Reading the tea leaves: quality markers for green tea

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  • Published: Feb 12, 2016
  • Author: Ryan De Vooght-Johnson
  • Channels: Laboratory Informatics / Chemometrics & Informatics
thumbnail image: Reading the tea leaves: quality markers for green tea

Green tea: beyond the hype

Tea is the most widely consumed beverage after water and has a special place in cultures worldwide. In recent years, green tea has grown in popularity, especially in the West. Green tea is regularly praised as a ‘superfood’, with studies showing benefits for everything from weight loss and allergies to cancer and heart disease.

Tea is the most widely consumed beverage after water and has a special place in cultures worldwide. In recent years, green tea has grown in popularity, especially in the West. Green tea is regularly praised as a ‘superfood’, with studies showing benefits for everything from weight loss and allergies to cancer and heart disease.

The tea hails from China, where it is thought to have been first brewed in 2737 BC. Today it is grown across Asia and has become a popular drink across the globe thanks to its well-reported health effects. It is produced from the leaves of the Camellia sinensis plant, which are freshly harvested and immediately steamed in order to minimise oxidation (chemical reactions that deepen the colour of tea).

There are three major chemical constituents of green tea leaves: xanthine (from which caffeine is derived), essential oils and polyphenols (known for their ability to protect cells from damage). One of the major phenols found in green tea is catechin, a natural antioxidant. There are various types of catechin in green tea, which, as well as antioxidant properties, are reported to have anti-bacterial effects, prevent the development of allergies and even protect against heart disease. These are just a few examples of the compounds in green tea, which contains a plethora of chemicals that contribute to its beneficial health effects.

Chromatography meets chemometrics

Identifying each individual compound in the huge mixture of chemicals found in green tea requires sophisticated analytical tools. In the attempt to develop such systems, a range of solvents have been proposed for extraction of the chemicals, but their relative efficacies are not fully known. There are also problems with chromatographic methods as the presence of a large number of overlapping peaks can make it difficult to recover useful and reliable information from chromatograms.

In a study published recently in the Journal of Chemometrics, researchers describe a new strategy to identify the metabolites in green tea. The researchers used a range of solvents, to identify which is best for extraction, and chemometrics tools to solve the problems of overlapping peaks.

The researchers conducted their studies in Brazil, where two-week periods of harvesting and production begin in September and continue until April, allowing 12–14 crops of green tea per year. They took leaf samples at two times during the year, at the beginning in September (the first pruned harvest) and 18 days later (when new shoots were ready to be harvested). They did this in 2011 and 2012, collecting a total of four samples.

Four different solvents (ethanol, ethyl acetate, dichloromethane and chloroform) were used to extract the metabolites from the crushed tea leaves. Ethanol, and mixtures containing ethanol, were found to be the most effective for extracting metabolites from the tea samples, which will aid further research in this area.

Next, the extracted metabolites were analysed by high-performance liquid chromatography (HPLC)-diode array detector (DAD)-electrospray ionization mass spectrometry (ESI/MS). To resolve the problem of overlapping peaks, the researchers used chemometrics tools, including multivariate curve resolution-alternating lest squares (MCR-ALS), a useful technique for resolving and quantifying the components in complex mixtures after analysis by chromatography.

Significantly, they were able to identify most of the metabolites without the need for analytical standards. This shows MCR-ALS to be a powerful method of resolving co-eluted peaks from complex natural mixtures, more effective than traditional approaches such as univariate peak detection.

Indicators of quality

Finally, a variation of the statistical method partial least squares regression (called OSC-PLS-DA) was applied to the peak areas of the constituents, which separated samples from different harvests.

In both years, epigallocatechin and caffeine showed the biggest differences between the two harvests. The results suggest the compounds may be indicative of environmental stress, such as changes to temperature between harvests and years. The researchers say these two metabolites could serve as quality markers for green tea, as they have significant health benefits and are sensitive to external conditions.

Related Links

J. Chemometrics, 2016, 30, 75-85, Analytical investigation of secondary metabolites extracted from Camellia sinensis L. leaves using a HPLC-DAD-ESI/MS data fusion strategy and chemometric methods.

Kew gardens: Camellia sinensis

Wiki: Green tea

Article by Ryan De Vooght-Johnson

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

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