Curcumin is the yellow pigment of turmeric. It is a vivid food colour and natural pH indicator, and although heavily researched, very little of it is absorbed by the body when eaten.
What it is¶
Curcumin is the bright yellow-orange pigment of turmeric, the dried rhizome of Curcuma longa, a member of the ginger family. It is the main member of a group called the curcuminoids, alongside demethoxycurcumin and bisdemethoxycurcumin; together they typically make up somewhere around 2–5% of dried turmeric, varying considerably with variety and origin.
Curcumin contributes colour rather than flavour. Turmeric's earthy, slightly bitter, gingery taste comes mostly from its essential oil, rich in sesquiterpenes called turmerones (see aroma-compounds).
It is approved as a food colour, E100, and colours mustards, cheeses, butter, margarine, sweets and curry-powder. English mustard owes its yellow partly to turmeric.
A natural pH indicator¶
🔬 Yellow to red
Curcumin changes colour with acidity. In acidic and neutral conditions it is yellow; in alkaline conditions, above roughly pH 8, it turns red-brown as the molecule loses hydrogen ions. Stir a pinch of turmeric into water with a little bicarbonate of soda and it goes orange-red; add lemon juice and it returns to yellow. Chemists once used turmeric paper as a test for alkalis and for boron.
The same chemistry explains why turmeric stains on soap-washed surfaces sometimes turn pink or red: many soaps are mildly alkaline.
Properties in the kitchen¶
- Fat-soluble. Curcumin dissolves poorly in water and well in oil and alcohol. Frying turmeric briefly in oil or ghee, as in most Indian cooking, disperses its colour evenly (see fat-and-flavour).
- Light-sensitive. Curcumin fades in sunlight. That is a nuisance for jars on a sunny shelf, but useful for stains: a turmeric-stained worktop or plastic tub left in bright daylight often fades noticeably within a day or two.
- Heat-tolerant within reason. It survives normal cooking, although long, high heat degrades it somewhat.
- A strong stain. Wash spills promptly; wooden boards and white clothes are the usual victims.
Bioavailability¶
Curcumin has been the subject of thousands of laboratory studies, and is widely marketed in supplements. The chemistry offers a caution.
When eaten, very little curcumin reaches the bloodstream intact. It is poorly soluble in the watery contents of the gut, poorly absorbed, and rapidly converted by the gut wall and liver into glucuronide and sulphate conjugates that are excreted. Levels of free curcumin in blood after even large oral doses are typically very low. Chemists call this low bioavailability.
Two approaches are commonly used to raise it:
- Piperine. The pungent compound of black-pepper inhibits the enzymes that conjugate curcumin. A frequently cited 1998 study found that taking piperine with curcumin greatly increased blood levels (see piperine).
- Formulation. Supplement makers use fats, emulsifiers, nanoparticles and other carriers to improve absorption.
Traditional cooking tends to combine turmeric with fat and often with pepper, which has led to speculation that the combination is no accident; that is a plausible but unproven idea.
⚠️ No health claims
Laboratory activity does not show that a compound helps people. A 2017 review by medicinal chemists argued that curcumin is chemically unstable, poorly absorbed and prone to giving misleading results in laboratory screens, and that clinical trials had not established it as a treatment for any condition. herbsandspices.food makes no health claims for curcumin. There have also been reports of liver injury linked to turmeric and curcumin supplements, particularly high-dose and high-absorption products; supplements are a very different dose from culinary turmeric.
Adulteration¶
Because curcumin's colour is a mark of quality, turmeric has a long history of adulteration. Investigations in South Asia have found turmeric coloured with lead chromate to brighten dull roots, a serious source of lead exposure. See lead-in-spices and spice-adulteration.
History¶
Turmeric has been used as a dye, cosmetic and food colour in South Asia for millennia, and gave yellow to the robes of some Buddhist monks. Curcumin was first isolated in 1815 by the chemists Vogel and Pelletier, and its structure was worked out in the early 20th century.
See also¶
- turmeric
- piperine
- fat-and-flavour
- curry-powder
- annatto and saffron — other natural yellow colours