A. Mulet

Drying Modelling and Water Diffusivity in Carrots and Potatoes

Modelling is a useful way to validate mechanisms of drying and to establish properties, and thus can be used to determine the reliability of calculated values of the effective diffusivity (De). Diffusive models are considered for the description of convective drying of particulate vegetables (carrots and potatoes). Four models with different degrees of complexity are discussed. The degree of complexity depends on consideration of shrinkage, heat transfer and particle temperature variation during drying, and the De dependence on moisture and temperature simultaneously. Experiments have been carried out to test the models and the assumed boundary conditions. Experimental results show that the effect of shrinkage cannot be neglected in establishing reliable values for De. The effect of the external resistance, which is affected by the velocity of the air used in drying, also appears to be an important factor for testing De values. Two methods are proposed to carry out this test, both of which could be readily applied if the drying curve is available. Experiments on drying carrots and potatoes are used to illustrate the procedures.

New Food Drying Technologies - Use of Ultrasound:

Reducing water availability is one way to preserve food. Water in solid foods is transferred to a fluid, either gas or liquid; during this process both internal and external resistance affect water transfer from the food. As a consequence, any means to reduce those resistances constitute an improvement of the process, and ultrasound appears to be a way to reduce those resistances. Ultrasound are mechanical waves that produce different effects when travelling through a medium. Among others, those related to mass transfer include micro-stirring at the interface, the so called “sponge effect” and cavitations. Ultrasound has so far been applied to dehydration in solid-gas systems like onion drying. Nevertheless, the difficulties for the propagation in the air have led to the development of specially adapted transducers that have been applied in the drying of carrots. In solid-liquid systems, ultrasound has been used in the treatment of products immersed in hypertonic solutions, either in sugar solutions for fruits like apples or in salt brine in the case of cheese or meat. An increase in mass transfer is achieved if the threshold power value for the product is attained.

High intensity ultrasound effects on meat brining

Pork loin (longissimus dorsi) samples of two different geometries, cylinders and slabs, were immersed in saturated NaCl brine for 45min under different conditions: without brine agitation (STAT), with brine agitation (AG) and with ultrasound application (US) at eight levels of ultrasonic intensity. Moisture content change and NaCl gain were considered in order to evaluate the difference in the brining treatments. No significant differences were found in moisture and NaCl content of samples treated under STAT conditions and AG conditions, while the influence of ultrasound on the mass transfer process during meat brining depended on the intensity applied. There was an ultrasonic intensity threshold above which the influence of ultrasound appeared. At the highest level of intensity studied, the water content of samples was significantly higher than the initial water content of meat. As regards NaCl transfer, once above the intensity threshold, the increase in the NaCl content was proportional to the applied ultrasonic intensity. Not statistically significant differences were found for sample geometry.

Influence of High-Intensity Ultrasound on Drying Kinetics of Persimmon

Drying persimmon pieces is recognized as a way to preserve and add value to the excess production of the fruit in Spain. To this end, air drying kinetics of persimmon cylinders (30 mm height and 13 mm diameter) were determined under different drying conditions: 8 air drying velocities (0.5, 1, 2, 4, 6, 8, 10, and 12 m/s) with and without application of high-intensity ultrasound (21.8 kHz and 154.3 dB). The drying process was modeled using two diffusion models with and without the influence of external resistance to drying. From the effective diffusivity and the mass transfer coefficient identified from the data it was concluded that high-intensity ultrasound increased the drying rate at the lowest air velocities tested, affecting both external and internal resistances.

Influence of the Applied Acoustic Energy on the Drying of Carrots and Lemon Peel

The application of power ultrasound could constitute a way of improving traditional convective drying systems. The different effects produced by the application of power ultrasound may influence the drying rate without provoking any significant increase in product temperature. Due to the fact that the effect of power ultrasound is product dependent, the aim of this work was to address the influence of the applied acoustic energy on the convective drying of carrot and lemon peel. Convective drying kinetics of carrot cubes (side 8.5 mm) and lemon peel slabs (thickness 7 mm) were carried out at 40°C and 1 m/s by applying different levels of acoustic power density: 0, 4, 8, 12, 16, 21, 25, 29, 33, and 37 (kW/m3). The application of power ultrasound during drying was carried out using an airborne ultrasonic transducer (21.7 kHz). Drying kinetics were described considering a diffusion model. In both products, the application of power ultrasound improved the effective moisture diffusivity (De ). The improvement was linearly proportional to the applied acoustic power density. In the case of lemon peel, the effects of power ultrasound were found over all the range tested (0–37 kW/m3), whereas in the case of carrot, it was necessary to apply an acoustic power density of over 8–12 kW/m3 to be able to observe the influence. The more intense effect of acoustic energy in lemon peel drying may be explained by the fact that lemon peel is a more porous product than carrot.

DRYING OF CARROTS. I. DRYING MODELS.

ABSTRACT Three models of different complexity are proposed to describe the falling rate period of the carrot drying process with shrinkage. A moving or fixed boundary problem as well as a constant or local moisture and temperature dependent effective diffusivity are considered. The moving boundary problem is solved by an explicit finite difference method. Heat transfer coefficient and effective diffusivity identification were carried out. The results of the heat transfer coefficient show a good agreement with other sources. Using experimental data and the models. describing the heat and mass transfer three different expressions for the effective diffusivity are established. Two of them are only temperature dependent considering or not particle shrinkage. The third one takes into account temperature and local moisture as well as shrinkage. Drying of foods is a complicated process involving simultaneous coupled heat and mass transfer phenomena which occur inside the material being dried (Chiang and Petersen...

Drying models for green peas

Abstract A fixed or moving boundary problem was considered for the drying of green peas. The first model was solved by separation of variables, assuming that sample size and geometry remain constant during the process. For the second model, a finite difference method was used. Using experimental data from experiments carried out at different air-drying temperatures (40, 60 and 80 °C), two different expressions, of Arrhenius type, for the effective diffusivity dependence on the air temperature were obtained. Throughout these expressions, it was possible to simulate the drying kinetics of green peas at temperatures (50, 70 and 90 °C) different from those used to obtain the models. The second model was found to be more precise (percentage of explained variance > 99.8%) than the first one (>98.4%).

Equilibrium isotherms and isosteric heats of morel (Morchella esculenta)

Abstract Moisture desorption isotherms of Morchella esculenta mushrooms were determined at 5, 15, 25 and 35 °C and within the range of 0.11–0.92 water activity using a standardized conductivity hygrometer. Five models were considered to describe isotherms (GAB, Oswin modified, Henderson modified, Halsey modified and Ratti) being fitted to the experimental data by direct non-linear regression. The GAB and Ratti equations gave the best fit of the experimental data. Isosteric heats of water desorption were obtained from the experimental data by applying the Clausius–Clapeyron equation within the range of 0.04–0.50 moisture content (dry basis).

Drying of shrinking cylinder-shaped bodies

Abstract A mathematical model has been developed for the prediction of sample temperature, average moisture and moisture distribution in a cylinder-shaped solid during the drying process. The effect of shrinkage was taken into account. The macroscopic heat balance and the microscopic mass balance combined with Ficks law were simultaneously solved using the Runge-Kutta-Merson method and a numerical finite difference method. The effective diffusion coefficient was expressed as a function of sample temperature and local moisture content. Using an experimental drying curve determined at 90 °C, the diffusional equation was identified for broccoli stems, and was used to predict the average and local moisture content variations during the process. Good agreement with experimental results was found (average percentage of explained variance was 99.8%).

Modelling of broccoli stems rehydration process

Abstract Using a diffusional model, mass transfer in rehydration of slab-shaped bodies was simulated. For that purpose the rehydration kinetics of broccoli stem slabs was investigated and the influence of temperature determined. It was observed that rehydration rate increased with water temperature but the final moisture content decreased. A good agreement between the model and the experimental data was obtained when D eff and W e values were identified for each temperature (average %var 99.3). The temperature influence on D eff was interpreted by the Arrhenius relationship, with an activation energy of 17.9 kJ/mol.