Effect of sucrose, isomalt and maltodextrin on texture and morphology of tapioca starch before and after ultrasound treatment

Abstract
Background and objectives: native starch might not be able to be used in some food applications, therefore it should be modified, physically or chemically. Food producers tend to use modified starch with desirable properties. Production of pregel starch as a modified starch obtained by gelatinization and drying of native starch and can be used as stabilizer in food products. Modified starch would increase water holding capacity, resistance to heat and viscosity and decrease the synersis of starch. Ultrasound treatment is one of the starch physical modifications, which is more appropriate, due to its low chemical application, low time consuming and high selectivity. Fractures, rupture of the polymer chains and mechanical damage occur in starch following ultrasound treatment, because of the disintegration of bubbles, existing high velocities of liquid layers near the starch granules, and shear forces, lead to rearrangement of intra- and inter-molecular hydrogen bonding between water and starch molecules, therefore changes in starch structure, size and shape, water absorption and solubility as well.

Materials and methods: Present research investigated the effect of different levels (4, 8 and 12%) of sucrose, isomalt and maltodextrin on the microstructural (by scanning electron microscopy (SEM)), textural properties (by texture analyzer (TA)) and freeze –thaw stability (synersis) of tapioca starch prior to and following ultrasonic-assisted pregelatinization (400 W, 10 min at 60 ºC). In a completely randomized design, the experimental data were obtained and Duncan's new multiple range test was applied (p ≤0.05). All statistical computations and analyses were carried out by SAS software Version 22 in triplicate.
Results: According to SEM analysis, the spots, deformations and cross-linking created by the polyols were more significant after ultrasound assisted pregelatinizion due to the disruption of starch granules and more permeable structures due to ultrasonic treatment, resulting in open structures and more exposed hydroxyl groups, and facilitating starch-sugar cross-linking, interactions and connections. Results from texture analysis revealed that by increasing the amount of sucrose, isomalt and maltodextrin from 4% to 12%, tapioca gel starch hardness and cohesiveness increased significantly and this affection is more obvious after ultrasound assisted pregelatinization. At all sugar levels, synersis decreased, due to the cross- linking of polyol-starch, and less amylose reassociation. On the other hand, by using polyols and ultrasonic treatment, stability to freeze-thaw increased. The rearrangement of intra- and inter-molecular hydrogen bonding between water and starch molecules resulted in the disruption of molecular orders, hence the more accessibility of hydroxyl groups.
Conclusions: the presence of polyols significantly affected the properties of starch granule. Polyol addition to tapioca starch following pregelatinization via ultrasonic process, created more obvious changes in microstructure, textural properties and freeze-thaw stability of starch gel due to the higher accessibility of hydroxyl groups, and more cross-linking and connections among the examined polyol sugars; isomalt was more efficient than sucrose and maltodextrin in manipulating all the starch properties.
Keywords: Starch, Polyol sugar, Ultrasound-assisted pregelatinization