"’ I’ve been through the mill, ground, and bolted, and come out a regular-built down-east johnny-cake"

bolt 2
vt.
5ME bulten < OFr buleter, ? dissimilated < *bureter < bure (< VL *bura), coarse cloth; akin to It burattare < buratto, sieve6
1 to sift (flour, grain, etc.) so as to separate and grade
2 [Archaic] to inspect and separate, as good from bad; examine closely