Shear starch/glycerol/water

Table 5.1 Results of main parameters calculated for potato starch/glycerol/water mixtures processed in the shear cell [25].

Figure 5.9 Microstructure of potato starch/glycerol/water treated in the shear cell without water addition a) 20% glycerol, b) 25% glycerol [25].

Starch/glycerol ratio [%) Water (%) Max torque (Nm) Max shear stress (Pa) MaxSME (kl/kg) Shear stress multiplied by time (Pa s)... 

Almost all parameters calculated for samples of starch/glycerol mixtures without water addition showed lower values than with 5% added water [26]. Much lower values in SME and in shear stress multiplied by time were noted. According to previous data [20] it may be concluded that increasing moisture content should lower values of maximal shear stress and should thus reduce the macromolecular degradation. This can be confirmed by intrinsic viscosity measurement There are some differences in the extrusion behavior of potato and of cornstarch, as reported by Della Valle et al. [27]. Molten potato starch under the same conditions... 

The microstructures of shear-heated samples differ with glycerol and water levels (Figure 5.9). Figure 5.10 shows the structures of starch/glycerol mixtures with 10% added water. The smaller starch granules, in comparison with Figure 5.9,... 

In its natural form, starch is not meltable and therefore caimot be processed as a thermoplastic. However, starch granules can be thermoplasticized through a gelatinization process. In this process, the granules are disrupted and the ordered crystalline structure is lost under the influence of plasticizers (e.g., water and glycerol), heat, and shear. The resultant melt-processable starch is often termed thermoplastic starch (TPS). Since the advent of TPS,... 

Figure 5.8 Effect of rotation speed on intrinsic viscosity during heat-shear treatment (87°C, lOmin) of a mixture of potato starch with 20% of glycerol and 20% of water added (unpublished data).

---