Fleur de Sel vs. Kosher Salt: A Finishing Salt and a Cooking Salt Compared

A teaspoon of Morton kosher salt weighs 4.8 grams. A teaspoon of Diamond Crystal weighs 2.8.¹ So “kosher salt” is really two products. Flavor, on the other hand, barely matters: America’s Test Kitchen found no flavor difference worth shopping for between salts once they’re mixed into food.² Fleur de sel is for finishing, kosher salt for everyday cooking, and what changes a dish is the crystal, the moisture, and how much salt a spoonful carries.

Side by side

The sodium row puts a lab measurement of four products next to a general figure from the AHA. They aren’t the same kind of number, and we found no lab analysis of either kosher brand.

How each one is made

Fleur de sel. The best-documented fleur de sel comes from Guérande, on France’s Atlantic coast, where it’s a protected geographical indication with a published specification. Two salts come from the same pans. Gray salt crystallizes on the clay bottom. Fleur de sel forms as crystals floating in sheets on the surface, only when there’s wind and sun, and is skimmed off by hand. Under the specification it can hold up to 12% moisture and can’t contain additives.³ Fleur de sel is made in other places too, under their own rules.

Kosher salt. Despite what you may read, it isn’t a mined salt with special processing. ATK says both big US brands are made from brine. Diamond Crystal uses the Alberger process, heating and stirring brine in open containers so hollow, pyramid-shaped crystals form on the surface. Morton’s brine goes into vacuum evaporators, and the salt is then pressed through rollers into large, thin flakes.¹ The name comes from how it’s used, not how it’s made: the coarse grains were used to draw blood from meat, as Jewish dietary law requires.

Texture, not flavor

ATK’s tasters found that sea salts vary in color but only slightly in flavor, and that sea salt tastes no different from table salt once mixed into food.² In its kosher salt test, broths seasoned with equal weights of Morton and Diamond Crystal tasted the same. On buttered bread, tasters preferred Diamond Crystal’s “delicate and fleeting crunch.”¹ So you pick either salt for how it handles and how it sits on food.

• Fleur de sel is fine, flaky and damp. It goes on at the end, on steak, fish, tomatoes, eggs or caramel. That’s how ATK recommends using sea salts.
• Kosher salt is easy to pinch and spread, which is why ATK’s testers reach for it to season meat. Its coarse grains dissolve more slowly than table salt.⁸

Two claims about the difference don’t hold up as stated. One says fleur de sel dissolves faster on the tongue because it contains magnesium chloride and calcium chloride. We found no source for that mechanism. The Swiss survey, the one lab analysis of fleur de sel we found, couldn’t quantify magnesium in any salt it tested.⁶ The other says that since neither salt is iodized, neither has the bitter note some people taste in iodized salt. ATK’s tasters couldn’t taste any difference between iodized and noniodized salt.²

Measuring: where the swap goes wrong

Most home recipes measure salt by the spoon, and a spoon holds very different weights of different salts.

Even the kosher conversions disagree. Here’s the disagreement as published on our kosher salt page:

Ratio (Disputed): we show all three instead of picking one. ATK itself recommends weighing salt.⁹ With fleur de sel there’s nothing to convert from at all, so the swaps below give starting points, not ratios.

Pickling

You’ll see a claim that both salts work for pickling without affecting fermentation. The National Center for Home Food Preservation recommends canning or pickling salt. It says flake salt varies in density and isn’t recommended, and it lists an anticaking agent or other additives in the salt as a cause of cloudy brine. Morton’s kosher salt contains an anticaking agent.⁴ The center also notes that you can make pickles safely with iodized or noniodized table salt.¹¹ See pickling salt vs. kosher salt for that comparison.

Can one stand in for the other?

For finishing, yes. For measured cooking, go by weight. These follow our fleur de sel substitute and kosher salt substitute pages.

How we got these. By weight: the Codex standard requires food-grade salt to be at least 97% sodium chloride on a dry basis,¹² and Guérande fleur de sel can hold up to 12% moisture,³ so a gram of fleur de sel can’t carry more salt than a gram of dry kosher salt. By volume: we didn’t find a published gram figure for a teaspoon of fleur de sel, so these are cautious starting points. Diamond Crystal is the lightest salt by the spoon in ATK’s figures, which makes it the safest to start at equal volume. Morton weighs 4.8 g a teaspoon against Diamond Crystal’s 2.8 g.¹

Health notes

• Sodium. The AHA says table salt, kosher salt and most sea salts are about 40% sodium, and a coarse salt holds less sodium per teaspoon only because fewer crystals fit in the spoon.⁷
• Iodine. The NIH Office of Dietary Supplements lists both fleur de sel and kosher salt among specialty salts that aren’t usually iodized.⁵ If these are the only salts you cook with and you count on salt for iodine, see iodized vs. non-iodized salt.

What isn’t known

• We found no published weight per teaspoon for fleur de sel.
• We found no lab analysis of Guérande fleur de sel, or of either kosher brand.
• We found no test of how fast either salt dissolves on the tongue.
• We found no dated, sourced price comparison.

Related

• Fleur de sel and kosher salt, the full profiles
• Fleur de sel substitutes and kosher salt substitutes
• Fleur de sel vs. Maldon salt, fleur de sel vs. sea salt and fleur de sel vs. Himalayan salt
• Maldon salt vs. kosher salt, kosher salt vs. sea salt and kosher salt vs. table salt
• Cooking with kosher salt and too much salt