Effects of wavelength range and light path-length on the accuracy of the constituent analysis of unhomogenized milk using near-infrared spectroscopy

"We carried out a preliminary experiment for the development of on-line monitoring of raw milk constituents using near-infrared spectroscopy. We took precise measurements of major constituents in raw milk such as fat, protein and lactose in order to investigate effects of wavelength range and light path length on the accuracy of near-infrared measurement. A spectrophotometer (NIRS 6500) was used for near-infrared measurement and MilkoScan S54A was used as a reference constituent analyzer. Light path length (sample cell thickness) was 1, 4 and 10mm and wavelength ranges were set to 400-1,098nm; 600-1,098nm; and 1,100-2,498nm. As a result, the following accuracies were obtained: For fat, SEP=0.17% for the light path-length of 1mm and a wavelength range 1,100-2,498nm, and SEP=0.38% for the path-length of 10mm and a wavelength range 400-1,098nm; for protein, SEP=0.15% for the path-length of 1 mm and a wavelength range 1,100-2,498nm, and SEP=0.14% for the path-length of 10mm and a wavelength range 400-1,098nm; and for lactose, SEP=0.10% for the path-length of 1 mm and a wavelength range 1,100-2,498nm, and SEP=0.07% for the path-length of 10 mm and a wavelength range 400-1,098nm. It was thus confirmed that accurate measurements could be obtained in the longer wavelength range (1,100-2,498nm) and in the shorter wavelength range (400-1,098nm) when longer path-length was used."