Sorption properties of modified potato starch

Background. Starch is one of the biopolymers most commonly used in the food industry. In its native form its applications are limited. In contrast, its structure is relatively readily modified using various methods, producing starch derivatives of greatly diverse properties. The primary methods used to change characteristics of starch include acetylation and oxidation. Thus obtained raw materials are used as food additives. Stability of these raw materials and food products to a considerable extent depends on their composition and parameters characterizing storage facilities, i.e. relative humidity and temperature. One of the methods to specify adequate ambient conditions is based on water activity supplemented by sorption isotherms. As a result studies investigating correlations between water activity and moisture content are of importance for the optimization of storage conditions and design of certain food processing procedures. For this reason the aim of this study was to assess the effect of modification (oxidation, acetylation and their combination) on sorption properties (described based on sorption isotherms) of potato starch. Material and methods. Analyses were conducted on potato starch and its derivatives produced by acetylation, oxidation and co-modification (acetylation coupled with oxidation). Starch was oxidized using sodium chlorate (I), while acetylation was run using anhydrous acetic acid. Sorption isotherms were determined by the static desiccator method. Results were described applying selected mathematical methods equipped with physical interpretations (BET and GAB) and empirical models (Halsey, Oswin, Henderson, Pelega, Lewicki, Blahovec-Yanniotis). Calculations were made using non-linear estimation. Results. The greatest water absorbability was found for native potato starch. Modifications caused a decrease in the amount of adsorbed water vapour, dependent on the applied modification. Acetylation resulted in significantly greater reduction of the amount of adsorbed water vapour in comparison to oxidation. Oxidation had a slight effect on sorption properties. The lowest hygroscopicity was recorded for co-modified starch, which resulted from the overlapping effect of oxidation and acetylation, reducing the capacity to adsorb water vapour. Four of the applied mathematical models (Pelega, Lewicki, GAB and Blahovec-Yanniotis) adequately reflected the course of isotherms and were characterized by comparable accuracy. Water contents in the monolayer ranged from 0.0738 to 0.0801 (the BET model) and from 0.0960 to 0.1010 (the GAB model). Based on the model proposed by Blahovec and Yanniotis it was found that modifications led to a decrease in the share of adsorbed water and an increase in the share of free water available for soluble components. Conclusions. Sorption isotherms for all tested starch forms (native and modified) had a sigmoid shape. Analyzed modifications led to a decrease in starch hygroscopicity, with acetylation causing a markedly greater reduction of water vapour adsorbability in comparison to oxidation. Among the eight mathematical models used to describe sorption properties the Pelega model proved to be the best, although three other (the Lewicki, GAB and Blahovec-Yanniotis models) described experimental data with comparable accuracy. In most cases chemical modification of starch resulted in a reduced monolayer absorbability. An exception in this respect was connected with the value established based on the GAB model for oxidized starch. The results may be used to optimize storage conditions and to design products containing starch and its derivatives.