Hot Air Convective Drying Research Articles

Influence of Drying on the Recoverable High-Value Products from Olive (cv. Arbequina) Waste Cake

Olive oil production in Chile has increased 900% in the last 10 years, resulting in a concomitant increase in waste products that need to be managed. In this study, drying assays were conducted in a convective hot-air dryer on Arbequina olive waste cakes between 40 and 90 °C and a constant airflow of 2 m/s. Proximate composition, dietary fiber, minerals, fatty acids, tocopherols, phytochemicals as well as antioxidant activity as DPPH, FRAP and ORAC values were determined. These six drying temperatures were found to have variable effects on general composition of the residues. Fatty acids were found in high concentration at low temperatures where oleic acid was the most abundant and α-tocopherol was the most prevalent form of vitamin E. The phytochemical content (total phenolics, flavonoids and flavanols) represented an average of 20.7% of the waste cake. Fresh cake showed the highest polyphenols and flavonoids content whereas flavanols content was higher at all drying temperatures. 3-Hydroxytyrosol was the most abundant phenolic compound, and found in higher abundance in samples dried at 90 °C. The antioxidant capacity measured by FRAP and DPPH assays decreased with drying or remained unchanged when measured by ORAC assay. Given the nutritional and commercial potential of waste cakes, a drying technology for the preservation of bioactive compounds from waste products is discussed.

Optimization of Process Parameters for Microwave Convective Drying of Grapes Using Response Surface Method

Drying is a key method to preserve agricultural food products. The longer drying time in many conventional drying methods at relatively elevated temperature during the falling rate periods leads to undesirable degradation of the finished products. Microwave drying shortens the drying time drastically and improves the desirable sensory quality of final product. In This study combination of convective hot air drying and microwave heating was applied to produce raisins from grapes in short time duration with desirable quality aspects. The influence of drying parameters such as specific microwave power, hot air temperature and their interaction were analyzed using response surface method. This research work was carried out by doing experiments in laboratory drying setup and optimal combination was determined as 45°C hot air temperature and specific microwave power as 0.25 W/g to obtain results in less than 8 hours to constraint the process for mass production in single industrial working shift. Optimal conditions obtained from lab scale setup were used on continuous conveyor drying setup to confirm the results for mass production.

Non-thermal hybrid drying of fruits and vegetables: A review of current technologies

Abstract Fruits and vegetables are very perishable commodities that have enormous industrial and commercial importance. To preserve its quality attributes, increase shelf life, and reduce transport weight, fruit and vegetable can be processed by drying. Over the years, conventional drying techniques have been widely applied, both industrially and commercially for the processing and preservation of fruits and vegetables. However, most of the conventional techniques are time and energy consuming, and affect important quality attributes of final products. Recently, there has been increased interest in the use of non-thermal drying methods in combination with other conventional drying techniques for preserving fruits and vegetables. These methods have been reported to enhance the quality attributes of dried products, reduce drying time and energy demand, and increase the overall drying efficiency. In this regard, the development of a cost effective non-thermal hybrid drying systems, such as combined ultrasound (US) and hot-air drying, ultraviolet (UV) and hot-air drying, and pulse electric field (PEF) and hot-air drying have recently been researched on. These drying techniques have become potential substitute for conventional industrial and commercial dryers, owing to their advantage of producing quality dried fruit and vegetable products, with reduced drying time and energy consumption. This study therefore attempts to highlight recent developments of the commonly used non-thermal combined convective hot-air drying (CHAD) techniques for fruits and vegetables, with emphasis on drying time, drying rate, quality attributes of products, and modelling approach. This study further highlights the primary constraints for industrial application of this technology as the inadequate medium of transmission for power ultrasound, cost of design and installation, and the limited study on (UV) and PEF assisted CHAD. The necessity for conducting more detailed studies on non-thermal assisted convective hot-air drying of fruits and vegetables was emphasized.

Osmotic dehydration with sorbitol combined with hot air convective drying of apple cubes.

The aim of the present work was to study the effect of the osmotic dehydration (OD) pre-treatment on the mass transfer kinetics and water activity (aw) of apple cubes during hot air drying. The adequacy of different mathematical models to describe the moisture content of the product during this process was also evaluated. Apple cubes were osmotically dehydrated with sucrose or sorbitol solutions at 60°C, and then dried by air at 25-80°C. Overall, the OD and rise of the air temperature resulted in an increased water loss rate and a reduction of the aw. The osmotic agent used in the OD was not relevant to the air drying kinetics, but the pre-treatment with sorbitol solutions produced dried samples with lower aw. Newton's, Page's, modified Page's, Henderson and Pabis', Two-term, Two-term exponential, Logarithmic, Midilli et al.'s models could describe the moisture content well during the air drying process.

Quality Assessment and Mathematical Modeling of Hot-Air Convective Drying of Persimmon (Diospyros kaki L.) Fruit

Abstract The influence of drying on the color, porosity, shrinkage and moisture of persimmon fruit during convective drying was determined by computer vision. The experiments were performed with persimmon fruit that were cut into slab 20 × 20 mm, which were arranged into a bigger slab, 60 × 60 mm. Drying process was carried out at 60 °C. Noticeable changes in quality parameters (color, porosity and shrinkage) could be observed during the drying process, where the central region of the sample evidenced less changes. Persimmon’s physical properties were experimentally obtained as the temperature function and heat and mass convective coefficients were adjusted as a time function. A numerical simulation using the Finite Volume Method allowed to describe the evolution of temperature and moisture content distributions during drying. The numerical and experimental results of temperature and moisture during persimmon drying were found to be in a good agreement.

Comparison of different drying methods on the chemical and sensory properties of chestnut (Castanea sativa M.) slices

The aim of this work was to determine the effect of hot-air convective drying (D), osmotic dehydration (OD), osmotic dehydration + drying (OD + D) and freeze-drying (FD) on chemical and sensorial characteristics of chestnut slices. Proximate composition, sugars, organic acids and lipid profiles were determined along 60 days of storage. Immediately after production, D and FD samples had similar proximate compositions, both with higher fat and protein contents than the osmodehydrated ones, the latter with increased sucrose contents. FD was the method that better preserved starch, amylose, ascorbic and citric acid molecules at day 0, while D originated samples with higher glucose and fructose contents. Along storage, the major variations were observed on organic acids: ascorbic acid decreased on all methods, while fumaric acid increased. Only small variations were observed on the fatty acids and vitamin E profiles and amounts. All samples presented similar and good overall sensorial acceptance with the exception of D. FD was the method that better preserved the sensorial characteristics until 60 days of storage, while D only preserved freshness until 15 days and OD + D until 30 days. In general terms, the most adequate and accepted preservation methods to apply to chestnuts would be FD and OD + D.

Effect of glass transition on the shrinkage of sugar kelp (Saccharina latissima) during hot air convective drying

Understanding the properties of a biomaterial such as seaweed is important for retaining product quality and improving shelf life through optimized post-harvest processing. An aquacultured sugar kelp (Saccharina latissima) was investigated to determine the effects of glass transition on the shrinkage of dried products. Freeze dried samples of varying moisture content were analyzed for glass transition and freezing point using a differential scanning calorimeter. An image processing algorithm was used for analyzing shrinkage while drying. Drying kinetics were studied at 40 and 70 °C, 10 m/s air velocity and at relative humidity of 25, 50 and 80%. When the drying temperature was 70 °C, above Tg, the shrinkage rate was greater, exhibiting higher matrix mobility in the rubbery state. The glass transition, freezing lines and moisture sorption isotherm can be utilized to extend the shelf-life of both low and high moisture sugar kelp and provide guidelines for other species of seaweed.

Mathematical modeling evaluation for convective hot air drying of poultry meat

Mathematical modeling evaluation for convective hot air drying of poultry meat

COLOR CHANGE KINETICS AND TOTAL CAROTENOID CONTENT OF PUMPKIN AS AFFECTED BY DRYING TEMPEARTURE

The color changes kinetics of pumpkin slices during convective hot air drying was investigated at drying temperatures of 50, 60, 70 and 80 °C. The hunter lab L* a* and b* color coordinates were used as assessment indicators. The total color change, chroma value, hue angle and brownness index (BI) of the pumpkin slices where also determined. To determine the most suitable kinetics model for the prediction of the color changes of pumpkin, the zero-order, first-order, and fractional conversion models were fitted to the experimental data, using linear regression analysis. The activation energy of the color change parameters (L*, a*, b* and ) was estimated and found to be 41.59 kJ/mol, 16.287 kJ/mol, 63.856 kJ/mol and 73.390 kJ/mol respectively. The fresh pumpkin samples contained a mean total carotenoid content of 25? g/g, while the total carotenoid content of samples dried at 50 °C, 60 °C,70 °C and 80 °C were 146? g/g, 56.4? g/g, 37.9? g/g and 102.5? g/g respectively. Further, the results of ANOVA showed there was significant difference between the total carotenoid content of the fresh pumpkin samples and those dried in convective hot air dryer at 5% (p<0.05) significant level.

Dehydration Characteristics of Whole Lemons in a Convective Hot Air Dryer

In this study, whole lemons were dried using a laboratory convective hot air dryer and the effects of drying temperature on dehydration behaviour and mass transfer characteristics of the lemons were investigated. The drying experiments were conducted using air temperatures of 50, 60 and 75 °C and air velocity of 1 m/s. It was observed that the drying temperature affected the drying time and the drying rate significantly. Drying rate represented no constant period and the entire dehydration process took place in the falling rate period. The usefulness of five mathematical models to simulate the drying kinetics was evaluated and Midilli and logarithmic models were found as the best models describing the drying curves. The effective moisture diffusivity values were obtained to be 1.15×10-10, 2.29×10-10, and 7.63×10-10 m2/s for the applied temperatures, respectively. The activation energy was also determined to be 71.32 kJ/mol. The convective mass transfer coefficient values were calculated by the analytical model and obtained to be 4.078×10-7, 1.023×10-7 and 4.346×10-8 m/s for drying temperatures of 75, 60 and 50 °C, respectively.

Drying kinetics of cotton based yarn bobbins in a pressurized hot-air convective dryer

In this study, the drying kinetics of cotton bobbin drying process in a pressurized hot-air convective bobbin dryer was investigated, and a drying model was introduced for the simulation of drying. Tests were conducted for drying temperatures of 70℃, 80℃, and 90℃; effective drying air pressures of 1, 2, and 3 bars; three volumetric flow rates of 42.5, 55, and 67.5 m3/h; and for three different bobbin diameters of 10, 14, and 18 cm. Optimum drying conditions were specified in terms of drying time and energy consumption. Results indicate that the total drying time depends significantly on the drying temperature, pressure, and volumetric flow rate. Results show that the minimum energy consumption is obtained for low values of drying air temperatures and pressures, and for moderate and high values of drying air volumetric flow rates. It was also found that the Page model is suitable for simulating the drying behavior of cotton yarn bobbins. Finally, results show that effective diffusion coefficient values are between 1.132 × 10−7 m2/s and 3.453×10−7 m2/s depending on the values of drying parameters.

Evaluation on the air-borne ultrasound-assisted hot air convection thin-layer drying performance of municipal sewage sludge

The thin-layer drying behavior of the municipal sewage sludge in a laboratory-scale hot air forced convective dryer assisted with air-borne ultrasound was investigated in between 70 and 130°C hot air temperatures. The drying kinetics in the convective process alone were compared to that for ultrasound-assist process at three ultrasound powers (30, 90, 150W). The average drying rates within whole drying temperature range at ultrasound powers of 30, 90 and 150W increased by about 22.6%, 27.8% and 32.2% compared with the convective drying alone (without ultrasound). As the temperature increasing from 70°C to 130°C, there were maximum increasing ratios for the effective moisture diffusivities of the sewage sludge in both falling rate periods at ultrasonic power of 30W in comparison with other two high powers. In between the ultrasound powers of 0 and 30W, the effect of the power on the drying rate was significant, while its effect was not obvious over 30W. Therefore, the low ultrasonic power can be just set in the drying process. The values of the apparent activation energy in the first falling rate period were down from 13.52 to 12.78kJmol−1, and from 17.21 to 15.10kJmol−1 for the second falling rate period with increasing the ultrasonic power from 30 to 150W. The values of the apparent activation energy in two falling rate periods with the ultrasound-assist were less than that for the hot air convective drying alone.

Microwave-assisted drying of blueberry (Vaccinium corymbosum L.) fruits: Drying kinetics, polyphenols, anthocyanins, antioxidant capacity, colour and texture

The aim of the study was to evaluate the effect of hot air convective drying (HACD), microwave vacuum drying (MWVD) and their combination (HACD+MWVD) on the drying kinetics, colour, total polyphenols, anthocyanins antioxidant capacity and texture of frozen/thawed blueberries. Drying resulted in reduction of total polyphenols content and antioxidant capacity (69 and 77%, respectively). The highest content of total polyphenols was noted after HACD at 90°C. Lower air temperature and prolonged exposure to oxygen resulted in greater degradation of polyphenols and antioxidant capacity. Drying processes caused a significant decrease (from 70 to 95%) in the content of anthocyanins. The highest content of anthocyanins and the strongest antioxidant capacity was found in blueberries dried using HACD at 90°C+MWVD. Among drying methods, HACD at 90°C+MWVD satisfied significant requirements for dried fruits i.e. short drying time and improved product quality.

Drying kinetics, rehydration and colour characteristics of convective hot-air drying of carrot slices

The effects of air drying temperature, slice thickness and pre-treatment application on the drying kinetics of carrot slices during convective drying in the range 50–70 °C were investigated. Results indicated that drying time, rehydration ratio and colour characteristics of carrot slices were more affected by drying air temperature, followed by pre-treatment applications. Five thin-layer drying models were applied to describe the drying kinetics. Midilli et al. model was the best model to characterize the drying kinetics of carrot slices. The moisture effective diffusivity calculated from the second Fick’s law of diffusion ranged from 3.46 × 10−10 to 1.02 × 10−9 m2/s. The values of activation energy determined from the slope of the Arrhenius plot, ln(D eff ) versus 1/(T + 273.15), were 35.53, 43.42, and 37.75 kJ/mol for blanch, potas and control samples, respectively.

Power ultrasound as a pretreatment to convective drying of mulberry (Morus alba L.) leaves: Impact on drying kinetics and selected quality properties

The effect of ultrasound pretreatment prior to convective drying on drying kinetics and selected quality properties of mulberry leaves was investigated in this study. Ultrasound pretreatment was carried out at 25.2–117.6W/L for 5–15min in a continuous mode. After sonication, mulberry leaves were dried in a hot-air convective dryer at 60°C. The results revealed that ultrasound pretreatment not only affected the weight of mulberry leaves, it also enhanced the convective drying kinetics and reduced total energy consumption. The drying kinetics was modeled using a diffusion model considering external resistance and effective diffusion coefficient De and mass transfer coefficient hm were identified. Both De and hm during convective drying increased with the increase of acoustic energy density (AED) and ultrasound duration. However, De and hm increased slowly at high AED levels. Furthermore, ultrasound pretreatment had a more profound influence on internal mass transfer resistance than on external mass transfer resistance during drying according to Sherwood numbers. Regarding the quality properties, the color, antioxidant activity and contents of several bioactive compounds of dried mulberry leaves pretreated by ultrasound at 63.0W/L for 10min were similar to that of mulberry leaves without any pretreatments. Overall, ultrasound pretreatment is effective to shorten the subsequent drying time of mulberry leaves without damaging the quality of final product.

Effect of hot air convective drying on the fatty acid and vitamin E composition of chestnut (Castanea sativa Mill.) slices

The aim of the present work was to study the effect of hot-air drying on the quality of chestnut slices, regarding fatty acid and vitamin E composition. Chestnut slices of two varieties, Longal and Judia, were dried in a tray dryer at 50 °C, for 1, 2, 4, 6, 8 and 10 h. Concerning fatty acids, at beginning both varieties presented significant different contents in C16:0, C18:0, C18:1, C18:2cc, C20:0 and C20:1, being C18:2cc the most abundant. Concerning vitamin E, both varieties had similar γ- and δ-tocopherol contents; however, after 10 h of drying their concentrations decreased in Longal (19.2 and 14.4 %, respectively). It was also in slices of Longal variety that a significant decrease was observed in C18:0 (15.0 %), C18:1 (19.4 %), C20:0 (14.3 %) and C20:1 (11.1 %) after 10 h of drying, suggesting this variety to be a little more heat sensible than Judia. Even though some variation on lipid composition was observed along drying of chestnuts, the variety showed to have a higher effect than the drying process itself. Thus, hot air convective drying seems to be an interesting process to apply in the future to this nut in order to produce a healthy snack, not causing potential losses from a nutritional point of view.

Optimization of Spirulina sp. Drying in Heat Pump: Effects on the Physicochemical Properties and Color Parameters

The aim of this work was to optimize the drying operation of Spirulina sp. using the heat pump to dehumidify the air. An experimental design using the drying air temperature (30, 40 and 50C) and the initial sample thickness (1, 3 and 5 mm) was performed. The responses were the physicochemical properties and the color parameters of the dried microalgae. The air temperature presented the highest effect on the phycocyanin content, while the sample thickness showed the highest influence on the other responses. The optimal operating conditions for the heat pump drying of Spirulina sp. were at air drying temperature of 50C and sample thickness of 5 mm. The phycocyanin content, total antioxidant activity and color difference were found to be 19.60 mg/g, 52.6% and 5.71, respectively. Thermograms (differential scanning calorimetry) showed that under these conditions, the product showed the highest thermal stability. Practical Applications The culture of Spirulina sp. is attractive for various commercial purposes, such as nutritional supplement for humans and animals and supply of active metabolites in the pharmaceutical and cosmetic industry. The conventional tray drying is a traditional method that is used in the microalgae drying. However, the heat pump drying enables operation in lower temperatures than the traditionally used in convective hot air drying. This operation occurs in moderate conditions of temperature and humidity due to air dehumidification, leading to retention of bioactive compounds in products that are heat sensitive. Therefore, the use of heat pump dryer for the production of dried Spirulina should be promising to preserve its phytochemicals and physical characteristics. The benefits of heat pump drying have been shown in different studies in the literature, but there are no reports about its use in Spirulina drying and its influence on the physicochemical and functional characteristics.

Fundamentals of Infrared Heating and Its Application in Drying of Food Materials: A Review

AbstractDrying is an energy‐intensive unit operation in chemical, food and allied industries. It is always desired to have low energy consumption for drying operation without compromising the quality of products. In this review, fundamental aspects of infrared heating are described in detail with the focus on its application to the drying of food materials. The modeling techniques are explained in detail to get emphasis on the design of large‐scale infrared dryers. This review also contains the investigations carried out by several researchers by experimentation and/or modeling to describe the infrared drying, infrared–convective drying, infrared–microwave drying and infrared–freeze drying. The present work describes the author's views regarding the capability and applicability of infrared source for drying of food materials. After going through a literature survey, it can be seen that infrared source especially in the far‐IR range has been widely used in convective hot air dryer than other types of dryers.Practical ApplicationsThe energy loss in convective air drying is found to be very large in chemical and allied food industries. Air, the drying medium, needs to be heated using air heaters, which generally have low thermal efficiency. On the contrary, as the infrared radiation is recognized as surface heater, it can be very suitable for drying of food materials whose drying is generally carried out in thin slices. Moreover, use of infrared heaters in other dryers is very beneficial to lower the drying time without significantly affecting the quality of the final products. Lowering the drying time can increase the productivity and decrease the cost of operation practically.

Drying kinetics and evolution of the sample's core temperature and moisture distribution of yam slices (Dioscorea alata L.) during convective hot-air drying

ABSTRACTIn the present work, the drying kinetics and evolution of sample's core temperature and moisture distribution of yam slices during convective hot-air-drying were investigated. In terms of drying kinetics, the effect of drying temperature (50, 55, 60, 65, 70°C), relative humidity (20, 30, 40, 50%), and sample thickness (5, 7, 9 mm) on drying characteristics of yam slices were studied. Results indicated that all the three factors had significant influence on the drying kinetics, whereas drying temperature gave the most significant effect, followed by relative humidity and sample thickness. Moisture-effective diffusivity and activation energy were calculated, and it was found that the diffusivity was in the range of 5.5454 × 10−10–1.0804 × 10−9 m2/s and the activation energy was 29.528 kJ/mol. Heat and mass transfer models were developed based on the finite element method to calculate the core temperature and moisture distribution of yam slices during drying. Model validation exhibited good agreement between predicted and experimental data, which illustrated that the developed models could precisely predict the core temperature profile and moisture distribution of the sample. The current work provides further insights to understand the characteristics and mechanism of drying process of yam slices.

Solar drying of henna (Lawsonia inermis) using different models of solar flat plate collectors: an experimental investigation in the region of Biskra (Algeria)

In this article, an experimental study is presented in two parts: First, the best performing solar dryer is selected between two models of flat plate solar collectors (FPCs). The second part investigates the drying behavior of henna (Lawsonia alba, syn. Lawsonia inermis Linn.) using the best selected FPC. The aim of this study is to get a clean product in a reduced time by using solar FPCs. Therefore, we first present the results of an experimental investigation on the thermal performances and pressure drops of two models of FPCs for indirect drying applications: i) simple-pass collector with trapezoidal obstacles (model I) and ii) double pass collector with trapezoidal obstacles in the air flow duct (model II). In the second part, we compare different methods of henna drying; we have proceeded to the application of the best indirect dryer (model II) for drying of henna under climatic conditions of Biskra, Algeria. As a result, some conclusions are made on the drying behavior of henna by the study of the influence of: drying process, air flow rate and masse of the product on the drying kinetic of the product for the forced convection hot air drying and their temperature dependence.