FUNDAMENTALS OF DRYING TECHNIQUES

The samples of sliced and mashed apples were freeze-dried by controlling their surface temperatures over the usual pressure range of commercial operations. The surface of sliced samples could not be maintained at above 10°C in order to prevent the frozen layer from melting, while that of mashed samples was allowed to heat up to 70°C.

Thermal conductivities and permeabilities were determined by applying the uniformly-retreating-ice front model to the dried layer of the samples undergoing freeze-drying. The values of permeability for the mashed samples were found to depend on the ice-crystallization time during freezing. The results indicated that the drying rate of sliced samples was limited by the transfer rate of water vapor flowing through the dried layer. A cellular structural model is proposed for predicting the permeability of the dried layer, based on the resistance of the cell membrane to molecular transfer of water vapor.     

STRUCTURAL MODELS RELATED TO TRANSPORT PROPERTIES FOR THE DRIED LAYER OF FOOD MATERIALS UNDERGOING FREEZE-DRYING

The values of thermal conductivity and permeability have been presented for the dried layer of raw beef, coffee solutions and sliced as well as mashed apples. A structural model for beef, coffee solution and mashed apple was developed for predicting the permeability of water-vapor flowing through the dried layer. In modeling the porous dried layer was assumed to be a bundle of capillary tubes with the pore space having an equivalent pore radius, porosity, and tortuosity factor. These results indicated that the structure for the solution systems was controllable by both solute concentration and freezing manners.

Another cellular structural model has been presented for predicting the permeability of a sliced apple. The resistance of a cell membrane to the molecular transfer of water vapor was determined from the value of permeability and the size as well as number of the cells existed in the dried layer. Since the drying rate of cellular materials was limited by the mass flux across the dried layer, the model was considered to play an important role in predicting the optimum heating program for the surface temperature of cellular food materials such as fresh fruits and vegetables.     

LOW TEMPERATURE OXIDATION OF LOW RANK COALS

The low temperature oxidation of a Montana subbituminous coal was investigated using round bottom 100 ml flasks in constant temperature baths. The experiments were carried out in normal and oxygen enriched air at 30°C, 45°C and 70°C with particle sizes ranging from 4 mesh to 100 mesh. Periodic analysis of gas samples from the flasks provided the rate data. The reactivity of the as received coal was compared with that of the same coal dried (i) in high pressure steam and (ii) in hot water

A rate equation has been proposed incorporating the effects of oxygen diffusion and surface reaction. For higher oxygen concentration and smaller particle sizes, the zero order surface reaction was found to be controlling. The temperature dependency of the reaction rate was found to be well represented by the Arrhenius equation. The activation energy varied slightly under various conditions between 15 to 20 kca)/g mole

The reactivity of the hot water dried coal was found to be similar to that of the as received coal. Steam dried coal however, was found to be much less reactive.     

Optimization of the Freeze-Drying Cycle: A New Model for Pressure Rise Analysis

The principal aim of this study was to evaluate the Pressure Rise Analysis (PRA) method as a nonintrusive method for monitoring the product temperature during primary drying of the freeze-drying process of model pharmaceutical formulations. The principle of this method, based on the MTM method initially published by Milton et al.1 consisted in interrupting rapidly the water vapor flow from the sublimation chamber to the condenser chamber and by analyzing the resulting dynamics of the chamber total pressure increase. A new physical model, named PRA model, based on elementary heat and mass balance equations and on constitutive equations expressing the elementary fluxes, was proposed and validated in this study for interpreting the experimental pressure rise data. It was possible to identify very precisely the values of some key parameters of the freeze-drying process such as the ice sublimation interface temperature, the mass transfer resistance of the dried layer and the overall heat transfer coefficient of the vial. The identified ice front temperatures were compared with experimental data obtained from vial bottom temperatures measured by thin thermocouples during freeze-drying runs of 5% w/v mannitol solutions. These two sets of data were found consistent with a maximum difference of no more than 2°C. The dried layer mass transfer resistance increased linearly as a function of its thickness, and the values were coherent with the few literature data published for this system. The method also led to reliable values of the vial overall heat transfer coefficient of approximately 20 Wm^-2 K^-1 in accordance with the published data for this type of vials and these experimental freeze-drying conditions.     

Surface Characterization of Ti and Ti (6% AI-4%, V) Metal Powders and Interaction with Primer Solutions

The surfaces of Ti and Ti (6% Al-4% V) powders were characterized by several techniques. BET surface areas as a function of temperature were measured using nitrogen adsorption. Heats of immersion (δ_wH) of these metal powders in water were measured after evacuation over the temperature range 100°-400°C. The δ_wH in water increased with increasing evacuation temperature and an anomalous increase was observed between 300° and 400°C. This was attributed to exposure of water to elemental titanium by cracking of the oxide layer at 400°C. XPS analysis did not support the possibility of metal migration through the oxide layer. Higher heats of immersion in water were determined for chemically pretreated compared to untreated Ti 6-4 powders. Water vapor adsorption isotherms were measured after evacuation of the metal powders at 100°C. Partial irreversibility of the water adsorption was observed on both powders. Water adsorption on Ti was temperature dependent. Heats of immersion measurements were used to study the interaction of primer solutions with these metal powders. Polyimide (LARC-13) and polyphenylquinoxaline (PPQ) interacted preferentially compared to the solvents with both powders. This polymer-metal interaction improved significantly after pretreatment of the Ti 6-4 powder by the Turco® 5578 and phosphate-fluoride processes. Again, an anomalous increase in the heat of immersion of Ti 6-4 evacuated at 400°C in the primer solution/solvents was observed. Anatase and rutile TiO₂ powders are not satisfactory models for the surface oxide layer on either Ti or Ti 6-4 powders.     

FREEZE-DRYING CHARACTERISTICS OF STRAWBERRIES

Drying kinetic, as well as color and volume variation, of whole and sliced strawberries were investigated after freeze-drying under various temperatures (30, 40, 50, 60 and 70°C). Dehydration time increased proportionally to the thickness of the product and heating plate temperature markedly reduced it. Freeze-drying caused a pronunciation in red color of strawberries. A decrease in hue angle by 22.5% (skin) and by 42.4% (pulp) was noted, with no significant effect of freeze-drying temperature up to 70°C. The strawberries had a volume reduction of 8% (whole) and 2% (sliced) due to freeze-drying although the level of shrinkage was also independent of freeze-drying temperature. However, the percentage of collapsed strawberries increased with process temperature. At heating temperatures higher than 50°C, the strawberry dry layer temperature was higher than the estimated glass transition temperature of dried fruit, increasing the risk of collapse.     

Breakthrough analysis for CO2 removal by activated hydrotalcite and soda ash

Carbon dioxide sorption rate parameters and sorption capacities on two different regenerable sorbents, namely hydrotalcite and activated trona, were investigated in a fixed bed flow adsorber, in the temperature range of 400–527 °C and 80–152 °C, respectively. Hydrotalcite, which was activated at 550 °C, was shown to give total and breakthrough CO₂ sorption capacities as high as 1.16 and 0.70 mmol/g, respectively, at 452 °C, in the absence of water vapor. In the presence of excess water vapor, the total CO₂ sorption capacity was not affected much, however a decrease in the breakthrough capacity and on the sorption rate constant was observed, especially at lower temperatures. In the presence of water vapor, activated trona was shown to give comparable total and breakthrough CO₂ sorption capacities, at much lower temperatures (T < 100 °C). The deactivation model gave good predictions of the CO₂ breakthrough curves and it was successfully used for the evaluation of the adsorption and the deactivation rate constants.     

EFFECT OF DRYING METHODS ON MOISTURE SORPTION ISOTHERMS OF FUFU AT THREE TEMPERATURES

Moisture sorption characteristics for dried fufu using three drying methods at 25°C, 32°C and 45°C were determined experimentally for water activity ranging from 0.11 to 0.96. At a given temperature and water activity, the rotary dried fufu had the highest equilibrium moisture content while the sun dried had the lowest.

Four linear and four nonlinear sorption models were used to analyze the data. The linear GAB model showed the best fit while the nonlinear Bradley model was the best over the whole range of water activity in their respective groups. Also estimates of the net isosteric heats of sorption and their dependence on moisture content were presented for each product.     

CONDUCTION DRYING OF RAW AND STEABED RICE BRAN

The effects of conduction heating in a steam jacketed cylindrical dryer-cum stabilizer at the drying surface temperatures ranging from 100° to 120°C on thin layer conduction drying characteristics of raw and steamed rice bran were studied. Results show that both raw and steamed rice bran at 13.7·14.2 and 14.7 ·16 ·· 3 percent (d b) initial moisture contants (imc; respectively dried uncer fillinq-rate period only, where as steamed bran at 30·3 per cent imc dried initially under constant rate period for a very short time and then dried under fall ing-rate period. The drying surface temperature of 115°C can be taken as optimumu for conduction drying of both raw. and steamed rice bran. The empirical drying equations, developed for the above drying conditions. can be used to predict the drying time with fairly good accuracy.     

DRYING CHARACTERISTICS OF RED CHILLI

Red chilli is widely consumed as a food additive throughout the world. It is blanched/treated to minimize quality loss during processing. This paper reports on various pre-treatments applied before drying and their influence on drying kinetics as well as product quality. Inactivation of peroxidase enzyme was achieved by blanching chillies at 90°C for 3 min in hot water. The physical appearance of the dried product was found to be the best when the blanched samples were soaked in gum acacia solution (0.2% m/v) for 15 min at room temperature. The pre-treated chillies were dried in a tray dryer at selected temperatures (55, 60, 65 and 70°C). Results indicated that drying took place in the falling rate period; the drying kinetics were adequately described by the Page's model. The activation energy for drying was determined to be 41.95 and 41.06 kJ/mol respectively, for blanched and gum-treated chillies. Total pigment content decreased while non-enzymatic browning increased with increase in drying air temperature.     

Drying of Chestnuts (Castanea sativa Mill.) after Osmotic Dehydration with Sucrose and Glucose Solutions

Chestnuts were dehydrated by using a combined method of osmotic dehydration followed by air drying. Samples were osmotically pretreated with sucrose (60% w/w) and glucose (56% w/w) for 8 h, air-dried at temperatures of 45, 55, and 65°C, at a relative humidity of 30% and at a velocity of 2.7 m·s^-1 and the experimental data of the drying kinetics were obtained. Whole samples were dried with different peelings: (a) removal of endocarp and pericarp (peeled) and (b) additionally the internal rough surface (cut). In all cases, cut chestnuts show greater drying rates than peeled samples, indicating that a significant mass transfer resistance in the layer nearest to the surface takes place. Peeled samples pretreated with sucrose solutions behave in a similar way to untreated samples. For the rest of the samples, the cut samples osmotically treated with sucrose solutions and all the samples treated with the glucose solution, the drying rates decrease during drying. Drying kinetics are successfully modeled by employing a diffusional model that takes the shrinkage into account. The effective coefficient of water diffusion was evaluated and correlated with temperature. The quality of the final product was monitored by color change. In spite of the fact that the total color difference is not modified by the osmotic treatment, the L*, a*, and b* color coordinates of cut samples treated with sucrose and glucose solutions do undergo changes; the L* and a* coordinates change less than the b*.     

EFFECT OF PRETREATMENT ON KINETICS OF CONVECTION DRYING OF ONION

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.     

DETERMINATION OF PARAMETERS AND PRETREATMENT SOLUTION FOR GRAPE DRYING

The drying of Iranian seedless white grapes (sultana) was investigated in a batch operation in a laboratory dryer. The effect of pretreatment and temperature on the drying rate of grapes at constant air velocity were studied. Pretreatment solutions contained different alkaline materials in different concentrations and temperatures. Dipping grapes in an alkaline solution increased the drying rate substantially. Increasing the air temperature in the dryer from 60 to 70°C accelerated the drying rate (for a decrease in drying time) of the pretreated grape. Grapes dried in 450-900 min depending on pretreatment and air temperature. The shorter drying time and best quality dried product were obtained with grapes dipped in a solution of potassium carbonate of 5% at 42°C. The mean apparent diffusivity of moisture for K₂CO₃ pretreatment was calculated at 50, 60, and 70°C, and the values were fitted to an Arrhenius type model. Moisture sorption isotherms of currants were determined at 20, 30, and 40°C using static gravimetric methods. A modified Halsay equation was used to predict experimental data for water activity in a range of 0.1-0.85.     

Air drying and colour characteristics of chestnuts pre-submitted to osmotic dehydration with sodium chloride

Air drying kinetics of chestnuts (Castanea sativa Mill.) submitted previously to osmotic dehydration with sodium chloride solutions (22%, w/w, 25 °C, 8 h) were experimentally determined. Drying experiments were carried out at 45, 55 and 65 °C during 32 h. Before osmotic dehydration operation, shell and tegument tissue were carefully removed in all samples. Rough external surface was maintained (peeled samples) or also removed (cut chestnuts) in different tests in order to evaluate the effect of the presence of this layer in the mass transfer rate. A diffusional model involving shrinkage was employed to simulate the drying kinetics of the all studied systems and values of the effective coefficient of diffusion of water were obtained. These coefficients were successfully correlated with temperature by means of an exponential equation and were employed to evaluate the additional mass transfer resistances due to the presence of layers and by the acquisition of osmotic solute in cut and peeled samples. The experimental determination of local salt and moisture contents during drying showed the existence of a drying front for water that moves along from surface to the centre of the sample and that salt accumulates near surface but also achieve the centre during the osmotic dehydration. Additionally, the analyses of the surface colour of air dried samples indicated that osmotic dehydration with salt keeps the characteristics observed for non-treated cut samples and deteriorates the colour quality in the case of peeled samples.     

Optimization of Drum Drying Parameters for Low Amylose Rice (KDML105) Starch and Flour

An attempt was made to study and model the effects of drum drying process variables on the physico-chemical properties of low amylose rice (KDML105) flour and starch. Drum surface temperature, holding time and solid content of the slurry were varied at three levels: 115-135°C, 14-84 s and 20-40%, w/w, respectively. The dependent variables were moisture content (MC), degree of gelatinization (DG), water absorption index (WAI), water solubility index (WSI) and pasting property. High solid content led to a decrease in DG, WAI and initial peak viscosity (IPV) and increase in WSI of dried samples. Longer holding time resulted in increased DG while surface temperature had no significant effect on all characteristics. Predictive correlations were developed using stepwise multiple linear regression to predict MC, DG, WAI, WSI, and IPV of dried products from drum drying variables.     

Ice formation in hardened cement paste, Part II — drying and resaturation on room temperature cured pastes

Ice formation in partly dried and resaturated room temperature cured mature Portland cement paste has been measured. The measurements were carried out continuously in the temperature range from +20° C to −60° C.

The results demonstrate that the drying-resaturation treatment increases the volume of large pores and the continuity of the pore system. The increase in the volume of large pores occurs at the expense of small pores and necks in the pore system.

As the relative water vapor pressure at which drying takes place is decreased to 0.58, the number of continuous large pores is increased. Drying at lower relative water vapor pressures does not further increase the effect.

The present findings point out the importance of the moisture history of concrete before testing such properties as water permeability and frost resistance.     

DEHYDRATION OF PEARS WITHOUT SULPHUR DIOXIDE

The experimental dehydration behaviour of Bon Chretien pears dehydrated without sulphites was investigated during isothermal (67°C) and simulated counter-flow dehydration (48°→60°C and 58°→67°C). Dehydration of peeled halves and quarters treated with sodium chloride retarded browning during isothermal dehydration relative to untreated halves. This retarding effect was enhanced by using dehydration conditions favouring high drying rates, low superficial product water activity and low superficial product temperatures Ascorbic acid-treated samples exhibited considerable browning, practically to the same degree as untreated samples-Objective colour measurements of dried products subjected to different dehydration conditions are presented. Constant rate drying periods were observed in counter-flow simulations (48°→60°C) at higher humidity (30% RH), but not at relatively low humidity (20% RH). It was found that enzymatic browning reactions could be controlled during dehydration by selecting dehydration conditions favouring low superficial product temperature and water activity (low humidity) in conjunction with a suitable inhibitor of polyphenol oxidase. Thus, it was found that simulated counter-flow dehydration (48°→60°C and 58°→67°C) of pear quarters gave significantly lighter coloured products when dehydrated at 20% RH in comparison to dehydration at 30% RH     

EQUILIBRIUM MOISTURE CONTENT OF DRIED BLOOD FLOUR FN THE TEMPERATURE RANGE OF 20 - 50°C

The paper presents results of water sorption tests of dried blood flour carried out tinder laboratory conditions and mathematical analyses of sorption isotherms obtained. Moisture equilibrium data were investigated at air temperatures in the range of 20-50°C and water activity ranging from 0 4 to 0 99. The experimental procedure used was a gravimetric dynamic method with continuous registration of sample weight changes. Sorption capacity decreases as temperature increases. Rehydration of the dried material results in hysteresis but this phenomenon was small. Four models of equilibrium moisture content/equilibrium relative air humidity (Chung—Pfost. Halsey. Henderson, and Oswin) were evaluated for their ability to fit data for the samples of dried blood. The modified Henderson equation was a good model for moisture adsorption and desorption of dried blood flour     

DRYING OF SPINACH WITH A HIGH ELECTRIC FIELD

An alternating current high electric field (HEF) of 430 kV/m generated by multiple point-to-plate-electrodes was used to dry spinach (Spinacia oleracea L). Point electrodes distributed above a layer of fresh spinach were effective in removing moisture. Drying continuously for 7 h removed 80.1% of the total moisture compared with 79.8% and 19.3% when dried in an oven (60°C) and in ambient air (25°C), respectively. The Hunter 'a' value for HEF-dried samples maintained most of the original green color compared with oven-drying, during which the spinach turned brown. Total chlorophyll (Chl), Chl a and Chl b contents in spinach were substantially higher after HEF-drying than oven-drying. The ascorbic acid content after six weeks of storage of the dried material was almost three times higher in HEF-dried samples than those oven-dried. HPLC determinations of organic acids and sugars indicated no formation of by-products in post-HEF-dried spinach.     

HEAT AND MASS TRANSFER DURING SAPWOOD DRYING ABOVE THE FIBRE SATURATION POINT

Pine sapwood was dried in an air convection kiln at temperatures between 60-80 °C. Temperature and weight measurements were used to calculate the position of the evaporation front beneath the surface. It was assumed that the drying during a first regime is controlled by the heat transfer to the evaporation front until irreducible saturation occurs. Comparisons were made with CT-scanned density pictures of the dry shell formation during initial stages of drying of boards.

The results indicate a receding evaporation front behaviour for sapwood above approximately 40-50% MC when the moisture flux is heat transfer controlled. After that we finally reach a period where bound water diffusion is assumed to control the drying rate.

The heat transfer from the circulating air to the evaporation front controls the migration flux. In many industrial kilns the heating coils therefore have too small heat transfer rates for batches of thin boards and boards with high sapwood content.