My grandaughter stays with me once or twice a week and I have a can of powder infant formula for her visits. She doesn't take much so we don't use it up fast. I keep it in the cabinet but am wondering if I need to refridgerate it? How long can I keep a can of the powder on hand before it starts to lose it's nutritional value? (She is 7 months old but was 16 weeks premature and is on special formula for premature babies if it makes a difference)