Reference point
Open a jar of chao, lift a piece onto a plate and smell from a hand’s span away.
Do this before you read on. An entry only works when you have just smelled the thing.
Salty, fatty, with a clear cheesy note and a layer of mould. Heavy, slow, and stays for a very long time.
Fermented bean curd is tofu fermented with mould, and in principle it is closer to blue cheese than to any other soy food. Both use fungi to break down protein, and both produce similar compounds.
Compared with fermented soybean paste it is fattier, with a clearer mouldy note. Both start with mould, but on different substrates and with different moulds: chao is fermented on tofu, tương on roasted soybeans with rice, and the paste then goes through a long brining stage that chao does not.
How to smell it
Lift a piece onto a plate, wait a minute, then smell from a hand’s span away. If you have blue cheese, smell it alongside — the resemblance surprises almost everyone.
Why this one is hard
Because of the urge to avoid it, and because few people think of comparing a Vietnamese food with a European one.
Don’t confuse with
This entry
Fermented bean curd
Fermented family
Both are fermented with moulds, so the difference is not a matter of fungus versus bacteria. Fermented bean curd is matured on tofu, so it is fatty with a clear mouldy note; the paste is fermented on roasted soybeans with rice and then brined for a long time, so it is drier, with a roasted note.
Scientific note
Fermented bean curd is tofu fermented by moulds and then matured in brine. The mould breaks down soy protein into amino acids and peptides, creating umami; at the same time it produces diacetyl for a buttery note and esters for a winey note. Chemically it is closer to aged cheese than to fresh tofu.
Diacetyl boils at 88 degrees, so it is very light and gone within minutes. It is marked as leading because it gives the characteristic buttery note, not because it decides how long the smell lasts.
This point is worth pausing on because it is the largest mismatch in the whole dictionary. Fermented bean curd is placed in the base stage, yet diacetyl boils at 88 degrees — lighter than any common essential-oil component and gone almost instantly.
The two do not contradict each other, and the reason is a trait shared by fermented foods: the compound you recognise is light, while the compound that stays is heavy. Diacetyl tells you it is fermented bean curd in the first second.
What stays for a long time comes from the peptides and amino acids formed as the mould breaks down soy protein — much heavier substances that hardly evaporate, most of which do not even show up on a routine volatile analysis.
So fermented bean curd has its identifying signal in the top stage and its body in the base stage. The volatility band can only draw the first half.
In Vietnamese life
Fermented bean curd is part of Vietnamese vegetarian cooking as a source of protein and depth. That it and blue cheese both use moulds to break down protein is one of the most interesting coincidences between two cuisines.