In this study, non-targeted metabolomics based on ultra-high performance liquid chromatography Q-Exactive-mass spectrometry (UPLC-Q-Exactive-MS) was used to explore the differences in the metabolite profiles of muscles from Bactrian camels slaughtered at different ages. The results showed that a total of 710 differentially expressed metabolites (DEMs) were identified in the longissimus dorsi muscle of camels from three age groups: Ⅰ (3–4 years old), Ⅱ (6–7 years old), and Ⅲ (9–10 years old). In total, 78 DEMs were found in groups Ⅰ vs Ⅱ, of which 47 were upregulated in group Ⅰ and 31 were upregulated in group Ⅱ. Totally, 49 DEMs were identified in groups Ⅱ vs Ⅲ, of which 18 were upregulated in group Ⅱ and 31 were upregulated in group Ⅲ. In addition, 65 DEMs were identified in groups Ⅰ vs Ⅲ, of which 29 were upregulated in group Ⅰ and 36 were upregulated in group Ⅲ. The results of Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis showed that the DEMs were mainly enriched in pathways such as protein and amino acid metabolism, fatty acid metabolism, and vitamin and mineral metabolism, indicating that the digestion and metabolism of various nutrients in camels were different at different growth stages. The contents of polyunsaturated fatty acids (PUFA) (especially n-6 PUFA and n-3 PUFA) and the PUFA/unsaturated fatty acid (UFA) ratio were significantly higher in group Ithan Ⅲ, which was mainly related to the significant upregulation of arachidonic acid, linoleic acid and 13-L-hydroperooleic acid concentrations in relevant metabolic pathways. Meanwhile, DEMs such as L-leucine, L-valine, and L-glutamine could serve as potential markers for evaluating the quality change of camel meat at different slaughter ages.

The differences in the proteomes of camel meat from different age groups were analyzed by tandem mass tag (TMT)-based quantitative proteomics and liquid chromatography tandem mass spectrometry (LC-MS/MS) to identify the key quality-related proteins of camel meat. The results showed that differentially expressed proteins (DEPs) had an important effect on the quality of camel meat. A total of 311 DEPs were identified in the longissimus dorsi muscle of camels from three age groups, 3-4 (Ⅰ), 6-7 (Ⅱ), and 9-10 (Ⅲ) years old. Altogether 245 DEPs were identified in groups Ⅰ versus Ⅱ, 16 in groups Ⅱ versus Ⅲ, and 139 in groups Ⅰ versus Ⅲ. In addition, 194, 1, and 110 DEPs were up-regulated, and 51, 15, and 29 DEPs were down-regulated in the three comparison groups, respectively. Gene Ontology (GO) functional annotation analysis indicated that structural proteins, metabolic proteins, and heat stress proteins could be used as biomarkers for discrimination among camel meat from different ages. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis showed that the DEPs were mainly involved in fatty acid metabolism, glycolysis/glucose production, amino acid biosynthesis, and the hypoxia inducible factor-1 (HIF-1) signaling pathway. Protein-protein interaction (PPI) analysis showed that metabolic enzyme proteins were key network-connecting proteins that affect camel meat from different ages. Correlation analysis showed that 16 DEPs were correlated closely with meat tenderness. The tenderness of camel meat from the three age groups was mainly influenced by actin, histone and protein kinases. The results of this study can provide a scientific basis for the grading and evaluation of camel meat, the selection of optimal slaughter age, and the study of camel meat quality characteristics.

In this study, we investigated the changes of physicochemical properties and structure of defatted camel’s milk proteins with four different heat treatments (65 ℃/30 min (T1), 85 ℃/15 s (T2), 120 ℃/15 s (T3), and 135 ℃/5 s (T4)) by measuring particle size, potential, turbidity, secondary structure, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) pattern, tertiary structure, surface hydrophobicity, free sulfhydryl group and disulfide bond content. The results showed that the particle size of camel’s milk proteins increased, the absolute value of the zeta potential decreased, and the turbidity increased with increasing temperature. Heat treatment had a significant effect on the secondary and tertiary structures, resulting in a decrease in the relative contents of α-helix and β-turn and an increase in the relative contents of β-fold and random coil in camel’s milk proteins. T3 had the greatest effect on the particle size, zeta potential, secondary structure and tertiary structure compared with the unheated control. SDS-PAGE profiles showed that the intensities of the soluble whey protein bands gradually decreased as the heating temperature increased. Observations by laser scanning confocal microscopy (LSCM) showed that the heat treatment caused protein aggregation, and T3 and T4 resulted in the production of large protein polymers. Under the T1 and T2 conditions, protein polymers were formed mainly through disulfide bonding. However, with the increase in heating temperature, the disulfide bonds of camel’s milk proteins were broken under the T3 and T4 conditions, resulting in the formation of free sulfhydryl groups and the exposure of the hydrophobic groups of camel’s milk proteins. The above results indicated that the structural and physicochemical properties of camel’s milk proteins can be changed to different degrees by different heat treatments, which in turn can affect the stability of camel milk. The results of this study provide a theoretical reference for the development of liquid camel’s milk products.

Whey protein is an important component of milk, but heat treatment during sterilization has a certain impact on the structure and properties of whey protein. Therefore, this study investigated the effect of different heat treatments on the denaturation degree, physicochemical properties and functionality of camel whey protein, which was separated from camel milk heat-treated under different conditions, 65 ℃/30 min, 85 ℃/15 s, 125 ℃/4 s and 135 ℃/4 s, after milk fat and casein precipitation. The results showed that whey protein treated at 65 ℃ for 30 min had the highest turbidity (0.14), particle size (0.24 μm), zeta potential (-9.54 mV), solubility (24.57 mg/mL), and relative content of α-helix (31%) as well as the lowest relative contents of β-turn (13%) and random coil (19%); the water-holding capacity (WHC), foaming capacity and emulsifying capacity were significantly increased by the heat treatment, which indicated that 65 ℃/30 min treatment resulted in the smallest denaturation degree of whey protein and improvements in its functional properties such as emulsifying capacity, foaming capacity, WHC and oil-holding capacity (OHC) without significantly changing its physicochemical properties. With an increase in the heating temperature, the particle size of whey protein under 85 ℃/15 s treatment was increased to 0.32 μm, and the solubility was reduced to 22.34 mg/mL, but its functionality was improved compared with that of raw milk whey protein.Treatment at 125 ℃ for 4 s or 135 ℃for 4 s led to a large number of whey protein denaturation, destroyed the secondary structure, increased the particle size and turbidity, decreased the solubility, and worsened the functional properties.