Three Curcuminoids, One Number

Turmeric rhizome contains curcumin, demethoxycurcumin and bisdemethoxycurcumin in a fairly constant natural ratio, roughly 75 : 20 : 5 in most Indian material with cultivar-dependent variation. “Total curcuminoids 95%” is the sum of the three; “curcumin 95%” in trade usage means the same thing, not 95% of the single molecule. A specification should say which it means.

Method 1: UV-Visible Spectrophotometry at 425 nm

The classical assay dissolves the extract in ethanol or methanol and reads absorbance at about 425 nm, where curcuminoids absorb strongly, against a curcumin reference or a fixed specific absorbance. It is fast and robust, and it is the basis of most trade certificates. It measures the yellow-orange chromophore, so it counts all three curcuminoids together and cannot separate them; anything else that absorbs near 425 nm, including some added colours, inflates the reading.

Method 2: HPLC with Diode-Array Detection

Reversed-phase HPLC or UPLC with detection at 425 nm resolves the three curcuminoids into separate peaks and quantifies each against its own standard. Validated UPLC-DAD methods run in minutes and have been used to discriminate turmeric by geographical origin from the curcuminoid profile alone. HPLC gives the ratio, which UV cannot, and the ratio is the first line of authentication.

Why the Two Disagree

On clean natural extract the methods usually agree within a few percent. They diverge when the material carries added colour, when a low-quality extract is spiked with purified curcumin (which shifts the ratio toward curcumin), or when reference standards differ between laboratories. A UV figure that is consistently a little above the HPLC figure is normal; a large gap is a question.

The Synthetic Curcumin Problem

Synthetic curcumin is cheap, colour-identical and, if the maker blends in the two minor curcuminoids, ratio-identical. Neither UV nor routine HPLC distinguishes it from natural. Two approaches do: carbon-14 analysis, which detects the petrochemical origin, and stable-isotope ratio analysis of carbon and hydrogen, which a 2023 study proposed as a cheaper screen with threshold values. Other analytical signatures, including impurity profiles specific to the synthetic route, have also been reported. A buyer who needs natural-origin assurance should ask for one of these tests on a reference lot, not on every drum, and should treat an unusually low price on 95% material as the first signal. Our adulteration guide goes deeper.

What to Specify

  1. “Total curcuminoids NLT 95% by HPLC, with curcumin, demethoxycurcumin and bisdemethoxycurcumin reported individually.”
  2. A natural-ratio window (for example curcumin 70% to 80% of total) as an authentication clause.
  3. Residual solvent by GC against ICH Q3C, since 95% material is a solvent-crystallised product.
  4. Origin assurance by C-14 or isotope ratio on a reference lot where the claim on pack is “natural”.

The EFSA acceptable daily intake for curcumin as a food additive, 3 mg per kg body weight, and the other regulatory anchors are set out in curcumin regulatory compliance.