MICROPARTICLE-ENHANCED NEPHELOMETRIC IMMUNOASSAY .1. MEASUREMENT OF ALPHA-S-CASEIN AND KAPPA-CASEIN

alpha-s-Casein and kappa-casein were measured in milk and curd by a microparticle-enhanced nephelometric immunoassay (NEPHELIA(R)). Specifically designed microspheres were coated with antigen (alpha-s-casein and kappa-casein) and then were agglutinated by specific antibodies. The light scattered by the agglutinates was quantified with a nephelometer. Antigen-coated microsphere agglutination was inhibited by free antigen solution, allowing its measurement. Calibration curves for alpha-s-casein and kappa-casein in milk and curd, performed with a low heat milk powder as standard, largely covered the usual concentrations of caseins. Accuracy (average ratios of recovery were 98.7 and 104.3%) and precision (coefficients of variation from 1.9 to 7.4%) assessed the fidelity of the method. NEPHELIA(R), applied to casein determination, offers many advantages over the classical methods of milk protein measurement: high dilution of the reagents, no pretreatment of the samples, enhanced sensitivity (few micrograms per liter), short reaction time (1 h), and easy use (no washing or phase separation).