The salts of the earth
I have noticed that lately, whenever salt is listed in a recipe it is "sea salt". Isn't all New Zealand salt sea salt, and can you substitute rock salt if you have it?
Funnily enough I had this same conversation in Berlin last weekend at the wonderful food hall on the top floor of the KaDeWe department store with a foodie friend. It got me thinking about the various deli counters around the world selling myriad types of salt, from kala namak (a black sulphur-tasting Indian and Pakistani salt mined in the Himalayas) through to the orange Murray River salts from Australia, Maldon sea salt flakes from Britain, and various fleur de sel salts from France.
Salt is simply made up of two ions essential for our life on earth - sodium and chloride - which in small quantities work well for us, but in excess can lead to high blood pressure and other worrying side effects. In New Zealand we produce the delicious flaky Marlborough sea salt, good enough to rival Maldon. But we also produce the more common finely ground free-flowing (usually due to the addition of anti-caking agents) table salt, and now of course you can also get large crystalline salts (both rock salt and sea salt) mixed with anything from horopito, karengo, chillies, and even delicious manuka smoked salt. I really love that for its lovely grunty smokiness - especially when ground over scrambled eggs and roast tomatoes for breakfast.
Sea salt is likely to have some other ions in there, probably magnesium or other bits and bobs from where it's sourced. Fleur de sel, one of the best foodie exports of France, is always a little moist and brown in colour, and like oysters, it tastes of the water and bay that it comes from. You're correct that all salts are sea salt - but a lot of salt, globally, is mined on land and is called rock salt. To harvest this type of salt it may well be mined as though it were coal, but sometimes water is pumped into the land mass to produce a brine which is then captured, filtered and purified to produce table salt.
Table salt generally has been purified and will be bright white and fairly pure. In Britain every winter we rely on rock salt to put on the pavements and some roads to stop people and cars skidding all over the place and a lot of this is harvested in Wales from mines that look like cathedrals - such is the light bouncing round in them as compared to pitch black coal mines.
Another salt that got a lot of press a decade or so back was kosher salt - used in many American books - but the reality is that (as almost all salt is kosher) it is simply salt with no impurities and no added iodine. So, to answer your question, and as someone who generally has four to five salts on the go at any one time, you can substitute whatever salt you like, whenever you want.
Black salt has a flavour unlike any other and it will affect the taste of the food you're cooking. Murray River salt has a subtle taste too - but if it's simply a matter of seasoning the water to boil your spuds - then use whatever is cheapest and available, and use the fancier ones to finish the dish as you serve it.
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