Dear Miss Ginsu,
I'm confused. What's the difference between sea salt, kosher salt and iodized salt? Are any of these salts better for a salt-reduced diet? And why do some recipes have unsalted butter?
— Brine on the Brain
That's a lot of questions! Let's number them as we go along...
1. What's the difference between all the salts on the market?
All three are variations on the same compound: sodium chloride. The difference between these salts is found in the flavor (from trace minerals) and the crystal size (from the manufacturing method).
The salt we use in kitchens comes in in several crystal sizes, from very fine (almost powdery salt used for popcorn); to fine-grain or granulated salt (like table salt); kosher salt (flaky, larger crystals); and coarse (the crystal size you see on pretzels). There's also rock salt, but that's only really used for making homemade ice cream and thawing your sidewalk, as far as I know.
Iodized salt is generally a table salt, which has a small crystal size that's meant to slip out of your shaker with ease. When compared directly with non-iodized salt, iodized salt will have a slightly bitter taste. It also makes your pickles go dark if you're doing cucumber pickling. Iodine was included for reasons of public health and not for culinary artistry, and that's why the top chefs never use iodized salt.
Chefs sometimes use sea salt because it has additional minerals that give it subtle flavors (and sometimes pretty pastel colors). These other minerals tag along when the salt is harvested from the saltflats in one of a handful of exotic areas around the globe. The very expensive sea salts should only be used as a sprinkled garnish, because the delicate flavors would be overpowered in most dishes.
For everyday kitchen use, I find that chefs generally prefer kosher salt, which has larger, flake-like crystals that make it easy to pinch, measure and sprinkle in a dish.
As a side note, you shouldn't measure out table salt (iodized or not) in a dish that calls for kosher salt. Because kosher salt crystals are larger, you'll use too much salt in the recipe if you substitute table salt.
Table salt's very small crystals actually fit together tighter in the teaspoon than kosher crystals, which leave some space. So you end up salting more than you'd reckoned on, and the dish can be too salty.
In short, it's most efficient to use a table salt (iodized or not) in your shaker, kosher salt for cooking, salting meats, etc., and sea salt for extra-fancy garnish.
2. Are any of these salts better for a salt-reduced diet?
Technically, no. But you might end up using less salt in a dish if you're using kosher salt, because it's easier to control.
Those "lite salt" mixtures are usually potassium chloride mixed with the standard sodium chloride. I personally feel that fresh herbs and spices, vinegars and citrus juices are better flavoring options for salt-restricted diets.
3. Why do some recipes have unsalted butter?
Again, that's about controlling the flavor of a dish. If you're using unsalted butter, you're responsible for how much salt you want to add, not Land o'Lakes or Hotel Bar.
Long ago, salt was added to butter as a preservative, but thanks to modern shipping and refrigeration, that's not generally necessary these days.
Incidentally, the salt level in a stick of butter varies from one dairy to another, so it's difficult to put a firm teaspoon amount on how much salt you're getting in a stick.
Hope that helps!