The findings suggest that the shelf life of black cohosh may be a lot longer than that currently included on supplement labeling.
Further, testing other herbal samples collected in the late 1800s and early 1900s could improve current knowledge on the stability of herbal supplements, says the research team from the New York Botanical Garden, Lehman College, City University of New York and Columbia University.
A decline in the growth of herbal supplement sales in recent years has been attributed to the products' loss of credibility, partially based on the ingredient content and partially on effectiveness.
"Both parameters are dependent on the stability of the active compounds," Dr Joerg Gruenwald from Phytopharm Consulting told NutraIngredients.com.
The US researchers studied pieces of powdered black cohosh root that were collected 85 years ago by the famous plant explorer and physician Henry Hurd Rusby. The specimen had been exhibited at the New York Botanical Garden until the 1930s before being removed to storage.
The sample was tested for its triterpene glycosidic and phenolic constituents - naturally occurring compounds thought to be responsible for the root's medicinal activity - using sophisticated laboratory analysis (high-performance liquid chromatography).
A comparison of the chemical constituents of the 85-year-old plant sample with those of a recently collected sample showed similar profiles, write the researchers in the 15 January issue of the Journal of Ethnopharmacology (96(3):521-528).
Both plant samples have similar amounts of the four major triterpene glycosides, although the total amount of the six major phenolic constituents measured in the 85-year-old plant material is lower than the amount measured in the modern plant material. This difference may be due to the natural chemical variation of these compounds in plants from different geographical areas and/or from different harvest seasons, or there may have been degradation over time, said the authors.
Antioxidant activity, tested in methanol extracts of the two samples, was also similar.
Michael J. Balick, director and philecology curator at the Institute for Economic Botany at botanical garden, pointed out that the compounds survived despite the fact that they were exposed to light, humidity and less than optimal storage conditions for decades.
"While many herb companies normally put a two to three year expiration date on their herbal supplements, this research suggests that some supplement ingredients might be active for many, many years beyond their expected shelf life," he said.
"It is time to use this same technique to look at other herbal samples that were collected in the late 1800s and early 1900s that still exist in The New York Botanical Garden's collections, and elsewhere, to investigate the stability of plant chemicals present in herbal medicines."
Mark Blumenthal, founder and executive director of the non-profit American Botanical Council, said the research supports what many scientists have known for a long time - that some herbs have biologically active compounds that are stable for many years.
"We have known this for the ginsenosides in Asian ginseng root and other compounds in other plants. Now we have confirmation that some of the key compounds in black cohosh do not break down over time."
He added that the results of the study are relevant to the rules for good manufacturing practices (GMPs) in supplements being finalized by the US Food and Drug Administration. The new regulations deal with expiration dates for herbal dietary supplement labels and how manufacturers determine the shelf life of these products.
However Professor Edzard Ernst, a herbal medicines expert, based at the University of Exeter's Peninsula Medical School, said that while the findings were not surprising, they were unlikely to be similar for other herbals.
"I don't think this particular publication would lead me to believe that shelflife for other samples would be as long as this," he told NutraIngredients.com.
"Perhaps it will have implications for black cohosh. But if all this information on conditions of storage is true, we have one example of a very time-resistant herbal. I don't think generalization across herbal remedies is a good thing."
He added that the issue of stability of herbal ingredients has been discussed for many years but testing century-old plant samples is unlikely to offer new support for herbal stability.
"It is an intriguing and interesting find, but not what I would call systematic research."
Black cohosh ranked eighth of all herbal supplements sold in mainstream retail outlets in the US during 2004, according to Information Resources in Chicago. It is also one of the leading supplements in the menopause category in Europe.
A number of clinical trials on black cohosh preparations suggest that they could reduce hot flushes, perspiration and mood swings.