Tannin extraction from grape stems through a solid-liquid process: optimizing efficiency by applying the response surface methodology / Extração de tanino do caule da uva por meio de um processo sólido-líquido: otimizando a eficiência aplicando a metodologia de superfície de resposta

Viticulture is an agro-industrial sector that produces large amounts of waste, with grape seeds, bagasse, and stems standing out among the main types of waste produced. This study evaluated the effects of extraction conditions of total and condensed tannins from grape stems through a solid-liquid process using the response surface methodology. Contact time between matrix and solvent and the concentration of ethanol in the solvent were monitored as independent variables. Contact time varied between 12 and 24 hours. The extraction solvent was composed of a mixture of ethanol and water in proportions of 0, 50, and 100% (v/v). Total tannins were quantified by the Folin-Ciocalteu method in tannic acid equivalents. Condensed tannins were quantified by the vanillin method in catechin equivalents. Response surface methodology pointed out similar conditions for the extraction of total and condensed tannins. Best yields for total tannins were achieved with a contact time of approximately 21.18 hours and a hydroalcoholic solvent composed of 51.65% ethanol. Regarding condensed tannins, the contact time was slightly inferior, with the best yields obtained in 19.31 hours with a hydroalcoholic solvent composed of 49.19% ethanol. The tannic extract obtained under optimized conditions according to the results of the response surface methodology was characterized by Fourier-transform infrared spectroscopy, allowing the identification of functional groups present in the extract.