Fortification of extrusion feed formulations with proteins from selected sources will improve the health-promoting quality of snack and breakfast foods. Molecular interactions among proteins, starch, lipids, water, and other constituents in the extrusion "melt" are the basis for extrudate appearance as well as textural, sensory, and physical properties. Covalent disulfide and sulfhydryl bonding as well as electrostatic, hydrogen, and hydrophobic affinity developing among molecules during extrusion determine extrudate sensory quality, digestibility, and nutrient availability. Extrudate quality is dependent on the physical profile of the feed material from particle size to moisture concentration, the mechanical profile of the twin-screw extruder from screw speed to die aperture, and the temperature, pressure, and shear leading to specific mechanical energy delivered in the extruder. Methods to assess extrudate physical and sensory quality are presented. Protein concentration, feed moisture, and extrusion temperatures are most important to expansion, density, and texture of extrudates. Fortification with whey and wheat proteins results in acceptable extrusion of snack and breakfast foods. Extrusion generally improves protein quality and digestibility while retaining active nutrients.
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