Mobilization of phenolic antioxidants from grape, apple and pitahaya residues via solid state fungal fermentation and carbohydrase treatment

This paper reports two strategies to mobilize phenolic antioxidants from lyophilized and oven-dried black grape (Vitis vinifera x (Vitis labrusca x Vitis riparia)) pomace, and apple (Malus domestica cv. Jonagold) and yellow pitahaya (Hylocereus megalanthus) peel, core, peduncle and seed mixtures: a solid-state Rhizomucor miehei NRRL 5282 fermentation-based process and a carbohydrate-cleaving enzyme treatment using R. miehei NRRL 5282 cellulase and Aspergillus niger pectinase cocktails. Both methods proved to be suitable to increase the extractable phenolic content and improve the antioxidant properties of the phenolics as determined by 1,1-diphenyl-2-picrylhydrazyl radical inhibition or ferric reducing antioxidant power analyses. During solid-state fermentation, maximal phenolic yields obtained in lyophilized grape, apple and pitahaya residues were 1956 ± 31, 477 ± 37 and 495 ± 27 mg gallic acid equivalents (GAE)/100 g dry matter (DM), respectively, while they were 1385 ± 71, 362 ± 27 and 615 ± 26 mg GAE/100 g DM in oven-dried samples, respectively. The major individual phenolics produced enzymatically from the substrates were identified by HPLC as gallic acid, 4-hydroxybenzoic acid, vanillic acid, (+)-catechin and (−)-epicatechin, with yields ranging from 0.58 ± 0.06 to 215.81 ± 17.17 mg/100 g DM depending on the substrate and the pretreatment. The obtained phenolic-enriched extracts could potentially be applicable as sources of natural antioxidants.