Apple Drying Research Articles

Convective air drying of apples as affected by blanching and calcium impregnation

The effect of previous blanching and calcium impregnation at atmospheric pressure (AI) or in vacuum (VI) on the rate of moisture movement during the first falling rate period of air drying of apples at 60 °C was studied. It was found that the effective diffusion coefficient of water ( D ef) calculated with Fick’s second law was strongly affected by heat pretreatment. With the exception of non-blanched tissues subjected to VI or VI followed by AI for 1.5 h, calcium uptake during impregnation step appeared to modify the matrix resistance to water flux only when tissue was previously heated. Light microscopy studies of apple tissues allowed explaining the observed drying behaviour.

Mass Transfer Modelling in an Acoustic-Assisted Osmotic Process

Ultrasounds are mechanical waves that produce different effects when travelling through a medium, some related to mass transfer (i.e. microstirring at the interface, the so called "sponge effect" and cavitations). Thus, ultrasound appears to be a way to reduce both the internal and external resistances in osmotic food drying processes. In this study, the influence of the ultrasounds on water and solute transports during osmotic processes of drying is evaluated. Two different systems have been studied, apple slabs immersed in 30ºBrix sucrose solution, and pork loin slabs in sodium chloride saturated brine. The mathematical modelling of the mass transfers has been carried out by assuming diffusional mechanism and considering the mutual effect between the two mass transfers, the water losses and solute gains. The mass transfer curves in the osmotic process of apple drying in sucrose solution were satisfactorily simulated by using a diffusional model considering independent mass fluxes. Nevertheless, this model did not allow for the accurate simulation of the water losses in the system constituted by pork-loin in saline solution. When the mass fluxes were considered mutually affected, the simulation was accurate for both cases water and solute transfer.

Moisture Loss Kinetics of Apple During Convective Hot Air and Isothermal Drying

Effective moisture diffusivity of cylindrical apple samples (0.7 and 1.4 cm diameter) was determined under convective hot air drying (40, 50, 60, and 70° C and 1.5 and 3 m/s air velocity). The effective moisture diffusivities obtained were in the range of reported values and were found to be temperature dependent according to the Arrhenius relationship. However, the prediction of moisture loss obtained from Fick's diffusion model failed to follow experimental drying curves. Temperature profiles during convective hot air drying showed temperature gradients. This lack of isothermal conditions may lead to inaccurate predictions of moisture loss. Therefore, a combined microwave-convective hot air apparatus, capable of providing isothermal drying conditions, was used to quantify the drying kinetics. Differences in effective diffusivities determined from convective (9.6–18.5 3 10210 m2/s) and isothermal (20.3–37.6 3 10210 m2/s) drying experiments were shown. Using effective diffusivities obtained under isothermal conditions, the Fickian model still did not predict moisture loss during isothermal drying. Due to the developing porous structure of apple during drying, it was hypothesized that drying of a hygroscopic porous material is limited by evaporation of water to water vapor. Therefore, an irreversible first-order kinetic model was proposed to predict isothermal drying of apple. Using the rate constant calculated from the slope of the normalized drying curves (0.08–0.17 min21), the model predicted accurate moisture loss at each temperature throughout the entire moisture range of 0.06–8.5 g/g dry solid.

Mathematical modeling of thin layer drying of Golden apples

In this study, a laboratory dryer was used for thin layer apple drying process and moisture ratio at any drying time were compared by Newton, Page, Modified Page, Henderson and Pabis, logarithmic, two-term, two-term exponential, Wang and Singh, Thompson, diffusion approximation, Modified Henderson and Pabis, Verma et al. and Midilli et al. models. The effect of drying air temperature and velocity on the coefficients of the best suited moisture ratio model were determined by multiple regression method. Root mean square error (RMSE) and chi-square ( χ 2) were used for the determination of the best suitable model. In addition to these statistical parameter, the modeling efficiency (EF) was also determined. According to the results, Midilli et al. model is superior to the others for explaining drying behaviour of apple. It is possible to predict moisture content of the product with generalised model showing the effect of drying air temperature and velocity on the model constants and coefficients.

Properties of modified atmosphere heat pump dried foods

In this study, apple, guava and potato were dried in a heat pump dryer under inert environmental (nitrogen or carbon dioxide) conditions and the impact on colour, surface porosity and rehydration abilities were investigated. Lemon juice and peel were used as natural inhibitors to prevent browning in air drying of apples. Comparisons were made between heat pump dried samples and those dried by freeze and vacuum drying. Results showed that MAHPD low temperature drying at about 45 °C and relative humidity around 10% led to better physical properties, such as reduced shrinkage, decreased firmness and more porous structure of the materials, which resulted in quicker rehydration. When using inert gas, the colour of heat pump dried food was similar to vacuum or freeze drying.

Simple modeling of infrared drying of fresh apple slices

The effect of drying temperature on drying rates of apple slice was examined at different temperature. The values of drying rate almost doubled when the drying temperature was increased from 50 to 80 °C. The experimental drying data were applied to various drying equation (Newton, Page, Modified Page, Wang and Singh, Henderson and Pabis, Logarithmic, Diffusion approach, Simlified Fick’s diffusion SFFD equation, Modified Page equation-II and Midilli equation). Midilli equation was best for characterizing drying behavior of apple for the whole range of temperature. The variation of these models parameters with temperature were described as Arrhenius and logarithmic type function of drying temperature. Sixty-six different model equations that describing the temperature and time dependence of infrared drying of apple were derived. Theoretical models describing temperature and time dependence of drying were fitted to experimental data and model parameters in equations were determined by multiple regression analysis in temperature range 50–80 °C. Model 50 derived from Modifiye Page II has lowest RMSE, MBE and chi-square and the highest modeling efficiency and regression coefficient. The moisture ratio change during infrared drying (0–240 min) of apple in the temperature range of 50–80 °C can be predicted by single equation: MR = exp - ( - 9 . 08244 + 1 . 580765 ln ( T ) ) ( t / ( - 0 . 628792 + 0 . 574354 ln ( T ) ) 2 ) ( 11 . 49544 - 1 . 74016 ln ( T ) ) .

Volatility of apples during air and freeze drying

The volatility of some representative aroma compounds during the freeze and convective drying of apples was analysed by gas chromatography/mass spectrometry (GC/MS). Among the 36 aroma compounds identified by GC–MS, three representative components of the apple flavor, ethyl acetate, ethyl butyrate and methyl anthranilate, were examined in detail. The retention of the above compounds and moisture content retention during freeze and conventional drying at three different temperatures was investigated. Drying kinetics of volatile compounds and moisture content were studied by introducing a first order kinetic model, involving a characteristic parameter (drying constant), as a function of the drying temperature. Most of the losses of the flavor compounds occurred during the early stages of drying. Retention of aroma was affected by the temperature of drying and the drying method used. Lower drying temperatures and freeze drying method are suggested for maximum retention of flavor in the dried product.

Determination of suitable thin layer drying curve model for some vegetables and fruits

This study presents a mathematical modeling of thin layer drying of potato, apple and pumpkin slices in a convective cyclone dryer. In order to estimate and select the appropriate drying curve equation, 13 different models, which are semi-theoretical and/or empirical, were applied to the experimental data and compared according to their coefficients of determination ( r, χ 2), which were predicted by non-linear regression analysis using the Statistica Computer Program. Moreover, the effects of drying air temperature, velocity and sample area on the model constants and coefficients were also studied by multiple regression analysis. Consequently, of all the drying models, a semi-theoretical Midilli–Kucuk model was selected as the best one, according to r and χ 2.

Thermodynamic analysis of the apple drying process

This paper is concerned with thermodynamic analysis of the process of single-layer drying of apple slices by means of a cyclone-type dryer. Using the first law of thermodynamics, energy analysis was performed to estimate the ratios of energy utilization throughout the drying process. Exergy analysis was carried out to determine exergy losses during the drying process by applying the second law of thermodynamics. It was concluded that the exergy losses increased with increasing energy utilization in both trays and the drying chamber. Values of the energy utilization ratio (EUR) of the drying chamber varied in the range 0−41.64 per cent depending on the drying conditions. The most exergy losses took place on the first tray (EUR = 0−21.06 per cent) during the drying of apple slices. It is emphasized that apple slices are sufficiently dried in the range 60−80°C with 20−10 per cent relative humidity and a drying air velocity of 1.5 m/s over a 3.33−5.33 h period in spite of exergy losses of 0−1.243 kj/s (EUR = 0−34.72 per cent).

Effect of hot air temperature on mechanical properties of dried apples

The aim of this work was to determine the effect of temperature and water content on the mechanical properties of convection dried apples. Apples v. Idared were cut into slices and then air-dried at temperature in the range from 50 to 80 °C. Mechanical properties were measured by compression–relaxation test. Samples were investigated after drying and also after 5 weeks of storage over anhydrous calcium chloride at constant temperature. Analysis showed that the observed differences between compression–relaxation curves of apples dried at 50, 60 and 70 °C were not significant. Decreasing of water content caused an increase of force needed to compress dried apples. Drying at 80 °C affected the texture of dried apples much more in comparison to those obtained at lower hot air temperature. The use of suitable temperature and drying of apples to definite water content could be used to design the mechanical properties of the final product.

Influence of airflow velocity on kinetics of convection apple drying

The aim of this experiment was to investigate airflow velocity influence (0.64, 1.00, 1.50, 2.00, 2.50 and 2.75 m s −1) on the kinetics of convection drying of Jonagold apple, heat transfer and average effective diffusion coefficients. Drying was conducted in a convection tray drier at drying temperature of 60 °C using rectangle-shaped (20 × 20 × 5 mm) apple samples. Temperature changes of dried samples, as well as relative humidity and temperature of drying air were measured during the drying process. Rehydratation ratio was used as a parameter for the dried sample quality. Kinetic equations were estimated by using an exponential mathematical model. The results of calculations corresponded well with experimental data. Two well-defined falling rate periods and a very short constant rate period at lower air velocities were observed. With an increase of the airflow velocity an increase of heat transfer coefficient and effective diffusion coefficient was found. During rehydratation, about 72% of water removed by the drying process was returned.

Modelling shrinkage during convective drying of food materials: a review

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.

Equilibrium distribution data for osmotic drying of apple cubes in sugar-water solution

Equilibrium distribution coefficient data were obtained for osmotic dehydration of apple cubes in sugar syrup of different concentrations and temperatures. The initial concentration of sucrose was varied from 30% to 70%, and temperature was varied from 22°C to 90°C. The distribution coefficients for solids ranged from 0.466 to 1.46, and for water it ranged from 0.621 to 1.80. The distribution coefficient for solids increased with the increase in temperature while it decreased with the increase in syrup concentration. However, the distribution coefficient for water decreased with increasing temperature and it increased with the increase in syrup concentration. A multiple regression analysis of experimental data was carried out to correlate distribution coefficients with dimensionless temperature and syrup concentration.

New drying systems for crops

Drying of agricultural products is an energy intensive operation. High prices and shortages in fossil fuels increase the emphasis on using solar energy as an alternative energy source, especially in developing countries. This paper deals with the construction of four different solar powered drying systems consisting of solar air heaters and drying chambers for small scale, natural convection, direct and indirect solar drying. Generally, solar drying systems which have two main parts, namely a solar air heater and a drying chamber, work during the day and they do not work at night and cloudy days. In this study, these drying systems were successfully tested for apple drying. The drying curves of those systems were compared with the traditional solar drying results. It is concluded that the using of the continuous drying system gave the best results with the reduction of drying time.

Temperature Measurement and Control during Quality Drying of Apple

In this paper a discussion on the temperature measurement in connection with apple quality drying is presented. The thermocouple as an ordinary temperature sensors was chosen in order to clarify their thermovoltage function as because the classical approximation function does not satisfy all the basic rules and the physical principles. With the help of the theoretical approaches and the Fermi-Dirac statistics of the electrons a new thermovoltage function was introduced. The result was compared to the classical approximations and tested with the thermocouple data presented by the thermocouple producers. Based on the results achieved a measuring circuit was developed without stabilised reference temperature.

Heat and mass transport in microwave drying of porous materials in a spouted bed

AbstractA heat‐ and mass‐transfer model was developed to simulate microwave and spouted‐bed combined (MWSB) drying of diced apples, a hygroscopic porous material. A total gas‐pressure equation was introduced to take into account internal vapor generation during microwave drying. The governing equations for heat and mass transfer were simplified using a scaling technique and numerically solved with the finite difference method. The physical, thermodynamic, thermal, transport and dielectric properties used in the simulation were either from our measurements or from the literature. Model predictions agreed well with experimental results. The pressure‐driven moisture migration during MWSB drying resulted in a high drying rate. The numerical analysis predicted a temperature‐leveling effect that was confirmed by experimental results. The unique temperature leveling in MWSB drying helps to control product temperature and improves product quality as compared to microwave‐assisted fixed‐bed hot‐air drying methods.

Effect of osmotic dewatering on rheological properties of apple subjected to convective drying

Apple v. Idared cut in cubes was dewatered by osmosis and subsequently dried by convection. Dewatering caused substantial changes in mechanical properties of apple tissue. Shrinkage due to osmotic pre-treatment was small but stiffness was half the value obtained by convective drying of apple cubes to the same final water content. In a viscous-elastic model of apple tissue, osmotic treatment increased the share of viscous element. Concentrated sugar solution in surface layers of pre-treated apple cubes modified the effect of convective drying on rheological properties of the tissue in comparison with those observed during drying of raw apple. Hardening of the surface was observed at the final stages of drying. The more water was removed the difference between osmosed and raw apple, both dried by convection, was the less evident. It is suggested that apple undergoing convective drying is composed by three compartments: surface layers, underneath tissue with lost turgor and structure defects and the core with properties similar to those of raw apple. Osmotic dewatering affects the surface compartment, hence its properties influence response of the material to stress.

Analytical and experimental study of a modular solar dryer

A plenty of recently established small Hungarian farms need new small scale agricultural equipment to expand the productivity and to fulfil the increasing quality control demand. As a response for this demand a modular solar dryer was developed and built which can help the quality saving with renewable energy. In this paper the features and the operation of the dryer is presented first. The effect of the solar air collector module for the physical properties of drying air was studied along with the calculation of efficiency, too. The mass flow of the drying air through the system is one of the most important factor concerning to the whole process, for that reason the velocity distribution is also presented. Finally some measuring results are presented for drying of apple.

Air drying behaviour of apples as affected by blanching and glucose impregnation

The effect of blanching and/or glucose impregnation at atmospheric pressure or in vaccum on the kinetics of moisture transfer during the first falling rate period of air drying of apple was analyzed. The moisture diffusivity of water (Deff) was strongly decreased by glucose uptake during impregnation step as well as volume shrinking. Studies of the cell structure using transmission electron microscopy revealed that both pretreatments did not modify in a great extension the cell wall resistance to water flux.

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

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