One mechanism, two outcomes
Dry tea leaf is porous, low in moisture and enormous in internal surface area — a rolled oolong pellet or a twisted sencha needle presents far more surface to the air than its size suggests. Aromatic molecules in the surrounding air are absorbed onto and into that surface, and because the leaf is dry it has capacity to spare. That is the whole mechanism. Placed beside jasmine blossoms it produces one of the great teas of southern China; placed beside a jar of cumin it produces a tea that tastes of cumin. Nothing distinguishes the two cases chemically. What distinguishes them is intent, control and whether the source of the aroma was chosen. Writers tend to present odour absorption as a storage fault and scenting as a production technique, which obscures the useful fact that they are the same event and respond to the same variables: proximity, time, temperature and how dry the leaf is. The symmetry has a useful diagnostic consequence. If a tea has acquired an unwanted smell, the question to ask is the same one a scenting workshop would ask: what was it next to, for how long, and how warm was it. The answer is almost always a specific object at a specific distance rather than a general failing of the cupboard, and identifying it stops the next tea going the same way. Odour transfer is a proximity problem, and proximity problems have addresses.
How fast it happens
Faster than most people expect and slower than a single exposure suggests. A tea sealed in a tin inside a cupboard that also holds ground coffee will usually be detectably affected within weeks rather than days, because the barrier is imperfect rather than absent. A tea in a permeable container — a paper bag, an unglazed jar, a loose-lidded caddy — sitting directly beside an open aromatic source can be affected within days. Warmth accelerates it, because it puts more of the aromatic compound into the air. Dryness increases it, because dry leaf has more free capacity. And the effect accumulates rather than plateauing quickly, which is why a tea that seemed fine at a month can be unusable at six. The practical implication is that the fix is spatial rather than temporal: nothing is gained by waiting, and the tea should be moved. Freshly opened tea is at its most vulnerable, which inverts the usual assumption that a new packet is safe. Dry leaf that has just come out of a sealed laminate pouch has taken up nothing and has its full capacity available, so the first weeks in a household cupboard are when most of the transfer happens. This is one more argument for deciding where tea will live before the first packet is opened, rather than discovering the problem in a tea that has already been ruined.
Coffee and spice, specifically
Coffee is the worst neighbour a tea has, and the reason is quantitative rather than mystical. Roasted coffee is an intensely aromatic material that actively releases volatile compounds for a long period after roasting, and ground coffee does so faster still. A tea and a coffee sharing a cupboard is a tea being scented with coffee. Ground spices are the next tier — cumin, fenugreek, cardamom, clove and cinnamon are all persistent and all common in the same cupboard as tea. After those come non-food sources that get overlooked entirely: scented candles, cleaning products, soap, scented bin liners and the cedar or camphor of certain storage furniture. A tea that has acquired a soap note is an unhappy thing to encounter and is not rare. The cheapest sound arrangement is a cupboard or a drawer that holds tea and nothing else. Two non-obvious sources round out the list. New cupboards and newly painted shelving off-gas for weeks and will scent anything porous stored in them, which catches households who have just moved or refitted a kitchen. And plastic storage boxes, particularly inexpensive ones, have a smell of their own that transfers readily to dry leaf; this is one reason the pu-erh storage community prefers food-grade boxes that have been aired thoroughly before use. Neither source is a foodstuff, which is exactly why neither gets considered.
Whether it can be undone
Largely not. Absorbed aromatics are held in the leaf and there is no domestic process that selectively removes them; airing an affected tea in an open dish reduces the most volatile fraction and leaves the rest, and it costs the tea its own aromatics at the same time. Light re-firing is the only intervention with a real record, and it belongs to roasted oolong rather than to general use: a gentle re-bake drives off moisture and some absorbed volatiles along with it, at the cost of moving the tea’s roast character. For anything delicate the honest answer is that the tea is now a different tea, and the useful decision is whether the new character is tolerable rather than how to reverse it. This asymmetry — easy to acquire, impossible to remove — is why odour deserves a place alongside oxygen and moisture rather than a footnote. There is one genuinely useful salvage, which is to change what the tea is for. A green tea that has taken on a faint cupboard note is usually still fine cold-brewed, where the low temperature collects a narrower and sweeter fraction, or iced with citrus, or used for cooking. Repurposing is not the same as repair and it is not a reason to be casual about storage, but it does mean an affected tea is rarely a total loss — and it removes the temptation to keep drinking a spoiled tea out of thrift.
Scenting done deliberately
Jasmine tea is the clearest case. Finished green tea is layered with freshly picked jasmine blossoms that open and release their aroma at night, held together for a period, then separated and the tea dried again; better grades repeat this over several successive nights with fresh flowers each time, which is what the trade means when it advertises a number of scentings. The flowers are normally removed at the end, so visible blossoms in a finished jasmine tea are often decorative rather than evidence of quality — the deliberate exception being certain Sichuan styles where the flower is intentionally left in. The same principle underlies osmanthus and chrysanthemum scenting, the rose congou style, and the pomelo- and citrus-flower scenting practised in parts of Guangdong. In every case the tea has been scented rather than flavoured, and no flavouring compound has been added to the leaf. One consequence for buyers follows directly from the mechanism. Because scenting works on dry leaf, the base tea has to be finished and stable before it is scented, and it is fired again afterwards — so a scented tea has been through more processing than an unscented one and its base has been chosen partly for its ability to survive that. This is why the base of a good jasmine tea is often a straightforward green rather than a delicate one, and why judging a scented tea by the standards of the plain tea underneath misreads what was being made.
What this implies for how a household stores tea
Three arrangements follow. Keep tea in a dedicated space, because a shared cupboard is a scenting chamber with an uncontrolled source. Keep categories apart within that space, since a heavily roasted oolong, a smoked souchong or a ventilated pu-erh cake will each contribute to the air around it — and a ventilated cake is by definition not sealed. And do not reuse a container that held a strongly aromatic tea for a delicate one; a tin that held Lapsang Souchong is a Lapsang tin permanently, and the correct response is to keep using it for smoked tea. None of this requires equipment. It requires deciding once where tea lives and not putting anything else there. A fourth arrangement is worth adding for anyone who stores dark tea alongside anything else. A cake being aged is ventilated by design, so it is continuously contributing its character to the air around it and continuously receiving whatever else is there. That makes it both the most vulnerable item in a mixed cupboard and the strongest source in it. Collectors who keep both halves of the rule in one house generally give dark tea its own enclosure entirely, which solves the ventilation requirement and the separation requirement in a single decision.