Friday February 12 2010
The research did not test the medicinal properties of rhubarb
The Daily Telegraph today said that rhubarb crumble is the “new cancer-busting superfood”.
This news story was based on research to determine how cooking rhubarb would affect the amount of antioxidant chemicals it contains. Some scientists believe that antioxidants offer protection from diseases such as cancer, although it should be noted that this research did not directly look at any aspect of human health. It will take further research to assess how cooking affects the breakdown of these antioxidant chemicals and how this may affect any health benefits from the food.
Where did the story come from?
This research was carried out by Dr Gordon McDougall and colleagues from Sheffield Hallam University and the Scottish Crop Research Institute in Dundee. The study was funded by Sheffield Hallam University’s Food Innovation Project. It was published in the peer-reviewed scientific journal Food Chemistry.
This research was inaccurately reported on by The Daily Telegraph. The published research did not investigate the effect of rhubarb extracts (or polyphenols) on cancer cells or human health in general. The study only looked at how the concentrations of these chemicals in rhubarb were affected by different cooking methods.
What kind of research was this?
This was a laboratory study looking at how different ways of cooking rhubarb affected the chemicals it contained. Some of these chemicals may be good for health, although this research did not assess if this was the case.
Certain species of the rhubarb family are reported to be rich in types of pigments called polyphenols that have antioxidant properties. The researchers say that some rhubarb species have been used in traditional Korean and Chinese medicine. They add that previous studies looked at the chemicals found in the roots of species, which are used for medicinal purposes.
This study analysed which chemicals were found in the stalk of a species of rhubarb commonly eaten in the UK, testing how different ways of cooking may alter its chemical composition.
What did the research involve?
The researchers used Crimson Crown rhubarb (Rheum rhaponticum) grown in Leeds under typical forced conditions. This particular variety was considered the premium form of rhubarb, and was selected due to its slender, straight stems and ruby red colour. The red colour comes from a type of polyphenol pigment called anthocyanin.
The rhubarb was either:
- blanched in boiling water (a common way to prepare rhubarb for freezing),
- slowly stewed at less than 80˚C,
- stewed rapidly at 100˚C,
- baked at 180˚C to mimic rhubarb being baked from raw in dishes such as pies, or
- left raw.
The researchers combined 115g of rhubarb with 15g of sugar and 200ml of water for each of the cooked recipes, adding nothing to the rhubarb left raw. The researchers cooked the rhubarb for 2, 5, 10 or 20 minutes using each technique, and included an additional measurement at 30 minutes for the baked rhubarb. The samples were then vacuum packed and frozen at -20˚C.
The researchers used a solvent (a substance used to dissolve another substance to create a solution) to extract the chemicals from the rhubarb. The chemical constituents of the rhubarb were then separated and identified using two analysis techniques called liquid chromatography and mass spectrometry. They also tested whether each extract still had functioning antioxidants.
What were the basic results?
The researchers found that polyphenol content was generally greater in cooked rhubarb than raw rhubarb. The highest polyphenol levels were found in slow-cooked and baked rhubarb.
The researchers could identify and measure the amounts of 42 different chemicals found in their rhubarb samples. They said that this ‘chemical profile’ was different to other species of rhubarb. They also found that cooking times had different effects on different types of polyphenols. Anthocyanins made up one fifth of the total polyphenol content of raw rhubarb. In baked rhubarb, the amount of anthocyanin was increased.
The antioxidant activity of cooked rhubarb had not decreased from when the rhubarb was raw. This activity appeared to correlate with the amount of polyphenol that the researchers measured.
The researchers suggested that increases in certain types of polyphenols may have occurred due to the chemical breakdown of other polyphenols or release of these polyphenols from the stalk when the rhubarb was cooked more.
How did the researchers interpret the results?
The researchers conclude that: “the increases in antioxidant capacity after cooking are probably the result of a balance between enhanced release of polyphenols from the plant matrix and degredation of the released components”. They also say that most polyphenols will initially degrade into other polyphenol components, leading to the antioxidant content of cooked samples becoming higher than in raw samples.
This study analysed the range of different polyphenols in the species of rhubarb most commonly consumed in the UK and how these chemicals were affected by different cooking methods. The results suggest that levels of some polyphenols are increased with increased cooking times, and that the antioxidant activity of rhubarb is not decreased by cooking.
However, the study is limited by the fact that the researchers did not publish any statistical analysis of their results. This means it is not possible to say the differences they observed with different cooking times and methods did not arise by chance.
It should be noted that the researchers did not look at how the polyphenols or other chemicals found in rhubarb might affect cancer. Although extracts from members of the rhubarb family are reportedly used in traditional Chinese and Korean medicine, the researchers said that there is great variability between the chemical compositions of the different types of rhubarb.
Further research is needed to assess how the chemicals found in rhubarb are broken down by cooking and whether these chemicals have any effect on disease.