Effect of lipoxygenase activity in defatted soybean flour on the gelling properties of soybean protein isolate

Soybean lipoxygenase was inactivated to different degrees by dry heating of defatted soybean flour for 0, 5, 10, 15, 20 and 25 min and soy protein isolates were prepared thereof by isoelectric precipitation of the water extract of the defatted soybean flour. The fluorescence emission intensity at 420 nm of the chloroform–methanol extract of soy protein isolates, which was indicator of the existence of peroxidized lipid, varied in parallel with the lipoxygenase residual activity in defatted soybean flours. The dispersion of soy protein isolate showed an increasing turbidity with the increase of lipoxygenase residual activity in the starting defatted soybean flour, suggesting an elevated tendency to form insoluble aggregates during the preparation of soy protein isolate. Small deformation rheological test revealed that the gelling times were shorter for those soy protein isolates derived from low lipoxygenase activity defatted soybean flours than that of high lipoxygenase activity. Frequency sweep showed that G′ of soy protein isolate derived from low lipoxygenase defatted soybean flour was independent of oscillation frequency in contrast to that of derived from non dry-heated defatted soybean flour, the latter showed a marked frequency dependence. Large deformation test revealed that the gel hardness increased about 10 times after dry heating of defatted soybean flour for 20 min. As the increase of the lipoxygenase residual activity, the gel permeability increased markedly, suggesting that soy protein isolate from high lipoxygenase defatted soybean flour produced coarser textured gel, which corresponded well with the results of scanning electron microscopy.