What commonly known as kosher salt is in actuality coarse kosher salt, also known as "kosher sea salts" because kosher sea salt is usually used in kosher cooking for followers of Jewish shochet (kosher food law). Some companies dropped the on kosher sea salt while others kept it. No matter why it became "normal" to use kosher salt, there are important differences between kosher salt and regular table salt. Here are the basics about kosher salt.
First, there is controversy over whether kosher sea salts contains any trace of lecithin, a substance that is found in all kosher foods, but which may inhibit the body's ability to absorb the fats found in fish and meats. It has been postulated that sea salt contains trace amounts of lecithin, but that the effects are insignificant, and that any reduction in eating habits would likely be short-lived, if it even took place at all. Most kosher chefs use kosher salt because it's cheap, but not because they trust in the value of trace amounts of an animal fat.
Second, the kosher salt that most supermarkets and cooks are using today has only been manufactured in recent years. Prior to World War II, kosher salt was made by women in Israel who processed their own salt. They were the first people to use kosher salt on a commercial scale, and they produced a remarkably uniform salt with a fine grain. It was harvested from the Dead Sea, and it was one of the simplest and cheapest natural sources of salt available.
Since kosher salt is much cheaper than table salt, it was quickly embraced as a low cost "alternative" to regular table salt. However, it's use as a seasoning agent was limited until the late fifties when the process of harvesting the salt became more standardized. Modern kosher salt has only been manufactured in the last twenty-five years or so, during which time its production efficiency has undoubtedly decreased. The table salt industry has relied on the cheaper kosher salt for many decades, while the kosher sea salt industry has relied on far more expensive substances.
Another problem faced by kosher salt consumers, who have recently switched to sea salt, is the lack of distinction between kosher salt and sea salt. Sea salt is harvested in different places around the world using completely different methods. For example, kosher salt harvested in Canada is often treated with ammonia to prevent bacterial growth, while sea salt has no such treatment.
This is what is known as "lagering". The process of using a salt without treating it with chemicals and stabilizers to make it less porous is known as "lagering". In fact, it is a major drawback of kosher salt. To compensate, today's sea salts are treated with various chemicals that help it attain its various "key features". Unfortunately, not all sea salts have undergone this process, making the difference between them and their "traditional" counterparts much less clear.
Perhaps the biggest drawback of kosher salt is its scarcity. As far as the supply goes, most of it is mined in salt mines around the world. The biggest drawback here is that only one salt variety per year can be mined since the process of mining for more takes a long time. This forces retailers to stock up on a limited number of salts, pushing prices up.
However, there are some places where sea and table salt can be used interchangeably. In areas where access to kosher salt is limited, such as the eastern Mediterranean, there is no difference between the two. As a result, kosher sea salt tends to be less expensive than table salt due to the scarcity of the mined varieties and the general quality of mined products.