Effect of the extent of conversion and retrogradation on the digestibility of potato starch

The effect of starch conversion on the susceptibility of potato granules to alpha -amylase was studied by direct sampling at different pasting times corresponding to different points on the RVA profile of a 6.4% (w/w) suspension of starch in distilled water. Native granules showed high resistance to alpha -amylase with 8.6 +/- 0.4% digestibility for a 6 h incubation period with the enzyme. When the suspension was heated to 60 degreesC, the digestibility increased to 53.5 +/- 0.7% although, at this temperature, there was still no noticeable increase in the measured viscosity (less than or equal to 0.040 Pa . s). The material sampled after a pasting time corresponding to the RVA peak viscosity showed a digestibility of 88.4 +/- 0.5%. This suggested, owing to the expected retrogradation of amylose on cooling, the quasi-total susceptibility of amylopectin to enzymatic digestion at this pasting stage. The effect of ions on the swelling of potato starch was used to assess whether the decrease of the swelling of the granules in the presence of NaCl was paralleled by an increase in resistance to alpha -amylase. A small (similar to 6.1%) but significant decrease in the digestibility of pasted starch was observed in the presence of salt. Finally, the effect of the retrogradation of the amylopectin fraction on its digestibility was assessed in extruded potato starch ribbons containing 35% (w/w) water and stored at different temperatures. After 14 days of storage, the digestibility decreased from 77.0 +/- 0.9% in the freshly extruded samples to between 28.0 +/- 1.7% and 42.1 +/- 0.3%, depending on the storage temperature. This suggested a measurable difference in the alpha -amylase susceptibility between the A and B polymorphs of retrograded amylopectin.