Drying Of Carrots Research Articles

Determination of diffusion and convective transfer coefficients in food drying revisited: A new methodological approach

It is usual to determine diffusion and convective transfer coefficients using classical approximate relationships involving dimensionless numbers. These approximate relationships were developed in the past, in order to avoid the difficulties associated with calculations of large series expansions and its corresponding expansion coefficients given by transcendental equations. However, the development of improved computing techniques has removed these difficulties, such that currently such calculations can be easily performed. An iterative methodology is proposed that takes advantage of current computational capabilities, avoiding to use approximate relationships. The proposed methodology is applied to generated data and also to experimental data from carrot drying. Additionally, a Matlab ® implementation of the proposed iterative methodology, along with the input data files corresponding to the results presented in this paper, is provided as supplementary material.

Enzymatic Pretreatment Effects on the Drying Process in Terms of Kinetics, Physical and Structural Aspects of the Vegetable Carrot <em>Daucus carota</em>

This study is part of a large comparative study of various pre-treatments used to improve the technological abilities of plants. This study is specifically designed to systematically analyze the intensification of vacuum drying of carrots using cellulase, pectinase and macerozyme as a way to change both structure and functional behavior. These enzymatic pretreatments resulted in major changes in porosity, drying time, capacity and rehydration kinetics and water activity. However, their effects were fundamentally different from physical pretreatments generally used to increase technological capability of drying carrots, such as Freezing/Thawing (FT) instantaneous controlled pressure drop (DIC), or Simple Steaming (SS). Thus, in enzymatic pretreatments no correlation was observed in terms of structure and functional characteristics. This completely contrasts with FT, DIC and SS as thermal/texturing pretreatments, where we found that functional properties strictly depend on structural modifications. Thus thermal/texturing pretreatment operations involve systematically a high correlation between porosity and the ability to dehydration and rehydration. The changes induced by the enzyme pretreatments were closely linked to irreversible microscopic changes of the polymer chains.

Ultrasound pre-treatment enhances the carrot drying and rehydration

The present work aimed to describe the mechanisms involved in the enhancement of the drying and rehydration process of carrot slices caused by the pre-treatment using the ultrasound technology. For that, carrot slices of 4mm of thickness were pre-treated for 30 and 60min using an ultrasonic bath (41W/L; 25kHz). The convective drying process was performed at 40 and 60°C with 2.0m/s of air velocity, while the rehydration process was performed at 25°C. The Henderson & Pabis model was used to describe the drying kinetics and the Peleg model to describe the rehydration process of the carrots slices. As a result, the drying and rehydration kinetics were described, at the different conditions of process, correlating the results with the main effects that the ultrasound cause as a pre-treatment (cell bloating and micro-channels) and the air-drying temperature. Depending on the length of the pre-treatment, the effects caused by the ultrasound in the following processes were different. In addition, it was corroborated that when the drying temperature is increased, less evidenced is the ultrasound effect. The ultrasound, when is applied for long times, enhanced the drying and further rehydration rate at low temperatures, due to the tissue damage. Moreover, vacuum-packed samples were pre-treated with ultrasound in order to exclude the water gain and to evaluate only the micro-channels formation effect. It was concluded that the ultrasound pre-treatment enhances the drying and rehydration processes; however, future optimization studies are recommended.

Model-based investigation of heat and mass transfer for selecting optimum intermediate moisture content in stepwise drying

We developed 3-D computer based model for heat and mass transfer during hot air drying of carrot cubes to predict the moisture content. Thermal energy, specific moisture evaporation rate (SMER) and drying time were predicted for stepwise-drying-air temperatures ranging from 60 to 70°C with various sizes of carrot cubes (10, 20 and 30mm). The cubes were dried at constant temperatures of 60°C and 70°C, step up temperature (60°C/70°C) and step down temperature (70°C/60°C). The drying temperature was changed at different intermediate moisture content levels (20–80%) of carrot. The lowest thermal energy consumption, the highest SMER and the shortest drying time were obtained at temperature in a step up mode associated with intermediate moisture content of 70% wet basis.

Effect of relative humidity on microwave drying of carrot

The aim of this study is to investigate the influence of the relative humidity on the product quality during microwave drying process. A new microwave drying system with humidity control was developed. Twelve schemes were planned in the experiments, for the purpose of changing the relative humidity of the convection air. The relationship between the humidity and the drying rate as well as the product quality were discussed. Nonparametric ANOVA and multiple comparisons were conducted. Results showed that the complementary water method could provide the best product quality, followed by the accelerated and low rate air flow schemes. High flow rate and decelerated air schemes should be avoid if the product quality is of utmost consideration. The accelerated air schemes should be a compromise proposal if the drying rate is also concerned.

Effects of magnetron arrangement and power combination on temperature field uniformity of microwave drying of carrot

ABSTRACTThe effects of magnetron arrangement and power combination on the temperature field uniformity of microwave (MW) drying of carrot were investigated in this study. Using one magnetron resulted in poor temperature field uniformity. However, when two magnetrons were applied, the Td (temperature dispersion between the maximum temperature value and the minimum temperature value among detectors) at different hot spots was lower than that obtained for the single magnetron. Td was 3–10°C when a combination of three magnetrons (α, β, and γ) was used at powers of 400, 500, and 600 W (except for the hot spot of “the highest temperature point”). This method of infrared temperature control-intermittent MW heating could reduce Td significantly in different hot spot positions and improve the uniformity of the temperature field.

Ultrasonic-Assisted Osmotic Dehydration of Carrot Followed by Convective Drying with Continuous and Intermittent Heating

The aim of the studies was to analyze the effect of ultrasound action on osmotic pretreatment and then on drying kinetics in continuous and intermittent drying conditions, and on the final product quality. This article presents the results of intermittent-convective drying of carrot preceded by ultrasonic assisted osmotic dehydration in fructose aqueous solutions. The theoretical drying kinetics developed from the numerical solution of mathematical model is validated using the experimental data. It has been shown that combination of ultrasonic assisted osmosis with intermittent-convective drying accelerates the drying process and improves the quality of dried biomaterial. A good adherence of the numerically determined kinetic curves confirms the usefulness of the presented model and its possible application to construction of controlled and optimized drying processes.

Comparison of carrot (Daucus carota) drying in microwave and in vacuum microwave

Drying is a single operation employed to prolong the life of a large quantity of vegetables. Carrot (Daucus carota) drying has been the subject of many studies. This plant has been highlighted in the human diet for having high nutritional value, mainly due to the high content of β-carotene. In this work, carrot drying behavior was studied in a regular microwave dryer and a vacuum microwave dryer. A vacuum of 450 mmHg was applied for drying of carrot in different geometrical shapes (cubes, discs and sticks). The samples were dried at power ratings of 1.0 W/g, 1.5 W/g and 2.0 W/g for both methods of drying. The evolution of physical properties such as density, volume and porosity was monitored and related to the moisture content of the sample and to the method of drying and power rating used. The geometric shape of the sample influenced the drying kinetics and it was verified that the cubic form was responsible for a slower drying. The application of vacuum showed no major changes in the drying kinetics in microwave but influenced the physical properties of the material. The influence of power ratings on the content of β-carotene was also evaluated and discussed. The main difference observed was the lower shrinkage of the samples dried in the vacuum microwave compared to those dried only in microwave.

Effect of a Modified Atmosphere on Drying and Quality Characteristics of Carrots

Many quality degradation problems are related to the high O2 content of normal air atmosphere during drying. To reduce O2 content in drying atmosphere and obtain food products with high quality, modified atmosphere drying was conducted. In this study, carrots were used as experimental materials to investigate the effects of drying parameters on the drying characteristics and product quality. Results showed that the increase in drying temperature and the decrease in O2 content positively influenced drying rate and effective moisture diffusivity. High carotenoid content, ascorbic acid retention ratio, and rehydration ratio were produced with low drying temperature and O2 content. The color parameters of products were highly correlated with carotenoid content, and low color difference could be achieved as drying temperature and O2 content decreased. Drying temperature and O2 significantly influenced carotenoid content, ascorbic acid content, rehydration, and color difference of dried products. Good quality parameters were obtained only at low drying temperature under the drying condition of normal atmosphere and could be achieved at drying temperatures of 40 to 70 °C when O2 content is 5 %. Therefore, the modified atmosphere drying is a promising method to protect the quality of dried products.

Ultrasound Assisted Osmotic Dehydration as Pretreatment for Hot-air Drying of Carrot

Ultrasound Assisted Osmotic Dehydration as Pretreatment for Hot-air Drying of Carrot

A method to estimate anisotropic diffusion coefficients for cylindrical solids: Application to the drying of carrot

In this study, a method to estimate water diffusivity in axial, radial and angular directions during drying of anisotropic cylindrical solids is proposed. The method is based on the analytical solution for non-steady state mass transfer equation in products shaped as longitudinal sections of finite anisotropic cylinders. A sequential approach is applied where radial and axial diffusion coefficients are first estimated from drying curves of whole cylinders with increasing height, while angular diffusivity is obtained thereafter from drying experiments carried out with longitudinal sections of fixed-height cylinders varying cut angle. Developed theory was applied to the analysis of two sets of convective drying data of carrot obtained at 80°C with an air velocity of 2m/s. The first experiment set was conducted with carrot cylinders of 2.2cm diameter and heights of 1, 2,…,10cm, while cylinders in the second set were longitudinally cut as halves, quarters, sixths and eighths with a constant height of 10cm. Under such experimental conditions, identified water diffusivities were in the range of 0.53–2.93×10−9m2/s. Differences in water diffusivity for studied mass transfer directions were significant (p<0.05), with SEM images revealing structural differences between longitudinal and transversal product cuts, thus further supporting numerical results. Present findings suggest that, depending on product, anisotropic diffusion should be included in drying modeling in order to obtain an accurate process description.

Optimisation of convective drying of carrots using selected processing and quality indicators

SummaryThe effect of drying temperature (40–65 °C) and air rate (2–6 m s−1) on the formation of Maillard reaction indicators and vitamins content of carrots dehydrated by convection has been investigated. The range of assayed processing conditions, based on a previous central composite face design (CCD), led to moderate changes in the studied parameters, even under the most severe conditions. In addition, the drying kinetic of the process was studied taking into account the experimental quantitation of shrinkage, which allowed the determination of a first drying period with a constant rate of water evaporation per unit of exchange surface. The slope of the first drying period, the moisture loss during the first hour of drying and the level of quality parameters (Maillard reaction indicators and vitamins) were correlated with processing conditions with high accuracy. For the prototype here used, the optima temperature and air rate to maximise the desirability function (0.77) were 46 °C and 4.9 m s−1.

Non-stationary drying of carrot: Effect on product quality

This paper presents the results of studies concerning the quality of food materials like vegetables subjected to convective drying in non-stationary conditions. The effect of different frequencies and amplitudes of the periodically changeable drying air temperature on the quality of dried carrot (Daucus carota L.) has been investigated. The indicators of the quality such as the color change, the water activity and the retention of β-carotene were analyzed. The slices of carrot root were used as the material for experimental tests. It has been shown that the non-stationary drying carried out in a prescribed variable air temperature significantly minimized the unfavorable effects in color changes, β-carotene degradation, and decay phenomena.

Design, experimental investigation and analysis of a solar drying system

In this experimental study, a new type solar dryer has been designed and manufactured by using double-pass solar air collector, heat pump and photovoltaic unit. The performance of solar dryer has been tested for carrot drying. In drying system, double-pass solar air collector (DPSAC) has provided the system heat energy requirement for drying process. Constant drying air temperature has been provided with PID (proportional-integral-derivative) control. Carrot slices were dried from initial moisture content 7.76gwater/gdry matter (dry basis) to final moisture content 0.1gwater/gdry matter (dry basis) at 50°C drying air temperature and variable air volume. Velocity of air has been changed depending upon air inlet temperature of drying cabinet. During the drying operation, the air velocity was measured 0.4–0.9m/s. The thermal efficiency of the double-pass collector was calculated from 60% to 78% according to experimental results. Carrot slices were dried at 220min by using double-pass solar air collector in solar-heat pump dryer. Consequently, system (solar-heat pump dryer) can be comfortably operated without the need to the heat pump under normal ambient air conditions.

Suitability for processing of carrot (Daucus carota L.) roots of different varieties

The article presents summary of studying the efficiency of dry carrot production when growing in the conditions of Ukraine’s Forest-Steppe subject to a variety. A complex assessment has been completed for fresh and dried products of 10 different varieties and hybrids of carrots on the content of basic biochemical, economic and biological indicators in order to highlight the best suited for drying. Varieties have been singled out, which roots display the highest yield, best marketability, accumulate the most of dry matter, sugars, etc. A close direct correlation between the weight of roots and their marketability has been established. An evaluation of the tested roots for complex organoleptic characteristics has been completed. It is proved that carrot drying is profitable. The yield and dry matter content in roots are key influence factors on profitability of dry carrot production. By the set of parameters under studying varieties most suitable for drying, Krasniy Velikan and Dlinnaia Krasnaia, have been selected. Profitability of their drying exceeds 40%.

Atmospheric freeze drying assisted by power ultrasound

Atmospheric freeze drying (AFD) is considered an alternative to vacuum freeze drying to keep the quality of fresh product. AFD allows continuous drying reducing fix and operating costs, but presents, as main disadvantage, a long drying time required. The application of power ultrasound (US) can accelerate AFD process. The main objective of the present study was to evaluate the application of power ultrasound to improve atmospheric freeze drying of carrot. For that purpose, AFD experiments were carried out with carrot cubes (10 mm side) at constant air velocity (2 ms−1), temperature (−10°C) and relative humidity (10%) with (20.5 kWm−3,USAFD) and without (AFD) ultrasonic application. A diffusion model was used in order to quantify the influence of US in drying kinetics. To evaluate the quality of dry products, rehydration capacity and textural properties were determined. The US application during AFD of carrot involved the increase of drying rate. The effective moisture diffusivity identified in USAFD was 73% higher than in AFD experiments. On the other hand, the rehydration capacity was higher in USAFD than in AFD and the hardness of dried samples did not show significant (p<0.05) differences. Therefore, US application during AFD significantly (p<0.05) sped-up the drying process preserving the quality properties of the dry product.

Infrared assisted dry-blanching and hybrid drying of carrot

Infrared (IR) blanching and IR assisted hot air (hybrid) drying of carrot slices were attempted and their performance (processing time, retention of vitamin C and rehydration characteristics) was compared with conventional blanching and drying techniques. Intermittent heating of carrot slices using IR radiation (chamber maintained at 180–240°C) for 8–15min resulted in desired level of enzyme inactivation. The time required for blanching of carrot slices (10mm thick) using hot water, steam and IR radiation was 5, 3 and 15min, respectively. Retention of water soluble vitamin C was higher (62%) in IR blanched carrot as compared to water (43%) and steam (49%) blanching. IR blanching reduced the moisture content by 13–23% (absolute). IR blanched samples dried by hybrid mode took ∼45% lesser time compared to water blanched–hot air dried samples. Higher rehydration moisture of dried samples indicated the retention of cell structure during IR blanching. Vitamin C retention was ∼39% higher in IR blanched–hybrid dried slices compared to water blanched–hot air dried. The study shows the potential application of dry-blanching and IR assisted hybrid drying in food processing for improving product quality.

Color Characteristics of Carrots: Effect of Drying and Rehydration

The aim of this study was to observe a change in color and a degradation of total carotenoids and β-carotene in carrots subjected to spout-fluidized bed drying at 60, 70, 80, and 90°C. Additionally, the color of carrots rehydrated at 95°C (10 min) was measured in order to estimate the reversibility of color changes caused by drying. Significant changes in a*, b*, ΔE*, and ΔC* were observed during drying and rehydration of carrots. The total difference in color and saturation ranged from 23.67 to 30.28, and from 22.97 to 29.90, depending on drying air temperature. The rate of deterioration of total carotenoids and β-carotene content influenced the color parameters of carrots. The first-order kinetic model adequately described evolution of a*, b*, ΔE*, and ΔC* derived for carrots during drying at 60, 70, 80, and 90°C. It was found that the spout-fluidized bed drying followed by rehydration changed the color of carrots irreversibly.

Computer Simulation of Drying of Food Products With Superheated Steam in a Rotary Kiln

The present work reports a computer simulation study of heat transfer in a rotary kiln used for drying and preheating food products such as fruits and vegetables with superheated steam at 1 bar. The heat transfer model includes radiation exchange among the superheated steam, refractory wall and the solid surface, conduction in the refractory wall, and the mass and energy balances of the steam and solids. The gas convection is also considered. Finite-difference techniques are used, and the steady state thermal conditions are assumed. The false transient approach is used to solve the wall conduction equation. The solution is initiated at the inlet of the kiln and proceeds to the exit. The output data consist of distributions of the refractory wall temperature, solid temperature, steam temperature, and the total kiln length. The inlet of the kiln is the outlet of the gas (superheated steam), since the gas flow is countercurrent to the solid. Thus, for a fixed solid and gas temperature at the kiln inlet, the program predicts the inlet temperature of the gas (i.e., at the kiln exit) in order to achieve the specified exit temperature of the gas. In the absence of experimental results for food drying in a rotary kiln, the present model has been satisfactorily validated against numerical results of Sass (1967, “Simulation of the Heat-Transfer Phenomena in a Rotary Kiln,” Ind. Eng. Chem. Process Des. Dev., 6(4), pp. 532–535) and limited measured gas temperature as reported by Sass (1967, “Simulation of the Heat-Transfer Phenomena in a Rotary Kiln,” Ind. Eng. Chem. Process Des. Dev., 6(4), pp. 532–535) for drying of wet iron ore in a rotary kiln. The results are presented for drying of apple and carrot pieces. A detailed parametric study indicates that the influence of controlling parameters such as percent water content (with respect to dry solids), solids flow rate, gas flow rate, kiln inclination angle, and the rotational speed of the kiln on the axial solids and gas temperature profiles and the total predicted kiln length is appreciable. The effects of inlet solid temperature and exit gas temperature on the predicted kiln length for carrot drying are also shown in this paper.

Shrinkage and Color Change during Microwave Vacuum Drying of Carrot

Microwave vacuum drying (MVD) was applied to carrot slices and the radial shrinkage and changes in color and shape of its main tissues were evaluated. Tissue dimensions were obtained using computer vision; their visual color was quantified from Hunter Lab coordinates and their carotenoids contents were determined spectrophotometrically. The drying time varied from 24 to 42 min when the microwave power was changed from lower to higher values. MVD samples showed high rehydration capacity. The radial shrinkage values ranged from 36.2 ± 5.0 to 46.1 ± 4.6%, where higher values were seen in the core and under high microwave power. Circularity, solidity, and major and minor axes were shape descriptors of carrot slices that varied significantly during drying, changing from 0.58 to 44.3% of their initial values. In both fresh and dried products the cortex color exhibited superior color than that of the core. MVD incurred a significant whitening index and carotenoids losses.