Counterion and microcrystalline cellulose effects on the structural properties of starch at low moisture contents

Abstract

Snacks are a rapidly growing sector of the food industry assisted by technological advances in the area of low-moisture materials. The major ingredient in snacks is starch, and different ingredients result in distinct physical properties of the final product including structure and expansion ratio during thermal processing. Along with starch type, increasing use of other ingredients particularly salts to achieve a variety of sensory profiles can have unexpected effects on the textural performance of commercial formulations. To date, molecular studies on these systems are scant in the literature, although the subject has important technological implications and affects the overall quality of preparations. This study examined the structural properties of condensed starch obtained from potato and cassava. Sample preparation included hot pressing at 120°C for 7 mins to produce extensive starch gelatinisation. Materials covered a range of moisture contents from 3.6 to 18.8%, which corresponded to relative humidity values of 11 and 75%. Salt addition in the form of sodium chloride and calcium chloride, with sodium and calcium ions being placed at opposite ends of Hofmeister series, was up to 6.0% in formulations. Instrumental work was carried out with dynamic mechanical analysis, modulated differential scanning calorimetry, Fourier transform infrared spectroscopy, scanning electron microscopy and X-ray diffraction. Experimental conditions ensured the development of amorphous matrices thus imitating textural consistency in food products. It was found that moisture content variation affects dramatically the characteristics of the glass transition temperature (Tg) in potato starch. Sodium ions interact with the hydroxyl and phosphate groups of potato starch to alter considerably the mechanical properties of high-solid preparations. These become softer than starch-water systems at conditions of elevated temperatures governed by molecular mobility. In contrast, salt addition creates hard matrices at conditions of low temperature, which are characteristic of the mechanical glassy state. Densely packed salt-polysaccharide segments in the glassy state have a high energy requirement to molecular mobility that raises the Tg with increasing counterion content. Calcium is a stronger electrolyte than the sodium cation because of the dense electric charge. Dissolved calcium atoms in our systems, as opposed to crystallised calcium chloride, form specific electrostatic interactions with the polar and negatively charged sequences of the potato starch molecule inducing an antiplasticising effect on potato starch that stabilises the polymeric matrix in the glassy state. Strikingly, counterion effects were minimal in the non-phosphorylated amylopectin sequences of cassava starch, which exhibited similar structural behaviour with cassava starch-water samples. The final chapter deals with the effect of addition of microcrystalline cellulose (MCC), up to 6.0% w/w, on the physicochemical properties of potato starch based films. Composite materials were treated as for single starch systems to reveal that the inclusion of MCC increases the mechanical strength of the starch films. In contrast to salt addition, both moisture content and presence of MCC have a plasticising effect producing a reduction in the Tg. Overall, results indicate that there is no specific and non-trivial interaction between starch and MCC, with the elongated fibres acting as inert filler in the polymeric composite