TIME‐DEPENDENT FLOW BEHAVIOR of STARCH PASTES, WITH FOOD PROCESS APPLICATIONS

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Abstract
The time dependency of shear stress has been investigated in completely swollen modified potato starch pastes during short periods of time (30 s) over wide ranges of shear rates, temperatures and starch concentrations (6‐450 s−1, 10‐90°C, 3‐10%). A special, stepwise measuring process made it possible to correlate the kind of time dependency to the shear stress level; thixotropy or rheodestruction occurred at low shear stress (below 10 Pa); rheopexy occurred at intermediate shear stress (from 10 to 150 Pa); and thixotropy occurred at high shear stress (above 150 Pa). Bearing these results in mind, similar results can be observed in other studies on potato starch, maize starch, modified tapioca starch and tomato juice. the size and the kind of hysteresis loop in the flow curves can be related to the kind of time dependency at each shear stress level and to the duration of the effect at each level. These phenomena limit the precision when the flow properties are predicted for starch pastes and many other viscous foods, as long as the time dependency factor is disregarded in the measuring and modelling procedures. It is important to consider the time dependency when holding tubes for aseptic processing are sized; if this is not done, the sterilization effect will be too low when rheopexy prevail close to the surface of the tube.