Chemically modified starches have a large number of hydroxyl groups which can be modified chemically by introducing functional groups such as carboxyl, acetyl, hydroxypropyl, amine, amide, etc. This modification gives starches specific properties that make them more suited for certain food applications and improve their functional and nutritional characteristics. Chemical modification can be achieved through decomposition methods such as acid hydrolysis and oxidation, or through derivatization methods such as etherification, esterification, crosslinking, and dual modification.
Acid Hydrolysis starch
Acid-thinned, thin-boiling, and fluidity starches are terms for starches that have undergone acid hydrolysis. This process uses H3O+ to break the α-1,4 glycosidic bond and hydrolyze the glucosidic linkages. The reaction generates a high-energy carbocation intermediate which reacts with water, leading to the regeneration of a hydroxyl group. Acid hydrolysis starts from the surface of the starch granule and the amorphous regions are more susceptible to it than the crystalline regions.
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Bleaching and Oxidation of Starch
In simple terms, oxidation of starch involves a reaction of the hydroxyl groups on the starch molecule to form carbonyl and carboxyl groups, which leads to the breakdown of the starch into amylose and amylopectin. The extent of oxidation depends on various factors such as the starch molecular structure, reaction conditions, and type of oxidizing reagents used. Oxidized starches have lower hot viscosity, improved clarity, film forming and binding properties, making them suitable for use in various food applications such as in batters and breading or as fillings in bakery products.
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Substitution (Stabilization)
Stabilized starches are produced by reacting starches with monofunctional reagents to convert hydroxyl groups into larger ester or ether groups. This results in more stable pastes and gels that have a reduced tendency to retrograde. The negative charge of some groups introduced through this process further reduces interchain associations to increase paste stability.
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Cross-linking
Starch cross-linking is a process that improves the strength of swollen starch granules during heating and makes them more resistant to viscosity breakdown from mechanical shear, acid, or high temperatures. The extent of cross-linking affects the properties of modified starch and low levels of cross-linking often result in higher swelling powers and paste peak viscosities compared to unmodified native starch.
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Dual Modification
Dual modification is a combined method of chemical and physical or chemical and enzymatic modification of starch. The reaction conditions during preparation of dual-modified cross-linked/hydroxypropylated starches are important and depend on the source of starch, the type of cross-linking reagent, the desired properties, and the efficiency of the cross-linking reaction. Dual-modified starches are widely used in food industry for stability against acid, thermal, mechanical degradation and retrogradation, particularly in salad dressings, canned foods, frozen foods, and puddings. For example, cross-linked and phosphorylated rice starch provides better freeze-thaw stability.
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