Rehydration Kinetics Research Articles

Mechanisms of formation and disintegration of alginate beads obtained by prilling

In this paper, compendial sodium alginate beads have been manufactured by laminar jet break-up technology. The effect of polymer concentration, viscosity and polymeric solution flow rate on the characteristics of beads was studied. Size distribution of alginate beads in the hydrated state was strongly dependent on the flow rate and viscosity of polymer solutions, since a transition from laminar jet break-up conditions to vibration-assisted dripping was observed. The re-hydration kinetics of dried beads in simulated gastric fluid (SGF) showed that the maximum swelling of beads was reached after 1–2 h, with an increase in volume of two to three times and a time lag dependent on the polymer concentration. The re-hydration swelling profiles in simulated intestinal fluid (SIF) showed no time lag and higher swelling volume; moreover, in this medium after the maximum swelling was reached, the bead structure was quickly disaggregated because of the presence in the medium of phosphate able to capture calcium ions present in the alginate gel structure.

Rehydration of Dehydrated Foods

: Dehydrated products readily take up moisture when immersed in a liquid medium, leading to significant changes in their thermophysical properties. The rehydration kinetics, the structural properties (apparent density, true density, specific volume and internal porosity), the viscoelastic behavior (compression tests), and the flavor losses were investigated during rehydration of various fruits and vegetables. The effect of temperature on the above properties was also investigated for some fruits and vegetables dehydrated by different drying methods (convective, vacuum, freeze, and osmotic drying). The results showed that the water temperature influences the rehydration kinetics and the equilibrium moisture content of the rehydrated. The structural properties of rehydrated foods appear to show a hysteresis compared to those measured during dehydration processes. The shrinkage that takes place during dehydration prevents rehydration and produces products with lower apparent density and higher porosity. Structural damages that occur during drying seem to affect also the viscoelastic behavior of the rehydrated foods and the hysteresis phenomenon is also observed at the textural properties, with a degree varying between the different drying methods. More specifically, freeze-dried materials present the highest hysteresis after rehydration, losing their elasticity and becoming more viscous. Osmotic pretreatment seems to help freeze-dried materials to keep their elastic nature probably due to solids gain. Air- and vacuum-dried materials showed the smallest hysteresis tense, keeping their viscoelastic characteristics during rehydration close to those of dried materials. Flavor losses seem to have lower rates during rehydration comparing to those observed during drying. The maximum retention of flavor has been observed in the rehydrated products after freeze-drying, and flavor levels are close to those observed for fresh boiled foods.

Combined effect of gamma-irradiation and osmotic treatment on mass transfer during rehydration of carrots

The exposure to irradiation pretreatment results in increase in cell wall permeabilization, leading to softening of tissue and the loss of turgor pressure, thereby affecting the reconstitution properties. Rehydration kinetics of gamma-irradiated (3.0–9.0 kGy) and water (0° Brix) or osmotic pretreated (up to 50° Brix) carrot samples were studied. The diffusion coefficient for water infusion and solute loss during rehydration of irradiated and osmotically pretreated and finally dried samples were estimated considering Fickian diffusion. The results shows that as increase in doses of gamma irradiation resulted in decrease in water and increase in solute diffusion coefficient. The pretreatment with water (0° Brix) resulted in increased water gain and reduced solute loss during rehydration for irradiated samples. The samples pretreated with osmotic solution of 10° Brix (concentration more than the natural cell water concentration) could not absorb more water in comparison with control because of cell permeabilization caused by the osmotic treatment due to osmotic stress. The water diffusion coefficient values decreased and solute diffusion coefficient increased with an increase in dose rates. The equilibrium moisture as well as equilibrium solid content were dependent on the treatment conditions.

Rehydration process of Boletus edulis mushroom: characteristics and modelling

AbstractRehydration of air‐dried Boletus edulis mushrooms was investigated at six temperatures (25, 30, 40, 50, 60 and 70 °C). To describe the rehydration kinetics, two empirical equations, Peleg and Weibull, and a diffusion model for a slab were considered. The empirical models described the rehydration process properly, while the diffusion model also described experimental data adequately when considering the moisture‐dependent effective diffusion coefficient. The equilibrium moisture content increased in line with temperature up to 60 °C, then decreased. The kinetics constants of the Peleg and Weibull models, k1 and β respectively, were affected by temperature. This influence of temperature can be expressed in term of an Arrhenius relationship, with an average activation energy of 19.2 kJ mol−1. Copyright © 2005 Society of Chemical Industry

Rehydration Characteristics of Osmotic Pretreated and Dried Onion

The effect of osmotic pretreatments (10% sodium chloride for 1 h or 50% sucrose for 3 h treatment) on rehydration kinetics of dried onion was studied over a range of temperatures (25–658C) and compared with an untreated (control) sample. The effective diffusion coefficients for water and solute were determined by considering the rehydration process to be governed by Fickian diffusion. The diffusion coefficients for water absorption and solute infusion were found to be in the range 1.96 � 1029 to 8.04 � 1029 and 2.94 � 1029 to 5.53 � 1029 m2 s21, respectively. Osmotic pretreatment resulted in a decrease in the diffusion coefficient of water as well as an increase in the diffusion coefficient of solute during rehydration. Decrease in the diffusion coefficient of water was due to an increase in the proportion of ruptured and shrunken cells caused by osmotic treatments, which in turn resulted in reduced ability of dried onion tissue to absorb water. The increase in diffusion coefficient for solid during rehydration was higher in the case of osmotic pre-treated material, some of the solids absorbed during osmotic dehydration were not retained in the cell matrix and dissolve faster than the constitutive onion dry matter.

Modelling the hydration of foodstuffs

The rehydration kinetics of dried foodstuffs is of critical importance to their sensory properties and delivery of flavour and functional molecules. Based on the dynamics of capillary flow in partially saturated porous media, a finite element model is developed to predict the infiltration of water into dried food products taking into account temperature effects. The finite element model is based on the mixed form of the mass conservation equation. The constitutive relationships of water retention and hydraulic conductivity are adopted from the fields of hydrology and soil science. The transfer properties of water in the porous medium depend on the moisture content and the microstructure. This is contrast to the constant transfer properties often used in the heat and mass transfer models developed for foods. Rehydration of green tea as a function of temperature has been simulated and results are compared with NMR measurements. There is good agreement.

Viscosity and starch particle size effects on rehydration of freeze‐dried carrots

AbstractA Correction has been published for this article in Journal of the Science of Food and Agriculture 85(4): 707 (2005).Rehydration is a critical step in the utilization of dry food particulates. The effects of viscosity and average particle size of four starches, namely corn, rice, potato and pre‐gelatinized Ultra Sperse 2000 were assessed and compared with water as the control. Average starch particle size diameters determined with image analysis showed that only the native starches swelled after 5 min treatment in 85 °C water. Rehydration curves were modelled utilizing a normalized Weibull distribution. The derived Weibull β‐shape parameter indicated that water uptake followed both imbibition and Fickian diffusion. Both viscosity and starch particles size distribution influenced the rehydration kinetics and their effect could not be explained by a simple mechanism. Medium uptake during rehydration could be improved and customized by choosing the most appropriate thickening agent. Copyright © 2004 Society of Chemical Industry

Influence of osmotic pre-treatments on rehydration characteristics of carrots

The effect of water (0 °Brix), as well as osmotic pre-treatment (5–60 °B), on the rehydration kinetics of carrot slices has been investigated, and compared with control samples. The diffusion coefficient for water infusion and solute loss during rehydration of osmotically pre-treated and finally dried samples were estimated considering unsteady state mass transfer during rehydration. The result shows that treatment up to 10 °B resulted in an increase in moisture diffusion and decrease in solid diffusion. It may be due to the lower concentration of sucrose, which imparts structure and mechanical strength to the tissue, as well as maintaining the general protein (present in plant cells membrane) structure in the dry state and hence, the protecting membrane shows its functionality upon rehydration. Maximum water diffusion and lowest solids diffusion was observed for samples treated with water (0 °B). The samples pre-treated with more than 20 °B could not absorb as much water, in comparison with the control, because of cell permeabilization, due to osmotic stress. At the same time, solute loss during rehydration increased, possibly due to structural changes induced by osmotic pre-treatment, and interaction of the osmoactive substance with the cell component.

Magnitude and kinetics of rehydration influence the viability of dehydrated E. coli K-12.

The influence of rehydration conditions on the recovery of Escherichia coli K-12 was studied. The results showed that the osmotic pressure gradient of rehydration shock realized before plating greatly affected cell viability. When rehydration occurred quickly from an hyperosmotic level of 133 MPa in glycerol solution before slow rehydration by plating on an agar surface to reach initial osmotic pressure (1.4 MPa), bacterial viability was strongly related to the intensity of the hypo-osmotic gradient used. Rehydration to 107 MPa resulted in a survival ratio of 41%, whereas strong rehydration to 1.4 MPa resulted in only 0.7% survival. These studies also demonstrated the influence of the rehydration kinetic on cell recovery. An optimal rehydration rate of 0.136 MPa x s(-1) increased cell recovery by a factor of 40 when compared with the faster and slower rates of 131.6 MPa x s(-1) and 0.006 MPa x s(-1), respectively.

Hydroxylation and Dehydroxylation Behavior of Silica Glass Fracture Surfaces

The hydroxylation and dehydroxylation behavior of amorphous silica fracture surfaces was studied using temperature‐programmed static SIMS. The results show that vacuum heat treatments result in more extensive condensation of silanol groups on the silica glass fracture surface as compared to fumed silica (Cabosil). This is attributed to differences in the distribution of silanol groups on the two silica surfaces. The rehydration kinetics of the dehydroxylated silica fracture surfaces showed two distinct reaction rates—an initial rapid increase in the silanol concentration, followed by a slower rehydration for longer dosing times. The slower rehydration reaction was shown to follow first‐order reaction kinetics with the reaction rate constant, suggesting hydrolysis of strained siloxane bonds on three‐membered silicate ring structures. The much faster initial rehydration is attributed to the hydrolysis of extremely strained siloxane bonds in two‐membered, edge‐shared tetrahedral rings. The effect of the dehydration time and temperature (i.e., thermal history of the surface) on the rehydration kinetics is also discussed.

Rehydration kinetics of dehydrated products

The rehydration kinetics of various dehydrated fruits and vegetables (apple, potato, carrot, banana, pepper, garlic, mushroom, onion, leek, pea, corn, pumpkin, and tomato) were examined during rehydration at different temperatures. Dehydrated products were produced in an air dryer at 70 °C and 10% air humidity, and subsequently they were rehydrated in a bath of water tempered at 40, 60 and 80 °C. A first order kinetic model was used, in which the rate constant and the equilibrium moisture content are functions of the process variable (water temperature). The parameters of the model considered were found to be greatly affected by the water temperature during rehydration. In particular the temperature increment increases the rate of rehydration and the equilibrium moisture content of the rehydrated products.

Rehydration of casein powders: effects of added mineral salts and salt addition methods on water transfer

Enrichment of milk with micellar casein decreases water transfer during the rehydration of milk powders. In this study, the effects of the ion environment and the ion addition method on the rehydration kinetics were found to be dependent on the changes in the micellar casein. For example, adding citrate or phosphate solution to the micellar casein suspension before drying considerably increased rehydration rates and this was related to the destruction of the micelle structure. Water transfer in the casein suspension was improved by adding NaCl during rehydration: this effect may be explained by the more hygroscopic nature of NaCl rather than by extensive modification of the micellar structure. The addition of CaCl2 considerably affected micelle organization and led to the formation of insoluble structures during spray drying.

Capillarity and sap ascent in a resurrection plant: does theory fit the facts?

Capillarity and sap ascent in a resurrection plant: does theory fit the facts?

Modelling of the rehydration process of brocolli florets

The aim was to study the rehydration kinetics of branched bodies such as broccoli florets, at different temperatures, and to this end a diffusional model was used considering the product structure as a sum of different sized cylinders. The rehydration rate was observed to increase with water temperature. The equilibrium moisture content of the product linearly decreased as the water temperature increased (r=0.95). A close fit between the model and the experimental data was obtained when the effective diffusion coefficient (Deff) and equilibrium moisture content (We) were identified at each temperature (average explained %var=99.65). The temperature influence on Deff was interpreted by the Arrhenius relationship, with an activation energy of 89.73 kJ/mol.

Rehydration Kinetics of High‐Pressure Pretreated and Osmotically Dehydrated Pineapple

ABSTRACT Rehydration kinetics of high‐pressure pretreated (100, 300, and 500 MPa for 10 min) and osmotically dehydrated pineapple (Ananas comsus) cubes (2 × 2 × 1 cm) were studied at different temperatures (5, 25, and 35°C), and compared with ordinary osmotically dehydrated samples. The effective diffusion coefficients for water and solute were determined, assuming the rehydration process to be governed by Fickian diffusion. Diffusion coefficients for water absorption into the tissue as well as for solute diffusion out of the tissue were found to be lower in the samples subjected to high‐pressure treatment. Further, the diffusion coefficients decreased with increase in treatment pressure. A possible explanation for the observed decrease in diffusion coefficients can be attributed to the permeabilization of cell membranes, the release of cellular components, and structural changes of the cell materials. The diffusion coefficients were correlated with rehydration temperature (T) and treatment pressure (P) by an Eq. of the form D = A exp[–(B.P + C/T)], where A, B, and C are constants.

Rehydration and Phase Changes of Potassium Acetate-Intercalated Halloysite at 298 K

The kinetics of rehydration of potassium acetate-intercalated halloysite are slow as determined by X-ray diffraction. Compared with the rehydration of potassium acetate-intercalated kaolinite where rehydration takes 20 min, the rehydration of the halloysite–intercalation complex takes 4 h. X-ray diffraction shows the halloysite with an initial d(001) spacing of 7.13 Å expands to 13.97 Å with the insertion of the potassium acetate through intercalation. Upon heating to 250°C, two expanded phases with d(001) spacings of 11.84 and 9.80 Å are observed which upon cooling to 25°C become 11.72 and 8.98 Å. Upon exposure to air, multiple expanded phases are observed. The molecular structure of the potassium acetate-intercalated halloysite during cooling from 250°C and upon rehydration has also been studied using Raman microscopy. Raman spectroscopy shows that although the halloysites have been completely expanded, intercalation is incomplete as evidenced by the intensity of the inner surface hydroxyl band at 3695 cm−1. Thermal treatment shifts the band attributed to the inner hydroxyl at 3620 cm−1 to 3632 cm−1 at 250°C, which upon cooling returns to the starting frequency. The interaction between the acetate and the inner surface hydroxyls is observed by the hydroxyl stretching frequency of 3605 cm−1. Upon dehydration the band shifts to 3599 cm−1. The observation of multiple expanded phases of halloysite during rehydration is strongly supported by the changes in the bands associated with the inserting acetate ion. Two C–C and two C=O stretching vibrations are observed. It is proposed that the acetate inserts into the halloysite layers with two different molecular structures.

Modelling of broccoli stems rehydration process

Using a diffusional model, mass transfer in rehydration of slab-shaped bodies was simulated. For that purpose the rehydration kinetics of broccoli stem slabs was investigated and the influence of temperature determined. It was observed that rehydration rate increased with water temperature but the final moisture content decreased. A good agreement between the model and the experimental data was obtained when Deff and We values were identified for each temperature (average %var 99.3). The temperature influence on Deff was interpreted by the Arrhenius relationship, with an activation energy of 17.9 kJ/mol.

Saccharomyces cerevisiae viability is strongly dependant on rehydration kinetics and the temperature of dried cells.

The effects of rehydration kinetics and temperature on the viability of Saccharomyces cerevisiae dehydrated by drying were studied. During rehydration, a water activity range of 0.117-0.455 must be crossed slowly in order to maintain cell viability. If this range is crossed rapidly, cell viability can be preserved if rehydration takes place at 50 degrees C. Several hypotheses have been proposed to explain previous results. One hypothesis, which relates cell mortality after rapid rehydration to water flow through the membrane in phase transition, is the more plausible and requires further investigation.

MICROWAVE and CONVECTIVE DRYING of POTATO SLICES

Potato slices were dried using microwave drying, combined microwave and convective drying, and convective drying. Rehydration kinetics were also studied. Drying rates of the different drying methods were determined and microwave drying was compared with convective drying. Microwave drying has a potential for producing better quality dried products while considerably reducing drying duration (i.e., 10 min vs. 10 h). Effective moisture diffusivity profiles were calculated using Fick's diffusion model in one dimension. Statistical analysis showed diffusivity to increase with increasing internal temperature but to decrease (in microwave drying) with increasing moisture content, with high correlation (R2≥ 0.973).

Cinética de rehidratación y deterioro biológico en Zea mays almacenado a una Humedad Relativa del Aire Intergranular (HRAI) del 92%

Maize grains ( Zea mays var indurata) «Plata» tipe destined to commercialization were submitted to a relative humidity of the intergranular air of 92% during 120 days to 25 °C. The kinetics of rehydration, humidity and biological damage («mouldy» «blue eye» and «spoiled») was determined. After 14 to 17 days, the sample rehydrates on the basis of commercialization of 14,5% of humidity and in the same period a degree of standard of commercial quality is lost. It is established that the basis of commercialization of 14,5% is very near the limit which favours the biological damage.