different ages and genders, and may supplement information on the characteristics of this breed.
4. Discussion
For this indigenous breed, which has not been systematically selected, the information on growth trend is lacking. In this study, we recorded the body weight of the population from 0 to 22 weeks. The results showed that the growth rate of this breed was slower compared with the broilers [
22,
23], especially the first 4 weeks of growth. This may be due to the lower feed intake of this breed. Overall, it indicates that the growth performance of this breed needs to be further improved. Nutrition and flavor are important indexes to evaluate meat quality. Previous studies revealed that the breed, age, gender and meat cut have effects on the meat quality of chicken. In this study, we determined the proximate composition of pectoralis and thigh muscle of Guangyuan grey chicken with different ages and genders. The content of CF was considered to be associated with tenderness and flavor of meat [
24,
25]. We found that the thigh muscle had a higher moisture and CF than those of the pectoral muscle in all age groups, and this result was more pronounced in the case of female chickens. These results were also observed in Panpipat’s study, while there was no difference between the genders in their study [
26]. Thus, the thigh muscle may have a better texture than the pectoralis of Guangyuan grey chicken. Considering the effect of gender, the highest CF was displayed in 120 d for male thigh muscle and 150 d for female thigh muscle, respectively. This result shows that the regular CF deposition may be different for chickens of different sexes. It was speculated that this phenomenon may be due mainly to sexual maturity, owning to the fact that the age at first egg of Guangyuan gray chickens is about 150 days. For pectoralis, the moisture and intramuscular fat in pectoralis showed an uptrend with the increase in age, which was also observed in Yuan’s study [
14]. It seems that the older female chicken had a better taste. Protein content is one of the indexes for evaluating the nutritional value of meat. As observed in the results, the content of crude protein was higher in the pectoral than thigh muscle (
p < 0.05). This was compatible with previous research [
26,
27]. It indicates that the protein nutritional value of the pectoral muscle is higher than that of the thigh muscle.
Amino acids are substrates for protein synthesis in organisms. In humans, there are nine amino acids that need to be provided by food, called EAAs. We identified eight EAAs in Guangyuan grey chicken via acid hydrolysis method, excluded Tryptophan. In this study, we identified that the content of EAA in the pectoral muscle was higher than that in the thigh muscle. This result was consistent with the results reported by Ou’s study [
11]. Nevertheless, the EAA/TAA ratios of all samples were greater than 0.4, exceeding the recommended value in the FAO/WHO. Compared with other species, duck [
28], pig [
29], fish [
30] and shrimp [
31], the ratio of EAA/TAA in chicken was higher [
9], as well as Guangyuan grey chicken. According to the results of AAS, the scores of amino acids were all higher than 100 except for Leu, Val and part of Lys. This result can be explained by breed and diet—it still needs to be further improved.
Free amino acid is one of the substances that contributes to the flavor of meat via the Maillard reaction with reducing sugar [
32]. In current research, the deproteinized free amino acid of the chicken was detected via LC-MS/MS to evaluate the flavor of Guangyuan grey chicken. The UTAAs were the main components of FAAs [
17]. Just as the results of the free amino acid, the content of UTAA was higher in 120 d male chicken, the lowest was present to 150 d female chicken. This indicated that the content of UTAA in chicken was not linearly related to age. The content of Glu was the highest of all free amino acids, which was combined with the results of other studies [
33,
34]. It demonstrated that Glu is one of the central substances to form meat flavor. Sweet-taste amino acids is also an important component of flavor. The range of STAAs was different in previous studies [
35,
36]. In this study, we referred to the results of Kirimura et al. [
17]. The differences in STAAs in different meat cuts were prominent, and the content of STAAs in thigh muscle was significantly higher than that in pectoral muscle. The highest content of STAAs was also presented in the thigh muscle of 120 d roosters. Within them, the most abundant content of STAAs was glutamine, which is in agreement with the results of previous studies [
37,
38]. Intriguingly, apart from the pectoralis of 150 d female chicken, the content of Gln in thigh muscle was significantly higher than that in pectoralis. In the study of Imanari et al., it was also observed that the content of Gln in vastus lateralis muscle was significantly higher than that in longissimus lumborum muscle [
39]. This would suggest that the thigh muscle of Guangyuan grey chicken may be better at sweet taste. We also observed that the total amount of FAAs was higher in 120 d chicken. Considering the content of EAAs and UTAAs, 120 d is more suitable as a reference for the marketable age of Guangyuan grey chicken. Moreover, the total amount of FAAs was higher in thigh muscle than in pectoral muscle, which was in agreement with the other study [
11].