Preparation of dry honey by microwave–vacuum drying

Microwave–vacuum (MWV) drying was investigated as a potential method for obtaining high-quality dried honey. Liquid honey was heated and dehydrated in a MWV dryer to a moisture content less than 2.5% within about 10 min. The drying curves and the temperature changes of samples were tested during MWV drying at a different of microwave power, vacuum pressure levels and sample thicknesses. Fructose, glucose, maltose and sucrose contents in the liquid and dry honey were determined by high-performance liquid chromatography (HPLC). The volatiles in the liquid and dry honey were concentrated by solid-phase microextraction (SPME), separated and identified by gas chromatography–mass spectrometry (GC–MS). A sample thickness of less than 8 mm and a vacuum pressure of 30 mbar were identified as the better parameters for the MWV drying. The core temperatures of the sample were about the same as the surface temperatures, the temperature changes were from 30 to 50 °C with higher dehydration rates while no darkening of the honey took place during MWV drying. There were no significant changes on the contents of fructose, glucose, maltose and sucrose in the honey after MWV drying. The volatile acids, alcohols, aldehydes and esters made up the bulk of the identified aroma compounds of the used liquid honey and the content of alcohols and the esters changed slightly. The acids decreased markedly whereas the aldehydes and the ketones increased remarkably in the honey dehydrated by MWV drying.