Several years ago, I wrote a recipe for chocolate chip cookies that called for kosher salt. Soon after the recipe was published, I received an email from an upset mom who said the cookies were so salty she couldn't serve them. I had tested those cookies 20 times (really!). I was determined to prove her wrong. I reached for my kosher salt (I always use Diamond Crystal, a habit I picked up in restaurant kitchens), and then realized I hadn't tested them with Morton's kosher salt. So I made them again with Morton's. It was like eating a salt lick.
That was the day I learned that salts are not created equal.
All salt, whether fine or coarse, sea or table, is sodium chloride. Five grams of coarse sea salt is as "salty" as five grams of fine salt. Sea salt often contains minerals picked up from the ocean, while table salt might contain additives such as iodine or anticaking agents. Sometimes these flavors are detectable.
The main difference lies in the crystal shape. Grains of table salt or fine sea salt are small and densely packed; kosher and coarse sea salts are large flakes with more air between them. This means that a tablespoon of fine salt weighs more (and is therefore saltier) than a tablespoon of larger salt crystals.
Diamond Crystal and Morton's are two popular brands of kosher salt. Side by side the crystals look about the same size, but their shapes are very different. Shirley O. Corriher, the science-food guru and author of Cookwise and Bakewise, explains that Morton's kosher salt is made by putting the salt granules through high-pressure rollers and flattening them into large flakes. One grain of Diamond Crystal kosher salt, on the other hand, is formed like several upside-down pyramids stacked one over the next to form a crystal—a method the company patented. To help visualize this, Corriher says, "table salt is like an ice cube; Diamond Crystal is like a snowflake." And Morton's, she continues, is like a flattened ice cube.
By volume, Diamond Crystal is lighter and less "salty" than Morton's and fine sea or table salt, and therefore more forgiving in the kitchen. Corriher says, "Major restaurants use Diamond Crystal because if they have some kid who throws too much salt into a dish, he's less likely to oversalt it." Chefs like the taste. They like the feel of the large flakes in their hands. They like the crunch and mouthfeel. Diamond Crystal's shape helps it crumble easily in your hands, stick to surfaces, and dissolve quickly—good qualities for seasoning meats and batters.
So what does Corriher use when writing recipes? For consistency's sake, she calls for fine sea salt, which has the same weight, salinity, and volume as table salt. I asked Rose Levy Beranbaum, author of The Cake Bible and Rose's Heavenly Cakes, the same question. Beranbaum said, "For baking, I like uniodized fine salt."
For baking this makes sense. But should recipe writers discourage people from using kosher salt because it can go so wrong? When the CHOW test kitchen writes a recipe that calls for sprinkling meat with salt or seasoning a dish, we like to recommend kosher salt for the same reasons chefs love it. But when writing a recipe for say, a brine, which calls for a lot of salt, it's important to know which salt the chef is using, otherwise you will end up with under- or oversalted meats.
Typically when a recipe calls for "salt," it's referring to fine sea or table salt. Most sources, including Bakewise and Cook's Illustrated, say that 1 teaspoon fine salt = 1 1/2 teaspoons Morton's kosher = 2 teaspoons Diamond Crystal kosher. But I found this ratio to be quite off. Look at my results after weighing the different salts:
1 teaspoon fine sea or table salt = roughly 1 1/4 teaspoons Morton's kosher salt = roughly 1 3/4 teaspoons Diamond Crystal kosher salt
My results show that it's better to think of Morton's and fine salt as roughly the same, and to nearly double the amount of Diamond Crystal.
So, even though all salt is salt, there is good reason why most recipes say "season to taste." But take that with a grain of salt.