Why Does Some Tea Get Better With Age? Microbes Doing the Work

The tea industry has long used fermentation to describe the oxidation that turns green leaves brown, which happens when the leaf is bruised and its own enzymes react with air, and which is what distinguishes black tea from green. That process involves no microorganisms at all and is not fermentation in any biological sense, and the industry has been criticised for the usage for a century without changing it. Genuine microbial fermentation happens in one category of tea, in which moulds, yeasts and bacteria act on the leaf over months or years in controlled damp conditions, transforming it in ways that enzymatic browning does not.

The genuinely fermented category divides into two processes:

• •Leaves are first dried and lightly processed into a rough raw material
• •Raw ageing stores compressed cakes in humid conditions for years or decades
• •The microbial community arrives from the leaf, the air and the storage room
• •Ripened processing piles wet leaves in heaps for weeks under supervision
• •Heat and moisture in the heap accelerate the same transformation enormously
• •That accelerated method was developed in the 1970s to imitate decades of ageing

The chemistry has been studied in detail and the changes are substantial. Bitter and astringent compounds present in the young leaf are broken down or converted, which is why aged material tastes smooth where fresh material tastes harsh. New compounds form that are not present in unfermented tea, giving the characteristic earthy and woody character. The microbial community changes over the course of ageing, with different organisms dominating at different stages, and particular moulds are associated with desirable results. Caffeine content changes relatively little. Storage conditions determine the outcome as much as the starting material, so the same cake ages differently in a dry northern climate and a humid southern one.

The best known category is not the only genuinely microbial tea and the others are worth listing. Several Chinese dark teas from Hunan and Guangxi are produced by comparable pile fermentation and are compressed into bricks, historically for transport into Tibet and Mongolia where they were a staple rather than a luxury. Japanese fermented teas from a few localities use lactic acid bacteria in a process closer to pickling, producing a sour result quite unlike anything else. Fermented tea leaves are eaten rather than drunk in Myanmar, served as a salad with nuts and oil. Kombucha ferments a brewed tea rather than the leaf itself, using a culture of yeast and bacteria, and is a different thing again.

Aged tea from particular mountains and particular years commands prices comparable to fine wine, and the market has behaved accordingly. Cakes are bought, stored and traded rather than drunk, with a serious secondary market and considerable price volatility, including a spectacular collapse in 2007 after speculation drove prices to unsustainable levels. Counterfeiting is widespread, with labels, wrappers and provenance documents forged, and authentication is genuinely difficult since the product looks much like any other compressed cake. Age is claimed and rarely verifiable. Buyers who intend to drink the tea rather than trade it are advised to buy from sellers who have stored it themselves, which is the only practical check on both storage and identity.

The industry calls enzymatic browning fermentation, which involves no microorganisms, and one category of tea is genuinely fermented by moulds, yeasts and bacteria over months or years. Raw ageing takes decades while a heaped process developed in the 1970s imitates it in weeks. Bitter compounds break down and new ones form, with storage conditions mattering as much as the leaf. The market speculates, and counterfeiting is widespread.