The Food Lab: Better Home Cooking Through Science

That is sausage meat that was ground in a warm meat
grinder. Appetizing, right? The problem is with the fat. As
fat gets warmer, it gets softer. Rather than cutting cleanly
through the grinder, warm fat will smear, turning your fine
grind into a mushy paste. Try cooking up this paste, and
rather than staying in your sausage in nice juicy little
pockets like good fat should, the smeared fat will flow out
of your meat in vast rivulets, leaving nothing behind but
dry, mealy, lean meat.
To prevent this from happening, it is vitally important that
both the meat and the grinder be cold. If the meat for my
sausage has not come straight out of the coldest part of my
fridge, I spread it on a rimmed baking sheet and place it in
the freezer for 15 minutes before attempting to grind it.