Solid-phase extraction of staphylococcal enterotoxin A in dairy products using an ion exchange resin

Abstract

In this study, we developed a solid-phase extraction method for staphylococcal enterotoxin (SE), especially SE type A (SEA), in milk and dairy products. Ion-exchange resin, a cation-exchanger, was applied for the extraction of SEA, which was then measured using a sensitive fluorescence immunoassay. First, the optimal pH for the binding of SEA to the cation exchanger was determined to be pH 4.0. Second, the optimal volume of elution buffer, consisting of phosphate buffer with 0.5 M sodium chloride at pH 8.0, was estimated to be 6 ml for a column containing 4 ml wet volume of the cation-exchanger. Food samples (10 ml each) were then studied for the application of the extraction method. High recovery of SEA was obtained from raw milk (approximately 90%), sterilized milk product (approximately 60%), and skimmed milk powder (approximately 60%), whereas the recovery from cheese (approximately 40%) was lower than from other samples. These recoveries were almost constant at different concentrations of spiked SEA. The minimum measurable level with this extraction method was very low, being 0.25 ng/g in skimmed milk powder for example. With a larger volume (60 ml) of sample, this level was as low as 0.025 ng/g SEA in skimmed milk powder. The present extraction method was very simple and time saving in comparison with the conventional method with chloroform and trichloroacetic acid. The results of this study indicate that this extraction method could be applicable for the measurement of SEA in milk and dairy products.