How to get your isoflavonoids

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  • Published: Feb 12, 2007
  • Author: Steve Down
  • Channels: Sample Preparation
thumbnail image: How to get your isoflavonoids

Flavonoid compounds are plentiful in plants, whether you consider their yield or diversity. Several thousand different structural variations of flavonoids, isoflavonoids and related compounds with the same phenyl-substituted benzopyranone structure have been isolated from a huge variety of plants. Along with the carotenes, they endow fruits, vegetables and herbs with their characteristic colouring.

However, these plant pigments have an equally attractive property in that many of them are antioxidants, protecting the human body from attack by reactive oxygen species. Their regular inclusion in the diet has also been associated with reduced levels of certain cancers, the prevention of heart disease and inflammation control. Plants high in flavonoids are used in traditional medicines and many individual flavonoids are under investigation as potential medicines.

Fruits, tea, soybean, wine and dark chocolate are some of the foodstuffs that are blessed with high levels of flavonoids. For researchers, pharmaceutical companies and herbal supplement producers, one critical step in the processing of the plant material is selection of the optimum extraction procedure. This should balance the quest for the highest possible flavonoid yields with the cost-effectiveness and green nature of the process.

A method such as solvent extraction might give a good yield but the cost of the solvent and the overhead in disposing of it safely must also be taken into account. Other, greener procedures which use reduced amounts of environmentally friendly solvents might not be so effective. In practice, it is unlikely that many organisations compare the performances of a range of different extraction procedures, preferring to use those with which they are familiar and which give a reasonable, rather than the optimum, yield.

Now, researchers from the Czech Republic have rigorously compared six extraction techniques for isoflavonoids in plants. Isoflavonoids are a flavonoid sub-group consisting of the 3-phenyl-substituted 1,4-benzopyranones, compared to the flavonoid sub-group of 2-phenyl substituted analogues. The team, led by Martin Adam from the University of Pardubice, compared the yields of the major isoflavonoids daidzein, genistein, apigenin and biochanin A from four plants.

After optimising the conditions for each technique with soybean flour, they applied the procedures to dried leaves of Matricaria recutita (chamomile), Rosmarinus officinalis (rosemary), Foeniculum vulgare (sweet fennel) and Agrimonia eupatoria (agrimony). All of the extracts were analysed by HPLC with UV detection at 260 nm to quantify the isoflavonoids.

Supercritical extraction (SFE) was conducted with carbon dioxide containing 5% methanol, with the sample placed between two layers of glass beads in the cell to prevent clogging of the restrictor valve.

For pressurised fluid extraction (PFE), the plant leaves were placed between two layers of quartz wool and acetonitrile was selected as the best of three solvents tested, the other two being methanol and acetone. Two modes of ultrasonic extraction, using an ultrasonic bath or an ultrasonic homogeniser were employed, again using acetonitrile in both cases.

For matrix solid-phase dispersion (MSPD), the better of two sorbents tested was the Oasis HLB material. Cartridges of the same material were used for solid-phase extraction (SPE).

The final method was Soxhlet extraction with methanol, although this procedure required further purification of the extract to remove interfering substances. This was achieved by SPE, Oasis HLB again giving the best recoveries from seven columns tested.

The results showed that no single method gave the maximum extraction yields for the four isoflavonoids, despite the fact that they are structurally very similar. The greatest yields for daidzein and genistein were achieved with Soxhlet extraction, followed by SFE and the ultrasonic techniques. Conversely, apigenin and biochanin A were best extracted by SFE.

When other considerations are given higher priority than the yield, SFE and ultrasonic homogenisation were quickest, PFE and SFE gave extracts that do not require subsequent filtration, whereas SFE used the least solvent.

This study will help researchers working with isoflavonoids who need to obtain their samples from plants, rather than a jar, and illustrates that there is no ideal technique for extracting all isoflavonoids from plants.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.


Lemon and tea are good sources of flavonoids

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