Automated refractive index measurement of catalyst‐laden edible oils undergoing partial hydrogenation

Abstract A planar optical waveguide, excited by monochromatic light, becomes a sensor for measuring the refractive index (RI) of edible oils. RI is commonly used to determine iodine value (IV). This process sensor operates in the presence of nickel catalyst and diatomaceous earth (DE) powders. An on‐line refractometer system, incorporating this sensor, demonstrates an accuracy of ±0.0001 RI units (RIU). Resolution is 0.00001 RIU. The waveguide refractometer is based on a loss in light transmission due to a temperature‐induced matching of waveguide and oil RIs. An algorithm adjusts the RI to conform to the temperature and wavelength as specified in AOCS Method Cc 7–25 ( Official Methods and Recommended Practices of the American Oil Chemists' Society , edited by D. Firestone, American Oil Chemists' Society, Champaign, 1989), the accepted procedure for RI measurement with an Abbé refractometer. The theory, construction and performance characteristics of a prototype waveguide refractometer are described. Samples of partially hydrogenated soybean oils (IV ranges=106–75) are first examined with the Abbé refractometer, then tested in the prototype waveguide refractometer. Catalyst and DE are subsequently added to each sample, and RI data are taken again with the waveguide refractometer. Results confirm the instrument's ±0.0001 RIU accuracy and its immunity to interference from catalyst and DE. Real‐time operation holds promise for automatic control, improved production efficiency, and better batch‐to‐batch consistency of the hydrogenation process.