Why does distilled liquor has a soft and harmonious flavor after long-time ageing? A thermodynamic analysis

Abstract

The effect of acid–ester equilibrium on the quality of aged distilled liquor (e.g., Baijiu) and techniques for the rational design of flavor are yet to be fully understood. In terms of thermodynamics, the long-term ageing is a spontaneous process in which the thermodynamically unstable state changes to the stable state. This study used gas chromatography, sensory evaluation, and thermodynamic strategies to explore the acid–ester equilibrium during the ageing process of Baijiu. The content of most esters in new Baijiu is significantly higher than that of acids and a consistent rule in the ester balance was detected, with the reaction quotients of esterification reactions gradually approaching the equilibrium constants. The thermodynamic data confirm that the contents of esters and acids become stable after ageing. And the stability of ester and acid content is the premise of the soft and harmonious flavor of Baijiu: after ageing, the Baijiu will keep its harmonious flavor which derives from the blending process. On the contrary, the blended but unaged Baijiu, will lose the harmonious flavor due to the change of the content of esters and acids.