[nanoPost] Starch nanocrystals

University USA

• Spherulitic crystallization of starch
• Starch nano-crystals.
Maize amylose, separated from granular starch by means of an aqueous leaching process, was used to investigate spherulite formation from concentrated mixtures of starch in water. Amylose (10-20%, w/w) was found to form a spherulitic semicrystalline morphology over a wide range of cooling rates (1-250 °C/min), provided it was first heated to >170 °C. This is explained through the effect of temperature on chain conformation. A maximum quench temperature of approximately 70 °C was required to produce spherulitic morphology. Quench temperatures between 70 and 110 °C produced a gel-like morphology. This is explained on the basis of the relative kinetics of liquid-liquid phase separation vis-a-vis crystallization.
Predictions can now be made about the tendency of a starchwater system to form a particular morphology, based both on the composition of the starch (ratio of amylose to amylopectin) and on the processing conditions. Within these factors, maximum heating temperature, cooling rate, quench temperature, and amylose content play major roles in determining the final morphology.
The formation of spherulites from amylose solutions occurred over a wide range of cooling rates and quench temperatures. The ability of native starches to form spherulites is highly variable, and this is likely due to the ratio of amylose to amylopectin as well as differences in the molecular architecture of the starch polymers. Differences in the solubility of amylase and amylopectin are well-known, and these differences may play a prominent role in determining the liquid-liquid demixing behavior that they conjecture plays a crucial role in the formation of the spherulitic morphology. Indeed, it has been previously recognized that amylopectin can hinder phase separation in starch containing both polymers.
Confirmation or refutation of the foregoing ideas awaits further investigation employing techniques sensitive to chain conformation, e.g., SAXS, CD, FTIR, or NMR. The challenge to investigating the starch-water system with these techniques is in working at high temperature with a volatile solvent. Further investigation should include the influence of the amylose chain length and concentration of morphology.