J. Bon

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.

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.

Water and salt diffusion during cheese ripening: effect of the external and internal resistances to mass transfer

Abstract High humidities of drying medium lead to lower drying rates, and both external or internal conditions determine the drying rate. Thus, a diffusional model has been developed assuming that the external resistance to mass transfer could not be neglected in these cases and solved by a finite difference method. The external mass transfer coefficient was estimated from the literature. This mathematical model was used to identify water and salt effective diffusivity coefficients by using experimental data of ripening experiments carried out on parallelepipedal Mahon cheeses of 0.14 m ×0.14 m ×0.09 m edges kept at 12°C and 85% RH. Using these identified values, 7.8×10 −12 m 2 / s for moisture diffusion and 5.3×10 −10 m 2 / s for salt diffusion, average moisture content and water and salt profiles during the ripening of 0.20 m ×0.20 m ×0.10 m cheeses ripened at 12°C and two different relative humidities, 70% and 80% RH, were accurately simulated.

EFFECT OF SHAPE ON POTATO AND CAULIFLOWER SHRINKAGE DURING DRYING

ABSTRACT Shrinkage of potato cubes, parallelepipeds and cylinders was addressed during drying. A basic methodology by using image analysis was set-up in order to measure and monitor geometrical changes along the drying process. Both measurements by image analysis and directly with a calliper were carried out. Results from both methods showed a good agreement with no bias. Particle shape was found to influence shrinkage in a different way according to the axis. In fact the larger dimension shrinks less than the shorter one. This could be linked to the influence of the core drying in shrinkage. Experiments were also carried out for cauliflower stems. The difference in shrinkage between directions was greater for cauliflowers, due probably to the effect of alignment of fibers along the axis. Good agreement with existing data in the literature for specific volume was found.

Drying characteristics of hemispherical solids

Abstract A diffusional model has been proposed to simulate the drying curves of hemispherical bodies. The equation representative of the mass transfer in terms of Ficks law for a hemispherical shaped body has been solved by separation of variables. By using this solution and the optimization capabilities included in Microsoft Excel 5.0™ spreadsheet, an effective diffusivity coefficient was identified for potatoes at different air drying temperatures (30, 50, 70 and 90 °C). Effective diffusivity varied with air drying temperature according to the Arrhenius equation. Experimental data obtained using the same and different experimental conditions of air drying temperature (from 30 to 90 °C) and sample size (hemispheres with radius from 0.0128 to 0.0230 m) could be predicted by using the proposed model (%var = 99.7%). The importance of taking into account the real geometry of the solid was evaluated by using the solution for a spherical shaped body. The model developed for a sphere provided less satisfactory results in the simulation of the drying experiments mentioned (%var = 96.6%).

Enthalpy-Driven Optimization of Intermittent Drying

Several optimization problems pertinent to intermittent drying of biological materials were analyzed and a tool to resolve each optimization problem was developed. As the product quality is related to the particularities of the thermal treatment, an average enthalpy gain of the product weighted by the maximum enthalpy gain was considered as the objective function to be minimized. A diffusion model was used to estimate the objective function, and process simulations were performed for batch drying of grains in the rotating jet spouted bed. The results show that the optimization of intermittent drying improves considerably the energy performance of such a drying process. Moreover, it offers better product quality due to lower enthalpy gain.

Effect of Blanching and Air Flow Rate on Turmeric Drying

Turmeric processing involves two main steps, blanching and drying. Blanching is a common step in the traditional processing of rhizomes, and hot air drying is an alternative to traditional solar drying. For this study, drying kinetics were performed at different air flow rates (0.2, 0.5, 0.7, 1.2, 2.1, 2.6, 3 and 4m/s) to determine the effect of air flow on the process. To examine the blanching effect, drying kinetics were carried out with blanched and unblanched rhizomes at different temperatures (60, 70, 80, 90 and 100ºC). A diffusion model and two empirical models (Weibull and Peleg) were used to describe mass transfer during drying. The effect of air flow rate on external resistance was observed, and the air velocity transition zone between the external and internal resistance control zone was identified (1–2m/s). Blanching previous to drying increased the process rate at all the temperatures tested, although its effect was reduced when the air drying temperature increased. Empirical models fitted better drying kinetics than the diffusion model, however, the diffusion model provides valuable information about the phenomenon of water removal and scaling up.

Review: Low intensity ultrasonics in food technology / Revisión: Ultrasonidos de baja intensidad en tecnología de alimentos

Ultrasonic applications can be classified into low intensity or high intensity applications. The latter are used to modify a process or product with ultrasonics, while in low intensity applications the process or product modifies the ultrasonic signal, thus providing information about the product. Low inten sity ultrasonics in food technology can be used to monitor a process (liquid level, flowmeters) or to determine the quality of food products. Since ultrasonic techniques are rapid, non-destructive, easy to automate and relatively inexpensive, the number of applications is rapidly growing in this field. Ultrasonics can also be considered for use in laboratory testing devices to determine physical and chemical properties of foods. Ultrasonics has been used to determine texture, composition and physical state in liquid and solid foods. The commonly measured ultrasonic parameters are velocity, attenua tion and frequency spectrum composition. Velocity is the parameter used most since it is the simplest and most reliable measurement. This paper reviews the basic principles of ultrasonics, the most suit able techniques for each type of application, the testing devices needed to make measurements and the most interesting applications.

Modelling Thermodynamic Properties of Banana Waste by Analytical Derivation of Desorption Isotherms

Banana is an agricultural product of great economic importance for various developing countries. The relationship between moisture content and water activity provides useful information for the processing and storage of banana waste. The water activity and moisture content of three banana (Mussa spp. Haploid AAB cv. Nanica) waste items were analyzed to determine the desorption isotherms at six different temperatures (20, 30, 40, 50, 60 and 70°C). The desorption isotherms of the peel, pedicel and pulp of overripe bananas were determined in wide ranges of moisture content (0.001–6.360 kg kg-1 d.b.) and water activity (0.02–0.907). The theoretical GAB model was used for modelling the desorption isotherms. An analytical solution of the Clausius-Clapeyron equation was proposed to compute the isosteric heat of sorption, the differential entropy and Gibbs’ free energy by way of the GAB model when the effect of temperature on the hygroscopic equilibrium was considered.