Effect of ohmic heating and vacuum impregnation on the quality and microbial stability of osmotically dehydrated strawberries (cv. Camarosa)

The combined effect of osmotic dehydration/ohmic heating (OD–OH) and vacuum impregnation/ohmic heating (VI–OH) on physicochemical and quality parameters of strawberry (a_w, color, firmness and microstructure), as well as on microbial stability of storage samples at 5 and 10 °C, was analyzed. Treatments were carried out with a 65% (w/w) sucrose solution at 30 °C, and ohmic heating at 9.2, 13, and 17 V/cm electric field strengths, corresponding to applied voltages of 70, 100, and 130 V. Dehydrated samples showed that water loss was greater in OD–OH treatments at 17 V/cm. The greatest solute gain, least firmness loss and least color loss were obtained in the VI–OH treatment at 13 V/cm. The shelf-life of strawberries treated with VI–OH at 13 V/cm and stored at 5 °C was extended from 12 d (control samples) to 25 d. Furthermore, the VI–OH treatment at 13 V/cm was the best processing condition for dehydrating strawberries.     

Dielectric spectroscopy of osmotic solutions and osmotically dehydrated tomato products

Complex permittivity is the physical property that describes the interaction between matter and an electromagnetic field. This property is related to the structural and physico-chemical properties such as water and soluble solids content or water activity of the material.

The complex permittivity of different aqueous solutions, fresh and osmotically dehydrated cherry tomatoes was measured and related to the products’ composition. Results showed that fruits osmotically dehydrated with sucrose solutions reached the lowest loss factors but the maximum penetration depth (D_p). On the other hand, fruits immersed in binary salt solutions showed the inverse tendency. For products impregnated with ternary solutions an intermediate behaviour was observed. Calcium lactate addition had no significant effects on complex permittivity at 2.45 GHz, but its presence increased the effective loss factor at low frequency.

The dielectric properties of fruits and vegetables can be modified by controlling osmotic process variables (time, temperature, pressure and osmotic solution composition) in order to improve their further drying with microwaves.     

Effect of osmotic dehydration and vacuum impregnation on respiration rate of cut strawberries

Respiration rate in terms of O₂ consumption and CO₂ production was determined in strawberry halves, both fresh-cut and vacuum impregnated with an isotonic solution. The experimental measurements were also carried out in osmodehydrated samples for different concentration levels (to 30 °Brix) at atmospheric pressure and by applying a vacuum pulse. Changes throughout time of O₂ and CO₂ concentration in the headspace of chambers containing the samples were analysed to determine respiration rates. The effect of temperature on respiration rate in fresh-cut and impregnated samples showed sigmoid behaviour where a sharp increase in respiration levels occurred between 5 and 10 °C. Osmodehydration treatments resulted in a great decrease in O₂ consumption but no notable changes in CO₂ generation, which suggests that anaerobic biochemical pathway became dominant respiration mode due to the treatments. Production of ethanol and acetaldehyde was detected in these cases in agreement with the anaerobic process.     

Effect of drying methods on osmotically dehydrated cashew apples

Matured ripe cashew apples were transversely cut into 10 mm, 15 mm and 20 mm slices and immersed in sugar solutions of 52°Brix, 60°Brix and 68°Brix, for 10 h. The osmotic temperature was maintained at 27 °C in a water bath. Osmosed samples were subsequently dried in either an air-oven (50 °C) or a vacuum-drier (50 °C), both for 6 h.The instantaneous moisture content (d.b) of osmosed cashew apples decreased with increasing immersion time and osmotic solution concentration, but also increased with increasing slice thickness. The water loss, solids gain and percentage weight reduction increased with increasing osmotic solution concentration and immersion time, but decreased with increasing slice thickness. Osmotically dried samples received high acceptability. Sample pre-osmosed in 60°Brix and 68°Brix solutions were significantly better (P>0.05) than those pre-osmosed in a 51°Brix solution. A significant difference (P>0.05) between the osmo-oven and osmo-vacuum dried cashew apples could not be ascertained (under the conditions of the investigation).     

Effects of brine concentration and temperature on color of vacuum pulse osmotically dehydrated sardine sheets

The effects of brine concentration (0.15-0.27 g NaCl/g) and temperature (30-38 °C) on the color parameters (L, a, b, ΔE) of vacuum pulse osmotically dehydrated sardine sheets were investigated. The results showed that osmotic dehydration has a significant effect on color of sardine sheets. Redness (a-value) and yellowness (b-value) decreased through dehydration time while lightness (L-value) and total color difference (ΔE) increased. The decreases in redness and yellowness were lesser with increasing temperature and brine concentration while increases in lightness and ΔE were higher. These changes would be predicted by simple models as a function of the temperature, brine concentration, and dehydration time.     

Characterisation of osmotically dehydrated papaya with further hot air drying and microwave vacuum drying

Papaya was subjected to osmotic dehydration (OD) prior to hot air drying (HA; 70 °C) and microwave vacuum drying (MVD; 3.75 W g^?1 and 13.3 kPa). An increase in immersion time in a sucrose solution [65/100 g (w/w)] at 40 ± 2 °C from 0 to 4 h decreased moisture content from 7.5802 to 1.2215 kg water kg dry solid^?1. During HA and MVD, effective moisture diffusivity was in the range 7.09 × 10^?8 to 9.13 × 10^?8 m² s^?1 and 2.85 × 10^?6 to 3.50 × 10^?6 m² s^?1, respectively, depending on immersion time. The hue angle of HA samples was 0.83–0.91, whereas that of MVD samples was 48.55–50.32. Both drying methods decreased springiness. Porosity was clearly observed in MVD samples. Rehydration rate was enhanced by increased immersion time and MVD (P ≤ 0.05). From preference mapping, MVD samples were more preferable than HA samples. Moreover, MVD samples with 1–3 h OD were more preferable than those with 4 h OD.     

Microwave vacuum drying of osmotically dehydrated mandarin cv. (Sai‐Namphaung)

Mandarin [mandarin cv. (Sai‐Namphaung)] was subjected to osmotic dehydration prior to microwave vacuum drying. Osmotic solutions were varied using different ratios of sucrose to glycerol (9:1, 7:3 and 5:5 w/w). Because of the decreased moisture content and solid gain during osmotic dehydration, dielectric properties of mandarin were changed significantly (P ≤ 0.05). The osmotically dehydrated mandarin was then dried further using microwave vacuum drying at 4.8 and 6.4 W g^?1. Among thin layer models, page model was the best to describe the drying of osmotically dehydrated mandarin. An increase in the microwave power could increase drying rate without significant effect on hardness of dried samples. Nonetheless, the hardness was significantly (P ≤ 0.05) reduced by an increase in the glycerol ratio in the osmotic solution. The increase in microwave power and glycerol ratio significantly (P ≤ 0.05) decreased β‐carotene content and thereby affected colour of the dried mandarin.     

Rehydration and sorption properties of osmotically pretreated freeze-dried strawberries

The aim of this work was to investigate the influence of osmotic dehydration and type of osmotic solution on selected physical properties of freeze-dried strawberries. Frozen Senga Sengana strawberries were dehydrated in osmotic solution with water activity of about 0.9 (sucrose and glucose solutions and starch syrup). Osmotically dehydrated fruits were frozen and freeze-dried at heating shelf temperature of 30 °C for 24 h.     

Process conditions effect on the quality of banana osmotically dehydrated

Banana is greatly perishable and does not resist freezing, hence dehydration is the preservation technique of choice. This paper deals with the effect of process conditions on both the dehydration kinetics and the final quality of banana osmotically dehydrated. Banana slices (5 mm thick, 23 mm diameter) were osmotically dehydrated for 4 h, following a 3² full factorial design (temperature (30, 40, and 50 °C), sucrose concentration (45, 55 and 65% w/w)) with experiments in triplicate. The kinetics of dehydration efficiency (DE) and mass loss (DM) and the quality of osmotic dehydrated banana, was analysed through the measurement of colour, final volume (V/V⁰) and shape changes (SF).Both temperature and sucrose concentration in the osmotic solution had a significant effect on DE but had no significant effect on DM. Peleg’s equation fitted well the data for DM and DE during OD. Sucrose concentration and temperature, had no significant effect on DM after 4 h OD or at the equilibrium mass loss (DM^e) (p > 0.05), however, temperature had a significant effect on the initial rate of DM (p < 0.05). Sucrose concentration and temperature, had no significant effect on the colour parameters (L, Chroma and Hue), but showed a significant effect on V/V⁰ and SF, with temperature having a more extensive negative impact on V/V⁰ and SF than sucrose concentration. Process temperature (low temperature, e.g., 30 °C) has to be carefully selected in order to reach a compromise between OD rate and an appropriate final product quality. The reduction in the kinetics caused by low temperature could be compensated by using highly concentrated sucrose solutions (as much as 65%), which would favour the compositional changes with a lesser impact on the product quality.     

Effect of calcium salts on the texture,structure and sensory acceptance of osmotically dehydrated guavas

The effect of additives, calcium chloride and calcium lactate (5–25 g kg^?1), on the osmotic dehydration of guavas with sucrose solutions was studied, aiming at the structural preservation of processed fruits. The osmotic process was evaluated from the reduction in weight of the guavas, water loss and solids gain, and the samples were analyzed with respect to calcium content, texture (stress and strain at failure, relaxation time and residual stress), structure by light microscopy and sensory acceptance. Calcium salts had a strong influence on the texture and structure of the processed guavas, resulting in the maintenance of tissue structure when calcium lactate was used at concentrations up to 15 g kg^?1, and calcium chloride was used at 5 g kg^?1. The sensory acceptability of guava was related to the structural and texture results. Calcium treatments did not improve guava's sensory acceptance. Guavas treated with calcium lactate showed good sensory acceptance, presenting slight inferior scores only at concentrations above 20g kg^?1, while CaCl₂ treated guavas showed average scores statistically equal to the sucrose and calcium lactate treated fruits only at 5 g kg^?1. Copyright © 2007 Society of Chemical Industry     

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.     

Modeling mass transfer during osmotic dehydration of strawberries under high hydrostatic pressure conditions

Simultaneous application of high hydrostatic pressure (200–400 MPa) during osmotic dehydration of strawberries was studied in this investigation. The high hydrostatic pressure treatment improved the diffusion coefficients of water and soluble solids compared to atmospheric pressure operation. Effects of process pressure on diffusion coefficients were achieved through an Arrhenius-type equation. Mathematical modeling of mass transfer was performed applying Newton, Henderson–Pabis, Page and Weibull models. Based on statistical results, the Weibull model gave the best goodness of fit on the experimental data under the studies' operative conditions.Industrial relevance: This article deals with the mathematical modeling of mass transfer during simultaneous high hydrostatic pressure treatment and osmotic dehydration of strawberries. Transfer of water and soluble solids during this combined process were satisfactorily simulated with the Weibull model. Results indicated that application of this innovative technology improved strawberries dehydration rates compared to atmospheric pressure operation resulting in a dried fruit with intermediate moisture content ready to be used as input material of further processes.     

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.     

Calcium lactate effect on the shelf life of osmotically dehydrated guavas

The effect of calcium lactate on osmodehydrated guavas in sucrose and maltose solutions was monitored during storage under passive modified atmosphere for 24 d at 5 °C. Sample texture and color characteristics, microbial spoilage, sensory acceptance, structural changes, and gas composition inside the packages were periodically evaluated. Calcium lactate inhibited microbial growth on guavas, with yeast and mold counts in the order of 10(2) CFU/g throughout storage. The calcium salt reduced respiration rate of guava products, showing O(2) and CO(2) concentrations around 18% and 3% inside the packages. A firming effect on fruit texture, with up to 5 and 2 times higher stress and strain at failure values and tissue structure preservation could also be attributed to calcium lactate use. However, fruits treated with calcium lactate, osmodehydrated in maltose and sucrose solutions, showed sensory acceptance scores below the acceptability limit (4.5) after 13 and 17 d of storage, respectively.     

The air drying behaviour of fresh and osmotically dehydrated banana slices

Ripe banana, cut to 10mm thick slabs were osmotically treated in sugar solutions of 35, 50 and 65° Brix for 36h. The initial moisture content fell from a value of 3.13kg H₂O DM to 2.19, 1.63 and 1.16kg H₂O kg⁻¹ for treatment in the three solutions, respectively. These slabs, with Total Soluble Solids (TSS) contents of 26, 34 and 39° Brix, respectively, as well as freshly cut but untreated slabs (15° Brix) were air dried in a cabinet type tray drier to near equilibrium conditions at fixed temperatures from 40 to 80°C and at a constant air speed of 0.62m s⁻¹. Drying was found to occur in the falling rate period only for both banana types and two drying constants K₁ and K₂ were established for a first and second falling rate period of drying. Increasing the drying air temperature significantly enhanced the drying rate and the K-values, except at 80°C when the rates fell, possibly because of case hardening of the slabs. Reducing the slab thickness also improved the drying rate, but increasing the air speed to 1.03m s⁻¹ did not have any profound effect. As the sugar content of the banana slabs increased through the osmotic treatment, drying rates fell. Calculated apparent moisture diffusivities at 60°C ranged from 34.8× 10⁻¹⁰ m² s⁻¹ (fresh slab) to 8.8×10⁻¹⁰ m² s⁻¹ for dried (39° Brix) slabs. The moisture diffusivity was significantly lowered as the moisture content dropped in drying and with increased levels of sugar. Previously osmosed and then air dried banana slabs showed appealing colour and texture compared to the fresh banana.     

Microscopic features, mechanical and thermal properties of osmotically dehydrated guavas

The effects of an osmotic dehydration process using sucrose and maltose solutions at 40 and 60 °Brix on microscopic features and some mechanical and thermal properties of guava tissue were studied. Also the addition of calcium lactate to the sugar solutions, aiming at preserving the structure of the processed fruits, was investigated. The guava texture (stress at failure) and the structure as observed by light microscopy were both evaluated, and differential scanning calorimetry (DSC) was used to verify the interaction between calcium ions and cell wall pectin in the guava tissue. The calcium content of the differently treated samples was also related to microscopic features, mechanical and thermal properties of guavas. The osmotic process using sucrose and maltose solutions caused severe structural damage to the guava tissue, and this effect was intensified at higher sugar concentrations and by the use of sucrose solutions. The addition of calcium lactate promoted maintenance of the guava structure, showing turgid cells with well-defined cellular contours, resulting in an increase in hardness and indicating bonding between the Ca²⁺ and cell wall pectin, which was confirmed by the DSC experiments.     

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.     

Modelling of mass transfer and texture evaluation during osmotic dehydration of melon under vacuum

The influence of vacuum time and solution concentration on mass transfer and mechanical properties of osmodehydrated melon cubes has been studied. Pulsed vacuum osmotic dehydration (PVOD) was carried out at 30 °C for 4 h, using sucrose solutions (40, 50 or 60°Brix) and applying a vacuum pulse (100 mbar for 5, 10 or 15 min). Kinetics of water loss, solid gain and stress at rupture were analysed, as well as effective diffusivities using the hydrodynamic model. The increase in solution concentration favoured water removal, but no significant effect of vacuum time was observed. The use of less concentrated solutions coupled to the action of vacuum pulse resulted in greater solid uptake. Samples subjected to PVOD using 60°Brix sucrose solution presented greater water loss, lower sugar uptake and better maintenance of fresh fruit texture throughout the process. Diffusion coefficients estimated by the hydrodynamic model showed a good fit to the experimental data.     

Use of pectinmethylesterase and calcium in osmotic dehydration and osmodehydrofreezing of strawberries

Strawberry samples of two varieties (Camarosa and Elsanta) were dehydrated using different osmotic solutions (60% glucose, fructose, sucrose and raftilose) and subsequently frozen by rapid and high-pressure shift freezing (HPSF). The effect of pectinmethylesterase (PME) and calcium (Ca⁺⁺) added to the osmotic solutions on several compositional parameters and the textural/structural quality of dehydrated and osmodehydrofrozen-then-thawed samples was studied. Due to the presence of PME and Ca⁺⁺ in the osmotic solutions, weight reduction upon dehydration was slightly decreased, which was correlated to a small positive effect on the net uptake of sugars and depression of the initial freezing point. Except for the Camarosa samples treated with sucrose, PME and Ca⁺⁺ in osmotic sugar solutions positively affected the relative hardness of dehydrated fruits, which was ascribed to the effect of PME and Ca⁺⁺ on the cell wall strength of the tissue. No cell wall damage and tissue particle alterations were observed upon dehydration. The effect of osmotic dehydration (OD) using different sugar solutions without PME and Ca⁺⁺ on the texture and structure of frozen-then-thawed samples was limited and sometimes negative. The added PME and Ca⁺⁺ however positively influenced the volume and shape of the thawed samples, which could be related to slightly higher relative hardness values and, for the Elsanta strawberry fruits, also to the reduced (up to 81%) drip loss upon thawing. Upon freezing the dehydrated fruits, no cell wall disruption was observed. Tissue distortion caused by freezing and indicated by a decrease in particle size, convexity and roundness, was compensated by the use of PME and Ca⁺⁺ during OD.