Advances in the Modification of Starch via Esterification for Enhanced Properties

Starch, a renewable polysaccharide, has attracted growing interest as an alternative industrial feedstock due to its biodegradability, low cost, and readily available hydroxyl (-OH) group, thereby according it the potential of possible modification. Despite its environmental suitability, several factors such as brittleness, sensitivity to enzymes and acids, insolubility in organic solvents etc., have been reported to limit its application as feedstock in some industrial processes. However, techniques of chemical modification aimed towards overcoming these shortcomings have been reported to proceed via substitution, degradation or cross-linking. The degree of substitution (DS) for modified starch is targeted within high (1.5-3), medium (0.2-1.5), and low (0.01-0.2). This is as it relates to their application as thickener or stabilizer in food industry, development of thermoplastic and biodegradable materials. Impact of reaction medium and catalyst type are examined as it affects the degree of substitution. During esterification, the reaction either occurs on the whole starch chains to form conventional starch esters or occurs at the outer surface of the starch molecule, leaving the internal crystalline structure intact. Consequently, the degree of rupturing with subsequent esterification of starch has been reported to be dependent on the agent used, hence classifying the esters formed as organic or inorganic. Therefore, this review examines various findings targeted at starch modification with a direct focus on improving the degree of substitution (DS) via double esterification.