Physico-chemical properties and digestibility of native and citrate starches change in different ways by synchrotron radiation.

The physico-chemical properties and digestibility of native and citrate cassava starches changed as a result of synchrotron radiation treatment. In this study, the native and citrate starch samples were exposed to radiation doses of 0.1, 0.4, 0.8 and 3.9 kGy. The granular morphology revealed that all samples were rupture and damage after radiation. As increasing radiation doses, the relative crystallinity as determined by WAXS and the ratio of 1047/1015 cm-1 from FTIR result decreased while the degree of degradation and solubility increased for all samples. The swelling power of radiated native starches decreased with higher radiation doses indicating that the cross-linking of starch was induced by synchrotron radiation which was related to an increase in the resistant starch content. On the contrary, for radiated citrate samples, the FTIR peak at 1724 cm-1 was observed. The ratio of 1724/2900 cm-1 and total esterified citric acid did not change. The swelling and degree of di-esterification were reduced while the degree of mono-esterification increased with higher doses. It implied that the cross-linking by ester bonds was broken into mono-ester bonds. This work demonstrated that synchrotron radiation changed the physical and chemical properties of native and citrate starches in different ways.

Structure of branching enzyme- and amylomaltase modified starch produced from well-defined amylose to amylopectin substrates.

Thermostable branching enzyme (BE, EC 2.4.1.18) from Rhodothermus obamensis in combination with amylomaltase (AM, EC 2.4.1.25) from Thermus thermophilus was used to modify starch structure exploring potentials to extensively increase the number of branch points in starch. Amylose is an important constituent in starch and the effect of amylose on enzyme catalysis was investigated using amylose-only barley starch (AO) and waxy maize starch (WX) in well-defined ratios. All products were analysed for amylopectin chain length distribution, α-1,6 glucosidic linkages content, molar mass distribution and digestibility by using rat intestinal α-glucosidases. For each enzyme treatment series, increased AO content resulted in a higher rate of α-1,6 glucosidic linkage formation but as an effect of the very low initial branching of the AO, the final content of α-1,6 glucosidic linkages was slightly lower as compared to the high amylopectin substrates. However, an increase specifically in short chains was produced at high AO levels. The molar mass distribution for the enzyme treated samples was lower as compared with substrate WX and AO, indicating the presence of hydrolytic activity as well as cyclisation of the substrate. For all samples, increased amylose substrate showed decreased α- and ß-amylolysis. Surprisingly, hydrolysis with rat intestinal α-glucosidases was higher with increasing α-1,6 glucosidic linkage content and decreasing M¯w indicating that steric hindrance towards the α-glucosidases was directed by the molar mass rather that the branching density of the glucan per se. Our data demonstrate that a higher amylose content in the substrate starch efficiently produces α-1,6 glucosidic linkages and that the present of amylose generates a higher M¯w and more resistant product than amylopectin. The combination of BE→AM→BE provided somewhat more resistant α-glucan products as compared to BE alone.

Synergistic amylomaltase and branching enzyme catalysis to suppress cassava starch digestibility.

Starch provides our main dietary caloric intake and over-consumption of starch-containing foods results in escalating life-style disease including diabetes. By increasing the content of α-1,6 branch points in starch, digestibility by human amylolytic enzymes is expected to be retarded. Aiming at generating a soluble and slowly digestible starch by increasing the content and changing the relative positioning of the branch points in the starch molecules, we treated cassava starch with amylomaltase (AM) and branching enzyme (BE). We performed a detailed molecular analysis of the products including amylopectin chain length distribution, content of α-1,6 glucosidic linkages, absolute molecular weight distribution and digestibility. Step-by-step enzyme catalysis was the most efficient treatment, and it generated branch structures even more extreme than those of glycogen. All AM- and BE-treated samples showed increased resistance to degradation by porcine pancreatic α-amylase and glucoamylase as compared to cassava starch. The amylolytic products showed chain lengths and branching patterns similar to the products obtained from glycogen. Our data demonstrate that combinatorial enzyme catalysis provides a strategy to generate potential novel soluble α-glucan ingredients with low dietary digestibility assets.