I'd be hesitant to use garbage bags if the food was going directly into them. They're just not designed to come in direct contact with food. (Not sure if that's what you meant, though.)
http://en.wikipedia.org/wiki/LDPEGenerally, the only difference between "industrial" plastic applications and food-grade plastic (if its "PE") is the food-grade has to use edible, food-grade lubricants in the manufacturing process.
Minus that difference, this is what they make many food containment products. Saran wrap, milk jugs, sandwich bags - you name it.
Its not really the plastic itself, but the FDA requirements of hair nets, factory conditions, and lubricants.
I'd be more concerned about the pesticides and "acceptable" contaminants like rat droppings than the off-gassing of the plastic. That same plastic is soda bottles, etc. Even the white pickle-buckets, polyethylene.
Just sayin'.
The only plsatic you should be concerned with (for long-term storage) is the PVC with the recycle code #3
Heres a handy-dandy chart..
http://en.wikipedia.org/wiki/Resin_identification_code