Unexpected health benefit of brewing tea

Research out of Northwestern University has revealed an unexpected benefit of drinking tea: when brewed, tea naturally adsorbs heavy metals like lead and cadmium, effectively filtering dangerous contaminants out of drinks.

This is because heavy metal ions stick to, or adsorb to, the surface of the tea leaves, where they stay trapped.

The findings have been published in ACS Food Science & Technology.

“We’re not suggesting that everyone starts using tea leaves as a water filter,” said Northwestern’s Vinayak P Dravid, the study’s senior author. “In fact, we often utilise model experiments and tweak diverse parameters to probe and understand the scientific principles and phenomena involved in capture/release cycles of contaminants.

“For this study, our goal was to measure tea’s ability to adsorb heavy metals. By quantifying this effect, our work highlights the unrecognised potential for tea consumption to passively contribute to reduced heavy metal exposure in populations worldwide.”

Benjamin Shindel, the study’s first author, said there wasn’t necessarily anything uniquely remarkable about tea leaves as a material.

“They have a high active surface area, which is a useful property for an adsorbent material and what makes tea leaves good at releasing flavour chemicals rapidly into your water,” he said.

“But what is special is that tea happens to be the most consumed beverage in the world. You could crush up all kinds of materials to get a similar metal-remediating effect, but that wouldn’t necessarily be practical. With tea, people don’t need to do anything extra. Just put the leaves in your water and steep them, and they naturally remove metals.”

Scanning electron microscope image of black tea leaves, magnified by 300 times. Black tea, which is wilted and fully oxidised, exhibits a wrinkled and surface, potentially increasing the available surface area for adsorption. Image credit: Vinayak P Dravid Group/Northwestern University.

To conduct the study, the Northwestern team explored how different types of tea, tea bags and brewing methods affect heavy metal adsorption. The various varieties tested included ‘true’ teas such as black, green, oolong and white, as well as chamomile and rooibos teas. They also examined the differences between loose-leaf and commercially bagged tea.

Cellulose bags are best

As well as testing the tea, the scientists also tested the effectiveness of the packaging. After multiple experiments testing different types of bags without tea inside, Dravid, Shindel and their team determined that cotton and nylon bags only absorbed trivial amounts of the contaminants. Cellulose bags, however, worked incredibly well.

“The cotton and nylon bags remove practically no heavy metals from water,” Shindel said. “Nylon tea bags are already problematic because they release microplastics, but the majority of tea bags used today are made from natural materials, such as cellulose. These may release microparticles of cellulose, but that’s just fibre, which our body can handle.”

Steeping time is significant

When comparing different varieties of tea, the researchers found that tea type and grind played minor roles in adsorbing contaminants. Finely ground tea leaves, particularly black tea leaves, absorbed slightly more metal ions than whole leaves.

“When tea leaves are processed into black tea, they wrinkle and their pores open,” Shindel explained. “Those wrinkles and pores add more surface area. Grinding up the leaves also increases surface area, providing even more capacity for binding.”

It was steeping time that played the most significant role in tea leaves’ ability to adsorb metal ions; the longer the steeping time, the more contaminants were adsorbed.

“Any tea that steeps for longer or has higher surface area will effectively remediate more heavy metals,” Shindel said. “Some people brew their tea for a matter of seconds, and they are not going to get a lot of remediation. But brewing tea for longer periods or even overnight — like iced tea — will recover most of the metal or maybe even close to all of the metal in the water.”

Real-world benefits

From their experiments, the researchers estimate that tea preparation can remediate about 15% of lead from drinking water, even up to lead concentrations as high as 10 parts per million. This estimate applies only to a ‘typical’ cup of tea, which includes one mug of water and one bag of tea, brewed for three to five minutes. Changing the parameters remediates different levels of lead. Steeping for longer than five minutes, for example, adsorbs more lead compared to the average steeping time.

While brewing tea will not solve the problem in the event of a water crisis, Shindel said the study’s results provide useful new information that could be applied to public health research.

“Across a population, if people drink an extra cup of tea per day, maybe over time we’d see declines in illnesses that are closely correlated with exposure to heavy metals,” he said. “Or it could help explain why populations that drink more tea may have lower incidence rates of heart disease and stroke than populations that have lower tea consumption.”

Top image credit: iStock.com/Ildar Imashev