The impact of freeze-drying on microstructure and rehydration properties of carrot Adrian Voda a, ⁎, Natalia Homan b , Magdalena Witek b, 1 , Arno Duijster c , Gerard van Dalen a , Ruud van der Sman d , Jaap Nijsse a , Lucas van Vliet c , Henk Van As b , John van Duynhoven a, b a Unilever Research and Development Vlaardingen, Olivier van Noortlaan, P.O. Box 114, 3130 AC Vlaardingen, The Netherlands b Laboratory of Biophysics and Wageningen NMR Centre, Wageningen University, Dreijenlaan 3, 6703 HA Wageningen, The Netherlands c Quantitative Imaging Group, Delft University of Technology, Lorentzweg 1, 2628 CJ Delft, The Netherlands d Food Process Engineering, Wageningen University, Bomenweg 2, 6703 HD Wageningen, The Netherlands abstract article info Article history: Received 18 June 2012 Accepted 22 August 2012 Keywords: MRI NMR Microtomography μCT Microscopy SEM Image analysis Pore size distribution Tortuosity Microstructure Winter carrot Freeze-drying The impact of freeze-drying, blanching and freezing rate pre-treatments on the microstructure and on the re- hydration properties of winter carrots were studied by μCT, SEM, MRI and NMR techniques. The freezing rate determines the size of ice crystals being formed that leave pores upon drying. Their average size (determined by μCT) can be predicted in a quantitative manner by considering dendritic growth and freezing rates. Blanching as a pre-treatment, however, did not affect pore size distribution induced by freeze-drying. Upon rehydration of the freeze-dried carrots, PFG NMR and MRI show that cellular compartments were not restored and instead a porous network with permeable barriers is formed. Blanching pre-treatment intro- duced a less connected and more anisotropic porous network if followed by fast freezing, indicating that more of the native cell wall morphology is preserved. © 2012 Elsevier Ltd. All rights reserved. 1. Introduction Consumers have a high appreciation for fruits and vegetables, which are an important dietary source of vitamins, phytochemicals, fibers and minerals (Hoffmann, Boeing, Volatier, & Becker, 2003). The intake of fruits and vegetables has been associated with a wide range of beneficial health effects (Pomerleau, Lock, & Mckee, 2006). A main hurdle for consumers to raise their daily intake is the lack of convenience in preparing meals. The food industry has addressed this by offering the consumer dried fruits and vegetables, which are rehydrated shortly before consumption. A major obstacle for further growth in this area is the relative poor quality of the rehydrated fruits and vegetables in the product after preparation. Another bottleneck is the poor compromise between convenience in meal preparation and textural quality (Jangam, 2011; Prothon, Ahrne, & Sjoholm, 2003). Most dehydrated fruits and vegetables are produced by air drying. A disadvantage of this method is a substantial degradation in quality, including appearance (shrinkage, drying-up, darkening), nutrients, flavor, and the low rate of rehydration (Devahastin & Niamnuy, 2010; Ratti, 2001). Higher quality products can be obtained using more expensive freeze-drying methods (Mujumdar & Law, 2010). Freeze-drying involves crystallization of water in ice crystals, which subsequently sublimate, thus leaving a porous dried product. This may lead to loss in texture and an increase in friability (Brown, 1976; Chassagne-Berces et al., 2009; Ratti, 2001; Van Buggenhout et al., 2006). Improvements in the freeze-drying process of foods have been driven by engineering, where technologies are being optimized to balance rehydration rate and final texture (Mujumdar, 2011; Sagar & Kumar, 2010). Considering the underlying microstructure and its role in rehydration may enhance the efficiency and rate of process in- novation (Mebatsion, 2008). A major barrier to embark on such an approach has been the lack of adequate quantitative measurement technologies that enable decision making based on sound microstruc- tural data. Hence we embarked on an approach where we quantita- tively assessed microstructural features of freeze-dried carrots as a model system. In this work the impact of thermal pre-treatments and freeze-drying on the microstructure of the cortical tissue of winter carrots was investigated. The purpose of this investigation was to quantitatively de- scribe the features of dry and rehydrated microstructures by means of dedicated image analysis and NMR parameters. To achieve this, a suite Food Research International 49 (2012) 687–693 ⁎ Corresponding author. Tel.: +31 10 4606708; fax: +31 10 4606545. E-mail address: adrian.voda@unilever.com (A. Voda). 1 Present address: Institute of Physics, Jagiellonian University, Reymonta 4, 30‐059 Kraków, Poland. 0963-9969/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.foodres.2012.08.019 Contents lists available at SciVerse ScienceDirect Food Research International journal homepage: www.elsevier.com/locate/foodres