Tuesday, September 02, 2014

Fermentation is Mold Magic

The web is full of stern admonition that boring low-grade sheng becomes boring low-grade aged pu.  But anyone that's ever tasted sauerkraut knows that boring sad limp cabbage can still transform into something wonderfully complex.  Why can't boring factory sheng transform into a mouth pleasing tongue tingling brew through the magic of fermentation?

No shortage of bloggers including myself have been disappointed with poorly aged stomach killing teas.  What could have gone wrong there?  Let us postulate ever so unscientifically as I am fond of doing in this blog.

Puerh fermentation rests on bioactivity of beneficial molds. Yes molds!

I've read in various studies that some of the following micro-critters are responsible for fermentation in puerh:
  • Aspergillus acidus - picture taken from NIH above.
  • Aspergillus fumigatus - not a friendly bugger. It's the mold commonly found in compost piles and can cause repiratory infections in individuals with immunodificiencies.  I really should think twice about huffing shu so indiscriminately.
  • Zygomycetes - is a pathogen rather than a friendly fermenter. Must read paper and update...
  • Penicillium - ditto
  • Aspergillus Luchensis - tasty koji mold

It's clear even a tiny chunk of puerh is a microbiome where various colonies of fungi eke out a balance. Just as sauerkraut fermentation can be ruined when the wrong kinds of bacteria and molds taken over, perhaps the badly aged tea suffers an imbalance of beneficial fungi. 

The paper I am hot to get my grubby paws on (Identification and quantification of fungi and mycotoxins from Pu-erh tea) mentions that 4 out of 36 puerh samples contained the dreaded mycotoxin Ochratoxin AOcrhatoxin A produced by unfriendly mold is a carcinogenic as well as a neurotoxin that may give you nightmares about pu drinking.  Actually Ochratoxin A is so prevalent in the food system that there is actually a TWI (Tolerable Weekly Intake) guideline esblished with some foods like licorice and ginger having elevated amounts. I probably ingested some today. Whew.  No need to give up my pu.

I'm so curious about the tea taste value of those 36 samples? Did those 4 teas taste differently in a negative way or were they still palatable or even interesting?   

Last weekend in the car I asked my husband, "Why do I like tea so much?" He said without missing a beat, "What's there not to like."  

UPDATE 9/4 Mathias has most generously shared the mycotoxin paper mentioned. My walnut is exploding with so much excitement.  Dear reader, I will take this weekend to digest this paper and report back to you. 


  1. The amateur mycologist in me just had a real geek out. Thank you for this awesome post!

    1. Dear Amanda,

      You are very welcome. We are at the mercy of the invisible world of microbes but we know so little about it.

      Perhaps in the future we could buy the beneficial kinds of Aspergillus to sprinkle on top on our cakes.


  2. I have heard of tea shops boiling shu, maybe that is a fail safe against customers getting sick. However, I think factories test and inject as needed to control the pile fermentation. Some of these microbes die off once the process is complete, which is what happens when sauerkraut finishes, the resulting vinegar type broth is inhospitable to bacteria. Otherwise, it is likely someone's dog poo in the pu.