A.E. Drouzas’s research while affiliated with National Technical University of Athens and other places

The effective thermal conductivity (keff of two granular starches was investigated at various bulk densities, temperatures and moistures, using the method of heated probe. Granular powders of high-amylose and high-amylopectin starches were used at bulk densities 500-800 kg/m3, temperatures 25–70°C. and moistures of 0–40%. The keff increased linearly with bulk density, from 0.065 to 0.220 W/mK. Temperature had a positive effect up to 6.5% moisture. At higher moistures keff increased sharply above 50°C, indicating starch-water interaction. The keff increased linearly with moisture at low (0–15%) and high (> 25%) moistures, remaining nearly constant at intermediate moistures. These changes were related to the porosity of the starches.

Combined microwave (MW)/vacuum drying of fruit materials has a promising potential for high-quality dehydrated products. A better knowledge of the drying kinetics of fruit products could improve the design and operation of efficient dehydration systems.A laboratory MW/vacuum drier was used for drying kinetics experiments with model fruit gels, simulating orange juice concentrate. The system was operated in the vacuum range of 30–50 mbar and MW power of 640–710 W. The distribution of the electromagnetic field in the cavity of the oven was determined from the drying rate of samples, placed at 5 different locations.The drying rate was determined by periodic weighing of the sample. The rate constant (K) of the single-layer model of drying was estimated by regression analysis of the experimental data. An empirical model is proposed for estimating the drying constant (K) as a function of the absolute pressure and the MW power of the system.

Pectin-sugar gels were dehydrated with four different drying methods: freeze, microwave, vacuum and conventional drying to a moisture content of about 5%. The effect of the drying method on the water sorption properties of dehydrated products was evaluated at 25 °C. Freeze-dried gel adsorbed more water vapour than microwave-dried gel, which had a higher sorption capacity than vacuum- and conventional-dried product. The sorption isotherms were in agreement with the reported shape for high sugar foodstuffs. Three different equations proposed in the literature (GAB, Oswin, Hasley) were used to fit the sorption data. The GAB equation gave the best fit to the experimental data. The porosity of the dehydrated products depended on the drying method, ranging between 0.2 and 0.5. Freeze- and vacuum-dried pectin developed the highest porosity, whereas the lowest porosity was obtained using conventional and microwave drying. Hunter colour parameters (L, a, b) also depended on the drying method. The colour of freeze- and vacuum-dried pectin was close to that of commercial pectin, while the colour of the conventional- and microwave-dried product changed significantly.

Microwave vacuum drying of banana slices was investigated experimentally. This type of drying procedure is preferable to conventional drying techniques in order to avoid product degradation due to high temperatures encountered in convective drying. The drying process was examined by introducing pulsegenerated microwave power in banana samples. The material temperature was monitored. Temperature peaks in the last stages of drying indicated that drying could be favoured if temperature was maintained below a maximum level, so that the final product should not be burned by hot spots during microwave drying. This procedure produced dehydrated products of excellent quality as examined by taste, aroma, smell and rehydration tests.

The effective thermal diffusivity (αeff) of two granular starches was determined directly and indirectly, using the heated probe method of unsteady-state heat conduction. In the direct method, αeff was determined from transient time-temperature data, obtained with a thermocouple placed near the heated line source. In the indirect method, αeff was calculated from the effective thermal conductivity (λeff), the bulk density (ϱb), and the specific heat (Cp) of the granular starch in the moisture range 0–30% (dry basis) and at ϱb = 700 kg/m3, T = 25°C. The specific heat of the starches, determined by differential scanning calorimetry, increased linearly from 1260 to 1800 kJ/kg K in the moisture range 0–30% (dry basis). The indirect method yielded more accurate values of αeff (near 1 × 10−7m2/s) than the direct measurement, as concluded from the F-test at level of significance 5%.

Prediction of the effective thermal conductivity of granular and porous foods is essential in engineering calculations and in modeling of food processes. Structural models are more useful than empirical equations, since they are based on the physical and transport properties of the components of the food system. Six structural (geometric) models were tested, using experimental data on the effective thermal conductivity of granular starches in the ranges of bulk density 500–800 kg/m3, moisture content 0–0.4 kg water/kg dry solids and temperature 25 °–70 °C.The parallel model of heat conduction in the granular starch (solid/gas phases) and in the starch granules (dry starch/sorbed water phases) yielded the lowest standard deviation between the experimental and the predicted values of thermal conductivity. Combinations of the parallel and the Maxwell models yielded acceptable results.