Mathematical modelling of thin layer hot air drying of apricot with combined heat and power dryer

Assessment of Drying Kinetics, Textural and Aroma Attributes of Mentha haplocalyx Leaves during the Hot Air Thin-Layer Drying Process

Since Mentha haplocalyx leaves are rich in bioactive constitutes, particularly volatile compounds, there are higher demands for high-quality dried medicinal and aromatic peppermint products. This study aimed to assess the drying kinetics of hot air thin layer drying Mentha haplocalyx leaves and exploring the effects of hot air-drying temperatures on the textural properties and sensory quality. According to our results, the Midilli model is the best model representing the hot air-drying process. The effective moisture diffusivity (Deff) and activation energy (Ea) of the hot air-drying process were determined as 7.51 × 10⁻⁹–3.03 × 10⁻⁸ m²/s and 57.98 KJ/moL, respectively. The changes of textural and aromatic profiles of dried Mentha haplocalyx leaves were subsequently evaluated by the SEM, GC–MS and E-nose technology. Changes in leaf cellular membrane structures were observed in this study, indicating that the loss of moisture content induced the shrinkage of leaf cells during the hot air-drying process. Moreover, the altered profile of volatile compounds was identified at the different drying temperatures. As a result of the GC-MS analysis, increasing the content of D-carvone from 61.89%, 69.25% and 78.2% resulted in drying temperatures of 35 °C, 45 °C and 55 °C, respectively; while a decreasing trend of other volatile compounds, including D-Limonene, cineole and l-caryophyllene was detected as drying temperature elevated. Finally, the aromatic profile was evaluated by E-nose, and results of the flavor radar fingerprint and PCA showed that aromatic profiles were significantly altered by the drying process. The overall results elucidated that the hot air thin layer drying at 35 °C efficiently improved the final quality of dried Mentha haplocalyx leaves by maintaining flavor properties.

Study on the splitting by hot-air drying of Camellia oleifera fruit

In order to explore the feasibility of hot air splitting of Camellia oleifera fruit, the effect of hot air temperature on peel splitting, the moisture state and moisture migration in peel, the peel microstructure and the seed color were studied. The results showed that higher hot air temperature could accelerate the splitting rate, the optimum temperature for splitting C. oleifera fruit was 90–110 °C considering the seed quality. Page model was the most suitable for describing the drying kinetic characteristics of C. oleifera fruit. Nuclear magnetic resonance (NMR) revealed the changing of the dehydration rate, the migration rate of bound water, immobilized water and free water in peel during hot air drying. The expansion of micro-channels in peel was conducive to moisture migration in the early splitting stage, but microstructure damaged in the late splitting stage accompanied by loose disorder of micro pores, serious shrinkage and deformation of peel.

The Effect of Pre-Drying Treatment and Drying Conditions on Quality and Energy Consumption of Hot Air-Dried Celeriac Slices: Optimisation

Celeriac is a good source of fibre, trace minerals, and phenolic compounds; it has a pleasant aroma but is a perishable material, prone to discolouration. This research investigated the optimisation of the quality and energy demand in hot-air dried celeriac slices. The experiment utilised the I-optimal design of response surface methodology with 30 experiment runs. Pre-drying treatments (blanching at 85 °C, three minutes; dipping in 1% citric acid solution, three minutes; no pre-drying treatment), drying temperatures (50, 60, and 70 °C), air velocities (1.5, 2.2, and 2.9 m/s), and thickness (three-, five, and seven-mm) were applied. The drying conditions affected drying time significantly (p < 0.0001). The model by Midilli and others and the logarithmic model fitted best with celeriac slices drying kinetics. Blanched samples had a higher ΔE*_ab (total colour difference) and BI (browning index) but lower WI (whiteness index) than samples with other pre-drying treatments. The rehydration ratio decreased with the increase of sample thickness and blanching (p < 0.0001). A quadratic model described the specific energy consumption (E_s) best. The dried samples compared with fresh samples had increased antioxidant activity but decreased total phenolic compound value. The optimisation solution chosen was 58 °C drying temperature, 2.9 m/s air velocity, and 4.6 mm sample thickness with acid pre-drying treatment.

Analysis of environmental factors for production of green raisins in Liang-fang

The commercial value of green raisins produced by the shade-air drying in Liang-fang was 2–3 times as high as that of yellow-brown raisins. This study systematically evaluated the effect of the temperature, humidity, and light of sun drying and shade-air drying on raisins. The sugar, acids, texture and color parament of raisins were compared with different drying conditions. The results indicated that shade-air drying can offer a dark and conducive light environment with the maximum temperature below 40 °C, and a lower humidity by segregating rainwater during the day as well. Then, the contents of acids of raisins in sun drying were lower compared with the shade-air drying and had a lower hardness and a* values of raisins. In summary, low light and ultraviolet intensity, suitable temperature and appropriate humidity environments were the basal and key factors for the formation of the green raisins. The present study not only revealed the crucial factors during shade-air drying in Liang-fang in Turpan, but also provided a scientific basis for improving the Liang-fang conditions and producing green raisins in other regions.

Modelling of moisture migration during convective drying of pineapple slice considering non-isotropic shrinkage and variable transport properties

The present work aims to develop a 3-dimensional finite element (FE) model to analyze moisture migration during drying of pineapple ring considering moisture dependent diffusion coefficient (D) and mass transfer coefficient (h_m) along with radial and longitudinal shrinkage. Pineapple rings were dried at 70 °C temperature and 0.6 m/s air velocity to study the moisture loss and shrinkage kinetics during drying. Thickness, outer radius and inner radius of hollow cylindrical pineapple slices were reduced by 79.3%, 32.2%, and 51.2%, respectively due to the occurrence of shrinkage during drying. Non-linear regression analysis showed the quadratic model to best fitted to the experimental moisture ratio data for explaining the shrinkage phenomenon in pineapple slice during drying. Shrinkage was accommodated into FE modelling using the arbitrary lagrange-eulerian method. Consideration of variable D showed better agreement with the experimental data than consideration of constant D, however constant and variable h_m predicted similar results. Incorporation of shrinkage phenomena during modelling led to prediction of more accurate result showing 0.06% deviation from experimental curve, but neglecting the shrinkage resulted in a 17% deviation. Hence, model developed with consideration of shrinkage along with variable D and h_m presented best fit with experimental drying curve. Developed model allowed the visualization of spatial moisture profile within the sample during drying, which would be useful for estimating the correct drying time, optimizing and designing of drying process.