Why not calcium chloride? It works and you don't have to worry about the probable toxicity of aluminum.
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Generations of cooks have used alum as part of their pickling process to pickle cucumbers. Alum is the agent in the pickling spices that gives the finished cucumbers their firmness and the satisfying crunch they gives off when eaten. Maintaining firmness is important in the early stages of pickling because once produce has softened, the firmness is lost for good. Many pickling recipes today still call for alum, but food safety experts say alum is not necessary for a successful batch of pickling and it is toxic at a certain dose.
Alum for pickling, also known as aluminum potassium sulfate or potassium aluminum sulfate, can be found readily on a grocery’s shelves in the spice aisle in the form of a powder. Food safety experts say alum for pickling is safe, but ingestion of a single ounce (28.34 grams) can be deadly for an adult. Alum for pickling can be discarded if the methods used are more modern and the vegetables and fruit called for in the recipes are fresh. If alum is employed, it should be thoroughly rinsed from the final liquid and product.
In traditional pickling recipes, alum is mixed with water. Canners and others present in the kitchen should be careful not to inhale the powder or the fumes because of aluminum’s toxicity. Food experts emphasize that alum for pickling can be safely employed but it is not recommended, and its use is not strictly necessary. Another ingredient traditionally used to preserve the firmness of the produce is lime, but food safety experts say lime, too, can be discarded from the recipe. Pickling lime, like alum, if used in the recipe should be entirely removed by a thorough rinse after the produce’s first soaking, and persons working on the recipe should avoid inhaling any dust from the lime.
There are different types of pickled fruits and vegetables, including watermelon rind, and different processes for making them. Fermented or brined produce is cured in a mixture of salt and water for a week or more. Another process, called fresh pack, employs a mixture of spices, vinegar and seasonings that has been boiled, but alum is generally not used with this method. Fruit pickling employs lemon juice or vinegar. Alum is used medicinally as a styptic, an astringent and an emetic, and it is also found in baking powder.