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Red food colouring
Also called red food coloring, red gel colouring, Allura Red, Red 40, E129
A concentrated red dye sold as a liquid, a gel or a paste. In the United States it is usually Allura Red (Red 40). In Europe it may be Allura Red, beetroot extract, or carmine, which is made from cochineal insects and is not vegetarian.
Origin
Synthetic food dyes came out of the coal-tar chemistry of the late 19th century and are now made from petroleum. Red velvet cake's bright red was popularised by the Adams Extract company of Texas, which sold red dye with printed cake recipes in the 1930s and 1940s.
Flavour
None at normal baking doses. Large amounts of cheap liquid colouring can leave a faint bitter note, which is one reason gels are preferred.
How to use it
Add it with the wet ingredients so it spreads evenly. Gels are far stronger than liquids: a teaspoon of gel does the work of two tablespoons of supermarket liquid. Acid in buttermilk and vinegar keeps red velvet redder; too much baking soda pushes it brown.
Storage
Keep it capped at room temperature away from light. It lasts years, though natural beet-based colours fade within months.