McDonald's Corp rolled out its new Crispy Chicken Sandwich in February. Wendy's Co is selling Crispy Chicken Nuggets. And chains like Shake Shack Inc are known for having some of the fast-food industry's crispiest french fries.

With delivery and to-go orders soaring during the pandemic, marketing "crisp" is risky if the food arrives at consumers' homes limp and listless. In a quest to keep the crunch as long as possible, many chains are tweaking their recipes, trying different additives, formulations, cooking techniques and temperatures...