Moisture distributions for different heating combinations

4.5.4

Moisture distributions for different heating combinations

Many safety (microbiological/chemical) and quality attributes of the final food product are related to the total moisture and moisture distribution after the heating/cooking pro- cess, making them critical information. The overall moisture losses from the samples heated by different combinations are shown in Figure 4.11. More moisture is lost from

74 76 78 80 82 84 0 200 400 600 800 1000 1200 1400 M o is tu re c o n te n t (% w b ) Time (s) Full microwave, convection and radiant Cycled microwave, convection and radiant

Convection and radiant

32 34 36 38 40 42 0 200 400 600 800 1000 1200 1400 M o is tu re c o n te n t (% w b ) Time (s) Full microwave, convection and radiant

Cycled microwave, convection and radiant

Convection and radiant

High initial moisture Low initial moisture

Figure 4.11: Computed average moisture content history for material with different initial moisture contents heated using three different combinations.

samples that are heated to higher temperatures since heating leads to increased evapo- ration and transport of water as water vapor in the porous medium. The moisture loss in 20 min from the high initial moisture samples are 7, 7.6 and 9.4% for convection and radiant, cycled microwave, convection and radiant, and full microwave, convection and radiant heating respectively. For the low initial moisture samples these are 15.4, 17.5 and 22.1% respectively, showing is a marked increase (more than two-fold) in the loss of moisture from the corresponding low initial moisture samples.

To determine the reason for the difference in moisture loss above, the total amount of water lost as liquid water and water vapor from the surface were calculated for the high and low moisture samples. At t = 20 min, for the high moisture sample heated using microwaves for the full time, the rate of loss of water as liquid and vapor was 3.0210 6 and 1.8410 7kg/s, respectively. For the low moisture material, they were 1.3610 6 and 1.2810 6 kg/s, respectively. It can be observed that although the rate of water lost as liquid water reduces by half at low moisture content as the capillary diffusivity decreases (see Table 4.1), there is a ten-fold increase in moisture loss as vapor. This is attributed to the increase in temperatures as discussed for Figure 4.8 which leads to increased evaporation in the overall domain and the binary diffusion and pressure driven flow of vapor from the interior to surface.

High initial moisture content Figure 4.12 presents the moisture distribution in the high initial moisture samples after 20 min of heating. For convection and radiant heat- ing, the moisture loss is mostly from the top surface with interior locations remaining at high moisture content even after 20 min of heating. The dominant mode of trans- port is capillarity which moves the liquid water to the surface and there is practically no pressure driven flow or binary diffusion of the vapor since the interior locations are not heated. The interior remains wet and the complete drying of the sample using only convective and radiant heating is therefore expected to take a very long time. In case of cycled microwave heating, there is additional mass transfer due to formation of vapor in the interior locations by evaporation and movement to the surface by binary diffusion and pressure driven flow. In case when the microwaves are on for the full time, very high heating rates are involved and there is increased evaporation leading to comparable moisture loss in the interior locations as well. In certain locations though, the moisture loss from the surface is not able to keep up with additional moisture coming from inte- rior and therefore the locations close to the surface have higher moisture content. This is generally not desired during food preparation since it leads to soggy surface and is one of the known drawbacks of microwave heating. Interestingly, this is not observed when cycled microwaves are added and hence, careful addition of heating modes by matching the relative rates of heating may help in development of custom products by keeping the surface moisture low (crisp).

Low initial moisture content For the low initial moisture material, there is more vari- ation in the moisture distribution even for convection and radiant heating (Figure 4.13). This is attributed to higher evaporation in the low moisture material discussed earlier. An interesting result observed here is that even when the microwaves are on for the full time, there is no accumulation of moisture in the surface unlike the high moisture ma-

Moisture content (% wb) Convection and radiant Cycled microwave,

convection and radiant

Full microwave, convection and radiant

Figure 4.12: Computed moisture content maps for material with high initial moisture content heated using different combinations after 20 min of heating.

Moisture content (% wb) Convection and radiant Cycled microwave,

convection and radiant

Full microwave, convection and radiant

Figure 4.13: Computed moisture content maps for material with low initial moisture content heated using different combinations after 20 min of heating.

terial. The rate of evaporation of moisture due to convection and radiant heating near the surface becomes higher than that due to microwave heating which in turn facilitates moisture removal from the surface. This is also discussed in the next section.

Gauge pressure (Pa) Convection and radiant Cycled microwave,

convection and radiant

Full microwave, convection and radiant

Figure 4.14: Computed pressure distributions for material with high initial moisture content heated using different combinations after 20 min of heating.