Air-Drying Kinetics of Osmotically Dehydrated Fruits

ABSTRACT

The air drying kinetics of fresh and osmotically dehydrated fruits (apples) was determined. Two sugars, glucose and sucrose, were used as osmotic dehydration agents. Three levels of sugar concentration (15%, 30% and 45%) and several times of immersion into the sugar solution were used. Following the osmotic preconcentration, the fruit samples were dried at 55°C and the weight of material was recorded. The effective water diffusivity of samples treated under various osmotic conditions was estimated and the results were related to the sugar content and the bulk porosity of the samples. The effective water diffusivity, resulting from the application of the diffusion equation to the drying kinetics of the apples was found to decrease significantly for the samples pretreated by a concentrated sugar solution (e.g. 45%), evidently due to the lower porosity and other physicochemical factors. The low diffusivity may be beneficial in the storage stability and utilization of dehydrated fruits.     

Effect of Pulsed Electric Field and Osmotic Dehydration Pretreatment on the Convective Drying of Carrot Tissue

The influence of pulsed electric field (PEF) and subsequent centrifugal osmotic dehydration (OD) on the convective drying behavior of carrot is investigated. The PEF was carried out at an intensity of E = 0.60 kV/cm and a treatment duration of t _PEF  = 50 ms. The following centrifugal OD was performed in a sucrose solution of 65% (w/w) at 40°C for 0, 1, 2, or 4 h under 2400 × g. The drying was performed after the centrifugal OD for temperatures 40–60°C and at constant air rate (6 m³/h).

With the increase of OD duration the air drying time is reduced spectacularly. The dimensionless moisture ratio Xr = 0.1 is reached for PEF-untreated carrots after 370 min of air drying at 60°C in absence of centrifugal OD against 90 min of air drying after the 240 min of centrifugal OD. The PEF treatment reduces additionally the air drying time. The total time of dehydration operations can be shortened when OD time is optimized. For instance, the minimal time required to dehydrate untreated carrots until Xr = 0.1 is 260 min (120 min of OD at 40°C and 140 min of drying at 60°C). It is reduced to 230 min with PEF-treated carrots.

The moisture effective diffusivity D _eff is calculated for the convective air drying based on Fick's law. The centrifugal OD pretreatment increases drastically the value of D _eff . For instance, 4 h of centrifugal OD permitted increasing the value of D _eff from 0.93 · 10^?9 to 3.85 · 10^?9 m²/s for untreated carrots and from 1.17 · 10^?9 to 5.10 · 10^?9 m²/s for PEF-treated carrots.     

EFFECT OF OSMOTIC DEHYDRATION ON VISCOELASTIC PROPERTIES OF APPLE AND BANANA

ABSTRACT

The rheological behavior of osmotically dehydrated apple and banana was examined under uniaxial compression and relaxation tests of cylindrical specimens. Compression and relaxation tests were performed, following air drying of osmotically pre-treated samples, at various moisture contents ranging from 0.1 to 1.5 kg/kg dry basis. The maximum stress and the corresponding strain were correlated to the moisture content using simple mathematical equations. It was shown that both parameters increase as water was removed and their values are significantly higher than the corresponding values for untreated air dried samples. The effect of moisture content on compressive behavior of osmotically dried materials was introduced through its effect on the four model parameters: the maximum stress (σ_max) the maximum strain (?_max), the elastic parameter (E) and the viscoelastic exponent (p). The stress relaxation data of osmotically treated samples were modeled using a two-term Maxwell model. It was shown that osmotic pre-treatment increased the remaining force and it decreased the relaxation time of dehydrated samples. The osmotic dehydration and therefore the sugar gain tend to increase the viscous nature of fruits and decrease their elasticity for both materials, causing plasticity of the structure.     

Effect of sucrose concentration of osmotic dehydration pretreatment on drying characteristics and texture of peach chips dried by infrared drying coupled with explosion puffing drying

Effects of osmotic dehydration (OD) pretreatment on the texture characteristics of peach chips after combination drying were investigated. Peach slices were immersed into 100, 300, and 500?g/L sucrose solution for 4?h, respectively, at room temperature and then predried to a critical moisture content of 0.5?kg water/kg dry matter that was determined by the effective moisture diffusivity (D_eff) curves under infrared drying at 80°C. The peach chips were then dried using explosion puffing drying (EPD). The sucrose solution with lower concentration (100?g/L) would improve the drying rate (DR) of peach slices during infrared drying. However, sucrose solution with higher concentration (500?g/L) might affect water diffusion, resulting in lower drying rate. The changes of texture characteristics of dehydrated peach were ascribed to sucrose uptake during the impregnation step. The content and constitutes of soluble sugars in peach tissue, which was significantly affected by OD treatment, were also detected in the research. The results indicated that the combined infrared and EPD drying, in which OD with appropriate concentration (300?g/L) was applied as pretreatment, could improve the drying characteristics and texture of peach chips.     

COMBINED OSMOTIC AND MICROWAVE DRYING OF STRAWBERRIES

ABSTRACT

Strawberries pretreated with 2% ethyl oleate and 0.5% NaOH were osmotically dehydrated and their osmotic dehydration rate is compared with untreated berries. It was found that treated berries dehydrated belter compared to untreated berries. Osmotically dehydrated berries were convective and microwave dried at different power levels and results were compared with respect to drying time and rate. The rehydration ratio, texture, color and sensory values are compared with freeze dried strawberries with the same pretreatment. It was found that microwave drying was short in time and the quality parameters of the microwave dried berries were comparable to those of freeze dried berries.     

EFFECT OF OSMOTIC DEHYDRATION ON VISCOELASTIC PROPERTIES OF APPLE AND BANANA

The rheological behavior of osmotically dehydrated apple and banana was examined under uniaxial compression and relaxation tests of cylindrical specimens. Compression and relaxation tests were performed, following air drying of osmotically pre-treated samples, at various moisture contents ranging from 0.1 to 1.5 kg/kg dry basis. The maximum stress and the corresponding strain were correlated to the moisture content using simple mathematical equations. It was shown that both parameters increase as water was removed and their values are significantly higher than the corresponding values for untreated air dried samples. The effect of moisture content on compressive behavior of osmotically dried materials was introduced through its effect on the four model parameters: the maximum stress (σ_max) the maximum strain (ε_max), the elastic parameter (E) and the viscoelastic exponent (p). The stress relaxation data of osmotically treated samples were modeled using a two-term Maxwell model. It was shown that osmotic pre-treatment increased the remaining force and it decreased the relaxation time of dehydrated samples. The osmotic dehydration and therefore the sugar gain tend to increase the viscous nature of fruits and decrease their elasticity for both materials, causing plasticity of the structure.     

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.     

Drying of Chestnuts (Castanea sativa Mill.) after Osmotic Dehydration with Sucrose and Glucose Solutions

Chestnuts were dehydrated by using a combined method of osmotic dehydration followed by air drying. Samples were osmotically pretreated with sucrose (60% w/w) and glucose (56% w/w) for 8 h, air-dried at temperatures of 45, 55, and 65°C, at a relative humidity of 30% and at a velocity of 2.7 m·s^?1 and the experimental data of the drying kinetics were obtained. Whole samples were dried with different peelings: (a) removal of endocarp and pericarp (peeled) and (b) additionally the internal rough surface (cut). In all cases, cut chestnuts show greater drying rates than peeled samples, indicating that a significant mass transfer resistance in the layer nearest to the surface takes place. Peeled samples pretreated with sucrose solutions behave in a similar way to untreated samples. For the rest of the samples, the cut samples osmotically treated with sucrose solutions and all the samples treated with the glucose solution, the drying rates decrease during drying. Drying kinetics are successfully modeled by employing a diffusional model that takes the shrinkage into account. The effective coefficient of water diffusion was evaluated and correlated with temperature. The quality of the final product was monitored by color change. In spite of the fact that the total color difference is not modified by the osmotic treatment, the L?, a?, and b? color coordinates of cut samples treated with sucrose and glucose solutions do undergo changes; the L? and a? coordinates change less than the b?.     

Application of Osmotic Dehydration Processes to Produce Apple Slices Enriched with β-Carotene

The objective of this work was the impregnation of β-carotene in apple slices by osmotic dehydration (OD) at atmospheric pressure and by pulsed vacuum osmotic dehydration (PVOD). Osmotic solutions were sucrose solutions of 30, 40, and 50 °Brix containing β-carotene (0.01 g/mL). Maximum impregnation using OD treatments was 1.5, 3.5, and 4.1 mg β-carotene/g ds. When using PVOD, significantly shorter processing times were required and impregnation reached 4.7, 5.5, and 6 mg β-carotene/g ds. Values of a_w obtained by OD were 0.973, 0.967, and 0.960 while by PVOD were 0.960, 0.930, and 0.880. Results showed that PVOD was a good option to impregnate apple with β-carotene.     

COMBINED OSMOTIC AND MICROWAVE DRYING OF STRAWBERRIES

Strawberries pretreated with 2% ethyl oleate and 0.5% NaOH were osmotically dehydrated and their osmotic dehydration rate is compared with untreated berries. It was found that treated berries dehydrated belter compared to untreated berries. Osmotically dehydrated berries were convective and microwave dried at different power levels and results were compared with respect to drying time and rate. The rehydration ratio, texture, color and sensory values are compared with freeze dried strawberries with the same pretreatment. It was found that microwave drying was short in time and the quality parameters of the microwave dried berries were comparable to those of freeze dried berries.     

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.     

Drying of Chestnuts (Castanea sativa Mill.) after Osmotic Dehydration with Sucrose and Glucose Solutions

Chestnuts were dehydrated by using a combined method of osmotic dehydration followed by air drying. Samples were osmotically pretreated with sucrose (60% w/w) and glucose (56% w/w) for 8 h, air-dried at temperatures of 45, 55, and 65°C, at a relative humidity of 30% and at a velocity of 2.7 m·s^-1 and the experimental data of the drying kinetics were obtained. Whole samples were dried with different peelings: (a) removal of endocarp and pericarp (peeled) and (b) additionally the internal rough surface (cut). In all cases, cut chestnuts show greater drying rates than peeled samples, indicating that a significant mass transfer resistance in the layer nearest to the surface takes place. Peeled samples pretreated with sucrose solutions behave in a similar way to untreated samples. For the rest of the samples, the cut samples osmotically treated with sucrose solutions and all the samples treated with the glucose solution, the drying rates decrease during drying. Drying kinetics are successfully modeled by employing a diffusional model that takes the shrinkage into account. The effective coefficient of water diffusion was evaluated and correlated with temperature. The quality of the final product was monitored by color change. In spite of the fact that the total color difference is not modified by the osmotic treatment, the L*, a*, and b* color coordinates of cut samples treated with sucrose and glucose solutions do undergo changes; the L* and a* coordinates change less than the b*.     

Ultrasound-assisted osmotic process on quality of microwave vacuum drying sweet potato

The effects of pretreatment before microwave vacuum drying (MVD) on texture, color, expansion, rehydration, drying rate, microstructure, sensory evaluation, and other properties of sweet potato were investigated in this study. The pretreatment consisted in five processing conditions, using blanching; osmotic dehydration at 35°Brix of sucrose (OD); ultrasound in distilled water (US); ultrasound in distilled water before osmotic dehydration (US?+?OD), and ultrasound-assisted osmotic dehydration (USOD). Pretreatments of sweet potato before MVD have shown success in reducing drying time with US treatment relatively more effective regarding drying time than other treatments. Compared with other treatments, US showed the highest rehydration ratio values. The osmotic group pretreatment exhibited a pronounced effect on water loss and solid gain, improved the color, aroma, and taste of dried sweet potato, whereas sucrose impregnation resulted in a hard texture observed with OD sample. USOD samples had a higher expansion ratio, lower hardness and color difference values, appeared less cell damaged, and recorded better overall quality than the other samples. There was a slight difference between USOD and US?+?OD samples. Combining osmotic dehydration with ultrasound as a pretreatment can significantly accelerate the heat transfer rate, reducing the dried time accordingly and increasing energy efficiency.     

Effects of ultrasound-assisted air-drying on vitamins and carotenoids of cherry tomatoes

This work examined the influence of the ultrasonic-assisted air-drying on the dehydration of cherry tomatoes (Solanum lycopersicum var. cerasiforme) and on the availability of vitamins B, E, and carotenoids in the dried product. This study allowed estimating the effective water diffusivity for the air-drying process and for the air-drying process subjected to ultrasonic waves. The water effective diffusivity increased by 33–89%, depending on the operating conditions, when subjected to ultrasound. The application of ultrasound increased the availability of vitamins B₁, B₂, B₃, B₆, and B₅, releasing the vitamins bounded to membrane, protein, or apoenzyme. The use of ultrasound allowed the retention of carotenoids in the dried product when drying was carried out at a low temperature (45°C) and low air velocities (1 m/s).     

Application of osmotic dehydration and microwave drying to strawberries coated with edible films

Osmotic dehydration (OD) is one of the conventional methods to increase the shelf-life of vegetables and fruits. However, the operating conditions can adversely affect the organoleptic and nutritional quality of fresh products due to the high sugar uptake during processing and the loss of water-soluble constituents to the osmotic medium. The application of edible films has attracted interest due to their ability to reduce the entry of solutes and simultaneously increase the removal of water during OD treatments. Microwave (MW) is one of the most effective emerging technologies to accelerate dehydration processes in vegetable matrices. This aspect is particularly relevant in strawberries, given its high content in bioactive and nutritional compounds. Thus, the aim of this paper was to assess the influence of edible film application (alginate–lactate) in strawberry slabs during combined OD-microwaves dehydration processes. Samples of 1?cm thickness were treated with sucrose solution (60°Bx, 40?°C, 4?h) and then were dehydrated in a microwave oven (1.2?W/g). The application of alginate–lactate edible films resulted in similar weight and water losses but lower solids gain compared to uncoated samples during OD treatment. OD pre-treated MW-dried strawberry slabs with moisture contents up to 0.15?kg dry basis were obtained after 100?min of MW drying. Besides, the effective diffusional coefficient (D_e) was estimated for MW drying process, values ranged from 4.5 to 8.8 10^?10 m²/s when shrinkage effect was considered, and from 1.1 to 2.3 10^?9 m²/s for constant thickness assumption.     

Enthalpy-Entropy Compensation and Water Transfer Mechanism in Osmotically Dehydrated Agar Gel

Enthalpy-entropy compensation and water transfer in osmotically dehydrated agar gel were studied by carrying out experiments at 30, 40, and 50°C in a 60% (w/w) sucrose solution. An additional experiment was carried out at the isokinetic temperature (T_B = 14°C) to confirm the physical meaning of T_B. When osmotic dehydration (OD) was carried out at the isokinetic temperature, the diffusion coefficient remained constant (≈0.54 × 10^?10 m²/s) during the entire process and the weight loss reached a limit (≈0.277 g/g) when the process was performed at T_B. Leffler's criterion indicated that diffusion mechanism was entropically controlled given the internal resistance developed during OD. Results were confirmed by the linear relationship found between the relaxation time and entropy variation according to the Adam and Gibbs equation.     

EFFECT OF OSMOTIC DEHYDRATION ON COLOR AND SORPTION CHARACTERISTICS OF APPLE AND BANANA

ABSTRACT

Color and sorption characteristics of osmotically treated and air dried apple and banana were studied during air drying at 70^°C. The color parameters: Lightness (L), Redness (a) and Yellowness (b) were studied, using a Hunter Lab chromatometer. A first order kinetic model was fitted to the experimental data adequately for color parameters, while sorption data for treated and air dried products were fitted to the GAB model. Untreated fruits showed an extensive browning which was monitored by a significant drop of the lightness (L) and an increase of redness (a) and yellowness (b). Osmotically pretreated samples did not brown as much as the untreated samples and the lightness L decreased only slightly while a, b increased slightly. Osmotic pretreatment resulted in a shift in sorption isotherm for both fruits. Osmotic dehydration prevented color damages and decreased the sorption capacity of dehydrated products.     

The Effects of Predrying Treatments and Different Drying Methods on Phytochemical Compound Retention and Drying Characteristics of Moringa Leaves (Moringa oleifera Lam.)

The most appropriate maturity stage of Moringa oleifera leaves was selected for drying based on phytochemical content, including quercetin and kaempferol. Desorption isotherms were developed and were best fit by the modified Henderson model. Prior to drying, samples were left untreated, blanched in boiling water, and blanched in NaHCO₃/MgO. The leaves were dried by hot air tray drying (TD) and heat pump–dehumidified drying air (HPD) at air temperatures of 40, 50, and 60°C. Alternatively, leaves were subject to microwave drying (MWD) at 150, 450, and 900 W and to freeze drying (FD). The moisture versus time data were fitted to five drying models. In general, a three-parameter model gave the best fit. The drying constant was related to the drying temperature or microwave power using an Arrhenius model. Effective moisture diffusivity (D _eff) increased with higher drying temperature, higher microwave power, or blanching treatments. Structural changes in the leaves after drying and upon rehydration were observed by scanning electron microscopy (SEM). Leaves blanched and dried using HPD at 50°C and fresh and dried using FD showed a partial breakdown of the tissue structure upon rehydration. HPD and blanching reduced the drying time by 8.3% and increased quercetin and kaempferol levels by 42.1 and 51.4%, respectively, compared to TD at 50°C. MWD provided the quickest drying followed by HPD and TD, respectively. HPD drying of M. oleifera after blanching resulted in relatively greater quality compared to TD and MWD.     

Effect of citric acid and blanching pre-treatments on drying and rehydration of Amasya red apples

Drying behaviour of red apples was experimentally examined in a laboratory dryer at drying temperatures of 55, 65 and 75 °C and a constant air velocity of 2.0 m/s. Two pre-treatments (blanching and 0.5% citric acid) were applied to prior to drying process. It was observed that both the drying temperature and pre-treatment affected the drying time. The shortest drying times were obtained from pre-treated samples with citric acid solution. Blanched samples have higher rehydration ratios than other samples. Five mathematical models namely, Newton, Henderson and Pabis, Page, Logarithmic and Parabolic models were evaluated in the kinetics research. The fit quality of the proposed models was evaluated by using the determination of coefficient (R²), reduced chi-square (χ²), root means square error (RMSE) and mean relative percent error (P). The Parabolic model provided the best representation of data. Effective moisture diffusivity (D_eff) computed on the basis of Fick's second law. D_eff value of pre-treated samples with citric acid solution was higher than the other samples.     

Effect of Four Drying Methods on the Quality of Osmotically Dehydrated Cranberries

Abstract

Partially dehydrated cranberries (osmotically dehydrated) were dried to low water contents using one of following four methods: hot air drying; microwave-assisted convective drying; freeze-drying; and vacuum drying. Quality evaluation was performed on all samples, including sensory evaluation (appearance and taste), texture, color, water activity, and rehydration ratio. Hot air drying produced dried cranberries with the best visual appearance while freeze-dried cranberries had the highest rehydration ratio. The other methods presented similar rehydration ratios. There was no significant difference in color measurements and water activity. Few differences in texture were found, except for freeze-dried cranberries, which had a lower toughness compared to the other drying methods including commercially available dried cranberries. Microwave-assisted to hot air drying rate ratios increased as the moisture content decreased.