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Dry-roasting of nuts using microwaves


Key words drying, roasting, hot air roasting, heating, microwave, nuts, Macadamia nut, Brazil nut, hazelnut, peanut
Latest version 2012/02/08
Completed by IRTA

How does it work?

Primary objective To reduce processing time during drying and dry-roasting, and save energy.
Working principle Since drying is a high energy demanding process, Microwave heating, installed as an assisting treatment of conventional hot air drying process, aims to reduce the drying time and increase the industrial yield.

Hybrid drying processes in the second drying stage of nuts can consist in combining hot air convection and microwave heating (1) in the existing production line, or using a microwave-infrared (Dry-roasting and pasteurization of nuts using infrared heating) combination oven (8). Traditionally drying operation starts just after harvesting to prevent any hydrolytic rancidity and mould development. Harvested hulled in-shell nuts with a moisture content from 20 to 30% (d.b.) are pre-dried by natural air convection in the field during 3-4 weeks time, till the moisture content is reduced to 10% (d.b.). The conventional second drying stage could consist in a hot air convection in a silo. Otherwirse, dry-roasting of nuts using MW can represent a time shortage from 87,5% (hazelnut) to 96,5% (macadamia nuts) of the drying operation, and in most cases the sensorial properties are acceptable (less rancidity).

Additional effects
  • Higher production yields
  • Lower processing costs
  • Improve kernel peeling, that is a major problem for many varieties, especially for Hazelnut
Important process parameters temperature, time, frequency, power
Important product parameters water activity (aw), moisture content, fatty acid composition, weight, product geometry, dielectric properties

What can it be used for?

Products Macadamia nut (1,4,6), Brazil nut (7), hazelnut (8), peanut (2,3)
Operations Drying, roasting
Solutions for short comings Shorting processing times

What can it NOT be used for?

Products High water content (>50%)
Operations -
Other limitations
  • Nuts need to be continuously moved in order to avoid the Maillard reaction due to overheating, and to get an efficient and uniform drying.
  • Depending on product geometry, uniformity of treatment could be limiting.
  • In case large amounts of nuts must be processed, running costs could be unaffordable.
  • Although combining hot air and microwave results in a faster process, it could require prohibitively high investment and operating costs (1).
  • Even a short period MW heating accelerates the formation of some undesirable and harmful compounds (f.i. oxidation, transformed pigments) (3).
Risks or hazards The magnetrons delivers a powerful electromagnetic field and microwave leakage needs to comply with the regulation.


Maturity Though industrial applications of microwaves are applied since the eighties, appropriate processing parameters for industrial dry-roasting need to be tested.
Modularity /Implementation MW heaters can be installed in front of a conventional hot air roasting process in the existing production line.
Consumer aspects Consumers are familiar with microwave heating, due to the domestic use of this technology.

No significant differences between the microwave and conventional drying processes have been found. Some studies have reported nuts with more rancid taste in case using conventional drying. (5)

Legal aspects Please check local legislation
Environmental aspects Energy saving

Further Information

Institutes IRTA-Mas Bover, Middle East Technical University, Mansoura University, National Research Centre El Dokki, University of Campinas
Companies Sairem, Petrie Technology, Stalam, Eodiss Systems, Remak, MEAC, Romill
References 1. Borompichaichartkul C., Luengsode K., Chinprahast N., Devahastin S., 2009. Improving quality of macadamia nut (Macadamia integrifolia) through the use of hybrid drying process. Journal of Food Engineering; Volume 93, Issue 3, 348-353

2. El-Badrawy, E.E.Y., El-Zainy, A.R.M., Shalaby, A.O. & El-Sayed, N.Y., 2005. Effect of microwave roasting on chemical composition of peanut seeds and comparing it with the ordinary roasting process.

3. M. G. Megahed, 2001. Microwave roasting of peanuts: Effects on oil characteristics and composition. Food / Nahrung (Molecular Nutrition and Food Research); Volume 45, Issue 4, 255–257

4. Silva F. A., Goncalves L. A. G., Damiani C., 2011. Oxidative stability of macadamia nuts dryed with hot air microwaves. Pesquisa Agropecuaria Tropical; Volume 41, Issue 2, 286-292

5. Silva, F.A.; Maximo, G. J.; Marsaioli, A. Jr.; et al, 2007. Impact of microwave drying on the sensory profile of macadamia nuts. Ciencia e Tecnologia de Alimentos; Volumen 27, Issue 3, 553-561

6. Silva, F.A. Marsaioli A. Jr., Maximo G.C., Silva M.A.A.P., Gonçalves L.A.G., 2006. Microwave assisted drying of macadamia nuts. Journal of Food Engineering; Volume 77, Issue 3, 550-558

7. Silva F. A., Marsaioli A. Jr., 2004. Comparative study of Brazil nuts ( Bertholletia excelsa) conservation dried conventionally and by microwave. Boletim do Centro de Pesquisa e Processamento de Alimentos; Volume 22, Issue 2, 387-404

8. Uysal N., Sumnu G., Sahin S., 2009. Optimization of microwave–infrared roasting of hazelnut. Journal of Food Engineering; Volume 90, Issue 2, 255-261

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Created by Hte irta on 8 February 2012, at 17:12