Natural food colours

Replacing synthetic dyes with natural pigments is not a like-for-like substitution. Synthetic azo dyes were selected for stability, low cost and predictable shade. Natural pigments are metabolites whose properties were shaped by their function inside an organism, and their instability in food is a consequence of that chemistry rather than a manufacturing shortcoming.

Anthocyanins: colour is a pH equilibrium

Anthocyanins provide most natural reds, purples and blues. The pigment exists as several interconverting forms whose proportions depend on pH: the red flavylium cation dominates in strong acid; toward neutral it hydrates to a colourless hemiketal, so colour fades; higher still, blue quinoidal bases appear.

Colour is therefore not a property of the molecule but of the solution, which is why an anthocyanin that gives a clean red in a soft drink at pH 3 goes dull and grey in a neutral dairy product. Stability improves through copigmentation — stacking with colourless phenolics or with metal ions shields the reactive position and holds the coloured form — and this is how some plants achieve stable blues that isolated anthocyanins cannot. Sulfites bleach anthocyanins, and ascorbic acid degrades them, so a fortified beverage is a hostile matrix.

Betalains: acid-tolerant, heat-fragile

Betalains, the red-violet pigments of beetroot and prickly pear, are nitrogen-containing and chemically unrelated to anthocyanins despite the similar colour. Their advantage is that colour is far less pH-dependent across the food range. Their weakness is thermal: betanin hydrolyses on heating, so retorting or extended baking destroys it. They also carry the earthy geosmin note of their source.

Carotenoids: lipophilic and oxidisable

Carotenoids give yellow to red and are fat-soluble, which immediately constrains where they can be used — aqueous applications need emulsification or encapsulation. Colour comes from a long conjugated polyene chain, and that same chain is the vulnerability: it is readily oxidised, especially with light, heat, oxygen and metal ions present, and oxidation destroys the chromophore. Heat and light also isomerise the double bonds from trans to cis, which shifts hue and reduces intensity without destroying the molecule.

Phycocyanin behaves like a protein

The main natural blue is a pigment covalently bound to a protein, so its colour depends on the protein staying folded. It denatures with mild heat and loses colour outside a narrow band near neutral pH. Blue therefore remains the hardest colour to deliver naturally.

What fermentation actually changes

Fungal and bacterial fermentation can produce pigments — including carotenoids and various polyketide-derived colours — in a tank rather than a field. What this changes is supply: consistent shade batch to batch, no crop failure, no seasonal variation, no land use, and no reliance on extracting a minor component from a large mass of plant material.

What it does not change is the chemistry. A fermentation-derived carotenoid oxidises exactly as a plant-extracted one does. Formulation still requires encapsulation, oxygen barrier packaging, antioxidants and light protection.

One genuine caution belongs here: some fungal pigment pathways are related to mycotoxin biosynthesis in the same or related organisms, so strain selection and toxin screening are a real part of qualifying a fermentation-derived colour, not a formality.