Blanching of green bean (Phaseolus vulgaris)

Experiments with one and two steps blanching of green beans have been carried out. Inactivation of the peroxydase requires more heating than inactivation of the enzymes which gives rise to off flavour from aldehydes. When blanching for about one minute to inactivate lipoxygenase, aldehyde formation of flavour ceases. The content of vitamin C decreases during blanching according to a first order reaction. Since considerable loss of vitamin C occurs during blanching, the treatment time should be reduced to a minimum. During preblanching at 65–75 °C and final blanching, chlorophyll is degraded to pheophytin and the surface colour expressed by the Hunter-values (–a/b) increases with time which means that the colour of the beans changes from green to yellow. The firmness of beans, which was measured by use of a tenderometer, decreases during blanching according to a first order reaction with 40 kcal/mole activation energy. Preblanching at 65–75 °C increases the firmness of the beans linearly with treatment time. This increase in firmness is stable after final blanching at 95 °C and even after thawing of frozen beans.