Why do some recipes call for alum?
The use of alum is no longer recommended. At one time it was added for crispness. If good quality ingredients are used and up-to-date methods are followed, firming agents are not needed. Alum has little crispness affect on quick-process pickles. It will increase firmness of fermented pickles when used at levels up to ¼ teaspoon per pint, but greater amounts will decrease firmness.
What about using lime?
Lime is a source of calcium. Calcium improves pickle firmness. Food-grade lime may be used as a lime-water solution for soaking fresh cucumbers 12 to 24 hours before pickling them. However, excess lime absorbed by the cucumbers must be removed to make safe pickles. To remove excess lime, drain the lime-water solution, rinse and then re-soak the cucumbers in fresh water for 1 hour. Repeat the rinsing and soaking steps two more times.
Is there a safer method than the addition of lime to cucumbers?
There is a calcium chloride product on the market called Pickle Crisp®. This product provides the calcium to help firm pectin but does not have the hydroxide component that lime has that can lower the acidity of pickled foods. Directions call for adding a small amount to each jar of pickles before sealing. Follow the manufacturer’s directions on the jar. Calcium chloride is an ingredient in some commercial pickle mixes.
For more info and a couple of good recipes: