Study On Ultrasonic Vacuum Drying Of Honey And Its Enhanced Drying Mechanism

In recent years,the drying of viscous materials has always been a problem in the food drying industry.With the development of ultrasonic technology,this article uses a combination of ultrasonic technology and traditional vacuum drying technology to dry viscous materials,such as liquid honey.Liquid honey has the inconvenience of carrying or storage limitation due to its transparent or translucent shape and sticky texture.Using honey as the experimental material,the heat and mass transfer rule of ultrasonic vacuum technique was explored by means of drying characteristic test comparing with vacuum drying technology.In order to determine the best combination of ultrasonic parameters for ultrasonic vacuum drying of honey,this paper also studied the influence of ultrasonic power and frequency on honey drying kinetics.Low field nuclear magnetic resonance(LF-NMR)and scanning electron microscope(SEM)experiments were used to investigate the enhanced effect of ultrasonic vacuum technique on the moisture distribution and microstructure of honey during drying.In addition,in order to further determine the role of ultrasound in the ultrasonic vacuum technique,this paper systematically investigated the effects of ultrasonic power and frequency on the rheological properties and the thermodynamic properties of honey.Moreover,in the study of the mechanism of the effect of ultrasound on honey,the changes in the composition of the main carbohydrates and molecular groups in honey under the action of ultrasound were comprehensively analyzed.Finally,through the drying process optimization experiment,the best process parameters of ultrasonic vacuum drying technique are obtained.The main achievements are summarized as follows:(1)Results showed that ultrasonic vacuum drying technique could reduce the time for drying honey by about 70%to 90%,compared with vacuum drying.The sonochemical effects of ultrasonic vacuum drying were enhanced with the increased ultrasonic power and were more obvious in the initial stage of drying.The nonlinear fitting analysis of experimental data on seven kinds of thin-layer drying mathematical models showed that Logarithmic model is more suitable for describing the law of moisture change in honey during ultrasonic vacuum drying than the other models because of its higher regression coefficient value and smaller reduced chi-square and root mean square error values Therefore,within the scope of experimental research,the Logarithmic model is more suitable than other models to describe the moisture change law of honey during ultrasonic vacuum drying,and can represent the behavior of ultrasonic vacuum drying honey.(2)Low field nuclear magnetic resonance(LF-NMR)experiment was used to study the dynamic distribution of water in honey.It was concluded that the combination of ultrasonic frequency and power of 40 k Hz 80 W could effectively remove the bound water in honey.Moreover,increasing the ultrasonic power can effectively improve the removal rate of bound water in honey while keeping the ultrasonic frequency unchanged.The increase in ultrasonic power not only weakens the binding force of water molecules in the bound water,but also promotes the migration of partially weakened bound water in the honey sample to fixed water(the water with weaker binding force to non-aqueous components).SEM results showed that the porous structure formed by increasing the ultrasonic power is also conducive to the rapid migration and drying of moisture in honey.At the same time,the observation of the microstructure of honey by scanning electron microscope confirmed that the degree of damage to honey tissue structure by ultrasonic effect was not only related to the ultrasonic frequency,but also increased with the increase of the ultrasonic power.(3)After studying the influence of ultrasonic power and frequency on the rheological properties of honey,it was found that the Newtonian fluid properties of honey did not change due to the effect of ultrasonic.However,under the influence of ultrasonic effect,the viscosity of honey sample increased with the increase of ultrasonic power and time.Moreover,the thermal effect of ultrasound reduced the moisture content of honey.The Arrhenius equation was applicable to describe the temperature dependence of the viscosity of honey treated with different ultrasonic power or without ultrasonic treatment.Nevertheless,the Willian–Landel–Ferry(WLF)equation appeared to be more accurate than Arrhenius equation(R~2>0.97)to describe the correlation between viscosity and temperature of honey with high ultrasonic power(>80 W)and low moisture content below 17.1%(R~2>0.98).(4)Oscillatory rheology data revealed that the enhancement of ultrasonic power lead to the decrease in the tendency of the loss modulus(G')to be larger than the storage modulus(G").That is,the fluid state of honey is gradually biased toward the solid state.Moreover,the ultrasonic frequency of 40 k Hz is more obvious than that of 28 k Hz.The differential scanning calorimetric(DSC)experiments showed that applying different ultrasonic power and frequency could significantly reduce the Tg value of honey(P<0.05),and the reduction of the Tg value is proportional to the degree of ultrasonic damage to the honey's internal structure.(5)After the mechanism of the effect of ultrasound on honey has been studied,it is found that the increase of ultrasound power promotes the continuous rise or fall of the sugar content of honey.Especially under the action of 40 k Hz ultrasonic frequency,the high ultrasonic power has a great influence on the main sugars of honey(fructose,glucose,sucrose and maltose).Moreover,the results show that the ultrasonic power plays a more active role in changing the sugar content in honey than the ultrasonic frequency.The infrared spectroscopy experiment indicated that the higher ultrasonic power is more obvious in changing the characteristic absorption peak wave number of the sugar molecule group.In addition,under the same ultrasonic power,the ultrasonic frequency of 40 k Hz has a more significant change in the number of characteristic absorption peaks of the O-H,C=O,C-H,and C-O groups in the honey molecule than28 k Hz.(6)After studying the mechanism of ultrasonic vacuum drying of honey,the best process parameters were selected for the quality test of solid honey.The test shows that:the color of honey after ultrasonic vacuum drying has a reddish and yellowish color;the HFM value also increased slightly,but it did not exceed the standard limit for honey products.In addition,the amount of fructose and glucose in solid honey increased slightly,while the amount of maltose and sucrose decreased slightly,and the Tg value increased.According to ultrasonic vacuum drying technology,the technological parameters of dried honey were optimized.Comprehensive sensory evaluation test results showed that under ultrasonic vacuum drying conditions with 40 k Hz ultrasonic frequency,25?drying temperature,vacuum degree-90 k Pa and ultrasonic power 100W,the drying time was the shortest and solid honey had the highest overall evaluation.