Causes for Sour Flavors in Puerh Tea

Recently I tried a tea recommended by a friend which I found to be very sour, to the point where I didn’t want to continue drinking it. I’m not a stranger to sour tea. But of course I feel bad when I think a tea is sour because I know the reasons why. The question is whether the tea is a loss or still has hope of further transformation. I’d like to recommend reading as much as you can about fermentation in general, along with puerh fermentation articles when you come across them. Using a very informative book published last year, Modernization of Traditional Food Processes and Products, (A. McElhatton and M. El Idrissi, eds. Springer, 2016), I will attempt to apply the fermentation concepts described in this book, and explain sourness in a more approachable manner in the context of puerh storage. Hopefully then I can pick out a cause for what I taste in my friend’s tea.

Sour flavors in puerh tea are due to acids which are secretions, or by-products, of bacteria and enzymes. In any fermentation process of vegetable matter, acid formations are normal and an important stage toward getting the finished product you want. A mild sourness is indicative of a desirable stage in slow-aging sheng puerh. Acid formations provide a favorable environment for growth of fungi like Aspergillus niger and Penicillum, but also provide an unfavorable environment for putrefactive bacteria so that the product does not spoil or rot.

Fermentation, to put it simply, is a three-stage process of bacteria->to acid-> to yeast in which all three are in enough of a balance that the correct bacteria and yeast will grow at each stage. Gallic acid, among others, is the primary compound responsible for a mild sour flavor when puerh is aging well. Aspergillus fungi such as niger, foetidus and Penicillum also produce a mild gluconic acid as the result of fermenting glucose sugars. In the middle of fermentation, a mild sourness is normal.

However, other acid products are produced during fermentation as well. When acids get overly abundant, beneficial bacteria and fungi cannot continue to develop and the next stage of fermentation cannot progress. Either the tea doesn’t make it to the stages responsible for breaking down bitterness and developing flavor, or the tea ferments too fast and breaks down before the good yeasts get a chance to add flavor. So, we can end up with tea that has too many acids and didn’t properly progress to the next microbial stage. Let’s take a look at some of the possibilities for overabundance of acids.

Too High Heat and Humidity

Several organisms can grow out of balance when tea is stored or fermented in too hot conditions. The enzyme Rhyzopus is active at temps of 32-40C and is important in breaking down the starchy cell walls and pectin in the tea. This will allow for an effective release of tea juices into an infusion, and add fragrance. So this enzyme Rhyzopus is important in fermentation of puerh tea. Rhyzopus secretes fumaric acid, lactic acid and succinic acid. While breaking down the cell walls of the tea leaf is the desired function, an overabundance of Rhyzopus will break open the structure of the tea completely. In a high-heat situation with too much Rhyzopus, your tea cake will develop mushy, melded spots because the leaves are breaking down into wet globs.

Too-wet tea that arrived moldy.
The globbing of the leaves is evident near the neifei

Unfortunately, this is how poorly controlled fast fermentation with high heat can result in a sour tea product. This is true regardless of the length of time the tea is kept in a too-high heat situation. The longer the high heat continues, the more Rhyzopus breaks down the tea making backtracking impossible. The tea will taste sour because it is browned and too broken down, and the brew is muddy or dirty looking. However, if the high heat conditions were for a shorter period of time, the tea may be able to rebalance as long as it’s not broken down too much. Alas my friend’s tea suffered from too much breakdown so I don’t think it has anywhere to go from its sour state.

Too Cold and Dry

Lactic acid is an important part of fermentation because it creates positive conditions for beneficial yeasts like Aspergillus niger. Aspergillus niger is a carbon source of food for Saccharomyces yeasts which are responsible for transforming the bitterness in tea into sweet and mellow flavors. As long as the acid environment is under control, in late-stage fermentation the Saccaronmyces yeasts will get their opportunity to convert the bitterness in the tea into nuanced flavors. This is because Rhyzopus has done its job to break open the cell walls and make the plants juices available to this yeast. But when a tea is too cold and dry at the start, Rhyzopus may not sufficiently grow to break down the cells of the tea. Fermentation cannot progress and the tea appears green and young.

Both Rhyzopus and Saccharomyces need sufficient heat and humidity to grow. A too-dry and too-cool environment means the tea is stuck in a state where the earlier yeasts and bacteria produced the correct acids, but fermentation stopped there. If kept in this state for too long Aspergillus and Rhyzopus will die out, and their waste carbon products needed for Sacchronmyces food are no longer available. Thus the Sacchromyces yeasts will never get their chance to develop those lovely flavors from the bitter plant juices. In addition, our friends Sacchromyces are responsible for clarifying the juices from cloudy to clear. The tea will remain bitter and acid sour, and the brew will be cloudy. Fragrance too is gone.

This is what produces the familiar “dry storage sour,” but unlike the too-high heat situation, if the tea is not cold and dry for too long, then heat and humidity can be applied to correct the stuck tea and get fermentation moving again. When puerh collectors complain about “too dry and cold” conditions in the west, they are referring to the possibility that western collectors cannot provide enough heat and humidity. Thus their tea will remain sour and dry until it passes a point of no return when all the microbes are killed off. The tea tastes flat in addition to dry and sour.

Charred Leaves in Chaqing

Another cause of dry storage sour is char produced by the wok in processing the maocha. Burnt smoky tea is an addition to the puerh cake that is not intrinsic to it. The tea is altered by the addition of burnt carbon. Heat and humidity can speed up the usage of this carbon by yeasts, but the fragrance and flavor produced by the char may overpower the subtle flavors produced by Saccahromyces. In my experience, char is the most common cause of bad flavor in puerh tea because it’s an addition to the tea that will not go away by the normal slow fermentation process. It requires a correction of high heat and humidity for the perfect amount of time, and the perfect time is rarely achieved. 

Puerh collectors are accustomed to accepting this processing flaw as “ordinary” when in fact it is not. “Retired smoke” is not caused by improper fermentation, but a condition imposed on the tea leaf during the wok process of chaqing used to stop the oxidation of the tea leaves prior to pressing. Another cause for smokiness is when chaqing is performed in a closed, smoky room such that the odors from wood burning permeate the leaves. I feel sad when I taste this, because a fine leaf may never get a chance to be what it is meant to be. Poor leaf, well, nothing lost and nothing gained. Yet many collectors are so accustomed to char or smoky flavors that it is accepted as a normal condition. Indeed, many people have learned to like it. Once in awhile I like it too, even though I think it is a flaw.

My favorite Manzhuan tea developed some flavor from char.
Breaking up the cake into a clay jar for a few months
corrected the problem, and I no longer taste it.

Setting aside a char factor, let us return to the main causes for sour tea during storage. A very mild sourness is normal when puerh tea is young and in a fermentation state under the correct conditions. But sourness can get out of control in two ways.  One way is too much heat and humidity, causing too high levels of acid by-products that will not allow late stage fungi to develop flavor, and the tea gets too broken down to retrieve it from the acidic condition. The other way to get sour tea is by slowing fermentation via too cold and too dry conditions, leaving acid by-products unavailable for further fermentation, cutting off the process mid-stage.

My friend’s tea suffered from the overly hot and humid wet storage condition. The tea has no remaining green and is not likely to change much after some airing. You will know when that sour problem can be corrected and when you have a loss on your hands. I have had many opportunities to experience these problems and it is always a learning experience to recognize them. Tasting your tea regularly is the best way to assess its progress. As long as you maintain your tea for as much of the year as possible at 68-80F (18-26C), and RH 60-70%, you will avoid a permanent storage-related sourness assuming you purchased good quality raw tea with excellent processing at the start.