I eat natto most mornings and I know what it is supposed to smell like. Opened a pack from the back of the fridge today and it was a sharp chemical ammonia smell, the sort you get off a cleaning product, rather than the usual funk.
Best before was about a week ago. The beans look right, the threads are there, no colour on them. Is ammonia a spoilage signal or just an old pack?
If it is only mildly ammoniac, there are three things that help and they are what most people do without thinking about it. Open the pack and leave it for a few minutes, because ammonia is volatile and some of it simply airs off. Then stir it properly, fifty strokes or more, which builds the threads and changes both aroma and texture.
Then the standard seasonings: the sachet, soy, karashi, spring onion, over hot rice. The warmth and the rice mellow it considerably.
Freeze the spare packs. It freezes well and it slows the fermentation right down. Thaw in the fridge.
The actual bin list for anybody unsure: green, black, pink or fuzzy mould. Note that a white powdery look is the bacteria and is normal, so do not throw out a good pack over that. Also dry crumbly gritty beans with a lot of crystal-like white spots, slimy liquid that is not the usual sticky threads, and sour or rotten smells rather than the earthy funk.
For anybody making it at home: strong ammonia means shorten the ferment or drop the temperature, and resting it in the fridge for a day afterwards calms it down noticeably.
@stir_fifty_times · 2w ago · 3 replies
Age rather than spoilage, and there is a mechanism behind it. Natto is fermented with Bacillus subtilis, which breaks down the protein in the soybeans, and part of that breakdown releases ammonia. A little is normal and it accumulates the longer fermentation runs.
The part people do not realise is that it carries on slowly fermenting in the fridge. So a pack a week past its date has had extra time to build it up, and if it was ever somewhere warm on the way home that accelerates it.
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@opened_the_pack · 2w ago
So is a sharp ammonia pack still edible, or just unpleasant?
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@stir_fifty_times · 2w ago
A mild note on opening is common and most regular eaters barely register it. A smell strong enough to make you pull your head back usually means it is past its best: not the flavour it is meant to have, and most people find it unpleasant rather than dangerous.
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