Hybrid Drying Research Articles

Recent Food Drying R&D at Jiangnan University: An Overview

Drying of fresh produce and marine products to develop high-quality dried products cost-effectively has been the theme of extensive research carried out at the School of Food Science & Technology of Jiangnan University, Wuxi, China. Both conventional and innovative hybrid drying techniques encompassing the enhancement of drying rates and quality through utilization of modern ultrasonic and nanotechnology have been examined in detail and reported in the open literature. Some of the work was funded by the industry and eventually utilized in industrial-scale processing. The objective of this article is to summarize the key developments for a wider global readership. An outline of ongoing drying R&D will be provided in the closing section.

Pineapple Drying Using a New Solar Hybrid Dryer

This paper presents the results of an experimental pineapple (Ananas comosusL.) drying study in a new solar hybrid dryer. The dryer is a direct and integrated type, with a black mate pan which measures 1.675 x 0.61 x 0.055 m (1.02 m2). It uses a copper helical tube that conducts hot water, at 80°C, generating extra heat for the drying process; the tube is located at the bottom of the black pan. A caliber 6, transparent vinyl film is used as a cover and the walls and base of the dryer are isolated by 0.0254 m thick fiberglass. It is inclined 23° south and uses pump to recirculate water. The variety of pineapple used was honey, 1/4 of grown age. Each slice was 0.005 m thick, resulting in a mass density of 2.83kg/m2 in the drying trays area. To be able to have useful comparisons, drying tests were performed during winter and spring of 2013, using both the hybrid dryer and the traditional solar dryer. Results showed that when initial humidity between pineapples is quite similar (this being one of the most influential factors in changing efficiency, time and other important process variables), evaporation efficiencies are higher in the traditional process; such efficiencies ranging between 22.7% and 24.0%; while the efficiency of the hybrid dryer ranges between 9.3% and 14.0%. This is basically due to the increase of energy loss when using both sunlight and heated water. On the other hand, the time the process took to reach the humidity goal (24.0% humid base or 0.32 dry based) was much faster when using the hybrid dryer. This dryer ended the process in a range of 6.0 to 6.8hours while the traditional solar process took between 8.0 and 8.8hours. It is also important to note that the final process was basically homogeneous throughout the dryer, especially when the drying trays on the top end of the dryer were placed 0.03 m further away from the top, and produced a fine quality product with only slight discoloration of the pineapples’ side which faced the sun.

Optimization of Combined Heat Pump and Microwave Drying of Yacon (S mallanthus sonchifolius ) Using Response Surface Methodology

Response surface methodology was employed to investigate the effects of drying temperature (15.0–45.0C), air velocity (1.00–2.00 m/s), moisture content at conversion point (40.0–60.0%) and microwave (MW) power (1.0–3.0 W/g) on the average drying rate (DR), specific moisture evaporation rate (SMER), total color difference (ΔE), rehydration ratio (RR) and shrinkage ratio (SR) of yacon dried by combined heat pump (HP) and MW methods. A central composite rotatable design was used to develop models for responses. The coefficients of determination R2 of DR, SMER and ΔE were higher than 0.7. However, R2 of RR and SR were 0.540 and 0.343, respectively. Based on response surface and desirability functions, the optimum conditions for combined HP and MW drying of yacon were drying temperature of 42.7C, air velocity of 1.69 m/s, moisture content at conversion point of 50% and MW power of 2.0 W/g. At this optimum point, DR, SMER and ΔE were 0.262 kgH2O/kg(d.b.)·h, 0.222 kg/kW·h and 23.59, respectively. Practical Applications Yacon are rich in essential amino acids, minerals and fructooligosaccharides (FOS). However, FOS hydrolyzation, browning and decay are the common phenomena during postharvest storage and transportation. Hybrid drying techniques are being developed to minimize weakness and maximize merits of different drying techniques to produce better quality of foodstuffs, reduce drying time, decrease energy consumption and increase drying efficiency. Therefore, response surface methodology was employed to investigate the effects of drying temperature, air velocity, moisture content at conversion point and MW power on the average DR, SMER, ΔE, RR and SR of yacon dried by combined HP and MW methods, and the optimum process conditions were determined to obtain the criteria of maximum DR and SMER, and minimum ΔE. This research will provide valuable information for industrialization of yacon dried by combined HP and MW methods.

Enhancement of sulforaphane content in cabbage outer leaves using hybrid drying technique and stepwise change of drying temperature

Sulforaphane is a hydrolysis product of glucosinolates, which are plant secondary metabolites found in various kinds of Brassica vegetables including white cabbages (Brassica oleracea L. var. capitata). Sulforaphane is of interest as it is claimed to possess chemoprotective effect against an array of cancers. The formation of sulforaphane is via enzymatic reactions and the rates of formation and degradation are strongly dependent on the temperature. Careful regulation of the material temperature during drying to maximize the formation and minimize the degradation of sulforaphane should therefore be possible. In this study, the effect of stepwise change of medium temperature during hot air drying, vacuum drying and hybrid drying, i.e., low-pressure superheated steam drying (LPSSD) followed by vacuum drying, on the evolution of sulforaphane in white cabbage outer leaves was investigated; the results were compared with those in the cases of constant drying medium temperature. Similar evolution patterns of sulforaphane were noted in all cases; sulforaphane first increased before reaching the maximum and then decreased towards the end of drying. Nevertheless, the cabbages undergone LPSSD at 60°C for 10min and then vacuum drying at 45°C until reaching the final moisture content exhibited the highest amount of sulforaphane.

Ultrasonic Convective Drying Kinetics of Clipfish During the Initial Drying Period

A hybrid drying system for high-intensity airborne ultrasound applied in convective drying was investigated for the drying of salted codfish (clipfish). Convective drying with ultrasonic assistance at 10, 20, and 30°C was compared to the same process without ultrasound. The Weibull model was used to model and investigate the drying behavior, and the effective diffusion in Fick's law was determined. The ultrasound decreased the drying time more at lower drying temperatures. The drying time was reduced by over 90% at a drying temperature of 10°C. For an industrial drying process at a temperature of 20°C, the drying time was reduced by 32.2%. The ultrasonic, convective drying of clipfish at a temperature of 20°C was faster than the same process without ultrasound at 30°C. The investigations showed a thermal effect for all products when ultrasound was applied. The specific moisture extraction ratio (SMER) in the investigated system was improved by 0.2 kgwater kWh−1. The heat transfer coefficient in the system used was increased by 32.6% for a heating process in a separate investigation, whereas for a cooling process no increased heat transfer coefficient was determined. The thermal effect might (at least partially) explain the faster drying of ultrasonic-assisted convective drying. The results obtained demonstrate the potential of airborne ultrasound in convective drying with regard to drying time, energy consumption, and product quality. Documentation of the thermal effect should be included in future R&D on this topic.

Model and process simulation of microwave assisted convective drying of clipfish

Microwave (MW) assisted convective drying of clipfish (cured cod) was investigated at different intensities. The determined kinetics show that the drying time can decrease by over 90% at a microwave intensity of 155 W kg−1. However, quality aspects limited the microwave application to maximum intensities of 65 W kg−1, where the drying time is still decreased by 35%. The determined drying kinetics where used to model a 40 ton tunnel dryer for clipfish in a dynamic simulation. The energy consumption of a 200 kW heat pump drying system (HPD) and a heated ambient air drying system (HAAD) were investigated for different microwave intensities. The energy consumption for the HPD system was 171 kW h t−1 and it was 445 kW h t−1 for the HAAD system. For the microwave assisted drying systems the energy consumption increased up to 4465 kW h t−1. Microwave hybrid drying of clipfish is therefore considered uneconomical despite the faster drying kinetics. Economical energy levels for MW-HPD would have to be below 1.6 W kg−1 and 6.2 W kg−1 for MW-HAAD, at a drying time reduction of 50%.

Efficiency of a hybrid solar–gas dryer

The thermal and drying efficiency of three operating configurations for a hybrid solar-gas dryer were calculated in transitory state. The hybrid dryer, comprising a solar collector, auxiliary LPG (liquid propane gas) combustion heater, and a drying chamber, can be operated through an LPG (GHS) heating system, a hybrid solar-gas (HHS) heating system, or a solar (SHS) heating system. Global efficiency was calculated considering the energetic contributions of the solar collector and/or the auxiliary heating system, in accordance with the mode of operation being evaluated. Losses resulting from reflection and absorption were considered in the analysis of the solar collector. The thermal efficiency of the collector was principally affected by air mass flow, collector angle of inclination, and the difference between ambient temperature and the collector’s internal temperature. A simulation varying air velocity parameters inside the solar collector was utilized to estimate the air mass flow needed to produce a thermal efficiency greater than the efficiency calculated under current design and operational conditions (26%). Maximum drying efficiencies were 86%, 71%, and 24% for GHS, HHS, and SHS, respectively. HHS and GHS exhibited similar drying rates in the constant period of the curve (∼0.030kg H2O/kg d.s. min). The efficiency of the hybrid drying system was similar to the LPG drying system, with the advantage of consuming 20% less fuel without sacrificing quality in the dried product.

Solar Drying of Sliced Potatoes. An Experimental Investigation

The work presented in this paper was aimed to investigate an indirect solar dryer, locally designed and constructed, In order to adjust appropriate conditions for safe store of sliced potatoes. Throughout this study, by the mean of several experiments performed at Ouargla City in southern Saharan of Algeria. We searched to improve the performance of our solar drying system. An electrical resistance supplied by a variable number of photovoltaic panels was used to enhance the thermal efficiency. On the other hand, we tried to follow the impact of various controlling factors of hybrid solar drying on the quality of dried potato slices. Our present findings showed that the best operating conditions to ensure best efficiency, low duration time and higher product quality were selected as follows:1- Hybrid drying (solar energy) with improving the delivery of solar panels, drying time and quality of the product. Used operating conditions were: air flow speed 0.51 m/s in duration of 2h 45min through the period between Mai 06th, 2012 and Mai 28th, 2012.2- Hybrid drying (conventional electric power) using a maximum speed, performed at: 0.5 m/s in duration of 1h 15min through the period between Mai 13th, 2012 to Mai 18th, 2012.

Emerging hybrid drying technologies for food products

Emerging hybrid drying technologies for food products

Changes of Chemical and Physical Quality Attributes of Macadamia Nuts during Hybrid Drying and Processing

Macadamia nuts are well known for their unique flavor and taste. They contain a high level of monounsaturated fatty acids such as oleic acid, which is claimed to have an effect on reducing blood cholesterol. This study focuses on monitoring the changes in the physical and chemical qualities of macadamia nuts during drying, roasting, and storage. A combination of heat pump drying and tunnel drying was selected for this study to provide a comparison with convectional hot air drying. After completion of all drying treatments, the dried nuts were subjected to roasting at 125°C for 20 min and storage in ethylene vinyl alcohol copolymer (EVOH) bags at 4°C for 3 months. After each stage of processing, nut samples were taken to assess the quality in terms of fatty acids, moisture content, water activity, peroxide value, free fatty acids, color, and volatile compounds. The experimental results showed that drying treatments using heat pump combined with tunnel drying can reduce drying time significantly. It appears that the combination of heat pump and tunnel drying resulted in maintaining the commercially accepted chemical and physical quality characteristics of macadamia nuts in terms of key fatty acid contents, peroxide value, free fatty acid content, and color. Moreover, drying with a heat pump dryer combined with tunnel drying did not increase the content of undesirable volatile compounds in dried macadamia nuts after processing.

Quality improvement of orthodox and CTC tea and performance enhancement by hybrid hot air–radio frequency (RF) dryer

Orthodox and CTC tea were dried in hot air, radio frequency and hybrid hot air – radio frequency (RF) dryers for comparisons of drying durations and quality parameters. Drying duration was the shortest for RF drying at 60min for orthodox tea where as for CTC tea it was 90min. Hybrid drying for CTC tea took total of 55min and the same for orthodox at 20kW power was 85min. Aroma index of made liquor increased at 16kW compared to 20kW RF power.

Infrared assisted dry-blanching and hybrid drying of carrot

Infrared (IR) blanching and IR assisted hot air (hybrid) drying of carrot slices were attempted and their performance (processing time, retention of vitamin C and rehydration characteristics) was compared with conventional blanching and drying techniques. Intermittent heating of carrot slices using IR radiation (chamber maintained at 180–240°C) for 8–15min resulted in desired level of enzyme inactivation. The time required for blanching of carrot slices (10mm thick) using hot water, steam and IR radiation was 5, 3 and 15min, respectively. Retention of water soluble vitamin C was higher (62%) in IR blanched carrot as compared to water (43%) and steam (49%) blanching. IR blanching reduced the moisture content by 13–23% (absolute). IR blanched samples dried by hybrid mode took ∼45% lesser time compared to water blanched–hot air dried samples. Higher rehydration moisture of dried samples indicated the retention of cell structure during IR blanching. Vitamin C retention was ∼39% higher in IR blanched–hybrid dried slices compared to water blanched–hot air dried. The study shows the potential application of dry-blanching and IR assisted hybrid drying in food processing for improving product quality.

Efficient Energy Recovery with Wood Drying Heat Pumps

This article presents a 13-m3 wood dryer coupled with a 5.6-kW (compressor power input) heat pump. Drying tests with hardwood species such as yellow birch and hard maple were completed in order to determine the system's energy performance. Supplementary heating to compensate for the dryer heat losses was supplied using electrical coils or steam exchangers. The heat pump running profiles and dehumidification performance in terms of volumes removed and water extraction rates, coefficients of performance, and specific moisture extraction rates were determined for two all-electrical and two hybrid drying tests. The hardwood drying curves, share of the final moisture content, and final quality of the dried wood stacks, as well as total drying energy consumption and costs, were determined for each drying run. Finally, the total energy consumption of the drying cycles using a heat pump was compared with that of a conventional drying cycle using natural gas as a single energy source.

Advancements in Drying Techniques for Food, Fiber, and Fuel

Removal of moisture from biological materials, popularly called drying, has numerous benefits, including ease of handling due to reduction in bulk, resulting in reduced handling costs. Moreover, drying prevents microbial growth and spoilage. Though different drying techniques share a common objective, conceptually they are different and require modification/adaptation based on the biomaterial that is dried. There have been significant scientific advancements in the past century in the field of drying of foods, fibers, and fuel. This article will provide an extensive review of various drying pretreatments and different hybrid drying techniques, including supercritical and fluidized bed concepts, microwave drying, superheated steam drying, and heat pump drying to meet tomorrow's food and energy needs.

A Mixed Integer Formulation for Energy-efficient Multistage Adsorption Dryer Design

This work presents a mixed integer nonlinear programming (MINLP) formulation for the design of energy-efficient multistage adsorption dryers within constraints on product temperature and moisture content. Apart from optimizing temperatures and flows, the aim is to select the most efficient adsorbent per stage and product to air flow configuration. Superstructure models consisting of commonly used adsorbents such as zeolite, alumina, and silica-gel are developed and optimized for a two-stage, low-temperature, adsorption drying system. Results show that the optimal configuration is a hybrid system with zeolite as the first-stage adsorbent and silica-gel as the second-stage adsorbent in counter-current flow between drying air and product. A specific energy consumption of 2,275 kJ/kg is achieved, which reduces to 1,730 kJ/kg with heat recovery by a heat exchanger. Compared to a conventional two-stage dryer at the same drying temperature, this represents a 59% reduction in energy consumption. The optimal system ensures the exhaust air temperature of the first-stage regenerator is high enough to regenerate the second-stage adsorbent so no utility energy is spent in the second stage. A higher second-stage adsorbent wheel speed favors energy performance as it becomes optimized for energy recovery while the first is optimized for dehumidification. Although this work considers three candidate adsorbents in a two-stage system, the same reasoning can be applied to systems with more stages and adsorbents. The developed superstructure optimization methodology can, by extension, be applied to optimize multistage hybrid drying systems in general for any objective.

Trends in Development of Dried Vegetable Products as Snacks

Diverse types of vegetable snacks made using different drying methods are becoming increasingly commercially important for the food processing industry worldwide because they are also recognized as healthy for human consumption. Here we consider four aspects of drying snacks: unit operations currently used in the industry, novel or emerging methods with nontraditional means, combined (or hybrid) drying methods, and the quality changes during storage. Each drying method has its own advantages and limitations. Hybrid drying techniques are being developed to maximize the benefits of different drying techniques to produce better quality vegetable snacks that are attractive to the consumer. The merits and limitations of more than 10 drying techniques used for making dried vegetable snacks are discussed. Moreover, several new vegetables snacks dried using new drying methods are presented and discussed. A comprehensive review of the recent progress in production of dried vegetable snacks is presented and recommendations are made for future research.

UTILIZATION OF A DEVELOPED SOLAR STORAGE AND DRYING SYSTEM FOR CONTINUOUS DEHYDRATION OF SOME LEAFY MEDICINAL PLANTS

A developed solar system was used for dehydration some leafy medicinal plants and for storage of solar energy in water. It consisted of a flat-plate collector, a reflector, a drying unit, an auxiliary electric heater and a water tank. The solar system was tested during June to September 2009 for drying Lemon balm (Melissa officinalis L.) and peppermint (Mentha pepperita). Inlet air temperature to the dryer was higher than the ambient temperature and it could be maintained as desired using a temperature controller. Drying was taken about 8 to 10 hours to reduce the moisture contents of Melissa from 70-72 to 9% (wb) and 33 hours to reduce it from 70-72 to 17% (wb) by the hybrid dryer and natural sun drying method, respectively. Moisture content of Peppermint was reduced from 69-71% (wb) to 8-9 % (wb) in 10-12 hours by hybrid dryer and 51 hours by natural sun drying to reduce the moisture contents of it to 10% (wb). The capacity of the dryer was to dry about 16-20 kg of fresh Lemon Balm and also about 16-20 kg of fresh Peppermint per batch for each plant. The color, oil content and appearances of the products dried in the hybrid dryer were better than sun drying method. The dryer was also occupied with an auxiliary heat source to be used whenever adverse weather conditions exist. Using the water tank, about 15-20°C can be maintained in water during the time of sun-shine. During night time,the stored head is transferred from the water to the drying air inside the solar dryer and controls the air temperature through the drying process at night.

RANCANG BANGUN UNIT PENGERING HIBRIDA HEMAT ENERGI UNTUK PENGERINGAN TEMBAKAU RAJANGAN

The general objective of this research is to design and making hybrid dryer unit is used drying the chopped tobacco, in particular is to examine the performance of the hybrid dryer, the temperatures found in each rack, and compare the results of drying tobacco with naturally dried tobacco or dried above the traditional rack. This research method by creating a hybrid construction of the dryer unit, as follows: (1) determining the size of the design of space heating, (2) determining the size of the rack and the many levels of shelving, (3) purchasing of necessary materials and equipment, (4) implementation of manufacturing components or major parts of the dryer according to its function, (5) assembling the components into the dryer construction. This study focus are the performance of the hybrid dryer unit that has been created, namely: (1) homogeneity of dried tobacco content of each rack, (2) production capacity of each generation, (3) a long drying process, (4) operational costs and financial analysis. The results of this study is a prototype unit of a viable hybrid dryer used to dry tobacco with a capacity of 50 kg of chopped wet tobacco to produce 10 kg of dried tobacco drying time is 18 hours. Hybrid dryer unit is fit for use based on the analysis of the structure of production and financial feasibility analysis based on investation of hybrid dryer Rp 15.000.000 and working capital Rp 10.000.000. The method of Net Present Value (NPV) = Rp 48.747412, Internal Rate of Return (IRR) = 60%, Net Benefit Cost Ratio (Net B / C) = 2, 95, and Pay Back Period (PBP) = 1.16 years.

An Overview of Recent Developments and Some R&D Challenges Related to Drying of Foods

Thermal dehydration is the most common and cost-effective technique for preservation of foods and for the production of traditional as well as innovative processed products such as snacks with desired functionalities. The basic intent of this article is to provide a global overview of emerging and innovative thermal drying technologies that are already commercialized or show potential of industrial exploitation upon successful R&D to sort out some limitations. New drying technologies are needed to enhance quality, reduce energy consumption, improve safety, and reduce environmental impact. Mathematical modeling can be used for cost-effective development of untested novel designs to reduce the cost and time required for innovation. As examples of emerging drying technologies we consider selected dehydration techniques with imminent commercialization potential. These include heat pump–assisted drying, microwave-assisted drying, low-pressure superheated steam drying, pulse combustion spray drying, pulsed and ultrasound-assisted osmotic dehydration, as well as novel gas–particle contactors such as impinging streams and pulsed fluidized beds. Multistage drying, intermittent drying, and the use of hybrid drying technologies—which combine advantages of different dryers without some of their limitations—will be outlined. This article also discusses various methods of energy minimization, and the potential for use of renewable energy will also be discussed briefly. Although this overview emphasizes food dehydration, the themes covered are applicable to other materials as well.

Hybrid Drying of Rubberwood Using Superheated Steam and Hot Air in a Pilot-Scale

A pilot-scale rubberwood dryer was constructed and injected with superheated steam and hot air to study the effect of the hybrid system on the drying rate and mechanical properties of the wood. A total of 300 pieces of rubberwood boards, each with dimensions of 1000 mm long × 76.2 mm wide × 25.4 mm thick, were stacked in 1.0 m × 1.0 m × 1.7 m (1.7 m3) pallet. The stack was impinged with alternating cycles of superheated steam and hot air. The time required for conventional drying was 168 hours, but the drying time for the hybrid system was only 64 hours, resulting in a 62% reduction. After being dried, the rubberwood boards were mechanically tested for static bending, compression strength, hardness, and shear strengths. From the mechanical tests, the hybrid drying system using superheated steam and hot air had no significant effect on the mean shear strength parallel-to-grain; however; the mean compression strength parallel-to-grain was reduced by 24.2% and the mean MOR by 21.4%. Nonetheless, the mean MOE was increased by 30.4% and the mean of hardness by 16.4%.