Saw palmetto, or Serena repens, is a palm indigenous to the southeastern US – Alabama, Florida, Georgia, Louisiana, Mississippi and South Carolina.
The fruits are eaten by an assortment of wild animals, livestock and people. When labelled as saw palmetto, Serenoa repens can be legally sold in the US as a herbal dietary supplement. The fruits are most frequently taken to ameliorate benign prostate hyperplasia. Saw palmetto oil inhibits the conversion of testosterone to dihydrotestosterone by 5a-reductases. Benign prostate hyperplasia is associated with elevated concentrations of dihydrotestosterone. Clinical studies report few adverse events from Serenoa repens consumption, which are mostly mild.
It has been reported that in two out of six tested commercially available samples of saw palmetto extract, the level of total fatty acids was found to be less than 20%, which clearly indicates product adulteration. It is obvious that the nutraceutical industry (especially the manufacturers of finished supplement forms) needs to have an ultimate strategy and supporting analytical methods to clearly identify adulterated (or counterfeit) saw palmetto extracts.
Identification methods
The commonly used identification methods include the ratio of the content of fatty acids, HPTLC and DNA.
■ The ratio of the concentration of lauric acid to the concentration of the respective fatty acids must be in the specified range. The ratio can even distinguish the saw palmetto oil prepared by solvent hexane or supercritical extraction with carbon dioxide.
■ A direct identification result can be achieved by comparing the sequence of the zones present in the HPTLC chromatograms obtained with the reference solution and the test solution. The essence of this method is analysing fatty acids and esters in the saw palmetto oil qualitatively. The advantage is that it can directly observe and compare the differences among several samples in a short time. Meanwhile, the disadvantage is that it is easy to be disturbed by human factors. For example, the spots that are unrecognisable to the naked eye may lead to incorrect identification conclusions. More often than not, the speckle of the reference substance extracted by solvent may be different with the product extracted by supercritical fluid extraction.
■ The protein coding plastid genes matK and rbcL have been sanctioned by the Consortium for the Barcode of Life (CBOL) for use in plant DNA barcoding. Polymerase chain reaction (PCR) amplifies a specific region of a DNA target for the extracted sample. Then, the DNA is sequenced by using a nextgeneration sequencer. The sequences are compared with reference DNA sequence database. However, the DNA can sometimes be undetectable due to degradation or processing.
Nuclear magnetic resonance
Nuclear magnetic resonance (NMR) spectroscopy has long been used in structural analysis and identification of organic compounds. PRISPEX SL used NMR to try to distinguish the saw palmetto oil and other cheap oils according to the special fatty acid composition. The test was carried out using a BRUKER AVANCE III 400MHz on a 9.0mg/batch sample and CDCl3 as a solvent. Scanning times were 16 ppm for the hydrogen spectrum and 1,024 for the carbon spectrum, with a spectral width of 20 ppm, 250 ppm, and a 1s pulse delay.
Review of the results
Taking a look at Figure 1 and 2, there are a number of points worth noting.
■ The characteristic peaks of the saw palmetto oil suspected at 65-68 ppm in the carbon spectrum of Figure 1 (red area).
■ It shows that the coconut oil has the same characteristic peaks with the saw palmetto oil at 7.55 and 7.75 ppm in the hydrogen spectrum of Figure 2 (orange area), which should be prevented from being misjudged in identification.
■ It can be seen that these spectras have more than 80% similarity. This is because the basic chemical constituents of vegetable oils are all advanced straight-chain fatty acids and glycerides.
■ Previous studies have shown that saw palmetto oil contains more free fatty acids and higher acid value compared with common plant oils. However, it is believed that immature fruits contain only 50% free fatty acids, while ripening fruits contain more than 90%. As a result, it is more difficult to choose the methods of identification.
The above research shows that several methods are not entirely useful for saw palmetto oil authentication. At present, the most practical identification method of the saw palmetto oil should be based on GC and HPTLC, which are in compliance with USP and EP.
PRISPEX SL aims to collect more vegetable oil samples, research the characteristic peaks to determine the compositions and source, eliminate the differences of harvesting time and processing of raw materials, and establish a database. In doing so, its actions will result in a method of NMR identification that will be more powerful and direct.
