Onion Drying Research Articles

Drying kinetics and quality aspects during heat pump drying of onion (Allium cepa L.)

A prototype heat pump dryer has been developed for drying of fruits and vegetables at low temperature and relative humidity to maintain the quality of dried product. Onions, of Nasik red variety were peeled, trimmed and sliced to 2 mm thickness. The onion slices were dried in the heat pump dryer at 35ºC (32 % R.H.), 40ºC (26 % R.H.), 45ºC (19 % R.H.) and 50ºC (15 % R.H.). Samples were also dried in a hot air dryer at 50ºC (52 % R.H.) for comparison. The drying rate increased with increase in drying air temperature, associated with reduced R.H., in the heat pump dryer. Drying took place mainly under the falling rate period. The Page equation, resulting in a higher coefficient of determination and lower root mean square error, better described the thin-layer drying of onion slices than the Henderson and Pabis equation. Heat pump drying took less drying time of 360 min and yielded better quality dried product, with higher retention of ascorbic acid and pyruvic acid and lower colour change, as compared to a hot air dryer at the same drying air temperature of 50ºC.

Kinetics Modeling and Moisture Diffusivity of Onion Slices in Fluidized Bed Drying

Drying characteristics of onion (Allium cepa L.) was studied with different combinations of air temperature (50, 60, 70 and 80C), air velocity (1, 2.5 and 4 m/s) and slice thickness (2, 4 and 6 mm) in a fluidized bed dryer. The drying behavior of onion such as drying rate and moisture diffusivities and their variation with change in drying air parameters were analyzed. Drying took place mainly under falling rate period and drying rate decreased with the moisture content of the samples. It was observed that the drying behavior of onion was better represented by the Page's model as compared with the exponential model. Moisture diffusivity values of onion ranged between 2.148 × 10−9 and 1.474 × 10−7 m2/s for the drying conditions under study. PRACTICAL APPLICATIONS The research results of the article will be useful for the industries involved in onion drying. Further, this article will help the researchers and academicians working in drying of agricultural commodities in general and onion in particular.

Energetic Analysis of Drying Biological Materials with High Moisture Content by Using Microwave Energy

One of the most important parameters in the drying process is the efficiency of energy transfer. Although it is well known that the absorbed component of the referred energy in the dehydration process under microwave conditions is theoretically lower—especially during the falling rate period of drying—by an order of magnitude than that of convective drying, the real efficiency of energy transfer in the microwave field depends on numerous—partly unknown—factors. In this study, the most significant relations between the energy transfer efficiency and the drying parameters were investigated. During the experiments, two power levels were used to study the effects of domain variables in apple, potato, and onion dehydration processes. The results showed that variations in microwave power level played an important role in overall drying kinetics. Based on the recorded data, we determined the typical energy efficiency factors for microwave drying of apple, potato, and onion. The specific energy consumption was calculated to compare the energy efficiency of a single measurement series. It was evident that by applying higher power levels, the energy utilization was more efficient. Furthermore, we theoretically proved and experimentally showed the nonthermal effects of the microwave energy.

Improving the Drying of Onion Using the Instant Controlled Pressure Drop Technology

In the present article, a new manner to use the instant controlled pressure drop (DIC) was adopted in the drying of onion. Instead of using it as a texturing treatment inserted in the second stage of hot air swell-drying operation and performed when water content is less than 30 g H2O/100 g dry matter, here the DIC was performed as a blanching–steaming pretreatment of fresh cut onion, with a steam pressure of 0.2–0.5 MPa, for 5–15 s, and an instant pressure drop (with a rate > 5 MPa s−1) toward a vacuum of 5 kPa. Afterward, drying was carried out with 40°C, 1 m s−1, and 267 Pa partially pressure humidity hot air. Although this DIC treatment aimed to avoid notable expansion, it could intensify mass transfer, reducing the total drying time, which reached 700 min instead of 3200 min for untreated samples; the effective diffusivity and the initial starting accessibility dramatically increased. Consequently, sensorial and nutritional attributes could be preserved with a perfect decontaminated end product.

EFFECT OF CURING AT DIFFERENT TEMPERATURES ON PHYTOHORMONE AND BIOCHEMICAL COMPOSITION OF ONION 'RED BARON' DURING LONG-TERM POSTHARVEST STORAGE

Long-term storability of onions is a result of first, dormancy induction and then sprout suppression, but little attention has been given to understanding the underlying physiological mechanisms of these processes. In the UK, bulb onion drying and storage technology is based on 1970's research and therefore was carried out on cultivars different from those used today. Thus, it is likely that alterations to current curing and drying methods, e.g., a reduction in the temperature and duration, could deliver benefits in the form of energy savings and reduced carbon emissions, whilst still producing onion bulbs of a satisfactory quality standard. Technologically advanced refrigerated and controlled atmosphere (CA) storage systems can maintain availability of onions for prolonged periods, but are energy intensive, and depend partly upon the use of the sprout suppressant, maleic hydrazide (MH). Increasing pressure from both consumers and retailers to eliminate residues in food, means that future use of MH as a sprout suppressant is uncertain. The system of topping onions in the field, lifting into store and then drying at 28°C for 6 weeks may not be suited to modern cultivars and could adversely affect storage life. Recent work has shown that a decline in endogenous abscisic acid (ABA) was correlated with storage life in both CA and regular atmosphere, and that the concentration of ABA in freshly harvested bulbs was ca. double that measured before versus after curing. Thus, maintaining ABA concentration could extend dormancy and storage life. This study examined the effect of curing at lower temperatures (20, 24 and 28°C with subsequent storage at 1°C) on the storability and biochemical characteristics (including ABA and other hormones measured using LC MS/MS) of onion bulbs. Storage life (determined by sprout growth) was not significantly affected by curing temperature. Sucrose concentration increased, and glucose concentration decreased during curing at all temperatures, however, immediately after curing the concentrations of glucose and fructose were less in those onions cured at 28°C, whereas the sucrose concentration was higher in onions cured at lower temperatures.

Varietal Effect on Drying Behaviour and Quality Characteristics of Onion

Experiments were carried out to study the drying behaviour and its effect on the quality characteristics of onions for three commonly grown varieties of onion. The quality characteristics studied were pyruvic acid concentration, colour and moisture content for each of the three varieties i.e. PBW-1, Punjab Naroya and PRO-6. The onions were dried for 495 min in the form of rings at constant drying temperature of 60°C in a through flow Satake dryer. The variety PBW-1 was found more suitable for commercial dehydration on account of lower drying ratio of 6.49 as compared to 7.26 for Pb Naroya and 7.90 for PRO-6, respectively. Constant drying rate period was not observed across all the experiments conducted for the three varieties. The whole process of drying of onions occurred in two to three falling rate drying period. The pyruvic acid, an important quality determining factor in onion, showed maximum loss of 71.20% for variety PRO-6 as compared to the loss of 63.61% in PBW-1 on dry weight basis during drying of onion rings at 60°C. It was observed that the total colour difference (ΔE) of 7.47 for the samples dried at 60°C was more in variety PRO6 as compared to variety PBW-1(ΔE = 6.13) and Pb Naroya (ΔE = 5.22).

Convective drying of onion: Kinetics and nutritional evaluation

The present work aimed, on one hand, to study of the drying of onions in terms of drying kinetics, which was evaluated at 30 °C, 50 °C and 60 °C. The experimental data was fitted to different empirical kinetic models from the literature, and this kinetic study was then complemented with the modelling if terms of Fick's diffusion equation, for estimation of the diffusion coefficients. On the other hand, the chemical characterization in fresh and dried onions at different temperatures (varying from 30 °C to 70 °C) was analysed, to evaluate the effect of drying and drying temperature on the chemical composition of the product. In this way, the analyses of moisture content, sugar content, crude protein, ash, fat, crude fibre, acidity and vitamin C were made and reported in this paper. From the results obtained it was verified that some chemical components of the onions are not affected by drying (ash, fat, protein and fibre) whereas some others are considerably influenced by drying (sugars, acidity and vitamin C). The present work allowed concluding that the three empirical models tested (Newton, Modified Page and Logarithmic) all describe relatively well the dehydration kinetics at the three temperatures analysed. Moreover, from the experimental data it was possible to estimate the diffusivities, which range between 3.33 × 10 −09 m 2/s at 30 °C and 8.55 × 10 −09 m 2/s at 60 °C.

A Study on the Changes in the Alliinase Activity during the Vacuum Freeze Drying of Onions(Allium cepa L.)

A Study on the Changes in the Alliinase Activity during the Vacuum Freeze Drying of Onions(Allium cepa L.)

A Study on the Changes in the Alliinase Activity during the Vacuum Freeze Drying of Onions(Allium cepa L.)

A Study on the Changes in the Alliinase Activity during the Vacuum Freeze Drying of Onions(Allium cepa L.)

Drying Behaviour and Energy Requirement for Dehydration of White Onion Slices

White onions were dried using an osmo-convective drying technique. Osmotic dehydration was done by dipping the onion slices in 10, 15 and 20 brix NaCl solution at ambient temperature for one hour. The increase in the concentration of NaCl increased the water loss, solid gain and weight reduction in onion slices. The drying times of pre-osmosed onions slices at drying air temperature of 50, 60 and 70°C were 6.75, 6.00 and 5.25 h respectively. The moisture diffusivity increased with increase in air temperature and ranged between 0.83 and 0.91x10-10 m2/s. Quality of dried product in respect to flavour strength and color was superior. A lowest specific energy consumption of about 1.98 kWh/kg was found in the air drying of onions pre-treated with 20 brix NaCl solution and at air temperature of 70°C.

Development of a solar-assisted dryer and evaluation of energy requirement for the drying of onion

A solar-assisted forced convection dryer was developed to study the effect of airflow rate (2.43, 5.25, 8.09 kg/min), air temperature (55, 65, 75 °C), and fraction of air recycled (up to 90%) on the total energy requirement of drying of onion slices. The dryer was provided with a flat plate solar air heater having both the corrugations and triangular fins to the absorber plate. For drying of onion slices from initial moisture content of about 86% (wet basis) to final moisture content of about 7% (wet basis), the energy required per unit mass of water removed during without using recirculation of air was found between 23.548 and 62.117 MJ/kg water. The percent energy contribution by the solar air heater, electrical heater, and blower was found between 24.5% and 44.5%, 40.2% and 66.9%, and 8.6% and 16.3%, respectively. The savings in total energy due to fraction of air recycled were determined at 65 and 75 °C air temperature for the above three airflow rates. The maximum saving in total energy up to 70.7% was achieved by recycling of the exhaust air. The energy required per unit mass of water removed was found between 12.040 and 38.777 MJ/kg water. The percent energy contribution by the solar air heater, auxiliary heater, and blower was found between 22.4% and 40.9%, 33.6% and 62.6%, and 11.2% and 37.2%, respectively.

Catalytic Infrared Dehydration of Onions

ABSTRACT: Dehydrated onions are commonly dried with convection heating, which is inefficient and costly. This study compared the drying and quality characteristics of onion dried with catalytic infrared (CIR) heating and forced air convection (FAC) heating. Sliced high‐solids onions were dehydrated under 9 conditions: CIR heating with and without air recirculation, and FAC each operated at 60, 70, and 80 °C. In general, CIR both with and without air recirculation had higher maximum drying rates, shorter drying times, and greater drying constants than FAC at moisture contents greater than 50% (d.b.). Dried onion quality, measured as pungency degradation, was similar for both the drying methods at 60 and 70 °C. The color analysis showed better product color (whiter and less yellow) at lower temperatures for CIR and higher temperatures for FAC. The browning could have been caused by the higher surface heat flux of the CIR heating and longer process times of FAC drying. Aerobic plate counts and coliform counts were not significantly different for either product from the CIR or FAC drying. However, samples dried by the CIR had significantly lower yeast and mold counts than those dried by the FAC. It is recommended that CIR be used in the early stages of onion drying.

Modeling the performance of the reversed absorber with packed bed thermal storage natural convection solar crop dryer

A transient analytical model has been presented to study the new concept of a solar crop dryer having reversed absorber plate type collector and thermal storage with natural airflow. The performance of 1 × 1 m 2 area of crop dryer with packed bed and airflow channel was evaluated for drying of onions. The model was solved to compute the air temperatures and various functional components of the drying systems for a day of the month of October for the climatic condition of Delhi (India). The parametric study involved the effect of width of airflow channel and height of packed bed on the crop temperature. The effect of thermal storage is observed on the natural mass flow rate in the drying system. A 30° inclined absorber plate with in-built thermal storage and 0.12 m width of airflow channel induced the mass flow rate in the range of 0.032–0.046 kg s −1 during the drying process. The thin layer drying equation was used to study the drying rate and hourly reduction in moisture content in the crop trays. It has been observed that the crop moisture content and drying rate decreases with the drying time of the day. A reversed absorber plate of 1 m length and 1 m breadth with 0.15 m packed bed could dry 95 kg of onion from a moisture content of 6.14–0.27 kg water/kg of dry matter in a 24 h drying period.

Kinematic Model for a Far Infrared Vacuum Dryer

Energy analyses in drying of welsh onion from one source of far infrared radiation at an absolute pressure of 2.6 and 5 kPa were carried out to determine the energy input at which the highest level of efficiency would be attained. The experimental results were used not only to formulate an advanced mathematical model for the prediction of moisture content of onion as a function of drying time but also to study the dryer efficiencies. Less than half of energy input was utilized for evaporating water from the onion. Approximately 73–99% of operating efficiency, which was dependent on the dryer configuration, was converted into radiant energy. The highest radiant efficiency was obtained at 40 W initial power supply, which was equivalent to the source temperature of about 100°C. An 80 W initial electrical power supply at a 10 cm distance of heater from the onion surface showed the highest drying efficiencies. At 80 W power, efficiency decreased with increasing distance from the heater source but at the expense of quality. For optimum drying efficiency without compromising quality, distance of heater from onion surface was proposed to be 10 cm.

Modeling the performance of greenhouse with packed bed thermal storage on crop drying application

This paper presents a transient analytical model to study the application of a greenhouse with packed bed thermal storage to crop drying. The performance of an even shape greenhouse with a packed bed and crop dryer was evaluated for drying of onions. The model was solved to compute the air temperatures and various functional components of the drying systems for a day of the month of May for the climatic condition of Delhi (India). The parametric study involved the effect of length and breadth of greenhouse and mass flow rate of air on the temperatures of crop. The thin layer drying equation was used to study the drying rate and hourly reduction in moisture content in the crop trays. It has been observed that the crop moisture content and drying rate decreases with the drying time of the day. A greenhouse of 6 m length, 4 m breadth with a 0.278 kg s −1 air mass flow rate with a 0.25 m height of packed bed could dry 2280 kg of onion from a moisture content of 6.14–0.21 kg water/kg of dry matter in a 24 h drying period.

New Food Drying Technologies - Use of Ultrasound

Reducing water availability is one way to preserve food. Water in solid foods is transferred to a fluid, either gas or liquid; during this process both internal and external resistance affect water transfer from the food. As a consequence, any means to reduce those resistances constitute an improvement of the process, and ultrasound appears to be a way to reduce those resistances. Ultrasound are mechanical waves that produce different effects when travelling through a medium. Among others, those related to mass transfer include micro-stirring at the interface, the so called “sponge effect” and cavitations. Ultrasound has so far been applied to dehydration in solid-gas systems like onion drying. Nevertheless, the difficulties for the propagation in the air have led to the development of specially adapted transducers that have been applied in the drying of carrots. In solid-liquid systems, ultrasound has been used in the treatment of products immersed in hypertonic solutions, either in sugar solutions for fruits like apples or in salt brine in the case of cheese or meat. An increase in mass transfer is achieved if the threshold power value for the product is attained.

Accelerated drying of welsh onion by far infrared radiation under vacuum conditions

Far infrared radiation has excellent radiation characteristics and high energy conversion rates can be achieved using ceramic coated radiators. Using such a radiator, the dehydrating synergy, generated by far infrared radiation heating under vacuum condition, the drying of welsh onion was studied. The radiation intensity levels influenced dramatically the drying rate and the product qualities: the a ∗ for rehydrated onions, and the L ∗ and ΔE ∗ ab for dehydrated onions. A rising-rate period, a constant-rate period, and a falling-rate period were ascribed to the drying behaviour. The radiation also had significant effects on chlorophyll content. The long time in drying and the high temperature may have contributed to a decrease in rehydration properties.

A SINGLE-LAYER MODEL FOR FAR-INFRARED RADIATION DRYING OF ONION SLICES

ABSTRACT Three mathematical models were investigated for the description thin-layer far-infrared drying characteristics of fruits and vegetables. The models considered were the exponential model, the Page model, and an approximation of the diffusion model. Tests were conducted using three levels of far-infrared radiation intensities (0.50, 0.80 and 1.00 kW/kg of initial mass of onion), three initial thicknesses of onion slices (2, 4 and 6 mm), three air velocities (0.10, 0.20 and 0.35 m/s) and three inlet air relative humidities (28.6, 36.8 and 43.1%), and with two replications for each treatment. Stepwise multiple linear regression analyses were performed to relate the parameters of the model with the drying conditions. The performance of these models is evaluated by comparing the coefficient of correlation (r 2 ), the residuals of moisture ratios, and the root mean square errors between the observed and predicted moisture ratios. The Page model MR = exp(−kt N ) was gave the best results in describing single-layer far-infrared radiation drying of onion. Drying parameter k of the Page model was correlated with radiation intensity, initial thickness, air velocity and air relative humidity, and N was found to be independent of the four drying conditions.

FAR INFRARED–VACUUM AND –CONVECTION DRYING OF WELSH ONION

The drying of welsh onion by a far–infrared (FIR) heater under reduced pressure and convective conditions were studied to understand the drying characteristics. The effects of radiant power input and vacuum levels on the drying phenomenon were investigated. In addition, drying rate as a function of moisture content was studied vis–a–vis drying qualities such as rehydration potential, shrinkage, and color change. When onion samples were dried from about 10.1 to 0.05 w/w dry basis moisture content, the three characteristic periods, namely, rising, constant, and falling rate periods, were obtained. The drying rate increased with increasing radiant power input and was higher in a reduced–pressure environment than under a convective condition. With 0.17 W/cm2 radiant power input, the drying rate was 50% higher in the vacuum operation than under the convective condition. There was a higher shrinkage with a concomitant higher rehydration potential and superior quality of onion at lower radiant power input with the FIR–vacuum system than with the FIR–convection system.

Modeling of a Far Infrared Vacuum Dryer on Welsh Onion

A study of the energy efficiencies in the drying of welsh onion from one source of far infrared radiation and at an absolute pressure of 5kPa was carried out to determine the energy input at which the highest level of efficiency would be attained. The experimental results were used not only to formulate a simple mathematical model for the prediction of moisture content of onion as a function of drying time but also to study the dryer efficiencies. At 70-watt energy input, the highest drying efficiencies were attained.