Thermal energy effect on the phase transition of a waxy corn starch, Amioca, with different amount of water was evaluated by using differcntial scanning calorimetry. For water content less than 40% (w/ w), the kinetics of the conversion of starch followed the pseudo-zeroth order. An activation energy of 33.78 kcal/mol for 80% Amioca and that of 44.25 kcal/mol for 75% Amioca in the Amioca-water system were obtained from the Arrhenius relation. A useful correlation betwcen the rat& constant of conversion and a nondimensional tempcraturc parameter, T/T,,, was found from various sources and with various moisture contents.

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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A drying system was designed and constructed to investigate the thin layer air drying of French fried Russet Burbank potatoes. This system permitted the weight loss of the sample to be monitored accurately and continuously without removal of the sample from the drying chamber. Constant drying conditions over a practical range were maintained by continuous control using an on-line computer. The drying conditions investigated in this study included dry bulb temperature ranging from 43 to 93"C, relative humidity ranging from 10 to 60%, and air velocity ranging from 0.79 to 1.77 m/sec. A logarithmic model was used to describe the drying curve. The factors affecting drying rate are discussed. The dry bulb temperature had a dominant effect at low relative humidity, whereas at high relative humidity other factors also affected the drying rate. Relative humidity affected the drying rate significantly, while air velocity had the least effect on the drying rate. The drying rate constant in the logarithmic model was correlated with relative humidity, dry bulb temperature and airflow rate and a good fit was obtained.

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 (