D. Marinos-Kouris

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.

Equilibrium moisture content and heat of desorption of some vegetables

Abstract 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.

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.

DEEP FAT FRYING OF POTATO STRIPS—QUALITY ISSUES

Moisture loss and oil adsorption kinetics, structural properties (apparent density, true density, specific volume and internal porosity), color changes and viscoelastic behavior (compression tests, crispness) were investigated during deep fat frying of french fries. The effect of frying conditions (oil temperature, sample thickness and oil type), drying pretreatment and osmotic dehydration pretreatment on the above properties was also examined. The results showed that oil temperature and thickness of potato strips have a significant effect on oil uptake, moisture loss and color parameters of french fries, while the use of hydrogenated oil in the frying medium does not affect these properties. The porosity of french fries increases with oil temperature increases and sample thickness and it is higher for products fried with hydrogenated oil. Maximum stress and maximum strain increase during frying, while crispness of potato strips is higher for hydrogenated oil, and lower for refined oil. Air drying and osmotic pretreatment increase porosity of fried potatoes but decrease their oil and moisture content. A negative effect on color development with drying time was also observed. Pre-fry drying as well as osmotic pre-treatment increases the maximum stress and maximum strain of french fries during frying. Air drying pre-treatment increases the crispness of potato strips while osmotic pre-treatment does not affect it, with the exception of sugar solutions.

Effect of osmotic dedydration pretreatment on quality of french fries

Abstract Osmotic dehydration as a pretreatment before frying in order to produce low-fat french fries was examined. A relationship between frying kinetics and the type of solution used for osmotic dehydration before frying of french fries has been developed. The effect of osmotic dehydration on the structural properties (apparent density, true density, specific volume and internal porosity) and color parameters (lightness, a, b) of french fries was also investigated. Four types of solutions (sugar, NaCl, maltodextrine 12 and maltodextrine 21) were used. The results showed that osmotic pretreatment has a significant effect on oil uptake and moisture loss of french fries, as well as on the structural properties and color parameters of french fries. Osmotic dehydration pretreatment decreases oil and moisture content of french fries, while the porosity and intermit of color increased.

Effect of pre-treatment on viscoelastic behaviour of potato strips

Abstract The viscoelastic behaviour of french fries during deep fat frying after two different types of drying pre-treatment (air drying and osmotic dehydration) was examined under uniaxial compression tests. The stress-strain data of the compression test were modelled using a simple mathematical model, containing parameters such as the maximum stress ( σ max ), the maximum strain ( e max ), the elasticity parameter ( E ) and the viscoelastic exponent ( p ). The effect of air drying duration and the type of osmotic solution used on the compression behaviour of fried potatoes was introduced through the corresponding effects upon the model parameters. Pre-fry drying as well as osmotic pre-treatment increase the maximum stress and maximum strain of french fries during frying. Air drying pre-treatment increases the crispness of potato strips while osmotic pre-treatment does not affect it, with the exception of sugar solutions.

THE EFFECT OF PROCESS CONDITIONS ON THE DRYING KINETICS AND REHYDRATION CHARACTERISTICS OF SOME MW-VACUUM DEHYDRATED FRUITS

ABSTRACT 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.

HEAT AND MASS TRANSFER COEFFICIENTS IN DRYING: COMPILATION OF LITERATURE DATA

ABSTRACT Recently reported heat and mass transfer coefficient data in drying were retrieved from literature and were classified per dryer type and material. Most of the data were available in the form of empirical equations using dimensionless numbers. All available empirical equations were transformed in the form of Heat or Mass Transfer Factors versus Reynolds Number (jH = aRe n or jM = aRe n ). In the case that more than one equations were reported for the same dryer type, a new similar equation was re-fitted to consolidate the existing literature equations. It is expected that the resulting equations are more representative and predict more accurately the heat and mass transfer coefficients. Equations for each dryer type are also proposed.

VISCOELASTIC BEHAVIOR OF DEHYDRATED CARROT AND POTATO

Abstract Viscoelastic properties for carrot and potato were experimentally determined by stress relaxation tests using uniaxial compressive tests of cylindrical specimens at various deformation rates (5,10 and 20 cm/min). Stress relaxation tests were performed following conventional drying at 70°C and 15% humidity for various moisture contents ranging from 10 to 80%. The tests were performed using a Zwick testing machine and the stress relaxation data were modeled using a two-term Maxwell model. It was shown that the relaxation behavior of carrot and potato was not affected by deformation rate, but it was sensitive to moisture content. The remaining force and relaxation time of the elastic component were found to be depended on moisture content. Drying tends to decrease the remaining force and the elastic relaxation time of carrot and potato until a certain moisture content (1.7 and 1.9 kg/kg db). Further uptake of water tends to increase the remaining force and the elastic relaxation time for both materi...