Abstract
Abstract The processing to which a material is subjected can affect its physical or chemical structures, resulting in products with different hygroscopic behaviours. The present work studied the water adsorption properties of sorghum subjected to different types of processing: raw flour (produced by milling whole grain), extrudates (obtained using a double screw extruder) and extruded flour (obtained by milling the extrudates). The isotherms were obtained using an automated instrumental method. The tests were run in duplicate at 25 °C with relative humidity values ranging between 11 and 84%. The water adsorption data fitted the GAB model well, showing high coefficients of determination. The estimated water contents of the adsorption monolayer ranged from 5.3 to 6.9 g of water per 100 g of dry material. The sorption isotherms were affected by the type of processing, extrusion cooking resulting in products with less water in the monolayer (less hygroscopic). The milling process yielded high water contents in the monolayer, probably due to the breakdown of some polymer-polymer interactions, which exposed the binding sites. To ensure microbiological stability, the water contents in the materials should not exceed 6.9 g of water per 100 g of dry material for raw sorghum flour, 5.3 g of water per 100 g for sorghum extrudates and 6.7 g of water per 100 g for extruded sorghum flour.
Highlights
The interaction of water vapour with solid materials has an impact on a wide range of industries, including the pharmaceutical (ÖZENGIN; ELMACI, 2016), food (FENNEMA; REIDE, 2010) and polymer (ANSELL, 2015) industries
The sorption isotherm describes the amount of water sorbed at equilibrium by a material with known water activity, constant temperature and constant pressure
The sorption isotherms of the sorghum materials were affected by the type of processing
Summary
The interaction of water vapour (moisture) with solid materials has an impact on a wide range of industries, including the pharmaceutical (ÖZENGIN; ELMACI, 2016), food (FENNEMA; REIDE, 2010) and polymer (ANSELL, 2015) industries. This interaction is measured by means of the sorption isotherms. The moisture sorption isotherm curve (moisture content versus water activity) is generated from adsorption or desorption processes and is obtained from the equilibrium moisture contents determined at several aw values (LABUZA; ALTUNAKAR, 2007). The Isotherms are important to determine the moisture barrier properties required by packaging materials to limit moisture gain or loss in a package (DAMODARAN et al, 2008; ALVES et al, 2015)
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