Temperature retrogradation rate

Retrogradation rate strongly depends on water content (Figure 6.27b), and the relation is similar to that with temperature. According to Eq. (6.14), the supersaturation will be greater for a higher volume fraction of starch. On the other hand, at very low water content the mobility of the polymer chains will be very small, which will reduce the rate at which crystallites are formed. For 20% water at room temperature the mobility is effectively zero and no retrogradation occurs. 

Likewise, many of the adhesive properties can be explained by reference to the amylose and amylopectin properties of the starch used. Jelly gums are usually made from waxy starches (100% amylopectin) and are stable at room temperature for many months, as would be expected from the slow retrogradation rate of this starch. A corrugating formulation must set to form a bond and become water resistant in a short period of time. The low molecular weight portion of the amylose would be expected to come out of solution fairly rapidly to make a temporary bond, and the high molecular weight portion would more slowly tend to create water resistance. 

Hydroxyethyl group introduction at low DS results in distinct modification of physical properties. Among these are decreased gelatinization temperature range (126), increased granule swelling rate (127), and decreased abiUty of starch pastes to gel and retrograde. 

The low temperature/low temperature cycle theoretically results in a low product temperature. However, the drying rate is low and there is a risk that primary drying is not over when the temperature of the shelves is raised. In this case, collapse or retrograde collapse may occur. In addition, final product moisture may be above specification. 

Superimposed on the temperature influence is an influence of the molecular weight of the amylose, as the rate of gelation (= retrogradation) increases with decreasing chain-length. ... 

The hypothesis advanced here is that the observed enhancement is due to thermocapillarity arising from the temperature dependence of the interfacial tension. Although we know of no theory for such a Marangoni effect in binary condensation per se, theory does exist for both mass transfer (M. 11) and heat transfer (16, 12) aeross an interface in the absence of bulk flow. We note that the sign of the derivative of the interfacial tension with respect to temperature is positive near a lower consolute point and that this is in the correct direction to sustain disturbances in condensation rate. Thus, in retrograde condensation, provided a critical temperature gradient normal to the interface is exceeded, a local increase in condensation flux toward the vapor liquid interface will result in its cooling. 

In Table 1 the results obtained with 5 prosthetic valves (HK 21, 23, 25, 27, 29 mm TAD) are shown. Water at room temperature was the circulatory fluid. Steady retrograde flow is not an icreasing function of the diameter, meaning that the working tolerances affect very much the clearance between disc and stent. It is possible, by knowing both pressure and flow rate across the valve, to calculate a hydraulic equivalent diameter which shows the size of the disc--stent clearance. The ratio between such a diameter and the nominal one provides a figure of merit for the comparative evaluation of different prostheses. 

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