Viscosity of starch pastes

Each of these amylases causes a rapid decrease in the viscosity of starch pastes and the rapid disappearance from its reaction mixtures of products that give color with iodine. During the early stages of the hydrolysis of starch, the relative decrease in the viscosity of the substrates for hydrolysates of equivalent reducing value is most marked... 

Subsequently a small, but defined effect of compression up to 8 X 107 Pa on the viscosity of starch pastes was reported.38 Paronen and Justin39 investigated this... 

In contrast to the /S-amylase, the malt a-amylase (and other liquefying amylases) very rapidly lower the viscosity of starch pastes. When the relative viscosity has dropped to half the original value, only about 0.1% of the D-glucosidic linkages in the starch have been hydrolyzed. The malt a-amylase is a typical liquefying enzyme. 

When heated with water to a temperature varying according to the origin of the starch, it swells up and forms a paste, which on cooling forms a jelly. The viscosity of starch paste varies widely, depending not only on the variety of starch used, but also on the treatment during j)rei)aration in j)urifying and drj ing. 

The changes that occur in a starch when it is heated in water can be studied in several ways. One way is to follow the changes under a microscope, another is to measure the viscosity of the paste. A Brabender amylograph is normally used to monitor the paste viscosity. 

Shi, X. H., BeMiller, J. M. (2002). Effects of food gums on viscosities of starch suspensions during pasting. Carbohydr. Polym., 50, 7-18. 

Classification.—Commercial starches are classified, according to the viscosity of the paste produced, as thick- or thin-boiling. Wheat starch is a typical thin-boiling starch, as a 5% mixture of wheat starch in water yields a thin, translucent syrup, scarcely gelatinous at boiling temperature. Corn starch, on the other hand, is a characteristic thick-boiling starch. Its 5% mixture with boiling water is practically non-fluid. 

Retrogradation Retrogradation is the change of starch pastes from low to high viscosity on standing. ... 

The dispersion of low-substituted, acetylated starches in water increased as the number of acetate groups was increased and as the temperature was raised. The viscosity of starch-acetate pastes was lower than that of native starch pastes at all temperatures. The viscosity decreased until an acetyl content of 2.36% was reached, after which it changed only slightly. The reduced ability of starch acetates to form gels could be expressed in terms of the ratio of the viscosities at low and high temperatures. A rotation viscosimeter Rheotest was used to study the viscosities and the rheological properties of starch acetates. 

Pasting Refers to the increase in viscosity of starch slurries subjected to heating due... 

The physically defined state of starch paste is the most employed application form of starches. It covers a broad range of different flow properties. Viscosity depends upon kind and concentration of starch, temperature of heating and holding, shearing force by agitation, stirring, flow through pipes, and on pressure. 

Thixotropy and Other Time Effects. In addition to the nonideal behavior described, many fluids exhibit time-dependent effects. Some fluids increase in viscosity (rheopexy) or decrease in viscosity (thixotropy) with time when sheared at a constant shear rate. These effects can occur in fluids with or without yield values. Rheopexy is a rare phenomenon, but thixotropic fluids are common. Examples of thixotropic materials are starch pastes, gelatin, mayoimaise, drilling muds, and latex paints. The thixotropic effect is shown in Figure 5, where the curves are for a specimen exposed first to increasing and then to decreasing shear rates. Because of the decrease in viscosity with time as weU as shear rate, the up-and-down flow curves do not superimpose. Instead, they form a hysteresis loop, often called a thixotropic loop. Because flow curves for thixotropic or rheopectic Hquids depend on the shear history of the sample, different curves for the same material can be obtained, depending on the experimental procedure. 

Starch acetates [9045-28-7] are made by reaction of starch with acetic anhydride. Starch acetates are used in foods to provide paste clarity and viscosity stabiHty at low temperatures. A waxy maize starch acetate is most commonly used. Waxy maize starch acetates for food use are often cross-linked. Acetylated starches are also widely used in warp sizing of textiles. 

DSC revealed that the XG and starch did not interact synergistically and hence did not promote the formation of three-dimensional network structures. However, the hydrocolloid significantly decreased the retrogradation and syneresis of the starch paste, particularly in blends with a starch/XG ratio of 8.5/1.5. Mixing 1% or 2% tamarind XG with 9% cornstarch resulted in an increase in the paste viscosity from 385 to 460 and 560 BU (Brabender units), respectively [298]. The XG is associated with starch, as was evident from the lowering of the pasting temperature and the synergistic increase in pseudoplasticity and yield value of the blend pastes. However, carboxymethylated and hydroxypropylated XGs showed a diminished interaction. 

On cooking maize starch the viscosity increases when the starch begins to gelatinise. As the temperature rises towards 95°C the viscosity falls. When the paste is cooled the viscosity rapidly increases. The variation of viscosity with temperature is characteristic for each different origin of starch. Potato starch, for example, has a lower gelatinisation temperature than maize starch but has a higher maximum viscosity. When cooled the viscosity of potato starch rises less. Once again amylopectin starches do not show this behaviour as they do not gel. 

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