Analysis of reaction kinetics occurring at thermal inactivation of the milk enzymes adenosine deaminase, alkaline phosphatase and gamma-glutamyltransferase

The pasteurizing steps for high-temperature and short-time pasteurization o f liquid milk for consumption as, determined in the German Milk Ordinance, are characterized by the inactivation of the milk enzymes alkaline phosphat ase (ALP, EC 3.1.3.1) and lactoperoxidase (POD, EC 1.11.1.7). The present s tudy did not only deal with the aforementioned enzymes, but also with the t hermal stability of g-glutamyl transferase (gamma-GT, EC 2.3.2.2) and adeno sine deaminase (ADA, EC 3.5.4.4). Except from POD, the inactivation kinetic s of the enzymes ALP, gamma-GT and ADA were described and compared. The mil k samples were produced in a pilot plant for indirect milk heating by using replaceable heat holders. During the first trial series, heating temperatu res from 60 degrees C to 90 degrees C and holding times from 20.2 to 163 s were selected. During the second trial series, in order to have a maximum A DA activation, the fat-adjusted initial milk was heat-treated for 2 min at 70 degrees C and subsequently submitted to temperatures from 82 degrees C t o 90 degrees C and to holding times from 2 to 3.7 s in a pilot heating plan t. As expected, the enzymatic activity of ALP and gamma-GT was reduced or inac tivated in the range of 60 degrees C to 75 degrees C. The inactivation of b oth enzymes within this temperature range could be described by first order reaction kinetics. in direct comparison, gamma-GT showed a higher thermal stability than ALP. This fact was also confirmed by a higher activation ene rgy for gamma-GT than for ALP. The ADA activity, when compared with the fat -adjusted initial milk, was essentially increased in the temperature range from 60 degrees C to 75 degrees C and with holding times up to 163 s. Thus the maximum ADA activity was achieved in the short-time heating phase. In t he temperature range from 82 degrees C to 90 degrees C, the ADA enzyme was inactivated; in this temperature range, the activation energy for the ADA i nactivation was similar to that of ALP and gamma-GT in the temperature rang e from 60 degrees C to 75 degrees C. Additionally, ADA had a higher thermal stability than POD. The results as regards the reaction kinetics presented in this study allowed the statement that apart from ALP gamma-GT is suited for characterizing the short-time heating range of liquid milk for consump tion. Furthermore, in accordance with results from an earlier study dealing with the ADA activity in commercially available milk, the milk enzyme ADA is an appropriate heat indicator for differentiating between short-time hea ted and high-heated market milk.