N. Sanjuán

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.

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).

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.

Effect of rehydration temperature on the cell wall components of broccoli (Brassica oleracea L. Var. italica) plant tissues

Broccoli florets and stems tissues were rehydrated at different temperatures, namely 25°C, 40°C, 55°C, 65°C and 80°C. Water sorption capacity (WSC) values exhibited by broccoli tissues largely depended on the temperature used. Broccoli tissues showed maximum WSC values when rehydration was performed at relatively low temperatures. A significant correlation was found between WSC and the total arabinose content of broccoli tissues. Further, sequential extraction of pectic polymers revealed that samples with lower WSC exhibited a larger solubilisation of neutral pectic side-chains, the florets sample being rehydrated at 40°C which better maintained the original pectin structure. Overall, the results suggest that arabinose-rich pectin neutral side-chains might play an important role on the water sorption process of dehydrated broccoli tissues.

Use of ultrasound to assess Cheddar cheese characteristics.

Blocks of Cheddar cheese were matured in temperature-controlled chambers at 5 and 12 degrees C. The ultrasonic velocity increased during maturation ranging from 1657 to 1677 ms-1 at 12 degrees C and from 1684 to 1693 ms-1 at 5 degrees C. The ultrasonic velocity was related to the square root of the deformability modulus and the slope in puncture. The increase of velocity during maturation shows the feasibility of using an ultrasonic device to non-destructively monitor Cheddar cheese maturity. Ultrasound velocity was measured at different temperatures. The velocity decreased with increasing temperature, and from the slope of the first part of the temperature-velocity curves it was possible to non-destructively assess the moisture content of different types of cheese.

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.

Modeling the Dehydration of Apple Slices by Deep Fat Frying

One of the objectives of immersion frying is to remove water from the food. Thus, predicting moisture loss is important when developing a model for that process. With the aim to model moisture transfer during apple frying, Granny Smith apple slabs were processed at 140, 150, 160, and 170°C. The moisture diffusivity was computed, ranging between 10.7 · 10−9 and 17.7 · 10−9 m2 · s−1. There was a close fit between the model and the experimental data (average %var 99.0). Model validation was carried out considering different slice thickness. Temperature influence was interpreted by the Arrhenius relationship with an activation energy of 25.4 kJ · mol−1.

Modelling drying kinetics of thyme (Thymus vulgaris L.): Theoretical and empirical models, and neural networks

The drying kinetics of thyme was analyzed by considering different conditions: air temperature of between 40 ℃ and 70 ℃, and air velocity of 1 m/s. A theoretical diffusion model and eight different empirical models were fitted to the experimental data. From the theoretical model application, the effective diffusivity per unit area of the thyme was estimated (between 3.68 × 10−5 and 2.12 × 10 −4 s−1). The temperature dependence of the effective diffusivity was described by the Arrhenius relationship with activation energy of 49.42 kJ/mol. Eight different empirical models were fitted to the experimental data. Additionally, the dependence of the parameters of each model on the drying temperature was determined, obtaining equations that allow estimating the evolution of the moisture content at any temperature in the established range. Furthermore, artificial neural networks were developed and compared with the theoretical and empirical models using the percentage of the relative errors and the explained variance. The artificial neural networks were found to be more accurate predictors of moisture evolution with VAR ≥ 99.3% and ER ≤ 8.7%.

Determination of Shrinkage Function for Pork Meat Drying

The shrinkage of pork meat cylinders was measured and correlated with moisture content. Samples of different sizes were dehydrated under different drying conditions: forced convection (25°C and 0.6, 2.0, and 2.8 m/s) and natural convection (5, 10, 15, and 20°C). A linear relationship was found between the quotients R/R0 and V/V0 and the moisture content. This linear relationship was not found to be dependent on the size of the samples, their salt content, or drying conditions. For the experimental conditions in this study, water losses are responsible for shrinkage.

Ultrasound Effects on the Mass Transfer Processes during Drying Kinetic of Olive Leaves (Olea Europea, var. Serrana)

The large amount of materials derived from olive trees pruning may constitute a source of compounds with antioxidant activity. The air drying of raw matter is a previous stage before extraction preserving the raw matter and avoiding the interference of water on the process but it constitutes a slow and high energy demanding process. In this sense, the application of high intensity ultrasound could affect the mass transfer rate. In order to address the influence of ultrasound on the drying process of olive leaves, air drying experiments were carried out (40°C and1 m/s) without (0 W) and with ultrasound application (8, 16, 25 and 33 kW/m3). The different drying conditions were tested by triplicate. To model the experimental data, diffusional models considering or not the external resistance to mass transfer were used. The model without considering external resistance did not fit well to the experimental data. However, the model including external resistance achieved percentages of explained variance above 99.7%. The influence of ultrasound on diffusivity was not significant but a significant dependence was found for the mass transfer coefficient, proportional to the power density applied.