My kitchen drain has a buildup. This same thing happened about 5-6 yrs ago. I replaced the entire 2" pvc across the basement ceiling back in 2006. And I replaced it again last week.
The only thing hooked up to this line is the dishwasher & the kitchen sink. No garbage disposal. I thought it could be undissolved dishwasher detergent. So, only used liquid since the last time I replaced this drain pipe.
But not. The substance is a light greasy substance and it will float in water. We are careful to screen particles in the drain. I think it's peanut butter from the knives and years of peanut butter jelly sandwiches!
I installed a giant 3" trap in the basement ceiling hooked together with rubber boot connectors so I can drop it next year in order to clean it out.