EFFECT OF PRETREATMENT ON KINETICS OF CONVECTION DRYING OF ONION

ABSTRACT

Onion of two varieties, i.e. Oporto and Blonska was dried by convection at 60°C and 2 m/s. Prior to drying sliced onion was subjected to following treatments: soaking in water, dipping in starch solution, dipping in starch solution supplemented either with ascorbic acid or CaCl₂, and dewatering by osmosis in sucrose solution. Drying of raw onion was taken as a reference.

It has been found that both variety and pretreatment affect the course and rate of drying. Soaking in water and dipping in starch solution either increase rate of drying or have no effect on the kinetics of the process. Supplementing the starch solution with ascorbic acid or CaCl₂ adversely affects the rate of drying. Unfavorable effect on the rate of drying has also osmotic dewatering preceding convective drying.     

EFFECT OF DRYING MODE ON DRYING KINETICS OF ONION

Onion slices 3 mm thick were dried in different modes. Drying at 60°C by convection was taken as a reference process. reviseddate=It has been shown that increase in air temperature increases the rate of drying. Effective diffusivity increases with increasing temperature, but it is strongly dependent on water content. Energy of activation for diffusion is also strongly affected by water content. acceptdate=Stepwise changes in temperature do not result in beneficial effects which could be expected. Extension of drying time was even observed in respect to that obtained at constant 60°C. printpubdate=Drying with infrared energy or assisting convective drying with microwave energy results in increased drying rates and substantial shortening of the drying time.     

DRYING OF ONION: EFFECTS OF PRETREATMENT ON MOISTURE TRANSPORT

ABSTRACT

This study investigated the drying of osmosed and fresh onions. Onion slices (0.8 × 0.8 × 0.15 cm) soaked in sodium chloride solutions (10 and 15% w/w) for 60 min at 22°C were submitted to air drying. The experimental kinetics data obtained were employed to determine effective diffusivity, using a mass transfer model based on Fick's law of diffusion applied to thin slabs. The results show those samples soaked in the 10% NaCl solution had faster drying rates and larger moisture diffusion coefficients. The drying time of onions can be reduced to less than half by introducing an hour of osmotic dehydration in a salt solution. The dried previously osmosed samples presented a more natural coloration than the untreated ones did.     

DRYING OF ONION: EFFECTS OF PRETREATMENT ON MOISTURE TRANSPORT

This study investigated the drying of osmosed and fresh onions. Onion slices (0.8 × 0.8 × 0.15 cm) soaked in sodium chloride solutions (10 and 15% w/w) for 60 min at 22°C were submitted to air drying. The experimental kinetics data obtained were employed to determine effective diffusivity, using a mass transfer model based on Fick's law of diffusion applied to thin slabs. The results show those samples soaked in the 10% NaCl solution had faster drying rates and larger moisture diffusion coefficients. The drying time of onions can be reduced to less than half by introducing an hour of osmotic dehydration in a salt solution. The dried previously osmosed samples presented a more natural coloration than the untreated ones did.     

DRYING KINETICS OF ONION AND GREEN PEPPER

ABSTRACT

Drying kinetics were examined by introducing one-parameter empirical mass aansfer model, where the characteristic parameter (drying constant), is a function of process variables. The model was tested with data produced in an experimental through dryer, using direct regression analysis. Investigation involved two vegetables (namely, green pepper and onion) and a wide range of characteristic dimensions of samples and air conditions (temperature, humidity, and velocity). The parameters of the model considered were found to be greatly affected by sample characteristic dimension and air temperature.     

DRYING KINETICS OF ONION AND GREEN PEPPER

Drying kinetics were examined by introducing one-parameter empirical mass aansfer model, where the characteristic parameter (drying constant), is a function of process variables. The model was tested with data produced in an experimental through dryer, using direct regression analysis. Investigation involved two vegetables (namely, green pepper and onion) and a wide range of characteristic dimensions of samples and air conditions (temperature, humidity, and velocity). The parameters of the model considered were found to be greatly affected by sample characteristic dimension and air temperature.     

KINETICS OF THE CLAY ROOFING TILE CONVECTION DRYING

ABSTRACT

Kinetics of the convection drying process of flat tile has been investigated experimentally in an industrial tunnel dryer. Several velocities of wet tile movement through the dryer were tested to obtain maximum allowable drying rate curve. As there are various models to describe the kinetics of convection drying, finding a model that would fairly well approximate the kinetics of the whole drying process was part of the research. Especially the polynomial and exponential models were tested.

It was found that exponential model of the type: B(t) = (a-B_e. EXP(?bt²) +B_et (?dB(t)/dt) = 2bt(B(t)?B_e significantly correlates the kinetics of the whole tile drying process.

Applying the maximum allowable drying rate curve obtained for flat tile in the first period of drying, a grapho–analytic model for the optimal conducting of the process has been developed.     

KINETICS OF THE CLAY ROOFING TILE CONVECTION DRYING

Kinetics of the convection drying process of flat tile has been investigated experimentally in an industrial tunnel dryer. Several velocities of wet tile movement through the dryer were tested to obtain maximum allowable drying rate curve. As there are various models to describe the kinetics of convection drying, finding a model that would fairly well approximate the kinetics of the whole drying process was part of the research. Especially the polynomial and exponential models were tested.

It was found that exponential model of the type: B(t) = (a-B_e. EXP(-bt²) +B_et (-dB(t)/dt) = 2bt(B(t)-B_e significantly correlates the kinetics of the whole tile drying process.

Applying the maximum allowable drying rate curve obtained for flat tile in the first period of drying, a grapho-analytic model for the optimal conducting of the process has been developed.     

ACOUSTIC DRYING OF ONION

ABSTRACT

Drying processes have long been used as a means of preserving foods and other biological materials. In recent years, the use of drying processes has had great impact on both the production and the processing phases of the agricultural industry. Dehydration of three thickness onion slices using sound waves was evaluated at two frequencies, two sound pressure levels and three temperatures. Compressed air served as the driving force of an aerodynamic siren ( sound source) as well as the carrier of the evaporated moisture. Experimental results showed that drying rates are increased by acoustic vibrations and the increase depended on the sound frequency. The loss of pungency in onion ( measured as the percent of remnant pyruvic acid) was not significantly different at a confidence level of 95% for all variables studied.     

EFFECT OF SOME PHYSICAL AND CHEMICAL PROPERTIES OF OILSEEDS ON DRYING KINETICS PARAMETERS

ABSTRACT

Physical and chemical characteristics of Argentine varieties of high oleic, confectionery and oil-type sunflower seeds, soybean and rapeseed were studied in relation to drying kinetics. Page equation provided the best fit to the data. A significant difference was found in half time response between soybean and the other oilseeds. Diffusivity was positively correlated with the size of the seed (r²=0·969) and negatively (r²=0·876) with the true density independently of the oil content. Half time response tended to increase with oil content according to the grain structure. Lower true density, equivalent diameter and oil seed content were detected in high oleic sunflower varieties with respect to the traditional ones, although a similar behaviour respect drying kinetics was observed. Confectionery sunflower showed the largest size but high diffusivity and low drying rate was obtained. Rapeseed drying behaviour was similar to sunflower varieties with low oil content and high true density.     

EFFECT OF SOME PHYSICAL AND CHEMICAL PROPERTIES OF OILSEEDS ON DRYING KINETICS PARAMETERS

Physical and chemical characteristics of Argentine varieties of high oleic, confectionery and oil-type sunflower seeds, soybean and rapeseed were studied in relation to drying kinetics. Page equation provided the best fit to the data. A significant difference was found in half time response between soybean and the other oilseeds. Diffusivity was positively correlated with the size of the seed (r²=0·969) and negatively (r²=0·876) with the true density independently of the oil content. Half time response tended to increase with oil content according to the grain structure. Lower true density, equivalent diameter and oil seed content were detected in high oleic sunflower varieties with respect to the traditional ones, although a similar behaviour respect drying kinetics was observed. Confectionery sunflower showed the largest size but high diffusivity and low drying rate was obtained. Rapeseed drying behaviour was similar to sunflower varieties with low oil content and high true density.     

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.     

EFFECT OF PRETREATMENT AND DRYING ON THE QUALITY OF OYSTER MUSHROOMS (PLEUROTUS OSTREATUS)

Fruiting bodies of oyster mushroom were dried by hot-air drying, vacuum-drying and freeze-drying in order to compare food qualities after drying. Prior to drying, mushrooms were subjected to blanching or dipping in sodium metabisulphite solution (1 or 5 g/L), or dipping in citric acid solution (1 or 5 g/L). Drying of raw mushrooms was taken as a control. Blanching reduced the attractiveness of dry mushrooms; sodium metabisulphite improved it. It has been found that pretreatment and drying method affect the course and rate of drying. Samples subjected to hot-air drying and vacuum-drying were darker than those freeze-dried, which were clearly more attractive. The hot-air and vacuum-dried mushrooms on rehydration were inferior in quality to the freeze-dried samples. Flavor of the freeze-dried mushroom was not significantly different from that of the hot-air dried mushrooms. Food quality of dried mushrooms depends significantly on the type of drier used.     

EFFECT OF PRETREATMENT AND DRYING ON THE QUALITY OF OYSTER MUSHROOMS (PLEUROTUS OSTREATUS)

Fruiting bodies of oyster mushroom were dried by hot-air drying, vacuum-drying and freeze-drying in order to compare food qualities after drying. Prior to drying, mushrooms were subjected to blanching or dipping in sodium metabisulphite solution (1 or 5 g/L), or dipping in citric acid solution (1 or 5 g/L). Drying of raw mushrooms was taken as a control. Blanching reduced the attractiveness of dry mushrooms; sodium metabisulphite improved it. It has been found that pretreatment and drying method affect the course and rate of drying. Samples subjected to hot-air drying and vacuum-drying were darker than those freeze-dried, which were clearly more attractive. The hot-air and vacuum-dried mushrooms on rehydration were inferior in quality to the freeze-dried samples. Flavor of the freeze-dried mushroom was not significantly different from that of the hot-air dried mushrooms. Food quality of dried mushrooms depends significantly on the type of drier used.

     

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.     

DETERMINATION OF DRYING CONSTANTS AND THEIR DEPENDENCE ON DRYING AIR PARAMETERS FOR THIN LAYER ONION DRYING

An experimental through dryer with continuous weighing system was developed for determining the drying rate constants for thin layer drying of onion (Allium cepa L, variety: Agnfound White- 1). Drying characteristics of onion were examined using ambient heated air for a temperature range of 50-80°C and air flow velocity range of 0.25-1.00 m/s. The influence of drying variables on drying rate constant was explained suitably hy Arrhenius type equation. This was also compared with Power model. The models were tested with data produced in an experimental through dryer, using nonlinear regression analysis. The Arrhenius type model predicted the moisture ratios well at the experimental drying conditions.     

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.     

DRYING KINETICS OF APRICOTS

The kinetics of apricot dehydration was examined using a factorial experimental design in order to evalute the effect of air temperature, air velocity and pretreatment of the sample on the drying time and the transport coefficients of apricots. A finite element method was used to solve the differential equations describing the transfer of moisture in the irregular shape of the apricot. The two transport coefficients of the system were estimated using non-linear regression analysis. It was found that the moisture transfer is entirely controlled by the external resistance to mass transfer for the air velocities examined. A new method is proposed for the evaluation of the relative significance of the external and the internal mass transfer during, which should repiace the well known Biot number criterion.     

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.     

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.