MODELING and EXPERIMENTAL STUDY ON DRYING of APPLE SLICES IN A CONVECTIVE CYCLONE DRYER

The main objective pursued in this paper is to experimentally investigate the single layer drying behavior of apple slices in a convective type cyclone dryer and also to perform the mathematical modeling by using single layer drying models in literature. the experiments were conducted at drying air temperatures of 60, 70 and 80C in drying air velocities of 1 and 1.5 m/s. It was concluded that apple slices with the thickness of 12.5 mm would perfectly dry in the ranges of 280–540 mm while those with the thickness of 8 mm would dry in the ranges of 180–320 min in these drying conditions by using convective type cyclone dryer. Additionally, the mathematical model describing the single layer drying curves was determined by nonlinear regression analysis, and the logarithmic model was selected as the most suitable model to obtain the drying curve equation of apple slices. Considering the parameters such as drying time, drying rate, moisture transfer, velocity and drying air temperature it is suggested that the apple slices be dried at the above optimum processing conditions.     

THIN-LAYER DRYING OF DATES

Thin‐layer drying rates of two date cultivars, namely, Sukkari and Sakie were experimentally determined at three drying temperatures 70, 80, and 90C. Three drying models were evaluated for the thin‐layer date data. These were the exponential model, Page equation, and the approximation of diffusion model. The Page equation fitted the data best. Two empirical expressions of Page equation for predicting moisture ratio as a function of drying time and drying temperature for the two date cultivars are presented.     

DRYING METHODS AND QUALITY OF SHRIMP DRIED IN A JET-SPOUTED BED DRYER

The objective of this study was to experimentally investigate the effects of various parameters, i.e., size of shrimp, level and pattern of inlet drying air temperature, on the drying kinetics and various quality attributes of dried shrimp viz. shrinkage, rehydration ability, texture and color during drying in a jet‐spouted bed dryer. It was found that the use of a constant inlet air temperature of 100C yielded dried shrimp of the best quality in terms of low percentage of shrinkage, high percentage of rehydration, low maximum shear force and high value of redness compared to shrimp dried using other conditions. A simple mathematical model that enables prediction of the drying behavior of shrimp in a jet‐spouted bed dryer has also been developed.     

MATHEMATICAL MODELLING OF THIN-LAYER DRYING OF KIWIFRUIT SLICES

The effect of temperature on the drying kinetics of kiwifruit was investigated. The drying process was carried out at temperatures of 50, 55 and 60C, air velocity of 2.4   m/s and relative humidity between 10–25%. Drying time decreased considerably with increased air temperature. Drying process took place in the falling rate period. Twelve mathematical models available in the literature were fitted to the experimental data. The models were compared by four statistical parameters; i.e., determination of coefficient, mean relative percent error, reduced chi-square and root mean square error, and the best-fit model was selected. The modified Henderson and Pabis and Verma et al. models were given the best results in describing thin-layer drying of kiwifruits. The effective diffusivity of water during air-drying varied from 1.743 to 2.241  ×  10 ^{− 10}   m²/s over the temperature range investigated, with activation energy equal to 22.48   kJ/mol.

PRACTICAL APPLICATIONS

Drying can be described as an industrial preservation method in which water content and activity of agricultural products are decreased by heated air to minimize biochemical, chemical and microbiological deterioration. Kiwifruit has a very short life because of softening and vitamin loss during cold storage. The use of drying prolongs the shelf-life of the kiwifruit, as the water content reduction slows down deterioration reactions. In this study, drying characteristics of kiwifruits were studied in a convectional hot-air dryer. The objectives of the present study were to determine experimentally the thin-layer drying characteristics and rehydration capacity of samples, and to fit the experimental data to 12 mathematical models available from the literature.     

CONVECTIVE DRYING OF PUMPKIN: INFLUENCE OF PRETREATMENT AND DRYING TEMPERATURE

The aim of this research was to study the influence of blanching as a pretreatment on the drying rate, color and rehydration rate of pumpkin slices ( Cucurbita maxima ) dried in a convective dryer at 50, 60 and 70C.
The different pretreatments did not modify drying time. The diffusion coefficient depended on temperature and moisture content, but not on pretreatments. It varied between 1.6 and 2.9  ×  10⁻⁹ m²/s and activation energy was approximately 35.6 kJ/mol. The rehydration rate of the dried sample, which was found to be influenced by drying temperature but not by the pretreatments, reached about 2 kg water/kg dry solid in about 150 s in boiling water. Color parameters " L " and " b " were influenced by drying temperature and pretreatments. Parameter L varied between 36 and 53 and parameter b varied between 20 and 31. Samples dried at 70C without pretreatments presented color parameter values very similar to those of the fresh sample.

PRACTICAL APPLICATIONS

This work evaluates the pretreatment and drying conditions of pumpkin. Five parameters of the optimal product are considered: pretreatment time, drying time, temperature, color and rehydration rate of the product. With these results, drying of pumpkin can be carried out in industrial convective dryers and the dried product may be used in the preparation of soups, premixed foods, snacks, etc.     

THIN-LAYER INFRARED RADIATION DRYING OF RED PEPPER SLICES

Drying characteristics and changing of quality parameters of red pepper were determined experimentally as a function of infrared power ( IP ) and air velocity during infrared drying. Drying experiments of red pepper slices were carried out at three levels of IP (300, 400 and 500 W) and at three air velocities (1.0, 1.5 and 2.0 m/s). The drying time at air velocity from 1.0 to 2.0 m/s ranged between 314 and 455 min, 213 and 297 min and 196 and 230 min at IP of 300, 400 and 500 W, respectively. Drying rate increased with increasing IP and decreasing air velocity. All drying experiments had a falling rate of drying period. The specific energy consumption was varied between 4.62 and 7.59 kW·h/kg for all the drying conditions. Thickness change/shrinkage was found to be in the range of 0.162 and 0.263. The color parameters and the rehydration ratio were significantly influenced by IP and air velocity.

PRACTICAL APPLICATIONS

Drying is an important operation in red pepper processing. In open-air condition, the drying time of red pepper is 8–10 days. Several researchers state that drying in open-air conditions also has some disadvantages such as lack of ability to control the drying operation properly, long drying time, weather uncertainties, high labor costs, large area requirement, insect infestation and contamination with dust and other foreign materials. The results from this study can be used as reference for drying of red pepper in similar applications. The drying method proposed in this study decreased the time required for drying of red pepper, reduced the space needed for equipment and protected the product quality. Therefore, the drying technique proposed in this study can be a good alternative to other drying techniques available. Also, the proposed drying method can be used as a reference to drying applications of various vegetables and fruits.     

全封闭热泵干燥系统冷凝温度对干燥能力的影响

本文针对全封闭热泵干燥系统不同冷凝温度下的干燥速度进行了分析,依据计算结果绘制出了不同风速时干燥速度随冷凝温度的变化曲线,提出了设计全封闭热泵干燥系统选取合适冷凝温度及风速的原则。     

THIN-LAYER DRYING KINETICS OF SESAME HULLS UNDER FORCED CONVECTION AND OPEN SUN DRYING

Sesame hulls are a useful by‐product of the sesame processing industry. The sesame hulls are produced at a high moisture content (68% wet basis) and need further drying to prevent deterioration. In this study, both open sun drying (OSD) and forced convection drying (FCD) at 42, 55, and 76C and 1.2 m/s air velocity were investigated. Six common thin‐layer drying models were fitted to the experimental data. Several statistical parameters were used to evaluate the performance of thin‐layer drying models, including r², x², root mean square error (RMSE) and residuals. Sesame hull drying was found to take place completely in the falling rate region. The modified Page model was found to describe OSD data well, while the Wang and Singh model was the best model for describing FCD. Effective diffusivity was found to be 1.89 × 10^‐8 m²/s and 7.36 × 10^‐10 to 1.20 × 10^‐9 m²/s for OSD and FCD, respectively. Activation energy was also found to be 12.95 kJ/mol for FCD.     

EVALUATION OF THIN-LAYER DRYING MODELS FOR DESCRIBING DRYING KINETICS OF BARBERRIES (BARBERRIES VULGARIS)

Barberry is known as a medicinal and ornamental plant throughout the world. Edible fruit from the barberry is used in medicine to treat liver, neck and stomach cancer, for purification of the blood and for breath freshness. In order to assess and select a suitable form of the drying curve, six different semitheoretical and/or empirical models were fitted to the experimental data. Experiments were performed at air temperatures of 50, 60 and 70C. At each temperature, three air velocity values were selected: 0.5, 1 and 2 m/s. Consequently, of all the drying models, the Page model was selected as the best mathematical model according to R ², χ ² and root mean square error parameters. Constants related to the Page model are reported and regressed against air condition using multiple regression analysis.

PRACTICAL APPLICATION

All commercial flow dryers are designed on thin-layer drying principles. Thin-layer drying means to dry as one layer of sample particles or slices. In this paper we present a mathematical model for thin-layer drying of barberry fruits (moisture ratio against time). Definition of moisture ratio as a function of time and drying condition has many advantages such as (1) easy control of dryer system, (2) lower energy consumption, (3) less thermal damage to the products and (4) lower investment. It is believed that the present study should be useful to people seeking (1) to optimize the design of drying systems and their components, and (2) to identify appropriate applications and optimal configurations for drying systems.     

PREDICTING DRYING BEHAVIOR OF SPAGHETTI IN A CONTINUOUS INDUSTRIAL DRYER WITH THE MODELS DETERMINED IN A LAB SCALE BATCH SYSTEM

In the first stage of this study experimental data were obtained by using a lab scale batch type dryer, operating under various constant temperature/constant relative humidity combinations; then a mathematical model was determined for the effective diffusivity of moisture in spaghetti. In the second stage of the study the model obtained in the lab scale batch type dryer was used to calculate the moisture content of the spaghetti leaving continuous industrial scale dryer, and its success was tested by comparing with the experimentally determined moisture content of the product. The model was not found successful and corrected. Mathematical models based on experimental data obtained in a bench-top laboratory dryer underestimated the apparent equilibrium moisture content and over estimated the effective diffusivity of water in spaghetti in industrial scale continuous drying equipment. The structural differences in the product, extrapolation of the data beyond the range of the original experiments, variations in mode (batch versus continuous), surface-to-volume ratios, flow patterns, and geometry associated with each drying system were anticipated as the cause of the scale up problem.     

OPTIMIZATION OF MARINE FISH DRYING USING SOLAR TUNNEL DRYER

Studies were conducted to optimize fish-drying process in a Hohenheim-type solar tunnel dryer to produce safe and high quality dried fish products. Five commercially important tropical marine fish species in the Bay of Bengal such as silver jew fish, Bombay duck, big-eye tuna, Chinese pomfret and ribbon fish were used and drying was performed at 45 to 50 and 50 to 55C temperature ranges. Moisture content of the fish samples reached 16% after 36 and 32 h of drying at temperature ranges of 45 to 50 and 50 to 55C, respectively. Products produced at 45 to 50C were found to be excellent on the basis of flavor, color and texture. Their rehydration ability ranged from 65 to 80% with minimum in big-eye tuna and maximum in silver jew fish. Values of total volatile base, peroxide and aerobic plate count of all the final dried products were within the acceptable limit.

PRACTICAL APPLICATIONS

In this experiment, we optimized the drying process in a Hohenheim-type solar tunnel dryer, constructed using locally available materials, for several commercially important marine fish species in the Bay of Bengal over a wide range of temperature, humidity and moisture content. The data obtained in this study are now successfully used for large-scale production of dried fish of high quality, providing better market price for its producers as well as greater acceptability to the consumers. We also developed another low cost solar tunnel dryer using bamboo, hemp, canvas, rope and polythene sheets, which became very popular among small-scale dry fish producers in the coastal areas of Bangladesh.