antioxidants Review Natural Phenol Polymers: Recent Advances in Food and Health Applications Lucia Panzella and Alessandra Napolitano * Department of Chemical Sciences, University of Naples “Federico II”, Via Cintia 4, Naples I-80126, Italy; [email protected] * Correspondence: [email protected]; Tel.: +39-081-674133 Academic Editor: Stanley Omaye Received: 17 March 2017; Accepted: 12 April 2017; Published: 14 April 2017 Abstract: Natural phenol polymers are widely represented in nature and include a variety of classes including tannins and lignins as the most prominent. Largely consumed foods are rich sources of phenol polymers, notably black foods traditionally used in East Asia, but other non-edible, easily accessible sources, e.g., seaweeds and wood, have been considered with increasing interest together with waste materials from agro-based industries, primarily grape pomace and other byproducts of fruit and coffee processing. Not in all cases were the main structural components of these materials identified because of their highly heterogeneous nature. The great beneficial effects of natural phenol-based polymers on human health and their potential in improving the quality of food were largely explored, and this review critically addresses the most interesting and innovative reports in the field of nutrition and biomedicine that have appeared in the last five years. Several in vivo human and animal trials supported the proposed use of these materials as food supplements and for amelioration of the health and production of livestock. Biocompatible and stable functional polymers prepared by peroxidase-catalyzed polymerization of natural phenols, as well as natural phenol polymers were exploited as conventional and green plastic additives in smart packaging and food-spoilage prevention applications. The potential of natural phenol polymers in regenerative biomedicine as additives of biomaterials to promote growth and differentiation of osteoblasts is also discussed. Keywords: tannins; lignins; grape pomace; spent coffee grounds; bioinspired phenolic polymers; black foods; food supplement; animal feed; food packaging; tissue engineering 1. Introduction Investigation of biomass as low cost sources of sustainable chemicals for direct exploitation or further to suitable transformations has become a main focus of academic and applied research over the last few years. Most of the vegetable sources currently investigated are rich in polymeric phenolic components that are amenable for several uses and applications. In the following part of this review, we will focus on selected classes of phenolic polymers with particular attention to those that can be obtained from abundant available sources by minimal processing. A considerable number of reports described the uses of such materials in different fields ranging from bioremediation [1] to material sciences, e.g., for preparation of phenolic resin, polyurethane, flocculants, gel, modified polymer materials or adhesives [2]. Apart from this, the great beneficial effects of natural phenol-based polymers on human health and their potential in improving the quality of food have been increasingly appreciated, and this work will critically review the most interesting and innovative reports in the field of nutrition and biomedicine that have appeared in the last five Antioxidants 2017, 6, 30; doi:10.3390/antiox6020030 www.mdpi.com/journal/antioxidants Antioxidants 2017, 6, 30 2 of 24 Antioxidants 2017, 6, 30 2 of 24 years.update Patents of those were applicatio excludedns that since have the maina potential aim of for this further review development is an update, ofbut those may applicationsnot be ready that for havea straightforward a potential for use further in industries. development, but may not be ready for a straightforward use in industries. After a short presentation of the main sources of the natural phenol polymers either dietary or not, withwith special special emphasis emphasis on on waste waste materials materials from fr agro-basedom agro-based industries, industries, polymers polymers obtained obtained through biomimeticthrough biomimetic procedures procedures from naturally-occurring from naturally phenol-occurring compounds phenol willcompounds be briefly will surveyed. be briefly The mainsurveyed. body The of the main review body will of addressthe review the will applications address ofthe phenol-based applications polymersof phenol of-based relevance polymers to food of andrelevance health. to food and health. 2. Main Sources of Natural Phenol Polymers 2.1. Dietary Sources Many largely consumed foods are rich sources of phenol polymers, most commonly tannins, traditionally categorized into thethe hydrolyzablehydrolyzable andand thethe condensedcondensed oror non-hydrolyzablenon-hydrolyzable classesclasses [[3].3]. One of the most investigated are those found in grapegrape skinskin andand seeds,seeds, mainlymainly polymericpolymeric proanthocyanidins belonging to the group of condensed tannins that can release monomeric phenols only upon depolymerization under acidic conditions (Figure1 1).). TheThe distributiondistribution andand levelslevels ofof thesethese tannins vary significantlysignificantly as a function of the grape variety and the degree of ripening and markedly affect thethe taste taste and and the the astringency astringency of the of winethe wine produced produced [4–8]. Of[4– the8]. sameOf the type same are tanninstype are occurring tannins inoccurring persimmon, in persimmon, a fruit largely a fruit consumed largely consumed in Japan [9 in]. Japan [9]. OH HO OH OH OH HO OH O O O HO O R =H, OH R1 1 O R =OH, gallate O 2 OH R2 O HO OH OH O OH OH O HO O OH OH O O O O HO O R1 Punicalagin R2 HO OH n HO OH OH OH OH OH OH OH HO O HO OH R1 OH HO R2 O OH O O Condensed tannins (proanthocyanidins) O O OH HO OH O OH O O HO HO OH OH O Punicalin Ellagitannins Figure 1. Structure of the main tannins found in grape, persimmon and pomegranate. Pomegranates are one of main highly valuable sources of ellagitannins that are also found in Pomegranates are one of main highly valuable sources of ellagitannins that are also found in other other fruits and nuts, e.g., strawberries, raspberries, blackberries, cloudberries, muscadine grapes, fruits and nuts, e.g., strawberries, raspberries, blackberries, cloudberries, muscadine grapes, almonds almonds and walnuts [10]. These tannins were formulated as hydrolyzable conjugates containing and walnuts [10]. These tannins were formulated as hydrolyzable conjugates containing one or more one or more hexahydroxydiphenoyl groups that esterify a sugar, usually glucose. The unique hexahydroxydiphenoyl groups that esterify a sugar, usually glucose. The unique gallagyl esters, gallagyl esters, punicalagin and punicalin, are the predominant ellagitannins of pomegranate punicalagin and punicalin, are the predominant ellagitannins of pomegranate (Figure1). They mainly (Figure 1). They mainly occur in the husk (pericarp) and peels (mesocarp) and are extracted into the juice upon commercial processing of the whole fruits. Analyses and quantitation of ellagitannins are Antioxidants 2017, 6, 30 3 of 24 Antioxidants 2017, 6, 30 3 of 24 occur in the husk (pericarp) and peels (mesocarp) and are extracted into the juice upon commercial particularlyprocessing ofdifficult the whole because fruits. they Analyses are only and partially quantitation solubilized of ellagitannins in the extraction are particularly solvent or difficultremain covalentlybecause they bound are only to cell partially walls and solubilized other macromolecules in the extraction of solvent the fruit or [11]. remain covalently bound to cell wallsThe and current other macromolecules burst of interest ofin thethe fruitblack [11 components]. of various foods or drinks of plant origin, includingThe current rice, tea, burst beans, of interest radish,in ginseng the black and components grapes, stems of various from the foods beneficial or drinks health of plant properties origin, attributedincluding to rice, the tea, pigments beans, as radish, evaluated ginseng in vitro and and grapes, in vivo stems studies from by the comparison beneficial with health the properties colorless varietiesattributed [12,13]. to the pigmentsActually, structural as evaluated studiesin vitro showedand in that, vivo thoughstudies all by appearing comparison black, with the the pigments colorless arevarieties very [different12,13]. Actually, in nature. structural Paradigmatic studies is showed the case that, of black though sesame all appearing seeds (Sesamum black, the indicum pigments L.), traditionallyare very different used in in Chinese nature. folk Paradigmatic medicine and is the as casefoodof in blackChina sesame and other seeds East (Sesamum Asian countries indicum andL.), largelytraditionally investigated used in for Chinese its potent folk medicine antioxidant and activity as food that in China is considered and other superior East Asian to countriesthat of white and sesamelargely investigatedseeds [14,15]. for The its black potent pigment antioxidant was activitypurified thatby hydrolytic is considered treatment superior of tothe that seeds of whiteafter removalsesame seedsof lipid [14 compone,15]. Thents, black but pigmentits insolubility was purified hampered by a hydrolytic detailed structural treatment analysis. of the seedsSolid state after spectroscopyremoval of lipid coupled components, to chemical but itsdegradation insolubility methodologies hampered a detailed and model structural studies analysis. indicated