The pioneers who traveled west would put silver coins in their water barrels to keep the water from spoiling. Back before there was refrigeration, people would put a silver coin in their milk jugs to keep the milk fresh longer.
These days we have no REAL silver coins in circulation. But you can buy "Silver Rounds" from any coin store. A "silver round" is one measured ounce of 99.9% silver. It's cast to look something like a silver dollar. These days they cost under $20, but can be used over and over again.
Real silver is a natural germ fighter. In tests, silver has kept milk fresh for 2 - 10 extra days. A Silver Round can also be put into water dispensers to keep not only the water fresh but also the spout clean. Keep milk and water fresh the natural way, by putting a 99.9 % silver round in your storage containers.
COIN stores care silver rounds! ... & some Pawn Shops too! ... Just look up "Coin Stores" or "Numismatic Stores" in your yellow pages. Sometimes Sports Collectibles stores also sell silver rounds. Just give them a call first & ask if they sell "One Ounce Silver Rounds"... Maybe these days with the price of Silver so high, they may even sell half-ounce rounds.
---> BUT: Make sure these are FINE silver (It'll say Fine Silver or 99.9 on the "coin" or "round") & NOT Sterling Silver... Because of the purity. Sterling would also work, but then you may be putting a tiny bit of copper in your milk or water. You don't want to use a Real Silver Coin (like a half dollar), because Coin silver is only 90% silver unless it's an emergency.