The scum will be broken down food particles and or limescale.
Use dishwasher salt - change the detergent - use a good quality rinse aid - run a cleaning tablet through the machine now and again - rinse the worst of the food debris off the crockery before loading the machine - remove and clean the filters in the bottom of the machine - check for blockages in the rotating arms - avoid the multi tabs.
Rinse Aid breaks down surface tension in the water - helps any dirt to drain off before drying - no rinse aid and the dirt particles get baked on hard.
Peccavi, January 2011
