Turmeric chemical constituents
Curcuminoids are naturally present in the rhizomes (underground stems) of turmeric. Turmeric rhizomes are used as a spice in Indian cooking, being one of the principal ingredients in curry powder. Turmeric is known for its bright yellow color, imparting that color to curries and American-style prepared mustard. The chemical constituents responsible for that color are curcuminoids, present in 3-5% of the rhizome.35 The turmeric rhizome also contains starch (45-55%, including the arabinogalactan ukonane A), essential oil (2.5-6%, with ar-turmerone as the main component), and small amounts of sugars, protein, vitamins (mostly vitamin C), and a resin.36
The term curcumin can be confusing because it can be used to describe a single compound or a family of compounds known as curcuminoids, of which the single compound curcumin is the most abundant. Curcumin is present at approximately 77% of the curcuminoid mixture. Curcumin is chemically known as a bis-alpha-beta-unsaturated beta-diketone [(1E,6E)-1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione], a linear diarylheptanoid compound with two oxy-substituted aryl moieties linked together through a seven-carbon chain. The second and third most abundant curcuminoid analogs are demethoxycurcumin at roughly 17% of the total, and bisdemethoxycurcumin, which is present at about 3%.3 The chemical structures of key curcuminoids are shown in Figure 1.
When possible, the terms curcumin and curcuminoid mixture are used in this publication to differentiate the pure compound from the mixture. However, due to the ambiguity of the use of the terms in the literature, this distinction is not always clear.
Pure curcumin (supplied by Sigma-Aldrich Co. LLC; St. Louis, Missouri) is insoluble in water and ether, but is soluble in ethanol, methanol, dimethyl sulfoxide (DMSO), and acetone. Experiments into the degradation of curcumin revealed that, when placed in a phosphate buffer (0.1 M, pH 7.2 at 37°C), it decomposed by approximately 90% within 30 minutes. Curcumin was found to be much more stable in human blood, in which less than 20% decomposed after one hour, and roughly 50% decomposed after eight hours. The major (nonenzymatic) degradation product of curcumin is a bicyclopentadione derivative, trans-6-(4’-hydroxy-3’-methoxyphenyl)-2,4-dioxo-5-hexenal.37,38 Minor degradation products have been identified as vanillin, ferulic acid, and feruloyl methane.
Figure 1. Chemical Structures of Curcuminoids
Curcumin: R1 = R2 = OCH3
Demethoxycurcumin: R1 = H, R2 = OCH3
Bisdemethoxycurcumin: R1 = R2 = H
In humans, curcumin is metabolized to glucuronide and sulfate conjugates, which can be measured in plasma samples.39 Reduction of curcumin to dihydrocurcumin and ultimately to tetrahydrocurcumin (THC; not to be confused with the “THC” that refers to tetrahydrocannabinol, found in Cannabis sativa, Cannabaceae) by microorganisms in the human gut also has been reported.40
Essential oil is present in turmeric rhizomes at 2-7%.41 The concentration of ar-turmerone depends on the turmeric cultivar. Analysis of an oil obtained from Kancor Ingredients Ltd. (Angamaly, Kerala, India) revealed it was composed of ar-turmerone (62%), curlone (12%), and ar-curcumene (6%), in addition to other monoterpenes and sesquiterpenes.41 Another study reported that alpha- and ar-turmerones were present in turmeric rhizome in yields of 0.03% and 0.027% (by weight), respectively.42 [Note: As noted above in the Description section, while there is no essential oil in the curcuminoid fraction of turmeric rhizome, and thus no essential oil in most turmeric extracts standardized to a relatively high level of curcuminoids, the BCM-95 extract is blended with turmeric essential oil specifically to enhance absorption and to provide the separate and claimed synergistic activity of the ar-turmerone in the essential oil, according to the US marketer of BCM-95 (C. Myers [EuroPharma] personal communication to M. Blumenthal, February 16, 2016).]