Juan A. Cárcel

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.

Composition assessment of raw meat mixtures using ultrasonics.

The use of ultrasonic velocity measurements to determine the composition of dry fermented sausages was assessed. Mixtures of ground lean and fatty tissues were prepared to cover a wide range of fat (2-90 wt.%), moisture (7-76 wt.%), and protein (2-21 wt.%) contents. The ultrasonic velocity in fat decreased on average 5.6 ms(-1) per °C increase in temperature, due to the negative temperature coefficient for fat and the fat melting, which is observed in (DSC) differential scanning calorimetry analysis. The ultrasonic velocity temperature dependence allowed the determining of the fat, moisture and protein+others content, by measuring the ultrasonic velocity in the mixtures at 4 and 25°C and using a semi-empirical equation. The explained variance was 99.6% for fat, 98.7% for moisture and 85.4% for protein+others. The results obtained show the feasibility of using ultrasonic velocity measurement to assess the composition of meat products such as dry fermented sausages, rapidly and non-destructively.

Improvement of Convective Drying of Carrot by Applying Power Ultrasound—Influence of Mass Load Density

Power ultrasound is considered to be a novel and promising technology with which to improve heat and mass transfer phenomena in drying processes. The aim of this work was to contribute to the knowledge of ultrasound application to air drying by addressing the influence of mass load density on the ultrasonically assisted air drying of carrot. Drying kinetics of carrot cubes were carried out (in triplicate) with or without power ultrasound application (75 W, 21.7 kHz) at 40°C, 1 m/s, and several mass load densities: 12, 24, 36, 42, 48, 60, 72, 84, 96, 108, and 120 kg/m3. The experimental results showed a significant (p < 0.05) influence of both factors, mass load density and power ultrasound application, on drying kinetics. As expected, the increase of mass load density did not affect the effective moisture diffusivity (De, m2/s) but produced a reduction of the mass transfer coefficient (k, kg water/m2/s). This was explained by considering perturbations in the air flow through the drying chamber thus creating preferential pathways and, as a consequence, increasing external mass transfer resistance. On the other hand, it was found that the power ultrasound application increased the mass transfer coefficient and the effective moisture diffusivity regardless of the mass load density used. However, the influence of power ultrasound was not significant at the highest mass load densities tested (108 and 120 kg/m3), which may be explained from the high ratio (acoustic energy/sample mass) found under those experimental conditions. Therefore, the application of ultrasound was considered as a useful technology with which to improve the convective drying, although its effects may be reduced at high mass load densities.

Modeling Ultrasonically Assisted Convective Drying of Eggplant

Modeling constitutes a fundamental tool with which to analyze the influence of ultrasound on mass transfer phenomena during drying. In this work, the study of the effect of power ultrasound application on the drying kinetics of eggplant was addressed by using different models based on theoretical (diffusion) or empirical approaches. Drying kinetics of eggplant cylinders (height 20 mm and diameter 24 mm) were carried at 40°C and 1 m/s applying different ultrasonic powers: 0, 6, 12, 19, 25, 31, and 37 kW/m3. The experiments were carried out at least three times at each different ultrasonic power. Shrinkage and sorption isotherms were also addressed in order to attain an optimal description of eggplant drying. Applying ultrasound sped up the drying kinetics. The ultrasonic power was identified as having a significant (p < 0.05) influence on both the effective moisture diffusivity and the mass transfer coefficient, which was well explained by linear relationships. The most complex model, which considered both external resistance and shrinkage to be significant phenomena, provided the best agreement with experimental data, giving percentages of explained variance of over 99.9% and mean relative errors of under 1.2% in every case. According to these results, ultrasound technology could have the potential to improve the convective drying of eggplant at an industrial scale.

Application of low intensity ultrasonics to cheese manufacturing processes

Ultrasound has been used to non-destructively assess the quality of many foods such as meat, fish, vegetables and dairy products. This paper addresses the applications of low intensity ultrasonics in the cheese manufacturing processes and highlights the areas where ultrasonics could be successfully implemented in the future. The decrease of ultrasonic attenuation during the renneting process can be used to determine the optimum cut time for cheese making. The ultrasonic velocity increases during maturation for those types of cheese that become harder during this manufacturing stage, thus being an indicator of the maturity degree. Moreover, ultrasonic measurements could be linked to sensory parameters. From the ultrasonic velocity measurements at two different temperatures, it is possible to assess cheese composition, thus allowing an improvement in the quality and uniformity of cheese commercialization. In addition, in pulse-echo mode it is possible to detect cracked pieces due to abnormal fermentations and also to assess the distance of the crack from the surface.

Intensification of Low-Temperature Drying by Using Ultrasound

The main aim of this work was to test the feasibility of power ultrasound to intensify low-temperature drying processes. For this purpose, the convective drying kinetics of carrot, eggplant, and apple cubes (side 10 mm) were carried out at atmospheric pressure, 2 m/s, −14°C, and 7% relative humidity with (acoustic power 19.5 kW/m3) and without ultrasound application. Under the same experimental conditions, kinetics studies of ethanol removal from a solid matrix were also performed. Diffusion models were used to describe drying curves and identify kinetic parameters in order to evaluate and quantify the process intensification attained by ultrasound application. The effect of ultrasound application was similar for all products tested; that is, the drying time was shortened between 65 and 70%. In the case of ethanol removal, the time reduction achieved by ultrasound application was 55%. The mass transfer coefficient and effective moisture diffusivity increased by 96 to 170% and by 407 to 428%, respectively, when ultrasound was applied.

Improvement of water transport mechanisms during potato drying by applying ultrasound

BACKGROUND The drying rate of vegetables is limited by internal moisture diffusion and convective transport mechanisms. The increase of drying air temperature leads to faster water mobility; however, it provokes quality loss in the product and presents a higher energy demand. Therefore, the search for new strategies to improve water mobility during convective drying constitutes a topic of relevant research. The aim of this work was to evaluate the use of power ultrasound to improve convective drying of potato and quantify the influence of the applied power in the water transport mechanisms. RESULTS Drying kinetics of potato cubes were increased by the ultrasonic application. The influence of power ultrasound was dependent on the ultrasonic power (from 0 to 37 kW m(-3) ), the higher the applied power, the faster the drying kinetic. The diffusion model considering external resistance to mass transfer provided a good fit of drying kinetics. From modelling, it was observed a proportional and significant (P < 0.05) influence of the applied ultrasonic power on the identified kinetic parameters: effective moisture diffusivity and mass transfer coefficient. CONCLUSIONS The ultrasonic application during drying represents an interesting alternative to traditional convective drying by shortening drying time, which may involve an energy saving concerning industrial applications. In addition, the ultrasonic effect in the water transport is based on mechanical phenomena with a low heating capacity, which is highly relevant for drying heat sensitive materials and also for obtaining high-quality dry products.