Since Jam making uses natural acids. Metal+Acid ------>salt and hydrogen. Since aluminum is high in the reactivity series it will react with the acid from the fruit quickly forming metal based salts in you jam. A steel pan which is lower in the reactivity series will not produce salt quickly so your jam wont be ruined.
A saucepan should be made out of a metal that conducts heat very well, that doesn't react with the food to make it taste differently and that isn't too expensive. Copper is one of the best conductors of heat but if you want a pan made fully out of copper it would cost a fortune. Aluminum is also a metal that conducts heat very well and isn't too expensive. Stainless steal is a good metal because it is tough, so withstands dishwashers and damages. It is non-reactive which means certain acidic foods won't be discolored, which may be the case with aluminum pans. A solution to this would be a stainless steal outside and have a copper or aluminum core. This way it still conducts heat well, doesn't cost too much and because it only has an aluminum core, the metal wont change the food.