The Collaborative International Dictionary of English v.0.48:

Yaw \Yaw\ (y[add]), v. i. [imp. & p. p. Yawed (y[add]d); p.
   pr. & vb. n. Yawing.] [Cf. Yew, v. i.]
   To rise in blisters, breaking in white froth, as cane juice
   in the clarifiers in sugar works.
   [1913 Webster]