Study On Kinetics And Quality Changes Of Illicium Verum By Far Infrared Radiation-Hot Air(FIR-HA) Drying

Because of high moisture,fresh Illicium verum needs to be dried in time to prevent deteriorating and keep high quality after being picked at the same time,so it's very important to focus on advanced drying method.In this study,the radiation characteristics of the far-infrared radiation?FIR?emission source were conducted,then the drying characteristics and quality of fresh Illicium verum by different drying methods,especially far infrared radiation-hot air?FIR-HA?drying,were investigated further.The main conclusions were as follows:?1?The radiation characteristics of FIR emitting sources were studied firstly.The power density of the semi-blocked?No.3?source was the highest.The power density 0 cm away reached 28.358 W·m^-2 at 70°C,where the emissivity and drying rate was the highest.Reducing the distance between the source and materials was able to increase power density.And a larger number of FIR emitting sources suggested higher power density,drying rate and better radiation uniformity during drying.The power density at the same wavelength increased if temperature went up.Maximum radiant peaks were shown at about 3?m and 9?m.At a lower temperature than 60°C,the power density of 9?m is higher than that of 3?m,indicating that the radiation of far-infrared is stronger than near-infrared.The maximum radiation wavelength at different temperatures was observed to drift slightly.Elongation of the preheating time of the sources might cause a slightly increasement of the drying rate.As for the sources distribution,the upper layer dispersed type showed a higher power density,drying rate and better radiation uniformity.?2?The effects of FIR on drying characteristics and kinetics were also studied.The absorption spectrum of the Illicium verum matches the FIR emitting spectrum,and the initial moisture content of the materials affected the drying rate of FIR-HA.Investigations displayed FIR was able to increase the temperature inside Illicium verum.Then the effect of FIR-HA at50?,60?,70?and 80?on the drying characteristics was investigated,by contract with hot-air drying?HA?at the same temperature,traditional natural drying?ND?and greenhouse drying?GD?.The drying rate was showed to go ascend or stay uniformed at the beginning and go descend later,and FIR-HA accelerate drying rates?DR?.The Page model fitted the drying data the best among 4 thin-layer drying models used for describing the drying of Illicium verum.FIR-HA increased the effective moisture diffusion coefficient(D_eff)and effectively decrease activation energy?E_a?.The E_a value reduced from 30.83 kJ·mol^-1?HA?to 27.63 kJ·mol^-1?FIR-HA?.Besides,the bound water proportion of the final products was increased after FIR-HA drying,suggesting a better storability.When dried at a higher temperature,water activity of Illicium verum was reduced more.Low field nuclear magnetic resonance?LF-NMR?analysis showed that the peaks at 1 ms and 10 ms stood for bound water and mobile water,respectively,while the peaks at 100 ms and 1000 ms stood for volatile oil.Results also showed that the peak at 100 ms mainly appeared in the spectrum of the kernel,but the peak at 1000 ms mainly in that of the husk.FIR-HA is stated beneficial for moisture removal and volatile oil retainment.Results from low-field magnetic resonance imaging?LF-MRI?indicated that FIR-HA accelerated the diffusion of internal moisture,which is beneficial to the drying rate acceleration.?3?Further,the effects of FIR on the comprehensive quality of Illicium verum after drying were studied.By quantitative descriptive analysis?QDA?,FIR-HA product was evaluated to own higher richness of aroma,better surface color but worse appearance than the HA ones.After FIR-HA,the surface micro-holes of Illicium verum increased and became thicker,resulting in the enhancement of hardness and fracturability.The change of microstructure was more significant at a higher drying temperature like 80?.Moreover,the retainment of volatile oil was the highest at 70?FIR-HA?8.86%?.FIR-HA realized better volatile oil retainment of Illicium verum.Compared with HA at the same temperature,60°C FIR-HA has the highest retention rate of volatile oil content,which is increased by 15.93%.Detection by headspace-solid phase microextraction-gas chromatography-mass spectrometry?HS-SPME-GC-MS?,results has showed that trans-anethole was the major constituent in volatile oil of Illicium verum,and the trans-anethole contents of FIR-HA were higher,and the its content was highest at 70?as well.Significant correlation was found among the richness of aroma,the volatile oil content and the characteristic components.Besides,spice,wood,sweet,sour and burnt smells were shown to own significant correlation with the contents of trans-anethole,?-bergamotene,linalool,limonene,and?3-carene respectively.