Transport of cellular water during drying: An understanding of cell rupturing mechanism in apple tissue
Graphical abstract
Introduction
Structural heterogeneity of food material makes it complex to understand the physicochemical changes that occur due to the simultaneous heat and mass transfer during drying. Drying with traditional methods is very time consuming and results in energy wastage and quality deterioration (Karim & Hawlader, 2005). Therefore, a large number of theoretical and experimental studies have been conducted to uncover the actual physical phenomena of heat and mass transfer during drying. Most of the studies investigated the drying kinetics at tissue or bulk level (Askari et al., 2013, Ben Haj Said et al., 2015, Hazervazifeh et al., 2016, Zarein et al., 2015). However, plant-based food tissues, particularly an apple tissue has highly porous and amorphous characteristics and contain different cellular environments (intracellular, intercellular, and cell wall) with various amount of water in these environments (Khan, Joardder, Kumar, & Karim, 2016). Intracellular water (ICW) is the water that resides inside a cell, and the water resides in a region surrounded by two or more cells is called intercellular or free water (FW) (Joardder et al., 2017, Khan, Joardder, et al., 2016), as shown in Fig. 1. The water that resides within the cell wall is known as cell wall water (Joardder, Brown, et al., 2015, Khan, Wellard, Nagy, et al., 2016). According to the bonding strengths, ICW, FW, and cell wall water are sometimes referred as loosely bound water (LBW), capillary water, and strongly bound water (SBW), respectively (Khan & Karim, 2017). It is argued that the major part of the water (about 80–90%) in an apple tissue is present in intracellular spaces (inside solid matrix) (Khan, Wellard, Nagy, et al., 2016).
Transport of different cellular water has a significant effect on the morphological behaviour of food material during drying. It is argued that physical processes such as cracking or shrinking as a result of drying are initiated at cell level (Konstankiewicz et al., 2002). Migration of FW has a minimum effect on the food quality; whereas, migration of ICW results in cellular shrinkage, pore formation and the collapse of the cell and pore structure (Joardder, Karim, et al., 2015, Prothon et al., 2003), and hence has major effect on food quality. Moreover, shrinkage (results of cell rupture) not only degrade the quality of dried food material, but it also influences the drying kinetics because of the changes in the drying surface and setting up of pressure gradients within the food material (Dincer, Ozalp, & Zamfirescu, 2015). In this context, understanding of the migration mechanism of ICW is crucial for accurate prediction of actual heat and mass during drying of food material. Considering this research gap, many theoretical and experimental works have been conducted to uncover the cellular level heat and mass transfer processes during drying. Theoretical studies have been conducted by developing mechanics-based models for cellular level water transport. The models have been developed based on the assumption that intracellular water is only transported from cell to cell through very fine capillaries (micro-channels), and it is referred to as symplastic transport (Prothon et al., 2003). Therefore, only simple Fickian diffusion has been used to develop the model (Aregawi et al., 2014, Ho et al., 2013, Marcotte et al., 1991). ICW can be migrated in two different ways: intracellular pathways (symplastic transport, as mentioned earlier) and extracellular pathways. When the ICW transported from intracellular spaces to the intercellular pathways through rupturing the cell membrane; it is defined as extracellular pathways, and the process is called apoplastic transport process (Khan and Karim, 2017, Prothon et al., 2003). Due to the insufficient understanding of cell rupturing phenomena, the existing food drying models have been developed only considering the one physics for transporting ICW (symplastic transport). Therefore, to understand the cell membrane rupturing phenomena during drying the present authors investigated the cellular water transport mechanism during drying of potato tissue in their previous study (Khan, Wellard, Nagy, Joardder, & Karim, 2017). It was found that intracellular water is mostly transported through rupturing the cell membrane. They also argued that cell membranes rupture at different stages of drying rather than collapsing at one time. However, Halder et al. (2011) postulated that all of the membranes of the cells would collapse at once after the specific temperature is reached. The result of this contradictory argument warrants further investigation about cell rupture mechanism. In addition to this research gap, the aim of the authors' previous study (Khan, Wellard, Nagy, et al., 2017) was to investigate the cell rupturing mechanism for a low porous food tissue, and therefore, potato was taken as a low porous food tissue. Porosity strongly affects the material shrinkage that happens during the rupture of cell membranes. Porous characteristics of food material depend on its cell dimensions and cell wall thickness. Larger cells in plant tissue generally are found loosely packed when compared with smaller cells which lead to making it more porous. The initial porosity of potato tissue is close to zero, therefore, the solid material content in the potato food tissue is higher than that of the high porous food tissue, like Apple (Khan, Wellard, Nagy, et al., 2016). This material characteristic indicates that the potato tissue has the smaller cell dimension as compared to highly porous food tissue, specifically granny smith apple tissue (Joardder, Brown, et al., 2015, Konstankiewicz et al., 2002). Moreover, the cell size and shape of inner core and outer core of potato tissue are significantly different as compared to apple (highly porous) tissue (Konstankiewicz et al., 2002). Generally, cell wall thickness and cell dimensions are the main factors that affect the ICW transport process. The cell wall thickness of apple tissue is about 6 μm (Joardder, Brown, et al., 2015), and the cell dimension is 39,465 μm2 (McAtee, Hallett, Johnston, & Schaffer, 2009). Instead, the cell dimension of potato tissue is 5500–11,200 μm2 (Konstankiewicz et al., 2002) which is much lower than apple tissue. As these two types of food material (highly porous and low porous) has diverse microstructural characteristics, therefore it can be expected that two different food tissue will show the different trends for cell level water transport. To make a general argument about the cell membrane rupturing phenomena, rigorous studies on different types of food materials (low and high porous) are required. The necessity of such research motivated the authors to extend their previous research to investigate the cellular level water transport phenomena during drying in a highly porous food tissue (apple) and compare with the previous findings.
There are several methods available that can be applied to investigate cellular level water transport including, differential scanning calorimetry (DSC), differential thermal analysis (DTA), bioelectric impedance analysis (BIA), and nuclear magnetic resonance (NMR) methods. Based on the limitations of different methods, none but NMR method is found suitable for investigating moisture migration mechanisms while drying is in progress (Khan and Karim, 2017, Khan, Wellard, Mahiuddin, et al., 2017).
NMR methods are considered as the most appropriate for this type of investigations because this method is a widely used for examining the distribution of different types of water in various cellular locations inside plant-based food material (Khan, Joardder, et al., 2016, Khan, Wellard, Nghia, et al., 2016). Proton nuclear magnetic resonance (1H NMR) relaxometry study has been proven to be a viable method in the study of plants and plant-based food materials submitted to stress reflecting anatomical details of the entire tissue and the water status in particular (Gambhir et al., 2005, Van Der Weerd et al., 2002). Since, the water protons dominate the 1H NMR proton signals, which are an average over the whole sample that provides the information about the water content in the plant tissue (Van der Weerd et al., 2001), and the proton NMR signal intensity is directly proportional to the proton density of the tissue (Westbrook, 1993). The water proton relaxation behaviour strongly depends on the water mobility in the microscopic (cellular) environment of the tissue, local magnetic field fluctuations (related to the molecular environment) and the strength of the applied magnetic field. The spin-spin (T2) relaxation is the transverse component of the magnetization vector, which exponentially decays towards its equilibrium value after excitation by radio frequency energy. From the T2 relaxation analysis, it is straightforward to see the number of decays in the relaxation curve and therefore it is possible to investigate different cellular level water based on their intensity of relaxation. Previously, this T2 relaxometry method was widely used to analyze the water distribution in the wood sample (Dvinskikh et al., 2011, Quick et al., 2007, Stenström et al., 2014). Moreover, the method was extensively used to investigate bound and free water in animal lung (Cutillo et al., 1992, Sedin et al., 2000), brain (Berenyi et al., 1998, Furuse et al., 1984, Inao et al., 1985, Sulyok et al., 2001, Vajda et al., 1999), liver (Moser et al., 1992, Moser et al., 1996), red blood cells (Besson, Wheatley, Skinner, & Foster, 1989). Sometime NMR is used to investigate the different quality aspect in food tissue such as sugar content in fruit tissue (Delgado-Goni et al., 2013), quality of fruits and vegetables (Chen et al., 1989, Van de Velde et al., 2016), and the maturity of fruits and vegetables (Chen et al., 1993, Ruan et al., 1999). Using this method, some studies have been conducted to investigate the changes in water compartmentation during drying of plant tissue (Hills and Nott, 1999, Hills and Remigereau, 1997), as well as drying of yellow poplar (Zhang, Wang, & Gazo, 2013), Douglas fir (Menon et al., 1987), red cedar (Menon et al., 1987, Quick et al., 2007), Norway spruce (Thygesen & Elder, 2009) and Scots pine (Rosenkilde & Glover, 2002). These studies found a strong relationship between different water T2 relaxation times and the percentage of moisture loss during drying. Recently, Khan, Wellard, Nagy, et al. (2016) investigated the proportions of water in different cellular environments of 11 various types of food material using NMR-T2 relaxometry. They concluded that NMR is the best approach for the quantitative analysis of biological tissue. Therefore, in this research, NMR T2 relaxometry has been used to investigate the cellular level water migration mechanism during drying of apple tissue.
Section snippets
Sample preparation and drying
Granny Smith apple samples were considered in this study. The samples were collected from a local market in Brisbane, Australia. Samples were stored in a refrigerator at 4 °C until the drying experiments. At the start of each experiment, the materials were washed and cut into cylindrical slices of 30 mm height and 20 mm diameter, as shown in Fig. 2. The prepared samples were then dried in a cabinet dryer (temperature range 35–70 °C). Each sample was taken from dryer after 30 mins of drying and then
Result and Discussions
Intracellular water migration mechanisms were investigated using 1H NMR-T2 relaxometry at different stages of drying. Fig. 3 shows the T2 relaxation decay curve obtained from samples at various drying times. It can be seen that the magnitude of proton intensity is varied at the various stages of drying. This change is not so significant at the initial stages of drying. However, in the later stages of drying (after 180 min of drying) the magnitude of intensity changes significantly (Fig. 3b).
Conclusion
This paper presents the mechanism of intracellular water transport during drying in highly porous food material taking of apple as a sample. 1H NMR T2 relaxometry was used to investigate the proportion of intercellular and intracellular water in apple tissue at different stages of drying. The experimental results were validated with literature data. The findings of this study were compared with the authors' previous findings on low porous food material (potato). It was observed that ICW
Future works
Different types of food material have the different microstructural diversity. Therefore, understanding of accurate ICW transport phenomena for different food material is very complex. The present authors, investigated the ICW transport phenomena for two different food materials: potato, as a low porous material (Khan, Wellard, Nagy, et al., 2017), and granny smith apple as a highly porous food material. To make this new finding about the cell rupturing mechanism during drying more general,
Acknowledgment
This research work is supported by Queensland Government Advanced Queensland Fellowship (AQF, the grant ID is 02073-2015-30358/15) and Queensland University of Technology, Australia for funding a QUTPRA scholarship and HDR tuition fee sponsorship. Thanks to Dr. Mark Wellard, for helping to NMR experimentation and suggestion about the data analysis. Moreover, S.A.N. was supported by the Hungarian Brain Research Program “B” (KTIA_NAP_13-2-2014-0019).
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