M.K. Krokida

Drying kinetics of some vegetables

Abstract The effect of air conditions (air temperature, air humidity and air velocity) and characteristic sample size on drying kinetics of various plant materials (potato, carrot, pepper, garlic, mushroom, onion, leek, pea, corn, celery, pumpkin, tomato) was examined during air drying. A first-order reaction kinetics model was used, in which the drying constant is function of the process variables, while the equilibrium moisture content of dried products within the range of 0.10–0.90 water activity at two temperatures (30 and 70 °C) was fitted to GAB equation. The parameters of the model considered were found to be greatly affected by the air conditions and sample size during drying. In particular the temperature increment increases the drying constant and decreases the equilibrium moisture content of the dehydrated products.

Rehydration kinetics of dehydrated products

The rehydration kinetics of various dehydrated fruits and vegetables (apple, potato, carrot, banana, pepper, garlic, mushroom, onion, leek, pea, corn, pumpkin, and tomato) were examined during rehydration at different temperatures. Dehydrated products were produced in an air dryer at 70 °C and 10% air humidity, and subsequently they were rehydrated in a bath of water tempered at 40, 60 and 80 °C. A first order kinetic model was used, in which the rate constant and the equilibrium moisture content are functions of the process variable (water temperature). The parameters of the model considered were found to be greatly affected by the water temperature during rehydration. In particular the temperature increment increases the rate of rehydration and the equilibrium moisture content of the rehydrated products.

Water loss and oil uptake as a function of frying time

Abstract A relationship between moisture loss and oil adsorption with frying time during deep fat frying of french fries has been developed. A first order kinetic equation was used, in which the rate constant is a function of the main process variables (oil temperature, sample thickness and oil type). This model was applied to a wide range of experimental data and its parameters were estimated using non-linear regression analysis. The results showed that oil temperature and thickness of potato strips have a significant effect on oil uptake and moisture loss of french fries, while the use of hydrogenated oil in the frying medium does not affect mass transfer phenomena.

KINETICS ON COLOR CHANGES DURING DRYING OF SOME FRUITS AND VEGETABLES

Abstract 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.

Effect of freeze-drying conditions on shrinkage and porosity of dehydrated agricultural products

Abstract Structural properties such as particle density, bulk (or apparent) density and porosity of apple, banana, carrot and potato were investigated after freeze-drying under various drying conditions. The materials were freeze-dried at various pressures, to achieve sample temperature ranging between −50 and −5 °C. For temperatures above the glass transition temperature of the concentrated amorphous solution ( T ′ g = −45 ° C ), the freeze-dried materials decreased in size. The bulk density of the freeze-dried materials was increased with temperature and an exponential equation was used to express this influence. The final porosity decreased as the sample temperature increased for all the materials examined, ranging between 70 and 95%. The final porosity was related to the complex viscosity of frozen apple and potato which decreased linearly as the temperature increased in the temperature range examined.

Colour changes during deep fat frying

The effect of frying conditions on the colour changes during deep fat frying of french fries has been investigated. The Hunter colour scale parameters redness, yellowness and lightness were used to estimate colour changes during frying as a function of the main process variables (oil temperature, oil type and sample thickness). A first-order kinetic equation was used for each one of the three colour parameters, in which the rate constant is a function of the main process variables. The results showed that oil temperature and thickness of potato strips have a significant effect on the colour parameters, which are not affected by the use of hydrogenated oil in the frying medium.

Effect of pre-drying on quality of french fries

Abstract The effect of duration of pre-drying on structural properties (apparent density, true density, specific volume and internal porosity) and color parameters (lightness, a, b) of fried potatoes was also investigated. The results showed that pre-drying pre-treatment had a significant effect on oil uptake and moisture loss, as well as structural properties and color parameters of french fries. Air drying decreased the oil and moisture contents of french fries, while the porosity increased. A negative effect on color development with drying time was also observed. A relationship between frying kinetics and drying time before frying of french fries has been developed.

EFFECT OF MICROWAVE DRYING ON SOME QUALITY PROPERTIES OF DEHYDRATED PRODUCTS

ABSTRACT The effect of microwave and microwave-vacuum drying on some quality properties such as density, porosity, color and viscoelastic behavior of dehydrated apple, banana, carrot and potato was investigated. It is concluded that microwave drying and moreover microwave-vacuum drying tends to increase the product porosity and to prevent the color damages during drying. Microwave drying seems to decrease the maximum stress and maximum strain of dehydrated products, while it increases their elasticity and decreases their viscous nature

RHEOLOGICAL PROPERTIES OF FLUID FRUIT AND VEGETABLE PUREE PRODUCTS: COMPILATION OF LITERATURE DATA

Recently published values of rheological properties of fluid fruit and vegetable puree products were retrieved from the literature and analyzed. The results of more than 10 materials are presented, concerning the reported ranges of variation of Consistency Coefficient and Flow Behavior Index, together with the corresponding ranges of variation of concentration and temperature. The related literature sources are presented for each material. Empirical models, relating both Consistency Coefficient and Flow Behavior Index to concentration and temperature, are proposed and fitted to all retrieved data for each material. The data were screened carefully, using residual analysis techniques.

Effect of drying method on the sorption characteristics of model fruit powders

Abstract 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.