Effects of high pressure processing on soymilk enzymes, proteins, and isoflavones
The objective of this research work was to investigate whether high pressure processing (HPP) could qualify as an alternative to the conventional thermal treatment of soymilk based on its effects on some nutritional and quality aspects. Thermal and pressure inactivation of lipoxygenase, [Beta]-glucosidase and trypsin inhibitors (Kunitz, KTI, and Bowman-Birk, BBI) in buffer solutions was described by a first order kinetic model. [Beta]-Glucosidase was more thermostable than lipoxygenase (55-65°C), but less pressure stable (450 and 500 MPa). While at 95 and 100°C KTI was more thermolabile than BBI, at 120°C their inactivation rate constants were similar. BBI was more pressure stable than KTI at 600-800 MPa and 75°C. The differences in stability of the enzymes and inhibitors may be attributed to their different disulfide bonds content. The enzymes behavior under thermal and HPP treatments differed due to different mechanisms of action of temperature and pressure on proteins. Raw soymilk was pressurized at 100-700 MPa and room temperature for 10 min. Above 400 MPa, [Beta]-glucosidase was more pressure stable than lipoxygenase. Pressure alone did not inactivate the inhibitors and combined pressure/temperature (75°C) treatments had to be applied. After soymilk was heated at 120°C for 10 min, the total trypsin inhibitor activity was 11% and that of BBI was 22%, suggesting that most of the activity left was due to BBI. The behavior of enzymes and inhibitors in soymilk and in buffer was different due to differences in environmental factors. Neither change in color nor in rheological properties of pressurized soymilk was observed. Native electrophoresis showed changes in proteins at [greater than or equal to] 300 MPa, suggesting dissociation into subunits and aggregation. Pressure denaturation occurred at 200 MPa for [Beta]-conglycinin and 300 MPa for glycinin. When soymilk was pressurized at 75°C, there was a shift in the distribution of isoflavones from 6"--O-malonylglucosides towards [Beta]-glucosides, which was related to the effect of adiabatic heating. HPP of soaked soybeans increased the total isoflavone concentration of soymilk by 19% at 700 MPa. However, 6"-0-malonylglucosides and [Beta]-glucosides decreased at 300 MPa. The decrease in aglucons content at [greater than or equal to] 400 MPa might be related to the decrease in protein extractability.