Evaluation of the Antioxidant Capacity of Portuguese Wine Grape Skins and Application in Cereal Bars Using Different Extraction Solutions

Grapes (Vitis sp) serve as the foundation for various products, particularly wines and juices. Research on the chemical composition of this fruit has indicated a high antioxidant capacity, making it functionally attractive and encouraging the use of grape skins in the development of new products. For this reason, this study aimed to evaluate the antioxidant capacity of Portuguese wine grape skins from the Verdelho (white) and Castelão (red) varieties and to analyze their added value when incorporated into cereal bars. Three cereal bar formulations were created: one containing 10% red grape skin, another with 10% white grape skin, and a third with 5% of each type. In this study, ethanol (50:50, v:v) and methanol (70:30, v:v) served as the extraction solvents. To increase the extraction efficiency, an ultrasonic bath was used under specific conditions: 50°C for 20 minutes for ethanol extraction and 60°C for 25 minutes for methanol extraction. The antioxidant capacity was determined using the DPPH• method (1,1-diphenyl-2-picrylhydrazyl), quantified by spectrophotometry and expressed as trolox equivalents in percentage terms. The inhibition percentage confirms the antioxidant capacity of the samples. Using ethanol as the extraction solvent, the skins of red and white grapes showed 37.74% and 18.87% inhibition, respectively. The cereal bars exhibited 20.46% (red), 15.84% (white), and 19.36% (mixed) inhibition. Using methanol, the red and white grape skins showed 77.46% and 34.61% inhibition, respectively, while the cereal bars exhibited 39.29% (red), 29.9% (white), and 34.61% (mixed) inhibition. The obtained results were promising, confirming the preservation of the antioxidant capacity of grape skins even after incorporation into cereal bars. Methanol demonstrated greater efficacy than etanol however, ethanol, being less toxic and more environmentally friendly, yielded proportional results, making it a viable alternative.