There are two species of white spiders common to Minnesota, the Goldenrod Crab Spider and the Whitebanded Crab Spider. In either case, it sounds like you saw the female of the species.
The female goldenrod spider can change color as part of a camouflage system, depending on which flower it is living or hunting upon. They prefer goldenrod flowers, hence their name, but will turn white if living on a white flower, though the color change process can take up to 3 weeks. These spiders can eat moths, bees and butterflies, but are harmless to humans.
The Whitebanded Crab Spider
This type of spider (pictured) are similar to their Goldenrod cousins, in that they can change color, although the process doesn't take so long, usually around 10 days. The Whitebanded spider also has a different facial structure to the Goldenrod, although they share much the same diet. The size range is slightly larger (5-12mm as opposed to 6-11mm) but are less likely to be found in northern US states than the Goldenrods.
Watch as a couple of a pet dogs get up close to a white crab spider: