Reference point
Lift the lid of a crock of mustard greens that have soured fully and smell what rises.
Do this before you read on. An entry only works when you have just smelled the thing.
Sour, salty, with a light sulphur note and a layer of the vegetable still there. Sharp, and it rises as soon as the lid comes off.
This is lactic fermentation, quite different from the alcoholic fermentation of fermented sticky rice and the protein fermentation of fish sauce. Lactic bacteria produce acid, not alcohol, so the smell leans sour rather than yeasty.
Freshly soured, fully soured and over-fermented pickles are three stages with three different smells. Cooks tell them apart by nose and decide which dish each goes into.
How to smell it
Smell a crock pickled two days ago, then one fully soured, then one over-fermented if you can find it. Three stages of the same process — a good exercise in reading time with your nose.
Why this one is hard
Because the word sour once again lumps taste and smell together. And because the smell of over-fermented pickles is unpleasant, people stop before smelling the whole range.
Don’t confuse with
This entry
Pickled mustard greens
Fermented family
Pickles are lactic-fermented, so they are brightly sour; fermented bean curd is mould-fermented, so it has a savoury, buttery note. One is sour, the other umami.
Scientific note
Pickled mustard greens undergo lactic fermentation: bacteria turn sugar into lactic acid, lowering the pH and preserving the vegetable. The smell changes clearly by stage because different bacterial species take turns to dominate. Over-fermentation is when acetic acid and sulphur compounds take over — not random spoilage but a predictable next stage.
In Vietnamese life
Pickled mustard greens are eaten in all three regions, pickled three different ways. It is also one of the oldest food-preservation techniques still used daily in Vietnam.