Total inactivation of lipoxygenase in whole soya bean by pulsed light and the effect of pulsed light on the chemical properties of soya milk produced from the treated soya beans

The inactivation of lipoxygenase (LOX) in the whole soya bean prevents lipid oxidation that produces an off‐flavour of soya food. The inactivation of lipoxygenase in the whole soya bean by pulsed light (PL) was examined with three distances (5, 7 and 9 cm) from the PL strobe and for different durations. Soya bean was treated with PL with and without ice surrounding the soya bean sample tray for limiting the rise in sample temperature. Results show that without ice surrounding the sample tray, the lowest LOX residual activity was 4.7%, 0.4% and 0.0% for 80‐s duration at 5 cm distance from the PL strobe, 110 s at 7 cm from the strobe and 150 s at 9 cm from the strobe, respectively; the soya bean temperature after treatment was 109.6, 116.3 and 114.8 °C, respectively. The instantaneous temperatures of the soya bean core measured during PL operating were above 100 °C. The lipoxygenase band was disappeared after longest PL treatments of each distance compared with the LOX band control as assessed by electrophoresis. The pulsed light had no negative effect on peroxide value of produced soya milk. However, PL reduced significantly the total solid amount and changed the colour of the produced soya milk. The residual activity with sample cooling by ice during treatment was 79.0%, 98.8% and 95.7%, with sample temperatures of 81.7, 91.2 and 66.9 °C, respectively. This study indicates that PL illumination could fully inactivate LOX in whole soya beans, with the photo‐thermal effect of PL as the main factor responsible for the inactivation of LOX. Percentage residual activity of LOX after soya bean treated with PL for cool samples and normal samples for the longest time of each distance. Numbers in Celsius are soya bean temperatures.