Despite my passion for history, anthropology and philosophy, my training background in daily life has always concerned scientific world; in particular agronomic and food technology studies have led me to be fascinated by microbiology of tea, which in the western world, however, is a subject with which we find ourselves dealing too little often.
I will not investigate the oxidative and fermentative processes, which will be the subject of other posts, but I will speak in particular of a fungus, an extraordinary being, what I would describe as the microbiological hero of Fuzhuan. Eurotium cristatum is a very vigorous ascomycota, typical of the Hunan region, able to survive at relatively high temperatures and in situations of high osmolarity. The origin of Fuzhuan brick tea is still very controversial today, but it can probably be traced back to the Ming dynasty. Already in ancient times the jin hua, the golden flowers due to the populations of E. cristatum, were used as a quality standard for heicha bricks.
This “flowering”, the Fa Hua, unavailable elsewhere, is a process due to the uniqueness of the microclimate of Hunan, and owes its color to the golden-yellow hue of the ascocarp of the fungus. But how does this condition arise, and how does it affect the taste of the tea?
The creation of Fucha is one of the most complicated production processes among dark teas, which involves three particular fermentation stages that differentiate it from the others. Once the rolling phase (sometimes 2 rolling phases) of the leaves has been completed, which favors the rupture of the cell membranes favoring the biological processes, the leaves are processed at a temperature between 100-200°C (depending on whether it is pile steaming or other methods), creating a selective condition (about 90% of fungi and bacteria are killed).
Follows a piling phase, a pre-fermentation phase in which the leaves are placed in piles and the temperature is lowered below 60°C and moisture content of 65% to 68% to favor microbial proliferation, creating an initial selection for some species of microorganisms.
The loose leaf tea is therefore left as raw material stage, the pile is turned at certain intervals. The reactions will take place and will develop the scent of the tea, such as various types of degradation of amino acids or hydrocarbons, Strecker degradation, enzyme-mediated oxidation and hydrolysis, as well as non-enzymatic degradation and oxidation. The pile fermented leaves were then dried; in old days the drying process was done in an oven with pinewood at around 70°C. Raw Fucha is now ready.
All this will end with the subsequent steam heating of the leaves at around 80°C and piled up again throughout the night, to make them suitable for pressing and further determine a selective environmental condition. At this point the pressing in bricks will take place once the temperature has reached 50°C and the bricks stored so that the third phase can begin, the flowering phase, known as fahua, under controlled moisture and temperature for at least 15–25 days, which marks the post-fermentation events.
We always keep in mind that these procedures, times and temperatures can vary according to the recipe and the various traditional methods.
Phases of a typical production process: (1) Fresh tea leaves, (2) Shaqing, (3) First rolling, (4) Pile fermentation, (5) Possible Second rolling, (6) Drying, Raw Fucha is ready, (7) Screening and sorting, (8) Blending, (9) Pile steaming, (10) Second short pile fermentation, (11) Pressing, (12) Fa Hua, Microbial fermentation, (13) Packaging and storage
E. cristatum together with other microbial agents acts on tea leaf molecules thanks to the activity of extracellular enzymes produced by fungal fermentation, such as α-amylases, polyphenol oxidase, cellulase, pectinase, xylanase, proteases and hemicellulase oxidase, contributing to the change of volatile substances and bioactive components during the fermentation process.
The dominant genres of microorganisms identified were Eurotium, Debaryomyces and Aspergillus, and three genres of bacteria such as Klebsiella, Lactococcus and Bacillus. Although all microorganisms contribute to the flavor profile, Eurotium cristatum was found to be deeply characterizing.
The degradation of proteins and polyphenols, as well as the production of metabolites, carbonaceous and nitrogenous compounds due to the action of E. cristatum and associated microorganisms lead to the formation of the distinguishable fungal and woody aroma of Fuzhuan tea, of stale, mentholated and undergrowth perception.
Furthermore, the traditional brick form must not be understood as a casual or merely hedonistic and cultural process, it has in fact emerged that the differences in the structure of the bacterial community between the different types of Fucha can be in part mainly attributed to the different degree of compression of the material.
Notable differences emerged in the microbial community in its taxonomic diversity and population size, in loose, unpressed, lightly pressed, hand-pressed, or machine-pressed Fuzhuan tea leaves. They all influenced the fermentative development in their own way, creating different tissue damage, thus modifying the release of leaf cell contents. This in turn plays an important role in influencing the subsequent fermentation stage.
The results consistently indicated that machine-pressed Fuzhuan tea contained the highest abundance and diversity of microorganisms.
The excessive mechanical damage caused by the strong pressing causes a greater degree of rupture of the membranes which is followed by a considerable release of the intracellular contents, as well as affected the humidity and air circulation inside the brick, significantly promoting the growth and the development of colonies of microorganisms.
The result is that the endogenous enzymatic and non-enzymatic action of the leaves, and to a greater extent the exogenous one of the microorganisms, leads to polyphenolic oxidation with a consequent greater production of thearubigins which characterizes the dark color of this heicha; the reactions involving polyphenols, carbohydrates and amino acids affect the silky texture and volatile compounds, as well as the oxidation of fatty acids.
The metabolic activity of eurotium cristatum and of the other microorganisms attenuates the green, herbaceous sensations, being replaced by floral, woody, stale ones towards the last part of fermentation, due to molecules such as methyl salicylate, derivatives of hexanol and of trimethoxybenzene. Furthermore, the gustatory characteristics are also changed.
Cathechins like EGCG, GCG, EGC, CG are positively correlated to the sensation of astringency, acidity and bitterness. It has been seen that during the fermentation process these substances tend to decrease proportionally to the microbial development and its metabolic activity, it can therefore be deduced that the latter is related to greater softness, less astringency and a higher sweetness with the continuation of the tea aging.
We have reached the end of this scientific journey into the world of this little golden wonder.
I remain available for further curiosities or material in case you want to learn more about the topic.