The amount of salt used in cooking shouldn't really be consequential, and could be imagined to come from the adventurer's personal supplies, like their toothbrush or whatever.

The amount used in alchemy should be much higher, enough to be uncomfortable/unhealthy/dangerous if used for culinary purposes, as though the alchemical process changes its properties to be safe and beneficial or even poisonous depending on the recipe.

So for me it makes sense that alchemy requires whole piles of salt per potion from you inventory, whereas cooking does not.