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.