Bovine casein (Sigma Chemical Company; St. Louis, MO) was dissolved by stirring in buffer at a concentration of 1%. The skim milk powder (SMP; supplied by Fonterra [Australia] Pty. Ltd. Cobden, Australia) substrate was prepared in a similar manner at a concentration of 36 mg mL-1 so that the substrate mixture contained 1%protein. The buffer used was Tris HCl at a concentration of 100 mM(to achieve a final pH in the assay of 7.0) or 3 M(to achieve final pH in the assay of either 7.0 or 8.0). The preservative used was 12 mMthiomersal, of which 92 mL was added per milliliter of assay mixture. The assay mixture comprised 900 mL of substrate and 300 mL of sample. Each mixture was shaken vigorously to ensure thorough mixing. Protease action is reported as the increase in free amino groups, measured by the fluorescamine method, during the incubation period of the assay. For quantification
of free amino groups, the results were compared
with a standard curve (0–40 mM) of the dipeptide Leu-Gly.
The azocasein (Christen and Marshall 1984) and FITCcasein
(Christen and Senica 1987) assays were carried out as
previously described using chemicals obtained from Sigma
Chemical Company. Unless stated otherwise, incubation for
24 h at 37C was used for both the azocasein and FITC-casein
assays. The FITC-casein assay mixture comprised 69 mL of
0.25% type III FITC-casein in 10 mM sodium phosphate
buffer at pH 8.0, and 17 mL of the UHT milk sample. Prior to
fluorometric measurement at a wavelength of 490 nm (excitation)
and 515 nm (emission), casein and unreacted
FITC-caseinwere precipitatedwith 14 mL of 306 mMtrichloroacetic
acid. Following centrifugation, 50 mL of the supernatant
was added to 1.2 mL of 300 mM phosphate buffer at
pH 8.5.