G.S.V. Raghavan

Microwave-assisted convective air drying of thin layer carrots

Abstract Dehydration characteristics of carrot cubes were evaluated in a domestic microwave oven (600 W) modified to allow passage of air at constant flow rate and a given air temperature. The parameters included inlet air at two temperatures (45 and 60 °C), and microwave oven operation at two power levels (2 and 4). Conventional air drying with microwave off (power level 0) served as the control. Drying time to achieve the desired moisture level in the final product, and rehydration characteristics of the product, were used to compare microwave-assisted drying with the conventional method. Microwave drying resulted in a substantial decrease (25–90%) in the drying time and the product quality was better when dried at the lower power level.

Comparison of heat transfer rates between a straight tube heat exchanger and a helically coiled heat exchanger

The purpose of this study was to determine the relative advantage of using a helically coiled heat exchanger versus a straight tube heat exchanger for heating liquids. The particular difference in this study compared to other similar studies was the boundary conditions for the helical coil. Most studies focus on constant wall temperature or constant heat flux, whereas in this study it was a fluid-to-fluid heat exchanger. Results showed that the heat transfer coefficient was effected by the geometry of the heat exchanger and the temperature of the water bath surrounding the heat exchanger. All tests were performed in the transitional and turbulent regimes.

Microwave-Convective and Microwave-Vacuum Drying of Cranberries: A Comparative Study

Abstract Two drying methods of cranberries (microwave-vacuum and microwave-convective) are reviewed, and their advantages and disadvantages regarding the quality of dried product and the process performance are presented. Mechanically and osmotically pretreated berries were subjected to drying and quality evaluation. Quality parameters are color (in Hunter L*a*b* coordinates), textural characteristics (toughness and Youngs modulus), and organoleptic properties (color, texture, taste, and overall appearance). Special emphasis was given to the energy performance of the process, monitoring of the real-time temperature profile, and the total microwave power-on time. Two microwave power densities are assessed, as well as different microwave power-on/power-off cycling periods. In almost all observed parameters, microwave-vacuum drying exhibited enhanced characteristics when compared to microwave-convective drying. Drying performance results (defined as mass of evaporated water per unit of supplied energy) showed that microwave-vacuum drying is more energy-efficient than microwave-convective. Tasting panel results exhibited slight preference in all parameters for microwave-convective dried samples.

DRYING CHARACTERISTICS OF RED CHILLI

ABSTRACT 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 Pages 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.

Microwave and Convective Drying of Grapes

Thompson seedless grapes were dried under forced convection and also combined convection and microwaves in a modified microwave (MW) oven. Effect of chemical pretreatment on drying rate was compared with fruits with no pretreatment. Different air temperatures and power levels were investigated. Dipping of fruits in hot alkaline solution of ethyl oleate considerably reduced drying time resulting in better quality raisins. MW drying reduced drying time resulting in raisins of comparable product quality even without chemical pretreatment. An air temperature of 50° C was found to be optimum for MW drying. Page’s equation fitted the MW drying data adequately well and the estimated parameters of the equation are presented.

Effect of four drying methods on the quality of osmotically dehydrated cranberries

Abstract Partially dehydrated cranberries (osmotically dehydrated) were dried to low water contents using one of following four methods: hot air drying; microwave-assisted convective drying; freeze-drying; and vacuum drying. Quality evaluation was performed on all samples, including sensory evaluation (appearance and taste), texture, color, water activity, and rehydration ratio. Hot air drying produced dried cranberries with the best visual appearance while freeze-dried cranberries had the highest rehydration ratio. The other methods presented similar rehydration ratios. There was no significant difference in color measurements and water activity. Few differences in texture were found, except for freeze-dried cranberries, which had a lower toughness compared to the other drying methods including commercially available dried cranberries. Microwave-assisted to hot air drying rate ratios increased as the moisture content decreased.

Rheological characteristics and kinetics of colour degradation of green chilli puree

Abstract Rheological characteristics as a function of particle size at 25°C and the effects of pre-treatment and temperature on colour degradation kinetics of green chilli puree were investigated. Green chilli puree behaved as a shear-thinning fluid and the power law described well the shear stress–shear rate behaviour. The consistency index ( K ) decreased while the flow behaviour index ( n ) increased as the particle size of the puree decreased. The results indicated that colour degradation during thermal processing of chilli puree followed a first-order reaction kinetics. The combination L × a × b closely described the colour degradation behaviour. Dependence of the rate constant during thermal processing of chilli puree obeyed the Arrhenius relationship. The activation energy values for water-blanched and lye-treated chilli puree were 11.4 and 16.0 kJ/g mol, respectively. Lye treatment resulted in retarding the colour degradation rate.

Overview of new techniques for drying biological materials with emphasis on energy aspects

With increasing concern about environmental degradation, it is desirable to decrease energy consumption in all sectors. Drying has been reported to account for anywhere from 12 to 20% of the energy consumption in the industrial sector. Drying processes are one of the most energy intensive unit operations. There are a number of approaches to reduce energy consumption in dryers. This paper reviews some novel strategies used to decrease energy consumption in drying operations. Drying conditions can be modified or the drying equipments can be modified to increase overall efficiencies. Hybrid drying techniques can also be used, such as combining vacuum or convective drying with electro-technologies (microwave, radio frequency, infrared heating). There is much debate on how to define drying and energy efficiencies. Some techniques to determine these efficiencies can be misleading when the goal is to take a holistic approach to determining energy consumption.

Radio-frequency treatment for ready-to-eat fresh carrots

This study was conducted to determine the potential of an RF thermal treatment to improve and extend the storability of vacuum-packaged carrot sticks stored at 5–6C. The results have shown that it is possible to treat carrot sticks to 60C in less than 2 min in a parallel plate RF applicator, and thus reduce the initial total microbial load. The RF treatments were compared to chlorinated water dipping, and hot water dipping. All storage trials of 7–14 days at 6C, have indicated that the reduction of the initial microbial load alone does not maintain the quality of carrot sticks for 14 days, since at this point the microbial loads in all cases studied were higher than 310 6 . Nonetheless, the quality of the RF treated samples was greater than for either the control samples (chlorinated water) or hot-water-treated carrot samples. The RF-treatments maintained colour, taste and the vacuum of the packages, which was not the case for control and hot-water-treated carrots. # 2001 Elsevier Science Ltd. All rights reserved.

A Mathematical Model for Constant and Intermittent Batch Drying of Grains in a Novel Rotating Jet Spouted bed

ABSTRACT A diffusion-bared mathdcal model is prswmed for batch drying of corn in a novel rotating jn spouted bcd device under constant as well as intermitted drying conditions. Such a device is suited for drying of large partides (e.g. grains, beam, seed, etc) for which internal heat and mass tranfer rates control the drying kinetics. Based on literature data for moisture diffusivities the model predictions are compared with experimental data for both continous and time-dependend air supply and/or heat input. Effects of dcvant parameter are evaluated and discussed in the light of potential pnrctieal applications.