The thermal behaviour of the milk alkaline proteinase, plasmin, was studied in acid and sweet (rennet) whey; indigenous (bovine) plasmin was studied in the former system, but endogenous porcine plasmin was added in the latter, due to the very low levels of residual plasmin. The inactivation of plasmin in both systems followed first-order inactivation kinetics, which was consistent with previous observations of plasmin inactivation in milk and model milk systems. The thermal inactivation of plasmin in acid whey (D-90 (C) = 108 + 29 min, z = 24.5 + 1.2 degrees C) was much slower than in the sweet whey system (D-90 (C) = 0.021 + 0.006 min, z = 7.3 +/- 0.3 degrees C). Similarly, denaturation of beta-lactoglobulin (beta-lg) followed a first-order inactivation profile and this protein was also more heat stable in acid whey (D-90 (C) = 86 +/- 76 min, z = 13.7 +/- 1.5 degrees C) than sweet whey (D-90 (C) = 0.81 +/- 0.29 min, z = 9.1 +/- 0.5 degrees C). While it is possible that the increased heat stability of plasmin in acid whey is due to reduced sulphydryl/ disulphide interchange reactions between plasmin and beta-lg, it also appears that structural changes in the plasmin molecule were a significant contributory effect on the thermal stability of plasmin in this system. Increasing the pH of acid whey decreased the heat stability of plasmin. However, adjusting the pH of sweet whey had little effect on the heat stability of plasmin. Overall, severe heat treatments may be required to ensure inactivation of the enzyme in acid whey, but a balance is required between reducing the activity of plasmin and maintaining the functionality of whey proteins as food ingredients. (c) 2005 Elsevier Ltd. All rights reserved.