INACTIVATION KINETICS OF LIPOXYGENASE IN PRESSURIZED RAW SOYMILK AND SOYMILK FROM HIGH-PRESSURE TREATED SOYBEANS

High hydrostatic pressure (HHP) treatment (275, 345, 483, 550 and 620 MPa), combined with thermal treatment (19, 50, 65, 70 and 80C), was applied for selected times to raw soymilk and to previously soaked soybeans before preparation of soymilk. Lipoxygenase (LOX) activity in HHP-treated raw soymilk was described using a first-order kinetics model. At low or moderate pressures, alone or in combination with thermal treatment, LOX activity exhibited high stability. Soymilks treated at 550 MPa exhibited high LOX inactivation rate constant values. Temperature dependence on the inactivation rate constants of soymilk treated at 550 MPa indicated that high activation energy was required to inactivate the enzyme in soymilk from HHP-treated soybeans compared with that for HHP-treated raw soymilk. Lipoxygenase activity was not detected in soymilk from HHP (620 MPa and 80C) treated soybeans.

PRACTICAL APPLICATIONS

Soybean processed products have to have the minimum amount of antinutritional factors. Soymilk has to be pasteurized to deliver a soy product with low microbial load and enzymes to avoid deterioration during storage. The traditional process to obtain this type of soy product is using thermal treatment; however, this treatment can change nutritional and sensory characteristics. An alternative approach to process soymilk is using high hydrostatic pressure treatments, alone or in combination with thermal processing, to deliver milk with fresh like sensory characteristics free from pathogens and deteriorative enzymes. Lipoxygenase could deteriorate soymilk fat to produce chemicals and off-flavor.     

EFFECT OF HIGH HYDROSTATIC PRESSURE ON SPORES OF GEOBACILLUS STEAROTHERMOPHILUS SUSPENDED IN SOYMILK

The inactivation of Geobacillus stearothermophilus spores (ATCC 7953) inoculated in soymilk was investigated using high hydrostatic pressure (550, 585 and 620 MPa) in combination with temperature (70, 80 and 90C) for selected times (2 s to 15 min). Inactivation of spores occurred at all selected treatments. Less than 10 CFU/mL of G. stearothermophilus were observed after 7 min of treatment at 620 MPa and 90C. An increase in the inactivation rate constant, at the highest pressure, was observed, resulting in a decrease in D values at all temperatures. D values were calculated as 10.6, 6.2 and 3.5 min for 70, 80 and 90C, respectively after pressurization at 620 MPa. z_p values decreased as temperature increased with values ranging from 142 to 238 MPa. The activation energy required for inactivation of G. stearothermophilus spores in soymilk, at the selected treatments, was in the range of 37.9–57.4 kJ/mol.