To alleviate that, make sure the source of your buying is reliable and that your DEF container is always sealed. One of the best outlets for DEF is commercial truck dealership shops. I don't use the stuff in my semi truck, as I did a end run around the regs and bought a 2013 truck and put a 2000 MY built engine in it. But a local Mack dealership near me will fill any vehicle or container that one brings in, and charges only $1.79 a gallon for DEF from their bulk tote. Truck stops have criminal DEF prices, even though they are lower than buying the 2.5 gallon stuff off the shelf.
Just make sure you use what you buy in a timely manner. DEF has a shelf life.