Investigation of molecular origin and characteristics of maltodextrin-surfactant interactions
The objective of this research was to extend the functionality of maltodextrin in foods, by forming maltodextrin-surfactant complexes that had novel properties not exhibited by maltodextrin alone. To achieve this objective it was necessary to understand the molecular origin and characteristics of maltodextrin-surfactant interactions. A variety of analytical techniques were therefore used to characterize maltodextrin-surfactant interactions.
Initial experiments using isothermal titration calorimetry (ITC) and surface tensiometry showed that an anionic surfactant (SDS) bound to maltodextrin when the surfactant concentration exceeded a critical value (∼0.05 mM). ITC showed that the interaction of SDS to maltodextrin was exothermic, which could have been due to an exothermic coil-helix transition and/or an exothermic binding reaction. Surfactant binding to maltodextrin only occurred when the number of monomers in the maltodextrin chain exceeded ∼24 glucose units. NMR studies showed that the interaction involved carbons 1 and 4 of the D-glucopyranose residues of maltodextrin and the surfactant hydrophobic tail, which suggested the formation of a helical inclusion complex.
The effect of surfactant type on maltodextrin-surfactant interactions was also investigated. ITC, surface tension and ultrasonic measurements indicated that the charge on the surfactant head group influenced their binding to maltodextrin. Similar amounts of anionic and cationic surfactant bound to maltodextrin, but a much smaller amount of non-ionic surfactant bound. The ITC indicated that surfactants with longer tail groups bound more strongly to maltodextrin than surfactants with shorter ones.
The effect of temperature, pH, and salt concentration on maltodextrin-surfactant interactions was studied using ITC. This study indicated that the enthalpy changes associated with surfactant demicellization were highly temperature-dependent. In contrast, the binding of surfactants to maltodextrin was exothermic and relatively temperature-independent. There was no effect of pH on the binding of surfactant to maltodextrin. In contrast, salt concentration affected both surfactant demicellization and surfactant binding to maltodextrin.
A potential application of maltodextrin-surfactant complexes was demonstrated by studying the rheology and thermal behavior of maltodextrin solutions in the presence and absence of SDS. The rheology and thermal properties of maltodextrin solutions changed significantly when surfactant was added, e.g. the viscosity increased, gelation occurred, and a melting transition was observed.