Which thermal process is not used as a lethality step?

In the context of food safety and preservation, lethality steps are processes designed to eliminate or significantly reduce pathogenic microorganisms to ensure food safety. Each of the other options (boiling, pasteurization, and sterilization) are methods that apply heat to food and are effective at killing bacteria and other pathogens.

Boiling involves heating food to its boiling point (100°C or 212°F at sea level) for a certain duration, effectively killing most pathogens. Pasteurization is a specific thermal process that heats liquid foods to a temperature that destroys harmful bacteria without significantly affecting the food's quality. Sterilization, on the other hand, involves heating food to extremely high temperatures (often above 121°C or 250°F) for sufficient time to kill all microbial life, including bacterial spores.

Conversely, drying is a process that primarily removes moisture from food rather than applying heat to achieve lethality. While drying can inhibit the growth of microorganisms by reducing available water, it does not necessarily kill them, particularly heat-resistant spores. Therefore, drying does not qualify as a lethality step as it does not guarantee the destruction of pathogens in the way the other methods do, making it the correct answer to the question.