Assessment of Quality Differences in Turmeric Finished Products by ¹H NMR and HPTLC

Reviewed: Chatzinasiou L, Booker A, MacLennan E, Mackonochie M, Heinrich M. The DNA-based authentication of commercial herbal products reveals their globally widespread adulteration. Front Pharmacol. 2019;10:1227.

Keywords: Adulteration, Curcuma longa, curcumin, HPLTC, ¹H NMR, ar-turmerone

In line with previous publications on quality of rhodiola (Rhodiola rosea, Crassulaceae),^1,2 St. John’s wort (Hypericum perforatum, Hypericaceae),^3,4 and turmeric (Curcuma longa, Zingiberaceae)⁵ bulk ingredients and finished dietary supplements, the researchers from University College of London used a combination of proton nuclear magnetic resonance (¹H NMR) spectrometry and high-performance thin-layer chromatography (HPTLC) to assess the quality of 50 commercial dietary supplement products mainly from pharmacies in the UK market, but also from Germany, the US, and from online purchases. In addition, 22 samples from herbal tea and dietary supplement manufacturer Pukka Herbs (Keynsham, UK) from different stages of the turmeric manufacturing process, i.e., ground roots, root powder, turmeric extracts, and finished turmeric dietary supplements, were analyzed. 

The ¹H NMR coupled to principle component analysis (PCA) metabolomics showed that most dietary supplement products clustered in proximity to the curcumin reference standard. Five products were a bit farther away due to the comparatively higher concentrations in curcuminoids. Another three products, which were composed of only curcumin/curcuminoids and piperine, aligned outside the PCA scores plot, marking then as clearly distinct from authentic turmeric extracts. The ¹H NMR data also allowed comparison of the relative amounts of curcuminoids, piperine, and ar-turmerone based on the signals at 6.73-6.84 ppm (curcuminoids), 6.85-7.00 ppm (piperine), and 7.04-7.13 ppm (ar-turmerone). Six samples were devoid of ar-turmerone, which could be due a manufacturing process aimed at eliminating the essential oil, improper sample storage, or the substitution of turmeric extract with curcumin or curcuminoids made by chemical synthesis. The latter seems to be the case for at least the three samples that placed outside the PCA scores plot.

Similar conclusions were obtained from the results of the HPTLC analysis. The authors placed the 56 (including six Pukka products) commercial dietary supplement products into six groups using conditions optimized for curcuminoid separation. Most (n = 47) of the commercial products were in the three main groups: curcuminoids with piperine (n = 19), curcuminoids without piperine (n = 19), and curcuminoids with piperine and an extra band at Rf = 0.81 (n = 9). One sample in this group showed additional bands possibly indicating admixture of an undeclared ingredient. Interestingly, 10 samples claiming to contain piperine appeared to contain no or minimal amounts of this alkaloid. The remaining three groups contained either curcumin and piperine alone (n = 2), curcuminoids alone with or without piperine (n = 5), or a turmeric extract with unidentified extra bands (n = 2).

The authors suggest that both methods are complementary and provide robust orthogonal approaches for turmeric extract quality control. Of the 56 commercial products, between three and six samples appear to have synthetic curcuminoids present, and one product contained an unknown additional substance.

Comment: According to the results of this investigation, it seems that a majority of turmeric supplements contain reasonable amounts of turmeric extracts. Adulteration with synthetic curcumin/curcuminoids was infrequent; the main deviation was the lack of piperine in 10 products despite the label indicating its presence. Given the general conclusions of this paper, it may be difficult to justify purchasing an NMR spectrometer for a QC facility unless there is a need for multivariate statistical analysis. While the authors reported only relative amounts of curcuminoids, piperine, and ar-turmerone from the ¹H NMR data, quantification of these components is relatively easy using NMR spectrometry. In particular for the determination of piperine, ¹H NMR has a distinct advantage, since the HPTLC conditions indicate some co-eluting peaks (given the fact that the color of the band in the piperine region in HPTLC varied between yellow and pink). Assuming that stronger magnetic field NMR instruments are capable of separating the curcuminoid signals, a rapid quantification of curcumin, demethoxycurcumin, bisdemethoxycurcumin, ar-turmerone, and piperine in a short time could be obtained by NMR in addition to the information about the similarity of the material to previous manufacturing batches.

References

1. Booker A, Jalil B, Frommenwiler D, Reich E, Zhai L, Kulic Z, Heinrich M. The authenticity and quality of Rhodiola rosea products. Phytomedicine. 2016;23(7):754-762.
2. Booker A, Zhai L, Gkouva C, Li S, Heinrich M. From traditional resource to global commodities:—A comparison of Rhodiola species using NMR spectroscopy—metabolomics and HPTLC. Front Pharmacol. 2016;7:254. doi:10.3389/fphar.2016.00254.
3. Booker A,Agapouda A,Frommenwiler DA, Scotti F, Reich E, Heinrich M. St John’s wort (Hypericum perforatum) products – an assessment of their authenticity and quality. Phytomedicine. 2018;40:158-164.
4. Scotti F, Löbel K, Booker A, Heinrich M. St. John’s wort products – how variable is the primary material? Front Plant Sci. 2019;9:1973.
5. Booker A, Frommenwiler D, Johnston D, Umealajekwu C, Reich E, Heinrich M. Chemical variability along the value chains of turmeric (Curcuma longa): A comparison of nuclear magnetic resonance spectroscopy and high performance thin layer chromatography. J Ethnopharmacol. 2014;152:292-301.