Why did my sauce get lumpy after being frozen?

Question

Two months ago, I made a fish sauce (roux + milk + fish stock + lemon juice + cheese + mushrooms). I made way too much, so I decided to put a good amount (0.5 l / about 2 cups) in the freezer.

I wanted to use it this week, so I got it out of the freezer, and I put it in a sauce pan on a low heat, when it was still frozen. However, the result after it was fully heated, was not so good: it contained lumps (but not roux-lumps) so not at all the desired consistency.

Why/how did the lumps got there? Was it because I didn't properly defrost the sauce? Or was it a consequence of the freezing itself? What can I do to avoid this the next time?

There were no lumps when I made it.

Answer 1

Assuming your roux is flour based, the original recipe mixture was able to be a sauce because the proteins in the flour formed gluten and created that lovely thickness we enjoy (your seasonings sound like a nice combo). The water from the other ingredients are trapped in the gluten. Unfortunately, when you freeze it, the water molecules form sharp shapes like knives and cut your gluten strands. This isn't a noticeble problem until your defrost it.

It doesn't matter how quickly or slowing you defrost. The damage was done when it was frozen.

To work around this issue, you have to freeze it faster! Professional establishments can super freeze things super fast. Freezing faster will make smaller knife-shaped ice crystals and that reduces the damage.

TIPS:

• freeze in smaller portions and shallower depth on a pre-frozen metal cookie sheet.

• Defrost and strain the sauce. Make a new roux and add the defrosted & strained sauce. The flavor comes from the frozen sauce and the new roux recreates the gluten.

Answer 2

My guess is that your defrosting method is a bit brutal, and caused the fat to separate. I would try to defrost it in the fridge over night instead of heating it up like you did. Hopefully that will make it smooth.