Fresh breakfast sausage, hollow centre

[jiggerjams]

Hi there, I've just made a batch a fresh breakfast sausage and after freezing and then later thawing them and cooking them I've noticed a few had hollow middles. Any particular reason for this to happen? Any help is appreciated.

[GusRobin]

Air pockets when you stuffed them; temps were too high too fast and it fatted out. Only 2 reasons I can think of - others may have more ideas