Chapter 4 Pore Generation in Food Materials by Application of Microwave Energy Under Sub-atmospheric Pressure Tim Durance, Mareike Ressing and Henning Ressing Food, Nutrition and Health Program, The University of British Columbia Vancouver, BC, Canada Abstract Vacuum microwave dehydration (VMD) has been shown to maintain porous structure in dried plant and animal tissues, resulting in microstructure appearance similar to that of freeze-dried tissues. Resonance chamber and travelling-wave designs of vacuum microwave equipment were compared for generation of porous structures. In part bubbles were generated by the absorption of microwave energy which raised the temperature of intracellular water above the boiling point, while cellular structures trapped the steam so generated. Wheat flour dough and potato starch gel were employed to investigate the impact of rheology, dielectric properties and microwave power on expansion and porosity of the dried materials. A finite element model was employed to test theories of expansion. Video monitoring of the VMD process suggests that expansion of entrapped non-condensable gases was also a factor that contributed to total porosity and this was confirmed with the finite element model. Apparently total porosity of VM dried gels and doughs increase up to a critical Young’s modulus (E), then declined as E increases further, with the critical E value varying substantially for different materials. Keywords: microwave, vacuum, dehydration, porosity, dielectric properties, biomaterials, snack foods
1. Introduction Microwave dehydration under vacuum is a relatively recent drying strategy that is currently used commercially for some food products, but on a limited scale. It can be viewed as a variant of vacuum drying in which radiant energy in the 1 cm to 1 m wavelength range is employed to accelerate drying (Scaman and Durance, 2005). While vacuum drying typically yields dried food materials of excellent quality due to low drying temperatures and low oxygen exposure, its commercial applications are severely limited by slow throughput and the attendant high costs. Slow throughput is predominately due to slow transfer of the energy required for the phase change. Most vacuum drying relies on conductive heat transfer that is slow unless very high temperature differentials are employed. Radiant energy transfer is of course not hindered by vacuum, is essentially instantaneous, and in the case of microwave, can often be delivered directly to the water within the food. While vacuum microwave dehydration of food was proposed as far back as 1945, and is still regularly reported in the food science literature, most papers have focused on retention of chemical quality (Yongswatdigul et al., 1996; Mui et al., 2002). Some early studies reported but did not necessarily exploit a unique feature of vacuum microwave (VM), the capacity to puff or expand some 37
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materials. First observed in fruits and vegetables (Figure 1), the phenomena have been attributed to bubbles of steam expanding within cellular plant or animal tissues. Since the latent heat of vaporization is generated by absorption of microwaves inside the food, much of the phase change takes place within the tissue, rather than at the surface as is seen in drying events driven by conductive or convective heat transfer. When steam pressure within the tissue exceeds the pressure of the external environment, the tissue tends to expand. If this differential is maintained until the material of the pore walls become rigid due to dehydration and/or cooling, the expanded structure is maintained. Several years ago we began experimenting with VM puffing as a means of creating porous structures of non-tissue materials. As well as possible applications in snack-foods, the technique has attracted some attention as a means of creating porous structures from biomaterials for medical and tissue engineering applications (Durance et al., 2006; Yang and Burt, 2006). These materials frequently must carry or incorporate temperature-sensitive drugs and other biologically active compounds. There is a shortage of simple, low-temperature techniques for generating stable composites of controlled porosity. The combination of microwave and vacuum may help fill this requirement. Therefore we selected a variety of food ingredients and hydrocolloid gelling agents for models of the VM dehydration process. Our goal was to improve our understanding of the VM puffing process and the factors that control total porosity and pore size distribution. Ultimately we wish to be able to predict puffing potential of materials from their fundamental properties for defined process parameters.
Figure 1. Comparison of structure of apple tissue dehydrated in hot-air (A) or in a microwave field under vacuum (B).
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2. Materials and Methods 2.1. Unleavened bread dough Commercial bread flour (1750 g) plus 0% to 1.5% salt, was mixed with water (20-25ÂşC; 34.3% by weight) using a dough hook in a 20 L mixer at 60 rpm for 10 minutes. Rested dough was rolled to 10 mm thickness. Plugs of dough were cut with a 10 mm diameter cylindrical cutter, and then manually rolled into balls.
2.2. Alginate-pectin-starch gels Gel was prepared by mixing sodium alginate 3%, high methoxy pectin 4%, corn starch 3%, and glycerol 0.5% (all w/w) with water at different concentrations in order to yield gels of different hardness. The viscous suspension was poured into cylindrical moulds (3 cm diameter Ă— 1.5 cm deep), quick-frozen, and suspended in 3% CaCl2 solution at room temperature overnight.
2.3. Vacuum microwave dehydration (VMD) Materials were dehydrated in a variable-power, vacuum microwave dehydrator (Model 1.8, EnWave Corp., Vancouver, BC) operated at 2250 MHz, 700 or 1300 W of microwave power and a chamber pressure of 28 mm Hg absolute pressure (Figures 2 and 3). Once loaded, the vacuum was applied for 20 s before the microwave was turned on to allow the chamber operating pressure to
Figure 2. Schematic of resonance-chamber Vacuum Microwave Dehydrator with agitation.
Figure 3. Model 1.8 vacuum microwave dehydrator, resonance chamber design.
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be reached. Actual VM drying time was 7 minutes. Samples (600 g wet weight) were loaded into a cylindrical, polyethylene drying basket which revolved on its horizontal axis at 4 rpm during the drying process, in order to continuously agitate the materials within the microwave field. The process was monitored continuously by means of a video camera within the VM chamber. Alternatively, samples were processed without agitation in a radiant energy vacuum dehydrator (Model REV-900, EnWave Corp., Vancouver, BC) operating at 2250 MHz, 300 W of microwave power and 30 mm Hg absolute pressure (Figure 4).
2.4. Finite Element model A 2D FE model was developed to simulate the puffing of dough balls under VM application. The modelling procedure solved the coupled heat transfer and solid mechanics equations in order to determine the puffing due to the externally applied vacuum and internal pressure build-up from water vapour. Dough balls were modelled as a 2-D circular geometry consisting of approximately 300 quadrilateral elements. For symmetry reasons, only a quarter of the dough ball was actually modelled. The FE mesh is depicted in Figure 11 in section 3.3. The details of the FE modelling process are described elsewhere (Ressing et al., 2006).
2.5. Dielectric properties measurement Relative dielectric constant ε’ and loss factor ε’’ of materials were determined with an openended coaxial probe and a network analyzer (Model HP 85070, Hewlett Packard, Houston, TX) (Colpitts et al., 1992).
2.6. Elastic modulus and fracture strength Elasticity and fracture strength of wheat doughs were measured in uniaxial extension (Figure 5), while the modulus of alginate-pectin-starch (APS) gels were measure in compression, both at 0.1 mm per second on a TA.XT2i Texture Analyser (Stable Microsystems Ltd., Surrey, UK).
Figure 4. Schematic of traveling wave microwave design (Metaxas & Meredith 1983).
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3. Results and Discussion 3.1. VM and REV dehydration Two related techniques were employed in this study. The term vacuum microwave (VM) is employed here to denote processing of materials in a resonance-chamber vacuum microwave oven, as is typically used for VM processing of foods. In this design, microwaves are generated and directed into a vacuum chamber containing the materials to be dried. In the design employed here, materials were tumbled in a rotating drum to ensure homogeneous exposure to microwaves and to prevent clumping. Resonance chamber designs are relatively easy to design and energy efficient, but it is essentially impossible to achieve a uniform microwave field throughout the chamber due to reflection and standing wave patterns. Some form of agitation or stirring is required to achieve uniform processing. Also, the load must be matched to the power of the system. The chamber is in a sense a deadend for the microwave power. If at any time the load is insufficient to absorb the power, undesirable events may occur, such as over-processing, overheating of the microwave generator or the electrical discharge known as arcing. In our case, the minimum load in the VM was about 50 g dry weight. For small loads, an alternate design was required. To avoid confusion, this process is referred to here as radiant energy vacuum (REV) A waveguide provides a zone of relatively uniform field strength in its central cross-section (Figure 4, insert). This is exploited in the so-called “travelling wave� microwave applicator designs (Figure 4). We developed a vacuum version of travelling wave applicator in cooperation with a microwave technology company. Microwave power was produced in a variable-power generator, fed through the wave guide containing the sample under vacuum, then surplus energy was directed into a water load at the end of the waveguide. Thus, unlike the resonance chamber design, microwaves passed through the sample only once. Very small samples could be processed safely and agitation was not required. In practice we found the REV design with its small, stationary load allowed finer control and monitoring of the process.
3.2. VM expansion and elastic properties of wheat dough As VM expansion is driven by expanding gases, we proposed that volume increase may be inversely proportional to elastic strength of the material. Experimental results with dough supported this. As salt concentration in dough was increased, resulting in harder dough that offered more resistance to expansion, final volume of the dry dough balls decreased (Figure 7). We measured
Figure 5. Schematic of uniaxial extension tests for wheat dough. Depicted is half the cross section of a Texture Analyser on the left, including the dough holder and the dough (broad lighter lines). On the right is the view from the top of the setup. L0 = original length of the dough; L = length after deformation; t0 = original thickness of the dough; t = thickness of the dough; F = force applied; d = diameter of the plunger; D = diameter of the holder.
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Young’s modulus (E) and Fracture Strength (Rm) in uniaxial extension (Figures 6 and 7). Dough Rm and E both increased with salt concentration in possibly linear fashion (Figures 8 and 9). When percent expansion of VM bread flour dough was compared directly to Rm, another possibly linear relationship was seen (Figure 10). E versus % expansion gave a similar but more variable relationship. Thus these results supported the hypothesis that VM expansion was inversely proportional to elastic strength. VM expansion experiments were routinely recorded on video and examination of these records revealed some interesting facts. Some expansion of the dough was seen to occur even before the microwave was turned on. This was attributed to expansion of air trapped in the dough due to
Figure 6. Sample stress—strain curve for wheat flour dough in extension. The fracture strength (Rm) is the maximum stress of the test. Young’s modulus (E) is the slope of a straight line fitted to the central 2/3 of the increasing curve.
Figure 7. VM expansion of unleavened bread wheat dough at different salt concentrations.
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Figure 8. Fracture strength in uniaxial extension as a function of salt concentration in bread wheat dough.
Figure 9. Young’s modulus of VM wheat bread dough as a function of salt concentration.
Figure 10. VMD dough volume increase as a function of wheat bread dough fracture strength.
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the reduced pressure in the vacuum chamber, air that could be expected to continue to expand once heating was begun. Also, in all cases maximum dough volume was achieve early in the drying process, within the first 3 minutes of the 7 minute drying process. Thereafter the samples continued to dehydrate but did not increase in volume.
3.3. FE model of VM expansion Figure 11 depicts the FE model developed to describe VM expansion of bread wheat dough. A unique feature of our FE model was the implementation of “wet” and “dry” dough elements. Differences between wet and dry dough elements lie primarily in their thermal and dielectric behaviour, while their structural properties are identical (Table 1). During the thermal analysis process, wet dough elements represented the water content in the dough mass. They were randomly distributed according to the mass ratio of water in the dough. This ensured that the FE model contained the right amount of water and flour mass that would be heated throughout the analysis. At the same time, the concentration of thermal properties of water in only certain elements and the random distribution of these elements allowed the simulation of uneven heating of the entire dough mass due to uneven distribution of water in the dough. This was possible, because the thermal properties, such as thermal conductivity and dielectric loss factor, of water and flour are well distinguished. In addition, the dielectric loss factor of wet dough elements depends on the dough’s salt content. The salt content was varied between 0 and 1.5 % with low salt content exhibiting lower loss factors than higher salt contents. On the other hand, structurally no distinction between wet and dry dough elements was feasible. In reality the structural stiffness of dough can not be estimated by a simple averaging of the material properties of flour and water as it arises from the interaction of the water with the flour components. For our experiment, dough stiffness was measured as a rheological property of actual
Figure 11. 2-D FE model of a dough ball. Yellow elements symbolize dry dough, blue elements symbolize wet dough. Wet dough elements are distributed randomly. For symmetry only one quarter was modeled.
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doughs. Hence, while heating and dielectric properties were allowed to assume different values in wet and dry elements, structural properties of wet and dry elements dough were not allowed to be different, but were considered equal for wet and dry dough elements. In order to simulate puffing of the dough balls due to the generation of water vapour inside the dough mass, the state of water contained in wet dough elements was checked at every step of the analysis. For this, the pressure inside the wet dough elements was determined and compared to the vapour pressure of water at the element temperature. If the element pressure was below the vapour pressure, part of the water mass was converted into steam filling the available element volume completely. At the same time, energy in the amount of latent heat for the converted water mass was removed from the system. Once a wet dough element was determined to contain water vapour, the vapour pressure was applied as internal pressure to the element edges during the structural analysis process to simulate the expansion of the dough ball due to vapour generation. Drying was simulated by water mass removal from wet dough elements containing water vapour. The water mass removal rate was determined in experiments and, for simplicity, was applied as a function of time ignoring the effects of temperature. Figure 12 depicts a simple reflection of the dough ball deformation, showing the initial and final volume superimposed. Figures 13 and 14 show the temperature and stress distribution of a dough ball with 0.0% salt concentration for times tdrying = 1, 10, 90 and 180 s of microwave exposure, respectively. In the early stages, uneven heating due to the non-uniform water distribution can Table 1. Material properties used for the FE model. Properties
Dry dough
Thermal Heat conduction λ [W/(m K)]1,2
0.33
Specific heat capacity c [J/(kg K)]1,2
908
Wet dough 0.59 4180
Dielectric loss factor ε”3 Salt concentration (%): 0.0
0.1
8.5
0.2
0.1
8.9
0.5
0.1
10.9
1.0
0.1
11.5
1.5
0.1
12.9
Drying rate r [%/s]
3 1
Latent heat h [MJ/kg] Density ρ [kg/m ]
3 1,2
0
0.175
0
2.26
740
1000
Structural Young’s modulus E [kPa]3 Salt concentration (%): 0.0
117.8
117.8
0.2
125.7
125.7
0.5
137.5
137.5
1.0
157.1
157.1
1.5
176.7
176.7
Poisson’s ratio μ
3
1
Anonymous (1982). 2 Rahman (1995). 3 Ressing (2005).
0.48
0.48
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be observed. As the heat penetrates through the dough ball at later points in time, the temperature distribution is primarily dependent on the penetration depth and the resulting lower heat generation in the centre of the dough ball. The resulting stress distribution shows the increased tensile stresses from higher vapour pressure in regions of higher temperature. Figure 15 summarizes predicted expansion of bread wheat dough at different salt concentrations. As in the experimental results, the model predicted lower volumes with higher salt and higher E. However, the FE model predicted less volume increase than was actually observed. This may be in part due to the fact that VM expansion occurred at material temperatures of about 30ºC while E (Young’s modulus) and Rm (fracture strength) were measured at 22 to 24ºC. Thus values of E and Rm in reality were less than the values used in the model.
3.4. REV alginate-pectin-starch (APS) gels Examination of experimental results with other materials tends to support the relationship between E and volume of microwave dehydrated gels. A series of APS gels of different initial elastic modulus were produced and dehydrated in the REV apparatus. In this experiment, total pore volume was determined by mercury porosimetry rather than increase in bulk volume. These results are summarized in Figure 16. In this material porosity decreased with increasing E above a critical value of about 0.27 MPa following the same pattern as seen for wheat dough. However, below the critical value, porosity actually increased with E. We propose that this may be typical behaviour for most materials under microwave expansion, whereas in the bread dough experiments we did not test dough of sufficiently low E to observe the rising portion of the curve.
Figure 12. Dough ball deformation under 35 mbar absolute pressure and 180 s of microwave application. The blue grid shows the original shape, the red grid depicts the expanded dough ball in its final form.
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3.5. Pore size and pore size distribution In addition to controlling total volume of pores, for most materials control of pore size and pore size distribution would be useful. Clearly different materials and microwave processes can lead to quite different pore sizes and distributions, as seen in the cross-sections of VM dough balls and REV APS gels (Figures 17 and 18). At this time we have very limited information as to the factors that determine pore size and the relationship between pore size and Young’s modulus, if any, is not clear. Further studies are required to elucidate this point.
4. Conclusions VM and REV techniques allowed the creation of expanded porous structures from a variety of hydrocolloid gels and doughs including wheat flour dough, starch, alginate and pectin gels, while avoiding destructive temperatures. Expansion is due to expanding entrapped gases and steam generated by heat associated with absorbed microwave energy. The resonance chamber VM design
Figure 13. Temperature distribution predicted by FE model.
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provided uniform bulk processing by agitating the material particles within the microwave field. The REV travelling wave apparatus allowed vacuum microwave processing of very small samples without agitation. A relationship was observed between strength of the gel, as measured by Young’s modulus or fracture strength, and the percent volume increase or total porosity. We propose as a hypothesis that for each suitable material there is a critical value of E, above and below which expansion potential decreases. A numerical model was developed which correctly predicted the influence of salt concentration and E on VM expansion of wheat dough in general terms, but was not quantitatively accurate. Further refinement and validation of the model is required. Average pore size and pore size distributions also vary with material and process variables. Further study is called for to identify all determinative factors and relationships between materials, processes and porosity.
Figure 14. Stress distribution predicted in VM bread wheat dough be FE model.
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Figure 15. Volume versus time as predicted by FE model for wheat dough of different salt content.
Figure 16. Total porosity as a function of Young’s modulus in compression for alginate—pectin— starch REV dried gels.
Figure 17. Porous structure of wheat VMD wheat flour dough.
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Figure 18. Porous structure of alginate/pectin/starch dough.
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