Insight on tannin extraction and mechanical changes during maceration from skins and seeds of Italian red grape varieties

This study investigated tannin extraction in four Italian red grape varieties — ‘Aglianico’, ‘Nebbiolo’, ‘Primitivo’, and ‘Sangiovese’. The grape initial tannin content was characterized through a potential extraction. Moreover, the extractable phenolic content was evaluated through maceration in wine-like solution of skins, seeds, and their combinations for 10 days, with ethanol concentration incrementally adjusted to simulate fermentation. Texture analysis of grape seeds and skins was conducted before and after the wine-like solution macerations. Results revealed variety-dependent differences in the mechanical and acoustic properties of grape skins and seeds, with seeds showing increased acoustic energy upon breakage. Grape varieties showed differences in skin and seed phenolic pools and extractability. Significant positive correlations were found between potential and extractable tannin content. Smaller, less galloylated flavan-3-ols were well extracted in model wine solutions. Extraction curves were examined from skins, seeds, and combined skins + seeds matrices, evidencing a faster skin phenolics extraction than seeds. Interestingly, tannin extraction from skins + seeds did not correspond to the sum of individual skin and seed extractions. Moreover, the skin-to-seed tannin ratio and tannin structural characteristics also varied between skin and seed joint and separate extractions.