Microparticle-Enhanced Nephelometric Immunoassay. 1. Measurement of _s-Casein and -Casein

¹ Laboratory of Applied Biochemistry, Associated with Institut National de la Recherche Agronomique, Faculty of Sciences, BP 239, 54506 Vandoeuvre les Nancy, France
² Laboratory of Immunology, Faculty of Medicine, BP 184, 54505 Vandoeuvre les Nancy, France
³ Sanofi Research, Montpellier, France

_s-Casein and -casein were measured in milk and curd by a microparticle-enhanced nephelometric immunoassay (NEPHELIA®). Specifically designed microspheres were coated with antigen (_s-casein and -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 _s-Casein and -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®, 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).

Key Words: nephelometric immunoassay • _s-casein • -casein

Submitted on January 22, 1991
Accepted on June 19, 1991