CONVECTIVE DRYING OF BANANA, GUAVA AND POTATO PIECES : EFFECT OF CYCLICAL VARIATIONS OF AIR TEMPERATURE ON DRYING KINETICS AND COLOR CHANGE

Pieces of banana, guava and potato were dried in a two-stage heat pump dryer capable of precise control of air humidity with predetermined cyclic variations of air temperature entering the drying chamber. The air temperature variations tested were : a cosine, a reversed cosine and three different square wave profiles with peak-to-valley variations from 20^°C to 40^°C. The cycle time was about 60 minutes with drying time of approximately 300 minutes. The drying samples were placed on trays in a thin layer. With appropriate choice of temperature-time variation, it is possible to reduce the overall color change while maintaining high drying rates.     

CONVECTIVE DRYING OF AGRICULTURAL PRODUCTS. EFFECT OF CONTINUOUS AND STEPWISE CHANGE IN DRYING AIR TEMPERATURE

Samples of banana were dried in a two-stage heat pump dryer capable of producing stepwise control of the inlet drying air temperature while keeping absolute humidity constant. Two stepwise air temperature profiles were tested. The incremental temperature step change in temperature of the drying air about the mean air temperature of 30 °C was 5 °C. The total drying time for each temperature-time profile was about 300 minutes. The drying kinetics and color change of the products dried under these stepwise variation of the inlet air temperature were measured and compared with constant air temperature drying. The stepwise air temperature variation was found to yield better quality product in terms of color of the dried product. Further, it was found that by employing a step-down temperature profile, it was possible to reduce the drying time to reach the desired moisture content.     

ON THE STUDY OF TIME-VARYING TEMPERATURE DRYING—EFFECT ON DRYING KINETICS AND PRODUCT QUALITY

The ability of heat pump dryer to produce controlled transient drying conditions, in terms of temperature, humidity and air velocity, has given it an edge over other drying systems. Exploiting this characteristic, we studied and compared the effect of different temperature-time profiles on the quality of agricultural products in a tunnel heat pump dryer capable of providing up to 14.6 kW of cooling capacity. The product quality refers to the color change of the products. Samples of banana and guava were dried in batches in a two-stage heat pump dryer. The effects of the starting temperature of a selected profile and the cycle time on both drying kinetics and product quality were studied. It was observed that by employing a step change in drying air temperature with the appropriate starting temperature and cycle time, it was possible to reduce significantly the drying time to reach the desired moisture content with improved product color.     

EFFECT OF OSMOTIC PRE-TREATMENT AND INFRARED RADIATION ON DRYING RATE AND COLOR CHANGES DURING DRYING OF POTATO AND PINEAPPLE

A combination of intermittent infrared and continuous convection heating was used to dry various osmotically pretreated sample of potato (in solutions of 10%, 20% and 30% NaCl) and pineapple (in solutions of 30%, 50%, 70% Brix). The effect of drying conditions on color changes of potato and pineapple was investigated. The Hunter color scale parameters (redness, yellowness and lightness) were measured to quantify the color changes. With appropriate choice of infrared intermittency as well as osmotic pretreatment, it is possible to reduce the overall color change while maintaining high drying rates. As expected, osmotic pretreatment resulted in a shift in the sorption isotherms for both products.     

EFFECT OF OSMOTIC PRE-TREATMENT AND INFRARED RADIATION ON DRYING RATE AND COLOR CHANGES DURING DRYING OF POTATO AND PINEAPPLE

A combination of intermittent infrared and continuous convection heating was used to dry various osmotically pretreated sample of potato (in solutions of 10%, 20% and 30% NaCl) and pineapple (in solutions of 30%, 50%, 70% Brix). The effect of drying conditions on color changes of potato and pineapple was investigated. The Hunter color scale parameters (redness, yellowness and lightness) were measured to quantify the color changes. With appropriate choice of infrared intermittency as well as osmotic pretreatment, it is possible to reduce the overall color change while maintaining high drying rates. As expected, osmotic pretreatment resulted in a shift in the sorption isotherms for both products.     

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.     

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

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.     

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.     

THE EFFECTS OF AIR TEMPERATURE,SPHERE DIAMETER AND PUFFING WITH C02 ON THE DRYING OF POTATO SPHERES

ABSTRACT

An experimental air dryer was used to investigate the effects of air temperature, sphere diameter and puffing with CO₂ on the drying of potato spheres. Accordingly, the experimental results showed only falling-rate behaviour and hence drying completely controlled by internal mass transfer was interpreted on the basis of Fick's diffusional model for non-steady state diffusion. Drying rate increased with increasing air temperature, and also increased with decreasing diameter of sphere. By considering the diffusion coefficients at different diameters, it was established that the drying occurred by a diffusion mechanism as opposed to a capillary mechanism. It was also found that CO₂ puffing had a positive effect on the drying rate.     

THE EFFECTS OF AIR TEMPERATURE, SPHERE DIAMETER AND PUFFING WITH C02 ON THE DRYING OF POTATO SPHERES

An experimental air dryer was used to investigate the effects of air temperature, sphere diameter and puffing with CO₂ on the drying of potato spheres. Accordingly, the experimental results showed only falling-rate behaviour and hence drying completely controlled by internal mass transfer was interpreted on the basis of Fick's diffusional model for non-steady state diffusion. Drying rate increased with increasing air temperature, and also increased with decreasing diameter of sphere. By considering the diffusion coefficients at different diameters, it was established that the drying occurred by a diffusion mechanism as opposed to a capillary mechanism. It was also found that CO₂ puffing had a positive effect on the drying rate.     

AIR-DRYING OF FRESH AND OSMOTICALLY PRE-TREATED PINEAPPLE SLICES: FIXED AIR TEMPERATURE VERSUS FIXED SLICE TEMPERATURE DRYING KINETICS

In this investigation, the air drying characteristics of fresh and osmotically pre-treated pineapple slices in a tray dryer were studied under different operating conditions. The air velocity varied from 1.5 to 2.5 m/s and the air temperature from 40 to 70°C. The analytical solution of the second Fick's law for an infinite slab was used to calculate effective diffusion coefficients and their temperature dependence could be well represented by an Arrhenius-type equation. Comparison of the results showed that the diffusion coefficients were lower for the pre-treated fruit. By means of automatic control, it was possible to obtain drying curves under conditions of constant product temperature, which showed to be an alternative to reduce the drying time of pineapple slices.     

THE INFLUENCE OF DRYING MEDIUM,TEMPERATURE, AND TIME ON THE RELEASE OF MONOTERPENES DURING CONVECTIVE DRYING OF WOOD CHIPS

ABSTRACT

The release of volatile organic components (VOC) during the drying of wood chips was studied experimentally. The drying medium was hot dry air with a pressure of 1 bar and velocity of 1 m/s. Four different temperature levels of the drying medium, i.e., 120, 140, 160 and 180°C, and two different materials, Scots Pine and Norway Spruce, were investigated. It was found that the main components released consist of various types of monoterpenes, with α-pinene dominating in each of the two materials. The amount released is time-dependent, showing a maximum at shorter times. The release rate is more intensive at higher drying temperatures. The mass-balance for α-pinene was checked by comparing the amount present in the wood chip before and after drying with the amount obtained by integrating the release curve. A good agreement was obtained. Environmental Scanning Electron Microscope pictures were taken of both the spruce and pine chip in order to investigate differences in the wood structure before and after drying with air at 160°C. No obvious structural change could, however, be observed. The influence of the drying medium on the release rate of monoterpenes (air and superheated steam) was also studied. The results showed that the release rate was faster when drying with superheated steam.     

发生器、吸收器传质效率对高温吸收式热泵性能的影响

研究分析了水／二甘醇高温吸收式热泵的发生器、吸收器传质过程及效率对系统性能的影响。当发生器、吸收器传质效率提高,系统性能系数COP、Yong效率η及Yong指标I均增大,而溶液循环比FR减小,通过回归分析,获得了评价热泵性能的关联式。     

THE EFFECT OF HETEROGENEITY ON WOOD DRYING,PART I: MODEL DEVELOPMENT AND PREDICTIONS

ABSTRACT

A two dimensional model which can predict the effects of the anisotropy and heterogeneity on the transport phenomena which occur in wood during drying is developed. It is shown that the appropriate driving potential for moisture transport is the ratio of the moisture content to the driving potential. In its one dimensional form, die model results compare favorably with experimental measurements for drying in the radial direction. In its two dimensional form the model is used to predict drying in a direction midway between the radial and the tangential. In this case free water moves in a diagonal direction because the low density earlywood dries faster than the latewood during the early stages of drying. The result is significant gradients in moisture content, not only in the drying direction, but also in the direction parallel to the drying surface.     

温度、气氛和淬冷对阿利特固溶和多晶转变的影响(英文)

利用X射线衍射和等离子电感耦合等离子发射光谱研究了烧成温度、烧成气氛、淬冷开始温度等参数对实验室熟料中阿利特的多晶转变和Al3+,Fe3+,Mg2+等主要杂质离子固溶量的影响。随煅烧温度升高,阿利特对称性升高,不掺杂熟料中阿利特晶型从MⅠ转变为MⅠ和MⅢ型的混合体;掺杂熟料中阿利特晶型从MⅠ转变为M和R型混合体。氧化气氛烧成熟料中阿利特的对称性高于还原气氛烧成的。还原气氛烧成熟料中阿利特无论掺杂与否,阿利特室温保留晶型都呈现MⅢ型晶体衍射特征。氧化气氛下烧成掺杂熟料中阿利特以R型为主并含有M型的混合体,不掺杂熟料中阿利特以MⅢ型晶体存在。掺杂熟料从1450℃淬冷可以保留较多的R型阿利特。在其它较低温度淬冷,掺杂熟料中阿利特更多呈现MⅠ型晶体的衍射特征。不掺杂熟料中阿利特在所有开始温度淬冷,都呈现M型衍射特征,其原因在于:随煅烧温度升高以及随淬冷起始温度的升高,Al3+在阿利特中的固溶量升高,Fe3+的固溶量有所降低,Mg2+的固溶量则没有明显变化。氧化气氛下烧成的阿利特中Al3+的固溶量要高于还原气氛下的,而还原气氛下烧成的阿利特中Fe3+/Fe2+的固溶量高于氧化气氛下的。     

DRYING OF BAKER'S YEAST BY A NEW METHOD: DEHYDRATION BY SUCCESSIVE PRESSURE DROPS (DDS). EFFECT ON CELL SURVIVAL AND ENZYMATIC ACTIVITIES

Abstract

Dehydration by Successive Pressure Drops (DDS) is a new unit operation, based on a repetition of compression/decompression cycles. In this work, we present the application of this drying process to baker's yeast. Drying rate, cycle efficiency, cell survival and the activities of four enzymes were evaluated. The effects of two parameters of DDS (PI and Dvp) on drying kinetics and product quality were studied, using the surface responses methodology. The results were also compared to those obtained with freeze-drying. PI had a positive effect on drying rate. Both PI and Dvp had an effect on cycle efficiency, particularly Dvp. Cell survival were much higher than that obtained with freeze-drying, with no significant impact of the parameters. The enzymatic activities were higher or close to the corresponding activities of freeze-dried sample, except the glucose-6-phosphate activity. The effects of PI and Dvp varied according to the enzyme.