Deconstructing thermal processing of cooperage oak: How oak species, toasting levels and charring levels collectively...

Oak-derived phenolic compounds significantly influence the color, taste, and overall quality of aged spirits, yet a comparative understanding of non-traditional oak species and the synergistic effects of cooperage thermal treatments is lacking. This research employed UHPLC-QqQ-MS/MS to systematically investigate the individual and interactive effects of oak species (Chinese, French, and American), toasting levels, and charring levels on hydrolysable tannins (HTs) and low-molecular-weight phenolic compounds (LMWPCs) in oak wood. Results revealed distinct phenolic baselines among oak species: While French oak dominated in HTs, and American oak was the richest source of key LMWPCs, Chinese oak matched French oak in most gallotannins and a characteristic HT-castacrenin E, with intermediate levels of LMWPCs content behind American oak. Both thermal processes universally transformed phenolics by degrading HTs and generating LMWPCs, primarily driven by toasting and modulated by charring's interactive effects, with distinct species variations. Chinese oak showed high HTs sensitivity to toasting but significant rebound with combined toasting-charring treatment, while maintaining continuous LMWPCs production. American oak displayed elevated HTs tolerance, yet its LMWPCs exhibited an increase with toasting and a decrease with charring. French oak demonstrated the greatest HT stability under varying thermal conditions while requiring synergistic toasting-charring to fully activate lignin degradation for optimal LMWPCs production. This work demonstrates that barrel chemistry is governed by oak species-thermal treatment interactions, offering practical guidance for optimizing oak selection and processing in spirit maturation.