Natural Blues: Structure Meets Function in Anthocyanins

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.

Beyond Purple Tomatoes: Combined Strategies Targeting Anthocyanins to Generate Crimson, Magenta, and Indigo Fruit

“…The chemical diversity and complexity of anthocyanins is a major obstacle. Nearly one thousand different naturally occurring compounds have been reported, whose structures differ in the combinations of sugar units, methoxyl groups, aliphatic and aromatic acyl groups decorating the basic structure of anthocyanins: a C6-C3-C6 skeleton structure consisting of two aromatic rings (A and B) and a central heterocyclic oxygen ring (C) with a positive charge (Figure 1), also known as a flavylium cation [9].…”

“…Horticulturae 2021, 7, 327 2 of 14 groups [9,10]. The final colour of plant tissues is determined by a combination of different factors (including vacuolar pH, the presence of co-pigments and metal ions, and the shape of the cell) but the B-ring hydroxylation pattern has a major impact.…”

“…The hydroxyl group at the 4′ position of the B-ring is usually present because its addition (from coumaroyl CoA) occurs early in the pathway, leading to the formation of the chalcone backbone, common to almost all classes of flavonoids. The mono-hydroxylated anthocyanin struc- The hydroxylation pattern of the B-ring has a major influence on colour and is associated with a shift from orange/red to violet/blue with an increasing number of hydroxyl groups [9,10]. The final colour of plant tissues is determined by a combination of different factors (including vacuolar pH, the presence of co-pigments and metal ions, and the shape of the cell) but the B-ring hydroxylation pattern has a major impact.…”

Anthocyanin Hybrid Nanopigments from Pomegranate Waste: Colour, Thermomechanical Stability and Environmental Impact of Polyester-Based Bionanocomposites

“…ACNs act as weak diacids and they become deprotonated with increasing pH values, increasing maximum absorption towards higher wavelengths. The pH-dependent dynamic equilibrium of the different ACN structural forms present in aqueous solutions was reported by Fedenko et al [15] and Houghton et al [44]. The red flavylium cation (AH + ) is favoured at very acid pH values (pH < 2-3), whereas at higher pH values some reversible (to some extent) chemical transformations could occur.…”

Spectral Characteristic, Storage Stability and Antioxidant Properties of Anthocyanin Extracts from Flowers of Butterfly Pea (Clitoria ternatea L.)

“…Due to the health concerns associated with synthetic pigments, the food industry has begun to invest heavily in natural food colorants [1,2]. Common plant pigments, including anthocyanins, betalains, and carotenoids, possess a red hue (from yellow to purple) and have been recommended for use as healthy natural food colorants [3,4].…”