plants Review Natural Blues: Structure Meets Function in Anthocyanins Alan Houghton 1 , Ingo Appelhagen 2 and Cathie Martin 1, *   Citation: Houghton, A.; Appelhagen, I.; Martin, C. Natural Blues: Structure Meets Function in Anthocyanins. Plants 2021, 10, 726. https://doi.org/ 10.3390/plants10040726 Academic Editor: Luisa Palmieri Received: 1 March 2021 Accepted: 31 March 2021 Published: 8 April 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 John Innes Centre, Department of Metabolic Biology and Biological Chemistry, Norwich Research Park, Norwich NR4 7UH, UK; Alan.Houghton@jic.ac.uk 2 Phyton Biotech, Alter Postweg 1, 22926 Ahrensburg, Germany; Ingo.Appelhagen@gmx.de * Correspondence: Cathie.Martin@jic.ac.uk; Tel.: +49-15771416896 Abstract: Choices of blue food colourants are extremely limited, with only two options in the USA, synthetic blue no. 1 and no. 2, and a third available in Europe, patent blue V. The food industry is investing heavily in finding naturally derived replacements, with limited success to date. Here, we review the complex and multifold mechanisms whereby blue pigmentation by anthocyanins is achieved in nature. Our aim is to explain how structure determines the functionality of anthocyanin pigments, particularly their colour and their stability. Where possible, we describe the impact of progressive decorations on colour and stability, drawn from extensive but diverse physico-chemical studies. We also consider briefly how this understanding could be harnessed to develop blue food colourants on the basis of the understanding of how anthocyanins create blues in nature. Keywords: blue; anthocyanin; food colourants; co-pigment; quinonoid base 1. Introduction Throughout history, our culture has been intrinsically linked to the colours of pigments and dyes accessible to us [1]. Colour is one of the primary visual indicators of food spoilage, so it comes as no surprise that we have evolved with innate psychological responses to abnormally coloured food; effects so strong that food not matching colour expectations can taste less appealing than an otherwise identical, “colour-correct” foods [2]. With recent trends towards use of natural pigments in food, manufacturers have invested heavily in developing replacements for synthetic colourants. Despite plenty of options for yellow to purple, natural blue colourants remain elusive [3]. Currently, only three synthetic blue pigments are approved in Europe as food additives. Brilliant blue FCF (derived from petrochemicals), Indigo carmine (synthetic derivative of indigo), and patent blue V. However, only one natural pigment is available—phycocyanin, derived from “Spirulina”, which is composed of a mixture of three species of cyanobacteria, namely, Arthrospira platensis, Arthrospira fusiformis, and Arthrospira maxima. Natural blue pigments are rare in nature, and only one known animal, a species of butterfly, Nessaea obrinus, can synthesise them. Instead, many apparently blue organisms create multi-layered nanostructures to reflect shorter wavelength light selectively, giving them their iridescent blue hues [4]. Unlike pigments however, the structural colour will change in these cases, depending on the viewing or illumination angle. Plants produce an array of natural colourants, which can be broadly separated into four major groups: chlorophyll (green), carotenoids (yellow-red), betalains (yellow and red), and flavonoids (yellow-blue). Plants utilise these pigments to perform essential functions such as photosynthesis, radical scavenging, and attracting pollinators. Many fruits and vegetables are coloured with red and orange pigments, which have the greatest contrast against green foliage. This increases the likelihood they will be seen and eaten, allowing their seeds to be dispersed widely. This seed dispersal strategy is so effective that three unique red pigments have evolved: lycopene (a carotenoid found in fruits such as tomatoes), betacyanins (betalains found in some of the Caryophyllales)[5], and anthocyanins (such as the major pigment in strawberries, pelargonidin 3-O-glucoside) [6]. Plants 2021, 10, 726. https://doi.org/10.3390/plants10040726 https://www.mdpi.com/journal/plants