Temperature effect starch dispersion

Efficiency of starch dispersion can be further improved when an excess quantity of steam is used97 or when the starch is held under pressure in a retention device (tank or coil) before discharge to the atmosphere. Both processes require a flash chamber (cyclone) to remove excess steam and steam relieved by the pressure drop. Excess steam refers to a multiple of the quantity required to reach target temperature and pressure. The quantity of saturated steam to reach a target temperature can be calculated from its heat content, the slurry temperature, the specific heats of starch and water, and the enthalpy of starch gelatinization. The action of excess steam in the dispersion zone between the jet and back-pressure valve causes a shear effect that enhances disintegration of the starch granules. In many applications, a three-fold excess of steam is used. 

The effect of different flow properties on heat transfer to canned dispersions undergoing intermittent axial rotation was studied by Tattiyakul et al. (2002b). In addition to the 3.5% cornstarch dispersion discussed above, a 5% CWM dispersion whose rheological data are shown in Figure 8-9 and a tomato concentrate that followed a simpler thermo-rheological behavior were studied. Because of the high apparent viscosity over a wide temperature range of the CWM dispersion, it had the slowest time-temperature profile (not shown here). Results on the effect of continuous axial rotation on heat transfer to a canned starch dispersion can be found in Tattiyakul (2001). 

Starch is dispersed in the paper mill in large stainless steel tanks by injection of steam or by heat transfer from a steam-heated jacket. The tanks are stirred and equipped with baffles to prevent formation of a single vortex at the agitator shaft. A minimum heating time of 20 minutes at 95°C is normally required. Steam injection dilutes the starch paste by condensate, which must be considered for concentration control. Pastes that are prone to retrogradation are held at a temperature above 91°C or quickly cooled to 66°C to prevent amylose formation. Attention to storage temperature and water balance is an essential requirement for the effective use of starch in a paper mill. 

Radiation radiation, depending on wavelength and dose, has been shown to have a variety of effects, including increased gelatinization temperature, improved stability and reduced viscosity, swelling power and enzyme susceptibility. Radiation has also been used to promote chemical modification. Marquette et al.118 have a patent for microwaving starch to improve rheological stability and hot water dispersibility. They... 

These are the commonest type of starch used in confectionery and are used to make jellies, pastilles and wine gums. They are made by heating a dispersion of the starch with a small quantity of acid at a temperature below the gelatinisation point, where the effect is to reduce the molecular weight by hydrolysing a few of the bonds. This decreases the viscosityof the starch pastes as might be expected from theory. Starch suppliers have devised a system where these starches are classified by fluidity, where the fluidity is the reciprocal of the viscosity. Thus, in a series of fluidity numbers 20, 40, 60, 75 and 80, the 80 fluidity gives the lowest viscosity paste. 

In some cases the solutions have exhibited structural effects, the nature or extent of which have not been fully established. Staudinger and Eilers° found that the viscosity of their starch triacetate solutions displayed little dependence on temperature and considered the acetate to be mono-dispersed. Higginbotham and Richardson, on the other hand, found a noticeable temperature dependence for viscosities of carefully prepared potato starch triacetate in tetrachloroethane solutions. For example, a 0.25% solution was thbcotropic at 25°, but not at 60°. The existence of structural effects in starch acetate solutions was likewise noted by Mullen and Pacsu for potato starch triacetate dissolved in tetrachloroethane. 

For starch to be effectively degraded it must first be heated above 65 °C. The starch grains then burst and they form a viscous gel. This must be thinned so that the viscosity is low enough for the dispersed starch to be handled. The a-amylase from barley is fairly stable to heat, and it is active at 65 °C. However some a-amylases from the organisms which grow at temperatures up to 80° or 90 °C (e.g. Bacillus stearothermophilus) are stable for short periods at temperatures above 100°C, and these heat-stable enzymes are very useful for the controlled thinning of starch gels. 

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