J.L. Kokini

Effect of Xanthan Gum and CMC on the Structure and Texture of Corn Flour Pellets Expanded by Microwave Heating

ABSTRACT Third-generation snacks obtained by microwave expansion of glassy cereal pellets are often hard and nonuniform and need to be improved to become acceptable to the consumer. Gums such as carboxymethyl cellulose (CMC) and xanthan gum (XG) have the ability to improve the volume, structure, and texture of expanded cereal products, due to their effects on moisture retention and rheological properties. This study investigates the effect of 1% addition of CMC and XG on the structural and mechanical properties of samples obtained by microwave expansion of glassy corn pellets. Unexpanded, glassy pellets were obtained by extrusion and subsequent equilibration at aw = 0.53 at room temperature. The equilibrated pellets were expanded by microwave heating. The addition of gums significantly improved the shape, structural and textural uniformity of the microwave-expanded samples. These effects were attributed to the contribution of gums to the rheology and moisture sorption capacity of the matrix. It is our hyp...

Factors That Influence the Microwave Expansion of Glassy Amylopectin Extrudates

ABSTRACT The microwave expansion of glassy, unexpanded amylopectin pellets was studied. Amylopectin was extruded at three levels of specific mechanical energy (483, 809, and 846 kJ/kg), and 35–40% moisture content, without expansion at the die. Glassy pellets were obtained by drying and equilibrating the extrudates at five water activities (aw 0, 0.11, 0.33, 0.67, and 0.75). The pellets were characterized by measuring volume, porosity, and moisture content. The pellets were then expanded in a constant power microwave oven to determine the degree of expansion. When subjected to microwave heating, regardless of extrusion condition and initial aw, the pellets expanded from the center where the highest temperature was recorded and then expansion advanced in the whole volume. Maximum expansion was reached after 30 sec of heating, after which samples started to burn from the center. Samples simultaneously expanded and lost moisture, both processes being faster and more intense for pellets of higher initial aw. ...

Influence of fat on expansion of glassy amylopectin extrudates by microwave heating

ABSTRACT The effect of solid fat (melting point 70°C) and liquid fat (melting point -20°C) on the microwave expansion of extruded glassy amylopectin pellets was investigated. Glassy, unexpanded amylopectin extrudates containing ≤10% fat were equilibrated at different water activity levels (aw) and then expanded by microwave heating. Expanded products were characterized by analyzing degree of expansion, structure, and texture. For the aw range studied, expansion increased with increasing aw of the pellets until a maximum at aw = 0.605 and then decreased. Completely dehydrated samples did not expand at all. This demonstrated that moisture was the driving force in the microwave expansion of the pellets. The addition of solid fat contributed greatly to expansion. The highest degree of expansion was obtained for pellets containing 6% solid fat at all aw levels. Due to the effect on expansion, the addition of solid fat also affected the microstructure and texture of the expanded products. Concentrations of soli...

Use of the Rubber Elasticity Theory to Characterize the Viscoelastic Properties of Wheat Flour Doughs

ABSTRACT The viscoelastic properties of durum wheat flour doughs were measured using the extensigraph in uniaxial extension and the Rheometrics mechanical spectrometer in oscillatory shear. The research examined the effect of increasing density of cross-links on rubber elasticity in these systems. The stress-strain behavior of durum wheat flour dough was not well simulated by Mooney-Rivlin type nonlinear elasticity. Addition of increasing amounts of iodate made the dough show appreciable strain thickening behavior, approximating the behavior of natural rubbers The estimated apparent molecular weight between cross-links ranged from 10,500 to 16,000, much larger than that of rubbers, for which values are in the range of 500–1,000. When the Mooney-Rivlin equation was tested, it appeared to approximate only moderately well the extensional behavior of iodate-added wheat flour doughs at finite but low extensions, where the finite extensibility of chains is not a factor. It is hypothesized that the cross-linked ...