Composición química y actividad antioxidante de la corteza plateada de tres variedades de coffea arábica (café)

Abstract

The silver skin (SS) of Coffea arabica, a unique byproduct of the roasting process, represents a potential source of bioactive compounds with functional properties. In Ecuador, where coffee cultivation is an economically and environmentally significant activity, the utilization of this residue remains limited despite its content of metabolites with potential antioxidant effects. This study evaluated the chemical composition and antioxidant capacity of SS from three locally grown coffee varieties—Sarchimor red (SR), Catucaí yellow (CY), and Acawa (AW)—using chromatographic and spectrophotometric techniques, for potential use in the pharmaceutical and food industries. A proximal study of the three SS varieties was conducted through residual moisture, total ash, total fats, proteins, and minerals assays, with results falling within established ranges and compared to data reported in specialized literature, showing significant differences among the varieties. The SS was subjected to two extraction methods: ultrasound and dynamic digestion for 60 minutes, using an ethanol:water (70:30 v/v) mixture as the solvent, with the extract dried (ED) via rotary evaporation. A preliminary analysis of the chemical composition of the three varieties was performed using thin-layer chromatography (TLC), revealing qualitatively similar chromatographic profiles. The presence of compounds with conjugated chromophore systems, phenolic metabolites, amino acids, and antioxidant activity was confirmed in the evaluated extracts. Total polyphenols, expressed as gallic acid equivalents (mg GAE/g ED), were quantified using the Folin-Ciocalteu assay. The SR variety showed the best results and, consequently, the highest antioxidant capacity, measured as Trolox equivalents using the ferric reducing antioxidant power (FRAP) and 2,2-diphenyl-1-picrylhydrazyl (DPPH) methods. Similarly, dynamic digestion yielded better results for all three raw materials compared to ultrasound-assisted extraction. Additionally, caffeine levels (mg/g ED) were quantified using HPLC-DAD, revealing significant differences among the varieties, with AW showing the highest alkaloid content. It was also observed that ultrasound-assisted extraction resulted in higher caffeine concentrations compared to dynamic digestion, attributed to ultrasonic cavitation, which enhances compound release without requiring high temperatures. These findings support the use of SS as a potential raw material for developing products in the pharmaceutical or food industries, while also contributing to environmental sustainability through the reuse of agroindustrial waste.