玫瑰花瓣红外喷动床干燥模型及品质变化

Drying models and quality changes of rose subjected to infrared assisted spouted bed drying

为提高玫瑰花瓣的干燥速率和品质,利用新型红外喷动床干燥设备,研究不同出风温度和风速下玫瑰花瓣的干燥特性并建立干燥动力学模型;对比不同干燥条件下玫瑰花瓣的品质变化。结果表明:提高出风温度和风速能够显著提高干燥速率和缩短干燥时间;玫瑰花瓣红外喷动床干燥过程主要为升速干燥和降速干燥,无明显恒速阶段;玫瑰花瓣红外喷动床干燥过程的有效水分扩散系数在6.703 85×10^-10~1.382 35×10^-9 m^2/s之间,随着出风温度和风速的增大而增大;通过模型拟合发现,Midilli模型能更好地反映玫瑰花瓣的红外喷动床干燥规律;温度和风速对复水比、总黄酮含量和总酚含量均有显著影响;风速对色泽和微观结构有显著影响。研究结果可为红外喷动床干燥的研究与应用提供参考。

Infrared drying presents several typical advantages, such as the high drying rate without loss of the quality of products, but there is a problem of uneven heat distribution in the drying process. In spouted bed drying, the strong points are efficient mass and heat transfer, particularly on the uniform heating. To combine infrared drying and spouted bed drying can be expected to maintain efficient heat and mass transfer, and concurrently ensure optimal quality of products. However, there is no research on the combination application of the two techniques for drying process until now. In order to improve the drying efficiency and quality of rose, an attempt has been made to investigate the effects of outlet temperature and wind speed on the drying characteristics using a new developed method of infrared drying assisted by spouted bed dryer, and then to establish an ideal model for the drying kinetics during drying. The rose color, rehydration ratio, total content of flavonoids, phenol content and microstructure were characterized under the different drying conditions. The results showed that the increase in the outlet temperature and wind speed can significantly improve the drying rate, while dramatically shorten the drying time. Specifically, the required drying time at 55 ℃ was 69% shorter than that at 45 ℃, and the wind speed of 23.1 m/s was 69% shorter than that of 16.4 m/s. The drying process can be divided into two steps: ascending and descending drying stage, without obvious stage of constant speed. In the early stage of drying, the phenomenon of "bubble bed" appeared, and then a good spouting state formed in the spouted bed as the drying process, where the loss of the material increased rapidly and the drying rate reached the maximum, finally in the descending stage. The effective moisture diffusivity coefficient ranged from 6.703 85×10^-10 to 1.382 35×10^-9 m^2/s, and increased with the increase of the outlet temperature and wind speed, indicating that the increase of outlet temperature and wind speed can lead to the enhancement of thermal effect, which was beneficial to the moisture diffusion. The Midilli model was determined to better describe the drying curves, after the comparison of drying models for 12 thin layers of rose petals. The maximum and minimum rehydration ratio of rose increased by 72.5% and 86.7% under different outlet temperatures and wind speeds, respectively. The high outlet temperature, high wind speed and long drying time can benefit to the reduction of total contents of flavonoids and phenols. It is also found that the proposed method of infrared assisted by spouted bed drying can be more beneficial to the retention of effective components than that in hot air drying, which is commonly-used media in the convention drying technology. The effect of wind speeds on rose color and microstructure was more significant than that of outlet temperatures. With the increase of outlet temperatures and wind speeds, the rose was subject to large deformation, more serious shrinkage during drying. This finding can provide a sound theoretical reference for the research and application of infrared assisted by spouted bed drying technique in plant drying.