Influence of Osmotic Pretreatment on Kinetics of Convective Drying and Quality of Apples

This article examines the influence of apple osmotic dehydration (OD) on the kinetics of convective drying and the quality of the products obtained. Fresh apples (Malus domestica), variety Ligol, were used as the experimental material. Samples were first dehydrated in three osmotic agents (glucose, fructose, and sucrose), each at a concentration of 40%, for 30, 60, and 120 min, and then dried convectively. Efficiency of osmotic dewatering was assessed on the basis of water loss (WL) and solids gain (SG). The influence of OD on convective drying kinetics was analyzed by comparing the drying curves and the basic kinetic parameters (time and drying rate). Quality of products was assessed on the basis of colorimetric measurements and sensory evaluation. It was found that OD significantly improved the quality of dried products through better retention of the original color, shape, texture, aroma, and taste, but it did not remarkably affect the rate of convective drying.     

Effect of Immersion Time in Osmosis and Ultrasound on Papaya Cell Structure during Dehydration

The effect of ultrasound-assisted osmotic dehydration applied at atmospheric pressure for different lengths of time on papaya tissue structure was evaluated. Ultrasound induced the loss of cellular adhesion, formation of large cell interspaces, and light rupture of the cell walls. The changes in the tissue structure caused by ultrasound application increased sugar loss, water loss, and effective water diffusivity. Ultrasound-assisted osmotic dehydration induced a gradual distortion in the shape of the cells, loss of cellular adhesion, and the formation of large channels caused by rupture of the cell walls. The changes caused by the application of osmotic dehydration resulted in high water loss and sugar gain.     

Ultrasound-Assisted Osmotic Dehydration of Strawberries: Effect of Pretreatment Time and Ultrasonic Frequency

Pretreatment of fruits prior to drying has shown success in reducing drying time and costs. In this work, ultrasound-assisted osmotic dehydration has been implemented as a method to increase water diffusivity and reduce drying time in strawberries. Strawberry halves were immersed in distilled water and in two different concentrations of sucrose solutions while pretreatment time and ultrasonic frequency levels were varied to determine their effect on drying time, water loss, and soluble solids gain. A microscopic analysis was carried out to evaluate the formation of microchannels and other changes to the fruit tissue structure. Greater sucrose concentration used in ultrasound-assisted osmotic dehydration resulted in greater water loss with greatest loss observed for the strawberry halves pretreated for 45 min in a 50% w/w sucrose solution. The pretreatment carried out for 30 min employing an osmotic solution of 50% w/w of sucrose resulted in the highest drying rate among the pretreatments. Osmotic dehydration used alone during pretreatment increased total processing time, whereas osmotic dehydration combined with ultrasonic energy during pretreatment reduced total processing time and increased effective water diffusivity. Cell distortion and breakdown were observed not only in pretreatments employing ultrasound-assisted osmotic dehydration but in conventional osmotic dehydration. Formation of microchannels through ultrasonic application and effects of osmotic pressure differential were considered to be largely responsible for reducing drying time for strawberry halves.     

Osmotic Dehydration Pretreatment in Drying of Fruits and Vegetables

Several vegetables and fruits, apple, ginger, carrot, and pumpkin were dehydrated under various osmotic conditions using sucrose and salt as the permeating agents. The dehydrated materials were then dried. The influence of solute concentration, process temperature and the type of solute on osmotic dehydration and further thermal drying were investigated. The nutrition loss during the osmotic process was measured using carotene as the nutrition index. The effect of calcium chloride present in osmotic solution on the product quality was also studied. A first order kinetic model was chosen to describe the mass transfer phenomena of the osmotic process. The equilibrium value of water loss, solute gained, kinetic constants K_WL and K_SG under various conditions are successfully predicted by the model. The relationship between the equilibrium value and four major factors that influence osmotic process of carrot was obtained based on the experimental data. The relations between the loss constant of carotene and the solute concentration in carrot and pumpkin were obtained based on the experimental data. The qualities of dried products are better for the osmotic dehydration pretreated samples than those dried directly.     

Effect of Osmotic Treatment on Drying Kinetics and Antioxidant Properties of Dried Mulberry

Air drying is widely used in fruit preservation. However, this causes a reduction in the nutritive value of the product. Osmotic treatment has been used as prior to drying. The objectives of this research were to study osmotic treatment in combination with air drying of mulberries and to evaluate the influence of different osmotic solutions on drying kinetics, reaction kinetics, and antioxidant capacity, with a focus on anthocyanins and phenolics. Mulberries, either untreated or osmotically treated with sucrose, sorbitol, and maltose, were dried in a tray dryer at 60°C with a velocity of 1 m/s. The drying kinetics of mulberries were explained using the Page model based on the R ² values, which ranged from 0.985 to 0.993, and E _s , which ranged from 0.031 to 0.091. Air drying caused degradation of anthoyanins and phenolics, which followed a zero-order reaction with R ² values ranged from 0.866 to 0.996. Osmotic treatment with maltose was found to be a good treatment for mulberry drying and preserved the phenolic and anthocyanin contents and provide high antioxidant capacity.     

Effects of Osmotic Treatment and Superheated Steam Puffing Temperature on Drying Characteristics and Texture Properties of Banana Slices

A porous structure is an important characteristic of crisp food and can be produced by a puffing process. However, puffed products may brown during puffing. To limit the browning reaction, food, in particular fruits, needs to be osmotically treated before puffing. This research therefore studied how osmotic treatment affects the quality of a puffed fruit sample, viz. banana. Banana with 20–23° Brix total soluble solid was immersed in sucrose solution concentrations at 30, 35, and 40° Brix and dried at 90°C using hot air until the sample moisture content was reduced to 30% dry basis. Then the banana slices were puffed using superheated steam at 180, 200, and 220°C for 150 s and dried again at 90°C until the sample moisture content reached 4% (db). It was found that osmotic dehydration could improve the color of puffed banana, with less browning than the non-osmotically treated puffed banana because the amounts of glucose and fructose in banana, which serve as important reagents for browning reactions, were decreased. The puffing temperature and osmotic concentrations did not enhance the browning rate. Sucrose impregnation resulted in longer drying times and limited banana cell wall expansion due to the interaction between the hydroxyl group of sucrose and that of banana tissue. This interaction further resulted in significantly higher shrinkage of the osmotically treated sample and a denser structure as viewed by scanning electron microscopy. The morphology of osmotically treated banana was a hard and brittle texture.     

Effect of Pretreatments on Drying and Rehydration Kinetics and Color of Sweet Potato Slices

The effect of air temperature and pretreatments (KMS: citric acid) on drying kinetics of sweet potato slices was investigated. Drying experiments were performed in a tray dryer. In falling rate period, moisture transfer from sweet potato slices was described by applying the unsteady-state Fickian diffusion model, and the rate constant (k) were calculated. The effect of temperature on k could be interpreted according to Arrhenius law. Drying rate and therefore k values were found to be affected by pretreatments. Rehydration rates of dried sweet potato slices at 25, 40, 80°C were also determined and found to be independent of drying conditions and rehydration temperature. The ΔE value was found to be the highest for slices treated at 50°C with 0.5:1.0% KMS and citric acid.     

Drying of Mushroom Slices Using Hot Air Combined with an Electrohydrodynamic (EHD) Drying System

In this study, hot air combined with EHD drying is examined as an improved method for drying mushroom slices. The effects of three levels of voltage (17, 19, and 21 kV) and electrode gap (5, 6, and 7 cm) on the drying kinetics, time, effective water diffusion coefficient, and energy consumption of the EHD, pure hot air (provided within a chamber), and hot air combined with EHD drying systems were investigated. ANOVA showed that there were significant differences between EHD treatments and control (pure hot-air-drying treatment) for all of the investigated responses. Voltage and electrode gap factors had significant effects on all investigated responses. The results confirmed that the combination of EHD and hot air can significantly reduce the drying period, resulting in a greater effective water diffusion coefficient and drying rate and reduced energy consumption. As such, this technique offers a promising solution to the considerable energy consumption of the drying industry.     

Water Diffusivity and Drying Kinetics of Air Drying of Figs

The aim of this study is to evaluate experimentally the effective diffusion coefficient and the drying kinetics of whole unpeeled figs (Ficus carica L. var. tsapela) in terms of drying conditions. Estimation of the effective diffusion coefficient was carried out employing Fick's law for unsteady diffusion, which was solved analytically and numerically. In both methods, shrinkage effect was considered. The results from the two methods were compared and presented together with the limited results from the literature. The estimated effective diffusion coefficient values by both methods were fitted to a modified Arrhenius equation. Finally, a model predicting drying kinetics was developed. The model's coefficients were associated to the experimental conditions.     

Application of Ultrasound and Ultrasound-Assisted Osmotic Dehydration in Drying of Fruits

This work examines the influence of ultrasonic and ultrasonic assisted osmotic dehydration pretreatments on the dehydration of eight fruits (banana, genipap, jambo, melon, papaya, pineapple, pinha, and sapota). An overview of the effects of ultrasound application on water loss, sugar gain, and effective diffusivity of water during the dehydration process is presented. The results showed significant differences for water loss and sugar gain among the fruits that were studied, which were analyzed based on the changes observed on the tissue structure of the fruit. The results also showed that the effective diffusivity of water in the fruit increased after application of ultrasound reducing air-drying time.     

Effects of Acetic Acid Pretreatment and Hot Air Drying on Resistance of Salmonella on Cabbage Slices

The heat resistance of Salmonella anatum inoculated onto the surface of cabbage slices as affected by acetic acid pretreatment (0.5–1.5% v/v) and hot air drying at 50–60°C was investigated. Approximately 1.5 log₁₀ of Salmonella numbers was reduced after soaking the vegetables in acetic acid solution. The inhibitory effect of acetic acid on the bacteria was more pronounced during drying. The heat resistance of Salmonella on the cabbage surface treated with acetic acid decreased considerably compared with untreated cabbage. Moreover, the vegetable treated with higher acetic acid concentration resulted in a higher degree of acid injury and hence increased the susceptibility to heat during drying. The results revealed that acetic acid pretreatment may be used as an additional processing step to increase the safety margin of dried vegetable products.     

Mechanical Means of Enhancing Drying Rates: Effect on Drying Kinetics and Quality

ABSTRACT

The effect of various pretreatments on drying kinetics as well as quality parameters of selected vegetables and fruits was studied experimentally in a convective dryer. The pretreatments tested include application of pinholes and drilled holes of different diameters and densities, blanching and freezing. Potato, cassava, dragon fruit and red chilli were used as the sample drying materials. As expected, increase of the diameter and density of the holes under fixed drying conditions increased the drying rate. Samples with drilled holes showed better drying performance than those with pinholes. The improvement of the drying rate was found to be dependent on the physical properties and the initial moisture content of the material. Results show that the drying performance of the products that possess a skin of low moisture diffusivity, such as chilli, can be improved significantly by blanching and making perforations in the skin. Overall changes of color and volume shrinkage during drying of the samples with and without holes were similar. The effect of different modes of heat input, such as convection, conduction and radiation, on the drying performance of the treated samples was also experimentally investigated. On-off type controllers were used to regulate the power of the heaters and maintain the product temperature within a range of pre-set values. Convection combined with radiation displayed the highest drying rate.     

Mechanical Means of Enhancing Drying Rates: Effect on Drying Kinetics and Quality

The effect of various pretreatments on drying kinetics as well as quality parameters of selected vegetables and fruits was studied experimentally in a convective dryer. The pretreatments tested include application of pinholes and drilled holes of different diameters and densities, blanching and freezing. Potato, cassava, dragon fruit and red chilli were used as the sample drying materials. As expected, increase of the diameter and density of the holes under fixed drying conditions increased the drying rate. Samples with drilled holes showed better drying performance than those with pinholes. The improvement of the drying rate was found to be dependent on the physical properties and the initial moisture content of the material. Results show that the drying performance of the products that possess a skin of low moisture diffusivity, such as chilli, can be improved significantly by blanching and making perforations in the skin. Overall changes of color and volume shrinkage during drying of the samples with and without holes were similar. The effect of different modes of heat input, such as convection, conduction and radiation, on the drying performance of the treated samples was also experimentally investigated. On-off type controllers were used to regulate the power of the heaters and maintain the product temperature within a range of pre-set values. Convection combined with radiation displayed the highest drying rate.     

Effect of Microwave Energy on Vacuum Drying Kinetics of Alginate-Starch Gel

Microwave energy drying under vacuum was investigated for alginate-starch hydrogel. Drying was conducted using 2450 MHz microwave energy at 25 mm Hg absolute pressure and different power levels; e.g., 300, 500, 700, 900, and 1100 watts. Drying was continued until final moisture content of the sample reached less than 1% wet basis. Moisture loss during drying was measured at 3-min intervals. Drying kinetics were used to describe both macroscopic and microscopic mechanisms of heat and mass transfer. Experimental drying kinetic data were fitted to a mathematical model. Experimental drying data points were fitted to an empirical model equation.     

Effect of Microwave Energy on Vacuum Drying Kinetics of Alginate-Starch Gel

Microwave energy drying under vacuum was investigated for alginate-starch hydrogel. Drying was conducted using 2450 MHz microwave energy at 25 mm Hg absolute pressure and different power levels; e.g., 300, 500, 700, 900, and 1100 watts. Drying was continued until final moisture content of the sample reached less than 1% wet basis. Moisture loss during drying was measured at 3-min intervals. Drying kinetics were used to describe both macroscopic and microscopic mechanisms of heat and mass transfer. Experimental drying kinetic data were fitted to a mathematical model. Experimental drying data points were fitted to an empirical model equation.     

Effect of Drying Temperature and Pretreatment on Protein Content and Color Changes during Fluidized Bed Drying of Finger Millets (Ragi,Eleusine coracana) Sprouts

Value addition to several agri-products such as grains, beans, and millets can be achieved by sprouting followed by thermal treatment. These value-added products can then be suitably formulated and fortified using cost-effective technologies so that they may be used as instant food, weaning food for babies, or functional food. Finger millets, used extensively by Asian and African communities, were studied. Sprouted finger millets were blanched (using hot water and steam) and dried using a fluidized bed dryer. This led to quality enhancement in terms of higher protein content, digestibility, and bioavailable micronutrients. The drying of sprouted millets (ragi in this case) over the temperature range of 40–70°C was studied. Later, the effect of infrared and pan roasting was studied to determine its effect on protein content and other sensory parameters. Dehydrated sprouted finger millets flour can be used for making noodles, papads, unleavened bread, and several other Indian cuisines such as idli, chakli, porridge, and gruel for newborns.     

Effect of Ultrasonically Induced Nucleation on the Drying Kinetics and Physical Properties of Freeze-Dried Strawberry

Power ultrasound has proved to be very useful in controlling crystallization processes since sonication can enhance both the nucleation rate and crystal growth rate by producing fresh and/or more nucleation sites. Therefore, in this study, power ultrasound was applied to control the freezing step of freeze-dried strawberry. The results showed that when power ultrasound was applied at different temperatures, it increased the nucleation temperature and shortened the characteristic freezing time. The application of power ultrasound in the freezing step increased the drying time in subsequently freeze-dried strawberry samples. This longer drying time was found to be due to increased resistance to moisture diffusion due to the formation of a network of small pores caused by sublimation of small ice crystals induced by the power ultrasound. Scanning electron microscopic images revealed that the freeze-dried sample frozen by ultrasound-assisted freezing (UAFD) had finer cellular structure compared to those frozen in other freezing conditions. UAFD samples had better textural hardness, while the rehydration capacity was lower compared to those of NRFD and NIFD samples.     

石灰调质污泥恒温干燥特性及动力学模型研究

对添加石灰的市政污泥进行恒温干燥实验,研究在不同的石灰添加量和不同的温度条件下污泥干燥特性、有效水分扩散系数和干燥活化能。结果表明:高温干燥时,石灰添加量的增加对平均干燥速率以及有效扩散系数的提高影响显著,低温干燥时影响不显著;石灰的添加可以降低干燥活化能;基于Modified Page模型建立的通用干燥模型能准确地描述石灰调质污泥的干燥特性;模型方程预测值和实验值吻合,均方根误差为0.32%。     

Process-Based Drying Temperature and Humidity Integration Control Enhances Drying Kinetics of Apricot Halves

The effects of drying temperature (50, 60, and 70°C) and absolute humidity (65, 90, 115, 140 g/kg at the initial stage) on drying kinetics and color attributes of apricot halves under process-based drying temperature and humidity integration control (PDTHIC) were investigated. Results indicated that appropriate PDTHIC could reduce the drying time by 18.75% compared to the control group. The absolute humidity parameter should be controlled well rather than continuously dehumidify for traditional practices. The moisture effective diffusivity (D_eff) at 70°C presented drastic fluctuations with increasing absolute humidity parameters, calculated using the Weibull distribution model. The activation energy (E_a) of samples treated by continuous dehumidification and PDTHIC were 31.40 and 74.18 kJ/mol, respectively. The variation trend of color parameters was different from the conventional hot air drying probably due to the dehydration mechanism. Observation of the microstructure of the dried samples indicated that the PDTHIC process can enhance the drying rate of apricot through generating a larger pore network compared to continuous dehumidification at the same temperature. The findings of the current work clearly indicated that process-based drying temperature and humidity integration control enhances the drying kinetics of apricot halves, leading to a promising technology for energy savings.     

A Comparative Study of Pork Drying Using Superheated Steam and Hot Air

Drying with superheated steam and hot air were comparatively studied for pork. Transport, physical, and chemical properties, i.e., effective diffusivity, color, microstructure, and rehydration ability, were investigated. The experimental results have shown that the decrease of pork moisture content in an early drying time was more rapid in superheated steam than in hot air and appeared to be lower in a latter time. The pork surface as examined by SEM was noticeably different for the samples dried by superheated steam and hot air. The fewer pores at the surface of superheated steam-dried pork caused a slower water penetration into the interior during rehydration, in addition to the lower drying rate. The color of the product from superheated steam was a relatively more intense brown than that obtained from hot air as presented by a lower L? value and a higher a? value.