The phase transition of a waxy corn starch, Amioca, with a limited amount of water available, dpon heating, was investigated by using experimentation and computer simulation. A model based upon the stoichiometric ratio of water to anhydro-glucose unit was developed lo simulate conversion of starch (gelatinization and/or melting) with different water contents. Simulation results showed a minimum ratio of 14 water molecules to one anhydrous glucose unit was required for complete gelatinization. A phase diagram based on this was constructed to relate water contents to gelatinization and melting of starch. INTRODUCTIONMANY STUDIES on extrusion cooking process in the food industry have focused on development of comprehensive models that describe the complex physical and chemical changes occurring in the operation of an extruder. The complexity of starch gelatinization and protein denaturation complicates the measurement of physical properties of dough such as viscosity, thermal conductivity and mass diffusivity. This makes such comprehensive modelling difficult. Several researchers (Mertier et al., 1980; Olkku and Vainionpaa, 1979) have been trying to relate physicochemical changes occurring in cerealbased doughs during extrusion to process variables. Others have been studying the reaction mechanisms of starch-water systems at the molecular level (Zobel, 1988a, b). For example, Lelievre (1976) used the theory of polymer crystal-amorphous phase transition to explain the mechanisms of starch gelatinization. For a comprehensive modelling work on extrusion, the formulation of meaningful kinetic equations based on a mechanistic model rather than an empirical one would be helpful. Donovan (1979) pointed out that phase transitions in starch was believed to follow two mechanisms; namely, gelatinization and melting. At higher water content, swelling of the amorphous region in a starch granule promoted the transformation of crystalline regions by pulling the crystallites apart in the process of gelatinization. At lower water content, crystallites melt at significantly higher temperatures. From experimental results, Donovan (1979) has indicated that, the stoichiometry for starch to undergo gelatinization only, upon heating, was 14 water molecules per glucose unit. The objective of our study was to characterize the phase transition of starch by using some DSC (differential scanning calorimetry) results and computer simulations based on the principle of stoichiometry and percent conversion of chemical engineering kinetics.

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Using a capillary rheometer and a single-screw extruder, waxy corn starch was extruded at a relatively low temperature range. Notable conversions induced by shear energy were obtained in very short residence times (from several set to 1 min). Conversions up to > 70% were also achieved. Kinetic studies showed that conversion caused by shear energy was more efficient than that by thermal energy. The shear rate constant, k, was an exponential function of shear stress and a linear function of shear rate. The minimum shear stress required to cause starch conversion was from 104 to lo6 N/m* for 100°C to 21°C. In such "shear" experiments, the maximum temperature rise due to viscous thermal dissipation, measured experimentally and calculated theoretically, was too low to cause any significant thermal cooking of starch. The shear converted starch extrudates were unpuffed and had shapes of traditional pasta.

Tribological (powdery friction) conversion (cooking) of starch as followed by DSC measurements was studied in a capillary rheometer. The crumbly extrudates with <20% conversion were analyzed to obtain a kinetic model and its parameter values suitable to describe the phenomena of tribological conversion of starch. Such conversion of starch followed zeroth order kinetics. An Arrhenius type activation energy plot described the shear energy initiated conversion well. The shear activation energy of starch conversions due to tribological shearing of powders was lower than that found for overall (tribological plus rheological) conversions. These findings can help develop more accurate and realistic models for starch conversion and similar processes and help elucidate their mechanisms.

The changes of size and size distribution of starch granules under extrusion conditions have been investigated by using a single screw extruder and an optical microscope coupled with an image analysis system. The mechanical energy input and material processing time were key factors influencing the degree of starch granular size reduction. In a high shear extrusion process (T=40°C), the weight-average granular size of extrudate was reduced from -1 2 . 4~ (raw waxy corn starch) to 1-2p, whereas in a low shear extrusion (T=9OoC), the granular size was reduced to -7,u. Fine particles (