The determination of intact β-casein in milk products by biosensor immunoassay

•Quantitative measurement of intact β-casein in milk.•Use of a label-free optical biosensor immunoassay.•Direct sample dilution in buffer.•Direct detection over immobilised commercially available anti-β-casein antibody.•Regenerable sensor surface stable for at least 200 cycles. A label-free optical biosensor immunoassay exploiting surface plasmon resonance detection for the estimation of the β-casein content in bovine milk and milk products is described. Samples were prepared by direct dilution with buffer and the protein was detected, under direct assay conditions, through binding with a commercially available anti-β-casein polyclonal antibody immobilised on the sensor surface. Assay conditions, selectivity and the potential for non-specific binding were defined. Analytical performance parameters included a method detection limit of 2.3 mg mL−1 for fluid milk, an intermediate precision RSDiR of 10.7 % for a skim milk powder and a mean recovery of 101.7 %, with a single functionalised flow cell being stable for at least 200 cycles. The method was found to be rapid, sensitive, precise and accurate, and is reliable for a range of milk products containing intact β-CN, and provides a routine complement to alternative conventional immunoassay and separation-based methods.