Mustard stain

In detail

Most prepared mustards are coloured with turmeric, and turmeric's pigment is curcumin, which behaves as a pH indicator: yellow in acid and neutral conditions, and red to reddish-brown in alkaline ones. That single fact explains the experience people describe as the stain getting worse when they treat it. An alkaline cleaner, a soapy solution or an ammonia-based product shifts curcumin into its red form, so a pale yellow mark on a shirt becomes a conspicuous orange one, and the change is often mistaken for the stain setting. It reverses on acidification, which is why the carpet institute's mustard entry includes a vinegar solution and why rinsing thoroughly matters as much as treating. The same chemistry governs curry and turmeric marks generally. Curcumin is also strongly light-sensitive, and prolonged daylight fades it more reliably than most household treatments — a genuinely useful last resort on a white cotton item. It stains plastics readily and more or less permanently, which is why a turmeric-coloured food container does not recover.

Why

• Mustard stain is explained by pH indicators in stains
• Mustard stain is explained by Fat-soluble pigments
• Mustard stain is explained by Photodegradation
• Mustard stain is explained by Pigment retention
• Mustard stain is explained by Photodegradation

Hacks and claims involving this

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