https://doi.org/10.15255/KUI.2009.022
Published: Kem. Ind. 59 (4) (2010) 169–177
Paper reference number: KUI-22/2009
Paper type: Review
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Application of high power ultrasound in drying of fruits and vegetables

A. Režek Jambrak, V. Lelas, Z. Herceg, M. Badanjak Sabolović and Z. Werner

Abstract

Ultrasound is a sound frequency in the range between 18 and 100 kHz that is above hearing of the human ear. High power ultrasound means application of intensities higher than 1 W cm–2 (usually in the range between I=10–1000Wcm–2). High power and low frequency ultrasound (f = 20 to 100 kHz) is considered as “power ultrasound” because its application causes cavitation and is applied in the food industry. High power ultrasound is applied for degassing of liquid food, for induction of oxidation/reduction reactions, for extraction of enzymes and proteins, for inactivation of enzymes and induction of nucleation for crystallization. Ultrasound is anticipating heat transfer; it is used for emulsifying, sterilization, extraction, degassing, filtrating, drying and induction of oxidation. Conventional hot air drying is a very energy- and cost-intensive process. Drying is a simultaneous operation of heat and mass exchange that is followed by phase changes. Application of different pretreatments, like osmotic dehydration, ultrasound and ultrasound assisted osmotic dehydration has shown different effects on fruits and vegetables. When the high intensity acoustic energy is passing through solid material, it causes several fast and successive compressions and rarefactions with speeds that depend on the frequency applied. Thus, material is exposed to a series of exchangeable squeezing and relaxations, very like continuous squeezing and releasing of the sponge. This mechanism known as “rectified diffusion” is very important in acoustic drying and migration of water. Application of ultrasound as a pretreatment has shown great influence on reducing afterward hot air drying thereby reducing total drying time. It is also shown that pretreatment before drying facilitates better mass transfer and water diffusivity than osmotic dehydration. Quality of the product after drying is better because ultrasound pretreatment is applied at room temperature thus reducing deteriorating alterations and nutritive loss of compounds in fruits and vegetables. In this paper, the basic theory of ultrasound has been described. In the figures, the range of sound by frequency is described, as is the amplitude of ultrasound and its effect on the material also l, the wavelength and attenuation coefficient have been explained. The most common usage of power ultrasound as probe type of high intensity ultrasound set system with piezoelectric transducer have been introduced as a system for ultrasound drying. When the ultrasound wave passes through material the basic effect occurs. It is called cavitation and is divided generally in two types: stable and transient cavitation. Also, when imploding cavitation bubble causes elevated temperatures and pressures several chemical reactions can happen. In the figure one can see the visualisation of sonochemical reactions that occur around the decomposed bubble. Several reactions of decomposition, polymerisations, formation of aggregates, breaking of aggregates, bonding, breaking of bonds, formation of radicals, hydroxyl radicals etc. The main aim of this paper was to introduce the new non-thermal pre-treatment or direct treatment of ultrasound probe, or set of probes in the drying of vegetables and fruits. The basic target of the paper was to improve the knowledge of experts in the food industry and technologists in the chemical and other industries to learn of the possibility of implementing new techniques in their facilities. One can observe the modern scheme of the system for drying of materials with direct contact and also the example of the ultrasound treatment of fruits, water diffusivity in fruit and total processing time of applying ultrasound. The most important thing is to conduct the drying process in the best way to reduce treatment time, and to optimize the system. The most commonly used mathematical models for determination of drying kinetics have been pointed out. The lack of information is about energy input, and the total quality of dried fruits and vegetables and for that purpose in the future several scientific projects need to be conducted to surely claim the benefit of ultrasound accelerated drying of foodstuffs.

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Keywords

high power ultrasound, solar drying, fruit, vegetables, modelling