Measurement ofβ‐casein in human milk by microparticle‐enhanced nephelometric immunoassay

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Abstract
A microparticle‐enhanced nephelometric immunoassay was developed for β‐casein quantification in human milk. It is based on the nephelometric measurement of the light scattered during the competitive immunoagglutination of a microparticle‐human whole casein conjugate with an anti‐bovine fi‐casein antiserum. This immunoassay is sensitive (detection limit in reaction mixture, 0.05 mg l‐1) and can be performed in diluted milk (1:6000 in reaction mixture), excluding any interference or sample pre‐treatment. It allowed the quantification of β‐casein over a large range of concentrations (0.6–18.3 g l‐1) with accuracy (linear recovery in dilution‐overloading assay) and precision (within‐ and between‐run coefficients of variation from 3 to 12%). The developed immunoassay was applied to the measurement of β‐casein concentration in 230 human colostrum, transitional and mature milk samples. In this preliminary study, the concentration was found to be significantly higher in transitional milk (3.1 g l‐1) than in colostrum (2.2 g l‐1). The ratio of β‐casein versus total protein was lower in colostrum (11%) than in transitional (23%) and in mature milk (22%).