A sericulture officer explains to a group of trainee farmers that after the mature silkworm larvae have finished spinning their cocoons, the cocoons cannot be directly reeled to extract silk, because the pupa inside is still alive and would eventually develop into a moth that emerges by cutting through and breaking the continuous silk filament. To prevent this and preserve the unbroken thread, the cocoons must first undergo a specific process that kills the pupa using heat, steam or hot air. Which process is being described here?