A seed full of oil
Sesame seeds are about half oil. Toasting them builds a deep, nutty smell. That smell dissolves into the oil inside each seed.
Grinding opens the seed
A whole seed keeps its oil and smell sealed in. Grinding cracks the hull. The oil and its aroma reach the air at once.
The smell starts leaving right away
Aroma compounds are light and drift off fast. The exposed oil also starts to go stale. Bagged ground sesame has lost most of its smell. It left before you opened the bag.
Half-ground is the target
Grind until about half the seeds are broken. You get a burst of smell and some crunch. Fully ground sesame turns into paste.
Shunta says:
Smell the pan, then smell the bag of ground sesame. The difference makes the point better than any chart.
