DRYING RELATED PROPERTIES OF APPLE

Drying related properties of apple are evaluated for various different drying methods (namely, convective, vacuum, microwave, osmotic and freeze drying), and their corresponding process conditions. The examined properties are drying kinetics, equilibrium material moisture content, density, porosity, color and viscoelastic characteristics. The effect of various process factors on these properties is described through particular mathematical models. The model parameters are estimated by fitting the corresponding model equations on a wide range of experimental data. Drying kinetics is greatly affected by the characteristic particle size and drying air temperature for convective drying, while for the case of microwave drying they are affected by the vacuum pressure and the emitted radiation power. Equilibrium material moisture content is affected by the temperature and the humidity of the surrounding air, while the osmotic pretreatment shifts the sorption isotherms to higher water activity levels. The quality properties examined, are significantly affected by the drying method. More specifically, osmotic dehydration decreases the porosity of the final product, while it prevents color deterioration and enchances the viscous nature of dehydrated apple. Freeze-dried apples develop the highest porosity, have the most elastic structure and the lowest rate of color deterioration.     

Three-stage hybrid osmotic–intermittent microwave–convective drying of apple at low temperature and short time

ABSTRACT

In recent years, intermittent microwave coupled with hot air-drying has been used increasingly, thanks to considerable improvements observed in drying properties. The aim of this study was to investigate the effect of process of drying apple pretreated osmotically with sucrose solution at five concentrations of 0 (control), 10, 30, 50, and 70% (w/w), using intermittent microwave at four power levels of 0 (control), 360, 600, and 900?W, four pulse ratios of 1, 2, 3, and 4, and convective hot air (40°C) on drying kinetics, effective moisture diffusion coefficient, shrinkage, bulk density, rehydration ratio, and energy consumption. Results showed that the three-stage hybrid osmotic–intermittent microwave–convective drying of apple at low temperature yielded higher drying rates (with 41.5% decrease in drying time) and improved quality of final product. The effective moisture diffusion coefficient increased with an increase in power, pulse ratio, and the concentration of osmotic solution. Furthermore, shrinkage, bulk density, and energy consumption of the samples decreased with an increase in power, pulse ratio, and the concentration of osmotic solution. In summary, the use of intermittent microwave coupled with forced convection of hot air (at low temperature) in drying of apple pretreated by sucrose osmotic solution led to products with improved properties in terms of both quality and quantity.     

EFFECT OF DRYING METHOD ON SHRINKAGE AND POROSITY

ABSTRACT

The effect of drying method on bulk density, particle density, specific volume and porosity of banana, apple, carrot and potato at various moisture contents was investigated, using a large set of experimental measurements. Samples were dehydrated with five different drying methods: conventional, vacuum, microwave, freeze and osmotic drying. A simple mathematical model was used In order to correlate the above properties with the material moisture content. Four parameters with physical meaning were incorporated in the model: the enclosed water density p_w, the dry solid density p_s, the dry solid bulk density p_bo and the volume shrinkage coefficient β'. The effect of drying method on the examined properties was taken into account through its effect on the corresponding parameters. Only, dry solid bulk density was dependent on both material and drying method. Freeze dried materials developed the highest porosity, whereas the lowest one was obtained using conventional air drying.     

EFFECT OF DRYING METHOD ON SHRINKAGE AND POROSITY

The effect of drying method on bulk density, particle density, specific volume and porosity of banana, apple, carrot and potato at various moisture contents was investigated, using a large set of experimental measurements. Samples were dehydrated with five different drying methods: conventional, vacuum, microwave, freeze and osmotic drying. A simple mathematical model was used In order to correlate the above properties with the material moisture content. Four parameters with physical meaning were incorporated in the model: the enclosed water density p_w, the dry solid density p_s, the dry solid bulk density p_bo and the volume shrinkage coefficient β'. The effect of drying method on the examined properties was taken into account through its effect on the corresponding parameters. Only, dry solid bulk density was dependent on both material and drying method. Freeze dried materials developed the highest porosity, whereas the lowest one was obtained using conventional air drying.     

EFFECT OF PRETREATMENT ON COLOR OF DEHYDRATED PRODUCTS

ABSTRACT

The effect of pretreatment on color of convective dried products (namely apple, banana, potato and carrot) was investigated. Five different types of pretreatment were taken into consideration; microwave, osmotic, sulfite, water blanching and steam blanching. Color characteristics were identified by measuring the color parameters (namely, Lightness (L), Redness (a) and Yellowness (b)) using a Hunter Lab chromatometer. The type of pretreatment was found to significantly affect the three color parameters. In addition, Redness (a) and Yellowness (b) were found to follow a first order kinetic model. Untreated dried materials showed an extensive browning, indicated by a significant drop of the L parameter and a corresponding increase of a, b parameters. Osmotically and microwave pretreated samples supressed browning compared to the untreated samples. In this case, lightness decreased slightly, while a, b increased slightly. Sulfite pretreatment prevented significantly color deterioration, while water and steam blanching also prevented enzymatic browning during convective drying.     

干燥方式对红枣动力学及挥发性成分的影响

采用热风干燥、真空冷冻干燥和微波真空干燥对红枣进行干燥处理，结合顶空固相微萃取-气相色谱-质谱（headspace solid-phase microextraction-gas chromatography-mass spectrometry，HS-SPME-GC-MS）和电子鼻对枣片挥发性成分进行了分析，并对比了3种干燥方式对红枣色泽、动力学和挥发性成分的影响。结果表明，热风干燥和微波真空干燥对枣片色泽影响较大，真空冷冻干燥能够更好的保持原始色泽；对动力学方程进行拟合，发现Page 模型为预测红枣3种干燥方式干燥特性最适合的数学模型；干燥对枣片挥发性成分种类及相对含量差异显著，其中干燥前后相同的挥发性成分有19种。挥发性物质主要有酸类、酯类、醇类、醛类、酮类、烃类、杂环类和其他化合物，干燥过后枣片中酸类和烷烃类挥发性成分相对含量显著增加，醛类和杂环类物质显著降低。电子鼻结果表明干燥后枣片香气特征与新鲜红枣有较大的区别，热风干燥和微波真空干燥香味特征较为接近，微波真空干燥枣片的焦甜香特征更为明显。相较于热风干燥和真空冷冻干燥，微波真空干燥效率较高、能耗较低、周期较短，更适于制备干燥枣片。     

EVALUATION AND MODELING OF TWO-STAGE OSMO-CONVECTIVE DRYING OF APPLE SLICES

ABSTRACT

Two-stage drying kinetics of cylindrical pieces of apples were evaluated by subjecting test samples first to various osmotic treatments and then to convective air drying to complete the drying process. Osmotic drying was carried out with cut apple cylinders of three different sizes (12, 17 and 20 mm diameter), all with a length to diameter ratio of 1 : 1, in a well agitated large tank containing the osmotic solution at the desired temperature. Solution to fruit volume ratio was kept greater than 60. After the osmotic treatment, apple slices were further dried in a cabinet drier at an average temperature 58°C. A central composite rotatable design (CCRD) with five levels of sucrose concentrations (34–63°Brix) and five temperatures (34–66°C) was used for osmotic treatment. Half-drying time and solids gain time were used as measures of rate of drying and associated diffusion coefficients for moisture loss and solids gain were evaluated. Half-drying time decreased with an increase in temperature or concentration, or a decrease in sample size. Diffusion coefficients were lower for smaller samples, and were higher for migration of moisture as compared to solids. For a given level of moisture removal, air drying times were shorter than osmotic drying times. Composite models were developed to describe the effect of process variables and particle size on the drying behavior of apple slices.     

MICROWAVE VACUUM DRYING KINETICS OF SOME FRUITS

ABSTRACT

Microwave vacuum drying kinetics of three fruits (namely, apple, kiwi and pear) were studied by introducing an one-parameter empirical mass transfer model, involving a characteristic parameter (drying constant), as a function of process variables. The model was tested with data produced in a microwave oven equipped with vacuum apparatus, using non-linear regression analysis. The investigation involved a wide range of microwave power and vacuum pressure levels. The parameters of the model considered were found to be greatly affected by the microwave power level while vacuum pressure affected slightly the process.     

Characteristics of Chard Leaves during Microwave, Convective, and Combined Microwave-Convective Drying

Chard leaves (Beta vulgaris L. var. cicla), which weighs 25 g with a moisture of 9.35 (db), were dried using three different drying methods, microwave, convective, and combined microwave-convective. Drying continued until leaf moisture fell down to 0.1 (db). Drying periods lasted 5-9.5, 22-195, and 1.5-7.5 min for microwave, convective, and combined microwave-convective drying, respectively, depending on the drying level. In this study, measured values were compared with predicted values obtained from Page's semi-empirical equation. Optimum drying period, color, and energy consumption were obtained for combined microwave and convective drying. The optimum combination level was 500 W microwave applications at 75°C.     

THERMAL DEWATERING OF DILUTED ORGANIC SUSPENSIONS: PROCESS MECHANISM AND DRYING KINETICS

ABSTRACT

Drying of raw hog manure, a highly diluted suspension of organic and inorganic matter was studied experimentally in the multistage screw-in-trough dryer. Laboratory tests included material characteristics (rheological properties, thermodynamic equilibrium, critical settling point), boiling pattern, and kinetics of convective drying at temperatures from 90 to 120°C and air velocity from 0.5 to 1.5 m/s. Three phases of manure drying were identified, namely boiling and release of volatile compounds, evaporation from viscous liquid, followed by drying of a soft paste, and finish drying of semi-dry granular product. Drying kinetics were quantified in terms of reduced moisture content and a generalized drying curve using the characteristic drying time concept. Field tests were carried out in the pilot multistage screw-in-trough dryer to validate design calculations.

     

Characteristics of Chard Leaves during Microwave,Convective, and Combined Microwave-Convective Drying

Chard leaves (Beta vulgaris L. var. cicla), which weighs 25 g with a moisture of 9.35 (db), were dried using three different drying methods, microwave, convective, and combined microwave-convective. Drying continued until leaf moisture fell down to 0.1 (db). Drying periods lasted 5–9.5, 22–195, and 1.5–7.5 min for microwave, convective, and combined microwave-convective drying, respectively, depending on the drying level. In this study, measured values were compared with predicted values obtained from Page's semi-empirical equation. Optimum drying period, color, and energy consumption were obtained for combined microwave and convective drying. The optimum combination level was 500 W microwave applications at 75°C.     

KINETICS ON COLOR CHANGES DURING DRYING OF SOME FRUITS AND VEGETABLES

Abstract

The effect of drying conditions on color changes of apple, banana, carrot and potato during conventional and vacuum drying was investigated. The Hunder color scale parameters redness, yellowness and lightness were used to estimate color changes during vacuum and conventional drying at 50, 70 and 90°C. Air humidity during conventional drying was regulated at 15, 30 and 40%. Air temperature and humidity affected redness and yellowness, but not lightness. A first order kinetic model was fitted to experimental data adequately for both redness and yellowness. The rate of color deterioration was found to increase as temperature increased and air humidity decreased, for both drying methods and all the examined materials.     

DRYING KINETICS FOR SOME FRUITS: PREDICTING OF POROSITY AND COLOR DURING DEHYDRATION.

ABSTRACT

Drying kinetics of four fruits (prune, quince, fig and strawberry) were studied by using a simple mass transfer mathematical model involving a characteristic parameter (K) as a function of process variables. The model was tested with data produced in a laboratory air dryer, using non-linear regression analysis. The investigation involved three values of sample thickness (5, 10, 15mm) and three different air temperatures (50, 60, 70?°C).

The parameters of the model were found to be greatly affected by sample thickness and air temperature. The effect of moisture content on the porosity of three fruits (namely avocado, prune and strawberry) was also investigated. A simple mathematical model was used to correlate porosity with moisture content. It was found that porosity increased with decreasing moisture content.

Samples of three fruits (avocado, prune and strawberry) were investigated to estimate color changes during conventional drying at 70?°C. A first order kinetic model was fitted to experimental data with great success. It is found that the color of avocado and strawberry change while the color of prune remains the same.

Furthermore, the water sorption isotherms of three fruits (avocado, prune and strawberry) at 25° temperature were determined experimentally and the parameters at the GAB equation were evaluated by means of non-linear regression analysis. The use of the above equation produced a very good fit.     

Production of Crispy Granules of Fish: A Comparative Study of Alternate Drying Techniques

Three alternate dying methods—vacuum drying (VD), vacuum microwave drying (VMD), and pulse-spouted vacuum–microwave drying (PSVMD)—were examined experimentally for their potential as an industrial scale technology to produce granules of fish. Drying kinetics as well as key quality parameters such as expansion ratio, texture, color, sensory characteristics, and microstructure of crispy granules produced by these drying techniques were examined. Results showed that the drying time is reduced with an increase in microwave power level and with reduced pressure as expected. The quality of the granules is affected by the drying method and the operating parameters employed. Granules obtained using PSVMD at a microwave power density of 6.0 W/g displayed optimal quality required for the commercial product accepted by consumers.     

Drying with Internal Vaporisation : Introducing the Concept of Identity Drying Card (IDC)

ABSTRACT

Different drying configurations (convective drying with moist air and superheated steam, microwave drying and vacuum drying) on different materials (isotropic and anisotropic) were experimentally studied in order to model and visualise the evolution of internal pressure and temperature. To be able to do so, in addition to measuring the average moisture, a method which can determine internal-local pressure and temperature simultanously by using specially designed sensors was developed. In combination with the experiments, the numerical code TRANSPORE has been used to simulate drying processes. A less comprehensive but more comprehensible analytical model was also provided to facilitate the better understanding of internal phenomena. Based on the results gained from measurement and numerical analysis, the dynamic distribution and development of local temperature and pressure inside seasoned medium are coupled together by a temperature-pressure graph, which is herewith called “Identity Drying Card” (IDC), a new concept initialid in the paper. By using IDC, the internal profile of temperature and pressure, the dominant transport properties (penncability and difisivity), the mechanism of transport (diffusion, convection or both) and the phase transitions during drying can be visualised. More specifically, the amount of dry air, the moisture content in the hygroscopic rcgion or the danger due to internal mechanical loads of handled materials can be figured out with the aid of IDC.     

COMBINED MICROWAVE AND SPOUTED BED DRYING OF DICED APPLES: EFFECT OF DRYING CONDITIONS ON DRYING KINETICS AND PRODUCT TEMPERATURE

ABSTRACT

The influence of microwave power (0 to 8.0 W/g, dry basis) and hot air temperature (25°C to 95 °C) on drying rate and product temperature of diced apples (from 31 % to 5% moisture content, dry basis) in a laboratory microwave and spouted-bed combined dryer was investigated. Product temperature initially increased sharply to a plateau about 12 to 15°C above the spouted bed air temperature at a microwave input power 6.4 W/g. This temperature remained almost constant thereafter. Uniform microwave heating was achieved as evidenced by uniform product color and product temperature. Drying rates increased with increasing spouted-bed air temperature or microwave power level, But higher microwave power caused more darkening of the product. Drying of the diced apples in the microwave and spouted bed drying system exhibited two falling rates periods. The influence of air temperature on effective moisture diffusivity followed an Arrhenius type equation. The activation energies were 23.7 kJ/mol and 26.7 kJ/mol for the first and second falling rate periods, respectively.     

Microwave,Vacuum, and Air Drying Characteristics of Collard Leaves

Collard leaves (Brassica oleracea L. var. acephala) with an initial moisture content of 6.65 on percentage dry basis (%db) were dried by three different drying methods: microwave, air, and vacuum. Samples of fresh leaves, 25 g each, were dried until their moisture was down to 0.1 on a dry basis. The following drying levels were used in each of the drying processes: 350, 500, 650, 750, 850, and 1000 W for microwave drying; 50, 75, 100, 125, 150, and 175°C for air drying; and 0.4, 50, and 100 mmHg at 50 and 75°C for vacuum drying, respectively. Drying times ranged between 2.5 to 7.5 min, 8 to 210 min, and 35 to 195 min for microwave, air, and vacuum drying, respectively. The data obtained compared well with a thin-layer drying model. Microwave drying at 750 W provided optimal results with respect to drying time, color, and ascorbic acid content (vitamin C).     

EVALUATION AND MODELING OF TWO-STAGE OSMO-CONVECTIVE DRYING OF APPLE SLICES

Two-stage drying kinetics of cylindrical pieces of apples were evaluated by subjecting test samples first to various osmotic treatments and then to convective air drying to complete the drying process. Osmotic drying was carried out with cut apple cylinders of three different sizes (12, 17 and 20 mm diameter), all with a length to diameter ratio of 1 : 1, in a well agitated large tank containing the osmotic solution at the desired temperature. Solution to fruit volume ratio was kept greater than 60. After the osmotic treatment, apple slices were further dried in a cabinet drier at an average temperature 58°C. A central composite rotatable design (CCRD) with five levels of sucrose concentrations (34-63°Brix) and five temperatures (34-66°C) was used for osmotic treatment. Half-drying time and solids gain time were used as measures of rate of drying and associated diffusion coefficients for moisture loss and solids gain were evaluated. Half-drying time decreased with an increase in temperature or concentration, or a decrease in sample size. Diffusion coefficients were lower for smaller samples, and were higher for migration of moisture as compared to solids. For a given level of moisture removal, air drying times were shorter than osmotic drying times. Composite models were developed to describe the effect of process variables and particle size on the drying behavior of apple slices.     

EFFECT OF PRETREATMENT ON COLOR OF DEHYDRATED PRODUCTS

The effect of pretreatment on color of convective dried products (namely apple, banana, potato and carrot) was investigated. Five different types of pretreatment were taken into consideration; microwave, osmotic, sulfite, water blanching and steam blanching. Color characteristics were identified by measuring the color parameters (namely, Lightness (L), Redness (a) and Yellowness (b)) using a Hunter Lab chromatometer. The type of pretreatment was found to significantly affect the three color parameters. In addition, Redness (a) and Yellowness (b) were found to follow a first order kinetic model. Untreated dried materials showed an extensive browning, indicated by a significant drop of the L parameter and a corresponding increase of a, b parameters. Osmotically and microwave pretreated samples supressed browning compared to the untreated samples. In this case, lightness decreased slightly, while a, b increased slightly. Sulfite pretreatment prevented significantly color deterioration, while water and steam blanching also prevented enzymatic browning during convective drying.     

DEEP FAT FRYING OF POTATO STRIPS—QUALITY ISSUES

Moisture loss and oil adsorption kinetics, structural properties (apparent density, true density, specific volume and internal porosity), color changes and viscoelastic behavior (compression tests, crispness) were investigated during deep fat frying of french fries. The effect of frying conditions (oil temperature, sample thickness and oil type), drying pretreatment and osmotic dehydration pretreatment on the above properties was also examined. The results showed that oil temperature and thickness of potato strips have a significant effect on oil uptake, moisture loss and color parameters of french fries, while the use of hydrogenated oil in the frying medium does not affect these properties. The porosity of french fries increases with oil temperature increases and sample thickness and it is higher for products fried with hydrogenated oil. Maximum stress and maximum strain increase during frying, while crispness of potato strips is higher for hydrogenated oil, and lower for refined oil. Air drying and osmotic pretreatment increase porosity of fried potatoes but decrease their oil and moisture content. A negative effect on color development with drying time was also observed. Pre-fry drying as well as osmotic pre-treatment increases the maximum stress and maximum strain of french fries during frying. Air drying pre-treatment increases the crispness of potato strips while osmotic pre-treatment does not affect it, with the exception of sugar solutions.