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.     

OSMOTIC DEHYDRATION OF PINEAPPLE

The effects of sugar type, sugar concentration, immersion time and temperature on the mass transfer of osmotic dehydration were studied using pie shape slices (7 mm thick with its center cork thrown away) of fresh pineapple (Queen variety, 90% maturity). The dehydration process was performed using two type of sugar as an osmotic agent (glucose and sucrose), three levels of sugar concentration (50, 60, and 70%), three levels of temperature (30, 50, and 70 °C), and three levels of immersion time (3, 6, and 9 hours). Following the osmotic dehydration process, the pineapple was dried at 70 °C for 48 hours. The mass transfer was then calculated and analyzed statistically. Sugar type, concentration, temperature, and length of immersion, have a significant effect on the mass transfer of osmotically dehydrated pineapple. The highest mass transfer of pineapple was found by using sucrose at the concentration of 70%, temperature 50 °C and 9 hours of immersion time.     

Use of Artificial Neural Network and Image Analysis to Predict Physical Properties of Osmotically Dehydrated Pumpkin

The objectives of this research were to predict, using neural networks, the color intensity (ΔE), percentage of shrinkage as well as the Heywood shape factor, which is the representative of deformation, of osmotically dehydrated and air dried pumpkin pieces. Several osmotic solutions were used including 50% (w/w) sorbitol solution, 50% (w/w) glucose solution, and 50% (w/w) sucrose solution. Optimum artificial neural network (ANN) models were developed based on one to two hidden layers and 10–20 neurons per hidden layer. The ANN models were then tested against an independent data set. The measured values of the color intensity, percentage of shrinkage, and the Heywood shape factor were predicted with R² > 0.90 in all cases, except when all the drying methods were combined in one data set.     

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*.     

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?.     

Influence of ohmic heating on rheological and electrical properties of reconstituted whey solutions

Reconstituted whey solutions (in the range of 8–24% w/v solute concentrations) were heated from 20 °C to prescribed temperatures (30, 40, 50, 60, 70 or 80 °C) ohmically by applying voltage gradients of 20, 30 or 40 V/cm, and conventionally at water bath. Electrical conductivity changes with increasing temperature were linear during ohmic heating. Whey solutions have non-Newtonian characteristics since Herschel–Bulkley model satisfactorily fitted the experimental shear stress–shear rate data. “n” values were in the range of 0.520–1.503. The whey solution having 24% concentration had a yield stress of 0.006–0.024 Pa at low temperatures. Although temperature and concentration were critical factors for the consistency and the electrical conductivity changes during heating (p < 0.01), the voltage gradient was not effective statistically. The high correlation (between ?0.910 and ?0.991) was obtained between changes of electrical conductivity and consistency coefficient values during ohmic heating. Since activation energies for ohmic heating (26.34–45.79 kJ/mol) depending on solute concentration were lower than conventional heating (26.70–50.04 kJ/mol), reconstituted whey solutions were less sensible to temperature changes during ohmic heating. It was recommended that ohmic heating could be applied as a faster alternative heating method in the whey processing.     

Effect of Osmotic Dehydration with Pulsed Vacuum on Hot-Air Drying Kinetics and Quality Attributes of Cherry Tomatoes

The effects of osmotic dehydration (OD) with or without pulsed vacuum (PV) on hot-air drying kinetics and quality attributes of cherry tomatoes were investigated. Both OD and PVOD pre-treatments were performed for 3 h at 50°C in 50 and 70^o Brix sucrose solutions with a solution-to-fruit mass ratio of 4:1, and PVOD was applied for 15 min before OD at atmospheric pressure. Samples were further dried at air temperature of 70°C. Effective moisture diffusivity (D _eff) of osmotically dehydrated samples increased gradually while the D_eff curve of fresh samples had a plateau stage during hot air drying. Lower glass transition temperature, T_g, values of osmotically dehydrated samples indicated that they needed a lower storage temperature. Both OD and PVOD pre-treatments had advantages in shortening drying cycles and improving quality of products. Compared with air drying, osmo-air drying decreased the total drying time, color change, and hardness of dried samples by 32.26%, 18.11%, and 88.21%, respectively, and increased volume ratio and vitamin C retention rate by 72.31% and 125.82%. As compared with OD, PVOD decreased color change and hardness by 28.48% and 45.17%, increased volume ratio and vitamin C retention rate by 27.41% and 17.77%, but there was no significant difference shown in drying time. Therefore, osmotic pre-treatment can shorten the total dehydration time, and improve the general quality of dried cherry tomatoes.     

Application of Osmotic Dehydration Technology on Jam Processing

A new jam technology was developed in which osmotically dehydrated fruits were used directly in jam processing. Since the fruits undergo a process at lower temperature (35-40°C) for short time during osmotic dehydration, the new jam product exhibits surperior natural colour, good flavour and overall quality. Sugar solution spent in osmotic dehydration treatment can be used in jam.     

Application of Osmotic Dehydration Technology on Jam Processing

ABSTRACT

A new jam technology was developed in which osmotically dehydrated fruits were used directly in jam processing. Since the fruits undergo a process at lower temperature (35-40°C) for short time during osmotic dehydration, the new jam product exhibits surperior natural colour, good flavour and overall quality. Sugar solution spent in osmotic dehydration treatment can be used in jam.     

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.     

SHRINKAGE CHARACTERISTICS OF STRAWBERRIES OSMOTICALLY DEHYDRATED IN COMBINATION WITH MICROWAVE DRYING

The subject of this paper is to study the shrinkage of strawberries as a function of the moisture ratio during microwave-convective drying. Strawberries were pre-treated and osmotically dehydrated and dried at power levels 0.1 and 0.2W/g, based on initial mass. The main results showed that the shrinkage has a linear relation with moisture ratio; the equivalent diameter of the strawberry has a reciprocal logarithmic function with moisture ratio; the change in volume was bigger for the fruits osmotically dehydrated than for the not osmotically dehydrated ones, power level exerts influence on shrinkage and equivalent diameter for pre-treated and osmotically dehydrated samples.     

Preparation and nutritional characteristics of a hydrolysate from pepitona (Arca zebra)

Two soluble products resulting from the hydrolysis of pepitona (Arca zebra) were prepared as flour. Papain at its optimum hydrolysis conditions, previously established, was the enzyme used (40 degrees C for two hours at a pH of 7 in the proportion of 0.3% weight/enzyme/100 g meat). The hydrolysate obtained was then subjected to two different dehydration techniques: drum drying at 121 degrees C and 18 seconds retention, and spray drying at 101 degrees C and 40 psi pressure. The products were then stored for a five-month period at a temperature of 25 degrees C +/- 2 degrees C, time during which chemical determinations were performed in both hydrolysates. Findings showed that the time of storage does exert a significant effect of deterioration on the products. The greater and more significant quality losses occur during the first two months. The dehydration techniques used also affect significantly the soluble nitrogen content, and non-protein nitrogen and soluble solids content, as well as color of pepitona hydrolysates. Spray-drying dehydration technique does not have a significant deteriorating effect. Biological studies undertaken demonstrated that the quality of both hydrolysates is satisfactory from the nutritional and amino acid composition points of view. A protein efficiency ratio (PER) of 2.27 and 2.29 was determined for the hydrolysate dehydrated by drum drier and for the dehydrated by spray drier, respectively. With regard to amino acid composition, both had satisfactory levels of essential amino acids, with a lysine content of 6.9 g/100 g protein for the hydrolysate dehydrated by drum drying, and 8.6 g/100 g protein for the other hydrolysate dehydrated by spray drying.     

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

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

Optimizing Drying Conditions for Microwave-Vacuum (MIVAC®) Drying of Russet Potatoes (Solanum tuberosum)

MIVAC® combines microwave heating with vacuum drying. Microwave power is modulated based upon product temperature and can limit overheating compared to other microwave-vacuum methods. Blanched potatoes were dried at 50, 60, and 70°C for 0 to 150 min. Potatoes dried at 70°C had a lower moisture content in less time compared to potatoes dried at 50 and 60°C, but the color of the dehydrated potatoes was affected due to overheating. Drying at 60°C for 150 min resulted in dried potatoes with acceptable color. Drying at 50°C resulted in dehydrated potatoes of acceptable color; however, it required more time.     

Optimizing Drying Conditions for Microwave-Vacuum (MIVAC®) Drying of Russet Potatoes (Solanum tuberosum)

MIVAC® combines microwave heating with vacuum drying. Microwave power is modulated based upon product temperature and can limit overheating compared to other microwave-vacuum methods. Blanched potatoes were dried at 50, 60, and 70°C for 0 to 150 min. Potatoes dried at 70°C had a lower moisture content in less time compared to potatoes dried at 50 and 60°C, but the color of the dehydrated potatoes was affected due to overheating. Drying at 60°C for 150 min resulted in dried potatoes with acceptable color. Drying at 50°C resulted in dehydrated potatoes of acceptable color; however, it required more time.     

Design and performance evaluation of an ohmic heating unit for thermal processing of highly viscous liquids

The overall objective of this study is to design, build and test a prototype automated ohmic heating cell by using a fluid jet to serve as a basis for modification and scale-up for industrial use. The new technology consists of applying an alternative electrical current to a falling jet between two electrodes. The length of the jet is the primary critical process factor in ohmic heating using a fluid jet. A radar level sensor was validated at different operating conditions. Two air valves, that depressurize or pressurize the ohmic cell, ensure the control of the jet length for precision and reliability. The thermal performances and the technical feasibility of this innovated automated ohmic heater were approved. Tests with carboxymethylcellulose (CMC) solutions (2%, w/w) demonstrated that this technology can be efficient for thermal processing of highly viscous liquids.     

Degradation kinetics of ascorbic acid during ohmic heating with stainless steel electrodes

Ascorbic acid degradation experiments were performed in buffer solution at pH 3.5 using a batch ohmic heater with uncoated stainless steel electrodes. The electrical conductivity of the buffer solution was adjusted using sodium chloride. The concentration of ascorbic acid was found using an HPLC technique. Kinetics of degradation can be described adequately by a first order model for both conventional and ohmic treatments, but unlike conventional heating, the temperature dependence of degradation for some ohmic treatments cannot be represented by the Arrhenius relation. During ohmic heating, power, temperature and NaCl content affect the degradation rate. A number of reactions, including electrode reactions, electrolysis of the solution, as well as reactions between electrode materials and electrolysis products may influence the reaction mechanism as well as kinetic parameters. At the highest power and salt content, citrate complexation and a significant loss of buffering capacity were noted, resulting in an increase in pH. The results underline the importance of inert electrode coatings, or the use of high frequency power for control of electrochemical reactions.     

Reaction of d-glucose in water at high temperatures (410 °C) and pressures (180 MPa) for the production of dyes and nano-particles

An autoclave (120-mL) and an optical micro-reactor (50-nL) were used to study the hydrothermal decomposition of d-glucose at high temperatures and high pressures. During slow heating (0.18 °C/s) to 350 °C in the autoclave, water-soluble glucose (0.9 M) began to decompose at 220 °C and reacted completely at 280 °C. The initial decomposition products were 5-(hydroxymethyl)furfural and levoglucosan, and these subsequently converted into oil and solid residue, and finally to solid particles at a 65 wt% yield at 350 °C. When the same heating rate and temperature were used on glucose solutions in the micro-reactor, yellow and orange materials decomposed from glucose were produced. Numerous particles precipitated at 251 °C, and at 350 °C, all the glucose changed to an orange film and solid particles, which were nanoparticles as confirmed by SEM. However, when the glucose solution was rapidly heated to 410 °C (9.5-17 °C/s), yellow, brown and orange sugar-like materials were produced. A homogeneous phase with yellow color still remained at temperatures as high as 380 °C, and few particles formed until 410 °C. It can be concluded that micron-sized particles and colored solutions can be produced by slow heating, while rapid heating resulted in the formation of dye-like substances with glucose-like structures. The formation of colored solutions and particles may have technological implications in food or materials formation processes that use high temperature water with biomass feedstocks.     

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.     

Osmotic Dehydration of Mango Cubes: Effect of Novel Gluten-Based Coating

Edible coating before osmotic dehydration (OD) reduces solute uptake, which can deteriorate product quality. Polysaccharide-based coatings have been reported for this purpose with limited success. Gluten, due to its inherent film formation property, has the potential to be used for this purpose. In this study, mango cubes of different maturity were osmotically dehydrated with and without gluten-based coating. An edible coating was prepared through fractionation of gluten. Osmotic dehydration was performed at 50°C for 2 h with osmotic solutions (OSs) at various concentrations (45, 55, 65 °Brix). Coating improved the dehydration efficiency index (DEI) due to a substantial decrease in solute gain (SG) and slight increase in water loss (WL). The coating modified the physicochemical properties and this effect was significantly observed at 55 °Brix. Coated mango cubes were harder and shrank more than uncoated mango. The physicochemical properties after OD were also found to be influenced by the maturity of raw mangoes.