100 120 140 160 180 Temperature (°C)
Figure 1. Effect of process temperature on product density. Source: Adapted with permission from Ref. 25.
variable, three-level fractional factorial design to measure the effect of extrusion of defatted soy flakes on residual trypsin inhibition activity (TIA), Warner Bratzler shear (WBS), and water absorption capacity (WAC) (26). Lower product moisture decreased residual TIA. Higher temperatures and lower moistures increased WAC, but increased feed moisture content decreased WBS.
A third study used both sensory and mechanical means to evaluate the texture of an extruded soybean meal product (27). A five step temperature range of 135-180°C was used in this experiment. A high correlation between the instrumental and sensory parameter was obtained. With increases in temperature, the product became less compact and spongy in appearance and had increased aligned fibers as reflected by cohesiveness. In a subsequent work, these investigators used a four-factor (temperature, screw speed, product moisture, and protein content) three-level RSM to evaluate the textural properties of extruded defatted soybean meal (28). Results from this study indicated that protein level and extrusion temperature were the most important factors affecting WBS values.
The effect of pH (5.5-10.5) of defatted soybean meal and soybean isolate on WBS values (20) indicated that at the extremes of the pH values, the extrudate appeared to have suffered a loss of structural integrity. The maximum shear force and sensory values were obtained at about a pH of 8, as indicated in Figure 2a and b. At pH < 5.5, the extrusion of the product becomes quite difficult, as pH is increased from 5.5 to 8.5 the product becomes tender, less chewy, and rehydrates rapidly (29). These effects have been attributed to the lower solubility of the proteins at acidic pH, which produced a denser product due to the formation of aggregates in the extruder (14).
Independent variables such as moisture content, temperature, and screw speed affect shear strain, stresses, and shear rate. The effect of shear environment on the textural properties of unroasted defatted soy flour was reported (30). Increasing strain enhanced cross-linking in the dough, as indicated by a higher work required to shear the sample. Higher product moisture reduced the density. A high shear rate (or shear stress) in the die achieved by increasing flow rate led to a denser and less absorbant product, caused by the disruption of cross-linking. When low levels of hydrocolloid (sodium alginate or methylcel-lulose) were added to defatted soy flour, sodium alginate increased chewiness, maximum force, WAC, and bulk density, whereas the addition of the same amount of methyl-cellulose decreased maximum force and chewiness (31).
A seven-factor, five-level experiment using RSM to optimize the process variables of temperature, screw speed, screw compression ratio, die diameter, and product moisture content during extrusion cooking of defatted soy grits studied the individual and interactive effects of the independent variables (32). Results indicated that high screw compression ratio and maximum temperature decrease between the barrel and the die favored good texture. In an investigation of the effects of product moisture, barrel temperature, and die temperature on the properties of extruded soy isolate and soy flour, it was found that soy isolate required higher pressure to extrude and had higher expansion and a narrower range of textures than did soy
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