Effect of high-pressure treatment on lipoxygenase activity

Solutions of commercial soybean lipoxygenase (100 mu g/mL) in 0.2 NI citrat e-phosphate and 0.2 M Tris buffer were subjected to pressures of 0.1, 200, 400, and 600 MPa for 20 min. The enzyme was stable at atmospheric pressure (0.1 MPa) over a wide pH range (5-9). In citrate-phosphate buffer, the enzy me had maximum stability over the pH range 5-8 in untreated samples and aft er treatment at 200 MPa, but with increasing pressure, the pH stability ran ge become narrower and centered around pH 7-8. The enzyme was more sensitiv e to acid than alkali, and at pH 9, it lost virtually all activity after pr essurization at 600 MPa for 20 min in both buffers. The activity of the cru de enzyme extracted from tomatoes treated at 200 and 300 MPa for 10 min was not significantly different from that of the untreated tomatoes, while a p ressure of 400 MPa for 10 min caused a significant decrease in activity and treatment at 600 MPa led to complete and irreversible activity loss. Compa red to unpressurized tomatoes, treatment at 600 MPa gave significantly redu ced levels of hexanal, cis-3-hexenal, and trans-2-hexenal, which are import ant contributors to 'fresh' tomato flavor, and this was attributed to the i nactivation of lipoxygenase.