Drying Of Foodstuffs Research Articles

A Semi-Empirical Approach for Predicting the Effects of Shrinkage on the Convective Mass Transfer Evolution During the Solar Drying of Foodstuffs

A Semi-Empirical Approach for Predicting the Effects of Shrinkage on the Convective Mass Transfer Evolution During the Solar Drying of Foodstuffs

Thermal evaluation of indirect type solar dryer using flat plate collector

Direct sun-drying has a slow drying rate and improper heating. Smallholders rely heavily on the sun because it is cheap and easy to dry post-harvest crops. This study aimed to evaluate thermally the dryer using a flat plate collector for use on a small scale. The test results show an increase in environmental temperature of 32.42%. This increased temperature is the temperature in the no-load drying chamber. The average ambient temperature is 32.26 (29.38-33.88 ℃) with a solar intensity of 700.06 W/m2 (422.2-833.4 W/m2) increasing to 47.74 ℃(38.25- 54.74 ℃) in the drying chamber. By utilizing local materials for the design of indirect solar dryers, it is hoped that small farmers will be easy to operate and maintain. The dryer is designed in an efficient manner and adapted to the needs of small farmers in the post-harvest drying of foodstuffs.

Experimental investigation and mathematical modeling of drying of green tea leaves in a multi-tray cabinet dryer

Although many experimental investigations have been carried out on the drying of foodstuffs in the tray dryers, little detailed analysis is available in the literature on the mathematical modeling of simultaneous heat and mass transfer in these types of dryers. In this work, drying behavior of green tea leaves was investigated in a multi-tray cabinet dryer. A mathematical model was developed for prediction of the moisture content and temperature of green tea leaves and also the humidity and temperature of the drying air, in the trays of the dryer. Each tray was considered as a fixed bed of tea leaves. The proposed model was solved, after dividing the bed into a series of thin layers, using a finite difference method and a trial and error technique. Equilibrium moisture contents and moisture diffusivity values, used in the mathematical model, were estimated and correlated for the green tea, used in this study. The predictions of the model showed a good agreement (MRE lower than 5%) with the experimental data, obtained from drying of green tea leaves in a domestic three-tray dryer at 60 and 70 °C. The effect of drying air velocity, temperature and the number of the green tea leaves in each tray was investigated on the drying behavior of green tea in the dryer.

Comparative Effects of Three Different Drying Methods on Drying Kinetics and Quality of Jerusalem Artichoke (Helianthus tuberosus L.)

Mid-infrared-assisted freeze drying (MIR-FD) of Jerusalem artichoke cubes were attempted and their performance (processing time, energy consumption, color, water activity, hardness and rehydration) was compared with single-stage freeze dried (FD, control) samples. Two mathematical models could be used to describe the drying curves. It was aimed to improve the drying parameters and product quality of MIR-FD dried material. The effect of mid-infrared pre-drying (MIR) on drying rates of Jerusalem artichoke cubes was examined at different infrared intensity (3 and 6 kW/m−2). The values of drying rate considerably changed when the infrared intensity and pre-drying time was increased from 3 to 6 kW/m−2 and from 4 to 5 min. The MIR-FD hybrid drying mode took 15–40% lesser time compared to FD dried samples. Therefore, MIR-FD drying is an effective and economical method. MIR application had a significant (P < 0.05) effect on the color of the final product. The FD resulted in a product with significantly lower firmness and slightly higher water uptake (P < 0.05) than samples dried in the MIR-FD. Hybrid drying gave better results – all of the quality parameters – over MIR alone. Practical Applications The Jerusalem artichoke (Helianthus tuberosus L) is available in fresh and processed forms (powder, chips and soft drinks), the Jerusalem artichoke is rich in inulin, sugars, protein, macro and micro elements. Freeze- and mid-infrared drying have been industrially applied in the drying of foodstuffs and pharmaceutical products. The aim of this article is to examine the effectiveness of freeze drying (FD) enhanced with two-stage mid-infrared-freeze drying. The application of the mid-infrared-freeze drying (MIR-FD) produces a fast drying rate and higher drying quality. Our research work confirmed that MIR-FD is a good alternative instead of the FD. The development of new drying possibility for vegetables with good physical, chemical properties may be of interest in regard to the market supply (food supplement, functional food, etc.).

Investigation of Hot Air- and Infrared-Assisted Freeze-Drying of Apple

The dehydration tests were conducted at three drying methods to evaluate the drying curves and the energy uptake. Apple (Malus domestica L.) cubes were dried under different processing conditions, applying freeze-drying (FD) method, supplemented by hot air (HA) and infrared radiation drying (IR) techniques. The IR heating method produced a higher drying rate than HA drying during the pre-dehydration. The water activity, color, firmness and rehydration ratio (RR) of finished products were measured. The dried material produced with IR-FD resulted the desirable color, moreover higher RR and lower firmness than the other samples dried by HA-FD ones. The quality of single-stage FD samples was close to fresh apple and IR-FD materials. It was observed that the IR-FD method produced a considerable decrease in the energy consumption, compared with FD and HA-FD drying treatments. The thin-layer models were applied, which were used to describe the drying kinetics of food materials. It was found that the applied mathematical models were suited to the requirements in predicting the changes of moisture ratio. Practical Applications Apple (Malus domestica L.) is one of the most widely grown fruit crops in the world. Available in fresh and processed forms, apples are rich source of vitamins, organic acids, polyphenols, anthocyanin and minerals. Currently, dehydration is a frequent practice as apples are components of numerous processed food products, such as snack, integral breakfast foods, etc. Freeze-, hot air- and infrared (IR) drying have been industrially applied in the drying of foodstuffs, and pharmaceutical products. The aim of this article is to examine the effectiveness of freeze-drying (FD) enhanced with two-stage hot air- and IR-FD. The application of the IR-FD produces a fast drying rate and higher drying quality. Our research work confirmed that IR-FD is a good alternative instead of the FD. The development of new drying possibility for apple with good physical and chemical properties may be of interest in the market supply (food supplement, functional food, etc.).

Model reduction technique for faster simulation of drying of spherical solid foods

To simulate drying of foods, the classical calculation based on finite differences, elements or volumes are quite time consuming, which may be an issue in an optimisation procedure where hundreds or thousands of simulations are necessary. A model with coupled heat and mass transfers, based on diffusion and convection, with non-linear properties and boundaries makes an accurate but slow simulator for drying of foodstuffs. In this work, it was shown that the model with coupled heat and mass transfer can be replaced by a simple and much faster, yet as accurate, two- or three-compartment model. Equivalent predictions of mean product temperature and moisture content are obtained by linking exchange coefficients (in compartmental model) to transfer coefficients (in diffusion convection model) and fine tuning the compartment volume sizes. The resulting simulations are very close with calculation time nearly a hundred times faster.

DETERMINATION OF DRYING PARAMETERS, ASCORBIC ACID CONTENTS AND COLOR CHARACTERISTICS OF NETTLE LEAVES DURING MICROWAVE‐, AIR‐ AND COMBINED MICROWAVE–AIR‐DRYING

ABSTRACT Nettle leaves (Urtica dioica L.), with a moisture of 4.41 (dry basis [d.b .]), were dried using three drying methods: microwave, air and combined microwave–air. Drying continued until leaf moisture decreased to 0.1 (d.b .). Three different microwave output powers, 160, 350 and 500 W, were used in the microwave‐drying. Drying treatments in air‐drying were 50, 75 and 100C, and 1 m/s fan speed. The combination drying, in which microwave‐ and air‐drying were applied together, was nine different combination levels. Drying periods lasted for 6–14, 40–120 and 2–12 min for microwave‐, air‐ and combined microwave–air‐drying, respectively, depending on the drying level. Energy consumption was 0.07–0.10, 0.25–0.34 and 0.08–0.16 kWh, respectively. In this study, measured values were compared with the predicted values obtained from Page's semiempirical equation. Optimum drying period, color and energy consumption were obtained when microwave‐ and air‐drying were applied simultaneously and the optimum combination level was 500‐W microwave applications at 50C.PRACTICAL APPLICATIONSMicrowave‐, air‐ and combined microwave–air‐drying have been industrially applied in the drying of foodstuffs, and pharmaceutical and biologic products. The use of a combined microwave–air dryer is common in the food industry. Applied examples include the serial or a parallel combination of bothmethods. The application of the microwave as a heat source produces a fast drying rate and higher drying quality.Tunnel‐type microwave–air dryers have been recently used in the industry, and they can be used for the drying of foods, wood and paper products, traditional medicine solid preparations, crude drugs and sliced herbal medicines. They have the virtues of rapid drying, well distributed, energy saving, high efficiency, easy to operate and control, safety and nonpollution, etc.

Influence of power ultrasound application on mass transport and microstructure of orange peel during hot air drying

Power ultrasound application on convective drying of foodstuffs may be considered an emergent technology. This work deals with the influence of power ultrasound on drying of natural materials addressing the kinetic as well as the product’s microstructure. Convective drying kinetics of orange peel slabs (thickness 5.95±0.41 mm) were carried out at 40 ∘C and 1 m/s with (US) and without (AIR) power ultrasound application. A diffusion model considering external resistance to mass transfer was considered to describe drying kinetics. Fresh, US and AIR dried samples were analyzed using Cryo-SEM. Results showed that drying kinetics of orange peel were significantly improved by the application of power ultrasound. From modeling, it was observed a significant (p¡0.05) increase in both mass transfer coefficient and effective moisture diffusivity. The effects on mass transfer properties were confirmed from microestructural observations. In the cuticle surface, the pores were obstructed by wax components scattering, which evidence the ultrasonic effects on the interfaces. The cells of the flavedo were compressed and large intercellular air spaces were generated in the albedo facilitating water transfer through it.

Supercritical-CO 2 drying of foodstuffs in packed beds: Experimental validation of a mathematical model and sensitive analysis

In this contribution, a mathematical model is built to predict the changes in water concentration in both a solid food matrix and a fluid carrier during supercritical carbon dioxide (SC–CO 2) drying. The mass balance equations of the model involve five dimensionless parameters: Peclet number modified Sherwood number, Fourier number, mass ratio and equilibrium constant. The differential equations were discretized using the finite explicit difference method. The resulting model was implemented and solved in Matlab/Simulink using an explicit Runge–Kutta solver. A very good agreement ( ARD = 7.2%) between experimental data, obtained by an independent group, and the present model was observed. The axial dispersion diffusion coefficient seems not to play a significant role during the drying process. A sensitivity analysis revealed that the predictions are relatively more sensitive to the equilibrium constant and the mass ratio than to Peclet and modified Sherwood numbers. Furthermore, in the case of Peclet and modified Sherwood numbers, the sensitivity and the uncertainty of the output are function of the final moisture content. The present model could be used as an optimization tool for kinetic studies to investigate the effects of different operation conditions on the performance and design of the supercritical drying technology.

Drying rates in shrinking medium: case study of banana

Drying of foodstuffs is used to improve product stability, but presents some potentially harmful side effects such as shrinkage. The reduction of the relative humidity of the environment was studied in this work to substitute heating during drying, also taking into account the effect on shrinkage. Drying experiments of cylindrical slices of banana were conducted in commercial equipment, with controlled temperature and relative humidity. The samples of banana, with diameter/thickness aspect ratio of approximately five, were used. Results showed the existence of a constant drying flux period is when the variation of the transfer area is taken into account, which is evidence that the shrinking is two-dimensional. Relative humidity, and not only temperature, influences the final characteristics of the product. With this, an equivalence project of drying rates is considered in different conditions for the optimization of energy costs and product quality.

Analysis of Freeze-Drying of Tropical Fruits

Freeze-drying is a process recommended for drying of heat-sensitive products. Some advantages of this technique are that it allows the shrinkage and the degrading reactions in the material, which are common in conventional drying operations, to be minimized. The knowledge of drying kinetics is essential for modeling and optimization of a freeze-drying process. In this work, drying kinetics for pineapple, guava, and mango pulps were investigated using five empirical equations commonly applied for convective drying of foodstuffs. Increasing constant- and falling-rate periods were observed during the drying of such pulps. The best fittings for the drying kinetics of all the materials were obtained using the Page and Chen and Douglas equations.

Drying Research ‐ Current State and Future Trends

AbstractResearch and development activity in the inter‐ and cross‐disciplinary area of thermal drying has been escalating rapidly over the past two decades. It is clearly motivated by economic incentive to improve the operation to produce a better quality product at a lower cost in terms of resources, energy consumption as well as environmental impact. The focus to date has been on improving the engineering aspects of design and operation to produce “engineered” products of desired characteristics. Much remains to be achieved in relation to the material science aspects of drying, e.g. the prediction of quality parameters which are product‐dependent. What is needed is a generalised drying theory that incorporates the transient transport phenomena involved in drying along with appropriate models to account for the morphological changes, as well as chemical processes that may occur during drying.This paper will focus on the current developments in drying as well as recent trends. Some areas where more R&amp;D is needed and which provide opportunities to make definitive contributions will be identified. For example, sophisticated analytical measurements at the micro‐scale are needed in order to examine the movement of moisture during drying, and also detect any changes in the physical structure as well as solute transport that can occur in drying of foodstuffs or solids containing a dissolved solid in the liquid phase. Even today, there is need to devise more efficient and compact dryers. Whether we will ever need “micro‐scale” dryers is a speculative question but certainly there will be the need to design dryers with high “volumetric” efficiency, unlike most dryers built today. Finally, there will be further developments and extended applications for superheated steam dryers, heat pump‐assisted hybrid dryers and multi‐stage dryers using different dryer types in each stage. Mathematical models for drying as well as for dryers will continue to be in demand to allow reliable design and scale‐up, as well as the control of industrial dryers.

Principles, Methods and Applications of the Convective Drying of Foodstuffs

Moisture removal from solids is an integral part of food processing. Moreover, convective drying is one of the most important techniques for preservation of biological products. An essential prerequisite for interpretation of the mass transport phenomena during drying is a knowledge of the equilibrium characteristics of the foodstuff. A further necessity is a knowledge of the drying kinetics and a detailed understanding of the moisture transport mechanism. During the drying process, a multiphase system undergoes simultaneous physical and structural modifications. Therefore, for accurate interpretation of this coupled heat and mass transport operation, moisture transport characteristics and physical property data is essential. This paper reviews more salient aspects of convective drying of foodstuffs.

Simulation of packed bed drying of foodstuffs with airflow reversal

A model for the simulation of batch dryers was developed. The effect of shrinkage of the materials during drying was incorporated into the model. This model was extended to include the case of airflow reversal. The results of the simulation for constant inlet air conditions were compared with experimental data obtained during batch drying of carrot cylinders. The effect of airflow reversal on the bed-averaged drying curves as well as on the local temperature and water content profiles was analysed. The results showed that both moisture and temperature profiles in the bed were flatter when airflow reversal was applied to the drying process while the average drying curves were practically unchanged. The effects of the starting time (t 1) and period of duration (Δt) of the airflow reversal were studied through numerical simulations for different lengths of the bed of drying.

Shrinkage during drying of foodstuffs

Experimental shrinkage data as a function of water content for particles of potatoes, apples and carrots of different geometry and under various drying conditions are presented. Volumetric shrinkage of individual particles is affected mainly by air velocity; other drying variables have negligible effect. Changes in surface area to volume ratio with water content are practically independent of drying conditions but dependent on sample geometry and the type of foodstuff. A simple model in which volumetric ratio is represented by one or two line segments is developed. The shrinkage of packed beds is also studied. Changes in bed volume during drying are almost only a function of bed-averaged water content. A simple way of predicting porosity in the bed is applied and its results are compared with experimental data.

Prediction of the sorptional equilibrium relationship for the drying of foodstuffs

AbstractAfter existing correlations were reviewed to predict the equilibrium of foodstuffs with moist air, it was found that they were not adequate. A new equilibrium expression is presented on the basis of equal chemical potentials in the external and internal phases. The resulting equation can be used over most of the range of moisture contents of sugar based foodstuffs including the high moisture region; it also provides simple analytical expressions for the partial derivatives of moisture content with respect to temperature and to water vapor activity.

Gold reflectors in infra-red heating

Heating by means of medium wave or short wave infra-red radiation is an essential feature of a wide variety of industrial processes. Its applications are to be found, for example, in the textile and papermaking industries, in printing, in the manufacture of plastics and of laminated glass, and in cooking and the drying of foodstuffs. Generally it is used with most effect in conveyor type ovens where continuous processing is involved and where poorly conducting materials are to be heated rapidly. The high temperature source employed is either a tungsten filament operating at around 2200°C—for short wave—or a nickel-chromium spiral filament running at 800°C or over for medium wave heating. In either case the filament is enclosed in a tubular envelope of quartz glass, a material which will withstand the high temperature and which is also transparent to infra-red. The envelope thus remains relatively cool and so minimises the loss of radiant heat to the surrounding atmosphere. This type of construction provides a simple and compact unit giving rapid heating and cooling times with ease of maintenance. In many cases these infra-red units are located above the material to be heated, and an efficient means of reflecting the heat must be employed. While external reflectors can be arranged, an improved technique recently developed involves applying a conventional gold coating to the reverse side of the quartz glass envelope. This gives a highly efficient reflector for infra-red, more than 99 per cent, particularly when employed in the form of twin tubes. Improved performance and of course the permanent maintenance of the high reflecting gold surface are ensured since it is not directly exposed to possibly unclean environments. For a more detailed account of the fundamentals of infra-red heating and of the types of equipment available an earlier paper by the writer may be found useful (Electrical Review, 1974, 194, (22), 681-684).

The dehydration of foods in edible oil in vacuo

Summary. Arachis oil, which was used as the medium for dehydration, showed considerable resistance to autoxidation when heated alone under the conditions employed in the drying of foodstuffs. the processing of cabbage, lean horse meat and herring had the effect of increasing the stability of the drying medium. the oil which adheres to the surface of the products was even more stabilized. Continuous use of the oil did not result in a rise in its content of free fatty acids. Neither the drying medium nor the adherent oil became rancid after storage for many weeks, although susceptibility to autoxidation was increased. the synthetic antioxidants BHT and sesamol, added to the arachis oil before heating, acted synergistically with the natural antioxidants of the foodstuffs, and gave rise to a striking improvement in the stability of the oil. the net protein utilization of herring dried in arachis oil in vacuo (73) was as high as that of freeze‐dried herring (70).

The vacuum contact‐plate dehydration of foodstuffs. I.—a first appraisal

AbstractFactors which must be considered in the drying of foodstuffs are discussed. The conventional hot‐air drying and the vacuum contact‐plate drying methods are described and compared. The latter yields better dried products than the former, both in appearance and palatability of the reconstituted, cooked material, and there is some evidence that the storage properties are also improved. Relevant factors are the absence of oxygen and the milder heat treatment of the foodstuff during vacuum contact‐plate dehydration. This technique is versatile and may be used to dry a wide range of commodities, many of which cannot be dried satisfactorily by conventional methods.