Suresh Prasad

Drying of garlic (Allium sativum) cloves by microwave–hot air combination

Abstract Garlic cloves were dried with hot air and combined microwave–hot air drying methods in an experimental dryer. The combined microwave–hot air drying experiments were carried out with 100 g sample sizes at temperatures of 40°C, 50°C, 60°C and 70°C at air velocities of 1.0 and 2.0 m/s, using continuous microwave power of 40 W. For comparison of hot air drying, the same sample sizes were taken for experiments and the drying air temperatures and air velocity were 60°C and 70°C, and 2.0 m/s respectively. The total drying time, the color and flavor strength of dried garlic cloves were used to evaluate the performance of the combined microwave–hot air drying and the conventional hot air drying processes. Combined microwave–hot air drying resulted in a reduction in the drying time to an extent of 80–90% in comparison to conventional hot air drying and a superior quality final product.

Modeling Microwave Vacuum Drying Kinetics and Moisture Diffusivity of Carrot Slices

Carrot slices of 3.5 mm thickness were dried in a laboratory microwave vacuum dryer at five different microwave power density levels of 2, 4.66, 7.33, 10, and 12.66 W/g and at three vacuum chamber pressure levels of 6.66, 19.98, and 33.3 kPa to 4–6% d.b. moisture content. Inside the dryer the sample holding plate was rotated with the speed of 4 rpm for uniform microwaves application. The drying rates were increased with the increase in microwave power density at all pressure levels and the Page model was found to be the most suitable model to predict the drying behavior of carrot slices at all process conditions. The Page model drying rate constant (k, min−1) showed high correlation with microwave power density at constant pressure by a power law equation and showed a logarithmic relationship with the microwave power density and pressure. Similar to the drying rate constant, the average moisture diffusivity at constant pressure was found to be function of microwave power density by power law equation as well as was also dependent on the power density and pressure by a logarithmic relationship.

Optimization of Microwave-Vacuum Drying of Button Mushrooms Using Response-Surface Methodology

Button mushrooms (Agaricus bisporous) were dried in a microwave-vacuum dryer up to a final moisture content of around 6% (d.b.). The effect of microwave power level (115 to 285 W), system pressure (6.5 to 23.5 kPa), and slice thickness (6 to 14 mm) on drying efficiency and some quality attributes (color, texture, rehydration ratio, and sensory attributes) of dehydrated mushrooms were analyzed by means of response surface methodology. A rotatable central composite design was used to develop models for the responses.Analysis of variance showed that a second-order polynomial model predicted well the experimental data. The system pressure strongly affected color, hardness, rehydration ratio, and sensory attributes of dehydrated mushrooms. A lower pressure during drying resulted in better quality products. Optimum drying conditions of 202 W microwave power level, 6.5 kPa pressure, and 7.7 mm slice thickness were established for microwave vacuum drying of button mushrooms. Separate validation experiment was conducted at the derived optimum conditions to verify the predictions and adequacy of the models.

Dielectric properties of garlic (Allium sativum L.) at 2450 MHz as function of temperature and moisture content

Abstract Dielectric properties of garlic were measured at 2450 MHz at moisture contents varying from 6% to 185% (dry basis) and at temperatures ranging between 35°C and 75°C, using the Resonant cavity perturbation technique. Both dielectric constant and loss factor were found to be directly proportional to the moisture content. The relationship of the dielectric constant as well as loss factor was found to be linear with temperature. Predictive models of the dielectric properties as functions of moisture content and temperature were developed using response surface methodology. Moisture content affected the penetration depth of microwaves significantly.

Finite element analysis of microwave heating of potato––transient temperature profiles

A two-dimensional finite element model was developed to predict temperature in solid food materials during microwave heating. Microwave heating of solids with rectangular and cylindrical geometries was analyzed using ALGOR commercial finite element analysis software. Absorbed microwave power density at any location in the test material was derived as a function of dielectric properties and geometry of the material. A program in C language was developed to find the amount of heat generated at any location in the developed model, using potato as a test material experimentally to validate the model. It is very difficult to compare the accurate temperature; hence rate of temperature rise was used to compare the predicted and experimental values. The temperature predictions by finite element analysis and the experimental measurement were very similar in slab-shaped samples. For slab-shaped potato sample, a cold spot occurred along the central axis. Edge and corner heating effects were also observed. The temperature decreased away from the corners to the edges, with a further decline to the center. In cylindrical samples, the experimental data and finite element predicted values of temperatures were close to each other at all regions except at the central region. A high concentration of absorbed power exists along the central line of the cylinder, because power density increased rapidly near the center.

Modeling Shrinkage and Density Changes During Microwave-Vacuum Drying of Button Mushroom

Shrinkage characteristics and apparent density of whole button mushrooms were determined at various moisture content levels (ranging from 5 to 92% wet basis) during microwave-vacuum drying at two different power (150 and 250 W) and pressure (10 and 20 kPa) levels. The above properties during convective hot air drying at 60°C were also measured for comparison. In both microwave-vacuum and air-drying methods, the shrinkage (volumetric and diametric) of mushroom showed a linear behavior with moisture content. Experimental data showed that the effect of the system pressure on shrinkage and density was more significant than the power level during microwave-vacuum drying. Moisture content and method of drying also affected shrinkage statistically. Microwave vacuum drying produced less shrinkage than air drying. Simple mathematical models were used to correlate the above properties with the material moisture content. The models were fitted to experimental data satisfactorily, and the parameters were estimated.

Optimization of Osmotic Dehydration of Potato Cubes Under Pulsed Microwave Vacuum Environment in Ternary Solution

Optimization of the process parameters for osmotic dehydration of 12.2-mm potato cubes was carried out using response surface methodology. The experiments were conducted using a central composite rotatable design (CCRD) with four factors, viz. sucrose concentration (27.5–42.5% w/w), salt concentration (7.5–12.5% w/w), total osmosis time (26.25–68.75 min), and microwave power density for the initial 4 min (0.375–1.125 W/g of total weight of solution and potato cubes) at two levels each to take into account the individual and interaction effects of the factors. A sample-to-solution ratio of 1:10 and pressure of 0.16 kPa for the initial 4 min were kept constant throughout all of the experiments. It was found that the linear effects of all factors on the water loss (WL) and solids gain (SG) were highly significant. The optimum condition was found at a sucrose concentration of 36.35%, salt concentration of 12.50%, osmosis time of 68.72 min, and microwave power density of 0.38 W/g for the initial 4 min, with a WL of 37.26% initial weight and SG of 8.74% initial weight. The drying of potato cubes was carried out using hot air, microwave–vacuum, and osmotic microwave–vacuum drying methods. It was found that potato cubes dried by combined osmotic microwave–vacuum had better sensory qualities.

Optimization of Osmotic Dehydration of Carrots under Atmospheric and Pulsed Microwave Vacuum Conditions

The osmotic dehydration of carrot slices was carried out by two methods: at atmospheric pressure and under pulsed microwave vacuum conditions. In the first part, 20 experiments were conducted using a central composite design (CCD) with sucrose concentration (23–57% w/w), solution temperature (29–71°C), and time of osmosis (6–74 min). The slice thickness of 3.5 mm and sample-to-solution ratio of 1:10 (w/w) was kept constant throughout all experiments. Numerical optimization was done for higher water loss (WL) and lower solids gain (SG) using developed correlations and optimum experimental conditions were found at 56.82% sucrose concentration, 29°C solution temperature, and 20-min osmosis time with water loss of 27.73% initial weight and 14.41% initial weight solids gain. In the second part of the investigation, osmotic dehydration of carrot samples under pulsed microwave–vacuum conditions was carried out at the optimum sucrose concentration and solution temperature obtained in the first part. The effect of microwave power and time on mass transfer was studied. The experiments were designed by using a CCD with two variables, viz. microwave time per minute (7 to 41 s) during initial 5 min of osmosis and total time of osmotic dehydration (7 to 41 min). During each experiment, vacuum at 6.66 kPa and microwave radiation of 1 W/g of osmotic solution and carrot slices at different pulsating levels were maintained during initial 5 min in the chamber and then osmotic dehydration was continued for different time intervals. Correlations were developed using a regression technique and the optimum conditions were found with 40.46% WL and 11.58% SG at microwave time per minute of 36 s during the initial 5 min and osmosis time of 12.48 min. In comparison to the osmotic dehydration of carrots at atmospheric pressure, the application of combined pulsed microwave vacuum during osmosis was found to increase WL.

Assessment of Quality of Pears Stored in Laminated Modified Atmosphere Packages

Various aspects of modified atmosphere packaging for storage of pears in perm-selective polymeric film packages were investigated. The dynamics of respiration-permeation interaction was analyzed with a view to facilitate self establishment of dynamic equilibrium of gaseous composition within the package and external atmospheres at a desired storage condition. By combining different polymeric films, three types of laminated modified atmosphere packages (LF1, LF2, and LF3) were developed. The modified atmosphere packaged pears were found to have retained freshness and quality for a longer period of time. The shelf life of MA packaged pears could be extended approximately 0.75 to 1.5 times in comparison to unpacked pears. The performance of package type LF1 was found to be the best, whereas LF2 was found to be the second best.

Respiration rate and heat of respiration of some fruits under controlled atmosphere conditions

Abstract Studies were conducted on green banana, mango and orange (mosumbi) to measure the rate of respiration and determine the heat of respiration at storage temperatures of 10, 15, 20, 25 and 30°C maintaining CO2 and O2 concentrations of 5 and 5%, 5 and 10%, 5 and 15%, 10 and 5%, 10 and 10%, 10 and 15%, 15 and 5%, 15% and 10%, and 15 and 15% for each temperature. The respiration rate and heat of respiration reduced with an increase in CO2 concentration and decrease in O2 level and storage temperature. Results revealed that the rate of respiration and also the heat of respiration varied exponentially with temperature for various concentrations of CO2 and O2.