Symposium Introduction pubs.acs.org/JAFC
Polyphenols: Physicochemical and Biological Properties and Perspectives of Their Use in a Zero-Waste Society ABSTRACT: The 28th International Conference on Polyphenols, held in Vienna (Austria) in July 2016, offered a venue for global exchanges on the active research on polyphenols and for the presentation and discussion of the latest advances. The multidisciplinary event attracted 280 scientists from four continents working in different fields, from analytical and biosynthetic chemistry through genetic and metabolic engineering, plant physiology, and ecology to food nutrition and health sciences as well as industrial applications. This special issue presents a selection of papers from oral presentations and poster contributions shown within the framework of sessions focusing on research in the fields of (1) chemistry and physicochemistry, (2) food, nutrition, and health, and (3) applied polyphenolics. This introductory paper also briefly summarizes general properties of this versatile and largest group of secondary metabolites and their use in bioeconomical approaches. he term “polyphenol” is inconsistently used and often simply subsumes molecules with more than one hydroxy group on an aromatic ring,1 but it can also be restricted to complex molecules possessing 12−16 phenolic hydroxy groups on five to seven aromatic rings.2 A more specific definition recently proposed that the term “polyphenol” should be used only for plant secondary metabolites derived exclusively from the phenylpropanoid and/or the polyketide pathway(s), featuring more than one phenolic ring and being devoid of any nitrogen-based functional group.3 Several thousand structures have been isolated and characterized from plants thus far,4 ranging from rather simple phenolic molecules to highly polymerized compounds with molecular weights of more than 30 000 Da.5 As a result of the huge diversity of structures, polyphenols possess diverse physicochemical properties.6 Over the years, the scientific community has been focusing on these fascinating molecules, trying to shed light on their chemistry, properties, and physiological relevance in plants, humans, and ecosystems. In 1972, the Groupe Polyphénols (GP) society was founded in France as an international association with the aim of promoting research on plant polyphenols while providing members worldwide with a unique forum to exchange information on all aspects of these fascinating natural products, from their most basic and fundamental physicochemical properties to their most diverse applications in food and agricultural, pharmaceutical, and cosmetic sciences and technologies (http://www. groupepolyphenols.com/). Since then, an international event was held every year and, as of 1980, every second year, on four continents thus far, under the auspices of the GP society. Since 2008, Recent Advances on Polyphenol Research, a book series containing chapters written by plenary lecturers at the International Conference on Polyphenols (ICP) and other invited contributors, presents updates on the most significant advances in the field.7 The aromatic and/or heterocyclic ring structure(s) of polyphenols determine their ultraviolet−visible light absorbance and resonance stabilization, which, in the presence of hydroxy groups, also enable the formation of quinonoid structures.8 Phenols are weak acids and, therefore, show a pH-dependent protonation/deprotonation status. The number and position of hydroxy groups have an important influence on properties, such as stability, light absorbance, free-radical
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© 2017 American Chemical Society
scavenging, antioxidant, prooxidant, and chelating activity.8 The basic structures are usually subjected to extensive modification reactions, such as glycosylation, methylation, acylation, and sulfation, which also strongly influence various chemical and physical properties.9 This, of course, also affects solubility, which is an important aspect in any application of polyphenols.10 Increasing numbers of hydroxy groups and sugar moieties increase water solubility, whereas methylation of the hydroxy groups decreases it.8 Free hydroxy groups are more reactive than glycosylated and methylated hydroxy groups, and this is notably essential for many of the biological activities related to the radical scavenging ability and antioxidant capacity of the flavonoids.10,11 Several structural elements of this class of polyphenols seem to contribute significantly to this: (i) the presence of vicinal hydroxy groups on the aromatic ring B attached to the heterocycle, (ii) a double bond between C2 and C3 in conjugation with an oxo function at position 4 of ring C, (iii) additional hydroxy groups adjacent to this oxo function11−13 (for atom numbering and ring labeling, refer to Figure 1). Polyphenols are ubiquitously present in plants and, therefore, also in human foods and drinks. The daily intake of flavonoids depends upon the composition and is estimated to range from 23 mg up to more than 1.5 g.14−16 Foods and drinks particularly high in flavonoid concentrations are onions, fruits,
Figure 1. Atom numbering and ring labeling in flavonoid structures using the example of the basic flavan structure. Special Issue: XXVIIIth International Conference on Polyphenols 2016 Published: July 3, 2017 6343
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Symposium Introduction
tea, cocoa, and wine.17 At least 437 individual polyphenols are regularly consumed, 94 of which at a level exceeding 1 mg/ day.14 Voluminous literature studying the impact of flavonoids on human health assessed via in vitro assays does exist. The bioavailability of flavonoids in humans remains, however, unclear, as does the question of whether nano- or micromolar concentrations of these substances need to be considered.18−20 Bioavailability aspects and specific effects of polyphenols after consumption are addressed in this issue (Almeida et al. and López de las Hazas et al.). The main known polyphenolic contributors in plant-derived food and beverages are phenolic acids and flavonoids.20 However, these molecules are highly unstable and yield numerous reaction products during processing and storage (Valero-Casesa et al.). Characterization of complex tannin polymers, including polyphenol reaction products, and determination of their organoleptic and nutritional properties are still major challenges.21 Several manuscripts in this issue address health-related effects of polyphenols from various sources (Tsai et al., López de las Hazas et al., Bernini et al., Perot et al., and Panzella et al.). Polyphenols have been suggested to play crucial roles in disease prevention, such as reducing the risks of cancer, cardiovascular disease, diabetes, stroke, Alzheimer’s disease, cataracts, and other functional disorders associated with aging.22 Many of the observed preventative effects are based on the radical scavenging and antioxidant properties of flavonoids and their interaction with various proteins.22 This issue provides new insights into ́ polyphenol analysis, structures, and reactions (Garcia-Esté vez et al., Gago et al., and Burtch et al.) as well as polyphenol− protein interactions (Silva et al., Pérot et al., Ferrer-Gallego et ́ al., Ramos-Pineda et al., and Garcia-Esté vez et al.). The interaction of proteins with tannins is of particular interest for food science because it is responsible for astringency perception (interactions with salivary proteins) (Silva et al., Ferre-Gallego et al., and Ramos-Pineda et al.) and the formation of haze and precipitates in beverages (Millet et al.).10 Tannins are a chemically diverse class of polyphenols defined by their ability to precipitate proteins. For example, the use of polyphenols as tanning agents belongs to the oldest technological applications of polyphenols.3 Several papers in this special issue address the role and concentrations of hydrolyzable and condensed tannins ́ (Tuominen and Salminen and Garcia-Esté vez et al.). Under the influence of limited natural resources and significant signs of climate change, the past years have seen a paradigm shift from waste disposal and simple combustion thereof toward the exploitation of plant-derived waste from food, feed, and raw material production via high-value biorefinery approaches to match the ever increasing demand for environmentally friendly raw materials and bioactive compounds. Polyphenols are used as bioactive substances in dietary supplements (synonym: nutraceuticals) and functional food,23,24 additives in food and cosmetic products (antioxidants, natural colors, and flavors), and raw material for innovative products, such as multifunctional polymer coatings or antibacterial packaging.25,26 Applications range from simple extracts from various plant sources to purified high-value products, with a potential value of more than 500 k€/ton.27 Valorization of polyphenols obtained as natural byproducts may require further chemical manipulation or modification to optimize solubility and other properties depending upon the specific applications. Polyphenols can be obtained directly from plant materials but may also arise as byproducts from the
lignocellulose industry or agroindustrial waste streams.28−30 The latter is increasingly preferred for economic reasons and to escape the food−fuel debate,27 which raises major concerns about the utilization of feed and food crops for energy production. Examples of polyphenol-based (by)products are antioxidants from pulp and paper processes,27 olive mill waste,31 wine industry,32 and pomegranate fruits33 as well as food and textile colorants from natural sources as alternatives to synthetic dyes from winery, black carrots, black bean, and elderberry34−37 or phytoestrogens from wood branches, red clover, and soybean.38 Several contributions in this issue focus on novel aspects within this emerging and steadily growing research area (Cruz et al., Panzella et al., Iacomino et al., and Bernini et al.) as well as on novel food sources (Vaštakaitė et al.). In 2016, the 28th International Conference on Polyphenols took place in Vienna at Technische Universität Wien from July 11 to 15. The conference saw the newest discoveries and innovations in the field of polyphenols discussed by nearly 300 scientists from all over the world. The lectures and posters covered the expansive field of polyphenols, delving into everything from chemistry, analysis, biosynthesis, genetics, metabolic engineering, role in plants and ecosystems to food, nutrition, health, and applied polyphenolics. Examples of topics of discussion are the newly discovered function of enzymes in secondary metabolism in cell nuclei, the design of more degradable lignin, and what mussels can teach us with regard to the production of multifunctional polyphenol coating. This special issue was created from conference contributions from the topics chemistry and physicochemistry, food, nutrition, and health, and applied polyphenolics. Considering the global active research in the field of polyphenols and the unbroken interest of academic and industrial researchers and users, we anticipate exciting results at the next conference, which will be organized by Jess Reed in Madison (WI, U.S.A.) in July 16−20, 2018, jointly with the 9th Tannin Conference (http://conferences. union.wisc.edu/icp/).
Véronique Cheynier† Heidi Halbwirth*,‡ †
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l’Unité Mixte de Recherches Sciences Pour l’Œnologie (UMR SPO), Institut National de la Recherche Agronomique (INRA), Montpellier SupAgro, Université de Montpellier, 34060 Montpellier, France ‡ Institute of Chemical, Environmental and Biological Engineering, Technische Universität Wien, 1060 Vienna, Austria
AUTHOR INFORMATION
Corresponding Author
*Telephone: +43-1-58801166559. Fax: +43-1-5880117399. Email:
[email protected]. ORCID
Heidi Halbwirth: 0000-0001-9059-5850 Notes
The authors declare no competing financial interest.
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REFERENCES
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