Influence of processing conditions on bioactive compound extraction from Vitis vinifera L. var. Alicante Bouschet grape pomace

Introduction and objectives: Alicante Bouschet is a red grape variety very rich in anthocyanins in both skin and pulp, which have been cultivated in the vineyards of the semi-arid region of Brazil, generating a large amount of residues rich in bioactive compounds. The aim of this study was to evaluate the effects of process conditions on the solid-liquid extraction of bioactive compounds from Alicante Bouschet grape pomace from red winemaking process. The influence of temperature (23 – 57 °C), ethanol concentration (16 – 84%) and citric acid concentration (0.8 – 3.2%) on the content of total phenolic compounds (TPC), monomeric anthocyanins (MAC) and on the antioxidant capacity (ABTS●+ and ORAC methods) of the extracts was investigated.
Methodology: Fresh pomace (skins, residual pulp and seeds) of Alicante Bouschet grape from red winemaking was supplied by the Rio Sol winery of the ViniBrasil group (Lagoa Grande, PE, Brazil). The pomace was dried in a convective tray dryer at 60 ºC for 24 hours and the dried material was processed in depulper to separate the seeds from the skins. The skins were grinded in a disc type mill resulting in grape skin “flour”. The grape skin flour was submitted to solid-liquid extraction process in a thermostatic bath using a hydroethanolic solution acidified with citric acid. The solid: liquid ratio (1:10), the stirring speed (200 rpm) and the extraction time (one hour) were fixed. After removing the suspended solids by filtration, the extracts were stored at -20°C in amber recipients for analysis of TPC, MAC and the antioxidant capacity (ABTS●+ and ORAC methods). The Response Surface Methodology (RSM) was used to evaluate the influence of temperature, ethanol concentration and citric acid concentration on the recovery of bioactive compounds from grape skin. The major effects, quadratic effects and effects of interaction were assessed using a 2^3 Rotatable Central Composite Design (RCCD). Optimum operation conditions were obtained by the combination of the individual responses in an optimized composite function: the desirability function. The mathematical models were validated by comparing the experimental and predicted values.
Results and discussion: Ethanol concentration and temperature had a significant influence on total phenolic compounds extraction and antioxidant capacity while monomeric anthocyanin extraction was only affected by ethanol concentration. The addition of citric acid caused a slight increase in the total phenolic content. The conditions selected as the most adequate for the extraction were: temperature of 40 °C, 50% v/v ethanol and 2% m/v citric acid. Total phenolic content presented a strong significant positive correlation (p < 0.05) (r = 0.906) with the antioxidant capacity measured by the ABTS●+ method. On the other hand, the antioxidant capacity measured by the ORAC method presented a significant positive correlation (r = 0.851) (p < 0.05) with monomeric anthocyanin content. Both methods of antioxidant capacity assessment were positively and significantly correlated (p < 0.05) (r = 0.847).
Conclusions: Because of its high content of phenolic compounds and anthocyanins, Alicante Bouschet grape pomace extract can be used as a promising natural additive (colorant and/or antioxidant) replacing synthetic additives in food formulations.