You need the dirt to get the good active.
According to the wall street journal "enzymes in Cascade detergent are designed to attach themselves to food particles.
Without food, the enzymes have nothing to latch on to.
So, basically, detergents won't know they need to clean if there isn't enough grime to tell them".
The rule of thumb is that one should scrap off all excess food from your plates before loading it into your dishwasher but don't actually clean the dishes before stacking them into the dishwasher because this will just create the opposite effect of having clean hygienic dishes.
The above theory is certainly food for thought, I would love to hear from you what are your thoughts and experiences.