A comparison of time-resolved fluoroimmunoassay and ELISA in the detection of casein in foodstuffs

With the aim of lowering the detection limit for casein in foods, three competitive assays are described: direct time-resolved fluoroimmunoassay (TR-FIA), using europium-conjugated antibody, indirect TR-FIA, using biotinylated antibody with europium-conjugated streptavidin and ELISA, using a HRP-conjugated secondary antibody. Food samples ( instant potato, flour mix, packet soup, spice-mix) were analysed. Standard curve sensitivities in direct and indirect TR-FIAs did not differ significantly ( p = 0.097), but both TR-FIAs were considerably less sensitive ( both p< 0.0001) than ELISA (LOQs 1.3, 1.5 and <1.0 mg kg(-1), respectively). The precision and working analyte range was similar in all three methods. Casein content measured in food products was comparable, using the three assays and rocket immunoelectrophoresis. The TR-FIA approach provided no improvement over the ELISA. All three assays allowed quantification of casein in foodstuffs in the order of 1 - 1.5 mg kg(-1), providing a basis for more rigorous validation and collaborative testing.