Authors: Heindl, Albert G.W.; Müller, Joachim

Source: Stewart Postharvest Review, Volume 3, Number 4, August 2007 , pp. 1-6(6)

Publisher: Stewart Postharvest Solutions

Abstract:

Purpose of review: Microwave energy can dramatically enhance drying of medicinal and aromatic plants, save energy and preserve product quality if properly applied in combination with hot air or vacuum drying. However, up to now there has been only limited industrial use of microwaves in the processing of agricultural crops and especially for medicinal and aromatic plants. This is due to the fact that there is a need for special knowledge on the application of specific microwave energy, the individual power absorption of each plant species, and on the polarity of the valuable ingredients like essential oils, which determine microwave absorption. Furthermore, the economically feasible portion of electromagnetic energy, of total energy for drying, has to be identified.

Main findings: In recent years, several studies have been published on microwave convective and vacuum drying of medicinal and aromatic plants. Most of the experiments were carried out on a small scale basis. However, energy evaluations and upscaling calculations, which decisively influence the economics of microwave drying, can hardly be found. For example, applying 500 W of microwave power to 25 g of a plant material for 5 min in a small microwave oven means a microwave energy input of 6 kWh/kg of product. For an industrial processing plant with a capacity of 1,000 kg/h of raw material, the microwave power would be 6,000 kW. Assuming an overall efficiency of 60%, the power requirements would be 10 MW - a power plant of its own! Often, technical details (eg, air speed, microwave power concentration or weight of sample) of the drying processes are missing. Furthermore, the convective drying experiments frequently used as controls in most studies are not always performed according the standards of the drying industry. For example, parsley leaves are dried with initial air temperatures of up to 120°C and high air speeds of up to 1 m/s to deactivate the colour changing enzyme chlorophyllase, but in laboratory studies much lower temperatures have been applied. On the other hand, quality aspects are described but not explained due to the lack of knowledge about microwave absorption of valuable ingredients or the lack of knowledge about inactivation of certain plant enzymes by microwaves.

Directions for future research: Each plant species has a specific range of moisture content where microwave absorption is optimal. More research has to be done to characterise the ranges of maximum efficiency of microwave application. Research should focus on valuable products whose market value justify high investment costs for microwave dryers. The microwave absorption of essential oil molecules or molecules of other valuable ingredients characterised by their loss factors and polarity has to be examined to decide whether microwave drying is feasible for high quality retention or not. Each plant species requires its own specific microwave application strategy due to structural differences. The future will see optimised combined drying systems of convective drying to remove the bigger portion of water and well dosed microwave drying - possibly assisted by vacuum - in moisture regions of highest absorption efficiency.

Keywords: DRYING; MICROWAVES; MEDICINAL AND AROMATIC PLANTS; QUALITY; ENERGY CONSUMPTION

Document Type: Research article

DOI: http://dx.doi.org/10.2212/spr.2007.4.3

Publication date: 2007-08-01

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