Microwave moisture loss

Adedeji et al. (2009) precooked chicken nuggets in microwave at different power levels and reported a tremendous reduction in oil uptake and a significant change in moisture loss in the fried products when a lower level of microwave power density... 

FIGURE 2.8 Effect of microwave precooking at different power densities on moisture loss in breading coating of chicken nuggets fried at 190°C. (From Adedeji, A.A. et al., J. Food Eng., 91, 146, 2009). 

Microwave analysis of moisture is also known as LOD (loss on drying), drying test, or total volatile solids. For many years, the standard oven test was used for moisture determination. Although very accurate, the test could take many hours to complete, depending on the sample. The time lapse rendered it practically useless for anything except a final quality control check. The new microwave moisture/solids analyzers were introduced about 20 years ago. They offer rapid, accurate moisture/solids analysis in a fraction of the time it took to run a standard oven-drying test. 

Moisture loss below freezable water range was mainly responsible, according to Vittadini et al. [145], for the thermomechanical behaviour of prebaked pizza dough reheated with a microwave oven as detected with DMA in three-point bending mode. Thermal transitions were associated to pizza texture characteristics thermograms presented two transitions a transition was detected at 60 40°C and was found responsible for the leathery texture in microwaved pizza, while the one detected at 0 30 °C was the dominating transition in the soft pizza reheated by conventional oven. 

Figure 2 shows typical drying curves for commercial p-hemihydrate that was preconditioned in three different environments to prepare samples with different amounts of free moisture. The curves exhibit a definite plateau after only a few minutes of exposure to microwaves, which is a strong indication that the weight loss is due to free water removal only. 

The loss factor (s") for higher- and medium-moisture content cheese increases gradually with temperature (5°C-85°C). The trend is opposite for low-moisture cheese. The increase in s" for high- and medium-moisture cheese could be attributed to ionic conduction. It was reported that the effect of temperature was more pronounced at lower frequencies than at higher frequencies (above 1 GHz) (Nelson and Bartley, 2000). Models were also developed to predict the effects of moisture and salt content. These models can provide the effects of frequency, temperature, and compositions on microwave processing of cheese. 

Also, copper in charcoal (Cu/C) is found to be an active catalyst for effecting click reactions between organic azides and terminal aUcynes [84]. Rates of cycloaddition are dramatically increased when carried out in the presence of triethylamine or using microwave assistance. There is no sensitivity to air or moisture and the catalyst can be recycled several times without loss of activity or selectivity for the 1,4-adduct. 

As an example, consider the drying of bread crumbs from 27 to 5% moisture, at a rate of 1000 lb of wet bread crumbs per hour. In such a system, it would be necessary to evaporate 231.6 lb of water per hour (1000 lb of bread crumbs contain 270 lb of water, but when dried to 5% contain only 38.4 lb of water). This would require about 77 kW to dry (231.6/3.0), plus an additional 20 kW to heat to the drying temperature, for 97 kW, neglecting all heat losses. If we assume this system to have a coupling efficiency of 75% (that is, the efficiency of coupling microwaves into the product), then a system of 130 kW is required, which would cost approximately one million dollars. On the other hand, if a conventional hot air dryer is used to reduce the moisture from 27 to 12% and the microwave dryer to finish drying it to 5%, then the amount of water to be evaporated by the microwave system is only 61.6 lb, which requires 20.4 kW (61.1/3.0). Since the product is already hot, we need to only increase the output to 27 kW to account for the 75% coupling efficiency (20.4/0.75). Such a system would cost of the order of 200,000 plus the cost of the less expensive conventional dryer. 

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