This is a basic list for one person. Some of the items can vary depending on personal taste, but better not stray too far from these items. Sorry, but chocolate won't make it across the plains in summer, but there will also be points along the way where you may be able to trade or purchase extra snacks, but it will be costly.
What's with all the coffee? There were places on the trail that the water was so bad with bugs and slime, and a general bad smell that the only way to get folks to drink it was to disguise it with the flavor of strong coffee. Everybody drank coffee, kids too!
Another clever idea is to bring along a milk cow. But don't forget, you have to feed and water her too, so add in for extra feed, plus it will slow you down some having to keep track of her.