Why does

[clive]

Why, when you freeze raw meat in, a sealed plastic bag, does it leak blood on the plate when you defrost it.

[andy hughes]

Because freezing causes the blood in some of the tiny capilliaries within the meat to expand, and rupture the vessels containing them. When thawed, this blood returns to liquid form, flows out of the meat, and onto the plate.

[moonacre]

I suspect it is because the plastic is not suitable for freezing. At minus 20C it becomes slightly brittle and contracts. Back at room temperature it relaxes but the stress of freezing leaves micropores (tiny holes) in the plastic which let liquids diffuse through. Buy and use purpose made freezer food bags and this won't happen.