Lately I've had several jugs of milk smell sweet after a week or so of being opened. As those microbes work to digest milk solids (lactose, proteins, and lipids), they sometimes create excess hydrogen sulfide. It's not going to suddenly flip a switch and just "turn" from one minute to the next. It is a sweet smell, and seems to taste ok, but the smell has me wondering as it's definitely not the normal milk smell. If it doesn't smell sour and it tastes sweet, it's good milk… … The scent is hard to miss and gets stronger with time. It is not sour smelling, nor is it lumpy. The odor and taste of grass or corn silage, legume hay, and brewer's grains are the ingredients causing most of the problems. It will smell like spoiled milk if it's bad. This is why, post-cheese, your mouth smells like a … The two methods which odor can be transferred to milk is: Nose or mouth → lungs → blood → milk; Mouth → digestive tract → blood → milk; Unclean Flavor. It is a gradual spoilage. Signs your milk has spoiled When milk begins to spoil, it develops an unpleasant, rancid odor.