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REA drying kinetics model has been widely examined for different materials and different operating conditions. Vacuum drying, due to its uniqueness, it presents a challenge for setting up the REA parameters, in particular the specification of the equilibrium activation energy value corresponding to each set of environmental condition. Here, the basic kinetic parameters derived from a convective drying run, is used while the effect of vacuum is captured by the equilibrium activation energy. Upon validation against the experimental data, REA is shown to be applicable. A comparison between the vacuum drying and the convective drying of the same material is also made. The vacuum drying at higher vacuum levels has been predicted, showing the expected behavior. It has been shown that REA is flexible and can be used in vacuum drying related design and optimization studies. [ABSTRACT FROM AUTHOR]

For better understanding of transport phenomena, local evaporation rate and water vapor concentration should be taken into consideration when predicting drying behavior. However, to the best of our knowledge, there is no model which explicitly represents this. Predictive modeling of combined infrared-heating and hot air drying (IR-HAD) is useful to assist in designing new dryer units and manufacturing the products with desirable quality. As the quality changes are basically local phenomena, the model should be able to capture the local evaporation rate. In this study, for the first, time, spatial reaction engineering approach (S-REA) was developed and used to describe infrared-heating drying of the food materials by using the reaction engineering approach (REA) to represent local evaporation rate. Benchmarks against the single-phase approach and the results showed that the S-REA gave high accuracy toward the experimental data (R2= 0.978–0.999). The S-REA successfully provided reasonable predictions of two-dimensional (2 D) profiles of moisture content, concentration of water vapor and temperature. This highlights the accuracy of the REA framework to describe the local evaporation rate during the infrared-heating. The REA-based predicted variables can then be applied to project the local quality parameters for quality assurance in food processing. The model is also ready to be used to audit energy demands in drying systems. [ABSTRACT FROM AUTHOR]

In this work, an online calibration system with an electric balance is integrated to a novel NMR/MRI system which has continuous and controlled air supply. The water status and distribution of the internal moisture content and total moisture content are measured simultaneously, respectively, by the NMR/MRI and the balance. Fresh and pretreated broccoli stalks and potatoes were employed as the food materials. Data obtained from the integrated balance were compared with the data obtained from NMR and MRI. The results show that the signal intensity is linear against the moisture content (R2>0.90). However, the linearity of the NMR data is much better than that of the MRI data, which is due to the device characteristics. The detection limits of the moisture content were about 11% and 20% for NMR and MRI, respectively. The study demonstrates that LF-NMR/MRI can be used as an online analytical method to quantify the moisture content and drying rates during drying of vegetables. [ABSTRACT FROM AUTHOR]

A reliable mathematical model is useful for predicting internal profiles inside materials during drying. In this study, for the first time, the spatial reaction engineering approach (S-REA) is employed to model the local profiles of food materials during drying. The REA is applied as the local rate of phase change and combined with a set of equations of conservation of heat and mass transfer to yield the spatial profiles of temperature and concentration during drying. The S-REA predictions are benchmarked against the Magnetic Resonance Imaging (MRI) data. The study indicates that the S-REA is applicable to model the internal profiles inside food materials during drying. The S-REA predictions also show closer agreement towards the experimental data than the effective diffusion model. While the S-REA predictions are accurate, it requires minimum number of experiments to generate the drying parameters. The S-REA has contributed to better analysis of transport phenomena inside food materials during drying through generation of local profiles. The S-REA predictions can potentially be implemented to interpret the sensory and quality matters during drying. [ABSTRACT FROM AUTHOR]

One of the most significant process intensification schemes in drying is microwave drying. Modeling the process of microwave drying is very useful. The lumped reaction engineering approach (REA) is now coupled with appropriate equations for modeling microwave heating. Here, a slight modification of the equilibrium activation energy is needed since the product temperature is higher than the ambient temperature. Unlike the diffusion-based approach, the REA drying parameters were generated from minimum number of drying runs. It has been found that the modifications lead to excellent agreements between the predicted and experimental data. The results of modeling match well with the experimental data. The overall model is accurate to describe the moisture content and temperature profiles. Comparisons with the diffusion-based approach indicate that the REA can achieve comparable or even better agreement toward the experimental data. This exercise has demonstrated that a simple combination of the lumped reaction engineering approach and the microwave energy absorption is versatile in predicting the microwave drying process accurately; thus, this worked example will be illustrative for future needed studies. [ABSTRACT FROM AUTHOR]

This article investigated the influence of drying rate on the shrinkage behavior of skim milk (SMP) droplets during convective drying, behavior which was then mapped using the Peclet number, Pe. The objective was to demonstrate the potential use of the Peclet number to predict the particle formation pathway during drying by defining a material-specific perfect shrinkage threshold using only the knowledge of initial process conditions and droplet characteristics. The shrinkage mapping was undertaken by drying relatively large and statically suspended droplets through adjustment of the drying air humidity. Its influence on droplet shrinkage behavior was observed through droplet diameter profile. Analysis of lactose, a non-skin-forming material, revealed perfect shrinkage behavior. Experiments with SMP, a skin-forming material, revealed that deviation from perfect shrinkage behavior occurred beyond the Peclet number threshold of 0.23–1.27 when droplets and the solid diffusion front were modelled as an equivalent sphere. The developed threshold was further used to predict the shrinkage behavior of atomized SMP droplets undergoing rapid drying in a spray tower. [ABSTRACT FROM PUBLISHER]

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]

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: Freeze-drying is a traditional approach of producing desiccated microorganisms. Industrial mass-production of desiccated microorganisms, however, pursues a more economic drying process, which leads to an increasing research interest in the thermal drying processes of microorganisms. These thermal drying processes include spray drying, fluidized bed drying, vacuum drying, air-convective drying, etc. Cells being thermally dried suffer from simultaneous heat and dehydration stresses. To maximize the cell survival and to study the mechanisms of cell inactivation during such processes, it is necessary to have understandings of both the chemical engineering principles involved and the biological properties of the microorganisms to be dried. This review attempts to give a balanced discussion on both aspects. Factors influencing cell viability during thermal drying are discussed in two groups, viz., intrinsic factors related to the microorganisms being dried and extrinsic factors related to process conditions. Finally, modeling of the inactivation kinetics is briefly reviewed. This article aims to bring together some common observations and findings for different organisms subjected to thermal drying, and discuss the mechanism underlying these observations. Efficient drying which is associated with fast removal of water content has economical benefits, but hurts cells. The desiccation of microorganism thus is a balance between these two considerations and needs to be strain-specifically optimized. [Copyright &y& Elsevier]

In this paper, a modified Biot and Lewis number analysis of air-drying of food and biological objects with small sizes has been carried out. It has been shown that the evaporative action broadens the range of the application of the conventional Biot assessment for uniform temperature approach. For evaluating whether heat or mass transfer is the limiting process for drying, the article has preliminarily demonstrated that the inclusion of the sorption characteristics of the material being dried in the calculation of the effective diffusivity for water vapor predicts larger Lewis numbers (greater than unity). Sample calculations are given in this article based on previous laboratory results. [ABSTRACT FROM AUTHOR]

Spray-drying is the prime process for manufacturing powders. Optimization of spray-drying processes requires both modeling the drying characteristics of a single milk droplet and the dryer-wide simulations incorporating computational fluid dynamics. In computational fluid dynamics simulations, it is desirable to have an accurate yet simple model for drying of single droplet, which does not require resolving partial differential equation. Here the experimental results on constant drying conditions with high humidity air flows are reported for the first time. Previously, a mathematical model using the reaction engineering approach (the REA model) was established using the very dry air conditions. The same REA model has been used to predict the current high humidity results. It has been shown that the predictions agree well with the experimental results, showing the good promise of this approach. [ABSTRACT FROM AUTHOR]

Magnetic resonance imaging (MRI) is a very powerful technique increasingly used in food engineering, yet examples where the local water content is quantified are scarce. Homogeneous whey protein hydrogels were utilized as a model system, far simpler than most foods. The normalized proton density intensity of hydrogels was predicted using experimental correlations of the spin-lattice and spin-spin relaxation times, T 1 and T 2 respectively. Using a typical echo time T E of 20 ms, the intensity is maximum at a volumetric swelling ratio Q ∼15, and MRI is suitable to study the drying of hydrogels at lower Q values. Swelling can be studied by adjusting T E to target the Q range of interest, e.g. ∼200 ms. Whereas reasonable agreement is found between predicted normalized intensities and from drying and swelling experiments, local quantification of Q in unknown conditions will suffer from noise and relatively poor repeatability. Deviations from predictions are observed in the swelling at high NaCl concentrations, and at high alkaline pH, that need to be studied further. Opaque particulate hydrogels can be studied equally well compared to stranded transparent gel, a clear advantage against optical techniques. [ABSTRACT FROM AUTHOR]

Many quality attributes of food products are influenced by the water status and the microstructure. Low-field nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) methods are applied to non-destructively monitor the water status and structure of food. The aim of this study is to investigate the water status and distribution inside broccoli tissues and the effects of hot-air drying on the water status by using NMR and MRI methods. Transverse relaxation times (T 2 ) provide the information of water status and water distribution. Results show that three water fractions with different T 2 relaxation times were detected inside broccoli, which corresponded to different cell compartments. Proton weighted imaging could monitor the spatial distribution of water. Image analysis indicates that the water distribution inside broccoli was heterogeneous and the water content reduced from the stalk to the buds. During hot-air drying experiments, different drying kinetics were observed in the florets and stalks, which were related to their different structures. In addition, a detection limit of the moisture content was calculated for LF-NMR (about 11.35%). The results of this study show that the low-field NMR and MRI methods can precisely provide the quantitative information of water status inside food materials, and can be used to investigate the effects of food processing on product quality. The method provided in this study can be used to monitor changes of water status and distribution in a sample non-destructively during drying process. [ABSTRACT FROM AUTHOR]

Encapsulation of docosahexaenoic acid (DHA) protects this environment-sensitive substance from oxidation and also masks its odor. In this study, DHA was encapsulated with octenyl succinic anhydride modified (OSA) starch and whey protein concentrate. Single droplets of emulsions with different compositions were suspended on a glass filament to observe the dissolution behavior of dried and semi-dried particles. At each drying stage, the drying process were stopped to monitor changes in the semi-dried particle’s morphology, either by attaching an additional water droplet or by keeping the semi-dried particle suspended in the drying chamber without air flow. Based on observations in the above-mentioned test, the crust formation phenomena of emulsion droplets with different compositions were compared at various drying stages. The microstructures of dried particles containing DHA were examined by scanning electron microscopy (SEM). The surface oil coverage and distribution were studied by observing the surface structure before and after oil extraction. The results obtained in this study indicate the underlying particle formation mechanisms during drying of DHA-containing emulsions formed with different wall materials. [ABSTRACT FROM AUTHOR]

Milk protein concentrate (MPC) powders are valuable dairy products, but the understanding of their drying behaviour and particle formation during convective dehydration especially at high concentrations is currently limited. This study aims to improve the understanding of the drying behaviour of MPC, particularly the influence of concentration on compositional redistribution, and particle formation from droplets. We utilised the glass-filament single droplet drying technique to monitor the thermal-moisture drying histories of MPC droplets and used the Reaction Engineering Approach (REA) to model their drying kinetics. Across the range of solids content tested, the rate of moisture removal and final moisture content of dried MPC particles achieved were relatively similar. However, increase in solids content of MPC droplets led to reduced shrinkage and increased rate of temperature rise during drying. The in situ rehydration study indicated slower rehydration with the increase of concentration. Surface composition of dried MPC particles was analysed using X-ray photoelectron spectroscopy (XPS) and the findings indicated the importance of bulk and surface compositions in determining droplet drying behaviour and particle formation during drying. [ABSTRACT FROM AUTHOR]

Abstract: Industrial spray drying of milk powder production utilises high solids feed at around 50wt.% or higher; however there are only a few fundamental studies investigating the droplet drying behaviour. The present work studied the shrinkage behaviour of 50wt.% skim milk droplet during drying. Temperature effects on both shrinkage kinetics and total diameter reduction were found to be different from low solids milk drying. A high drying temperature at 110°C led to negligible droplet shrinkage at X range between 1 and 0.3kg/kg. An increase in temperature resulted in larger particles after drying. The high initial solids could sustain the droplet shape and hinder solute diffusion; as such droplet shrinkage behaviour deviates from the description of ideal shrinkage kinetics. A general correlation of shrinkage coefficient b was established for skim milk with initial solids content of 10–50wt.%, useful for estimating droplet shrinkage kinetics within this range. [Copyright &y& Elsevier]

Abstract: The effect of shear rate and oxygen injury during atomization and the combination of these factors on the survival of Lactococcus lactis subsp. cremoris in spray drying was studied using laboratory and pilot scale spray dryers. The atomization was carried out using a two-fluid nozzle in the laboratory study and a two-fluid nozzle or rotary atomizer in the pilot scale study. The extent of oxygen-induced death was determined using ascorbic acid in the feed and atomizing the feed with gaseous nitrogen. The lowest levels of bacterial death were observed at lowest characteristic shear rate and in the presence of nitrogen and ascorbic acid. Quantitative analysis showed that lower shear rate, creating an oxygen-limiting environment during atomization and drying, and using oxygen scavengers in the feed were successful in enhancing bacterial survival in spray drying. We also report for the first time that, at least for L. lactis, the extent of death during the atomization stage far outweighs death during the drying stage, and that the majority of bacterial death (up to 93%) occurs during the atomization stage. The death of bacteria was found to be less when using a rotary atomizer or when using a two-fluid nozzle atomizer at lower flow rate. This work shows that bacterial death during spray drying can be minimized by using oxygen scavengers such as ascorbic acid and/or an anaerobic atomizing medium (such as nitrogen), and by altering the spraying conditions. [Copyright &y& Elsevier]