The molecular size and water dynamics effects of polyethylene glycol on starch gelatinization properties

Abstract

The mechanisms by which molecules influence the gelatinization temperature (T_gel) of starch are of both scientific and practical interest, since gelatinization affects the structure, texture, and functionality of starch-containing food and industrial applications. This study was designed to understand how polyethylene glycol (PEG, chosen for its unique physicochemical properties and pure size fractions) affects the gelatinization parameters of normal amylose-containing starches from four botanical sources (wheat, corn, tapioca, and potato) with differing amylopectin fine structures. Sucrose was also studied, since it is more effective at antiplasticizing starch, exhibited by higher T_gels, than many other small molecules which has posed challenges for reducing added sugars in foods. Differential scanning calorimetry was used to measure starch gelatinization in water and 15–45 % w/w solutions of sucrose and PEG molecules ranging in size from 200 to 10,000 g/mol. Solution density, water activity, and dynamic viscosity were also measured, and samples were viewed by confocal image analysis. The T_gels of all starch types were higher in 45 % w/w solutions of all PEGs, regardless of size, than of sucrose. Smaller PEGs (<1000 g/mol, the size exclusion limit of starch granules) increased the T_gel more than the larger PEGs across all starch types and concentrations. Starch structural differences were associated with different extents of T_gel variations and ranges, with cereal starches exhibiting greater variation in the presence of PEG. Smaller PEG resulted in higher T_gels attributed to diffusion into starch granules and resultant stabilization via intermolecular hydrogen bonding. Larger PEG exerted effects on T_gel, even though they were too large to enter the starch granules, by limiting solvent plasticization. The findings highlight how PEGs of different sizes are more effective antiplasticizers of starch than sucrose and elucidate how additive molecular size dictates the gelatinization behavior and, by extension, the end-use properties of starch in food and non-food systems.