I find that powdered milk stored in zip-locks (just from the grocery store) for more than about a week pick up a odd plastic flavor. You need long-term food storage quality bags or mylar bags. Something you will use every day (adding a spoonful here and there) does not do will in zip-locks which get gummed up after a half dozen uses. Tying a loose knot in the top of a bag is better. I would tend to agree with the screw-cap bottle. Just re-use something like an Ovaltine bottle (which is already made not to pick up flavors). Fill up the jar; it may not weigh exactly one pound- just write the actual poundage on the bottle.