Why does the packet of kool-aid advise against storing this nectar in metal containers in fridges?
Because Kool-Aid, and other drinks like it, have a LOT of acids in them. The acids eat through metal over time.
When I was in the Navy, we used unsweetened Kool-Aid to eat the rust off metal. The specific item being cleaned determined which flavor of Kool-Aid we used, because of the varying acid content.
Robin
While MsRobyn is right that it is because of the acid content, the reason is not that Kool-Aid will eat through the container (think how long that would take) but rather than the Kool-Aid will taste metallic. Stainless steel doesn’t have this leeching problem, but some metal containers do. Same reason you don’t want to make up a batch of tomato sauce in just any old metal pot.
When I wsa in the Boy Scouts, one of my fellow Scouts showed up for a hike with an aluminum canteen full of Kool-Aid. The Scoutmaster made him dump it all out.
If it’s an aluminum container, it’ll be pretty unpleasant. See Cecil on the subject of aluminum and acidic foods.
Robin