I have kept food in my car routinely for a long time. If you think it gets hot in Iowa, try Arizona or California. I have always used common supermarket stuff for this purpose - canned goods, foil wrapped bars, etc - nothing fancy or particularly expensive. It is true the heat will degrade them over time. The solution is simple - switch them out on a periodic basis - about every six months seems to work for the situations I have experienced. If you carry water (far more important) it should be changed regularly as well.