<title>Optical device for sensing the index of refraction of liquids with high turbidity</title>

Abstract

We discuss the use of photo-reflectance near the critical angle (PRCA) to monitor small changes of the RI of highly turbid liquids. The theory of the reflectance of a laser beam near the critical angle for an external medium with a complex RI is summarized. The applicability of PRCA to sense highly turbid media is demonstrated experimentally on bovine milk samples. We give experimental results showing the temporal variation of the refractive index (RI) during three different processes in bovine milk: 1) Mechanical stirring, 2) temperature changes, and 3) pH variations around the isoelectric point of the casein micelles (micelle aggregation). RI changes in the order of a few times 1 x iO are observed during the experiments. The experimental results show that the RI of milk can be used to track physico-chemical changes in time allowing one to measure the time constant of the different process. The design of a compact RI probe for insitu applications is discussed. The miniaturization of such a probe will probably limited by factors other than the loss of sensitivity. A novel angle-of-incidence control which requires only linear displacements of some of the optical components (no rotation) is proposed and shown to be feasible. Such an optical probe may be used in the dairy industry and in general in the food industry or food science research laboratories. It could give additional analytical power to the food scientist, engineer, or technician.