What Is Kosher Salt, and Why Do Recipes Name the Brand?

Kosher salt is not a kind of salt. It is a crystal shape, and the two American brands are so different that a teaspoon of one carries almost twice the sodium of the other.

What makes salt "kosher"?

The job, not the salt. The name is short for koshering salt — a coarse, flat crystal that sits on the surface of meat and draws blood out during kashering, rather than dissolving straight in. The Orthodox Union puts it plainly: “Kosher salt is really a misnomer: it should be called koshering salt.”

Salt is a mineral. It is inherently kosher and needs no certification to be permitted. Both major American brands happen to carry OU certification anyway, but that is not what the word on the box is describing.

Why does the brand matter so much?

Because the two brands are made by completely different processes and the crystals are not the same object.

Diamond Crystal is made by the Alberger process at St. Clair, Michigan — the only plant of its kind. Brine is flashed into an open evaporation pan and salt forms as hollow pyramids at the surface. They trap air, they crush between your fingers, and they are light.

Morton Coarse Kosher is vacuum-evaporated granular salt pressed between rollers into flat flakes. Morton's own description: “granulated salt compacted into coarse flakes.” Dense, hard, and it does not crush.

How much difference does that actually make?

A great deal. These are the manufacturers' own Nutrition Facts figures, scaled up:

A teaspoon of Diamond Crystal delivers less than half the salt of a teaspoon of table salt. Measured by weight all three are the same thing — about 393 mg of sodium per gram. Measured by the spoon they are three different ingredients.

Is the "two to one" rule correct?

For one pairing, yes. For the pairing people usually apply it to, no.

Two to one is the Diamond Crystal to table salt ratio, and it is accurate to within about seven per cent. Applied to Diamond Crystal against Morton it is wrong: the real figure is 1.71, so halving a Diamond Crystal quantity under-salts by roughly fifteen per cent.

Two pieces of standard advice that contradict their own sources

America's Test Kitchen publishes the chain “1 teaspoon table salt equals 1½ teaspoons Morton kosher equals 2 teaspoons Diamond Crystal.” Their own published gram weights — 6.0, 4.8 and 2.8 — give 1.25 for the Morton step, not 1.5. Following the 1.5 over-salts by about twenty per cent.

Morton's own FAQ advises that “for general use in teaspoon amounts, use the same amount of Morton Coarse Kosher Salt as table salt.” By Morton's own label weights that under-salts by twenty per cent.

Neither is a conspiracy. Both are what happens when a volume rule of thumb outlives the measurement it came from.

What is actually in the box?

Diamond Crystal's ingredient statement is one word: Salt. No anti-caking agent, no iodide, nothing else — which is why the packaging warns that caking may occur and tells you to shake the container.

Morton Coarse Kosher lists Salt, Yellow Prussiate of Soda. That is sodium ferrocyanide, an anti-caking agent capped by regulation at 13 parts per million. It is a tightly bound iron complex, not free cyanide, and 13 ppm is the legal ceiling for the whole category.

Neither is iodized.