What sanitation problem would baffles create inside a tank used to haul milk?

49 CFR 383.119(c); AAMVA CDL Manual Section 8.2.5, "Un-baffled Tanks," p.8-8-2

Hygiene requirements for food-grade cargo can directly override a design that would otherwise improve ride stability. Recognizing this tradeoff helps a driver understand why some tankers are inherently harder to control even though a safer baffled design exists.