Rock salt

Salt from the ground

Rock salt is salt that was mined, not evaporated from water at the surface. The mineral is halite, sodium chloride, NaCl, which forms cubic crystals and occurs in crystalline, granular and massive forms. A seam of rock salt can be colourless, white, pink or red, depending on the impurities locked in as it formed. The same chemical compound is sold as table salt, sea salt and flaked finishing salt, and what separates them is how they were obtained, how clean and how finely divided they are, and what has been added.

Because "rock salt" describes an origin and not a specification, the name tells you less than it appears to. A coarse grey block sold for road de-icing and a pink crystal sold in a spice jar can both be rock salt. The Kansas deposit mined near Hutchinson is a good example: the salt it yields is used mostly for de-icing, while its highest-purity output, over 99.8 percent sodium chloride, is used for table salt, food processing and animal feed. The international standard for food-grade salt requires at least 97 percent sodium chloride on a dry matter basis, excluding additives, and sets limits for other substances, so what matters in the kitchen is whether the packet is sold for human food and what it contains.

Rock salt is a mineral, not an organism, so it has no species, variety or microflora of its own. What it brings to a preparation is sodium chloride and, depending on the deposit, small amounts of other minerals.

How a salt bed forms

Every rock-salt deposit begins as a body of salty water that dries faster than it is refilled. In Kansas, the Hutchinson Salt Member was laid down in the Permian period about 275 million years ago in a restricted sea under a hot, dry climate, where evaporation exceeded the inflow of water and salt built up on the sea floor. Producing one foot of salt took about 80 feet of seawater, so the bed took thousands of years to accumulate. Younger rock then buried it. Today the deposit covers about 37,000 square miles under central and south-central Kansas, reaches more than 500 feet in thickness and is about 80 percent salt.

Other deposits are older still, and no two are the same. The Salt Range of Punjab, in Pakistan, holds the Khewra deposit, which its operator dates to the Precambrian, as far back as 600 million years. The geology matters to the cook for a simple reason: how pure a salt is, and what colour, depends on what else was in the ancient water and mud. The age of a bed does not tell you how clean a particular packet of its salt is.

Getting it out of the ground

Rock salt is taken from the earth in two ways. In room-and-pillar mining, a shaft reaches the bed and the salt is cut away in a checkerboard pattern, leaving pillars to hold up the roof. In Kansas about 75 percent of the salt is removed and 25 percent is left as support, in mines 600 to 1,000 feet deep. The Khewra mine is also worked room-and-pillar, on 17 levels, with seven thick salt seams adding up to about 150 metres of salt.

The second way is solution mining. Water is pumped down a well into the deposit, the salt dissolves, and the brine is brought to the surface, where it is evaporated to recover the salt. The salt that results is then crystallised, as it would be from seawater, and no longer looks like the rock that was underground. A packet that begins in a buried bed can therefore be a evaporated crystal, and a rough chunk that looks natural can be a quarried piece. Whether a product is food grade depends on how it was handled after extraction.

Khewra, Mandi and South Asian rock salt

The best-known rock salt in the kitchens of South Asia comes from the Salt Range of Punjab. The Khewra mine, about 160 kilometres south of Islamabad in the foothills of the range, produced 314,397 tons in 2022 to 2023. Its operator, the Pakistan Mineral Development Corporation, gives its average purity as 98 percent and describes salt ranging from transparent white through pink to red. The corporation sells edible salt in white, pink and dark-pink grades, in granular and powder form, alongside material for de-icing, decoration, animal licks and industry.

India has its own rock salt. Hindustan Salts Limited operates mines at Mandi in Himachal Pradesh, which it took over on a long-term lease in 1963. The Indian Bureau of Mines yearbook for 2012 records that the rock salt from the Mandi mines contained 67.81 percent sodium chloride, and that it was not suitable for human consumption, since edible salt must contain at least 96 percent; it was generally used to feed cattle. A portion of rock salt produced in India is iodised to supply regions where goitre is endemic. The figures are a useful reminder that the same word can cover a food ingredient and a feed supplement, and that the percentage of sodium chloride is the test.

Pink, black and white: the kinds of rock salt

In Indian cooking the term covers two salts with different jobs. The national dietary guidelines describe rock salt as of two types: pink salt, called sendha namak, and black salt, called kala namak, which can be told apart by colour, texture and smell. Pink salt is described as one of the purest forms of salt.

Kala namak is not simply dark rock salt. It is made by firing raw salt, traditionally from Himalayan-region mines or salt lakes, in a sealed ceramic jar with charcoal and small amounts of ingredients such as harad, amla, bahera and babul bark. The heat converts part of the salt's sodium sulphate into sulphur compounds, which give it the egg-like smell that is the point of the ingredient in chaat masala and in vegan egg dishes. The national guidelines add that black salt darkens in storage and that its spices, charcoal, seeds and bark are part of its preparation. Its sodium content is, however, much like that of other salts.

Rock salt differs from solar salt, which is crystallised from seawater in shallow pans and keeps small amounts of magnesium, calcium, sulphur, potassium, iron and zinc, and from purified salt, which is nearly pure sodium chloride with no additives. The mineral content of a rock salt depends on its bed. Colour is a poor guide: a pink crystal can be as pure as a white one, and neither colour reveals iodine, trace-mineral content or origin.

What salt does in preserving and fermenting

Salt preserves by taking water away from microbes. Water activity measures how much of the water in a food is free for microorganisms to use, on a scale from 0 for no available water to 1 for pure water. Salt and sugar bind water, so adding them lowers it. Fresh meat and fish sit at 0.99 and raw vegetables at 0.99. Most spoilage bacteria need a water activity of about 0.90 or above, Staphylococcus aureus can grow down to 0.91 without air and 0.86 with air, Clostridium botulinum needs about 0.93, and moulds and yeasts can grow as low as 0.61. A high-salt food such as soy sauce is at about 0.8. This is why a salted food can hold off most bacteria while moulds and yeasts, which tolerate the lowest water activity, are the hardest to stop.

Salt does a second, finer job in a ferment: it chooses the winners. A study of kimchi describes this as inhibiting harmful microorganisms by osmotic shock while regulating the selective growth of salt-resistant lactic acid bacteria, with optimal growth and acid production at a salt concentration of about 2 to 3 percent or less. In sauerkraut, a final concentration of 2.25 to 2.5 percent salt draws juice from the cabbage, helps keep undesirable bacteria from growing and favours Leuconostoc, the lactic acid bacteria that begin the fermentation, which tolerate less acid and give way to the more acid-tolerant lactobacilli. The amount of salt in a tested recipe is therefore chosen for the food and the organisms it is meant to favour, not raised for extra safety.

The kind of salt changes the result more than its colour suggests. Kimchi made with solar salt aged for one year, solar salt aged for three years, dehydrated solar salt and purified salt developed different microbial communities. Leuconostoc and Weissella dominated the kimchi made with solar salt, while Latilactobacillus sakei dominated the batches made with purified and dehydrated salt, and the sugars, organic acids and amino acids differed as well. The study did not test mined rock salt, so it shows that salt type matters and not what rock salt in particular would do. Home-preservation guidance asks for pickling salt, notes that iodized or non-iodized table salt can be used, that anti-caking additives can cloud the brine, that reduced-sodium salt is not recommended for fermented pickles, and that flake salt varies in density, which is why recipes should be followed by weight where they give it.

Rock salt in the kitchen: amla murabba and other uses

Rock salt turns up in cooking more often as a seasoning than as a preservative. In an amla murabba recipe attributed to the chef Reetu Uday Kugaji, a pinch of rock salt is added late in the cooking of the gooseberries in sugar syrup. The recipe gives no measure, does not present the pinch as a preservative, and leaves colour, grain size and mine unspecified. Another method for the same preserve calls for kala namak, a reminder that a recipe's "rock salt" and "black salt" are different choices.

Beyond the preserve, sendha namak and kala namak season fruit, chutneys, raitas and chaats, and plain rock salt serves in the same ways as other salts in curing, brining and cooking. A large crystal can be crushed in a mortar or bought ground. Ground and chunked salt weigh differently by volume, so a "pinch" or a spoonful can mean quite different masses.

When a recipe says only "rock salt", the practical questions are which of the three it means: plain pink or white sendha namak, the sulphurous kala namak, or a coarse salt meant for a brine. The answer changes the flavour, since kala namak carries sulphur notes that plain salt does not, and it changes the quantity, since a recipe written for fine table salt by weight will be undersalted if the same volume of coarse crystals is used, since they pack less salt into a spoon. Where a recipe gives grams, use grams, and where it does not, add salt by taste in a cooked dish and not by the spoonful in a preserve or ferment.

Choosing, storing and health

Buy rock salt that is sold for food. The label should say it is edible, and for fermentation, where the salt amount matters, it should say whether it is iodised or contains anti-caking agents. A decorative block or a mine sample is not a food ingredient because it looks like the crystals in the spice aisle. Keep salt dry, in a sealed container, away from rain and humidity; the international standard advises that iodised salt not be exposed to rain, excess humidity or direct sunlight, since iodine is lost under those conditions.

Salt is a source of sodium whatever its colour. The World Health Organization identifies sodium and chloride as the main components of regular salt, recommends reducing sodium intake, and recommends iodised salt for consumption. A pink colour or a claim about trace minerals does not take away the sodium in a serving. Salt is not among the major food allergens named by the US Food and Drug Administration; anyone with an allergy should still read the label of a purchased blend, since spiced or fired salts such as kala namak contain other ingredients.