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Hydrogels are polymeric materials with specific mechanical handling and encapsulation properties. Despite their widespread application, the modelling of the drying behaviour of hydrogels, particularly the evolution of texture stiffness with moisture loss, requires further development. This work aimed to develop numerical models to predict the moisture and deformation of cornstarch–alginate hydrogels under convective drying at 60 °C and 0.5 m/s. Cylindrical solids were used, and a transient three-dimensional FEM model predicted drying profiles via diffusion–convection mass transport. Texture analysis evaluating the hyperelastic coefficients of the hydrogels was performed for moisture contents ranging from 0.91 to 0.55 kg∙kg−1 w.b., yielding Young's modulus values from 24 to 147 kPa. A dimensionless relationship between the moisture ratio and elastic modulus produced a stiffness coefficient, used to adjust the moving boundary velocity and predict volumetric deformation. The model fitting returned an R2 higher than 0.95 and an RMSE lower than 0.04. The FEM model simulated hydrogel shrinkage by assessing the molar flux of water and mesh deformation at the boundaries, with mass diffusivity ranging from 2.38 to 5.46 × 10−10 m2∙s−1. Shrinkage reduced the surface area of solids during drying, revealing a pseudo-constant rate period in the drying profiles. The developed models effectively describe the drying of food materials with high shrinkage ratios. [ABSTRACT FROM AUTHOR]

In this work, a model sensitive to changes of process parameters such as temperature, air velocity, air initial relative humidity, and wet material layer thickness has been developed and verified by two different experimental methods. A laboratory convective batch dryer was used to measure drying behavior of 11 types of products at various process conditions to determine the model parameters. An independent solar drying system was used to experimentally verify the model. The model can predict drying kinetics at any point within a defined range of process conditions: model parameters were determined for temperature range of 40-50 °C, air velocity of 0.6-1.2 m/s, initial air relative humidity of 27-45%, and layer thickness of 4-12 mm. However, the model was verified also for extrapolated air velocity and humidity with good agreement between experimental and model data despite the use of a different independent experimental system. [ABSTRACT FROM AUTHOR]

Moisture transport during food drying can be phenomenologically described by Fick's second law and by the so-called anomalous diffusion model. However, in the literature, many studies have shown the extensive use of empirical/semiempirical models (EMs/SEMs) to adjust experimental data for the drying of thin-layer foods. This research aims to perform a critical analysis of the most commonly used EMs/SEMs and compare them with Fick's second law and an anomalous diffusion model using two different sets of hot-air drying data. Two waste byproducts from the food industry, spent coffee grounds and passion fruit peels, were selected for analysis. The selected EMs/SEMs were found to be mathematically interrelated (i.e., some are a subset of others), and their appropriateness was incorrectly justified mainly by their statistical goodness-of-fit. As shown, it is highly recommended that researchers start analyzing drying data with phenomenological models. The extensive use of EMs and SEMs can be replaced by the anomalous diffusion model, which has a high capacity to adjust empirical data and a sound phenomenological description of the process. [ABSTRACT FROM AUTHOR]

This work aimed to develop numerical models to predict the moisture content and deformation of potato slices during convective drying (40–80 °C, 0.5 m·s−1). Three-dimensional slices were considered in cylindrical, cubic, parallelepiped, and prism geometries. The first classic model coupled the linear constant drying rate period with the analytical solution of Fick's law in spherical coordinates, evaluating the mass diffusion coefficients (4.2–15.5·10−10 m2·s−1), critical drying time (1640–5085 s), and critical moisture content (1.8–2.4 kg·kg−1). The Finite Element Method (FEM) was a more robust model, that combined momentum and mass transfer to three-dimensional solid deformation of polyhedrons by ALE method, evaluating the mass diffusivity (1.4–6.5·10−10 m2·s−1). The FEM model could predict the shrinkage due to water molar flux removal on moving solid boundaries and explain the pseudo-constant drying rate detected in experimental data. The developed models accurately described the drying of food materials with a high shrinkage ratio. [ABSTRACT FROM AUTHOR]

Solar collectors are thermal devices that can trap solar energy and convert it to heat. This heat can be used for different industrial applications, for example, the drying of food is one of the most useful applications of solar collectors. This work aims to design and build a solar collector using nanofluids for the convective drying of food. The dimensions of the solar collector were 1 m² by 20 cm with an angle of inclination of 45°. The collector was composed of 9-mm thick tempered glass and a heat exchanger in which the nanofluids circulate. Nanofluids were designed based on canola oil and nanopowders (>50 nm) of Al2O3, CuO, and a 1:1 (w/w) mixture of both. Thermal profiles were determined using differential scanning calorimetry (DSC). The solar collector temperatures were recorded using an Agricos® unit. The maximum temperatures of the air leaving the collector were 39.1°C, 44°C, 54°C, and 47.1°C for canola oil, and the nanofluids composed of Al2O3, CuO, and the 1:1 mixture, respectively, with a maximum efficiency of 65.09%. An increase in the outlet air temperature was observed using the nanofluids compared to canola oil alone [ABSTRACT FROM AUTHOR]

Intermittent microwave–convective drying (IMCD) is an advanced drying technology that overcomes the shortcomings of microwave, convective, and microwave–convective drying. Research on the feasibility study of solar-assisted IMCD along with investigating its microstructure change, nutritional analysis, and appearance of dried food materials is inadequate. This research aims to investigate the effects of microwave intermittency—on quality attributes and structural changes of potato slices. In addition, optimization of pulse ratio has been performed in this study. Drying experiments, namely convective drying (CD) and IMCD, were conducted to assess the quality of dried potato slices. It was found that IMCD took only 12 min to complete the drying process, whereas CD took approximately 300 min. The optical (colour) and nutritional (vitamin C) properties of the IMCD dried potato slices appeared better than those of hot air-dried samples. The overall techno-economic analysis indicates that the proposed solar-assisted IMCD can dry an equal amount of potato while consuming one-tenth of the required energy of CD. Therefore, successful industrial application of the proposed drying system might be a stepping stone in the way to the advancement of energy-efficient food drying systems. [ABSTRACT FROM AUTHOR]

A computational model is generated to analyse the influence of shrinkage on convective drying problems. The object considered here is cranberry which is assumed as 100% spherical. The drying air temperatures were considered from 313 to 348 K. There were four models used in this study, namely one-dimensional models without and with shrinkage, two-dimensional models without and with shrinkage. A finite difference scheme was used to discretize the heat and mass transport equations. Four separate computer codes were written in MATLAB to solve the discretized equations. The Arrhenius model is used to couple the heat and moisture transport equations and solve them simultaneously. The Gauss–Seidel iterative method was used to solve the equations. An empirical shrinkage model was used to calculate the shrinkage effect. The temperature and moisture distributions of cranberry were obtained with drying time. Moisture distributions without and with shrinkage results were calculated and compared to identify the effect of shrinkage. The cranberry lost a maximum of 35% of its size during drying. There was not much variation found in the results between 1- and 2D models. The maximum difference in centre moisture content of cranberry without shrinkage model was 56%, and the difference in mean moisture content was 49.2%. The numerical outcomes were validated with experimental data, and the model with shrinkage was a good fit with them than without shrinkage. Therefore, shrinkage needs to be considered in convective drying phenomena. [ABSTRACT FROM AUTHOR]

Modeling of the hot air dehydration period is very important to increase food property and reduce energy consumption. The objective of the study is to examine the drying behaviour of banana slices by using convective hot air drying method. Drying kinetics of the banana slices was investigated experimentally for different air temperatures (40, 50, and 60 °C). The drying models were fitted to the experimental data. The model which gives the nearest results to the experimental information was predicted as the Midilli model (R²=0.99). It was determined that the L (brightness) colour parameter value declined as the moisture content of the food decreased. Effective diffusion coefficients (2.02×10-10, 5.05×10-10, and 8.08×10-10 m² s-1), activation energy (61.1 kJ (mol)-1) and shrinkage coefficients (23%, 32% and 40%) were calculated and it was seen to be compatible with the data given in the literature on food drying. The temperature and moisture content obtained from the experimental study were compared with numerical analysis and found to be compatible. [ABSTRACT FROM AUTHOR]

Convective drying is an ancient technique to dry moist materials. Many experimental and numerical studies on convective drying enriched the knowledge of researchers and industrialists for further research and developments. In this study, a thorough review is made on convective drying of food materials. A brief history of convective drying, its importance, methods, applications, and statistical data are clearly reviewed. The impact of important drying parameters such as shrinkage and influence of pretreatment during convective drying have been critically appraised. Instruments used, experimental setup developed, and properties measured and estimated have been listed and analyzed. A novel technology of taking snapshots during drying using the thermal camera is discussed. Different mathematical models such as analytical and numerical methods and its present challenges are extensively reviewed and discussed. An advanced level solution for convective drying at present is explained. Gaps in numerical methods to solve convective drying problems have been discussed and effective summaries are drawn. [ABSTRACT FROM AUTHOR]

The cumulated thermal load (CTL) combines the effects of temperature and drying time on the quality of dried products and therefore highly improves the comparability of different drying processes. To make it more practicable, it was adjusted in this work. The adjusted CTLad was developed to better match the obtained results on an experimental level. Since the CTLad also stands in relation to the amount of energy consumed, a minimum of the CTLad is sought for drying processes. In practical experiments, the theoretical approach was partially verified for different quality parameters for mangoes and pineapples. It could be shown that the comparability of different drying experiments improved with the use of the CTLad. Optimal drying parameters concerning low changes in color, ascorbic acid, and polyphenols, as well as a low consumption in energy and time (CTLad), were found to be air temperatures Tair of 60 °C and relative humidities RH of 12% for pineapples and air temperatures Tair of 50 °C and relative humidities RH of 7% for mangoes. [ABSTRACT FROM AUTHOR]

This study reports the formation of 2-furomethyl-amino acids (2-FM-AA) as indicators of Maillard reaction (MR) in black garlic elaboration, followed by the determination of furosine by ion-pair RP-HPLC-UV. The method was assessed for accuracy, repeatability and detection and quantitation limits indicating its adequacy. Traditional procedure of black garlic obtainment and the inclusion of convective drying (CDP) and ohmic heating (OHP) were assayed. For comparison purposes, three commercial black garlic samples were used. Together with furosine (2-FM-lysine), 2-furoylmethyl-γ-aminobutyric acid and 2-FM-arginine were detected. Levels of furosine were higher in CDP (46.6–110.1 mg/100 g protein) than in OHP (13.7–42.0 mg/100 g protein) samples, probably due to the most severe processing conditions used in the former. These results highlight the suitability of 2-FM-AA as chemical indicators to monitor the process of black garlic elaboration in order to obtain high quality products. [ABSTRACT FROM AUTHOR]

The finite element method was employed to model the simultaneous heat and mass transfer in cylindrical samples of banana during convective drying. The Arbitrary Lagrangian-Eulerian (ALE) approach was implemented to incorporate the axial and radial shrinkage effects in the FEM model. The results showed that the FEM model was satisfactorily able to predict the moisture content (R² > 0.97) and core temperature (R² > 0.98). Therefore, the proposed model can be used as a promising tool to optimise the drying process of banana from quality and efficiency points of view. Also, the model can be easily extended for drying of similar products with cylindrical shape. [ABSTRACT FROM AUTHOR]

The shrinkage of biological materials (agri-food), is a common phenomenon developed during a drying operation that directly affects the product quality. In this work a dimensionless mathematical model including the reduction at the interface of a high moisture content food (tuber) in 1D geometry was developed. The mass and heat conservation equations coupled to the Leibniz-Reynolds Transport Theorem (LRTT) at the interface of a high moisture content food were solved. The system of nonlinear unsteady state equations was solved by using a multi-frontal massively parallel sparse direct Solver. In order to validate this model, tuberous crop slices were dried at three temperatures: 40, 50 and 60 °C, relative humidity of 25% and a constant airflow of 1.5 m/s. Drying kinetics and the evolution of temperature within the product were logged. The model is able to simulate the moisture loss and predict the highest thickness change. The simulations show a good agreement with the experimental drying kinetics, as well as for the evolution of temperature. Simulations show 1-D shrinkage is not linear and the model improves its prediction when the drying temperature is higher (intensive mass flux). [ABSTRACT FROM AUTHOR]

Ultrasound application represents an alternative means of improving heat and mass transfer. This study explored the combined application of ultrasound (US) during both the osmotic dehydration (OD) pretreatment and the convective drying of pineapple. For that purpose, fresh and pretreated samples (20 or 40 min, with (55.5 kW/m 3 , 40 kHz) and without ultrasound application) in an osmotic solution of sucrose (40% w /w) were dried (40 °C and 70 °C) with (21.8 kHz, 31 kW/m 3 ) and without ultrasound application. A diffusion model permitted to quantify the influence of the factors studied (time of pretreatment, ultrasound application during pretreatment, drying temperature and ultrasound application during drying) in drying kinetics. The increase in drying temperature and the application of ultrasound during drying significantly accelerated the drying process by reducing both the internal and the external mass transport resistance. On the contrary, the osmotic pretreatments reduced the drying rate by increasing the external resistance. [ABSTRACT FROM AUTHOR]

Maqui ( Aristotelia chilensis [Molina] Stuntz) berry dehydration, drying kinetics (40-80°C), and drying temperature effects on texture profile, rehydration properties, dietary fiber, and sugar contents were evaluated. The Midilli-Kucuk model presented the best fitting to the drying curves. The effective diffusion coefficients varied from 0.34 × 10−10 m2 s−1 at 40°C to 2.14 × 10−10 m2 s−1 at 80°C. The texture profile analysis showed a slight difference in some parameters (springiness, gumminess cohesiveness, and chewiness). The WHC parameters had no significant differences in the range 50-80°C. The insoluble dietary fiber content was greater than the soluble one in all the samples. Also, the dietary fiber, fructose, and glucose contents increased in value with an increase in temperature. The results could be used to estimate the best drying conditions for producing a dehydrated maqui product. Practical application This native berry is categorized as a 'superfruit' due to its great number of wholesome properties. However, it is highly perishable and is primarily marketed as a powder since it is stable after drying. This study is the first one that provides results on both quality changes and energy consumption of the drying process. This information, here, can help engineers and marketers to determine the best drying conditions for maqui berries so that they retain their antioxidant properties and dietary fiber contents as well as maintain proper rehydration and texture properties, all of them interesting for to functional food marketing. [ABSTRACT FROM AUTHOR]

Physiochemical parameters with constant values are employed for the mass-heat transfer modeling of the air drying process. However, structural properties are not constant under drying conditions. Empirical, semi-theoretical, and theoretical models have been proposed to describe true density (ρp). These models only consider the ideal behavior and assume a linear relationship between ρp and moisture content (X); nevertheless, some materials exhibit a nonlinear behavior of ρp as a function of X with a tendency toward being concave-down. This comportment, which can be observed in garlic and carrots, has been difficult to model mathematically. This work proposes a semi-theoretical model for predicting ρp values, taking into account the concave-down comportment that occurs at the end of the drying process. The model includes the ρs dependency on external conditions (air drying temperature ( Ta)), the inside temperature of the garlic slices ( Ti), and the moisture content ( X) obtained from experimental data on the drying process. Calculations show that the dry solid density (ρs) is not a linear function of Ta, X, and Ti. An empirical correlation for ρs is proposed as a function of Ti and X. The adjustment equation for Ti is proposed as a function of Ta and X. The proposed model for ρp was validated using experimental data on the sliced garlic and was compared with theoretical and empirical models that are available in the scientific literature. Deviation between the experimental and predicted data was determined. An explanation of the nonlinear behavior of ρs and ρp in the function of X, taking into account second-order phase changes, are then presented. [ABSTRACT FROM AUTHOR]

Blueberry pulp was dried using hot air (HA) and refractance window (RW) methods at 55–95 °C, the drying kinetics were described by the Page (k and n) and Fick (D e f f 1 ) Models. In addition, mass transfer parameters (drying coefficient (S), lag factor (G), Biot number (B i), effective diffusivity (D e f f 2 ), external mass transfer (h m)); and the activation energy were analysed. As results, the Page model showed the best fit, D e f f 1 and k parameters increased with temperature, especially for RW drying at 95 °C. RW method reduced the HA drying time by more than 30%. Furthermore, the 0 < B i < 0.4 values were related to variations in internal resistance with temperature, except for HA drying at 95 °C, where the external resistance was also reduced. In conclusion, mass transfer parameters suggested that internal resistance was more important during HA drying, while RW was the most effective method since it needed lower time to dry samples. • Hot air (HA) and refractance window (RW) were applied for drying blueberry pulp. • Effective diffusivity and the kinetic parameters increased with temperature. • RW drying reduced the HA drying times by more than 30%. • The Biot numbers were in the range of 0 < B i < 0.4 , being lower for RW method. • The internal resistance was more important during HA drying. [ABSTRACT FROM AUTHOR]

In this study the effectiveness of drying methods - vacuum microwave drying (VMD), hot air drying (AD), convective air drying combined with vacuum microwave drying (AD-VMD) and osmotic dehydration followed by vacuum microwave drying (OD-VMD) - and their effects on both physicochemical and structural changes in strawberries are compared. Drying performance was assessed by drying rate, moisture content and water activity, while changes in quality attributes of strawberries were determined by measuring color, texture, microstructure, shrinkage and rehydration ratio. A maximum drying rate of 6.74×10−4kg water/(kg dry matter s) was found for microwave-assisted drying, which results into a product of intermediate water activity. Combining VMD with osmotic dehydration or air drying did not improve drying efficiency. VMD yields an elastic product of improved mechanical resistance with just a slight loss of color and an appropriate rehydration performance. Preliminary dehydration using hot air or sucrose solution was not able to improve most of the quality attributes of strawberries. [ABSTRACT FROM AUTHOR]

In the present work, the effect of drying was evaluated on some chemical and physical properties of apples, and the functions were modelled using feed-forward artificial neural networks. The drying kinetics and the mass transfer properties were also studied. The results indicated that acidity and sugars were significantly reduced by drying. Regarding colour lightness decreases, whereas redness and yellowness increased. As for texture, the dried samples were softer and less cohesive as compared to the fresh ones. Mass diffusivity increased with temperature, from 4.4×10−10 m2/s at 30°C to 1.4×10−9 m2/s at 60°C, and so did the mass transfer coefficient, increasing from 3.7×10−10 m/s at 30°C to 7.4×10−9 m/s at 60°C. As to the activation energy, it was found to be 34 kJ/mol. Neural network modelling showed that all properties can be correctly predicted by feed-forward neural networks. The analysis of the networks' behaviours input layer weight values also shows which properties are more affected by dehydration or more dependent on variety. [ABSTRACT FROM AUTHOR]

Abstract: A numerical model is developed for prediction of transient moisture content of food materials. The moisture transfer is modeled considering diffusion of liquid water from inner layer to outer surface of the food material followed by evaporation of water from the surface to the dry air which flows over the moist food material. Discretization of transient heat and mass transfer governing equations are done using the finite-volume method (FVM). A 3-D code in MATLAB is developed to solve the simultaneous heat and mass transfer equations. The flow field over the moist food material is assumed to be turbulent and SST k–ω turbulence model is used for prediction of heat transfer coefficient using a computational fluid dynamics (CFD) commercial code. The sample moist food material is considered to be a rectangular shaped potato and the effects of temperature and velocity on drying behavior of the same are predicted. Different drying rate periods are identified. The numerical model is validated with experimental data with a reasonable agreement. [Copyright &y& Elsevier]

Highlights: [•] The analysis of transport phenomena occurring during drying is performed. [•] Results demonstrated that front and rear faces have significant influences on the drying. [•] The heat and mass transfer equations within the object are analytically solved. [•] The external flow is predicted numerically. [ABSTRACT FROM AUTHOR]

Highlights: [•] Denaturation of WPI was higher in convective drying (CD) than in isothermal heat treatment (IHT) ⩽65°C. [•] The stress due to dehydration denatured significant (p <0.05) amount of WPI during CD ⩽65°C. [•] Both the dehydration and thermal stresses were dominant during CD of WPI ⩾65°C. [•] Denaturation of WPI was higher in IHT than in CD at >65°C. [•] Denaturation sensitivity of whey proteins during CD followed β-Lg>α-Lac>BSA. [ABSTRACT FROM AUTHOR]

Abstract: The paper deals with modeling the convective drying process. A relevant and reliable mathematical model that captures the history and distribution of temperature is presented. The attention is focused on the simultaneous heat and mass transfer occurring during drying where dry and hot air flows about the food. In the present study, external resistance to mass transfer is considered negligible. As a result, the drying curve is almost independent of the boundary conditions, which means that drying is diffusion-controlled. The main connotation of present study regards to undertake analytical procedure to establish the novel model for practical applications. The results show that the temperature evolution can be evaluated from an advanced analytical solution in a quick and efficient manner. The model is validated with the literature experimental data obtained for carrot and mango slabs. A good agreement is obtained between the model predictions and the available experimental results. [Copyright &y& Elsevier]

This study was conducted to evaluate the quality of and structural changes in parsley (Petroselinum crispum Mill.) during vacuum, microwave, and convective (hot air) drying. The selected quality attributes of the dehydrated parsley samples were investigated on the basis of ascorbic acid and chlorophyll retention, as well as color change and overall appearance. Ascorbic acid is an important indicator of quality, and its selection was due to its heat-labile nature. Ascorbic acid deterioration depends on air temperature, microwave power, and drying time. The lowest ascorbic acid value was found after convective drying, followed by vacuum drying and microwave drying. With convective drying, the appearance values of the dried parsley decreased when the drying period was extended. However, with microwave drying, appearance values were not drastically changed. The color values obtained from leaves supported the chlorophyll findings. At the end of the study, microwave drying at 750–850 W ensured the shortest drying time and the best overall quality of parsley; thus, it was chosen as the most appropriate technique for parsley drying. [ABSTRACT FROM AUTHOR]

Drying is a complex process which involves simultaneous heat and mass transfer. Complicated structure and heterogeneity of food and biological materials add to the complexity of drying. Drying models are important for improving dryer design and for evaluating dryer performance. The lumped reaction engineering approach ( L-REA) has been shown to be an accurate and robust alternative for cost-effective simulations of challenging drying systems. However, more insightful physics has to be shown spatially. In this study, the REA is coupled with the standard mechanistic drying models to yield the spatial- REA ( S-REA) as nonequilibrium multiphase mass-transfer model. The S-REA consists of a system of equations of conservation with the REA representing the local evaporation and wetting rate. Results of the modeling using the S-REA match well with the experimental data reported previously. This is the first comprehensive REA approach to model the profiles of water vapor concentration during drying of food and biological materials. This study indicates that the S-REA can be an accurate nonequilibrium multiphase mass-transfer model with appropriate account of the local evaporation rate. The overall REA concept is expected to contribute substantially for better and cost-effective representation of transport phenomena of drying process. © 2012 American Institute of Chemical Engineers AIChE J, 59: 55-67, 2013 [ABSTRACT FROM AUTHOR]

Thin-layer drying characteristics of whole pistachio were investigated by using a hot air convective dryer at a constant airflow velocity of 2 m.s-1 and air temperature in the range of 40-70°C. The experimental drying data were fitted to the eight well-known drying models i.e. the Newton, Page, Modified Page, Henderson and Pabis, Logarithmic, Diffusion, Thomson and New models. A predictive model using artificial neural network was proposed in order to obtain on-line predictions of moisture kinetics during drying of pistachio nut. For monitoring the drying process of pistachio, different activation function of neural networks such as tangent hyperbolic (tanh) and logarithmic sigmoid (logsig) were utilized. Drying time and air drying temperature were considered as network input and moisture ratio was as network output. The result indicated that tanh activation function gave better results than logsig activation function for monitoring the moisture ratio. Generally, perceptron neural network with logsig activation function as a goodness activation function was able to predict moisture ratio with 7 neuron in first and second hidden layer with R2 value equal 0.994. Investigation of validation data demonstrated that the predicted and experimental dying data were in good agreement. Comparing the R2 (coefficient of determination) and MAE using the developed ANN model it was concluded that the neural network could be used for on-line state estimation of drying characteristics and control of drying processes. [ABSTRACT FROM AUTHOR]

In the present work, the effects of drying air temperature and pre-treatment on color changes and texture of Boletus edulis (Bull.: Fr.) mushrooms were investigated during convective drying. The drying experiments were conducted in a laboratory hot-air dryer at temperatures of 50, 60, and 70 °C under thermal (water or steam blanching) and chemical (solution of potassium metabisulfite or citric acid) treatments. Changes in color were evaluated by the CIELAB color system and the experimental color parameters (L* a* b*) were fitted to a first order kinetic model. Values of total color difference (ΔE*), hue angle (h*), and chroma (C*) were also calculated. The texture of dried mushrooms was measured by an Instron texture analyzer and expressed as hardness. The model fitted the experimental a* and b* values sufficiently. The results showed that during convective drying, lightness decreased slightly while yellowness and redness increased. Chemical pre-treatments did not influence the color of mushrooms positively while blanching caused intensive color deterioration. Moreover, the ΔE* value could contribute to the assessment of color change effectively. Hardness increased as drying proceeded and was found to be greater at higher temperatures. Mushrooms subjected to water or steam blanching were harder in texture. A temperature limit of 60°C is imposed for the dehydration of B. edulis mushrooms. Thermal treatment prior to drying is not recommended for B. edulis mushrooms when color and texture are desirable quality criteria. [ABSTRACT FROM AUTHOR]

Food materials are highly perishable. Drying is necessary to restrict biological and chemical activity to extend shelf life. A good drying model is useful for design of a better dryer, evaluation of dryer performance, prediction of product quality, and optimization. The reaction engineering approach (REA) is a simple-lumped parameter model revealed to be accurate and robust to model drying of various thin layers or small objects. Modeling drying behavior of different sizes is essential for a good drying model, yet it is still very challenging, even for a traditional diffusion-based model, which requires several sets of experiments to generate the diffusivity function. The REA is implemented in this study, for the first time, to model drying of rather thick samples of food materials. An approximate spatial distribution of sample temperature is introduced and combined with the REA to model drying kinetics. Results have indicated that the REA can model both moisture content and temperature profiles. The accuracy and effectiveness of the REA to model drying of thick samples of food materials are revealed in this study. This has extended the application of REA substantially. The application of the REA is currently not restricted for thin layesr or small objects but also for thick samples. [ABSTRACT FROM AUTHOR]

Abstract: The aim of this work was to obtain a technological and economic alternative for mushroom and parsley dehydration combining convective and vacuum drying. Depending of product, this combination of technologies allows minimization of total drying time and avoids negative effects on quality of thermo-sensitive products during drying. Experimental drying curves were determined in a cross-flow convective dryer and in a cabinet vacuum dryer at 35, 45 and 55°C. The most appropriate theoretical models were obtained and applied for combined processes in order to minimize the overall drying time and avoid final product damage. For parsley at the highest temperature (45°C), reductions of 63% and 16% in drying time were observed with the combined drying process compared to the sole convective and sole vacuum drying, respectively. This reduction in process time was obtained when dryer change was done at the intermediate moisture condition that determines the highest drying rate during the whole combined process of convective and vacuum drying. For mushrooms, convective drying throughout the process, at the highest temperature (55°C) compatible with product visual quality, minimized drying time. [ABSTRACT FROM AUTHOR]

Abstract: The effect of the conditions of drying air on the quality of lemon balm (Melissa officinalis L.) leaves such as colour, rosmarinic acid and essential oil content was investigated in this research. Fresh leaves with a moisture content of approximately 80% wet basis were dried to a final moisture content of 10% wet basis. The thin-layer drying experiments were conducted in a high precision through flow laboratory dryer at air temperatures of 30, 35, 40, 45, 50, 60 and 70°C and the corresponding relative humidity by maintaining 10g water per kg of dry air. The effect of the increased humidity of drying air was also investigated at 15, 20, 25 and 30g/kg specific humidity. For all drying trials the air velocity flowing through the sample was kept constant at 0.2 m/s. Although drying of leaves at 30°C preserved their medicinal qualities and colour, the duration of the process was comparably long. On the contrary, high drying air temperatures caused considerable colour degradation, a decrease in rosmarinic acid content and significant essential oil losses. The effect of relative humidity of drying air on the overall quality was found to be insignificant. A temperature limit of 40°C can be imposed for convective drying of lemon balm in order to protect the heat-sensitive active ingredients and maintain the green colour of the leaves. [Copyright &y& Elsevier]

Abstract: The internal moisture distribution that arise in food products during drying, is a key factor for the retention of quality attributes. To reveal the course of moisture content in a product, internal moisture profiles in broccoli florets are measured by MRI imaging during drying experiments with controlled air flow and temperature. The 3D images concern a matrix size of 64×64×64 elements. Signal intensity is converted to product moisture content with a linear relationship, while taking a minimum detectable moisture content of 0.3kg water/ kg dry matter into account. Moisture content as a function of time is presented for a 2D cross sectional area in the middle of a broccoli sample. The average moisture contents for the cross sectional area obtained from the MRI imaging are compared with spatial model simulations for the moisture distribution. In that model the effective diffusion coefficient is based on the Free Volume Theory. This theory has the advantage that the changed mobility of water in the product during drying is taken into account and the theory also predicts the moisture transport in the porous broccoli floret. Key parameters for the Free Volume Theory are estimated by fitting to the experimental MRI results and the effective diffusion coefficient is given as a function of the product water content. [Copyright &y& Elsevier]

Abstract: Cranberries belong to a group of evergreen dwarf shrubs or trailing vines in the genus Vaccinium subgenus Oxycoccus. Traditionally they grow in acidic bogs throughout the cooler parts of the world; when cultivated are grown on low trailing vines in great sandy bogs. The research focuses on the study of vitamin C changes in Latvia grown wild (Vaccinium oxycoccus L.) and cultivated (Vaccinium macrocarpon Ait.) cranberries during convective and microwave vacuum drying. Latvian wild cranberries and cultivated cranberry varieties ‘Early Black’, ‘Ben Lear’, ‘Stevens’, ‘Bergman’ and ‘Pilgrim’ from Kurzeme region harvested in 2010 were used for experiments. Mechanical and thermal pre-treatment of fresh berries was applied for better water evaporation in drying process. During experiments the berries were dried: in a convective air dryer by controlled hot air stream circulation velocity of 1.2 m s-1 at temperature +50±2°C. The berries were placed on the perforated sieve (diameter 0.185 m, diameter of holes – 0.002 m); the moisture content of dried berries was 9.0±0.1%. Following quality parameters were analysed during experiments: moisture content (oven-drying method) and vitamin C content (LVS EN 14130:2003). Experimentally it was ascertained that drying time of berries by various drying methods mainly depends on the pre-treatment manner. During the drying process moisture content of cranberries decreased from on average 86.94% to 9.00%, about ∼10.00 times. Vitamin C is the least stable of all vitamins and it can be easily degraded during processing and storage. The most harmful factors to vitamin C content are the presence of oxygen, prolonged heating in the air ambiance and exposure to light. The initial content of vitamin C in wild and cultivated fresh cranberries was differing, which mainly depends on varieties’ individuality. Current research proved that vitamin C content loss in cranberries processed by microwave vacuum drying method comparing with drying in convective cabinet type dryer was smaller. [Copyright &y& Elsevier]

Abstract: In many productions, Arthrospira sp. biomass is dried in cylinders or layers form. Drying involves shrinkage that changes organoleptic qualities and aspects. To understand the change, similar production forms and sizes were convective dried at 45°C with 2–6mm initial diameter cylinders and 1 and 4mm thin layers. Total layer size- and cylinder length shrinkage ratio is only 95%. Thicknesses direction shrinkage is the most significant with 28–65%. Cylinders present anisotropicity and shrink more in the perpendicular direction to the support than in the parallel one. All curves present several stages. The first ideal shrinkage stage of layers goes until the half of initial moisture content that involves lower final porosity by layers than by cylinders. End of ideal shrinkage moisture content decreases when cylinder initial diameter decreases. Cylinders initial porosity is around 15–20% and increases only during the 2nd stage up to final porosities between 68% and 80%. [Copyright &y& Elsevier]

Abstract: The paper analyses the results of the research on drying a variety of giant pumpkin (Justynka – 957). The research involved drying kinetics of pumpkin as well as changes in the volume of 10mm pumpkin cubes, dried at the temperature of 80°C, using free (natural) convection and in the forced convection in the tunnel dryer, with the drying factor speed of 1.2ms−1 and in the fluid bed dryer. Drying kinetics of pumpkin pieces dried in the tunnel dryer, fluid bed dryer and during two-stage initial drying in the tunnel, and in the fluid bed dryer were examined. Measurements results were used to verify the theoretical models of drying kinetics of the first period taking volume shrinkage into consideration, and the second period of the examined processes. [Copyright &y& Elsevier]

Average effective moisture diffusivities for both the whole- and cut-rosehips were obtained during convective drying. The effects of process variables such as air temperature, air velocity and air absolute humidity on effective mois- ture diffusivity were studied. The average effective moisture diffusivity in rosehip ranged between 1.45 × 10-9 and 10.3 × 10-10 m2/s for whole-rosehip and between 1.44 x 10-9 and 5.13 × 10-9 m2/s for cut-rosehip at the temperatures studied. Activation energies for convective drying were found to be 62 kJ/mol for whole-rosehips and 58 kJ/mol for cut-rosehips. [ABSTRACT FROM AUTHOR]

Abstract: The drying characteristics of carrots are investigated experimentally and theoretically for different drying conditions. Carrot slices with three different thicknesses are used. Initially, sorption isotherms of the dried carrot slices are determined for different temperatures and equilibrium relative himidity (e.r.h.). Experiments are conducted for drying air temperatures of 35, 45 and 55°C, mean velocities of 0.2, 0.4 and 0.6 m/s and, relative humidity values of 40, 55 and 70%. In the covered ranges, the values of the effective moisture diffusivity, Deff , are obtained from the Fick''s diffusion model varying from 1.257×10−9 to 2.200×10−9. The effects of the governing drying parameters on the total drying time are determined. A mathematical model based on the Fickian model was established to describe the mass transfer in a carrot slice. Comparison of experimental and calculated results shows good agreement. [Copyright &y& Elsevier]

Abstract: Kinetics of osmotic dehydration (OD) and effects of sucrose impregnation on thermal air-drying of pumpkin slices were investigated. A simplified model based on the solution of Fick’s Law was used to estimate effective diffusion coefficients during OD and air-drying. In order to take into account shrinkage, average and variable thicknesses were considered. Pumpkin slices were dehydrated in sucrose solutions (40%, 50% and 60%, w/w, 27°C). The effective water diffusion coefficients were higher than the sucrose, and low diffusivity dependence with solution concentration was observed. Samples non-treated and pre-treated in 60% osmotic solutions during one hour were dried in a hot-air-dryer at 50 and 70°C (2m/s) until equilibrium was achieved. Pre-treatment enhanced mass transfer during air-drying. Great volume reduction was observed in pre and non-treated dried samples. Using variable thickness in the model diminished the relative deviations between predicted and experimental OD and drying data. [Copyright &y& Elsevier]

Dahi is one of the most popular fermented milk products consumed in India with excellent therapeutic properties.The initial moisture content of dahi is approximately 5.7 kg water kg dry solid−1. The shelf life of dahi is limited and hence the long term storage is possible in the form of dahi powder which can further be used as a base for formulation of health drink mix. Dahi was dried in a laboratory scale recirculatory convective air dryer to a final moisture content of 0.04 kg water kg dry solid−1. Drying characteristics of dahi were investigated under varying conditions of dahi thickness (0.003 m, 0.004 m and 0.005 m) and drying air temperatures (45°C, 50°C and 55°C) and velocities (1.5 m s−1, 2.0 m s−1 and 2.5 m s−1). Different drying models were used to simulate the observed drying data. The mathematical models were compared based on R-square and reduced chi-square values. The drying characteristics were satisfactorily described by Page, modified page, logarithmic and Midilli et al. models. The Midilli et al. model followed by modified page model provided the best representation of data. Effective moisture diffusivity computed on the basis of Fick's law varied between 2.52×10−10 m2 s−1 and 1.3×10−9 m2 s−1 under experimental drying air temperatures and sample thicknesses at air velocity 2.5 m s−1. The temperature and thickness dependence of effective moisture diffusivity was expressed by an Arrhenius type of equation. The equation showed best fit for diffusivity data at 2.5 m s−1 and varying temperatures and sample thickness. [ABSTRACT FROM AUTHOR]

Abstract: A newly developed Biot number–Dincer number correlation was applied to determine moisture transport parameters of broccoli drying. Broccoli florets were dried in a single layer with suction flow changing the temperature and flow of drying air. Experimental dimensionless moisture content data were used to calculate the drying coefficients and lag factors, which were then incorporated into the analytical model for slab geometry. In a relatively simple manner, Biot number, effective moisture diffusivity and mass transfer coefficient were computed and ranged between 0.2272–0.3030, 3.5780×10−6–16.6670×10−6 m2 s−1 and 1.9215×10−4–8.7256×10−4 ms−1, respectively. Effective moisture diffusivities calculated by newly developed equation which incorporated lag factor and drying coefficient differed from literature values. Experimental and predicted moisture profiles showed an excellent agreement for all drying conditions with mean errors ranging from ±1.26% to ±5.57%. [Copyright &y& Elsevier]

Starch pearls are extremely sensitive to the rate of drying. Fast drying leads to poor quality of final cooked (rehydrated) products. The objectives of this study were to reduce the lengthy drying time by incorporating low-power microwave finish drying and to evaluate the rehydrated quality of starch pearls. The effects of temperature and relative humidity on the drying rates of starch pearls in the constant-rate period were determined. When evaluating the saving of time, microwave energy efficiency and rehydration quality, the best results were obtained when combined natural convection and microwave finish drying were carried out at low power levels (119–143 W). Microwave finish drying reduced the natural convective drying time by about 50–75%, with an improvement in quality, as indicated by higher unbroken ratio, less cooking loss and shorter cooking time. [ABSTRACT FROM AUTHOR]

This study was conducted to evaluate the quality and structural changes in potatoes during microwave and convective drying. A modified microwave oven, operated in either the microwave or convective drying mode, was used to dry the samples. The quality attributes of the dehydrated potato samples were investigated on the basis of the ascorbic acid retention (vitamin C) and rehydratibility, and the structure in terms of the shrinkage behaviour. Ascorbic acid is an important indicator of quality and its selection was due to its heat labile nature. Ascorbic acid deterioration demonstrated first-order kinetic behaviour, and was further found to depend on air temperature, microwave power and moisture content. Reduced vitamin C destruction was found in the microwave dried samples. The volumetric shrinkage of the samples exhibited a linear relation with moisture content. With convective processing, the samples exhibited uniform shrinkage throughout, however, with microwave drying two shrinkage periods were observed. Microwave dried samples had higher rehydration potential. [Copyright &y& Elsevier]

Shrinkage of foodstuffs is a common physical phenomenon observed during different dehydration processes. These changes affect the quality of the dehydrated product and should be taken into consideration when predicting moisture and temperature profiles in the dried material. The aim of this work is to give a physical description of the shrinkage mechanism and present a classification of the different models proposed to describe this behaviour in food materials undergoing dehydration. The models were classified in two main groups: empirical and fundamental models. Empirical models are obtained by means of regression analysis of shrinkage data. Fundamental models are based on a physical interpretation of the structure of food materials and try to predict dimensional changes due to volume variation of the different phases in the food system along the drying process. Several models referred to in this work were compared with experimental data on air drying of apple, carrot, potato and squid flesh. Average relative deviations between experimental and predicted values of shrinkage found were in most cases less than 10%. For some materials, models that neglect porosity change tend to show larger deviations. [Copyright &y& Elsevier]

For the first time, the ethanol as pre-treatment to the ultrasound-assisted convective drying of food was evaluated. Pre-treatments were performed by immersion of apple slices in ethanol (0–30 min). Pre-treated samples were convectively dried (50 °C, 1 m s−1), without/with ultrasound (21.77 kHz, 20.5 kW/m3). As results, if both technologies were considered, conventional drying time reduction reached 70 ± 2%. From drying kinetics modelling, it was identified that ethanol pre-treatments mainly reduced the external resistance to mass transfer, while ultrasound had a greater influence on the internal one. In dried samples, as the ethanol pre-treatment time increased, the shrinkage decreased, and their rehydration capacity was greater. After rehydration, samples showed a decrease of >85% in viscoelastic characteristics. The antioxidant capacity and total phenolic content were better retained with ultrasound application. The obtained results corroborate that the proposed technologies are complementary significantly accelerating the drying without negative effects on physical properties. • Ethanol treatment and US-assisted drying shorten 70% the conventional drying time. • Ethanol pre-treatment mainly affected the external resistances to mass transfer. • US application mainly reduced the internal resistances. • Shrinkage in dried samples decreased as the ethanol pre-treatment time increased. • Rehydration properties were improved in the ethanol pre-treated samples. [ABSTRACT FROM AUTHOR]