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The application of power ultrasound sped up the drying kinetics at every temperature tested, achieving drying time reductions of up to 77%, which was linked to the improvement in diffusion and convective mass transport. In overall terms, ultrasound application involved a greater degradation of polyphenol and flavonoid contents and a reduction of the antioxidant capacity, which was related to the cell disruption caused by the mechanical stress of acoustic waves. 43

Low temperature drying (LTD) allows high-quality dried products to be obtained, preserving the nutritional properties of fresh foods better than conventional drying, but it is a time-consuming operation. Power ultrasound (US) could be used to intensify LTD, but it should be taken into account that process variables, such as the level of applied power, have an influence on the magnitude and extension of the ultrasonic effects. Therefore, the aim of this work was to assess the influence of the level of applied ultrasonic power on the LTD of apple, analyzing the drying kinetics and the quality of the dried product. For that purpose, apple (Malus domestica cv. Granny Smith) cubes (8.8mm side) were dried (2m/s) at two different temperatures (10 and -10°C), without and with (25, 50 and 75 W) US application. In the dried apple, the rehydration kinetics, hardness, total phenolic content, antioxidant capacity and microstructure were analyzed to evaluate the impact of the level of applied ultrasonic power. At both temperatures, 10 and -10°C, the higher the ultrasonic power level, the shorter the drying time; the maximum shortening of the drying time achieved was 80.3% (at -10°C and 75 W). The ultrasonic power level did not significantly (p<0.05) affect the quality parameters analyzed. Therefore, US could be considered a non-thermal method of intensifying the LTD of fruits, like apple, with only a mild impact on the quality of the dried product.

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The aim of this work was to evaluate the feasibility of power ultrasound (US) application during the low temperature drying (LTD) of eggplant, analyzing the influence of the process variables linked to the air flow (velocity and temperature) on the drying kinetics and different quality aspects of the dehydrated product. For that purpose, eggplant (Solanum melongena var. Black Enorma) cubes (8.6 mm side) were dried at different air velocities (1, 2, 4 and 6 m/s) and temperatures (10, 0 and-10ºC) without (AIR) and with (AIR+US) US application. The rise in the air velocity and temperature led to an increase in the drying rate in AIR experiments. US application accelerated the drying process under every experimental condition tested, shortening the drying time by up to 87%. As for the quality parameters, no remarkable influence of the process variables (US application, air velocity and temperature) on the rehydration, reconstitution in olive oil or hardness of the rehydrated product was observed.

The relevance of the people you love most is trully revealed when they are apart. Life goes on, memories remain.To my brother, Toño. La relevancia de las personas que más quieres se manifiesta realmente cuando no están. La vida sigue, los recuerdos permanecen.A mi hermano, Toño. AknowledgementsIt has been a long time since, after being involved for the first time in research during the realization of my Master Thesis, Víctor Sánchez gave me the oportunity to work with him to investigate subharmonic generation in resonant cavities. Soon after, he proposed me to start my PhD in the context of wave propagation in sonic crystals. I remember quite well my doubts and lack of confidence to start a PhD on this topic at that time. However, it was easy for Víctor to convince me, as science is a truly passion for him and he transmits it to everyone in his sorroundings. I am really grateful to him for sharing this years working together and totally convinced that I made the right choice. I also appreciate my other PhD supervisor, Rubén Picó, for his outstanding contribution and attitude, always open to discussion and helpful.Fortunately, by the time I started my PhD studies, our research group experimented a quick growth and several researchers gradually started to work with us, enriching my work deeply. Vicent Romero worked in Gandía for two years, and still collaborate with us. He made a huge contribution to our research group and to this thesis. He helped me in many ways and situations, including numerical simulations, experimental techniques, and any doubt I might have. I also want to acknowledge Lluis García Raffi, who collaborated a lot with me too. He is always ready to share his knowledge in mathematics and physics with me and able to generate a huge amount of ideas and proposals. I also appreciate the contributions from other members of the group, such as Javier Redondo, and the people from ICREA in Terrasa. I appreciate the contributions from Kestutis Staliunas, whose astonishing knowledge of physics is always an inspiration to us, and Yu-Chieh Cheng, our collaborations working in periodic structures in optics and acoustics were of great interest for me and her extreme efficiency in every task she performs is amazing.Chapter 5 of this thesis, and other works that are not included here, were developed in the University of Colorado at Boulder, in the USA, were I spent almost one year. I would like to acknowledge Mahmoud Hussein, from Dpt. of Aerospace Engineering Sciences, for giving me the oportunity to integrate myself in his group to work in the development of numerical techniques for elastic band structure calculation, and also for pushing hard to start and mantain a collaboration with our group in Gandía. It was an amazing experience were all the members of the group made me feel at home. Osama R. Bilal, Michael Frazier, Romik Khajehtourian, Dimitri Krattiger, Clémence Basquet and Hossein Honarvar, thank you all.Además de las personas con las que he trabajado en tareas directamente relacionadas con mi tesis, mi...

28The application of US increased the drying rate at every temperature tested, 29 shortening the drying time by 16% at 0ºC and up to 60% at -10ºC. The ultrasonically 30 assisted dried samples presented a rehydration rate which was slightly lower than that 31 of those that had been conventionally dried, but they were harder and whiter, which is 32 more suited to consumer preferences. Therefore, power ultrasound could be 33 considered an affordable technology with which to accelerate LTD of desalted cod, 34 providing high quality dried products. 35 36

Atmospheric freeze drying assisted by power ultrasoundTo cite this article: J V Santacatalina et al 2012 IOP Conf. Ser.: Mater. Sci , USAFD) and without (AFD) ultrasonic application. A diffusion model was used in order to quantify the influence of US in drying kinetics. To evaluate the quality of dry products, rehydration capacity and textural properties were determined. The US application during AFD of carrot involved the increase of drying rate. The effective moisture diffusivity identified in USAFD was 73% higher than in AFD experiments. On the other hand, the rehydration capacity was higher in USAFD than in AFD and the hardness of dried samples did not show significant (p<0.05) differences. Therefore, US application during AFD significantly (p<0.05) sped-up the drying process preserving the quality properties of the dry product. IntroductionThe increasing need to achieve an equilibrium between high quality of dried products and lowoperating cost has induced researchers to investigate the use of new techniques to intensify drying processes. From a quality point of view, freeze drying is considered to be one of the best food drying methods, but it is quite expensive due to freezing and the low pressures applied [1]. Atmospheric freeze drying (AFD) is an alternative consisting in the use of cold gas with low water vapor pressure as drying medium to cause direct moisture sublimation from frozen material at atmospheric pressure [2]. This technique could combine the advantages of both freeze drying (high product quality) and convective drying (low cost and continuous process). However, it presents as main drawback longer drying times than in traditional freeze drying processes.Power ultrasound (US) has been recently applied in hot air drying of different products reducing both the drying times and energy costs. The potential of power ultrasound to improve mass transfer phenomena during drying of several fruits and vegetables has been showed in previous works [3] [4]. Ultrasonic energy could be applied during AFD to accelerate the drying process due to the mechanical energy introduced in the medium, which could contribute to reduce both the external and the internal mass transfer resistance.