Doesn't seem to be anywhere, so...

7* items: 1 grinder.
8* items: 1 grinder.
9* items: 1 grinder.
10* items: 2 grinders.
11* items: 4 grinders.
12* items: 5 grinders.

B1/2 - Beginner
B3-6 - Normal
B7-13 - Hard
B14-29 - Very Hard
B30-32 - Time Attack
B33 - Solo Play

That's right, area specific blocks have gone boom.