干燥介质相对湿度对胡萝卜片热风干燥特性的影响

为了探究相对湿度和阶段降湿对热风干燥过程的影响,该文在干燥温度60℃、风速3.0 m/s条件下,研究了相对湿度(20%、30%、40%、50%)及第一阶段相对湿度50%保持不同时间(10、30、60、90 min),第二阶段相对湿度20%下,胡萝卜片的干燥特性和温度变化规律;利用Weibull分布函数对干燥曲线进行拟合并分析干燥过程,结合尺度参数估算水分有效扩散系数;基于复水比、色泽、干燥时间和能耗对不同相对湿度条件下的干燥过程进行评价。研究结果表明:相对湿度保持恒定条件下,干燥速率先上升后下降,且相对湿度越低干燥速率越大。降低相对湿度有利于缩短干燥时间,热风相对湿度20%比50%条件下干燥时间缩短了27.6%;分段降湿干燥条件下,热风相对湿度50%保持30min后降低为20%,其干燥时间比相对湿度恒定为20%条件下缩短了18.5%,干燥过程出现2个升速阶段;Weibull分布函数可以很好地描述胡萝卜恒定湿度和阶段降湿干燥过程。尺度参数α范围在1.864~3.635 h之间,形状参数β值在1.296~1.713之间,水分有效扩散系数在1.17×10-9~2.92×10-9 m2/s之间。对绿红值、复水率、能耗和干燥时间进行综合评价显示,热风相对湿度50%保持30 min干燥条件下绿红值最高为41.4,能耗相比于恒定相对湿度20%条件下减少了6.0%,复水比较高为3.81,综合评分较高为0.91。该文揭示了干燥介质相对湿度对胡萝卜片干燥特性的影响规律,对于优化干燥介质湿度控制策略以提高干燥速率和品质,降低干燥能耗提供了科学依据和技术支持。

Effect of hot air relative humidity on drying characteristics of carrot slabs

Drying is one of the most common processing methods for fruits and vegetables. Enhancing drying rate and dried products' quality as well as energy efficiency is very tempting for drying industry. Recently, a lot of investigations have illustrated that drying temperature, air velocity and sample's thickness have a significant influence on drying process. High drying temperature and velocity can extensively reduce drying time and enhance drying rate. However, few investigations take into account hot air relative humidity (RH) as important drying parameter in drying process. Some literatures showed that decreasing RH would lead to a steeper slope of moisture ratio decreasing. Whereas, some researches pointed out that there was no effect of RH on the drying rate in specified range of RH. In general, high RH can contribute to a rapid increase of product temperature. It is expected that with high RH pretreatment, the temperature and moisture tend to be equivalent and after that the RH can be reduced to enhance drying rate. In this circumstance, the temperature and moisture gradient between sample and drying air become consistent and the high efficient drying process could be achieved. Drying process refers to heat and mass transfer efficiency, energy consuming and products quality. Theoretical models, semi-theoretical and empirical models are widely used to describe drying process. However, those models neglect the fundamentals of the drying process and their parameters have no physical meaning and do not help in the optimization. Recently, Weibull model was used to describe drying process because of its applicability. Based on Weibull model, the relationship between scale parameters as well as shape parameters and drying process could be developed. So in current work, carrot slabs were used to explore the drying characteristics under the drying condition of constant RH and of decreasing RH step by step, together with constant drying temperature and constant air velocity. Result showed that under the condition of constant RH, drying rate would rise up in the initial drying period and then drop afterwards, additionally, drying rate was more rapid under lower RH. Drying time was decreased when reducing RH, and drying time under 20% RH was reduced by 27.6% compared to that under 50% RH. However, material's temperature would be the highest under 50% RH drying condition. Besides, under the drying condition of decreasing the RH step by step, when 50% RH was kept for 30 min and then was changed to 20% RH, the drying time was saved by 18.5% compared to constant 20% RH drying condition. When 50% RH was kept for 30 min or more than 30 min, there were 2 accelerating periods of drying rate. The material's temperature would decrease as the RH decreased because of moisture evaporation. Weibull model could fit the drying curves under different conditions accurately. The scale parameter ranged from 1.864 to 3.635 and the shape parameter ranged from 1.296 to 1.713. So there was a lag period according to the shape parameter value. Based on Weibull model, the moisture effective diffusivity calculated was in the range of 1.17×10-9-2.92×10-9 m2/s. Taking rehydration ratio, color value, drying time and energy consuming into account, 50% RH kept for 30 min and then reduced to 20% RH was proposed as the favorable condition for drying carrot slabs. The present work contributes to a better understanding of the effect of relative humidity on drying characteristics of carrot slabs, which is useful for enhancing drying rate and dried products' quality as well as energy efficiency.