actually that's a bit of a myth (that baking soda keeps things fresh):

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Baking soda is actually not a very good odor absorber. Activated carbon is better - it's the stuff that's in Brita filters and aquarium filters. You can get a big carton (like a 1/2 gallon of milk size) at a pet store for 7-9 bucks and put a little in a bowl or cheesecloth bundle.