Infrared estimation of fat, protein, and lactose in milk: evaluation of multispec instrument.

Thirty-eight milks (including homogenized and unhomogenized herd milks, individual cow milks, and packaged milks) were pre-analyzed by accepted standard methods and then analyzed in duplicate with a Multispec instrument on 2 different days. For the combined data, mean differences between duplicates were 0.01% or less within days and 0.025% or less between days. Standard deviations of difference between duplicates were 0.02% or less. Overall mean differences between Multispec and standard results were 0.02% or less with standard deviations of difference of 0.041%, but means for protein in packaged milks were 0.038% lower and means for lactose in individual cow milks were 0.04% higher than standards. Standard deviations of difference for fat were lower (0.016%) than average for packaged milks and higher (0.06%) than average for individual cow milks. Differences between Multispec results for homogenized and unhomogenized samples of the same herd milks were small, averaging 0.013% for fat, 0.005% for protein, and 0.011% for lactose. Regression equations for the combined data gave standard errors of estimate of about the same magnitude as the standard deviations of difference. Single calibrations would give good estimates for all of the types of milk analyzed, but for best accuracy the instrument should be calibrated with the type of milk to be analyzed.