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.

Introduction

Sweet cherry (Prunus avium L.) is a highly valued fruit crop of temperate regions. Although countries of the southern hemisphere (mainly Chile, South Africa, Australia and Argentina) contribute with only 5% of the world's production of sweet cherry, this crop is economically important for local development and has counter-seasonal marketing advantages (San Martino, Manavella, Garcia, & Salato, 2008; Coriolis Report, 2018).

For the fresh market, sweet cherries are hand-harvested, often mechanically sorted and frequently in transit for several weeks to distant markets. The growers' ability to competitively sell fresh cherries may be restricted by over-production and an excess in the market. In addition, since market intermediaries indicate a willingness to pay producers more per pound for fruit greater than 2.5 cm in diameter, firmness above 300 g/mm, and solid soluble contents above 18°Brix (Piaskowski et al., 2018), cherries without these features are usually discarded (Jara-Rojas, Guerra, Adasme-Berrios, Engler, & Valdés, 2015). However, these discarded cherries can be upgraded to powders highly enriched in dietary fiber and valuable natural antioxidants, which can be useful as additives and/or ingredients for food preservation during storage. This possibility is based on the fact that antioxidants are co-extracted with the cell wall biopolymers (fibers) from fruits and vegetables rich in phenolic compounds (Basanta et al., 2016; Renard, Watrelot, & Le Bourvellec, 2015), carotenoids and tocopherols (Idrovo Encalada et al., 2019). This adds an important functionality to the healthy effects of fibers within the gastrointestinal tract (Saura-Calixto, 2011) and their rheological effects (gelling, thickening, water absorption) (Idrovo Encalada, Basanta, Fissore, De’Nobili, & Rojas, 2016; Idrovo Encalada et al., 2019).

Before processing for fiber extraction, discarded fruits and vegetables need to be immediately stabilized by blanching (May 1990), a process that consists of heating the fruit for a short time prior to cooling and subsequent freezing. Blanching allows stabilizing the color, texture, flavor and nutritional quality of fruits by inactivating peroxidase (POX) and polyphenoloxidase (PPO), which are enzymes that catalyze the degradation of phenolic compounds and thus lead to subsequent browning. Blanching can extend the shelf life of the extracted fiber powders during their storage and use as additives or ingredients in foods of high water activity. Blanching also shows associated side benefits (Reyes de Corcuera, Cavalieri, & Powers, 2004; Sezer & Demirdöven, 2015), including its contribution to pathogen inactivation (Burke, 2019).

The usefulness of cherry fibers as functional food antioxidant preservatives can be determined not only through the 2,2-diphenyl-1-picrylhydrazyl radical (DPPH) scavenging and ferric antioxidant power (FRAP) assays but also by evaluation of their antioxidant capacity in real food systems. Although the study of food oxidation has mostly been focused on lipid oxidation (Li, Wang, Zhang, Shao, & Guo, 2019), during the past few years, protein oxidation has also been studied. Protein oxidation is due to the off flavor caused by the oxidation of specific amino acids, with simultaneous changes in functionality, and a decrease in digestibility and nutritional quality (Fernández, Ganan, Guerra, & Hierro, 2014). The effects of protein oxidation on essential enzymatic pathways involved in processes such as fermentations and cheese ripening are also of high relevance for the food industry (Dalsgaard, Otzen, Nielsen, & Larsen, 2007). The most harmful light-induced oxidation reactions in dairy products, meat and meat products, frozen fish, vegetable oils, beer and other alcoholic beverages are photo-oxidation and photoisomerization, which are due to the presence of photosensitizers such as myoglobin, riboflavin, bilirubin, and chlorophylls (Gordon, 2001; Lund, 2010).

Based on the above, the aim of the present work was to upgrade sweet cherries discarded at harvesting as antioxidant fibers for natural food preservation. We also aimed to analyze whether blanching affected the chemical composition of fibers and phenolic compounds, or the functionality and performance of cherry fibers as antioxidants to preserve milk proteins from photo-oxidation under UV-C radiation (200–280 nm).