The Collaborative International Dictionary of English v.0.48:

Crumb \Crumb\, v. t. [imp. & p. p. Crumbed (kr[u^]md); p. pr.
   & vb. n. Crumbing (kr[u^]m"[i^]ng).]
   To break into crumbs or small pieces with the fingers; as, to
   crumb bread. [Written also crum.]
   [1913 Webster]