The green chemistry of tea

And coffee, if you're into that

I confess to being a bit…selective when it comes to a proper cuppa. I have Opinions About Tea, to the point of choosing tea by region of origin—Keemun is one of my favourites. You won’t find Yorkshire Tea or PG Tips in my house, though I do keep a box of Twinings English Breakfast bags next to my loose-leaf strainers, as a concession to my friend who responds to the question “what kind of tea?” with “just regular tea, like normal people drink.” And I flatly refuse to order green tea at any cafe, as brewing it with boiling water ruins the flavour (if you’re curious, it should be brewed at 65-80°C).

So when I decided to stop drinking caffeine this year, it was a major blow to my delicate sensibilities. Herbal teas—which technically aren’t teas, as they’re not made from the tea plant—are often not to my taste, as people will insist on adding chamomile or licorice to them. I’ve slowly built up a collection of nice herbal options, but I still find myself craving a black tea to dip a biscuit in, or a soothing green tea while I work.

Luckily, decaffeinated tea (and coffee) is a thing you can buy. Unluckily, as a solvent chemist, I know how decaf tea is made, why it never tastes quite as good, and the sustainability issues with most decaffeination processes.

Decaffeination is, at its core, the attempt to remove caffeine molecules from tea while leaving all of the interesting flavour molecules behind. The technology has evolved over time, and there are now three main processes used in industry:

Methylene chloride decaffeination

Methylene chloride, or dichloromethane (DCM) is the traditional decaffeination solvent, and still used for many decaf products. The tea leaves or coffee beans are soaked in DCM, which extracts the caffeine—and some other molecules. The DCM is then removed via evaporation, leaving behind only trace amounts.

DCM is a hazardous solvent that can cause cancer, and has been much in the media in the last decade for causing worker deaths in certain uses, like paint stripping in poorly ventilated spaces. Realistically, the trace amounts left in the beverage are probably not enough to harm consumers, but workers in the decaffeination plant can be exposed to it at higher levels. And PR-wise, offering tea with a carcinogen in it (even a little bit of a carcinogen) is not great. It’s also derived from fossil fuels and harms the ozone layer, so all around not good from a green chemistry perspective.

Still, DCM is cheap and very effective in decaffeination, so it continues to be used.

Ethyl acetate decaffeination

Ethyl acetate is a drop-in replacement for DCM—that is, it can be swapped in easily without changing any of the processing equipment. That makes it a popular replacement for companies that want to go greener. It’s a bit less selective for caffeine than DCM, and a bit fruitier-smelling, so the tea ends up with a different flavour profile.

Ethyl acetate is found in nature, so you’ll often see teas decaffeinated with it labeled as “naturally decaffeinated”. However, cheap ethyl acetate is usually made from fossil fuels, and there’s nothing natural about the decaffeination process itself, so the label is somewhat misleading. Does wonders for marketing, though.

Replacing DCM with ethyl acetate is a simple move towards green chemistry, as it’s much safer in most respects, and can even be made from renewable sources (theoretically). However, ethyl acetate is flammable, while DCM is not, so there is a trade-off in terms of process safety.

Supercritical carbon dioxide (scCO2) decaffeination

scCO2 has been quite trendy in the academic solvents world for a few years now. Basically, CO2 gas is compressed until it becomes a fluid, and the temperature/pressure conditions can be tweaked to make it especially good at dissolving caffeine.

This is definitely not a drop-in replacement; adopting scCO2 extraction means replacing your standard vats with a system of high-pressure tanks (read: significant capital investment, and workforce re-training). But it can produce a better result in terms of flavour, and definitely leaves no trace of hazardous or fruity chemical behind.

No matter which process you use, the flavour of decaf tea (and coffee) will be different. There are so many different chemical compounds involved, and removing just caffeine without touching anything else is currently impossible.

Still, it’s an interesting case study in gradual improvement with green chemistry. There’s a drop-in replacement option, a capital-intensive option, and new technologies in development, like absorption with various materials. Each option has trade-offs, and which one is the right choice will depend on the business. How picky is your market about price vs. flavour? How much capital do you have on hand? Are you decaffeinating your tea yourself, or can you just switch to a supplier with a different technology?

All of these technologies are currently used in the UK market, which goes to show that there’s no perfect answer. And from a sustainability perspective, I have yet to see an LCA comparing the three options—please share if you know of one! Without that, it’s hard to say which one is actually the greenest option.