Ultrasound-assisted drying

As water is entrapped in the intact cells of a plant tissue, ultrasound-mediated effects depend on cell damage due to enhanced dehydration. It is likely, that a cell disintegration by PEF prior to ultrasound-assisted drying would make the... 

Ultrasound-assisted drying processes can be based on sound transmission in air (airborne ultrasound), liquids (osmotic solutions) or solids (contact ultrasound). Sound transmission and sound emitter designs must be adapted to the respective applications and have, together vdth the transmission medium, a major influence on process design and performance. 

Although the application of ultrasound in osmotic dehydration is, technically speaking, not a contact ultrasound application, its analysis provides an interesting insight into the principles of ultrasound-assisted drying and the different approaches to minimize mass transfer barriers applying PEF and ultrasound. It is therefore briefly discussed here. 

Further reductions in the energy consumption of contact ultrasound-assisted drying can be realized by applying an intermittent ultrasound treatment. Whereas, the maximum impact of ultrasound is achieved when the sound waves are... 

M. C. Yebra and S. Cancela, Continuous ultrasound-assisted extraction of cadmium from legumes and dried fruit samples coupled with on-line preconcentration-flame atomic absorption spectrometry, Anal. Bioanal. Chem., 382(4), 2005, 1093-1098. 

The ultrasound-assisted extraction of freeze-dried plant-based materials normally takes 2 hr. If the extract is evaporated to dryness, a total of 3 hr is necessary. As an additional sample preparation step, 2 to 4 days should be allotted for freeze-drying fresh plant materials, depending on the quantity of the material. Homogenizer-assisted extraction of fresh fruit takes <4 hr. 

Ultrasound assisted aerosol formation or nebulization prior to sample insertion into an atomic detector is dealt with in Chapter 8 on the grounds of its close relationship to the instrument in spatial and temporal terms. Also, as a step of spray drying, known as "atomization ", is discussed in Chapter 2 inasmuch it can be used for sample conservation purposes and hence as an analytical operation preceding sample preparation. This section is concerned with other non-analytical uses of US-assisted aerosol formation that are closely related to the analytical field and can open new avenues for the development of previously unexplored analytical uses. 

Ultrasonic nebulization is known to provide a higher analyte transport efficiency than pneumatic nebulization (normally 8-15 times higher) this results in improved sensitivity and lower detection limits, which is especially important for the analysis of species at trace or ultratrace levels [31-35]. Ultrasound-assisted generation of smaller drops and the use of a desolvation system to remove most of the solvent load allow the production of fine, dry analyte-enriched aerosol for insertion into a detection system some authors, however, ascribe most of the sensitivity increase of USNs to the desoivation system aione [36]. 

As noted earlier, not all open-vessel systems (viz. those that operate at atmospheric pressure) are of the focused type. A number of reported applications use a domestic multi-mode oven to process samples for analytical purposes, usually with a view to coupling the microwave treatment to some other step of the analytical process (generally the determination step). Below are described the most common on-line systems used so far, including domestic ovens (multi-mode systems) and open-vessel focused systems, which operate at atmospheric pressure and are thus much more flexible for coupling to subsequent steps of the analytical process. On the other hand, the increased flexibility of open-vessel systems has promoted the design of new microwave-assisted sample treatment units based on focused or multi-mode (domestic) ovens adapted to the particular purpose. Examples of these new units include the microwave-ultrasound combined extractor, the focused microwave-assisted Soxhlet extractor, the microwave-assisted drying system and the microwave-assisted distillation extractor, which are also dealt with in this section. Finally, the usefulness of the microwave-assisted sample treatment modules incorporated in robot stations is also commented on, albeit briefly as such devices are discussed in greater detail in Chapter 10. 

Karabegovic IT, Veljkovic VB, Lazic ML (2011) Ultrasound-assisted extraction of total phenols and flavonoids from dry tobacco Nicotiana tabacurri) leaves. Nat Prod Commun 6(12) 1855-1856... 

Fig. 5.15 Scanningelectron microscopy images ofalbedo cells obtained from orange peel in case of (a) hot-air drying and (b) ultrasound-assisted hot-air drying. Illustration courtesyof Prof. Antonio Mulet and Juan A. Carcel, Universidad Polit nica de Valencia, Spain.

Fig. 7.9 Assembly of the ultrasonic system used in contact ultrasound-assisted air drying. 1, samples 2, drying screen 3, ring sonotrode RIS 200 4, ultrasound processor UIS250L. Reproduced with permission from Schossler et a/. (2012a).

Fig. 7.11 Laboratory-scale system for contact ultrasound-assisted freeze-drying. A, Ac7lic lid with ultrasonic system B, screen for control samples C, freeze-d er 1, ultrasound processor UIPIOOO 2, sonotrodes BS2d34 3, vibration-free flange ...

The fact that the mechanism of action of ultrasound in tissues seems to be primarily mechanical, and perhaps partly thermal at the tissue level (Schossler et ol., 2011 Schossler et id., 2012c), may explain this limited effect on product quality. Yet, as long as the harsh conditions of cavitation can be avoided the product quahty can be maintained when the parameters of the ultrasound treatment are well chosen. Nonetheless, detailed investigations of product quahty must accompany the optimization of process parameters, especially in the case of critical applications, such as ultrasound-assisted osmotic dehydration, or apphcations involving special conditions, such as vacuum freeze-drying. 

As can be observed in Fig. 8.10, in the case of conventional drying, the dependence of effective diffusivity fitted an Arrhenius-type equation adequately, the activation energy being a characteristic of the product. For ultrasonically assisted dried samples an Arrhenius behavior was observed at low temperatures however, when the temperature reached 70 °C the identified diffusivity was quite similar for both methods of drying, with and without ultrasound application. 

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