The drying of fruits and vegetables is a complex operation that demands much energy and time. In practice, the drying of fruits and vegetables increases product shelf-life and reduces the bulk and weight of the product, thus simplifying transport. Occasionally, drying may lead to a great decrease in the volume of the product, leading to a decrease in storage space requirements. Studies have shown that dependence purely on experimental drying practices, without mathematical considerations of the drying kinetics, can significantly affect the efficiency of dryers, increase the cost of production, and reduce the quality of the dried product. Thus, the use of mathematical models in estimating the drying kinetics, the behavior, and the energy needed in the drying of agricultural and food products becomes indispensable. This paper presents a comprehensive review of modeling thin-layer drying of fruits and vegetables with particular focus on thin-layer theories, models, and applications since the year 2005. The thin-layer drying behavior of fruits and vegetables is also highlighted. The most frequently used of the newly developed mathematical models for thinlayer drying of fruits and vegetables in the last 10 years are shown. Subsequently, the equations and various conditions used in the estimation of the effective moisture diffusivity, shrinkage effects, and minimum energy requirement are displayed. The authors hope that this review will be of use for future research in terms of modeling, analysis, design, and the optimization of the drying process of fruits and vegetables.

This study examined the performance of different combined infrared (IR) and hot-air drying (HAD) strategies for sweet potato. Experiments were conducted for simultaneous infrared and hot-air drying, two-stage sequential hot-air and infrared drying, two-stage sequential infrared and hot-air drying, and intermittent infrared and hot-air drying in a laboratory scale combined infrared and hot-air dryer. The drying air temperature varied between 50 and 70 °C, the infrared intensity was 1100 W/m2, the air-velocity was 1.5 m/s, and the pulse ratio (PR) ranged from 1 to 3. Results indicated that the drying rate, drying time, effective moisture diffusivity, shrinkage, specific energy consumption (SEC), colour attributes and phytochemical compounds of sweet potato were affected by the different drying combination strategies. The drying kinetics, product shrinkage, and sample temperature were also influenced by drying time and air temperature. The two-term exponential model adequately explained the drying behaviour of sweet potato for all the different combination strategies. The intermittent IR and HAD combination strategy proved to be the most suitable based on the combined effect of total drying time (113-120 min), SEC (27.67-41.44 kWh/kg), total colour change (17.15-26.48) and bioactive compounds.

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This study investigated the drying kinetics, specific energy consumption (SEC), color, and microstructural changes of sweet potato (Ipomoea batatas L.) based on experimental set‐up of convective hot‐air drying (CHAD), infrared drying (IRD), and combined infrared and convective‐hot‐air drying (IR‐CHAD). The experiments were carried out at three air temperatures (50, 60 and 70 °C) and two IR intensities (1,100 and 1,400 W/m2) for sweet potato slices of 4 and 6 mm, respectively. The experimental results showed that the drying kinetics and mass transfer characteristic were significantly affected by drying air temperature, IR intensity, and thickness of the product. Combined IR‐CHAD provided a higher drying rate with the shortest drying time when compared with CHAD and IRD. The IRD resulted in the lowest SEC values. The combined IR‐CHAD resulted in 69.34–85.59% reduction in the SEC of CHAD. For combined IR‐CHAD, an increase in the IR intensity at each temperature and slice thickness caused a decrease in the total SEC value. Dried sweet potato slices using CHAD and IR1‐CHAD at intensity of 1,100 W/m2 showed the best color attributes. Combined IR‐CHAD proved to be a very efficient drying method for the drying sweet potato and can be used for both industrial and commercial purposes. Practical applications The rising concern regarding a more efficient drying method has rapidly increased the demand for novel drying techniques that can be used to produce premium dried sweet potato. However, most of these novel drying methods have not been thoroughly investigated and technical comparison with the common conventional methods have not been widely reported. In this respect, this study has shown that the novel combined IR‐CHAD is a very promising drying method for the industrial drying of sweet potato. This drying method could reduce the over drying time by 63–74% and SEC by 85% when compared with drying using conventional CHAD method. The combined IR‐CHAD at a lower IR intensity of 1,100 W/m2 could also provide dried sweet potato of better quality. The will amount to a reduction of the overall production cost which adversely could affect the prices of the final quality products.

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