Research On Preparation Of Dry Honey By Microwave-vacuum Drying

The microwave-vacuum drying procedure and the formula of dried honey were studied in this paper. Meanwhile, the qualities and functionality of dried honey are tested by experiments.The microwave-vacuum drying process of honey experienced two periods. The constant drying rate period can be described by a theoretical model based on the energy conservation. However the model wasn't fit for the period when the drying rate declined. Here an empirical model is chosen and the effect of process variables was embodied in the expression of the phenomenological parameters. The regression method is used to estimate the best model parameters. The drying curves were tested and found that the drying rate increased with the improving of the microwave power and the vacuum pressure.Temperature distribution and changes of honey were tested during microwave-vacuum drying at a wide range of thickness of samples. The experiments showed that for sample thickness less than 8 mm, the core temperature of the sample was the same as its surface temperature, with uniform temperature distribution within the sample. However for sample thickness more than 8 mm, temperature gradient developed along the thickness of the sample. For samples with thickness≤8 mm, the drying process of honey experienced three distinct periods: a warming-up period without removal of moisture when the product temperature increased linearly with drying time until it reached the corresponding saturation temperature of water at the vacuum pressure; a constant period in which most of the moisture evaporated and flowed out of the sample efficiently with little resistance; and a heating-up period in which the drying rate decreased and sample temperature increased rapidly.The product did not became darker as honey dried by microwave-vacuum drying. The fructose, glucose, maltose and sucrose were determined by HPLC for liquid and dry honey, and the volatile of liquid honey and dry honey were concentrated by solid-phase microextraction (SPME), separated and identified by gas chromatography–mass spectrometry (GC-MS). By comparison the experiment dates, it can be concluded that the dry honey, by the current methods, was in good-quality with almost no alteration of color, no changes of the contents of fructose, glucose, maltose and sucrose, and a little changes of the compositions and contents of flavors.The equilibrium moisture content for dried honey under different moisture contents and temperatures was tested by a static weigh in method. The experiments showed that the hygroscopy of dried honey was effected by temperature and water activity. The equilibrium moisture content decreased with the improvement of temperature and increased with the elevation of water activity. The effect of water activity was more significant than temperature. 6 sorption isotherm model were simulated using non-linear regression analysis in SAS system and the GAB model was proved to be suitable.Honey's fructose content which makes it hygroscopic tends to make dried honey products"cake."For this reason, processing aids such as malt dextrine and orange fiber are used in the formulation of most dried honey products in powder to keep the product free-flowing or to modify its functionality. But for preparation honey candy, no processing aids or ingredients are needed.