I would recommend a cool dry place. Personally I put my Koi fodd into a Home Depot 5 gallon bucket with an air tight lid, for a large quantity, and I use Tupperware for the food I pull from everyday.
If it is not air tight it will absorb moisture, and can start to spoil or get mold. Since many Koi foods are expensive, I take care to store and seal them as best I can, I also would not use "old" food. I like to buy the freshest possible, i.e. longest expiration date. You dont really know this when purchasing on the internet so I like foods I know are popular and made fresh, Like Blackwater Koi, or Kodama Koi food, there are many others, but some are just out of my budget.