In chemical terms, an acid is a substance that readily releases free protons, the small reactive nuclei of hydrogen atoms. Water is a weak acid, and living cells are designed to operate while bathed in it. But strong acids flood living cells with more protons than they can handle, and cripple their chemical machinery. This is why acids are good at preserving foods: they cripple microbes. In the case of acidifying fish, a happy side benefit is that it leaves the fish with a distinctive, almost fresh aroma. Acid conditions cause heavy-smelling aldehydes, which accentuate the fishiness of TMA, to react with water molecules and become nonvolatile, so that lighter alcohols dominate the aroma. Pickled herring and other fish can be surprisingly delicate.