Osmotic Dehydration of Tomato in Sucrose Solutions: Fick's Law Classical Modeling

ABSTRACT:  Osmotic dehydration of tomato was modeled by the classical Fick's law including shrinkage, convective resistance at the interface and the presence of water bulk flow. Tomato slices having 8 mm thickness were osmotically dehydrated in sucrose solutions at 50, 60, and 70 °Brix and at 35, 45, and 55 °C. Other experiments were done in a 70 °Brix sucrose solution at 35 °C with tomato slices of 4, 6, and 8 mm thickness and at different motion levels (velocities 0, 0.053, and 0.107 m/s). Tomato weight, water content, and °Brix of the products were measured as a function of processing time (20, 40, 80, 160, and 320 min). Results showed that temperature, concentration, thickness, and solution movement significantly influenced water loss and sucrose gain during the osmotic dehydration of tomato. The model predicted the modifications of soluble solid content and water content as a function of time in close agreement with the experimental data. Experimental Sherwood number correlations for sucrose and water were determined as Sh _s = 1.3 Re ^0.5 Sc _s^0.15 and Sh _w = 0.11 Re ^0.5 Sc _w^0.5, respectively. The effective diffusion coefficients of water (4.97 10⁻¹¹– 2.10 10⁻¹⁰ m²/s) and sucrose (3.18 10⁻¹¹– 1.69 10⁻¹⁰ m²/s) depended only on temperature through an Arrhenius-type relationship.     

Influence of osmotic dehydration and freezing on the volatile profile of kiwi fruit

The effect of osmotic dehydration on the volatile fraction of kiwi fruit was studied, as well as the effect of freezing and frozen storage. Osmotic treatments were carried out in sucrose solutions until the kiwi fruit reached 30°Brix, at atmospheric pressure (OD) and by applying a vacuum pulse (PVOD), by using 45 and 65°Brix sucrose. Volatile compounds of fresh, dehydrated and frozen-stored (at −18 °C for 1 month) samples were obtained by simultaneous distillation-extraction, and analyzed by GC-MS. Osmotic dehydration provoked formation of esters and a decrease in aldehydes and alcohols, depending on the dehydration treatment applied, which is similar to what occurs during kiwi ripening. A severe reduction of all volatile compounds occurred after one month in frozen storage, which smoothes the changes induced by osmotic treatments. Only small differences between dehydrated and non-pretreated frozen/thawed samples could be recognized.     

Changes in optical and mechanical properties during osmodehydrofreezing of kiwi fruit

The influence of osmotic dehydration and freezing–thawing on optical (colour and translucency) and mechanical properties of kiwi slices were analysed. Osmotic treatments were carried out in sucrose solutions up till the soluble solids in kiwi fruit reached 30 °Brix, both at atmospheric pressure (OD) and by applying a vacuum pulse (PVOD). Analyses were carried out on fresh and dehydrated samples before and after frozen storage (at −18 °C for 1 and 30 days). Reflexion spectra (400–700 nm) were measured to obtain the Kubelka–Munk coefficients and CIE-L*a*b* colour co-ordinates. Mechanical properties were analysed through the compression test. A transparency gain was observed in PVOD treated samples and in frozen–thawed samples, which implied a reduction in product clarity and chrome. Colour hue did not change notably, due to either osmotic treatments or freezing. Samples treated with 45 °Brix osmotic solution at atmospheric pressure were the best preserved in mechanical properties after freezing–thawing.     

BOUND WATER IN FRUIT PRODUCTS BY THE FREEZING METHOD

SUMMARY— An insulated heat-sink containing Freon-12® (N.B.P. −21.6°F) provides a reproducible system for measurement of thermal properties of fruit products. The difference in time required to remove the latent heat of fusion of the "eutectic" mixture in comparison with distilled water measures the "bound" water. The 34-37%"bound" water in papaya (var. solo) pulp is unaffected by varying pH in the range 3.0–6.0. Less than 10% water is bound in guava, passion fruit and pineapple juice products with up to 35% sucrose added. Thermal conductivity of the solid phase "eutectic" mixture in the 5–50% soluble solids range fits the regression Y = AB^x, Y = (3.69) (0.96)^x where x = percent total soluble solids (TSS) and Y = thermal conductivity (Cal x 10³/(cm²) (sec) (°C/cm)). The relationship of freezing point to TSS values as 1.10 + 0.64 (TSS) in the test system is significant (P < 0.01) in the range 5–50% TSS. The time for fusion (θ_w) of sucrose/water in the range 5–50% TSS fits the regression: log θ_w= (−0.8325) (−0.0165) (TSS) or θ_w= (0.984) (0.164)^TSS sec. Addition of solutes such as sucrose will inhibit gelation without affecting "bound" water values. Pectin gel structures apparently are dependent on secondary bonding and independent of "bound" water per se.     

RHEOLOGICAL BEHAVIOUR OF KIWI FRUIT JUICE CONCENTRATES

The rheological behaviour of diluted kiwi concentrates (22.5–63.0 °Brix) over the temperature range 4 to 70C, was studied. The values below 55.7 °Brix fit the power law while above 55.7 °Brix fit the Herschel-Bulkley equation. Maximum and minimum consistency index values were 62225 and 16 mPa. s ⁿ . Concentrates showed a yield stress at 55.7 °Brix, from 5 to 11 Pa, and 63.0 °Brix, from 19 to 53 Pa. Yield stress was correlated with temperature by means of a linear equation. Arrhenius activation energies were in the range of 28.97 to 33.89 kJ/mol. The average value of the flow behaviour indexes varied from 0.85 (sample at 22.5 °Brix) to 0.59 (sample at 63.0 °Brix). Simple equations are proposed to describe the combined effect of temperature and soluble solids content on consistency index and rheological behaviour index.     

Effect of Osmotic Dehydration and Vacuum-Frying Parameters to Produce High-Quality Mango Chips

ABSTRACT:  Mango ( Mangifera indica  L.) is a fruit rich in flavor and nutritional values, which is an excellent candidate for producing chips. The objective of this study was to develop high-quality mango chips using vacuum frying. Mango (" Tommy Atkins ") slices were pretreated with different maltodextrin concentrations (40, 50, and 65, w/v), osmotic dehydration times (45, 60, and 70 min), and solution temperatures (22 and 40 °C). Pretreated slices were vacuum fried at 120, 130, and 138 °C and product quality attributes (oil content, texture, color, carotenoid content) determined. The effect of frying temperatures at optimum osmotic dehydration times (65 [w/v] at 40 °C) was assessed. All samples were acceptable (scores > 5) to consumer panelists. The best mango chips were those pretreated with 65 (w/v) concentration for 60 min and vacuum fried at 120 °C. Mango chips under atmospheric frying had less carotenoid retention (32%) than those under vacuum frying (up to 65%). These results may help further optimize vacuum-frying processing of high-quality fruit-based snacks.     

Storage stability of a stimulant coconut water–acerola fruit juice beverage

The objective of this work was to study the stability of a beverage formulated with acerola fruit juice and green coconut water with added caffeine. The beverage was prepared with 25% acerola pulp, 75% green coconut water and sugar up to 12°Brix, and caffeine (125 mg L⁻¹), heat processed at 90 °C for 30 s and packed in 250-mL glass bottles. Chemical, physicochemical, microbiological and sensory analyses of the beverage were performed just after processing and during 6 months of storage at room temperature (27 °C). The vitamin C content decreased significantly throughout storage, from 399.5 to 189.6 mg 100 mL⁻¹, although it has remained relatively high. The anthocyanins initially present (0.025 mg 100 mL⁻¹) were completely lost during the storage at a mean rate of 4 μg 100 mL⁻¹ month⁻¹. The product was microbiologically stable during storage. Colour changes were also observed with absorbance at 420 nm, with average values ranging from 0.19 to 0.24. However, according to the sensory analyses the product was acceptable during the 6 months of storage, presenting sensory scores (colour, taste and global acceptance) from 6.5 to 5.5, which suggests its potential for market.     

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.     

Physiology of pre-cut mango. I. ACC and ACC oxidase activity of slices subjected to osmotic dehydration

Physiologically mature mangoes were ripened for 6 days at 24°C and 98% relative humidity. Slices from these fruits were osmotically dehydrated by immersion in sucrose 65°Brix at 30°C and 211 mbar vacuum during 30 min. Slices not subjected to osmotic dehydration (NOD) and slices with osmotic dehydration (OD) were stored at 24, 13 or 5°C. The respiration rate of both slice types was affected by the storage temperature. 1-Aminocyclopropane-1-carboxylic acid (ACC) synthesis indicated activity of ACC synthase in both slices as well as in the whole fruit. ACC oxidase activity was greater in OD slices as compared to NOD and that was associated to better membrane stability. OD favored compaction of external layer in slices. No ethylene was detected in slices; however, the tissues did not lose their ability to synthesize ethylene. Results suggest that OD under vacuum may be beneficial as a pre-treatment of mango slices for longer shelf life under refrigeration.     

Osmotic Dehydration and Vacuum Impregnation on Physicochemical Properties of Chilean Papaya (Carica candamarcensis)

ABSTRACT: The effect of osmotic dehydration (OD) at atmospheric pressure and vacuum impregnation (VI) treatments on some physiochemical parameters of papaya (aw, pH, color, firmness, and microstructure) was analyzed. Osmotic treatments were carried out on papaya with 55°Brix and 65°Brix sucrose solutions at 30 °C. VI with 65°Brix osmotic solution was the most effective in reducing aw due to the highest sucrose gain during osmotic treatment. Color differences were associated to loss of clarity in line with transparency gain. Scanning electron microscopy observations show that osmotic dehydration caused shape changes and size reduction of papaya cells; also differences in microstructural features were observed between OD-treated and VI-treated samples. Moreover, the largest firmness observed in VI samples compared with OD treatments was associated with the thickness of the middle lamella between cells, which was greater in VI than OD treatments. Improvement in texture and palatability of papaya was obtained with VI rather than OD treatment compared with fresh papaya.     

Ascorbic acid loss and sensory changes in intermediate moisture pineapple during storage at 30–40°C

Kinetics of ascorbic acid loss as a function of storage temperature (30, 35 and 40°C) and water activity of intermediate moisture pineapple slices at a _W of 0.92, 0.95 and 0.97 was modelled. Loss was faster at higher a _W but storage temperatures of 30 - 40°C had little effect at the same a _w. Colour changes were larger than flavour changes. Spoilage was only a problem at high a _w.     

Mass transfer kinetics and regressional‐desirability optimisation during osmotic dehydration of pumpkin,kiwi and pear

Mass transfer kinetics and optimisation of osmotic dehydration (OD) of fruits and vegetables with diverse structures were studied. Different concentrations of sucrose (20–60 °Brix) and process times (0–24 h) were used. Magee’s model was appropriate for predicting water loss (WL), while Azuara’s model fitted well solids gain (SG) data and represented more accurately the evolution of the complete process close to equilibrium. Polynomial equations for each kinetic variable [WL, SG and weight reduction (WR) – for pumpkin, kiwi and pear] using multiple linear regression were fitted for a selected range of experimental data (30–240 min, 20–60 °Brix). A complete solution algorithm for desirability function was coded in Matlab^® 7.2 (Mathworks, Natick, MA, USA) with the aim to optimise osmotic dehydration process in terms of WL, SG and WR; optimal conditions were found for each fruit. Besides, an optimal common zone was identified for OD corresponding to process time from 114 to 240 min and sucrose concentration from 54 to 60 °Brix.     

Rheological behaviour of blueberry cloudy juice (Vaccinium corymbosum L.)

The rheological behaviour of blueberry cloudy juice ( Vaccinium corymbosum L.), prepared from fresh berries, was studied as a function of solids concentration, in the range 25–61 °Brix, at different temperatures (5–50 °C), with a control rate (CR) rheometer. Cloudy juice was concentrated by vacuum evaporation, and solutions at other concentrations were prepared from the concentrate by adding the liquid collected as distillate. The juices were slightly thixotropic between 45 and 61°Brix. For lower concentrations, the thixotropy was low and could be easily eliminated by shearing. The power law model was applied to fit the pseudoplastic behaviour. The activation energy for flow, evaluated with the Arrhenius–Guzman equation, increased with solids concentration and ranged from 4.6 to 24 kJ gmol^–1. A non-linear correlation, accounting for temperature and concentration effects on the consistency coefficient, was identified.     

Production, purification and thermal characterisation of invertase from a newly isolated Fusarium sp. under solid-state fermentation

Production of invertase employing a newly isolated Fusarium sp. under solid-state fermentation was optimised. Different process parameters were optimised. The maximum enzyme activity under optimum conditions was 47.23 ± 2.12 U gds⁻¹ with nitrogen additives. The enzyme was purified by ammonium sulphate precipitation, diethylaminoethyl cellulose ion-exchange chromatography and Sephadex gel filtration. This protocol gave 20.25-fold purification and 5.53% recovery. The optimum pH and temperature for activity were 5.0 and 50 °C. The K _m and V _max values for the enzyme were 8.33 m m and 21.48 μmol min⁻¹, respectively. A detailed kinetic study of thermal inactivation has been carried out. Enthalpy of activation (Δ H *) decreased when entropy (Δ S *) of activation increased at higher temperatures. Moreover, free energy of denaturation (Δ G *) increased at higher temperature making the enzyme thermally stable. A possible explanation for the thermal inactivation of invertase at higher temperatures is also discussed.     

Inverse Method to Estimate Kinetic Degradation Parameters of Grape Anthocyanins in Wheat Flour Under Simultaneously Changing Temperature and Moisture

ABSTRACT:  Thermal and moisture effects on grape anthocyanin degradation were investigated using solid media to simulate processing at temperatures above 100 °C. Grape pomace (anthocyanin source) mixed with wheat pastry flour (1: 3, w/w dry basis) was used in both isothermal and nonisothermal experiments by heating the same mixture at 43% (db) initial moisture in steel cells in an oil bath at 80, 105, and 145 °C. To determine the effect of moisture on anthocyanin degradation, the grape pomace–wheat flour mixture was heated isothermally at 80 °C at constant moisture contents of 10%, 20%, and 43% (db). Anthocyanin degradation followed a pseudo first-order reaction with moisture. Anthocyanins degraded more rapidly with increasing temperature and moisture. The effects of temperature and moisture on the rate constant were modeled according to the Arrhenius and an exponential relationship, respectively. The nonisothermal reaction rate constant and activation energy (mean ± standard error) were k _{80 °C, 43% (db) moisture} = 2.81 × 10⁻⁴± 1.1 × 10⁻⁶ s⁻¹ and Δ E  = 75273 ± 197 J/g mol, respectively. The moisture parameter for the exponential model was 4.28 (dry basis moisture content)⁻¹. One possible application of this study is as a tool to predict the loss of anthocyanins in nutraceutical products containing grape pomace. For example, if the process temperature history and moisture history in an extruded snack fortified with grape pomace is known, the percentage anthocyanin loss can be predicted.     

KINETICS OF OSMOTIC DEHYDRATION IN ORANGE AND MANDARIN PEELS

The nutritional and health properties of some citrus peel components such as pectin, flavonoids, carotenoids or limonene make interesting developing processing methods to obtain peel stable products, maintaining its quality attributes, increasing its sweetness and improving its sensory acceptability. In this sense, osmotic dehydration represents a useful alternative by using sugar solutions at mild temperature. Kinetics of osmotic treatments of orange and mandarin peels carried out at atmospheric pressure and by applying a vacuum pulse at the beginning of the process were analysed at 30, 40 and 50C, in 65 °Brix sucrose, 55 °Brix glucose and 60 °Brix rectified grape must. Vacuum pulse greatly affected mass transfer behavior of peels due to the greatly porous structure of albedo. So, PVOD treatments greatly accelerate the changes in the product composition in line with an increase in the peel sample thickness. In osmotic processes at atmospheric pressure, sample impregnation occurs coupled with osmotic process, but much longer treatments are required to achieve a reasonable concentration degree which assures sample stability. Low viscosity osmotic solutions seems recommendable in order to promote both diffusional and hydrodynamic transport, in vacuum pulsed pretreatments at mild temperatures.     

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.     

Optimisation of osmotic dehydration process of guavas by response surface methodology and desirability function

Response surface methodology was used to assess the effects of osmotic solution concentration (40–60°Brix), process temperature (20–40 °C) and vacuum pulse application time (0–20 min) at 100 mbar on water loss (WL), weight reduction (WR), solid gain (SG), water activity (a_w), colour parameters and mechanical properties of guava slices. Optimal process conditions were determined through the desirability function approach and quality characteristics of osmotically dehydrated guavas were analysed. Only models obtained for WL, WR and a_w were suitable to describe the experimental data. The desirability function showed that optimal conditions for osmotic dehydration of guavas were: osmotic solution concentration at 60°Brix, process temperature at 32 °C and 20 min of vacuum pulse application. Under optimal conditions, colour and mechanical properties of treated guavas were similar to fresh fruit, presenting WL of 29.01 g/100 g, WR of 25.91 g/100 g, SG of 3.10 g/100 g and a_w of 0.979.     

Pineapple Candying at Mild Temperature by Applying Vacuum Impregnation

ABSTRACT: The candying process usually takes a long time and is accomplished at high temperatures, implying damage in the product sensory properties. Sample vacuum impregnation, followed by successive osmotic steps (2 or more) in sucrose solutions of increasing concentrations, at 15 and 30 °C, has been used in order to improve pineapple candying process. Sample weight (and volume in some cases) has been controlled throughout the process till no notable changes occurred. Then, sample composition and water activity were analyzed. A 3-step process (VI with 25 °Brix followed by successive immersions in 55 and 65 °Brix solutions) provided good process yield, with better sensory properties when working at 15 °C.     

Effect of thermal processing on the texture of canned apricots

The effect of thermal processing on the texture of canned apricots was studied by using the main cultivar canned in Greece, Bebecou. The test temperature ranged from 82 to 95 °C. The loss of hardness was tested immediately after processing and after 3 months storage. The z -value was 16.7 °C and the energy of activation 116.5 kJ mol⁻¹. Some restoration in the hardness was found after 3 months storage, which might be attributable to the absorption of sucrose by the fruit.