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Synthesis and Characterization of Starch Acetates with High Substitution 1

Cereal Chem. 81(6):735–740 Acetylation of high-amylose (70%) maize starch to high degree of substitution (DS) was studied by reacting starch with acetic anhydride using 50% aqueous NaOH as the catalyst. DS increased with increasing reaction times and increasing ratios of acetic anhydride to starch. Reaction efficiency (RE) increased with longer reaction times and decreased with increases in the ratios of acetic anhydride to starch for extended reaction times. Increasing the amount of NaOH increased both DS and RE. A series of starch acetates with DS values of 0.57–2.23 were prepared and their crystalline structures, chemical structures, thermal stability, and morphological properties were investigated. After acetylation, and as DS increased from 0.57 to 2.23, the crystalline structures of starch steadily disappeared. The carbonyl group’s peak at 1,740 cm appeared in the FTIR spectra. The intensity of this peak increased with a decrease in the peak intensity of the hydroxyl groups at 3,000-3,600 cm, indicating that the hydroxyl groups on starch were replaced by the acetyl groups. Thermal stability of starch acetates increased. The smooth surface of the starch granules became rough with acetylation. Further acetylation led to the loss of the starch granules and the formation of beehiveand fibrouslike structures. In recent years, interest in finding environmentally friendly alternatives to nondegradable plastic material made from petroleum has placed an emphasis on natural macromolecular materials from agricultural sources. Starch has been one of the best choices because of its availability and low cost. However, the strong hydrophilicity and poor mechanical properties of starch prevent its application in the plastic products. As a result, it is logical to chemically modify starch to improve its properties for certain applications. Of various chemical modifications, starch acetylation can be performed with relative ease to improve significantly the physiochemical and functional properties of the starch, even with a low DS (Moorthy 1985). During acetylation, three free hydroxyl groups on C2, C3, and C6 of the starch molecule can be substituted with acetyl groups. Therefore, the theoretical maximum DS is 3. Starch acetate with DS 0.01–0.2 is approved by the FDA for food use to improve binding, thickening, stability, and texturizing (de Graaf et al 1998). On the other hand, high-DS starch acetates have been prepared since the middle of the last century with the aim of obtaining a replacement for cellulose acetate (Mullen II and Pacsu 1942b; Whistler and Hilbert 1944; Wolff et al 1951). Pyridine was a good catalyst in the preparation of the high-DS starch acetate because it not only led to complete esterification, but also did not cause starch degradation during the reaction. Pretreatment was necessary (Mullen II and Pacsu 1942a; Lohmar and Rist 1950), and the influences of some pretreatments on the reaction rate were compared (Mark and Mehltretter 1970). High conversion efficiencies were obtained when starch was pretreated with DMSO or hot water. However, the high cost, difficult recovery, and volatility of pyridine, along with the inconvenience of pretreatment, limited commercial development of this technology. Mark and Mehltretter (1972) developed a simple method to prepare relatively undegraded starch triacetate using NaOH as the catalyst without pretreatment. The expected chemical reactions are illustrated in Fig. 1. Reaction 1 is the main rate-controlled reaction involving a base catalyst, where starch is acetylated with acetic anhydride and a base catalyst. This is an example of nucleophilic substitution at an unsaturated carbon atom of acetic anhydride. The substitution takes place by an addition-elimination mechanism (Roberts 1965). Furthermore, the three free OH groups have different reactivities. The primary OH on C6 is more reactive and is acetylated more readily than the secondary ones on C2 and C3 due to steric hindrance. The primary OH located at the exterior surface of the starch molecules reacts readily with the acetic groups, while the two secondary ones located within the interior surface of starch form hydrogen bonds with the OH groups on the neighboring glucose unit. Of the two secondary OH groups, the OH on C2 is more reactive than the one on C3, mainly because the former is closer to the hemi-acetal and more acidic than the later (Fedorova and Rogovin 1963). Side reactions 2 and 3 rarely occur because almost no water exists in this reaction system. Shogren (1996) used this method to prepare starch acetates with DS 1.5, 2.0, and 2.5. However, no research has been published on the influences of reaction conditions on starch acetylation. Therefore, the first part of this study was to investigate the influences of selected reaction conditions on extent of starch acetylation. 1 Journal Series No. 14421, Agricultural Research Division, Institute of Agriculture and Natural Resources, University of Nebraska-Lincoln. This study was conducted at the Industrial Agricultural Products Center, University of Nebraska, Lincoln, NE. 2 Industrial Agricultural Products Center and Department of Food Science & Technology, University of Nebraska, Lincoln, NE 68583-0730. 3 Cadbury Adams, Morristown, NJ 07960. 4 Corresponding author. Phone: +1-402-472-1634. Fax: +1-402-472-6338. E-mail: