Determining the quality of milk due to the presence of mastitis by monitoring the oxidation reduction potential (ORP) of the raw (unpasteurized) milk. The ORP of the milk is tested by a sensor placed into the milk soon after having been removed from the animal (e.g., cow) and prior to the milk being combined in a bulk tank with milk from other animals. The sensor can be positioned in the line carrying the milk from the cow, allowing real-time testing of the milk. Each mammary gland (e.g., quarter) is tested separately, and the ORP levels are compared. Any gland (e.g., quarter) having a level that deviates from the other levels by a predetermined amount is considered suspect for mastitis.