Impact of Acid Hydrolysis on Morphology, Rheology, Mechanical Properties, and Processing of Thermoplastic Starch

TL;DR

This study investigates how acid hydrolysis modifies native wheat starch, leading to lower processing temperatures and altered mechanical and structural properties of thermoplastic starch, with potential energy-saving benefits.

Contribution

It demonstrates that acid hydrolysis reduces molecular weight and viscosity, enabling lower processing temperatures and improved properties of thermoplastic starch.

Findings

AH decreases molecular weight in amorphous regions.

AH-treated starches show higher crystallinity.

Lower viscosity during melt mixing reduces processing temperature.

Abstract

We modified native wheat starch using 15, 30, and 60 min of acid hydrolysis (AH). The non-modified and AH-modified starches were converted to highly-homogeneous thermo-plastic starches (TPS) using our two-step preparation protocol consisting of solution cast-ing and melt-mixing. Our main objective was to verify if the AH can decrease the pro-cessing temperature of TPS. All samples were characterized in detail by microscopic, spectroscopic, diffraction, thermomechanical, rheological, and micromechanical methods, including in situ measurements of torque and temperature during the final melt-mixing step. The experimental results showed that: (i) the AH decreased the average molecular weight preferentially in the amorphous regions, (ii) the lower-viscosity matrix in the AH-treated starches resulted in slightly higher crystallinity, and (iii) all AH-modified TPS with less viscous amorphous phase and higher content of crystalline phase exhibited similar properties. The effect of the higher crystallinity predominated at laboratory tem-perature and low deformations, resulting in slightly stiffer material. The effect of the low-er-viscosity dominated during the melt mixing, where the shorter molecules acted as a lubricant and decreased the in situ measured processing temperature. The AH-induced decrease in the processing temperature could be beneficial for energy savings and/or pos-sible temperature-sensitive admixtures to TPS systems.