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

The water adsorption isotherms of LM and HM pectins and pectin gels (made from HM pectin with the addition of sugar) were determined at 25C, using the standard gravimetric method developed by the European Cooperation Project COST 90. the water-binding properties of these materials were studied through examination of the sorption data. the experimental curves were fitted to one three-parameter equation (GAB equation), eight two-parameter equations reported in the literature, and a two-parameter equation developed in this work. the GAB equation gave the best fitting; however, the results showed that it should not be used for the estimation of the monolayer moisture content.

The water adsorption isotherms of Sultana raisins were determined using the static method of saturated salt solutions at 20°, 25°, 30°, and 35°C. The equilibrium moisture content at water activities up to 0.70 decreased as the temperature was increased from 20° to 35°C. At higher water activities the moisture content increased sharply as the temperature was increased, resulting in crossing of the isotherm curves. This behavior is attributed to the increased solubility of fruit sugars at higher temperatures. The experimental data were fitted well with the 3-parameter GAB and Halsey equations, and best with the 5-parameter D'Arcy-Watt equation. The thermodynamic implications are discussed.

Drying kinetics of four fruits (prune, quince, fig and strawberry) were studied by using a simple mass transfer mathematical model involving a characteristic parameter (K) as a function of process variables. The model was tested with data produced in a laboratory air dryer, using non-linear regression analysis. The investigation involved three values of sample thickness (5, 10, 15mm) and three different air temperatures (50, 60, 70 °C).The parameters of the model were found to be greatly affected by sample thickness and air temperature. The effect of moisture content on the porosity of three fruits (namely avocado, prune and strawberry) was also investigated. A simple mathematical model was used to correlate porosity with moisture content. It was found that porosity increased with decreasing moisture content.Samples of three fruits (avocado, prune and strawberry) were investigated to estimate color changes during conventional drying at 70 °C. A first order kinetic model was fitted to experimental data with great success. It is found that the color of avocado and strawberry change while the color of prune remains the same.Furthermore, the water sorption isotherms of three fruits (avocado, prune and strawberry) at 25° temperature were determined experimentally and the parameters at the GAB equation were evaluated by means of non-linear regression analysis. The use of the above equation produced a very good fit.

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.

The effect of drying conditions, namely, microwave power, vacuum pressure, as well as sample geometry, on the drying kinetics and rehydration characteristics of five microwave vacuum dehydrated fruits (apple, avocado, mushroom, pear and strawberry) was studied. The investigation involved a wide range of microwave power, vacuum pressure and sample size levels. The drying rate and the rehydration ratio, a measure of rehydration characteristics, were found to depend on the drying conditions. An empirical mass transfer model, involving a characteristic parameter for each process (drying and rehydration( as a function of process variables, namely, microwave power, vacuum pressure and sample size, was also tested with the data obtained in a microwave oven equipped with vacuum apparatus. Furthermore, a comparison of rehydration properties of microwave vacuum and conventionally dehydrated products, was conducted.

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.

The effect of drying conditions on color changes of apple, banana, carrot and potato during conventional and vacuum drying was investigated. The Hunder color scale parameters redness, yellowness and lightness were used to estimate color changes during vacuum and conventional drying at 50, 70 and 90°C. Air humidity during conventional drying was regulated at 15, 30 and 40%. Air temperature and humidity affected redness and yellowness, but not lightness. A first order kinetic model was fitted to experimental data adequately for both redness and yellowness. The rate of color deterioration was found to increase as temperature increased and air humidity decreased, for both drying methods and all the examined materials.

Microwave vacuum drying kinetics of three fruits (namely, apple, kiwi and pear) were studied by introducing an one-parameter empirical mass transfer model, involving a characteristic parameter (drying constant), as a function of process variables. The model was tested with data produced in a microwave oven equipped with vacuum apparatus, using non-linear regression analysis. The investigation involved a wide range of microwave power and vacuum pressure levels. The parameters of the model considered were found to be greatly affected by the microwave power level while vacuum pressure affected slightly the process.

Drying kinetics of four Fruits (namely, apple, pear, kiwi and banana) were studied by introducing an one-parameter empirical mass transfer model, involving a characteristic parameter (drying constant), as a function of process variables. The model was tested with data produced in a laboratory dryer, using non-linear regression analysis. The investigation involved a wide range of characteristic dimensions of samples and air conditions (temperature, humidity, and superficial velocity). The parameters of the model considered were found to be greatly affected by sample characteristic dimension and air temperature. The model also involved the equilibrium material moisture content which was formulated in the form of the GAB equation. Its parameters were evaluated by means of non-linear regression analysis to independent experiments covering two process variables (temperature and water activity of the surrounding air).

The equilibrium moisture content of potato, carrot, tomato, green pepper and onion was determined within the range of 10 to 90% water activity at three different temperatures (30, 45 and 60°C), using the standard static gravimetric method developed by the European Cooperation Project COST 90. The GAB equation was fitted to the experimental data, using the direct nonlinear regression analysis method and the agreement between experimental and calculated values was satisfactory. The net isosteric heat of desorption of water was determined from the equilibrium desorption data, using the Clausius-Clapeyron equation. An empirical exponential relationship between the net isosteric heat of sorption and the moisture content was proposed and validated.