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"Green Tea Polyphenols: comparison of microwave-assisted- vs. conventional water extraction".

Green tea (Camellia sinensis L.) is a rich source of antioxidants. Tea is rich in polyphenols, flavones, caffeine and phenolic acids. Predominant tea antioxidants are the catechins: e.g. epicatechin (EC), epigallocatechin (EGC), gallocatechin (GC), epigallocatechin-3-O-gallate (EGCG). Pouring boiling water on the tea and steeping it is the conventional extraction of tea.

Based on previous research, Nkhili et al. (2009) studied the effect of microwave-assisted (MWE) vs. conventional hot water extraction (CWE) on the amount tea antioxidants released by either extraction. In the conventional method, the authors infused 6.0 g tea in 120 ml water at 80 or 100ºC for 60 minutes. In the microwave-assisted tea extraction irradiation power was set at 600 W and 6.0 g tea was infused in 120 ml at 80 or 100ºC for 60 minutes. Nikhili et al (2009) sampled at intervals from 5 min to 60 minutes to determine the optimum extraction time for green tea antioxidants. Data was collected on polyphenols by injecting aliquots on a HPLC apparatus. Total phenolic content was determined by spectrophotometer.

The authors found total phenolic content in green tea to peak at 100 ºC at 20 min for the MWE (111.28 mg gallic acid equivalent (GAE)). The total phenolic content in CWE peaked at 100 ºC at 40 min duration (102.2 mg GAE). At 80ºC both MWE and CWE took more time (30 and 45 min) to peak and this temperature resulted in less total phenolic acid (84.1 and 87.09 mg GAE respectively).

Nkhili et al (2009) HPLC analysis of the samples showed EGCG as the most prevalent phenolic acid in green tea. The MWE method at 80ºC for 30 min resulted in 77.14 mg EGCG, while CWE at 80 ºC for 45 min yielded 64.18 mg. Both extraction methods at 100ºC yielded less EGCG and EGC compared to the 80ºC temperature. Furthermore, antioxidant activity as measured by inhibition of peroxidation of linoleic acid was highest in tea extracted at 80ºC with the MWE procedure.

Nkhili et al (2009) conclude that microwave-assisted extraction (MWE) of tea at 80 ºC is quite feasible since this temperature results in the highest concentration of EGC and EGCG. The MWE method consumed 0.25kW vs. 0.75kW for the CWE to obtain the highest antioxidant extraction in green tea. Thus authors suggest that MWE can result in energy savings and a reduced environmental impact (less CO2 production) compared to the conventional water extraction method.

Reference:


Nkhili, E. V. Tomao, H. El Hajji, E-S El Boustani, F. Chemat, and O. Dangles. 2009. Microwave-assisted water extraction of green tea polyphenols. Phytochem. Anal. 20:408-415.

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