Contents lists available at ScienceDirect Industrial Crops & Products journal homepage: www.elsevier.com/locate/indcrop Research paper Citrus reticulata Blanco peels as a source of antioxidant and anti-proliferative phenolic compounds Sandrine S. Ferreira a,b,1 , Amélia M. Silva b,c,1 , Fernando M. Nunes a,d, ⁎ a CQ-VR, Chemistry Research Center, University of Trás-os-Montes and Alto Douro, Quinta de Prados, 5000-801 Vila Real, Portugal b Department of Biology and Environment, UTAD, 5001-801 Vila Real, Portugal c Centre for Research and Technology of Agro-Environmental and Biological Sciences (CITAB-UTAD), University of Trás-os-Montes e Alto Douro, Quinta de Prados, 5001- 801 Vila Real, Portugal d Department of Chemistry, UTAD, 5001-801 Vila Real, Portugal ARTICLE INFO Keywords: Citrus reticulata Peels Polyphenols Antioxidant activity Anti-proliferative activity BT-474 cells ABSTRACT Citrus reticulata Blanco industrial use worldwide generates millions of tons of by-products, mainly peels, with negative environmental impact. The main purpose of this work was to compare the extraction efficiency of water and 70% ethanol, for recovering C. reticulata peel phenolic compounds. A simple solid phase extraction method was used for obtaining enriched phenolic extracts. There were no significant differences in the extraction effi- ciency for the two solvents used. The main components were hesperidin, naringin, tangeritin, and rutin, that accounted for nearly 86% of the total phenolics extracted. Solid phase extraction allowed a 4.5-fold enrichment in phenolics and antioxidant activity of the extracts. The anti-proliferative activity of the extracts was found to be dose-dependent but also dependent on the cell line. The solid phase extraction enriched phenolic extracts after 48 h exposure presented an IC 50 of 174.5 ± 5.8 μg/mL (BT-474), 391.9 ± 15 μg/mL (Caco-2) and > 500.00 μg/mL (HepG2). These results show that C. reticulata peels are a cheap and abundant source of antioxidant and potentially bioactive phenolic compounds. 1. Introduction Citrus genus is one of the most economically relevant fruit crop in the world. It includes about 17 species of citrus fruits such as Citrus reticulata Blanco (mandarin orange, tangerine), Citrus sinensis L. (sweet orange), Citrus aurantium L. (bitter orange), Citrus lemon L. (lemon), Citrus paradise M. (grapefruit) (Chutia et al., 2009). These crops of Asian origin, are nowadays distributed throughout the world, mainly in the tropical, subtropical and temperate regions, like Brazil, Portugal, Spain, Italy, Greece, Morocco, Turkey and Egypt (Boluda-Aguilar et al., 2010; Chutia et al., 2009). Their worldwide annual production is near 88 million tons (Negro et al., 2016). The industrial processing of citrus fruits generates millions of tons of organic waste residues such as peels, seeds, pulp and membranes (Boluda-Aguilar et al., 2010; Hiasa et al., 2016; Negro et al., 2016). These industrial by-products are an en- vironmental problem and a product waste. Increased awareness of food industries of the economic and environmental impacts of waste gen- eration and disposal are calling for transition towards more sustainable practices and the adoption of the circular economy concepts (Jurgilevich et al., 2016). Circular economy means reuse, repair, refurbishing, and recycling of the existing materials and products, what was earlier considered to be waste becomes a resource. The recovery of phenolic compounds present in considerable amounts in agricultural by-products for pharmaceutical and cosmetic applications has been studied as an attractive way of industrial by-products valorization (Melgar et al., 2017; Silva et al., 2016). Recycling of citrus by-products has been mainly performed, fresh or after ensilage or dehydration, for cattle feed (Bampidis and Robinson, 2006; Caparra et al., 2007). Other potential re-uses of citrus fruit by-products, include bioethanol pro- duction due to its high carbohydrate content (Boluda-Aguilar et al., 2010), essential oil production/recovery, mainly composed by D-li- monene (Negro et al., 2016), as a substrate for multienzyme production (Mamma et al., 2008). Due to their high content of phenolic acids, polyphenols (e.g., polymethoxylated flavones, glycosylated flavanones, flavonoids and limonoids), and carotenoids (Boluda-Aguilar et al., 2010; Hiasa et al., 2016; Ho and Lin, 2008; Huang and Ho, 2010; Moulehi et al., 2012), citrus peels can also constitute a source of po- tential pharmacological molecules. In fact, citrus peel medicinal use is known for centuries, as dried peels of C. reticulata “chen pi” have been used in the Traditional Chinese Medicine for treating indigestion, http://dx.doi.org/10.1016/j.indcrop.2017.10.009 Received 5 February 2017; Received in revised form 15 August 2017; Accepted 8 October 2017 ⁎ Corresponding author at: CQ-VR, Chemistry Research Centre, Chemistry Department, University of Trás-os-Montes and Alto Douro, Quinta de Prados; 5000-801; Vila Real, Portugal. 1 Both authors contributed equally to this work. E-mail address: fnunes@utad.pt (F.M. Nunes). Industrial Crops & Products 111 (2018) 141–148 Available online 16 October 2017 0926-6690/ © 2017 Elsevier B.V. All rights reserved. T