Snark and sarcasm are two of the things that the internet does best, and salt – surprisingly – is sometimes a target. It's not uncommon to see pictures circulating on social media of a package of salt with its expiration date prominently displayed and a tart comment about a freshness date that's a million-plus years away. In the case of sea salt, though, there's actually a reason for this.
Pure Salt Doesn't Go Bad
The idea of putting an expiration date on salt does seem silly at first. Sodium chloride, the salt we eat – there are a great many others, mostly of interest to chemists – is a mineral, which is to say it's essentially a stone. It's a stone with some interesting properties, since it dissolves into water in minutes rather than centuries, but it's still a mineral and "freshness" isn't a concept that makes sense with minerals.
Other foods go bad and spoil because they're organic. Molds and yeasts and bacteria of various kinds can feast on them, and their own natural enzymes start to break them down into compost over time. None of those things apply to salt. In fact, salt disrupts and prevents microbial activity, which is why it's so widely used as a preservative.
Sea Salt Isn't Necessarily Pure
Sea salt is different, because it's not formulated in a laboratory. As the name suggests, it's made by evaporating sea water until all that's left is the salt. In warmer climates, the water can be drawn into shallow lagoons and left to evaporate in the sun, while colder and rainier countries have traditionally relied on boiling the water. Modern operations typically speed the process by using large gas-fired boilers.
The thing with sea water is that it isn't a simple solution of salt in water. It contains a lot of other minerals and – of course – a lot of aquatic life, which all introduce impurities into the salt. These impurities give the salt much of its character, which is why a chef might choose to finish a dish with, say, Hawaiian black salt in one case and French sel de Guérande in another. While salt itself doesn't go bad, those impurities may sometimes be more perishable.
Shelf Life of Salt
The shelf life of a given type of salt, then, is determined by the kinds of impurities it contains and how high a percentage of impurities it contains. The more impurities, the greater the likelihood that some spoilage, or at least some deterioration, might occur.
In the simplest case, this might just mean that the salt's distinctive flavor notes will fade over time. This is slightly disappointing, especially if you've paid a premium price for exactly those notes, but the salt is still salty and still safe to eat and use normally as long as its taste doesn't become actually unpleasant. This is the closest salt comes to spoilage in the conventional sense.
Salt also attracts moisture from the air, and sea salt doesn't contain the anti-clumping ingredients you'll find in table salt. This means that it can absorb odors and flavors from the steamy kitchen air, and can also harden into a lump from the moisture. This doesn't spoil it, but does make it difficult to use.
Some sea salts also contain mold spores, which can be more problematic. While salt attracts moisture, it's unlikely to become damp enough for the mold spores to come to life before you add it to food. If you add it at the table, or while you're cooking, the spores –again – won't have time to come to life and reproduce before you eat them. If you use that salt for preserving vegetables or curing meat, though, they may contaminate your food.
Salt Expiration Date
So the date on a given type of salt, then, depends on its impurities. Some are dated solely because supermarkets won't stock a product without some sort of "use by" date. Others have a "best-before" date because their characteristic flavor will dissipate over time, or because they may develop unwanted flavors over time.
Ironically, even table salt – which begins as laboratory-pure sodium chloride, with an infinite shelf life – has a best-before date, because of the iodine and anti-clumping agents added to it in manufacturing. These dates are all assigned for the sake of quality, not for food safety, so they're just a guideline. If you want to know the reasoning behind the date on your own specific brand of salt, try reaching out to the manufacturer. If it's there purely to please the supermarkets, you can continue using it as long as you like.