Optimisation of osmotic dehydration of carrot cubes in sucrose-salt solutions using response surface methodology

Osmotic dehydration of carrot cubes in ternary solution of water, sucrose and sodium chloride at different solution concentrations, temperatures and process durations were analysed for water loss and solute gain during osmotic dehydration. The osmotically pre-treated carrot cubes were further dehydrated in a cabinet dryer at 65 °C and were then rehydrated in water at ambient temperature of water for 10–12 h and were analysed for rehydration ratio, shrinkage and overall acceptability after rehydration. The process was optimised for maximum water loss, rehydration ratio and overall acceptability of the rehydrated product, and for minimum solute gain and shrinkage of rehydrated product by response surface methodology. The optimum conditions of various process parameters are 50°B+10% w/v aqueous sodium chloride concentration, 46.5 °C solution temperature and 180 min process duration.     

OPTIMIZATION OF OSMOTIC DEHYDRATION PROCESS OF CARROT CUBES IN SUCROSE SOLUTION

ABSTRACT

For optimization of the osmotic dehydration process of carrot cubes in sucrose solution by response surface methodology (RSM), the experiments were conducted according to face‐centered central composite design. The independent process variables for the osmotic dehydration process were osmotic solution concentrations (45–55°Brix), temperature (35–55C) and process durations (120–240 min). Statistical analysis of results showed that all the process variables had a significant effect on all the responses at 5% level of significance (P < 0.05). The osmotic dehydration process was optimized by RSM for maximum water loss, rehydration ratio, retention of color, sensory score and minimum solute gain. The optimum process conditions were 52.5°Brix sucrose syrup concentration, 49C osmotic solution temperature and 150‐min process duration.

PRACTICAL APPLICATIONS

The process of osmotic dehydration can be used for the preparation of shelf‐stable products for the purpose of use during off‐season. The quality of preosmosed carrots is much superior to the product dehydrated with the convectional method of convective dehydration. The osmotically dehydrated carrots can be used for cooking as vegetables after rehydration or can be added directly into soups, stews or casseroles before cooking. If the product is blanched before osmotic dehydration, the process can be used successfully for the preparation of carrot candy.     

APPLICATION OF RESPONSE SURFACE METHODOLOGY FOR THE OSMOTIC DEHYDRATION OF CARROTS

Osmotic dehydrations of carrot cubes in sodium chloride salt solutions at different solution concentrations, temperatures and process durations were analyzed for water loss and solute gain. The osmotically pretreated carrot cubes were further dehydrated in a cabinet dryer at 65C and were then rehydrated in water at ambient temperature for 8–10 h and analyzed for rehydration ratio, color and overall acceptability of the rehydrated product. The process was optimized for maximum water loss, rehydration ratio and overall acceptability of rehydrated product, and for minimum solute gain and shrinkage of rehydrated product by response surface methodology. The optimum conditions of various process parameters were 11% salt concentration, 30C osmotic solution temperature and process duration of 120 min.     

Original article: Effect of sucrose and glycerol mixtures in the osmotic solution on characteristics of osmotically dehydrated mandarin cv. (Sai‐Namphaung)

This study investigated the effects of a series of osmotic solutions consisting of sucrose and glycerol on the quality of osmotically dehydrated mandarin, namely mandarin cv. (Sai‐Namphaung). Mandarin samples were peeled and osmotically dehydrated at 55 °C with agitation at 3.5776 × 10^?1g in five osmotic solutions containing various mixtures of 60% sucrose and 60% glycerol (9:1, 8:2, 7:3, 6:4 and 5:5 w/w, respectively). The osmotically dehydrated mandarin was further dried using hot‐air drying at 70 °C for 360 min. Increasing the glycerol ratio in the mixtures significantly (P ≤ 0.05) increased water loss and solid gain during osmotic dehydration, and significantly (P ≤ 0.05) affected kinetic rate constants during drying. An increase in the glycerol ratio in the mixtures caused a significant decrease in the quality factors of hardness, moisture content, water activity and reducing sugar. However, the increase resulted in an increase in the darkness of the dried mandarin, compared with increasing the sucrose ratio in the mixtures (P ≤ 0.05). The increase had an insignificant (P > 0.05) effect on vitamin C content.     

Effect of temperature and pretreatment on water diffusion during rehydration of dehydrated mangoes

The kinetics associated with rehydrating dehydrated mangoes was studied at three temperatures: 25, 40, and 60 °C. Besides, we studied how rehydration was affected by pretreating the fruit with osmodehydration in either sucrose or glucose before it was thermally dehydrated. We show that rehydration can be interpreted by Fickian diffusion and that the effective water diffusion coefficient is larger at 40 °C than at either 25 or 60 °C. Consequently, during rehydration of untreated samples at 40 °C, the weight gain, water gain, and loss of solids attain optimal values. We found that the rehydration kinetics of mango was not affected by osmodehydration pretreatments in sucrose. However, pretreatment in glucose significantly improved rehydration; for example, the effective diffusion coefficients of the glucose-treated samples were about twice as large as those of the untreated samples.     

Effect of the Osmotic Pre-Treatment on the Convective Air Drying Kinetics of Pear Var. Blanquilla

Kinetics of air drying of fresh and osmotically pretreated pear slices was analyzed. Osmotic treatments were applied for 0.5, 3, and 48 hours in 50°Brix sucrose solutions at 30°C. Air drying was carried out at 45, 55, and 65°C, 2.2 m/s air rate. Drying curves were modelled through a Fickian equation, obtaining the effective diffusion coefficient in each case. This coefficient was markedly affected by the temperature and by the osmotic pretreatment, ranging between 6.5 × 10^{? 12} to 5.8 × 10 ^{? 10} m²/s. A significant relationship between the D_e, and the inverse of the initial solute content of the samples was found. Activation energy of the drying process was similar for fresh and osmotically pretreated samples for 0.5 and 3 hours, but increased considerably when long osmotic treatment time was applied. Total drying time increased in osmodehydrated samples, but process yield, in terms of sample weight loss, increased. These effects were more marked when long osmotic pretreatment times were used.     

Concentration profiles and effective diffusion coefficients of sucrose and water in osmo-dehydrated apples

The spatial distribution of water and sugars in half-fresh apples dehydrated in sucrose solutions (30% and 50% w/w, 27 °C) for 2, 4 and 8 h, was determined. Each half was sliced as from the exposed surface. The density, water and sugar contents were determined for each piece. A mathematical model was fitted to the experimental data of the water and sucrose contents considering the overall flux and tissue shrinkage. A numerical method of finite differences permitted the calculation of the effective diffusion coefficients as a function of concentration, using material coordinates and integrating the two differential equations (for water and sucrose) simultaneously. The coefficients obtained were one or even two orders of magnitude lower than those for pure solutions and presented unusual concentration dependence. The behaviour of the apple tissue was also studied using light microscopy techniques to obtain images of the osmotically treated pieces (20%, 30% and 50% w/w sucrose solutions for 2, 4 and 8 h).     

SORPTION ISOTHERM BEHAVIOR OF OSMOCONVECTIVELY DEHYDRATED CARROT CUBES

Carrot cubes, osmotically pretreated with aqueous sodium chloride (10%, w/v), sucrose syrup (55°Brix), and mixture of sucrose and sodium chloride (50°Brix + 10%, w/v), were convectively dehydrated at 65C temperature up to final moisture content of 4–5% (wet basis). To study their equilibrium moisture content (EMC) behavior, the dehydrated carrot cubes were stored at temperature ranging from 10 to 50C and relative humidity ranging from 15–95% using static desiccator technique. Five isotherm equations, viz, Chung–Pfost, modified Henderson, modified Halsey, modified Oswin and modified exponential were applied to examine the data. Among the applied models, modified Oswin model was best fit for control (untreated) and salt‐treated samples, modified Hesley model for sucrose‐treated samples and two‐term exponential model for mixture of sucrose–salt treatment over the entire range of relative humidity and temperature. The EMC values of cubes osmotically pretreated with sodium chloride solution were highest among all pretreatments, and were lowest for control (unosmosed) samples.     

Effect of the solute on the development of compositional profiles in osmotic dehydrated apple slices

Apple (cv. Granny Smith) slices, 30-mm thick, were osmotically dehydrated for 9 h at 30 °C using glucose, sucrose and trehalose solutions with the same water activity (a_w = 0.96). After OD treatment, water and solute content were analysed in 1.5-mm thick serial disks of the apple slices to determine the effect of osmotic dehydration on the compositional profiles. Diffusional and “Advancing Disturbance Front” (ADF) models were applied to the experimental data, both showing a good fit. Changes in the compositional profiles of osmotically dehydrated slices were also analysed throughout storage time. For this purpose, the 30-mm thick dehydrated slices were kept at 10 °C for 7 days in hermetic plastic bags and compositional profiles were analysed after 1, 2, 3 and 7 days and modelled using Fermi's equation. Throughout storage, the profiles became flatter due to the counter-current migration of water and solutes associated to the concentration gradients. Mass transfer rate during dehydration was faster when sucrose or glucose was used, but trehalose implied an increase in the mass transfer resistance of the tissue. This behaviour was also observed in the mass transfer processes during storage. This effect was attributed to the changes induced by trehalose in the permeability of cell membranes through component interactions.     

Water Diffusion and Concentration Profiles During Osmodehydration and Storage of Apple Tissue

The objective of this work was to study the mobility of water and sucrose during osmotic dehydration and storage of apple tissue and to conduct an analysis of the behavior of the effective diffusion coefficients determined from concentration profiles. Osmotic dehydration (OD) of apple was carried out at 40°C for 1 h, and the solution: sample ratio was 20:1 (w/w). Samples of 20-mm diameter were extracted from the dehydrated apple immediately after the OD process and after 4 and 24 h of storage at 25 °C. Moisture of these samples and soluble solids content were analyzed. Our results showed, after 1 h of OD, the outer layer of the apple sample lost 0.37 kg water/kg apple and gained 0.30 kg sucrose/kg apple. These values decreased toward the internal layers of the apple. A fine layer of greatly dehydrated cells was formed on the surface around the sample, which determined the mass transfer rate in the whole tissue. Smaller mass transport rates were observed in the development of concentration profiles during storage. Diffusion coefficients obtained for the outer layer after 1 h of OD were 1.53 × 10⁻¹⁰ and 1.05 × 10⁻¹⁰ m²/s for water and sucrose, respectively. The analysis of compositional profiles developed during osmodehydration was a useful tool to get a better understanding of the changes in the water activity of the outer layer of the apple tissue.     

Diffusion in Heated Potato Tissues

Data to optimize nutrient retention during processing of potatoes were obtained by measuring loss of solutes (glucose, fructose, citric acid, potassium, calcium and magnesium) by diffusion into blanch water. Apparent diffusivities were calculated based on Fickian diffusion. Magnesium and calcium did not follow the Fickian approach with the same accuracy as the others. A hindrance factor (K), the ratio of apparent diffusivity in potato to a published value was used. The hindrance factor was calculated at reference temperature 25°C to separate preheating effects from the direct effect of temperature on diffusion coefficients. A mathematical model for K(T,t) was fitted to experimental data for each solute. All solutes showed a sharp decrease of hindrance factor in the region 50–60°C. These models showed, however, that heating of potato affected diffusivities of the solutes to different degrees. Ions had a limit solution retention in the tissue higher than glucose, fructose or citric acid.     

Effect of Freezing on Diffusion of Ascorbic Acid during Water Heating of Potato Tissue

The effect of freezing on ascorbic acid losses during water heating of potato cylinders at 50°, 65° and 85°C was studied at different periods of time. Loss of ascorbic acid in frozen potato was mainly by diffusion and greater than for fresh potato. The estimated apparent diffusivities of ascorbic acid in frozen potato during water blanching were 13.64 × 10^-10 m²/sec at 50°C, 17.78 × 10^-10 m²/sec at 65°C and 20.30 × 10^-10 m²/sec at 80°C. The activation energy of the diffusion process was 3,007 cal/mol. Ascorbic acid aparent diffusion coefficients in potato tissue with pre-freezing treatment were between 80 and 90% of ascorbic acid diffusion coefficients in water in the 50–80°C range.     

Rehydration Studies on Pretreated and Osmotically Dehydrated Apple Slices

ABSTRACT: The influence of different pretreatments (freezing, blanching, high electric field pulse, and high pressure) and osmotic dehydration (OD) times (0 to 6 h) on some characteristics of rehydrated apple was investigated. Pretreated apple slices were osmotically dehydrated, oven dried, and rehydrated in distilled water at room temperature. Rehydration capacity (RC) increased with OD time. Blanched and prefrozen samples had higher RC, firmer rehydrated samples, and greater dry-matter loss than the other pretreated samples. There was no change (P > 0.05) in the color of the samples before and after rehydration. The electrical conductivity of the immersion medium increased with rehydration time but decreased with OD time.     

REHYDRATION OF FREEZE-DRIED STRAWBERRIES AT VARYING TEMPERATURES

Strawberries (var. Seascape), cut in halves or 5‐mm slices, were freeze‐dried at a heating plate temperature of 55C for 28 h. Freeze‐dried products were then rehydrated at 0, 20, 40 and 80C in distilled water. The progression of the rehydration coefficient (RC) was followed as a function of time (up to 25 min). Less than 2 min were necessary to fully rehydrate the slices and less than 5 min for half strawberries. The results showed that halved and sliced freeze‐dried strawberries had higher RCs when rehydrated at a temperature near 0C. A simple diffusive‐type equation was used to represent water uptake during rehydration. Effective diffusion coefficients were modeled as a function of temperature using an Arrhenius‐type relationship.     

Effect of osmotic pretreatment on equilibrium moisture content of dehydrated carrot cubes

The equilibrium moisture contents were determined for carrot cubes osmotically pretreated in salt, sucrose and salt plus sucrose combined solution using static method at 10, 25, 40 and 50 °C over a range of relative humidities from 14% to 95%. Six isotherm equations were applied for analysing the experimental data. Modified exponential equation, is the best equation for predicting the equilibrium moisture contents (EMC) of the dehydrated carrot cubes preosmosed in salt and sucrose plus salt solution, whereas modified Hasley equation is suitable for dehydrated carrot cubes preosmosed in sucrose solution over the relative humidity range of 14–95%. The EMC of carrot cubes osmotically pretreated with salt was highest among all pretreatments, and was lowest for un‐osmosed samples.     

Mass Transfer Modeling During Osmotic Dehydration of Hexahedral Pineapple Slices in Limited Volume Solutions

The study of mass transfer during osmotic dehydration process in limited volume solutions was carried out to evaluate the diffusion coefficients of sucrose and water in the osmotic treatment of hexahedral pineapple slices. The experimental osmotic dehydration kinetics for pineapple slices of two different sizes were conducted at 25 °C using a 1:1 solution to fruit weight ratio. The analytical solution of a 3D mass transfer model considering a limited volume of osmotic solution (i.e., an osmotic media of variable solute concentration) was used for describing the mass transfer in osmotic dehydration of pineapple slices. This model was fitted to the experimental kinetics by means of nonlinear regression to obtain the diffusion coefficients. Additionally, the diffusion coefficients were evaluated considering an infinite volume of osmotic solution (i.e., an osmotic media of constant solute concentration). Results showed that the proposed model may be fitted accurately to the experimental osmotic dehydration kinetics and allows the estimation of diffusion coefficients when solute concentration in the osmotic media varies along the process.     

Dehydration kinetics of red pepper (Capsicum annuum L var Jaranda)

Shredded and whole red pepper samples were dehydrated in a laboratory drier with a through‐flow air velocity of 0.5 m s^?1 at 50, 55, 60 and 70 °C. Shredded peppers dried faster than whole peppers. The drying behaviour of whole samples was characterised by a constant‐ and a falling‐rate drying period, whilst that of shredded samples was characterised by a falling‐rate drying period only. The mass transfer coefficient for whole samples during the constant‐rate period was computed experimentally. The effect of temperature on the mass transfer coefficient was described by the Arrhenius model. The activation energy was 58 kJ mol^?1. In the falling‐rate period the mass transfer was described by a diffusional model, and the effective diffusion coefficient at each temperature was determined. Diffusion coefficients were estimated to lie between 4.38 × 10^?11 and 10.99 × 10^?11 m² s^?1 for whole peppers and between 37.23 × 10^?11 and 99.61 × 10^?11 m² s^?1 for shredded peppers. The effect of temperature on the effective diffusion coefficient was described by the Arrhenius equation, with an activation energy of 44 kJ mol^?1 for whole peppers and 56 kJ mol^?1 for shredded peppers. © 2003 Society of Chemical Industry     

EFFECT OF CENTRIFUGAL FORCE ON THE AQUEOUS EXTRACTION OF SOLUTE FROM SUGAR BEET TISSUE PRETREATED BY A PULSED ELECTRIC FIELD

In this article, the centrifugal aqueous extraction of solute from sugar beet tissue is investigated at ambient temperature after a pulsed electric field (PEF) treatment. Two kinds of samples of fresh sugar beet were used: a sample with a determined discoid shape and gratings. Both samples were pretreated by a PEF with 250 rectangular pulses of 100 µS each. The PEF intensity was fixed at 940 V/cm for the disk samples and 670 V/cm for gratings. The pretreated samples were placed in distilled water at ambient temperature with a water‐to‐solid ratio equal to 3 and subjected to different centrifugal accelerations. The centrifugal field significantly enhanced the kinetics of extraction from the electrically pretreated tissues of sugar beet. However, the increase of centrifugal acceleration was only effective up to a certain value (5430 × g for disk samples and 600 × g for gratings). The centrifugal extraction can be assumed to proceed in two stages: a first rapid washing followed by a slow diffusion stage. A two‐exponential kinetics model taking into account these two stages was applied and correctly described the centrifugal extraction from beet samples (disks and gratings).     

Rehydration of Freeze‐Dried and Convective Dried Boletus edulis Mushrooms: Effect on Some Quality Parameters

ABSTRACT: Quality of rehydrated products is a key aspect linked to rehydration conditions. To assess the effect of rehydration temperature on some quality parameters, experiments at 20 and 70 °C were performed with convective dried and freeze‐dried Boletus edulis mushrooms. Rehydration characteristics (through Peleg's parameter, k₁, and equilibrium moisture, W_e), texture (Kramer), and microstructure (Cryo‐Scanning Electron Microscopy) were evaluated. Freeze‐dried samples absorbed water more quickly and attained higher W_e values than convective dried ones. Convective dehydrated samples rehydrated at 20 °C showed significantly lower textural values (11.9 ± 3.3 N/g) than those rehydrated at 70 °C (15.7 ± 1.2 N/g). For the freeze‐dried Boletus edulis, the textural values also exhibited significant differences, being 8.2 ± 1.3 and 10.5 ± 2.3 N/g for 20 and 70 °C, respectively. Freeze‐dried samples showed a porous structure that allows rehydration to take place mainly at the extracellular level. This explains the fact that, regardless of temperature, freeze‐dried mushrooms absorbed water more quickly and reached higher W_e values than convective dried ones. Whatever the dehydration technique used, rehydration at 70 °C produced a structural damage that hindered water absorption; consequently lower W_e values and higher textural values were attained than when rehydrating at 20 °C.     

An Improvement of the Cell Diffusion Method for the Rapid Determination of Diffusion Constants in Gels or Foods

A diffusion cell was designed to study the apparent diffusion coefficient of solutes through foodstuffs. The mathematical model corresponding to the experimental setup did not assume quasi-steady state diffusion within the slice of food. It was used to obtain the effective diffusion coefficient of NaCl through 3% Agar gels. The results were compared to those obtained by using two agar gel cylinders at different concentrations. The diffusion coefficient of Cl^? in gel at 25°C was obtained with at least the same precision (D = 1.30 × 10^?9 m²/sec, S_D= 0.03 × 10^?9 m²/sec) within less than 110 min compared to a few hours with the two cylinder technique. With the mathematical model used to determine diffusion coefficients, the equilibrium ratio between solute concentration in solution and at the interface of foodstuff can be determined.