Methods of determining the quality of milk due to the presence of mastitis, by monitoring the concentration of various ions, and ratios of those ions, in the milk. As the level of mastitic infection progresses, the concentrations of sodium and chloride ions increase and the concentrations of potassium and calcium ions decrease. A ratio of sodium to potassium is more sensitive to infection detection than either ion concentration alone. Similarly, sodium to calcium, chloride to potassium and chloride to calcium is more sensitive than any of the ions alone.