Kinetics of ultrasound pretreated apple cubes dried in fluidized bed dryer

Abstract

This work evaluated the effect of ultrasonic pretreatment on the production of dehydrated apples (Malus domestica L. var Granny Smith) in a fluidized bed dryer. Cube-shaped apple samples were subjected to ultrasound in an ultrasonic bath and dried in a fluidized bed drier. The experimental design evaluated the effect of ultrasound pretreatment time (0 to 30?min) on the soluble solids loss during pretreatment and on the drying time. The ultrasonic pretreatment was carried out in a bath ultrasound operating at 25?kHz and outputting 55?W/m³ of power density. Distilled water was applied in the pretreatment to produce low-calorie apple cubes. Fluidized bed drying was carried out at 30, 40, and 50?°C. Fick’s law was used to model the drying process and to determine the apparent water diffusivity. The soluble solid loss ranged between 8.7 and 21.2% during the pretreatment, and the apparent water diffusivity during air drying ranged from 1.09?×?10^?6 to 2.81?×?10^?6 m²/min. Ultrasound pretreatment increased the apparent water diffusivity up to 58%. Apple cubes subjected to 20?min of ultrasound pretreatment and dried at 50?°C presented the highest apparent water diffusivity and dried to achieve a water activity of 0.4 in 100?min.     

Drying Behavior of Carrots Dried in a Spout–Fluidized Bed Dryer

The effect of water blanching treatment and the inlet air temperature on drying kinetics as well as the quality attributes of carrot cubes dried in a spout–fluidized bed dryer at 60, 70, 80, and 90°C were analyzed. The material shrinkage and the rehydration potential were calculated to assess the changes in quality of dried carrots. It was found that the value of the air velocity during the drying of carrot cubes in a spout–fluidized bed dryer should be related to the moisture content of the carrot particles. A high value of air velocity at the beginning of the drying cycle and a lower value for the later stages were also required. The linear equation was correlated to the data of shrinkage of raw and blanched carrots. Blanching significantly influenced the coefficients in the shrinkage model derived for drying of carrot cubes in a spout–fluidized bed dryer, while drying temperature did not influence the shrinkage of carrot particles. The intensity of heat and mass transfer during spout–fluidized drying of carrot cubes was dependent on the drying temperature. A correlation was developed to calculate the values of effective moisture diffusivity of dried carrot cubes as a function of the moisture content and temperature of the material. It was observed that for any given time of rehydration, both the moisture content and the rehydration ratio calculated for samples dried at 60°C were higher than for samples dried at temperatures of 60, 70, 80, and 90°C.     

Drying Characteristics of Carrots Immersed in a Fluidized Bed of Fluidizing Particles Under Reduced Pressure

The drying characteristics and properties (color and shrinkage) of carrots (as a representative agricultural product) were experimentally examined in a fluidized bed under reduced pressure. Dry hot air and superheated steam were used as the drying gases. Rice and carrot powders (0.125–0.355 mm in diameter) were used as the fluidizing particles, in addition to glass beads (0.12 mm in diameter).

It was confirmed that the drying rate using a fluidized bed was much higher than without a fluidized bed (hot-air drying), regardless of the type of fluidizing particles used. Under reduced pressure, both with and without a fluidized bed, the drying rate was higher than that at atmospheric pressure using hot air. The drying rate was sufficiently high for fluidized-bed drying with superheated steam, though the drying rate was higher with hot air than with superheated steam. As the drying temperature increased, the volume ratio (befor/after drying) of the sample increased. At high drying temperatures (373 and 423 K in the present study), the color of the sample changed; in other words, a heat-induced change in the properties of the carrot was observed. At a low drying temperature (333 K in the present study), the drying method did not affect the color of the carrot; i.e., the color of the dried material was maintained even in a fluidized bed under reduced pressure when the drying rate was higher.     

Drying kinetics and quality of carrots subjected to microwave-assisted drying preceded by combined pulsed electric field and ultrasound treatment

Abstract

In the study the impact of combined pulsed electric field and ultrasound pretreatment on microwave-assisted air drying kinetics and quality of dried carrot was evaluated. Pretreatment of samples has been realized through sequential application of PEF followed by US or vice versa. Sonication was performed using either the contact or immersive method whereas PEF treatment was carried out at 5?kV·cm^?1 and 10 pulses. After pretreatment the samples were subjected to microwave-assisted convective drying. Combined pretreatment reduced drying time by 27–49%. The highest retention of carotenoids (93.7%) was noted for samples subjected to PEF treatment followed by contact sonication. Optical properties of pretreated carrots were similar to reference samples (without pretreatment) and total color change ranged from 2.8 to 5.8. Application of immersive sonication, regardless of the sequence, resulted in the highest rehydration capacity and the highest loss of soluble solids. Despite samples subjected to combined pretreatment exhibited the most porous structure, the hygroscopic properties remained almost unchanged in comparison to the reference sample.     

OSMO-CONVECTIVE DRYING OF GRAPES

ABSTRACT

In order to reduce browning of grapes during drying, a special drying method was developed and evaluated using a laboratory scale fluidized bed dryer. Fresh Thompson seedless grapes were initially dried by immersion in a fluidized bed of sugar. The mass ratio of grapes to sugar was 1:1. The flow rate of hot air (at 45 and 60^°C) was sufficient to fluidize the sugar bed, while grapes placed on the screen, 3 cm above the drying air distributor, remained generally stationary.

Due to the simultaneous osmotic and convection drying effects, the drying time was reduced by factor ～1.5 as compared to drying under the similar conditions without added sugar. A special pre-treatment of dipping of grapes in ethyl oleate (2% solution in 0.5% sodium hydroxide) at 80?deg; C for 30 s further reduced the drying time by factor 2 in both cases. The color of osmo-convective dried grapes were comparable to that of sulfur dioxide treated grapes. The texture of osmo-convective dried grapes was more pliable (softer) than convective dried samples. The major problem associated with die osmo-convective drying of grapes on a sugar bed was the stickiness, caused by sugar, on the fruit surface. This was reduced by partially substituting sugar with semolina (maintaining a 1:1 ratio) to create fluidized bed.     

Drying of Carrots in a Fluidized Bed. II. Design of a Model Based on a Modular Neural Network Approach

Abstract

The present paper deals with the design of a neural network type model for drying of carrots, which includes the associated transport mechanisms of the process. The model uses the operational variables and the time as input parameters. Two sub-layers of linear and sigmoidal nodes make up the hidden layer, to represent the external and internal resistances to the diffusion of water vapors during the drying process. The single output node weights the contribution of each mechanism of the drying process to predict the exit moisture content of the product. This model was used to predict the drying of carrot particles in a mechanically fluidized bed dryer reported in a previous paper [Reyes, A.; Alvarez, P.; Marquardt, F. Drying of carrots in a fluidized bed: I.- effects of drying conditions and modeling. Drying Technology 2002, 20 (7), 1463–1483.]. Simulated drying curves obtained with this model fits adequately the curves determined experimentally for the most operation conditions, which would indicate that this model is appropriate to be used for rough estimations in the design, the selection of optimal operational conditions, and the scaling up of dryers.     

Drying of Inorganic Particulate Compounds

ABSTRACT

Inorganic particulates are usually dried in a fixed bed, fluidized bed, or spray dryers. These compounds are easy to dry, once their physical structure, with high porosity, allows moisture content removal with low resistances. For fluidized bed of alumina particle evaluations, a laboratory-scale drying unit was built. The drying experiments were carried out with alumina particles with different diameters to evaluate temperature and air flow rate effects on drying kinetics and bed height. In another case, the dehydration of a mixture of rare-earth chlorides in a fluidized bed was studied, aiming at the production of anhydrous rare-earth chlorides, used to obtain mischmetal by electrolytic and metallothermic processes. The spray drying experiments were carried out in a pilot plant. Spray drying is a technique largely applied in industrial processes to dry solutions or suspensions, converting their solid parts into a dried powder. A set of rare-earth drying experiments was carried out, aiming at the development of techniques to obtain a powder that could satisfy international morphological requirements. The results allowed evaluating the effects of air flow rate, feed concentration, atomizer model, rotation velocity, and atomization pressure on powder density and particle size distribution.     

Evaluation Of Fluidized Versus Spouted Bed Drying Of Baker'S Yeast

ABSTRACT

Drying of baker's yeast (Saccharomyces cerevisiae) was examined experimentally in laboratory scale fluidized bed and spouted bed dryers. Pressedyeast samples (70% moisture content, wb) were prepared in granular form (0.8–1 mm diameter) and dried first in a fluidized bed to characterize their fluidizationand drying properties.

It was observed that high moisture yeast particles (up to 35% wb) belong to group C and D of Geldart's classification, especially for higher (>200 mm) bed heights making their fluidization very difficult. On the other hand, fluidization of pre-dried yeast particles with moisture content under 35%, wb (group B) was very easy. This characterization suggests a new concept for drying of baker's yeast i.e. spouted bed drying in the first stage and fluidized bed in the second stage or spouted bed drying for the entire process. The quality parameter examined during the drying process was viability of yeast cells at a standard moisture content (6–8%, wb).     

Effects of acid pretreatments on some physicochemical properties of carrot undergoing hot air drying

Carrot is one of the important root crops cultivated worldwide. In many cases, carrot must be dried prior to its use. Although hot air drying is one of the most common methods for drying carrots, it is well known that hot air drying leads to much quality degradation of a dried product. To alleviate the adverse effect of hot air drying many pretreatment methods have been proposed. In this study, the effects of pretreatments with citric acid, which is used as an anti-darkening agent and a texture-modifier, were investigated. Carrots were soaked in citric acid to pH of either 4 or 5 or blanched in citric acid to pH of either 4 or 5 prior to hot air drying. Untreated carrots, carrots soaked in water and carrots blanched in water were used as control samples. The effects of pretreatments, in combination with hot air drying at 70, 80 and 90 °C, on selected physicochemical properties of dried carrots, namely, color, shrinkage, rehydration ability, β-carotene content and cis–trans isomerization of β-carotene, were evaluated. It was noted that carrots, which were soaked and blanched in citric acid, were redder than the untreated sample. Although shrinkage of pretreated carrots was higher, the rehydration ability of the pretreated carrots was higher than that of the untreated sample. The β-carotene content of carrots with no pretreatment decreased continuously, whereas the contents of β-carotene in citric acid soaked and blanched carrots tended to be unchanged. However, isomerization of β-carotene in all samples was not significantly different.     

The effect of freezing on the hot air and microwave vacuum drying kinetics and texture of whole cranberries

Abstract

The aim of this study was to evaluate the effect of convective and cryogenic freezing, hot air convective drying (HACD) at 60, 70, 80, and 90?°C and microwave vacuum drying (MWVD) at 100, 150, 200, 300, 450, and 500?W on the drying kinetics and texture of whole cranberries. Effective moisture diffusivities and drying rates were higher, whereas drying times were shorter for the samples dried by MWVD compared with the samples processed by HACD. The drying kinetics of cranberries during MWVD was discussed based on the hypothesis postulating that changes in the drying rate of cranberries during MWVD can be explained by and correlated with changes in the pressure gradient on material surface. Cranberries processed by MWVD were characterized by significantly greater hardness, gumminess, and chewiness in comparison with HACD samples. MWVD was found to be an effective method for producing dried snacks characterized by hard and crispy texture and considerable resistance to stress associated with manufacturing, packaging, storage, and delivery. HACD produced brittle fruit that were difficult to store and transport and were not fully suitable for direct consumption. Convective freezing before MWVD improved the overall appearance of cranberries, whereas cryogenic freezing combined with high temperature HACD adversely influenced the drying rate and produced dried cranberries with suboptimal overall appearance.     

内热式惰性粒子流化床中膏状物料干燥模型

通过对惰性粒子流化床中膏状物料干燥机理的分析,得到了干燥时间及单位面积床层水分汽化量的数学计算式,可对干燥器的性能进行预测。采用带浸没加热管的惰性粒子流化床对膏状钛白物料进行干燥中试研究,采用气流式喷嘴将膏状物料分散成200～400μm的小液滴喷洒在惰性粒子表面进行干燥,探索了适宜的干燥条件,测定和确定了最佳的干燥工艺参数、操作参数和设备参数。结果表明:该干燥工艺能强化床内传热传质,促进高黏性膏糊状物料很好地分散,床层温度分布均匀,干燥器的操作弹性大,热量消耗低,干燥强度高,传热系数可达300W·m-2·K-1以上。     

Drying Characteristics of Porous Materials in a Fluidized Bed under Reduced Pressure

Abstract

The drying of porous materials immersed in the fluidized bed under reduced pressure was performed, and the results were compared with those of hot air drying. The pressure in drying chamber was changed (5.0–101.3 kPa) and the effect of it was examined.

The temperature of the sample center becomes lower as the pressure in drying chamber decreases, and the temperature in fluidized-bed drying is higher than that in hot air drying at the same pressure. The effect of pressure in drying chamber on the sample temperature is significant for different temperatures of drying gas.     

THESES/DISSERTATIONS/1963-1983, POLAND

ABSTRACT

A fluidized bed of inert particles lpar;packing)--> can be used advantageously for the drying of paste-like materials of high moisture content. Wet pasty material is fed into a fluidized bed of chemically inert coarse particles. The wet material coats the surface of the inert particles. Drying takes place mainly in the thin layer formed on the surface of particles. After reaching a certain moisture content, the dried material film breaks off the surface of the packing particles, and leave the fluidized bed as a fine powder in the exit gas stream.

Experiments were performed using different organic and inorganic materials e.g. raw materials from human and veterinary     

OSMO-CONVECTIVE DRYING OF GRAPES

In order to reduce browning of grapes during drying, a special drying method was developed and evaluated using a laboratory scale fluidized bed dryer. Fresh Thompson seedless grapes were initially dried by immersion in a fluidized bed of sugar. The mass ratio of grapes to sugar was 1:1. The flow rate of hot air (at 45 and 60^°C) was sufficient to fluidize the sugar bed, while grapes placed on the screen, 3 cm above the drying air distributor, remained generally stationary.

Due to the simultaneous osmotic and convection drying effects, the drying time was reduced by factor ∼1.5 as compared to drying under the similar conditions without added sugar. A special pre-treatment of dipping of grapes in ethyl oleate (2% solution in 0.5% sodium hydroxide) at 80†deg; C for 30 s further reduced the drying time by factor 2 in both cases. The color of osmo-convective dried grapes were comparable to that of sulfur dioxide treated grapes. The texture of osmo-convective dried grapes was more pliable (softer) than convective dried samples. The major problem associated with die osmo-convective drying of grapes on a sugar bed was the stickiness, caused by sugar, on the fruit surface. This was reduced by partially substituting sugar with semolina (maintaining a 1:1 ratio) to create fluidized bed.     

Design and performance evaluation of a pilot-scale pulsed vacuum infrared drying (PVID) system for drying of berries

Abstract

The objective of current work was to develop a new pilot-scale pulsed vacuum infrared drying (PVID) system for the drying of berries. The system design and drying performance evaluation for grape and goji berries are reported here. The PVID system consisted of three major sections, including infrared heating section, vacuum section, and a controlling section. Electrical infrared (IR) emitter was made with carbon fiber sheet at the thickness of 2–4?mm, which emitted IR wavelength of 1–30?μm. The control system was used to achieve paused pressure by switching the drying chamber pressure between atmospheric pressure (101?kPa) and vacuum (3–10?kPa) successively and maintaining the pressure for different time periods. Grapes and goji berries were dried in a single layer at three different temperatures (55, 65, and 75?°C) with an atmospheric duration ranging from 1 to 12?min and vacuum duration from 10?min to constant vacuum. The drying characteristic and quality of PVID dried berries were investigated and compared with that of hot air (HA) dried berries. Results showed that the IR heating temperature, atmospheric duration, and vacuum duration had significant effects (p?<?.05) on the drying time. The optimum drying conditions for grapes and goji berries were IR heating at 65?°C, vacuum duration of 15?min, and atmospheric duration of 4 and 2?min, respectively. The corresponding drying time is nearly 720?min for grapes, and 450?min for goji berries. The PVID dried grapes and goji berries had more attractive color than that of HA dried ones. These findings demonstrated that PVID should be a promising method to produce high-quality dried berries. This study laid a good foundation for scaling up the technology in the future.     

Drying of Carrots in a Fluidized Bed. II. Design of a Model Based on a Modular Neural Network Approach

The present paper deals with the design of a neural network type model for drying of carrots, which includes the associated transport mechanisms of the process. The model uses the operational variables and the time as input parameters. Two sub-layers of linear and sigmoidal nodes make up the hidden layer, to represent the external and internal resistances to the diffusion of water vapors during the drying process. The single output node weights the contribution of each mechanism of the drying process to predict the exit moisture content of the product. This model was used to predict the drying of carrot particles in a mechanically fluidized bed dryer reported in a previous paper [Reyes, A.; Alvarez, P.; Marquardt, F. Drying of carrots in a fluidized bed: I.- effects of drying conditions and modeling. Drying Technology 2002, 20 (7), 1463-1483.]. Simulated drying curves obtained with this model fits adequately the curves determined experimentally for the most operation conditions, which would indicate that this model is appropriate to be used for rough estimations in the design, the selection of optimal operational conditions, and the scaling up of dryers.     

Vibrofluidized Bed Drying of Adipic Acid

ABSTRACT

Adipic acid is a white crystalline solid and is the most industrially important of all aliphatic dicarboxylic acids. Drying is the principal unit operation after its crystallization. The acid has a tendency to agglomerate when moistened, due to its high cohesiveness. The fluidization regime does not occur and the particles cannot be adequately treated in a conventional fluidized bed. This article deals with the drying of adipic acid in a vibrofluidized bed. The equipment used in the drying tests was specially designed and constructed to maintain the fluidized regime of fine particles of moistened adipic acid. The drying curves presented the constant and the falling rate periods, yielding final moisture contents that depend mainly on the drying temperature. The quality of the dried acid was also verified by the crystals' granulometric analysis, which showed breakage of crystals for drying at higher air temperatures. Results showed to be satisfactory for drying adipic acid with moisture content up to 5.1% w.b. in the vibrofluidized bed, keeping a high degree of bed homogeneity and quality of the fluidized state.     

Drying of Inorganic Particulate Compounds

Inorganic particulates are usually dried in a fixed bed, fluidized bed, or spray dryers. These compounds are easy to dry, once their physical structure, with high porosity, allows moisture content removal with low resistances. For fluidized bed of alumina particle evaluations, a laboratory-scale drying unit was built. The drying experiments were carried out with alumina particles with different diameters to evaluate temperature and air flow rate effects on drying kinetics and bed height. In another case, the dehydration of a mixture of rare-earth chlorides in a fluidized bed was studied, aiming at the production of anhydrous rare-earth chlorides, used to obtain mischmetal by electrolytic and metallothermic processes. The spray drying experiments were carried out in a pilot plant. Spray drying is a technique largely applied in industrial processes to dry solutions or suspensions, converting their solid parts into a dried powder. A set of rare-earth drying experiments was carried out, aiming at the development of techniques to obtain a powder that could satisfy international morphological requirements. The results allowed evaluating the effects of air flow rate, feed concentration, atomizer model, rotation velocity, and atomization pressure on powder density and particle size distribution.     

Effects of High-Temperature Fluidized Bed Drying and Tempering on Kernel Cracking and Milling Quality of Vietnamese Rice Varieties

Studies on the effects of high-temperature fluidized bed drying and tempering on physical properties and milling quality of two long-grain freshly harvested Vietnamese rice varieties, A10 (32±1% wet basis moisture) and OM2717 (24.5±0.5% wet basis moisture), were undertaken. Rice samples were fluidized bed dried at 80 and 90°C for 2.5 and 3.0 min, then tempered at 75 and 86°C for up to 1 h, followed by final drying to below 14% moisture (wet basis) at 35°C by thin-layer drying method. Head rice yield significantly improved with extended tempering time to 40 min. Head rice yield tended to increase with decreasing cracked (fissured) kernels. The hardness and stiffness of sound fluidized bed dried rice kernels (in the range of 30–55 N and 162–168 N/mm, respectively) were higher than that of conventionally dried ones (thin layer dried at 35°C). The color of milled rice was significantly (P < 0.05) affected by high-temperature fluidized bed drying, but the absolute change in the value was very small.     

THE DESIGN AND PERFORMANCE OF CONTINUOUS WELL-MIXED FLUIDIZED BED DRYERS - AN ANALYTICAL APPROACH

Abstract

In a previous study, a unified approach to the calculation of the size and energy consumption of well-mixed fluidized bed dryers was developed. A computer code was written and a parametric study undertaken to determine the effects of the principal operating variables. The results were shown to depend on the form of the drying kinetics. The present paper describes the formulation of an analytical model of a well-mixed fluidized bed dryer, which applies for materials that exhibit linear falling-rate drying. These materials are typified by small hygroscopic particles, which are frequently dried in such equipment. Equations are presented that enable the drying time, specific energy consumption, heater duty and bed area to be calculated. The predictions of these equations agreed well with previously published experimental findings and with the results of computer simulations for a typical solid (ion exchange resin) exhibiting the required drying characteristics.