Green Solvent Trick Spots Cancer-Linked Pollutants in Black Tea Drinks

Every year, laboratories around the world grind through mountains of food and drink samples, hunting for trace contaminants that could pose a risk to human health. Among the most closely watched of these unwanted guests are polycyclic aromatic hydrocarbons, a family of organic compounds formed whenever organic matter is burned or heated incompletely. Because several members of this family are classified as carcinogenic, regulators in Europe and elsewhere have set strict maximum limits for the four most concerning compounds, known collectively as PAH4. Now, a team of South Korean researchers has unveiled a faster, greener and remarkably sensitive way to check whether these pollutants have crept into one of the world’s most popular drinks: black tea.

The study, published in Food Science and Biotechnology, was led by Seo-Jeong Cho and Tae Gyu Nam of Kyonggi University, together with Kwan Joong Kim and Dae-Ok Kim of Kyung Hee University. Their goal was deceptively simple: build an analytical method that can pull vanishingly small amounts of PAH4 out of sugary, complex tea beverages without drowning the procedure in toxic organic solvents. What they produced is a textbook example of green analytical chemistry in action, combining a designer solvent with a microextraction technique and the workhorse sensitivity of high-performance liquid chromatography coupled to fluorescence detection.

To understand why this matters, it helps to know what PAH4 actually are. The quartet comprises benzo[a]anthracene, chrysene, benzo[b]fluoranthene and benzo[a]pyrene, four fused-ring molecules that the International Agency for Research on Cancer has flagged for their carcinogenic potential. They arise during processes such as smoking, grilling, roasting and drying, which means they can contaminate tea leaves during processing, particularly when leaves are dried over direct flames or in facilities where combustion fumes circulate. When those leaves are brewed into ready-to-drink beverages, any PAHs present can transfer into the liquid that consumers ultimately swallow. The European Union’s Commission Regulation 2023/915 sets maximum levels for these compounds in foods, making reliable monitoring a legal as well as a scientific necessity.

The catch for analysts is concentration. PAH4 levels in beverages, when present at all, sit in the low microgram-per-litre range, far below what a chromatograph can detect directly. Sample preparation is therefore the heart of any PAH assay, and traditional approaches lean heavily on solvents such as hexane, dichloromethane or acetonitrile, often in volumes that generate hazardous waste. Dispersive liquid–liquid microextraction, or DLLME, offered a way out. In DLLME, a few tens of microlitres of an extracting solvent are rapidly injected into the aqueous sample together with a disperser solvent, creating a cloudy emulsion of tiny droplets. Because the droplets are so small, the surface area available for extraction is enormous, and the target molecules migrate into the extractant within seconds. The extractant is then separated by centrifugation and injected straight into the instrument.

The Korean team’s innovation lay in choosing the extraction solvent itself. Instead of a classical organic solvent, they turned to a hydrophobic deep eutectic solvent, or HDES. Deep eutectic solvents are formed by mixing a hydrogen-bond donor and a hydrogen-bond acceptor in specific ratios; the resulting mixture melts at a temperature far lower than either component alone, behaving like a liquid while remaining tunable in its properties. Hydrophobic versions, typically built from combinations such as terpenes, fatty acids or quaternary ammonium salts, repel water and can therefore be used to extract non-polar analytes like PAHs from aqueous matrices. Because these solvents can be formulated from relatively benign, often bio-derived ingredients, they are widely promoted as greener substitutes for conventional extraction media, and they align with the twelve principles of green analytical chemistry that guide modern laboratory practice.

Getting the method to work in black tea beverages was not trivial. Tea drinks contain sugars, acids, colouring compounds and a host of polyphenols that can interfere with extraction or foul the chromatographic system. The researchers therefore systematically optimised every key variable: the composition of the HDES, the volume of extraction solvent, the type and volume of the disperser solvent, and the addition of salt to tune the ionic strength of the sample. Each of these parameters shifts the partitioning equilibrium between the aqueous beverage and the tiny solvent droplets, and the team’s careful optimisation allowed them to maximise recovery of the four PAHs while suppressing the matrix effects that plague food analysis. The result is a method tuned precisely to the chemistry of a commercially relevant liquid food.

The analytical performance they reported is impressive by any standard. Under optimised conditions, the method showed good linearity across the calibration range, with limits of detection between 0.02 and 0.06 micrograms per litre and limits of quantification between 0.07 and 0.17 micrograms per litre. Recoveries ranged from 98.9 to 119.8 percent, all within the acceptance criteria used in regulated food analysis, and relative standard deviations stayed between 1.4 and 7.3 percent, indicating excellent repeatability. In practical terms, this means the method can reliably detect PAH4 at levels far below the regulatory thresholds, while producing results consistent enough to support enforcement decisions. The validation follows the spirit of international harmonised guidelines for analytical procedure validation, lending the numbers additional credibility.

So what did the method find when pointed at real products? The researchers applied it to fifteen commercial black tea beverages purchased on the market, and the answer was reassuring: none of the four PAH4 compounds was detected in any sample. That is good news for tea drinkers, and it is consistent with earlier surveys of beverages and dairy products in South Korea, which generally found low or undetectable PAH levels in drinks. It also demonstrates that the method performs well in genuine commercial matrices, not just in laboratory-spiked water. A screening tool is only as valuable as its ability to deliver a trustworthy negative, and this study provides exactly that.

Beyond the immediate findings, the work fits into a broader movement reshaping food safety chemistry. Hydrophobic deep eutectic solvents have been deployed in recent years to extract everything from neonicotinoid insecticides in water, soil and egg yolk to organochlorine pesticides in apple juice, bisphenols in food samples and PAHs in hot beverages, often in combination with DLLME or related microextraction formats. Computational tools such as COSMO-RS screening are increasingly used to predict which solvent combinations will work best before a single experiment is run. The Korean study adds a carefully validated application in a high-consumption product category, and its combination of low solvent volume, low detection limits and simple operation makes it attractive for routine monitoring laboratories that process large sample loads.

For consumers, the takeaway is twofold. First, the fifteen black tea beverages tested were free of the four regulated PAHs, offering no evidence of contamination in the products examined. Second, the analytical machinery guarding the food supply keeps getting cleaner itself: methods that once required flasks of hazardous solvent now achieve better sensitivity with droplets of a designer solvent that can be formulated from benign ingredients. As regulators tighten limits on process contaminants and laboratories face pressure to reduce their environmental footprint, approaches like HDES-based DLLME are likely to spread from tea to coffee, soft drinks, edible oils and beyond. The humble cup of black tea, it turns out, has become a proving ground for the greener future of chemical analysis.

Subject of Research: Hydrophobic deep eutectic solvent-based dispersive liquid–liquid microextraction for detecting polycyclic aromatic hydrocarbons in black tea beverages

Article Title: Hydrophobic deep eutectic solvent-based dispersive liquid–liquid microextraction for the analysis of polycyclic aromatic hydrocarbons in black tea beverages

Article References: Cho, S.-J., Kim, K. J., Kim, D.-O., & Nam, T. G. (2026). Hydrophobic deep eutectic solvent-based dispersive liquid–liquid microextraction for the analysis of polycyclic aromatic hydrocarbons in black tea beverages. Food Science and Biotechnology. https://doi.org/10.1007/s10068-026-02331-5

Image Credits: AI Generated

DOI: 10.1007/s10068-026-02331-5

Keywords: polycyclic aromatic hydrocarbons, PAH4, deep eutectic solvents, dispersive liquid–liquid microextraction, black tea beverages, HPLC-fluorescence detection, green analytical chemistry, food safety, sample preparation, contaminant monitoring, Hydrophobic, deep