Effect of high-pressure treatment on lipoxygenase activity

Solutions of commercial soybean lipoxygenase (100 mu g/mL) in 0.2 NI citrate-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 enzyme had maximum stability over the pH range 5-8 in untreated samples and after treatment at 200 MPa, but with increasing pressure, the pH stability range become narrower and centered around pH 7-8. The enzyme was more sensitive to acid than alkali, and at pH 9, it lost virtually all activity after pressurization at 600 MPa for 20 min in both buffers. The activity of the crude 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 pressure of 400 MPa for 10 min caused a significant decrease in activity and treatment at 600 MPa led to complete and irreversible activity loss. Compared to unpressurized tomatoes, treatment at 600 MPa gave significantly reduced levels of hexanal, cis-3-hexenal, and trans-2-hexenal, which are important contributors to 'fresh' tomato flavor, and this was attributed to the inactivation of lipoxygenase.