An alternative method for the industrial monitoring of osmotic solution during dehydration of fruit and vegetables: A test-case for tomatoes A. Cataldo a,⇑ , G. Cannazza a , E. De Benedetto a , C. Severini b , A. Derossi b a Department of Engineering for Innovation, University of Salento, Complesso Ecotekne – Edificio Corpo O, via Monteroni, 73100 Lecce, Italy b Department of Food Science, University of Foggia, via Napoli 25, 71100 Foggia, Italy article info Article history: Received 7 October 2010 Received in revised form 3 February 2011 Accepted 12 February 2011 Available online 18 February 2011 Keywords: Dielectric properties Food industry Microwave reflectometry Osmotic dehydration Quality control Time domain reflectometry abstract In this paper, an alternative procedure for in situ and continuous monitoring of the status of the hyper- tonic solutions used in industrial osmotic dehydration (OD) process of fruit and vegetables is presented and validated. The proposed method, which is based on microwave reflectometry technique, circumvents the limitation of traditional control methods, since it discovers the relation between the characteristic parameters of osmotic solutions and the corresponding measured reflection coefficients. In fact, as the OD proceeds, the dielectric characteristics of the solution, associated to the reflectometry measurements, can be directly related to the variation of the water activity (a w ), which is the parameter that is typically considered to assess the dewatering capacity of the solution. As a result, through the method presented herein, a continuous monitoring of the dewatering capacity of the osmotic solution, associated to its qual- itative status, is possible, which, in turn, is useful also for optimizing the OD industrial process. The pro- posed method was preliminarily validated on reference aqueous solutions of sucrose. Successively, the method was tested also in a typical industrial application, involving the OD process of tomatoes. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Osmotic dehydration (OD) is an effective technique for the par- tial removal of water from fruit and vegetables obtaining a signif- icant increase of their shelf-life; in fact, by reducing water content, the growth of microorganisms is inhibited and the rate of degrada- tion reactions is reduced. OD used as pretreatment to conventional air drying not only removes water from the food, but also reduces aroma losses (Warczok et al., 2007) and lowers energy consump- tion (Derossi et al., 2008, 2010; Hui et al., 2007; Torreggiani and Bertolo, 2001). In the OD process, the product is immersed in a hypertonic solu- tion, thus promoting two counter-current flows: (1) Water motion from biological tissues to hypertonic solution; (2) Osmotic agents flow toward the product. OD has been extensively used for pre- dehydration of pineapples (Lombard et al., 2008), mushrooms (Torringa et al., 2001), melons (Rodrigues and Fernandes, 2007), bananas (Fernandes et al., 2006) and many other perishable goods in general. Nevertheless, as reported in Dalla Rosa and Giroux (2001), the adoption of OD in industrial production processes is hindered by problems with the overall management of the concen- trated solutions. In fact, there are still some practical aspects that make managing and controlling the osmotic solution the bottleneck of the OD process. In particular, there are still some major concerns regarding the appropriate individuation of the loss in dewatering capacity and the possibility of re-using the spent solution, so as to make the process more economically advanta- geous (Warczok et al., 2007). On such bases, problems related to the osmotic dehydration control and osmotic solution management are still unsolved and, currently, no methodologies can successfully and rapidly represent the solution to the aforementioned needs. As a matter of fact, the osmotic pressure (p) might represent the primary parameter whose estimation would lead to the identification of the quality status of the osmotic solution. However, many food-industry pro- cesses typically involve osmotic solutions with high value of p (up to 250–300 bar), which, in practice, seriously limits the possi- bility of realizing specific devices for measurements. Additionally, the high uncertainties and the long times needed for measurement are other drawbacks that make the direct measurement of p extre- mely difficult. On the other hand, water activity (a w ) is a reliable indicator of the status of the osmotic solution, since it is strictly related to the dewatering capacity of the solution. In fact, the OD proceeds until the osmotic solution and the food product have reached the same a w value (Barbosa-Canovas and Vega-Mercado, 1996). Never- theless, although this parameter can be a suitable indicator in the OD management, not only the instrumentation for measuring it is quite costly, but also measurements usually have to be performed 0260-8774/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2011.02.026 ⇑ Corresponding author. Tel.: +39 0832 297823; fax: +39 0832 297733. E-mail address: andrea.cataldo@unisalento.it (A. Cataldo). Journal of Food Engineering 105 (2011) 186–192 Contents lists available at ScienceDirect Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng