Osmotic Concentration – Drying of Mango Slices

ABSTRACT

The effect of initial water activity (a_w) of mango slices obtained by osmotic concentration pre-treatments with sucrose syrups on their air-drying behavior at 50, 60 and 70°C were evaluated calculating the apparent diffusion coefficients with second Fick's law. The increase in sucrose concentration or reduction in e_w of mango slices affects the moisture transport during the air drying step. The diffusion coefficients decrease with the increasing in solid content during the osmosis and increase with the drying temperature. The time needed to reach a final moisture content of 0.7 g water/g ds was 50 to 75% lower in osmosed mangoes than those obtained for fresh mango.     

THE INFLUENCE OF CONSTANT AND VARIABLE CONDITIONS ON THE DRYING KINETICS OF APPLES

Abstract

The purpose of this work was to examine the influence of constant and variable temperatures or velocities on apples' drying kinetics. The step changes of temperature influenced forcefully on apples1 temperature in all situations. The drying curves -6f apples showed the influence of air's temperature as well velocity on the water content. The largest difference between the apple drying curves and absolute value of drying rate are obtained after introducing step changes or rectangle impulses of air's temperature or velocity at the beginning of the process.     

THESIS SUMMARY

Abstract

In this thesis a brief literature review recalls the importance of a_w concerning the transport phenomena and physico-chemical transformations in food products during a heat treatment, and shows the absence of reliable methods to measure the a_w above 100°C. We present a new method to determine the sorption curves at h¯igh temperatures. This method is manomctric and dynamic. It has been used.

- to determine sorption isotherms and isobars for 4 different products (MCC, potato starch, cake dough, lactose), in the following temperature and pressure ranges, respectively 80 to 140°C, and 0.065 to 1 bar. Except the lactose sorption curves which have got an irregular shape because of some physico chemical transformations such as : crystallisation and changes of the crystal form, experimental sorption data have been fitted for each product with a good precision with a modified Oswin's model.

For products which curves can be modelling, sorption curves have been used to determine the isostcric heat with respect to moisture content and temperature. The difference between net isosteric heat and water evaporation heat is significant.

- to study the influence of a_w on the kinetics of cake coloration during the cooking. We found that kinetic reaction increases with temperature, but decreases with a_w. We have modelled the kinetics coloration at high temperature, and calculated the activation energies, for colour parameters, which decrease when a_wincreases.

This study could be carried out to determine sorption curves (isotherms + isobars) for other food products, and to enhance the final product quality during a heat treatment at high temperature.     

Impact of Swell-Drying Process on Water Activity and Drying Kinetics of Moroccan Pepper (Capsicum annum)

The impacts of total hot air drying (THD) and the swell-drying process (SD), which is hot air drying coupled with an instant controlled pressure drop DIC process, on the drying kinetics and adsorption isotherm behavior, were determined in the case of Moroccan peppers. Drying kinetics were studied through a model that takes into account the starting accessibility and the internal diffusion model with Fick's law. Adsorption experimental data were obtained by using the gravimetric method at 25, 40, and 50°C. Data were fitted to the GAB, Halsey, and Oswin models. Studied responses were: effective diffusivity (D_eff), starting accessibility (δW_s), monolayer moisture content (X_m), specific surface area (σ), internal porosity (?), and microstructure. Mean relative percentage deviation (E) and root mean square (RMS) were used to evaluate the fitting of models. Results showed that: pepper adsorption isotherms followed the type II isotherm curve;the equilibrium moisture content (X_eq) depended on the temperature and the drying method; the GAB and Halsey models fitted well the experimental data; at selected conditions of the DIC treatment (0.35 MPa and 5 s), the monolayer moisture content (X_m), the specific surface area (σ), and the internal porosity values were higher than those of THD samples; the DIC process intensified the drying operation, by increasing the effective diffusivity and the starting accessibility. Although the DIC process improves the drying kinetics, this behavior could be ignored related to a higher a_w. This study shows that, thanks to the new expanded structure obtained by the DIC process (higher porosity), the adsorption capacities of products are improved (a_w is reduced). Thus, the SD process improves both the drying operation and the stability of dried pepper products.     

OSMOTIC DEHYDRATION OP PAPAYA WITH CORN SYRDP SOLIDS

Abstract

The water activity (a_w) lowering capacity of four corn syrup solids (CSS) with nominal dextrose equivalent (DE) values of 10, 20, 30, and 40, and the osmotic concentration kinetics of papaya with solutions (50% w/w) of these solutes were studied. Apparent molecular weights (MW_a) were calculated and the a_wlowering capacity was characterized for each solute with Norrish's equation calculating a specific constant (K). The a_wlowering capacity increased as the DE value increased. Logarithmic relationships were found between the MW_aand the DE value and between K and DE (r ≥ 0.993).. The predicted a_wvalues using the MW_aand K of each CSS were in good agreement (average root mean square deviation ≤0.002) with the experimental data. Osmotic concentration was evaluated in terms of water loss (WL), loss of weight (LW) and solid gain (SG). The final values of SG increased and WL and LW decreased as the DE decreased. SG, WL and LW varied from 7.3 to 21.1% 76.1 to 49.1% and 72.0 to 28.0%, respectively for the CSS of 10 and 40 DE.     

Air impingement and intermittent drying: Application to apple and to mango

An original drying process combining air impingement and intermittent drying was studied on apple slices and mango cubes. The influence of four operating parameters (air velocity, drying/tempering periods, upper height, and air temperature) on the drying time and on the drying rate was evaluated. Continuous and intermittent drying were compared. The intermittency α = 1/7 (τ_on = 10 seconds and τ_off = 60 seconds) gave the best results. A time savings of 54% for apple and 67% for mango was reached. In continuous drying, a time savings of 4620 seconds was observed by increasing the air velocity from 6 to 40 m s^?1 for apple. Air temperatures of 328 K for apple and of 328 K or 338 K for mango were determined as optimum to prevent case‐hardening. Experimental results were fitted with the analytical solution of Fick's second law and the modified Page equation (average values R² = 0.985 and 0.961, for apple and mango, respectively). For both products, the apparent moisture diffusivity D_app, the drying constant k, the drying coefficient n, and the activation energy E_a, were identified. Activation energies calculated from the analytical solution were 30.3 and 36.8 kJ mol^?1 and were 25.4 and 30.0 kJ mol^?1 using the modified Page equation for apple and mango, respectively. Mango has an increased temperature sensitivity and thus will need less energy for drying than apple.     

Use of hyperspectral imaging for the prediction of moisture content and chromaticity of raw and pretreated apple slices during convection drying

The feasibility of using spectral reflectance information in the visible—near infrared (400–1,000?nm) region to estimate moisture content (g_W/g_DM) and chromaticity (CIELAB) of apple slices was investigated during convection drying. Apple slices were pretreated with hot water blanching (50 and 70°C), acid application (citric and ascorbic), and combinations thereof before drying at 50 and 70°C. Prediction models for the space-averaged spectral reflectance curves were built using the partial least square regression method. A three-component partial least square regression (PLSR) model satisfied the minimal root mean square error (RMSE) criterion for predicting moisture content (avg. RMSEP?=?0.13, r²?=?0.99); importantly, the critical wavelengths remained the same across all pretreatments (540, 817, 977?nm). Similarly, PLSR modeling showed that the optimal set of wavelengths (in terms of RMSE) were invariant across pretreatment for CIELAB a* prediction (543, 966?nm) and CIELAB b* prediction (510, 664, 714, 914, 969?nm). The stability of the information content of these wavelengths across pretreatments indicates their independence of color changes. Additionally, the spatial information in the hyperspectral images was exploited to visualize the performance of the predictive models by pseudo-coloring their values for each pixel in a single apple slice across different drying times. This visualization of spatial distribution of predicted moisture content and chromaticity changes shows significant potential for use in online monitoring of the drying process.     

Effect of water activity on autoxidation of raw peanut oil

The effect of water activity (a_w) on rate of autoxidation of raw peanut oil at 37 C was studied. Up to 3 weeks of storage, no marked effect of a_w on the rate of peroxide formation was noticeable. However, with prolonged storage, the rate of peroxide formation decreased rapidly at a_w of 0.67 and higher as compared to a_w of 0.50 and lower. At the end of 9 weeks’ storage, the peroxide value at a_w=0.02-0.50 was in the range of 31–35, while it was only ca. 10 at 0.79 and 0.92 a_w. There was no noticeable influence of a_w on the rate of free fatty acid formation. The oil stored for 7 weeks at 0.02 and 0.11 a_w had fresh flavor, although the peroxide values were high; whereas the oils at 0.79 and 0.92 a_w were stale, although the peroxide values were less than 10. The anisidine value at 0.02 a_w (348) was higher than at 0.92 a_w (62). The carbonyl values at 0.02 and 0.11 a_w were also higher (1090, 1021) than at 0.79 and 0.92 a_w (573, 780). The results indicate that the protective effect on the oil against peroxide formation at high a_w may be due to certain components present in raw peanut oil.     

A general method of predicting the water activity of ternary aqueous solutions from binary data

An isopiestic method has been developed to predict a_w for ternary solutions from binary a_w data. Another method, previously outlined by Robinson and Bower, has been extended to include non-electrolytes. Both methods have been used to predict a_w's for 51 representative systems. The results show that the isopiestic method is as good as, or better than Robinson and Bower's in predicting a_w for ternary aqueous electrolyte/ electrolyte, electrolyte/non-electrolyte and non-electrolyte/non-dectrolyte systems. Calculated a_w values are in good agreement (<0.5%> error on the average) with experiment. The methods are applicable to relatively concentrated solutions up to 12 m.     

ANALYSIS AND MODELING OF BROWNING OF THE GRANNY SMITH APPLE DURING DRYING

ABSTRACT

After proposing simple empirical models for drying kinetics and for sorption isotherms, the authors show that browning during drying of Granny-Smith apple samples can be calculated from models developped in static conditions. The time at which the browning begins (t_B) is similar to the induction time deduced from the static conditions ; it corresponds to a very low sample water content (around 1% dry basis) and to a sample temperature equal to the heating temperature. The “thermic past” (drying method, drying rate, heating temperature before t_B) has no influence on the browning rate, only the heating temperature after t_B is important     

Development of cantaloupe (Cucumis melo) pulp powder using foam-mat drying method: Effects of drying conditions on microstructural of mat and physicochemical properties of powder

In this study, the effects of drying conditions on moisture content, water activity (a_w), dissolution time, solubility, hygroscopicity, β-carotene, color, glass transition temperature (T_g), and sticky point temperature (T_s) of foam-mat-dried cantaloupe pulp powders and microstructure of dried cantaloupe pulp foams were investigated. Drying was performed in three temperatures (40, 55, and 70°C) on 3- and 5-mm thicknesses. The analysis of scanning electron microscopy micrographs with grey-level co-occurrence matrix showed that there is wide porous structure of dried foams at higher speeds drying. The temperature increase reduced moisture content and a_w, and increased hygroscopicity, and thickness rise increased moisture content and a_w and consequently decreased powders’ hygroscopicity under the same thickness and drying temperature, respectively. Increase in drying temperature would increase the reconstitution speed of powders into water and therefore the dissolution time decreased. In addition, results showed that the powder produced at 40°C have higher β-carotene content than those of produced at 55 and 70°C. With increasing drying temperature from 40 to 70°C, Lightness parameter (L) was increased while redness parameter (a) was decreased. The T_g and T_s were compared by plotting them in a graph against moisture content. For all drying processes the T_s was higher than the T_g. The drying conditions at 70°C (higher drying temperature) and 3?mm (lower thickness) led to a shorter drying time and consequent lower energy demand to produce a powdered cantaloupe pulp with high stability (low moisture content, a_w, and high T_g and T_s) and reconstitution speed of powder into water.     

EFFECT OF DRYING METHOD ON SHRINKAGE AND POROSITY

ABSTRACT

The effect of drying method on bulk density, particle density, specific volume and porosity of banana, apple, carrot and potato at various moisture contents was investigated, using a large set of experimental measurements. Samples were dehydrated with five different drying methods: conventional, vacuum, microwave, freeze and osmotic drying. A simple mathematical model was used In order to correlate the above properties with the material moisture content. Four parameters with physical meaning were incorporated in the model: the enclosed water density p_w, the dry solid density p_s, the dry solid bulk density p_bo and the volume shrinkage coefficient β'. The effect of drying method on the examined properties was taken into account through its effect on the corresponding parameters. Only, dry solid bulk density was dependent on both material and drying method. Freeze dried materials developed the highest porosity, whereas the lowest one was obtained using conventional air drying.     

Effect of outlet drying temperature and milk fat content on the physicochemical characteristics of spray-dried camel milk powder

Abstract

A composite face-centered experimental design was used to investigate the influence of spray drying conditions on the physicochemical characteristics of camel and cow milk powders. Response surface methodology (RSM) was deployed to appraise the effects of these processing parameters (the outlet drying temperature and the milk fat content) on water activity (a_w), glass transition temperature (T_g), bulk density, and free fat quantity. According to RSM analysis, it was noticed that the a_w and the T_g were primarily influenced by the outlet drying temperature instead of by milk fat content. Our results highlighted the negative effects of milk fat content and of the outlet drying temperature on the bulk density as well as on the free fat quantity of camel milk powder. Likewise, our findings underlined the negative effect of the outlet drying temperature on the bulk density of cow milk powder. However, the increase of fat content has led to the overexposure of fat at the free surface of the cow milk powder. Our results suggested a marked similarity of the overall thermodynamic behavior of both milks, during drying. Nevertheless, some differences were highlighted regarding the structuring of the particles of camel milk powder.     

Moisture Diffusivity in Rice Components During Absorption and Desorption

Moisture diffusivity values of different rice kernel components, namely endosperm, bran and husk, are required to solve mathematical models describing absorption and desorption processes. In addition to the rice variety and temperature, the moisture diffusivity also depends on its instantaneous moisture content or water activity (a_w) and whether rice is absorbing or desorbing moisture. This research was undertaken to determine moisture diffusivity values of rough rice components in different a_w ranges during absorption and desorption. Experiments were performed to measure sorption rates of different rice forms, including white rice, brown rice, and rough rice kernels. Mathematical models were developed to predict their moisture distribution during moisture sorption processes. These models were solved by finite element method using Comsol Multiphysics® simulation program. Moisture diffusivity values of different rice components were calculated and found to be different during absorption and desorption. Diffusivity of rice endosperm was higher during desorption than absorption at a_w higher than 0.20 and increased with an increase in a_w in 0.20–0.80 a_w range. Diffusivity of bran remained almost the same with a_w while diffusivity of husk decreased with an increase in a_w. Results obtained in this research demonstrated that the moisture diffusivity of different rice components varies significantly with the change in water activity or moisture and should be accounted in the mathematical models.     

Investigation of the Effects of Microwave Treatment on the Optical Properties of Apple Slices During Drying

The objective of this work was to study the changes of optical properties of apple (Golden Delicious) slices during drying. The optical parameters compared on the basis of Hunter values (L, a, b) changes as well as total color difference (Δ E) and browning index (BI). The effect of coating materials including carboxymethylcellulose (CMC), starch, and pectin as well as microwave treatment on optical properties and microstructure of dried samples were investigated. To analyze the effects of these processes on microstructure, scanning electron microscopy (SEM) was employed. Results showed that optical properties as well as microstructure of apple were affected by coating material and drying condition. Coated samples by CMC had a lower L and higher BI, but different results in the presence of starch were obtained. Microwave treatment in the presence of appropriate coating materials could improve the optical properties of dried apple slices.     

Evaluation of the usefulness of serial combination processes for drying of apples

Abstract

Serial combination drying processes are currently studied as alternatives to conventional drying processes. Compared to freeze-drying (FD), serial combination drying appears to be faster and less expensive, while providing better product quality than hot-air drying (HAD) and microwave-vacuum drying (MVD) as previously demonstrated for carrots. Using the example of carrots, it has also been shown that the drying front moves radially outwards over the course of FD. This unexpected behavior was suggested to originate from the carrots’ heterogeneous structure. It was hypothesized that apple pieces behave differently. Here, this hypothesis was proven by using micro-computed tomography measurements of partly freeze-dried apple pieces. In order to improve the drying process of apple pieces, several single and combination drying processes were carried out. Processes were evaluated by using drying time and sample quality as relevant parameters. Sample quality was determined by analyzing the 3D-structure, rehydration behavior, color, and ingredient retention. Results showed that single MVD is a well suitable drying technique for apple pieces, producing dried products of equal quality to FD. Different from carrots, serial combinations are not necessary to improve the quality of dried apple pieces. Nonetheless, especially a combination of HAD and MVD was useful to obtain specific structures such as puffed pores that did not result from single MVD.     

Temperature Changes during Microwave-Vacuum Drying of Sliced Carrots

Abstract

The temperature changes during microwave-vacuum drying of sliced carrots were investigated. Sliced samples were dried to 7–10% moisture content (wet basis) at a wide range of microwave power and vacuum pressure levels. The experiments showed that for sample thickness less than 8 mm, the core temperature of the sample was the same as its surface temperature, with uniform temperature distribution within the sample. However, for sample thickness more than 8 mm, temperature gradient developed along the thickness of the sample. The experiments also showed that, with the decrease of moisture content X _w (dry basis), for samples with thickness ≤ 8 mm, the drying process of sliced carrots experienced three distinct periods: a warming-up period (X _w  = 7.68) without removal of moisture when the product temperature increased linearly with drying time until it reached the corresponding saturation temperature of water in the food at the vacuum pressure; a constant temperature period (2 ≤ X _w  < 7.68) in which most of moisture evaporated and flowed out of the sample efficiently with little resistance; and a heating-up period (X _w  < 2) in which the drying rate decreased and sample temperature increased rapidly. The mathematical models for predicting sliced sample temperature were also developed based on the energy conservation and regression of the experimental date.     

Bioactive and Physicochemical Properties of Persimmon as Affected by Drying Methods

In this study, the effects of three different drying methods (freeze drying, oven drying, and vacuum oven drying) on bioactive (total phenolics, total flavonoids, condensed tannin and total hydrolyzable tannin contents, antiradical activity, and antidiabetic activity) and some physicochemical (dry matter, ash, water activity (a _w ), color, protein, hydroxymethylfurfural, glucose and fructose content) properties of persimmon fruit were investigated. Simplex lattice mixture design methodology was applied to determine the best solvent mixture for the extraction of phenolics from the samples. It was found that the mixture of acetone:water at the ratio of 50:50 % (v/v) was the best solvent mixture for the extraction. The persimmon powder sample obtained from freeze drying showed significantly (p <0.05) higher bioactivity values than oven- and vacuum-oven-dried samples. Antiradical activity changed significantly depending on the drying method employed and it was superior in freeze-dried samples than that of the other drying methods.     

From Laboratory Experiments to Design of a Conveyor-Belt Dryer via Mathematical Modeling

Abstract

A conveyor-belt dryer for picrite has been modeled mathematically in this work. The necessary parameters for the system of equations were obtained from regression analysis of thin-layer drying data. The convective drying experiments were carried out at temperatures of 40, 60, 80, and 100°C and air velocities of 0.5 and 1.5 m/sec. To analyze the drying behavior, the drying curves were fitted to different semi-theoretical drying kinetics models such as those of Lewis, Page, Henderson and Pabis, Wang and Singh, and the decay models. The decay function (for second order reactions) gives better results and describes the thin layer drying curves quite well. The effective diffusivity was also determined from the integrated Fick's second law equation and correlated with temperature using an Arrhenius-type model. External heat and mass transfer coefficients were refitted to the empirical correlation using dimensionless numbers (J _h , J _D  = m · Re ⁿ ) and their new coefficients were optimized as a function of temperature. The internal mass transfer coefficient was also correlated as a function of moisture content, air temperature, and velocity.     

Influence of Drying Conditions on the Hazelnut Quality. III. Browning

ABSTRACT

The influence of drying conditions on browning in hazelnut (Corylus avellana L.) has been studied by means of color measurement. Samples of unshelled and shelled hazelnuts were dried under six air temperatures (30. 40, 50, 60, 70 and 80°C). The drying experiments were done with forced air circulation, and two varieties of hazelnut were investigated, Negret and Pauetet. CiELab system was used to evaluate changes in total color (δE^?), lightness (L^?), chroma (C^?) and hue angle (H^?) on dried hazelnuts. Browning of hazelnut was observed to occur a1 all temperatures studied. The rate of pigment formation was determined from the δE' values with zero-order kinetic model, and temperature dependence was described by the Arrhenius equation with an activation energy (E_a) varying from 28 to 39 kJlmol. Negret variety showed the highest rate of browning.