It is well known that green tea has many medicinal properties — antibacterial activity, protective effect against many types of cancer, anti-diabetic and anti-inflammatory properties, to name a few. But all these benefits have been based on studying the infusion that is got when tea bags are dipped into hot water for about 3-4 minutes to allow the active components to seep into the water.
But a study published in January 2016 in the journal Scientific Reports has gone beyond studying the infusion and found the answers to what gives the green tea its medicinal properties. The researchers looked at the tiny particles suspended in the tea infusion to understand if these particles played a positive or negative role in the well-established bioactivity of green tea.
“After seeping a bag of green tea we can always find fine particles suspended in the tea infusion. These fine particles are of three different sizes — macro, micro and nano. This raised my curiosity to investigate the role of these particles in the green tea effect,” says Judy Gopal, one of the lead investigators of the paper in an email to me. Dr. Judy is a Chennai-born and educated researcher now working as an Assistant Professor at the Department of Bioresource and Food Science, Konkuk University, Seoul, South Korea.
Dr. Judy and Dr. Manikandan Muthu, the lead investigators of the team, studied the suspended particles by assessing their antibacterial activity against oral microflora. The researchers chose oral microflora to test the effectiveness of the suspended particles as oral microflora are the first to come in contact with green tea in the mouth. Also, green tea is known to prevent dental caries.
The antibacterial activity seen in green tea comes from catechins, phenolics, and flavonoids, with catechins playing a predominant role compared with the other two. “This is the reason why green tea has more antibacterial activity than black tea. Both teas have phenolics and flavonoids, but catechins are unique to green tea. This additionally suggests that catechins play a dominating role compared with phenolics and flavonoids,” says Dr. Judy.
Dr. Judy and her team studied the effect of all three sizes of suspended particles for their antibacterial properties. The researchers found that the higher the epigallocatechin gallate or EGCG (the major bioactive ingredient in green tea) content the better the antibacterial effect against the oral microflora.
Compared with macro and micro suspended particles, the nano particles had “significant antibacterial activity against S. mutans and the human dental bacterial samples”. The macro particles in the infusion did not have any distinct antibacterial property or EGCG conserved in them. When the researchers removed the macro particles from the infusion, there was no change in the bioactivity of the extract.
Likewise, the micro particles too did not appear to contribute much towards antimicrobial activity. Though the bioactive components and EGCG contents were relatively higher than the macro particles, the removal of micro particles from the infusion did not affect the antimicrobial properties significantly.
However, the nano-sized particles were found to be “packed with bioactive components, namely flavonoids, total phenols, catechins, EGCG and exemplified enhanced antioxidation and antimicrobial activity compared with its counterparts,” they write. “These results clearly indicated that the bioactivity of green tea was not just a green tea extract-based phenomenon, but a synergistic phenomenon of the EGCG in the extract together with the EGCG packed near nano to nano-sized green tea particles in the extract.”
Explaining why the nano-sized particles have greater antimicrobial activity, Dr. Judy says the higher surface area apparently provides greater surface attachment sites for the EGCG-catechin moieties. “This has not been directly observed so far, but nano-sized particles encasing more EGCG has been confirmed by analysis,” she says.
Though the nano particles are way less than macro particles in terms of weight, they are undoubtedly more in terms of numbers. She does not feel that increasing the amount of nano particles in green tea would lead to any major improvement in antibacterial properties.
“When we started working on this project we thought the nano particles in the extract will deter the activity of the extract and, hence, we will make a recommendation to the tea bag manufacturers to pack the tea leaves in bags with finer meshes to avoid the release of these particles into the extract. But, the results we got were diametrically opposite. The nano particles have a huge role to play and ought to be retained,” she says.
Science Chronicle 