Noun
(food) bittern: the liquor remaining after halite (common salt) has been harvested from saline water (brine), used as a coagulating agent in the manufacture of tofu.
salt derived from bittern, a powdered form also used in making tofu
Source: en.wiktionary.orgPlaces in Shikoku such as central parts of Kagawa Prefecture citation pickle the peeled young shoots by weighting them down in salt mixed with 10% nigari (magnesium chloride). Source: Internet
It is said (though no authority is cited) that the magnesium of the nigari binds with the oxalic acid thus mitigating its hazard. Source: Internet