Tomato stain

In detail

A tomato stain is more complicated than its colour suggests. The pigment responsible is lycopene, a carotenoid, and carotenoids are fat-soluble rather than water-soluble — which is why a tomato mark rinsed in water often looks better and then reappears, and why it can survive a wash and be permanently set by the heat of a dryer. Ketchup and cooked sauces add sugar, vinegar and oil to that, so the deposit is simultaneously a dye, a greasy soil and a sugar residue. The carpet institute treats it on the aqueous-tannin branch — detergent solution, vinegar solution, warm water, spot removal kit, then an ammonia solution — which addresses the water-soluble fraction and the alkalinity, in that order. On fabric the practical rule that follows from the chemistry is to treat cold, to include something that will handle the oily fraction rather than water alone, and above all not to tumble-dry until the mark has genuinely gone, because heat is what makes a carotenoid mark permanent. Sunlight fades lycopene, which is why an old tomato mark on a white cotton item sometimes clears where nothing else worked.

Why

• Tomato stain is explained by Fat-soluble pigments
• Tomato stain is explained by Protein denaturation
• Tomato stain is explained by Heat-set stains
• Tomato stain is explained by Pigment retention

Hacks and claims involving this

• Leaving a stain in the sun to fade it Plausible mechanism, untested in this application