Pickling salt is a type of salt that is designed specifically for pickling and canning. To make it most suitable for its intended uses, this type of salt is especially pure, and it typically is very fine-grained to ensure that it dissolves easily. Many markets carry pickling salt, although people in some cities might have to do a bit of searching for it. In a pinch, this type of salt can be replaced with kosher salt or table salt, although these salts might not perform as well.
The fine grain of pickling salt is important because it makes the salt more easily dissolved. This means that the salt will be distributed evenly throughout the brine, creating the required salinity to make pickled foods safe to store and eat. Without a fine-grained salt, cooks would need to be extra careful to make sure that all of the salt had dissolved, which would create an even suspension in the brine.
The purity of pickling salt is important from an aesthetic point of view, but not necessarily from a food safety standpoint. Iodized salt, such as typical table salt, will turn pickled foods dark, which can make them unsightly, and the anti-caking agents used in most salts will cloud the brine. Neither of these problems are dangerous, but most people don't like to eat discolored food that has been fished from a cloudy brine, so cooks use pickling salt instead.
Pickling salt doesn't just have to be used for pickling. It also can be used just like regular table salt in recipes, although it tends to cake, so people should be careful with pickling salt around moisture. To prevent caking in stored pickling salt, a few grains of rice can be added to the container. Caked salt also can be roasted in the oven to bake out the moisture, which makes it easy to break apart the caked salt.
In addition to salt, pickling brine usually includes an assortment of spices that slowly leach into the food as it pickles, infusing it with flavor. Depending on the spices used, the pickled food might be hot, savory or fairly neutral in flavor. The intensity of the salt flavor varies, depending on whether the food is pickled in vinegar with salt as a preservative or whether a salt brine alone was used for pickling.
When a salt brine alone is used, cooks can use a certain test called an egg test to see whether the brine is sufficiently strong. In the egg test, an egg is dropped into the brine. If it floats, the brine is good, but if it sinks, the brine is not salty enough.