EFFECTS OF pH, PROTEIN, FAT, AND CALCIUM ON DIFFUSE REFLECTANCE OF MILK

Cottage cheese and yogurt product quality and process control would improve if plant operators had a stable and sanitary device for pH measurement. The objectives of this research were to determine the effects of pH, protein, fat, and added calcium on diffuse reflectance of milk in the wavelength range of 350 to 990 nm and determine if milk pH correlates with diffuse reflectance over the range of treatments tested. Diffuse reflectance of milk was measured at pH levels in the range of 5.5 to 7.0 and at varying levels of fat, protein, and added calcium concentrations. Diffuse reflectance was found to decrease with pH, increase linearly with protein and fat concentration, be affected by milk pretreatment, and be affected by the interaction between calcium and protein. A pH prediction model which included reflectance data and product variables (fat, protein, pre-heat treatment) explained 74.4% of the variance and had a standard error of 0.29 pH units. Because milk constituents and pretreatment affect diffuse reflectance, this method would be limited in applicability to systems where factors such as fat, protein, calcium, and pretreatment would be relatively constant, such as in yogurt and cottage cheese production.