Final answer:
A critical grain quality requirement for popcorn is the presence of a hard endosperm, which allows the kernel to pop into fluffy popcorn.
Step-by-step explanation:
A critical grain quality requirement for popcorn is the presence of a hard endosperm. The endosperm is the starchy part of the corn kernel and is responsible for the expansion of the kernel when heated. Popcorn kernels with a hard endosperm are able to withstand the pressure from steam and heat, allowing them to pop into fluffy popcorn.