Difference of Free Amino Acids in Different Primary Processed Yunnan Arabica Coffee and Its Taste Forming Mechanism

To explore the influence of three different primary processing methods (wet-processed, honey-processed and sun-exposed processed) of Yunnan Arabica coffee on the free amino acid and its taste forming mechanisms, the total protein content, free amino acids content and composition were measured by using a Dumas nitrogen analyzer (Dumas) and high-performance liquid chromatography (HPLC). And, principal component analysis (PCA) and orthogonal partial least squares-discriminant analysis (OPLS-DA) were used to screening characteristic taste amino acids and employed molecular docking to verify its taste forming mechanism. The findings indicated that the protein content was highest in wet-processed coffee beans, followed by honey-processed coffee, and lowest in sun-exposed samples. The soluble protein content in the coffee extraction was highest in the washed process (0.40%), followed by sun-exposed (0.36%) and honey-processed (0.33%). All three samples were found to contain 17 free amino acids, but the content, distribution, and proportion were different. The total free amino acid content of the wet-processed samples reached 314.20 mg/L, followed by the sun-exposed samples at 304.99 mg/L, and the honey-treated samples had the lowest content at 278.08 mg/L. Among them, the wet-processed samples had the highest total content of umami and sweet amino acids, while the sun-exposed samples had the highest total content of bitter amino acids. The honey-treated samples had the lowest total content of umami, bitter, and sweet amino acids. PCA and OPLS-DA further identified Thr, Tyr, and Phe as the principal taste amino acids in wet-processed, honey-processed and sun-exposed processed coffee samples. The results of molecular docking indicated that the three specific free amino acids could strongly bind to the T1R2/T1R3 and T2R14 receptors through hydrogen bonds, Van der Waals forces, hydrophobic, and electrostatic interactions, demonstrating their taste characteristics.