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Microbial post-fermentation in heicha - how microbes make dark tea

Most teas are made thanks to the enzymes of the leaf itself - it is they that drive the oxidation of green, oolong or black. But there is one family that goes a completely different way: heicha, that is dark tea. Here the flavour is decided not by the enzymes of the leaf but by living microorganisms - bacteria, yeasts and fungi, which after the initial processing transform the tea over weeks, months and even years. It is a true microbial fermentation, closer to cheese or a pickle than to classic tea. From it come the famous pu-erh, liu bao and fu zhuan with their deep, earthy, forest flavour. Here is a guide to post-fermentation in heicha: how it differs from oxidation, what pile fermentation is and where the mysterious golden flowers come from.

What heicha is

Heicha, literally black tea in Chinese (though in European naming it is dark tea, so as not to confuse it with our black), is a family of teas put through microbial post-fermentation. To it belong pu-erh shou, liu bao, fu zhuan and other teas of an earthy, woody, sometimes sweet profile. The common feature is that after the initial processing the leaf is subjected to the action of microorganisms, which over time transform its compounds. This sets heicha apart from all other teas: it does not end at ordinary processing but goes through an additional, living transformation. The result is flavours deep, earthy and mature, impossible to get otherwise. Understanding that heicha is a tea transformed by microbes, and not only by the enzymes of the leaf, is the starting point for the rest. We cover the kinds of tea more in types of tea.

Post-fermentation versus oxidation

The key is to tell post-fermentation from oxidation, because they are two completely different processes. Oxidation is an enzymatic reaction, driven by the enzymes of the leaf itself in contact with oxygen - it is what decides whether a tea is green, oolong or black. Post-fermentation in heicha, by contrast, is the action of living microorganisms: bacteria, yeasts and fungi, which break down and transform the compounds of the leaf. It is a true fermentation, as with cheese, wine or a pickle, and not just oxidation. Importantly, in heicha the leaf first goes through fixing, which deactivates its own enzymes, so the further transformation is already the work of the microbes, not the leaf. This is why the term fermentation is here apt in the literal sense, unlike the misleading naming of oxidation as fermentation. They are two different machineries of flavour. We cover oxidation more in tea oxidation.

Pile fermentation - wo dui

The heart of the production of modern heicha is pile fermentation, in Chinese wo dui, that is wet piling. The leaves are arranged in great heaps, moistened and kept in controlled conditions of warmth and humidity for days or weeks. In these conditions microorganisms develop, which accelerate the fermentation and transform the tea. The heap is regularly turned and controlled, so the fermentation runs evenly and does not get out of hand. It is a technique developed in the 1970s precisely to speed up maturation - earlier heicha had to rest for years. Wo dui makes it possible to get a mature, earthy profile in weeks instead of years. This is why pu-erh shou (ripe) is made fast, while sheng (raw) matures slowly, naturally. Pile fermentation is the industrial taming of microbes.

Who works: the microbes of heicha

The flavour of heicha is the responsibility of a whole community of microorganisms, and research has identified its key players. In the fermentation take part numerous kinds of fungi, yeasts and bacteria - among them Aspergillus, Eurotium, Debaryomyces, Bacillus and many others. Interestingly, different microbes dominate at different stages: Aspergillus is the main creator of flavour in the early phase of fermentation, and others, like Bacillus or Debaryomyces, take over later. Research also shows that it is the fungi that contribute more to the characteristic profile of heicha than the bacteria. This shifting, staged work of the microbes explains the complexity of the flavour of dark tea. It is not one organism but a whole ecosystem, working in a particular order. Understanding that heicha is the work of a community of microbes shows why its flavour is so deep and many-layered. It is biology, not just technique.

A table: heicha versus ordinary tea

Let us gather the differences in one place:

The table shows the heart of it: ordinary tea is made fast thanks to the enzymes of the leaf, and heicha slowly thanks to the microbes, which gives a completely different, mature profile.

Golden flowers - jin hua

One of the most famous features of some heicha is the golden flowers, in Chinese jin hua. They are small, yellow specks, which appear naturally in well-stored bricks of tea, especially in fu zhuan. It is not a fault nor mould in the bad sense, but a beneficial fungus (Eurotium cristatum), considered a sign of proper fermentation and high quality. The golden flowers develop in the right conditions of humidity and temperature during maturation, adding to the tea sweet, mushroomy, gentle notes and deepening its character. Producers deliberately aim for their appearance, because it is a mark of a successful, healthy fermentation. It is a good example of how in heicha a desired microorganism becomes a mark of quality, not of contamination. Golden flowers are the tea equivalent of the noble mould in cheese.

The flavour of post-fermentation

Post-fermentation gives heicha a characteristic, unrepeatable profile. Earthy, woody notes dominate, of forest floor, wet earth after rain, sometimes mushrooms, dried fruit, and in mature examples sweet, balsamic tones. The raw bitterness and sharpness of the young leaf give way to gentleness, smoothness and depth. It is a mature, soothing and rounded flavour, far from the fresh green or floralness of other teas. The longer and better-managed the fermentation and maturation, the deeper and more harmonious the profile. A well-fermented heicha is clean and pleasant, without an unpleasant mustiness. It is precisely this earthy, forest character that the lovers of pu-erh and liu bao have come to love. The flavour of heicha is literally the flavour of time and the work of microbes written into the leaf.

Maturing with time

Heicha is one of the few teas that deliberately mature with time, like wine. In the case of pu-erh sheng (raw) the fermentation happens slowly and naturally over years of storage, and the tea gradually transforms and mellows. Old, well-stored bricks of pu-erh reach high prices precisely thanks to decades of slow transformation. Pu-erh shou (ripe) shortens this road through pile fermentation, giving a similar effect in weeks, but it too can mature further. The key to good maturation is storage: the right humidity and temperature let the microbes work healthily, and bad conditions spoil the tea. This is why heicha is sometimes treated as collectible and investment. Time here is not an enemy but an ally of flavour. We cover pu-erh more in pu-erh sheng and shou.

How to sense it in the brew

Heicha is easy to recognise by its deep, earthy character. The brew is usually dark, from red-brown to almost black, and in flavour notes of earth, wood, forest floor, mushrooms and mature, sweet tones dominate. Unlike a fresh green or a floral oolong, heicha is gentle, smooth, soothing and free of sharp bitterness. A well-fermented one tastes clean and pleasant, not musty. It is worth brewing side by side a young green tea and a mature pu-erh shou, to feel the whole gulf created by microbial post-fermentation. Over time you will start to recognise the characteristic forest flavour of heicha already in the aroma of the dry leaf. We cover the whole process of making tea more in how tea is made.

The essentials in brief

Let us gather it up. Heicha, that is dark tea, is a family of teas put through microbial post-fermentation - transformed by living bacteria, yeasts and fungi, and not only by the enzymes of the leaf. It is a true fermentation, different from oxidation. Its heart is pile fermentation (wo dui), that is wet piling that accelerates maturation, developed in the 1970s. The flavour is the responsibility of a whole community of microbes, dominating at different stages, with fungi contributing the most. The desired fungus of golden flowers (jin hua) is a mark of quality. The result is earthy, woody, forest and mature notes, and heicha, as one of the few teas, deliberately matures with time, like pu-erh. Now you know how microbes make dark tea and where its deep, forest flavour comes from.

Note every tea in GustoNote - including the kind and the post-fermentation notes you sense. Over time you will start to recognise the earthy character of heicha and understand more deeply how microbes shape flavour.