Preparation, characterization, and applications of highly substituted starch derivatives

Abstract

Starch esterification and etherification have been known for decades and are of interests to academic and industrial researchers. While lightly substituted starches, primarily used in food applications, have been extensively studied, highly substituted starches could find many industrial applications but have not been widely commercialized. In this dissertation, recent advances in preparation and properties of intermediately and highly substituted starches were reviewed. Experiments were conducted to: (1) develop methods of synthesizing highly substituted starch esters and ethers without using conventional organic solvents, (2) characterize the starch derivatives synthesized from solvent-free methods, and (3) plasticize the starch derivatives and explore their potential application as chewing gum bases. Waxy, normal, and high-amylose maize starches were acetylated to degree of substitution (DS) 0.1-1.7 in aqueous medium. Reaction efficiency (RE) of acetylation was in the order of high-amylose>waxy>normal starch. Distribution of acetyl groups on anhydrous glucose monomer level was determined by ¹H-NMR after peracetylation (with deuterated acetic anhydride) or perpropionylation. Acetylation was greatly preferred in C2 position in all DS levels regardless of amylose/amylopectin ratio of the base starch. Selected starch acetates were pre-gelatinized, dried, and melted in excessive octenylsuccinic anhydride (OSA) at 160 °C, to produce octenylsuccinylated acetylated starch (OS-Ac-starch) at three different scale levels (10 mg, 5 g, and 60 g). The representative OS-Ac-starch achieved a combined DS up to 2.85 (1.71 acetyl and 1.14 octenylsuccinyl) and contained less than 0.2% unreacted OSA. RE of octenylsuccinylation was positively related to acetyl DS and was between 0.5 to 16.2%. Pregelatinization significantly improved RE of OSA when acetyl DS was below 0.88; however, was not helpful when initial acetyl DS was higher. Triacetin was tested in OSA-starch acetate melting reaction as a diluent to reduce the usage of OSA. Addition of triacetin thinned the reaction melt and prevented acidity build-up during reaction. OS-Ac-starch produced in triacetin-added reaction achieved combined DS up to 2.55 and was structurally similar to starch ester synthesized in pyridine medium. RE of OSA was improved from 16.2 to 37.7% by triacetin addition. OS-Ac-starches had glass transition temperature (T[subscript g]) around 56 °C and were partially to fully soluble in organic solvents such as acetone and chloroform, suggesting their potential uses as water-resistant thermoplastic materials. Chewing gum base made from high-amylose starch ester had great stretch (360% elongation before breaking) and was superior to waxy and normal starch-based gums. In another approach, high-amylose maize starch was hydroxypropylated in aqueous isopropanol with propylene oxide to various molar substitution (MS) levels (0.56-1.64) and further acetylated to a range of DS (0.09-1.97), by either reacted in anhydrous acetic anhydride (dry heat melting reaction) or aqueous acetylation, to prepare hydroxypropylated acetylated starches (HPAcS). T[subscript g] of HPAcS was synergistically lowered by hydroxypropylation (HP) and acetylation (Ac) and was between -11 to 110 °C. Solubility of HPAcS in water was greatly enhanced by hydroxypropylation but significantly reduced by acetylation. From varying substitution levels of HP and Ac, it was practical to prepare thermoplastic starch materials of different water resistance. Chewing gum bases formulated from water insoluble HPAcS withstood different stretching forces and showed no sign to break at 750% elongation.