Effects of operating practices on performance of a fixed‐bed convection dryer and quality of dried longan

Bulk layer shifting during industrial drying of whole longan was evaluated. Analysis of drying conditions and product quality of samples from different areas of the dryer was conducted. The study found that both temperature and air velocity were non‐uniformly distributed during drying, which affected drying kinetics and product quality. High humidity and convective cooling of the drying air passing through the bulk caused condensation on fruits surfaces in the upper layer during the initial hours of drying. Those samples towards the sides of dryer, those initially in the upper layers, and especially those near the air inlet showed consistently lower quality. Four shifting schemes were tested, one of which was found to be exceptional in terms of end‐product uniformity. Issues concerning the improvement of industrial longan drying are addressed and an optimal shifting procedure is proposed.     

Applying CFD for designing a new fruit cabinet dryer

Cabinet dryers are the most popular equipment for fruit drying. One of the drawback of this dryer can be non-uniformity in the desired moisture content of end product. A new cabinet dryer with a side mounted plenum chamber was designed, constructed and evaluated. To obtain a uniform distribution of drying air flow and temperature taking into account the overall operating conditions, seven different geometries of cabinet dryer were envisaged theoretically (computational fluid dynamics (CFD) by using fluent software) and experimentally. Experiments were conducted on the most appropriate sketch with acceptable uniform air flow and temperature distribution. The experimental results showed that the new cabinet dryer illustrated an even distribution of air velocity and temperature throughout the dryer. Comparing the experimental and predicted (extracted for the CFD analysis) data revealed a very good correlation coefficient of 99.9% and 86.5% for drying air temperature and air velocity in the drying chamber, respectively.     

Industrial paddy drying and energy saving options

Paddy drying is an energy-intensive process and influences rice quality. In this study, the energy consumption of paddy drying in a large-scale milling plant was investigated. Furthermore, some drying experiments were conducted in a laboratory. The aims were to gain practical information and to propose more economical drying options to the industry. The results indicated that the current drying in the plant consumed specific primary energy of between 3.874 and 4.421 MJ/kg of water evaporated. The experimental results showed that two-stage drying with tempering by using a fluidised bed dryer at 100–110 °C in the first stage and drying with ambient air using a solar dryer for the second stage provided rice quality that was comparable to that of the plant. Also, from the calculation, the energy cost of the plant could be reduced if an in-store dryer was used after the first-stage drying by LSU dryer.     

Analysis of dryer performance for the improvement of small‐scale litchi processing

Litchi is a significant cash crop in Thailand, but marketing is constrained by the perishable nature of the fruit. Drying can extend the shelf life of the fruit; however, in Thailand drying technology is still in its early stages of development. With the aim of improving locally available small‐scale drying equipment, the performance of a litchi batch‐dryer by a farmers’ cooperative near Chiang Mai was studied. Drying conditions, energy consumption and product quality were monitored over three trials. Energy performance was analysed using instantaneous indices, leading to suggestions for possible design‐modifications. About 29.8% of the heat input was used for moisture evaporation, 38.4% was lost via unsaturated exhaust air and 17.6% was lost to the ambient environment. An optimum air flow rate, one that minimises exhaust heat losses without increasing the drying time, was calculated. Temperature and air flow within the drying chamber varied, resulting in a non‐uniform batch. To improve performance, a new design for the drying chamber air inlet is recommended.     

Drying performance and economic analysis of novel hybrid passive-mode and active-mode solar dryers for drying fruits in East Africa

In East Africa, post-harvest loss of fresh perishable fruits is enormous, causing recurrent seasonal deficits. In response, a modified passive-mode hybrid solar dryer hereafter called; ‘an Improved Solar Dryer (ISD)’, was developed as a potential low-cost technology alternative for drying fruits in Uganda. An improved PV-assisted hot air dryer using active-mode hybrid solar dryer hereafter called; ‘Solar Photovoltaic and Electric (SPE)’ was also fabricated. The ISD dryer utilises freely-available solar energy to dry the produce. In contrast, the SPE dryer uses both solar photovoltaic and electricity sources. In this study, the drying performance of the ISD and SPE dryers were performed and compared to that of the traditional open sun drying (OSD) method. Relative to the OSD method, the economic performance of the ISD and SPE dryers was analysed. The drying performance results show that the mean drying air temperatures achieved by the ISD and SPE dryers were 31.9 and 41.1 °C respectively; relative to the 27.6 °C for the OSD method. On average, the thermal energy attained by the ISD and SPE dryers were 3551 and 5757 Watts (W) respectively, as compared to 2952 (W) obtained for the OSD method. It took the ISD and SPE dryers 10 and 18 h to effectively dry the fruit (pineapple) products, respectively as opposed to the 30 h taken by the OSD method. The results confirm the superior performance of the ISD and SPE dryers than the OSD method. Results for the economic performance analysis show more reduced costs of drying the products using the ISD than the SPE dryer. Similarly, the pay-back period for the ISD was 2.4 times shorter than that of the SPE dryer. Joint results for economic analysis and drying performance confirm the ISD as a better drying technology than the SPE dryer in the context of the resource-constrained farmers/agro-processors without electricity in Uganda and East Africa.     

Application of simulation in determining suitable operating parameters for industrial scale fluidized bed dryer during drying of high impurity moist paddy

A systematic approach has been developed for selecting the suitable drying parameters to be used for drying of high moisture and high impurity paddy with an industrial fluidized bed paddy dryer (10–20 t h⁻¹ capacity) based on targeted specific air flow rate and residence time during two typical paddy drying seasons. A mathematical model was developed by modifying an existing model and was simulated and validated with observed industrial drying data as well as data reported in the literature. Comparison between the observed and simulated results showed that the mathematical model is capable of predicting outlet paddy moisture content and air temperature well. Suitable operating parameters were determined for reducing any initial paddy moisture content (mc) down to 24–25% dry basis (db), the safe mc level after fluidized bed drying to maintain rice quality, to achieve maximum possible throughput capacity of the dryer with corresponding energy consumption. Based on these criteria, bed thickness at 10 cm, specific air flow rate of 0.05 kg kg⁻¹ s⁻¹ (for corresponding bed air velocity of 2.3 m s⁻¹), air temperature of 150 °C and residence time of 1.0 min were found to be suitable drying conditions for reducing paddy mc from 30 to 24.30% (db) in one season while the maximum throughput capacity of 15.7 tonne per hour (t h⁻¹) might be achieved. The specific electrical and thermal energy were 0.48 and 6.15 MJ kg⁻¹ water evaporated, respectively. On the other hand, the dryer capacity was found to be limited to 7.4 t h⁻¹ during drying paddy of higher initial mc (35% db). This approach might provide easy and comprehensive guidelines for selecting suitable sets of operating parameters for any industrial fluidized bed dryer at its possible maximum throughput capacity for drying of freshly harvested high moist paddy with a high level of impurities.     

小颗粒（含粉料）物料射频干燥机设计与分析

针对小颗粒(含粉料)状物料干燥方式简单,热利用率低、获得的物料干制品质量得不到有效保障的问题,利用射频给热技术,设计了针对小颗粒(含粉料)物料中试射频干燥机。该设备由给热系统和输送系统两部分组成,给热系统主要包含射频发生器和翅片加热管,输送系统的主要传送部件为输送带。同时为保证射频干燥的均匀性,设计了物料厚度的限制装置,并以玉米颗粒为试验物料,对所设计的射频干燥机进行物料衡算及能耗分析。结果表明,干燥1 000kg玉米颗粒,平均耗时40.83min,平均干燥能耗4 241.438 9kJ/kg·H2O。该机可以较大幅度地提高热能利用率,保障物料干制品质量。     

Improved energy efficiency of farmers' cooperative litchi drying operation in Northern Thailand

In 2007, Hmong farmers growing litchi in the mountainous region of Northern Thailand started to dry the fruit in response to its declining market price. However, the locally available dryer, although affordable and of a size appropriate for smallholder farmers' cooperatives, showed excessive gas consumption, and this, along with rising fuel costs, imposed a threat to the drying operation. The objective of this study was to test low‐cost, easy‐to‐implement modifications to a locally available convection dryer, to improve its energy efficiency. Experiments were conducted in cooperation with a Hmong farmers' cooperative in Northern Thailand. Insulation of the air ducts and control of the air recirculation rate led to energy cost savings of US$2.45 per dryer‐load. With the modifications required being simple, the investment needed was recovered after only approximately 20 loads, while the energy efficiency of the dryer increased from 33% to 39%.     

Thin layer drying model for simulating the drying of Tilapia fish (Oreochromis niloticus) in a solar tunnel dryer

Mathematical models for predicting the plenum chamber temperatures developed by a solar tunnel dryer and the drying of Tilapia fish (Oreochromis niloticus) in the solar tunnel dryer was developed, and simulated in Visual Basic 6 (Microsoft Visual Basic 6.0™). Based on Student’s t-test, the simulated and actual data for both plenum chamber temperature and moisture ratio did not differ significant at 5% level of significance. In addition, the simulated and actual moisture ratios showed similar trends, and reduced exponentially with drying time. Further, the performances of models at 10% residual error interval were 83% and 81% for plenum chamber temperature and moisture ratio, respectively. Finally, strong linear correlations existed between simulated and actual data for plenum chamber temperature (R² = 0.961), and for moisture ratio (R² = 0.995). Therefore, the model can be used to predict the drying of Tilapia fish in a solar tunnel dryer.     

Evaluation of high pressure pretreatment for enhancing the drying rates of carrot,apple, and green bean

Preservation of fresh produce by drying dates back to ancient times and is still an indispensible technique. Conventional drying of fruits and vegetables is often accompanied by changes in color, texture, and taste. Suitable pretreatments can improve the drying process by reducing the drying time, yielding higher-quality products, and energy savings. In this study, two varieties of apples, Amasya and red delicious, green beans and carrots were pretreated with high hydrostatic pressure (HHP) at different pressure–time–temperature combinations (100–300 MPa for 5–45 min at 20 and 35 °C) prior to drying. The drying experiments were carried out by using a hot-air tunnel dryer at different temperatures (27–85 °C) and air velocities of 0.4 and 0.8 m/s with constant external conditions. Improving the drying conditions by increasing the drying temperature generally masked the effect of HHP pretreatment on drying rate. Generally, pressures of more than 100 MPa caused cell permeabilization resulting in higher drying rates. Among 14 models, the modified Page model was found to best explain the drying behaviors and model constants were evaluated accordingly. The Tukey multiple comparison test was applied on characteristic drying times to evaluate the relative effects of different pretreatments and drying conditions.     

Intermittent drying simulation in a deep bed dryer of yerba maté

In the present research, the reduction of energy consumption was evaluated during drying of yerba maté branches when tempering periods were applied. An empirical model was validated describing the moisture variation with time and process temperature, using different operation conditions. This model was used to simulate an industrial dryer in order to minimize energy consumption, maintaining the final moisture content constant. Best results were obtained working in the following conditions: application of heat for 15 min, 15 min of tempering and continuous application of heat. The bed height (1 m) was reduced in 10% maintaining the dryer production increasing the belt velocity. Working with these conditions a 10% of energy consumption can be reduced.     

Drying characteristics and quality of taegeuk ginseng (Panax ginseng C.A. Meyer) using far‐infrared rays

The purpose of this study is to investigate the drying characteristics of taegeuk ginseng using a far‐infrared dryer. The dryer used for this experiment can control the drying parameters such as far‐infrared heater temperature and air velocity. The far‐infrared drying tests of taegeuk ginseng were performed using large, medium and small ginseng at an air velocity of 0.6 m s^?1, and drying chamber temperatures of 45, 55 and 65 °C. The results were compared with those of the hot‐air drying method. The drying characteristics were analysed based on factors such as drying rate, colour changes, internal tissue structure of ginseng, saponin contents and antioxidant activities, and energy consumption. In general, the hot‐air drying time was 3–3.5 days longer than that of far‐infrared drying under the same conditions. The colour changes of far‐infrared drying were also smaller than that of hot‐air drying. Far‐infrared dryer required less energy consumption about 9.67–14.8% less than that required by hot‐air drying. At the same time, the saponin and polyphenol contents of taegeuk ginseng subjected to far‐infrared drying were 1.01 mg g^?1, 7.81 mg/100 g higher than that subjected to hot‐air drying, respectively. The results showed that the far‐infrared drying increases the capacity and quality of taegeuk ginseng.     

Effect of superheated steam drying on properties of foodstuffs and kinetic modeling

Conventional hot air drying is an energy intensive technique which consumes around 15–25% of national industrial energy in most of the countries. It also often results in unacceptable product quality, nutrient degradation and non-uniform drying. Using air as a drying media leads to oxidation and combustion reaction and releases undesirable components creating environmental issues. Drying operation also needs continuous improvements to reduce energy consumption and preserve quality. Superheated steam drying (SSD) is an innovative drying technology, utilizing heated steam beyond its boiling point as a drying medium in a dryer to remove excess water from the material. Researchers on comparing conventional and SSD claimed that SSD supports product, environment, and energy saving benefits as well as it overcomes many constraints of hot air and other conventional drying techniques. This review will provide comprehensive detail about the effect of SSD on different properties (texture, microstructure, color, nutrient retention, shrinkage) of foodstuffs. Mathematical modeling and simulation of the SSD process as well as product characteristics undertaken by researchers are also compiled briefly in this article.Industrial relevanceSSD offers many advantages over hot air drying which includes low net energy consumption, utilization of exhaust steam, no oxidative reactions and no hazardous gas, dust, dirt emission into environment. Kinetic modeling may prove to be useful for optimization and designing of the process.     

Formation of amorphous sugar in the syrup film – a key factor in modelling of industrial sugar drying

A mathematical model of the steady-state sugar drying operation in a cross-flow rotating drum dryer is proposed. Although the model is based on the classic two-film concept of simultaneous heat and mass transfer, it also allows for the formation and growth of a thin layer of amorphous sugar due to local supersaturation of the sugar syrup. The formation of amorphous sugar is a result of the competition between crystallization and drying kinetics. The onset of amorphization is assumed to occur when the system moves from the metastable zone to the labile zone on the sucrose–water phase diagram. The rate of sucrose amorphization is controlled by the conditions prevailing on the metastability limit. Diffusivity of water through the amorphous sugar, the only adjustable parameter of the proposed model, has been determined by fitting sugar moisture data from an industrial Louvre-type dryer. The layer of amorphous sugar plays a critical role during the falling-rate period of drying. Generally, models ignoring its presence tend to overestimate the moisture removal rate. The presented model predicts a dramatic decline of the evaporation rate once the amorphous sugar begins to form which is observed in the industrial practice. The effect of key operating parameters such as air flow rate, hot air temperature, and crystal size on the dryer performance is also discussed.     

Effect of drying air temperature and storage on industrial and chemical quality of rice grains

The drying air temperature and storage time can change the milling yield and chemical composition of rice grains. Therefore, the objective was to verify the industrial quality and chemical composition of rice grains, white and integral subgroups, in response to drying air temperature and storage time. For such, the rice grains were subjected to drying in a column dryer with a grain capacity of 1 m³, using 55 and 65 °C drying air temperatures. After, they were stored in PP Woven Bags under environmental conditions for 240 days. After drying and every 60 days the milling was performed, aiming to evaluate the industrial quality and chemical composition of the grains. The whole grains yield was negatively affected by the increase in drying air temperature for both subgroups. The increase in the drying air temperature led to a reduction in the lipid content and an increase in the ash content for grains of white subgroup. The storage time promoted an increase in the whole grains yield in the white subgroup when the grains were dried with the highest drying air temperature. The lipids and proteins contents decreased, while the fibers and ash contents increased during storage for the white subgroup grains. The chemical composition of grains from the brown subgroup did not change during storage, regardless of drying air temperature.     

Drying of edamames by hot air and vacuum microwave combination

The characteristics of hot air and vacuum microwave drying were compared using edamame as the raw material, and an optimized combination drying process was then established thereof so as to achieve increased drying rate and enhanced product quality. Edamame was subjected to 70 °C hot air drying for 20 min, and then microwave dried at power intensity 9.33 W/g for 15 min under −95 kPa (gauge pressure). The optimized combination drying process exhibited significantly shortened drying time as compared with conventional hot air drying, and greatly decreased mass loads on the vacuum microwave dryer.     

Qualitative and quantitative changes in the essential oil of Laurus nobilis L. leaves as affected by different drying methods

The effect of six different drying methods on the content and chemical composition of the essential oil of Laurus nobilis L. leaves was studied. The essential oils from fresh and dried samples were isolated by hydrodistillation in a Clevenger apparatus and analysed by gas chromatography–mass spectrometry (GC–MS). Results showed that air drying at ambient temperature and infrared drying at 45 °C increased significantly the essential oil content. Forty-seven components were determined in essential oils, which were mostly oxygenated monoterpenes. This class of compounds is not significantly affected by the method of drying except for air drying at ambient temperature. The main components 1,8-cineole, methyl eugenol, terpinen-4-ol, linalool and eugenol showed significant variations with drying methods. The concentrations of these compounds increased significantly in the case of air drying at ambient temperature. These results allowed considering this method of drying as the method that produced the best results in terms of essential oil and bioactive compounds contents.     

Drying characteristics of textured soy protein

Textured soy proteins (TSPs) are commonly used as a functional ingredient in several food applications; its process involves a drying step which is one of the most relevant and challenging processes in the food industry. The drying curves of two commercial types of TSPs were determined in a pilot air‐dryer operated at three different drying air temperatures and velocities, with two different heights of product layer. The results showed that drying air temperature significantly influenced all the experimental curves. The height of the product layer influenced only the drying curves of TSP Type I. The effect of drying air velocity was found to be lower. A two‐constant exponential model and a ten‐constant mathematical model were used in the development of a unified model capable of predicting the drying curves of these types of TSP. The experimental curves were successfully predicted in the range of humidity of industrial interest.     

A novel system for drying of agro-industrial acerola (Malpighia emarginata D. C.) waste for use as bioactive compound source

The industrial processing of acerola generates a large amount of waste that is usually discarded or underutilized, causing losses of raw material and energy, as well as environmental impact. As these agro-industrial residues are rich in antioxidant compounds, they can be better used. In this work the drying of acerola residues in a novel device (roto-aerated dryer) was investigated. The high moisture content of this residue demanded multiple drying stages to achieve the suitable final moisture. The effects of the number of drying stages and operating conditions on product quality and drying performance were analyzed. The bioactive compounds analyzed were: phenolic, flavonoid, ascorbic acid and citric acid. The results shown that the new drying system proved to be a good alternative for dehydration of acerola residues, due to the drying performance and the good quality of the product after drying.     

Drying kinetics and survival studies of dairy fermentation bacteria in convective air drying environment using single droplet drying

The drying and survival kinetics of Lactococcus lactis ssp. cremoris in a convective air drying environment were measured using single droplet drying experiments. Tests were carried out at five different drying temperatures (45–95 °C) at a constant air velocity (0.5 m/s) and within 2.4–11% relative humidity. The effect of protective agents (10% w/w) of lactose, sodium caseinate and lactose:sodium caseinate (3:1) was also evaluated. The thermal inactivation kinetics parameters in convective air drying and isothermal water bath heating were determined and compared. The results showed that the final temperature attained by the droplet affected the survival of the bacteria significantly, however, most of the bacterial death occurred in early stage of drying while evaporative cooling kept the drop temperature relatively low. At higher droplet temperatures (?65 °C) the bacterial cultures were inactivated by both dehydration and thermal stresses. At lower droplet temperatures (?55 °C) the rate of change in droplet moisture content had much stronger effect on the bacterial survival. Lactose and sodium caseinate, as protective agents, enhanced the survival of bacterial cells significantly at all the test conditions. The lactose:sodium caseinate (3:1) mixture synergistically enhanced the survival of the bacterial cultures. The death of these bacteria followed first-order kinetics during convective single droplet drying as well as during isothermal water-bath heating. However, the inactivation energy in convective single droplet drying (181.3 kJ/mol) was much higher than the inactivation energy in isothermal water bath heating (16.8 kJ/mol) within the medium temperature of 45–95 °C.