Sweet cherry (Prunus avium) fibers extracted from microwave- and steam-blanched recovered fruits: Photo-antioxidant activity in milk proteins

Abstract

Sweet cherries (Prunus avium L.) discarded at harvesting were upgraded as powerful antioxidant fiber powders for food preservation, and their photo-antioxidant effect on milk proteins evaluated. To stabilize the powders during their processing and use, cherries were first blanched through microwaves or saturated water vapor for comparison. The blanching condition (determined as that required to achieve enough peroxidase (POX) inactivation) was 6 min and 90 °C in the fruit center, and POX was the enzyme selected as indicator of blanching efficiency because it was more resistant than polyphenoloxidase in both processes. Fibers, the water insoluble residues, were then separately extracted from the microwave- or steam-blanched cherries. The fibers obtained after freeze-drying exhibited good hydration properties, as they were essentially constituted by cell wall biopolymers (uronic acids, neutral sugars, cellulose, lignin, proteins). Phenolic, mainly proanthocyanidins (≈600 mg/100 g fiber), were co-extracted together with lower amounts of extractable phenolics (≈120 mg/100 g), mainly cyanidin-3-(6′-p-coumaroyl) glucoside (dark violet color). Powders showed high 2,2-diphenyl-1-picrylhydrazyl radical scavenging (≈485 mg/100 g fiber) and ferric reducing antioxidant power (835-674 mg/100 g) capabilities (expressed as ascorbic acid). In whole milk, independently of the blanching process used, the fiber powders obtained exerted almost the same concentration (0–9.0 g fiber/100 mL milk)-dependent protective effect against protein photo-oxidation under UV-C radiation (200–280 nm), measured as carbonyl production. We concluded that the antioxidant power of the cherry fibers was mainly due to phenolics. These powders may be useful for the food industry as natural additives for functional antioxidant preservation of milk protein products.