Oh man, sour milk is one of the hardest smells to deal with because the dairy fats go rancid deep inside the cushion padding! Surface sprays won't work because they can't penetrate the foam where the bacteria is growing. I had a similar accident with a milk formula spill and hired Chicago Couch Cleaning to sanitize the entire section. Their deep steam extraction reaches right into the core of the cushion to flush out the sour milk residues completely. For parents seeking thorough professional upholstery cleaning services in Chicago, they are an absolute lifesaver. The sour odor vanished completely, and the sofa smelled fresh again. Definitely let them deep-clean it!