48 54

Protein solubility (%)

Figure 3. Correlation between OTMS texture and protein solubility for soya isolate. Source: Adapted with permission from Ref. 33.

starch extrusion. Starch consists of a-d-glucose units that are linked to form large macromolecules. Native starch is a mixture of two different glucose polymers—the linear amylose and the branched amylopectin. Under the severe shear environment inside the extruder, the starch molecule loses its granular and crystalline structure (42) and undergoes macromolecular degradation (43-48). The insoluble native starch is partially solubilized after extrusion at room temperature. The magnitude of the transformation is a function of multiple processing parameters and their interactions as well as the type of starches.

The extrudate characteristics of several varieties indicated that wheat starch had the highest expansion ratio at 135 and 225°C (Fig. 4) (49). Increasing product moisture content decreased the expansion ratio (49-54). Increasing temperature increased the expansion ratio up to a point after which it decreased with further increase in temperature (49,55). This decrease can be attributed to the degradation of starch molecules at higher temperatures. Expansion is affected by the dimensions of the die or nozzle (56,57) and product composition. Increasing the length-to-diameter ratio of the nozzle increased the expansion ratio. Increasing the amylose content decreased the expansion ratio (49,53,58). Presence of lipids, proteins, salts, and

123456 123456

Samples number

123456 123456

Samples number

Figure 4. Expansion of products extruded at (a) 135°C and (b) 225°C. Starches are (1) waxy corn; (2) corn; (3) common wheat; (4) rice; (5) amylon 5 (6) amylon 7. Initial moisture content was 22% by weight. Source: Adapted with permission from Ref. 49.

sugar can also affect the puff ratio (50,54,59-61). Decreasing the lipid content increased the expansion ratio of the product whereas increasing the salt and sugar concentration increased the expansion ratio.

Increasing moisture content increased the breaking strength of extrusion-cooked cornmeal (53). Increasing temperature was found to decrease the breaking strength for nonwaxy corn, but the reverse was true for waxy corn. Increasing the concentration of salt and sugar in the extrudate reduced the breaking strength (54). Results of two studies indicated an inverse relationship between expansion ratio and shear strength (54,55).

Native starches are insoluble in water at room temperature. However, on extrusion these materials are soluble in water and could be added as ingredients in other food mixes to provide an acceptable consistency. The solubility of starches increase with increasing severity of treatment. At lower moisture content, the solubility index is higher (49,62-64). Higher temperature also increases solubility.

A small portion of a considerable body of literature that addresses the effect of process and product variables on properties of extrudates has been cited. Some researchers have used the RSM to locate optimum conditions for the best product texture. Empirical predictive models have been developed to relate product quality to independent parameters. These models or equations suffer from two major drawbacks: (1) they are machine-dependent in that the data obtained from one extruder cannot be extrapolated to other machines, and (2) they do not explain the physical phenomena. Understanding of how the basic physical properties control texture formation is limited.

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