what differences if any, would assay show if performed using milk that was past its expiration date and had an off-putting odor.

The color will disappear.

Explanation:

Due to the absence of oxygen in the expired milk, the actual color of milk disappear. There are certain bacteria such as Pseudomonas present in the milk which is responsible for the change in color. With the passage of time, Pseudomonas starts growing in the milk and release certain toxin which spoils milk. They grow in both condition i. e. in the presence and absence of oxygen. Change in color is due to chemical releases by Pseudomonas.