Irradiation of maize and bean flours: effects on starch physicochemical properties

In an earlier study (Rombo GO et al, J Sci Food Agric 81: 497-502 (2001)) it was found that the in vitro starch digestibility of raw and cooked maize and bean flours was increased by irradiation of the flours at 2.5 kGy, but there was a reduction in starch digestibility at doses above 2.5 kGy. Experiments were performed to determine what effects irradiation had on the molecular properties of the starch in maize and bean flours. Increasing irradiation dose caused an increased proportion of (beta)(1-3)- and (beta)(1-4)-bonded starch in bean and maize flours. Starch containing (beta)-bonds is only partially digestible by porcine pancreatic (alpha)-amylase, and this may in part explain the reduction in starch digestibility at higher doses. Size exclusion-high-performance liquid chromatography showed that higher irradiation doses led to a reduction in the molecular size of amylopectin in both bean and maize starches, which presumably involved debranching and an increase in the production of short, straight-chain molecules. With increased irradiation dose there was an increase in the crystallinity of the amylopectin fraction of the bean starch as shown by differential scanning calorimetry, presumably at least in part due to (beta)-bonding and amylopectin depolymerisation. These two factors and their interrelated effects are probably responsible for the observed slight reduction in maize and bean porridge starch digestibility reported previously.