Alum (potassium aluminum sulfate) is a food additive that is ideal for pickling and canning. It helps create crisp pickled fruits or vegetables. Wondering what exactly alum is used for? This cooking essential is a crystalline powder that helps keep fruits and vegetables firm and crispy when cooking. Try adding a bit of alum when pickling for that perfect crunch. Dissolve alum in water using amounts specified in your recipe. Soak produce in the alum solution according to the specified time. Drain and discard the solution. Rinse the produce well then continue with pickling and canning process.