Properties of pullulanase debranched cassava starch and type-III resistant starch.

The objective of this study was to produce type-III resistant starch (RS-III) by the pullulanase reaction. A 10% (dry weight) cassava starch suspension adjusted to either pH 5.0 or 5.5 was gelatinized at 120°C for 30 min. Pullulanase of 3%, 5% and 10% (v/w) of starch weight was added to debranch the starch and placed in a water bath of 50°C for hydrolysis periods of 2 to 24 h. The resulting starch was retrograded by a cooling of 4°C and dried by hot air at 40°C. The starch products were determined for reducing sugars, amylose, RS-III, and for in vitro starch digestibility, as well as changes in the structural properties. The results showed that the reducing sugars obtained with any treatments tended to increase with the length of the reaction time. The starches treated at pH 5.0 or pH 5.5 and hydrolyzed with 5% pullulanase for 8 h had significantly higher reducing sugars of 1.57±0.1 and 3.43±1.2 g/100g, respectively, than the initial content (0.62±0.0 g/100g) and showed more effect than the 3% pullulanase. At pH 5.0, the starches hydrolyzed with 3 and 5% pullulanase for 8 h gave a higher amylose content than those treated at pH 5.5. An acidity effect of pH 5.0 related to the content of RS-III formed over the reaction time of 8 h, showing a high value of 12.8±1.3 and 17.4±1.5 g/100g for the 3% and 5% pullulanase, respectively. When the starch was reacted with 10% pullulanase for 8, 16 and 24 h, a significant increase in the RS-III from the gelatinized cassava starch (9.2 ± 0.0 g/100g) resulted with a value of 41.2±3.5, 45.8±2.5 and 42.5±1.3 g/100g, respectively. This result also related to the in vitro starch digestibility of the RS-III samples being about 20 to 30 % slower than the starting starch after 90 min of amylase digestion. Finally, the structural changes to the type-B crystallites via type-C and a mixture of the Vtype, as well as the scanning electron micrographs of the RS-III could confirm its property of slow enzymatic digestion. Thus, conditions of pH 5.0, hydrolysis of 10% pullulanase for 24 h and hot air drying were suitable for partially debranching amylopectin of the cassava starch, consequently providing small linear fragments and small clusters of the amylopectin for recrystallization and formation of the RS-III.