Microwave drying of food materials is now an attractive technique for drying foods, mainly raw produce such as fruit, herbs and vegetables. Drying is essentially the reduction in moisture content, the removal of water for the purposes of preservation. Microwave drying is especially suited for products with high levels of moisture such as mushrooms, cabbage and carrots. It is also effective for the retention of nutrient factors. Ideally, any stored food should retain as much goodness as possible over its shelf-life. The method has also been used successfully in the pharmaceutical and semi-conductor industry.
Traditionally, food drying relies on conduction, convection and infra-red radiation with the simplest being solar or sun drying. Without drying, up to 40% of the produce quality is lost if the moisture content is not reduced after harvesting as this prevents molds in particular from taking hold and ruining the goods.
Current Issues Of Convective Drying (CD) That Need Overcoming
Convective drying is still the most popular industrial method for reducing the moisture content of vegetables, herbs and fruits. The method is bedeviled by issues and severe limitations, mainly associated with overly high temperatures and long processing times. The contact of dried material with hot air causes losses in important flavour compounds, colour and nutrients. Sometimes, browning occurs.
There is also considerable shrinkage as the cells in the tissue collapse. Any new process method should address this.
Benefits Of Microwave Drying
With microwave or dielectric drying, the heat penetration is higher than other methods. The microwave energy applied is between 915 and 2400 MHz which is absorbed by water. Any heat is generated deep inside the material where the applied electromagnetic radiation is converted to kinetic energy of water molecules, which then evaporate off.
The advantages can be listed thus:-
-rapid energy dissipation throughout the food
-low heating effects
-effective and efficient drying in the period of the falling rate
-low migration of water-soluble nutrients
-the microwave focusing effect which allows selective heating of certain interior portions
– case hardening is minimised because there is little moisture pumping effect.
Some early examples improved upon conventional drying of potato and apple pieces (Huxsoll and Morgan, 1968). The kinetics of drying has been studied for a range of fruit and vegetable products – raisins (Kostaropoulos and Saravacos, 1995), carrot slices (Cui et al., 2004a and b), mushrooms (Giri and Prasad, 2007), potato (Bouraout et al., 1994) and is advised for other foodstuffs such as algae such as Spirulina.
Vacuum microwave drying has been applied to peanuts (Delwiche et al., 1986), carrots (Lin et al., 1998), apples (Sham et al., 2001), bananas (Mousa and Farid, 2002), tomatoes (Durance and Wang, 2002), cranberries (Sunjka et al., 2004), strawberries as we’ll see later, and garlic (Figiel, 2009).
Commercially, strawberries are often dried for the baking industry using a variety of methods including convection drying, vacuum drying and freeze-drying. One study conducted at the Wrocław University of Environmental and Life Sciences in Poland demonstrated that strawberries dried by these industrial and standard methods lost significant quantities of antioxidants and virtually all the anthocyanins, flavanols, and ascorbic acid, although colour was retained (Wojdyło et al., 2009). Microwave vacuum drying on the other hand showed far better retention of key components. Three outputs were analysed, 240, 360, and 480 W, and the highest wattage produced the best results of all.
A recent example looked at the drying of shredded carrot where the β-carotene retention was 87% and with virtually no solid loss. This paper reported on a response surface method for optimising the drying parameters based on microwave power, the level of vacuum and carrot sample thickness. Of the standard mathematical models available for assessing the data, the Page model gave the best fit (Chaughule and Thorat, 2011). The optimum drying condition was a microwave power of 200 W, vacuum pressure of 10 kPa and a carrot slice thickness of 7.1 mm.
There are some disadvantages associated with heterogenous distribution of heating within the processing chamber which leads to uneven drying (Risman et al., 1987). Cost can be high at first and sample size is dependent on the chamber volume. One other drawback is the change to product texture as the water moves through the matrix and in some cases product shrinkage.
Combinations of conventional and microwave vacuum drying have been examined. A study on beetroot cubes demonstrated dehydration by convective drying in hot air at 60 °C and by the combination of convective pre-drying (CPD) to a defined moisture content and then vacuum-microwave finish drying (VMFD) at 240, 360 or 480 W (Figiel, 2010). The combination method improved upon the rate of drying with retention of colour and a reduction in the level of shrinkage.
If the issues of uneven heating can be solved, microwave drying could be the method of choice for drying and hence preserving most fruit and vegetables. This post will continue to identify new developments and applications for the method in the food industry.
References
Bouraout, M., Richard, P., & Durance, T. (1994). Microwave and convective drying of potato slices. J. Food Proc. Eng., 17, pp. 353–363.
Chaughule, V.A. and Thorat, B.N. (2011) Microwave Vacuum Drying of Shredded Carrots and Its Nutritional Evaluation. Int. J. Food Eng. 7 (4), Article 8. DOI: 10.2202/1556-3758.2235
Cui, Z.-W., Xu, S.-Y., & Sun, D.-W. (2004a). Effect of microwave-vacuum drying on the carotenoids retention of carrot slices and chlorophyll retention of Chinese chive leaves. Drying Technology, 22(3) pp.563–565.
Cui, Z.-W., Xu, S.-Y., & Sun, D.-W. (2004b). Microwave-vacuum drying kinetics of carrot slices. J. Food Eng., 65(3), pp. 164–175
Delwiche, S.R., Shupe, W.L., Pearson, J.L., Wilson, D.M., 1986. Microwave vacuum drying effect on peanut quality. Peanut Science 13 (1), pp. 21–27.
Drouzas, A. H., & Schubert, H. (1996). Microwave application in vacuum drying of fruits. J. Food Eng. 28 pp. 203 -209
Durance, T.D., Wang, J.H., (2002) Energy consumption, density, and rehydration rate of vacuum-microwave and hot-air convection-dehydrated tomatoes. J. Food Sci. 67 (6), pp. 2212–2216.
Figiel, A. (2010). Drying kinetics and quality of beetroots dehydrated by combination of convective and vacuum-microwave methods. J. Food Eng., 98(4), pp. 461-470.
Giri, S. K., & Prasad, S. (2007). Drying kinetics and rehydration characteristics of microwave-vacuum and convective hot-air dried mushrooms. J. Food Eng., 78(2), pp. 512–521.
Huxsoll, C. C., & Morgan, A. J. (1968). Microwave dehydration of potatoes and apples. Food Technol., 22, pp. 47-51
Lin, T.M., Durance, T.D., Scaman, C.H., (1998) Characterization of vacuum microwave, air and freeze dried carrot slices. Food Research Int. 31 (2), pp. 111–117
Mousa, N., Farid, M., (2002) Microwave vacuum drying of banana slices. Drying Technology 20, pp. 2055–2066.
Kostaropoulos, A. E., & Saravacos, G. D. (1995). Microwave pretreatment for sun-dried raisins. J. Food Sci., 60, pp. 344–347.
Risman, P. O., Ohlsson, T., & Wass, B. (1987). Principles and models of power density distribution in microwave oven loads. J. Microwave Power, E2, pp. 193-198
Sharma, G.P., Prasad, S., (2001) Drying of garlic (Allium sativum) cloves by microwave-hot air combination. J. Food Eng. 50, pp. 99–105.
Sunjka, P.S., Rennie, T.J., Beaudry, C., Raghavan, G.S.V., 2004. Microwave-convective and microwave-vacuum drying of cranberries: a comparative study. Drying Technology 22 (5), pp. 1217–1231.
Wojdyło, A., Figiel, A., & Oszmiański, J. (2009). Effect of drying methods with the application of vacuum microwaves on the bioactive compounds, color, and antioxidant activity of strawberry fruits. J. Agric. Food Chem., 57(4), pp. 1337-1343.
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