The use of enzymes for thermal process monitoring: modification of milk alkaline phosphatase heat resistance by means of an immobilization technique

Milk alkaline phosphatase (AP) was immobilized by adsorption on to a synthetic polymer that is reversibly soluble–insoluble depending on the medium pH value. Immobilization experiments were conducted using different carrier concentrations. The relationship between carrier concentration and amount of added AP was expressed in terms of Molar Ratio (MR) which ranged from 0.6 to 16.0 mol of carrier available for each mole of enzyme. The effect of MR value on the activity and on the heat stability of the immobilized enzyme was investigated. The amount of active adsorbed enzyme depended on MR value and ranged from 13.3 ± 1.4 to 8.0 ± 0.3 Enzymatic Units (EU) for MR value of 2.0 and 16.0, respectively. An average D63 °C value of 130.0 ± 2.5 s was calculated for the enzyme free form and values equal to 148.0 ± 4.4, 122.7 ± 5.2, 154.3 ± 7.7, 113.3 ± 6.1 and 77.0 ± 2.9 s were found for adsorbed enzyme samples with MR of 0.6, 2.0, 3.8, 9.8 and 16.0, respectively. Over the temperature from 63 to 71 °C the following z values were computed: 6.5 °C for the free enzyme, 11.4 and 4.6 °C for the adsorbed form with MR of 0.6 and 3.8 respectively.