Near infrared spectrometry of milk in its heterogeneous state

Composition analysis by NIR spectrometry usually uses models that assume th e sample to be homogeneous. However, agricultural fluid products are natura lly heterogeneous, and in some cases of on-line sensing, it is very difficu lt to fulfill the homogeneity requirement, and some precautions are needed. The how itself might create a non-uniform distribution of the measured ing redient concentrations across the cross-section of the flow. A modified dis crete model was developed, which incorporates the concentration distributio n of the ingredients. The effects of model parameters (number of layers and of iterations, valid ranges of absorbency and specular reflectance factors ) were studied and evaluated. Samples of fresh raw milk were tested, in ord er to create a known distribution of the fat content in the sample, and wer e scanned by a NIR spectrometer. In the case of monotonic variation, along the sample, of the concentration of the measured ingredient (fat), the effe ct on the measured spectrum was similar to that of changes in the average c oncentration of a homogeneous sample. Comparison of the errors predicted by the model with the result of NIR measurements in heterogeneous conditions of fresh raw cow's milk gave close correlation in their direction and their relative size. This created the basis for a technique to evaluate other fl uids, in which the distribution of the concentration of the measured ingred ient could be evaluated and incorporated in the model simulations. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.