Effect of expansion by instantaneous controlled pressure drop on dielectric properties of fruits and vegetables Magdalena Kristiawan ⇑ , Vaclav Sobolik, Ladislav Klíma, Karim Allaf LEBTIAB, University of La Rochelle, Av Michel Crepeau, 17042 La Rochelle, France article info Article history: Received 15 November 2009 Received in revised form 14 September 2010 Accepted 22 September 2010 Available online 29 September 2010 Keywords: Microwave Dielectric properties Swell drying Penetration depth DIC Coaxial probe abstract The instantaneous controlled pressure drop (DIC) treatment is used for creation of a porous structure during drying of fruits and vegetables. DIC is based on high temperature, short time heating followed by an abrupt pressure drop into a vacuum. This abrupt pressure drop provokes auto-vaporization of the superheated liquid, expansion and breaking of the cell walls and instantaneous cooling. This process step is inserted between two drying stages at a moisture content of about 20% wet basis. The use of microwave radiation would provide more rapid and homogeneous heating than using steam in the DIC treatment and hot air during the final stage of drying. For that purpose the dielectric properties of the raw and DIC treated products were measured. The measurements were carried out with an open-ended coaxial probe at a frequency of 915 MHz in the range of temperatures 20–90 °C and moisture content 5–80% w.b. Three regions were revealed for the dependences of the dielectric constant and loss factor on moisture content. At low moisture content, these properties increased linearly with moisture content. At the middle moisture content, the increase was also linear but much steeper. At high moisture content, the dielectric properties were constant. The limits of these regions were different for e 0 and e 00 as well as for the products. The dielectric properties were slightly temperature dependent. The penetration depth increased with decreasing moisture content. The DIC treated products exhibited slightly lower dielectric properties than the raw products. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Drying is one of the oldest and most efficient methods for food preservation. Hot air drying can be divided into three periods: initial, constant rate and falling rate periods. In this paper, the latter is defined as the post-drying stage whereas the first two are the pre-drying stages. Coupled transfer of heat and mass is characteristic to almost all drying processes. Water elimination produces shrinkage and deformation of the final product. The shrinkage induces the increase of thermal conductivity, and thus of internal heat transfer, and the decrease of the pore size and thus of the water diffusivity within the product. Therefore, internal mass transfer becomes the limiting factor that considerably slows down the drying rate and consequently amplifies thermal degradation. A solution for pre- venting this degradation and for correcting the deformations could be through a texture modification by expanding the product. In this paper, the dielectric properties of several fruit and vegetables pertinent for a swell (expansion) drying process are investigated. This new dehydration process has three-stage. The instantaneous controlled pressure drop (DIC) treatment is inserted between the pre-drying and post-drying stage at the product mois- ture content of about 20% w.b. in order to remedy the product shrinkage. The DIC consist in a controlled expansion of oversatu- rated product humidity into a vacuum. The created porous struc- ture improves the kinetics of the post-drying stage and final quality of products (Louka and Allaf, 2004; Al Haddad et al., 2008). The DIC texturing consists in steam heating of materials in an autoclave for short time (less than 1 min), followed by fast expansion (Dp/Dt > 0.5 MPa/s) into a vacuum (50 kPa) (Louka and Allaf, 2004). The vapour, which evolves in oversaturated liquid, mechanically strains the material. This strain creates porous struc- tures as shown in Fig. 1. It can also break the cell walls and destroy micro-organisms. The temperature of the material before the expansion is usually higher than 100 °C and the pressure in the autoclave is equal or higher than the saturated vapour pressure. The DIC treatment at a moisture content about of about 20% (wet basis) results in a good porous structure of the treated material (Al Haddad et al., 2008). Direct penetration of microwave radiation into the core of the heated product enables fast increase in temperature throughout the entire volume (Drouzas et al., 1999; Ferhat et al., 2006). A rapid heating is essential for the treatment of temperature sensitive 0260-8774/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2010.09.014 ⇑ Corresponding author. E-mail addresses: magdanana@yahoo.com (M. Kristiawan), vsobolik@univ-lr.fr (V. Sobolik), lklima@mp-produkt.cz (L. Klíma), kallaf@univ-lr.fr (K. Allaf). Journal of Food Engineering 102 (2011) 361–368 Contents lists available at ScienceDirect Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng