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Effect of Cell Wall Properties on Porosityand Shrinkage of Dried AppleMohammad U. H. Joardderab, Richard J. Browna, Chandan Kumara &M.A. Karimaa Faculty of Engineering and Science, Queensland University ofTechnology, Brisbane, Queensland, Australiab Department of Mechanical Engineering, Rajshahi University ofEngineering and Technology, BangladeshAccepted author version posted online: 25 Feb 2015.

To cite this article: Mohammad U. H. Joardder, Richard J. Brown, Chandan Kumar & M.A. Karim(2015) Effect of Cell Wall Properties on Porosity and Shrinkage of Dried Apple, International Journal ofFood Properties, 18:10, 2327-2337, DOI: 10.1080/10942912.2014.980945

To link to this article: http://dx.doi.org/10.1080/10942912.2014.980945

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Effect of Cell Wall Properties on Porosity and Shrinkage ofDried Apple

Mohammad U. H. Joardder1,2, Richard J. Brown1, Chandan Kumar1, and M.A. Karim1

1Faculty of Engineering and Science, Queensland University of Technology, Brisbane,Queensland, Australia

2Department of Mechanical Engineering, Rajshahi University of Engineering and Technology,Bangladesh

Water removal during drying depends on the pathway of water migration from food materials.Moreover, the water removal rate also depends on the characteristics of the cell wall of plant tissue.In this study, the influence of cell wall properties on porosity and shrinkage of dried product wasinvestigated. Cell wall stiffness depends on a complex combination of plant cell microstructure,composition of food materials and the water-holding capacity of the cell. In this work, a preliminaryinvestigation of the cell wall properties of apple was conducted in order to predict changes of porosityand shrinkage during drying. Cell wall characteristics of two types of apple (Granny Smith and RedDelicious) were investigated under convective drying to correlate with porosity and shrinkage. Ascanning electron microscope (SEM), 2kN Intron, pycnometer and ImageJ software were used inorder to measure and analyse cell characteristics, water holding capacity of cell walls, porosity andshrinkage. The cell firmness of the Red Delicious apple was found to be higher than for Granny Smithapples. A remarkable relationship was observed between cell wall characteristics when compare withheat and mass transfer characteristics. It was also found that the evolution of porosity and shrinkage arenoticeably influenced by the nature of the cell wall during convective drying. This study has revealed abetter understanding of porosity and the shrinkage of dried food at microscopy (cell) level, and willprovide better insights to attain energy-effective drying processes and improved quality of dried foods.

Keywords: Cell wall characteristics, Compressive deformation, Drying kinetics, Porosity, Shrinkage.

INTRODUCTION

Since the moisture content of fresh fruits and vegetables is more than 80%, they are classified asperishable commodities.[1] Food is one of the most complex materials in natural form and thefundamental understanding of food drying has not been fully established.[2] Lack of properprocessing causes considerable damage and wastage of seasonal fruits in many countries, whichis estimated to be 3040% seasonal fruits in developing countries.[3] Drying of foodstuffs is

Received 5 May 2014; accepted 22 October 2014Address correspondence to Mohammad U. H. Joardder, Department of Chemistry, Physics, and Mechanical Engineering,

Faculty of Engineering and Science, 2 George Street, CBD, Queensland University of Technology, Brisbane 4001, Australia.E-mail: [email protected]

International Journal of Food Properties, 18:23272337, 2015Copyright Taylor & Francis Group, LLCISSN: 1094-2912 print/1532-2386 onlineDOI: 10.1080/10942912.2014.980945

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important, and is indeed the oldest method of food processing. Many physical and chemicalchanges occur in foods during the drying process. The quality of the dehydrated product is affectedby a number of factors including quality of raw material, method of preparing, processingtreatments, and drying conditions.[46] The first objective of drying is to remove water and henceto stabilize the food product. However, during food drying, many physico-chemical changes occursimultaneously, resulting in change in the overall quality of the dried food.[7] Moreover, dryingkinetics depends on the physical properties of food including food structure, components, maturityof the food sample, and drying conditions.[8] In order to achieve better quality food and optimumdrying conditions, deep insight into the drying phenomenon is essential.

Biological materials, especially plant tissues, are very complex in nature when one considerstheir anisotropy, porosity, and hygroscopic attributes. Fresh apple has a large amount of inter-cellular space, with 2238% air spaces of its total tissue volume.[9] In other words, fresh apple fleshis intensely porous, with an initial porosity 0.220.38.[10] Among the different types of apples, theGranny Smith has one of the highest levels of initial porosity (0.33), due to its larger celldimensions and thinner cell wall thickness. Larger cells in plant tissue generally are found looselypacked when compared with smaller cells. Despite having almost the same moisture content (0.86and 0.87 g/g of sample for Granny Smith and Red Delicious, respectively) and density (789 and766 kg/m3 for Granny Smith and Red Delicious, respectively), Granny Smith and Red Deliciousapples show different drying rates. Moreover, the literature confirms the role of the mechanicalproperties of plant-based food on flavor, bio-availability of nutrients, and textural perception.[11]

Plant cell membranes and walls are semi-permeable and are regarded as the components thataffect the transfer of water to and from parenchyma cells.[12,13] The determination of the moisturecontent in fruits and vegetables is a complex task, as it involves both free and bound water. Thedetermination of the separate types of water is even more critical than the estimation of the totalwater. The drying rate of the later falling rate period is too slower due to the internal mass transferresistance increases and water is transported through a thicker layer of the dry solid matrix.[14] Theliterature shows that in order to remove the last 10% of water from food material, it takes almostthe same amount of time that is required to remove the first 90% of the water content. In otherwords, bound water migration from food materials during drying takes significant time and energy.In plant tissue, water distribution is intercellular, intracellular, and within the cell wall.[15] Of thetotal water in plant tissue, more than 90% of water is intracellular and cell wall water.[16]

Therefore, macroscopic structural behavior depends on turgor pressure, cell wall properties, andcell size.[1719] Generally, the cell wall of plants comprises approximately 60% water, 28% pectinsubstances, 515% hemicellulose, 1015% cellulose, 12% protein, and 0.53% lipids.[20] Theseproportions vary due to a variety of factors, such as environmental conditions, stage of maturity,processing after harvest, and botanical origin of the plant.[21] Determining the size and shape of thecell is one of the most important functions of the cell walls in plant tissue. In addition to this, themechanical properties of the tissue depends on the cell wall properties.[22] In addition to this, intactcell walls and cell membranes are the main hindrance for water migration from the cell.[23] Dietaryfiber is a non-homogeneous mixture of polysaccharides and lignin; and this mixture is mainlyfound in plant cell walls.[24] This dietary fiber is mainly classified into soluble dietary fibre (SDF)such as pectin, gum, etc., and insoluble dietary fibre (IDF), such as lignin, cellulose, andhemicellulose.[25,26] These non-soluble compounds build the cell wall as a porous structure.[27,28]

Of the dietary fibers, only the soluble can hold water.[29] On the other hand, IDF causes lesspermeability of water due to its insoluble nature when compared with the SDF. Thus, the presenceof more IDF leads to the development of a denser solid matrix cell wall. So, it can be seen that aratio of soluble and IDF is an important factor when considering the permeability of the cell wall.More IDF means that there are more obstacles in the rigid skeleton for the path of water flowduring drying.

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Knowledge of the exact amount of bound water in the cell wall of fruit and vegetable tissue iscrucial for realistic modeling of heat and mass transfer as well as in determining more efficientdrying systems.[30] However, there is currently insufficient study concerning the determination ofcell wall bound water. In addition to bound water, recognition of the stiffness of the cell wall isessential to correlate the deformation (shrinkage/expansion) during drying. The compression testprovides the mechanical response of the plant tissue.[31] As stiffness is one of the structuralproperties of the tissue, it can be derived from the materials properties and by considering theshape of the sample. In addition to stiffness, Poissons ratio and Youngs modulus are importantapparent elastic properties of biomaterials for the anticipation of load-deformation behaviour ofthose materials. These elastic properties can be used in order to compare the relative strength ofvarious plant tissues.[32]

In general, the mechanical properties of plant tissues are assessed to predict post-harvestingprocess parameters.[33] However, researchers have not dealt with the mechanical properties of cellwalls (in general these are proportional to the mechanical properties of the tissue) in order toestablish relationships between drying kinetics and physico-chemical characteristics and mechan-ical properties. In this study, two types of apples, namely Granny Smith and Red Delicious, wereinvestigated to understand the characteristics of the cell wall. In addition, Influence of cell wallcharacteristics in terms of cell wall elastic properties (Young modulus, Poissons ratio and stiff-ness), cell wall thickness, cell diameter, and water distribution within the cell wall on the dryingrate and dried food physical properties such as porosity and shrinkage were investigated.

MATERIALS AND METHODS

Sample Preparation

Granny Smith and Red Delicious apples were purchased from the local supermarket. First theapples were sliced into 11 mm diameter and 12 mm height cylinders for compression tests (Fig. 1),where each of the samples was subject to 3.5 mm deformation. Then 3.5 mm thick apple sampleswere dried in a convective dryer at 70C for 180 min. Real-time mass measurements were providedwith KERN ABT 220-5DM analytical balance with 0.0001-g sensitivity. The compression speedwas 2, 5, 10, and 20 mm/min, as proposed in ASAE standard 368.1.[34] Experiments were carriedout using the 2kN Instron universal testing machine. The increasing force caused a loosening of therigidity of the sample, and led to a rupture at the rupture point. Force and deformation data fromthe compression test were recorded on a connected computer.

FIGURE 1 Preparation of sample for mechanical properties (compression test).

EFFECT OF CELL WALL PROPERTIES ON POROSITY 2329

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Youngs modulus (apparent elastic modulus, ASAE standard 368.1) was obtained from the slopeof the force-deformation curve at the point of its highest gradient.[35] The Poissons ratio wascalculated by measuring the diminution before and after deformation under compression test.Youngs modulus, Poissons ratio and stiffness were calculated using the following formulas:[3638]

Youngs modulus; E StressStrain

F=AL=L

F

D=22lL

Poissons ratio; d=Dl=L

where, d is transverse deformation (mm), D is sample width (mm), l is axial deformation (mm), andL is sample length (mm). From Youngs modulus, Hookes law can be derived, which describes thestiffness of the material.

Stiffness; k AEL F

l

where, A is the cross-sectional area of the sample.A Quanta 200 scanning electron microscope (SEM) was used in order to analyze the micro-

structure of both fresh and dried apple slices. Meanwhile, a pycnometer (Pentapyc 52000e) wasused to obtain the porosity of fresh and dried samples. Also, ImageJ 1.47v software was used toanalyse the microstructure of the cell wall thickness and bulk shrinkage before and after drying,and to measure cell dimensions.

Cell Wall Water Estimation

An assumption is made that all cell wall water is treated as physically bound water due to its easyavailability during the drying and moisture removal process. Therefore, measuring the cell wallthickness before and after drying can give the amount of water it holds. Some other assumptions ofcalculating cell wall water are as follows:

1. Shape of the cells is spherical. However, the ratio of intracellular and cell wall waterminimizes the error of this assumption.

2. As most of the plant tissue shows the heterogeneous nature of cell orientation and content,average numbers of cell and intercellular spaces will be considered.

3. The density of cell wall water is the same as the density of intracellular water.4. Cell dimension varies within the plant tissue, but the average cell dimension has been

considered here.

Thus, it is assumed that the difference in the cell wall thicknesses of fresh and complete driedsample represents the amount of bound water. After calculating the volume difference of the cellwalls, the following equation can be used to give the amount of bound water in the cell wall:

Mcw 4t r2 rt 13 2t2

(1)

where, r and r1 are the radius of the fresh and dried plant tissue cell (Fig. 2), t is the initialthickness and t1 is the final thickness of the cell wall. Shrinkage coefficient of the cell wallthickness,

t1t


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From Eq. (1), it is clearly apparent that the amount of cell water depends directly on the cell wallthickness, cell dimension, and cell wall shrinkage coefficient. Therefore, it is not enough to consideronly cell wall water to represent the amount of water in food tissue rather than the ratio of intracellularand cell wall bound water, which can provide more insight regarding the overall bound water presentin the plant tissue. So, the ratio of intracellular and cell wall water can be obtained from Eq. (2).

MiwMcw

r t 3

t3r2 3rt 2t2 (2)

Therefore, it is possible to obtain quite accurate amounts of bound water from examining themicrostructure of the fresh and dried products.

RESULTS AND DISCUSSION

Cell Stiffness and Drying Rate

As shown in Fig. 3, in order to achieve the same amount of compressive deformation, GrannySmith apples require less energy than Red Delicious apples, as Red Delicious apple tissue is stiffer(23.03 kN/m) than Granny Smith apple (21.8 kN/m). The average Youngs modulus is also higherfor Red Delicious (3.10 MPa), while Granny Smith shows Youngs modulus 2.81 MPa. It wasfound that the value of Poissons ratio is 0.1690.344 for Granny Smith[36] and 0.1550.25 for RedDelicious apples.[39] Overall, the cell wall stiffness is presumed higher in Red Delicious, as itstissue is stiffer than Granny Smith. Consequently, this higher stiffness affects the rate of moisturemigration from the cells of the apple flesh.

There is a significant similarity between the compressive test and the drying process. The result,as shown in Fig. 4, indicates that the moisture migration rate is higher for Granny Smith whencompared to Red Delicious. This result confirmed that in order to remove the same amount ofwater, the Red Delicious apple took more heat energy than the Granny Smith. Therefore, a positiverelationship was found between water migration by mechanical and thermal energy as demon-strated by these similar trends. Interestingly, this correlation can be explained by the nature of thecell wall stiffness that facilitates or hinders the water migration in mechanical and thermal energyapplications.

FIGURE 2 Estimation of cell wall bound water of plant tissue.


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Cell Dimension and Porosity

Cell dimension influenced the rigidity of the cell wall and tissue of plant materials. Greater celldimensions caused the loose packing of cells, and consequently density, rigidity, intercellular spaceswere affected significantly. Fig. 5 compares the microstructures of fresh and dried Granny Smith andRed Delicious apples. The results obtained from microstructure analysis of the samples shows that theGranny Smith cells are larger than those of the Red Delicious, as shown in Fig. 6. This is consistentwith previous literature, which maintained that the Granny Smith has a high initial porosity of is0.33.[9] It was also found from the pycnometer data that dried Granny Smith apple has a higherporosity due to its larger cell dimensions and the consequent loose packing of cells.

Cell Wall Bound Water

Cell wall thickness analysis, presented in Table 1, shows that the Red Delicious has thicker cellwalls when compared with Granny Smith cell wall thickness in fresh state (9.31 3.1 and 11.405

FIGURE 3 Compressive extension characteristics of apples.

FIGURE 4 Weight loss of apple slice in convective drying at 70C.


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3.2 m for Granny Smith and Red Delicious, respectively). On the other hand, after drying the cellwall shrinks more (on average 78.68% shrinkage) in Red Delicious compared to Granny Smith(50% shrinkage) as shown in Fig. 6. This result indicates the presence of more bound water withinthe cell wall in Red Delicious.

These findings concerning cell wall characteristics indicate that the amount of bound water incell walls was higher in the Red Delicious apple. In addition, from Eq. (3), the ratio of intracellularand cell wall water shows Red Delicious cell water was quite a deal higher than for the GrannySmith apple. This result might be an explanation as to why Red Delicious apple takes more energyfor compression and drying to give certain moisture content. It is relevant to compare the nature ofthese cell walls with the dietary fiber of apples. The ratio of IDF and SDF in Red Delicious is 4.37,whereas it is 3.21 for Granny Smith,[40] which indicates that the cell wall of the Red Delicious ishighly dense with insoluble fiber. In addition to this, it is a possible explanation as to why moreenergy is required to remove the same amount of water from Red Delicious apple when comparedwith the Granny Smith variety.

FIGURE 5 Microstructure of fresh and dried apple samples (500).


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Bound Water and Bulk Shrinkage

The amount of bound water influenced the trend of drying kinetics and physicochemicalchanges,[41] in particular during the latter part of drying kinetics, as shown in Fig. 7.Shrinkage of the Granny Smith apple increased steadily with the decrease of moisture contentwhen compared with the Red Delicious due to its higher intercellular and cell-to-cell moistureflux, up to intermediate moisture content. On the other hand, with respect to time and moisturecontent as shown in Fig. 8, shrinkage in the Red Delicious apple sharply increased at the end ofthe drying process, due to cell wall collapse, which began in that period.

This property of the cell walls also can explain why the cell wall collapses more in Red Deliciousapple than in the Granny Smith apple. Taken together, the findings from the compression test,drying kinetics, shrinkage in the cell wall, and bulk shrinkage of tissue manifest that Red Deliciousapple contains more trapped water in the cell wall than does the Granny Smith apple. Moreover, itemerged from the data that the porosity and shrinkage of dried food significantly depended on thenature of the cell wall. Further investigation is required to gain more insight into the cell wallproperties in order to achieve optimum drying conditions and a better quality of dried food.

FIGURE 6 Cell dimension of two types of apple.

TABLE 1Cell wall thickness of fresh and dried apple

Granny Smith apple Red Delicious apple

Cell wallthickness

Fresh(m)

Dried(m)

Cell wallshrinkage

Coefficient

Ratio ofintracellular andcell wall water

Fresh(m)

Dried(m)

Cell wallshrinkage

Coefficient

Ratio ofintracellular andcell wall water

Avg. 9.312 4.685 0.4969 4.2853 11.405 2.432 78.68 2.13Min. 6.734 3.769 0.4403 7.678 1.65 78.51Max. 11.785 6.281 0.4670 14.458 3.527 75.61


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CONCLUSION

Drying kinetics and dried food quality are subject to drying conditions and fresh food properties. Inthis study, cell wall characteristics in terms of stiffness, wall thickness, bound water, and celldimension were investigated in order to establish a relationship between with porosity andshrinkage. Drying kinetics and compressive deformation were found to trend similarly for thetwo varieties of apple slices. In other words, both drying kinetics and compressive drying confirmthat Red Delicious apple took more energy than Granny Smith apple to release the same amount ofwater. This finding suggests that drying kinetics noticeably depends on the cell wall characteristicsof plant tissues. Moreover, dried food quality, especially physical attributes such as porosity,shrinkage, and microstructure, are significantly affected by the cell wall characteristics. Thefindings of this study will bring new understanding of the relationship of the mechanical propertiesof plant tissue with drying kinetics and dried food quality.

FIGURE 7 Cell wall thickness of dried apple (1220).

FIGURE 8 Shrinkage of apple with moisture content (left) and time (right).


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ACKNOWLEDGMENT

This article was first presented in the First International Conference on Food Properties (iCFP 1)held in Kuala Lumpur, Malaysia on January 2426, 2014 and it received recognition as one of theinnovative papers.

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AbstractIntroductionMaterialS and MethodSSample PreparationCell Wall Water Estimation

Results and DiscussionCell Stiffness and Drying RateCell Dimension and PorosityCell Wall Bound WaterBound Water and Bulk Shrinkage

ConclusionACKNOWLEDGMENTReferences

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