Proposing a Seven-parameter Polynomial Equation to Fit the Moisture Sorption Isotherms of Helium Cold Plasma-treated Wheat Flours

Abstract

For seeking the cause of the increase in dough water absorption rate after cold plasma treatment, this study adopted low-pressure, radiofrequency 120 W helium cold plasma (CP) to treat wheat flours from 0 s to 180 s. Results showed that water absorption rate, conductivity, values of water and sucrose solvent retention capacity (SRC), and paste breakdown value in wheat flours increased significantly with increasing CP treatment time. To further explore the change causes in these physiochemical indexes, we developed a seven- parameter polynomial , (M was equilibrium moisture (%), ERH was equilibrium relative humidity (decimal), t was temperature (℃), and A~G were parameters. This polynomial could fit moisture sorption isotherms (20-35 ℃ and ERH 10%-90%) of helium CP treated flours, and the goodness-of-fit indexes were superior to those of modified Chung-Pfost (commonly used) and four-parameter polynomial (without temperature term) equations. The seven-parameter polynomial equation clearly showed the hysteresis loops between moisture adsorption and desorption isotherms of flour samples, which shifted downward with increasing temperature. The hysteresis degree showed a parabolic shape curve with increasing ERH, and the vertex of the parabola was at the range of 40-50% ERH. CP treatment shifted the monolayer adsorption sites to multi-molecular adsorption sites ahead of schedule, from ERH 50% to 40%. Further analysis using Dent model showed that the helium CP treatment was able to reduce the mono-layer moisture content and spreading pressure in wheat flour, and increase the elasticity of a capillary wall. The ability of water molecules was increased to form hydrogen bonds with the chemical components, which promoted the uniform distribution and permeation of water molecules into flour particles. Infrared spectral scanning showed that the degree of short-range order at the surface of starch granules and the interaction between protein and starch tended to increase with increasing CP time. Helium cold plasma technology has potential application in improving flour processing quality.

Keywords

• polynomial equilibrium moisture equation
• flour
• helium-cold plasma
• hysteresis loop
• extended pressure