ABSTRACT
Alkaline phosphate (ALP) from bovine milk was inactivated by pulses of high intensity electric fields (PEF). Alkaline phosphate is an indicator of the adequacy of thermal pasteurization of milk. Fluorometric analysis of ALP was used to assay the activity of ALP from raw whole milk, 2% milk, nonfat milk and modified simulated milk ultrafiltrate (MSMUF). Milk and MSMUF were treated with electric field intensities of 18.8 and 22.0 kV/cm, respectively. The activity of ALP was reduced by 65% in milk and MSMUF and 59% in raw milk and pasteurized, homogenized 2% milk. The temperature of milk treated with 70 pulses of 21.8 kV/cm increased from 22°C to 43.9°C. The temperature of MSMUF treated with 70 pulses of 22.3 kV/cm increased from 4°C to 8.4°C. The temperature increase did not contribute to the inactivation of ALP. Maximum velocity (Vmax) and Michaelis constant (Km) of ALP treated with PEF were determined. Seventy 0.46 msec pulses of 21.8 kV/cm reduced the Vmax of ALP in milk from 51.4 to 42.9 mg of fluoroyellow/min/50 mL and increased the Km of ALP in milk from 0.69 to 1.77. Seventy 0.78 msec pulses of 22.3 kV/cm increased the Vmax of ALP in MSMUF from 13.3 to 24.4 mg of fluoroyellow/min/50 mL and increased the Km of ALP in MSMUF from 0.12 to 0.54. Native ALP is resistant to trypsin proteolysis. ALP treated with seventy 0.78 pulses of 22.3 kV/cm was susceptible to partial proteolytic digestion by trypsin.
