Preparation Of Tri-modified Cassava Starch By Cross-linking And Esterification Pregelatinization Methods And Its Application

In this paper tri-modified cassava starch, used especially for Udon noodles, was prepared firstly by cross-linking the native cassava starch with sodium trimetaphosphate (STMP) employed as the cross-linking agent, and then esterified in the presence of acetic anhydride, subsequently this dual-modified starch was further pregelatinized to obtain target modified starch. Processing conditions of starch modification were optimized, besides, physico-chemical, morphological, crystalline properties of various modified starch prepared in the experiment were studied systematically, as well as their application in Udon noodles. The proper operating conditions of preparing cross-linked cassava starch with stabilization ability, of which the effect was evaluated in terms of combined phosphorus content, were illustrated as follows: concentration of native starch slurry was 40%, the amount of STMP 0.1% (w/w dry starch basis), pH=10.5, 50℃, reaction time 2.5h, dosage of sodium chloride 3%, and under such conditions the combined phosphorus content was 0.019%.Dual modified cassava starch, attained by a combination of modification methods of cross-linking and esterification, was made on the base of cross-linked cassava starch, and acetyl group content was selected as an indicator for esterification process. Optimum operating conditions of this process were showed as follows: concentration of cross-linked cassava starch slurry 40%, dosage of acetic anhydride 3% (w/w dry starch basis), pH=9.0, 30℃, and the reaction continued 2.0h. Under the treatment of acetylation, paste clarity, swelling power, solubility, and freeze-thaw stability of native starch could be enhanced. Modified starch used especially for Udon noodles was prepared by further pregelatinization of the dual modified cassava starch with 0.019% combined phosphorus content and 1.06% acetyl group content. This dedicated modified starch had advantages of higher paste stability, clarity, water-holding capacity, and lower initial gelatinization temperature and retrogradation, and thus significantly improved properties of native cassava starch.Testing results derived from polarization microscope and Scanning Electron Microscopy (SEM) showed that the birefringence of barely cross-linked starch granules and dual-modified starch granules were clear and the structures of them were also well retained. Infrared spectra of this cross-linked and esterified starch indicated that after dual-modification treatment, new chemical group was introduced into the molecular structure, and there was a characteristic absorption peak of carbonyl group at 1728cm-1. While the X-ray diffraction analysis graph illustrated that crystallinity of dual-modified starch occurred only a slight change, which verified that chemical reaction merely happened in the amorphous zone of starch granules. When dual-modified starch went through further pregelatinization processing, its granule structure was broken, the crystalline area was destroyed, birefringence disappeared, and a"bread peak"occurred in the X-ray diffractogram.Udon noodles added with dedicatedly modified cassava starch acquired a series of excellent qualities described as follows: they were white and shining in color and luster, a better ductibility during processing, hard to break down, and good water-holding capacity after rehydration, strong ability in boiling resistance during cooking, and after cooking they wouldn't stuck together, in addition to those, the noodles gained a wonderful taste with delicate texture and smoothness, and the products were stable in storage. Therefore, this tri-modified cassava starch could be used as desirable additive in the production of Udon noodles.