I have prepared silk fibroin from cocoons following the protocol established by the Kaplan group. However, in their protocol, they specified that "silk fibroin can be frozen under -80 or -20 before lyophilization".

I froze the silk fibroin solution under -80 and then freeze-dried it, but the lyophilization product looks like layered sheets and has almost no solubility in water.

Is it because -80 has induced insoluble beta-sheet formation? Or can anyone suggest where did I do wrong?