Ilse

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.

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.

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.