| Due to the high moisture content of fresh daylilies,the quality deterioration is easy to occur,and the picking period is short,so the dehydration and drying have become an important processing link of daylilies.In view of the serious disadvantages including the present most time-consuming drying methods and quality deterioration,the drying kinetics,drying model,water migration and quality changes of daylily were studied in depth by combinating drying method of hot air and mid-infared,in order to provide a theoretical basis for improving the drying efficiency of daylily and optimizing the drying process of daylily.The results were as follows:(1)The changes of environmental relative humidity,water ratio and drying rate with drying time were studied under different drying conditions(drying temperature,dehumidification wind speed).The effective water diffusion coefficient and drying activation energy were calculated,and the drying process of daylily was fit with 10 common drying models.The results showed that with the increase of drying temperature,the time needed to reach the maximum environmental relative humidity was significantly shortened,the time needed to dry to the end point was significantly reduced,and the average drying rate was increased.With the increase of the dehumidification wind speed,the time needed to reach the maximum environmental relative humidity also shortened,but the time needed to dry to the end point and the average drying rate showed a trend of decreasing at first and then increasing.With the increase of drying time,the overall drying rate of daylily increased rapidly at first and then decreased slowly,which was a typical internal water diffusion control characteristic without constant drying stage.The effective water diffusion coefficient of daylilies was in the range of 1.4671×10-9～4.1386×10-9,and the drying activation energy Ea was 50.01kJ/moI.Compared with most agricultural products,the difficulty of water removal was lower.According to chi-square χ2,residual square and RSS and determination coefficient R2,the fitting degree of each model was analyzed and judged.It was found that Weibull model could better describe the daylily drying process.The model parameters a and β were regression analyzed with the drying temperature and the humidity expending wind speed,and the combined drying model of hot air and mid-infrared was obtained.Meanwhile,the scale parameter a value varied from 37.65 to 78.49 under different drying conditions,accounting for 32.45%to 44.30%of the total drying time,and the average value was 37.32%.The results showed that most of the whole drying process was spent in the drying stage of the last third of water,and the shape parameter β was always greater than 1,which indicated that there was a lag in the early drying stage of daylily,and the drying rate showed a trend of first increase and then decline,which was consistent with the research results.(2)LF-NMR technology was used to detect different water states and their migration changes in the drying process of daylilies.The results showed that the content of A23 in free water was the highest after the fresh day lily was finished,accounting for about 91.50%of the total water,followed by A22 in non-mobile water,accounting for about 6.73%,and A21 in combined water was the least,accounting for about 1.77%.During the drying process,the total water content decreased continuously,and the A23 content took the lead in the rapid decline,part of which was converted to A22 and A21.After most of the A23 was removed,A22 and A21 were successively removed.A kinetic model was established based on the constant changes of the water in the three states with the drying process,and the R2 values were all greater than 0.95,indicating high reliability.It can be used to predict the change of water state in daylily at any time during the drying process.At the same time,multiple linear regression was used to explore the relationship between the wet base moisture content of day llower(Y)and the total signal value of LF-NMR(X)under different drying time t,drying temperature t and humidity removal wind speed V.The established multiple linear regression model was Y=-1.3961+0.0029x+0.0045t+0.0973v+0.0161t.The determination coefficient R2 was 0.9891,and the significant statistic of F was 0.0161,which was less than 0.05.Therefore,the model could be used to effectively monitor the water content in real time during the drying process of daylily.(3)By measuring the physical and chemical indexes of daylilies under different drying conditions,the quality change rule of daylilies under different drying conditions was studied,and the comprehensive evaluation of multiple indexes was conducted by TOPSIS method.The results showed that the effect of drying temperature on the quality of daylilies was more significant than that of dehumidification wind speed.When the drying temperature is constant,with the increase of the dehumidification wind speed,Vitamin C content,total sugar content,protein content,total flavonoids content,L*value,b*value,rehydration ratio,brittleness,elasticity and sensory score of day lily all showed a trend of first increasing and then decreasing,while reducing sugar content,a*value,AE,volume shrinkage,hardness and chewability all showed a trend of first decreasing and then increasing.When the dehumidification wind speed is constant,with the increase of drying temperature,vitamin C content,reducing sugar content,protein content,L*value,a*value,rehydration ratio,brittleness and sensory score of day loll all showed a trend of first increasing and then decreasing,b*value,ΔE and chewability showed a trend of first decreasing and then increasing,and volume shrinkage and hardness showed a trend of continuously increasing.Total sugar content,total flavonoids content and elasticity showed a decreasing trend.The indexes of vitamin C content,protein content,total flavonoids content,△E,volume shrinkage,rehydration ratio and sensory score were comprehensively evaluated by TOPSIS.The optimum drying conditions were determined as drying temperature of 70℃ and moisture removal wind speed of 12m/s. |
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