Alum, or potassium aluminum sulfate, is a mineral salt occasionally used in food preparation as a firming agent, most familiarly in home pickling to help keep vegetables crisp.
Despite its traditional use, alum does not act as a preservative or improve food safety; the USDA no longer recommends it for canning, and modern pickling relies on salt, vinegar, and proper processing instead. It has also seen limited historical use as a coagulant in some tofu-making, though gypsum (calcium sulfate) is the more common and reliable coagulant in modern practice. Because alum isn't needed for safe preservation, most current recipes that want crisp pickles use calcium chloride ('pickle crisp') or an ice-water soak instead.