Microwave Thin Layer Drying of Untreated and Osmotic-Pretreated Indian Gooseberries (Phyllanthus Emblica)

Table of Contents

1. Introduction:

2. Materials and methods:
2.1 Materials :
2.2 Experimental setup:
2.3 Experimental procedure:
2.4 Analysis of drying data:

3.Results and discussion
3.1 Estimation of parameters and comparison of drying curve models
3.2 Determination of effective diffusivities
3.3 Drying rate curves:
3.4 FMC versus RATE:
3.5 Development of model :
3.6 Determination of drying coefficient and Biot number

4. Conclusion

References :

AN EXPERIMENTAL INVESTIGATION ON MICROWAVE THIN LAYER DRYING OF UNTREATED AND OSMOTIC-PRETREATED INDIAN GOOSEBERRIES (PHYLLANTHUS EMBLICA ).

G B RADHIKA *, N ANNAPURNA DEVI **,

* Department of chemical engineering, Dr B V Raju Institute Of Technology, Jawaharlal Nehru Technological University, Narsapur, Medak, Andhra Pradesh, India.

** Department of chemical engineering, MVGR College of Engineering, Vizianagaram, Andhra Pradesh, India.

Abstract:

The thin layer drying characteristics of untreated and osmotic-pretreated Indian gooseberries was examined using a laboratory microwave dryer at five different powers. Hypertonic solutions like 10%NaCl and 20%NaCl were used for osmotic-pretreatment of Indian gooseberries. Drying was carried out until constant values were obtained. The drying curves obtained from the experimental data were fitted to different thin layer drying models. All the models were compared according to three statistical parameter χ2, RMSE and R2.

The results shown that gooseberries pretreated in 20%NaCl solution took lesser drying time than gooseberries pretreated in 10%NaCl solution and untreated samples. Midilli model was found to be the best model for describing the drying curve of Indian gooseberries. Effective diffusivities and drying coefficient (k) for untreated and pretreated Indian gooseberries were calculated at five microwave powers using linear regression method. Biot number for untreated and pretreated Indian gooseberries was calculated using standard mass transfer equations.

Keywords: Microwave drying, Indian gooseberries, Midilli model, statistical parameters, Biot number, effective diffusivity.

1. Introduction:

Microwave drying is a powerful drying technique used in food industry. Using this method of drying more uniform, hygienic and attractive colored dried product can be produced. Microwave drying is more efficient than conventional hot air drying as microwave penetrates to the interior of the food causing water to get heated within the food. This results in a greatly increased vapor pressure differential between the centre and the surface of the product, allowing rapid removal of moisture from food. Application of microwave energy to dry food materials is a good approach for copying with certain drawbacks of conventional drying (Sutar & Suresh Prasad,2007).

The osmotic dehydration has become not only a very attractive method for fruits and vegetables pretreatment before conventional drying but often is used as the independent dewatering process. Osmotic dehydration is provoked by a higher osmotic pressure in a hypertonic aqueous solution than in fruit or vegetable samples immersed in this solution. Water in the cells of biological materials permeates into the solution through the cell membrane due to high osmotic pressure (Kowalski et al, 2009).

Aonla (Phyllanthus emblica) or Indian gooseberry, an important fruit, is highly valued among indigenous medicine. Indian gooseberries are a natural source of vitamin-C and work better than synthetic ascorbic acid in cure of deficiency diseases. Due to high astringency, the Indian gooseberry are not popular as table fruit and also due to its high perishable nature its storability is also limited (Kumar & Nath 1993). The other methods of extending shelf life are by cold storage, sun drying and hot air drying or by processing to murabba, pickle, juice syrup, squash and dehydrated powder (Kalra, 1990). Only limited efforts have so far been made to process Indian gooseberry into dehydrated product (Palodkar et al 2003). Therefore the present study was conducted with the following objectives:

1) To study and compare the thin layer drying characteristics of pretreated and untreated Indian Gooseberries using microwave dryer at 5 powers.
2) To investigate the effect of pretreatment solutions on the drying rates of gooseberries.
3) To fit the experimental data obtained to theoretical models widely used to describe thin layer drying models and finding effective diffusivities using the data.
4) To develop a drying model for untreated and pretreated Indian gooseberries.
5) To calculate the drying coefficient and Biot number for untreated and pretreated Indian gooseberries using basic mass transfer equation.

2. Materials and methods:

2.1 Materials :

Indian gooseberries used in this study were obtained from a local grower. In order to preserve its original quality, they were stored in refrigerator until drying experiments. The initial moisture content of untreated and pre treated gooseberries was determined using oven method at 1050c for 24hrs. After drying, the untreated sample was found to have an initial moisture content of 87.01%wet basis. Whereas for samples pretreated with 10%NaCl solution for 4 hours and 20%NaCl solution for 30 minutes, the initial moisture content was 82.77% w.b , 82.4% w.b .

2.2 Experimental setup:

Microwave dryer was used for determining thin layer drying behavior of untreated and pretreated gooseberries. Whereas for pre treatment methods 10% NaCl solution and 20%NaCl solution were used and the drying was carried out in microwave. A digital balance with an accuracy of 0.01g was used to weigh the sample at each interval of time.

2.3 Experimental procedure:

Microwave drying experiments were conducted at 900, 720, 540, 360 and 180Watts. In each experiment, about 30g of untreated Indian gooseberries slices of 14mm and pre treated Indian gooseberries slices of 13mm and 12mm thickness were used. Whereas for osmotic-pre treatment methods, the samples were dipped in 10% NaCl solution for 4 hours and in 20%NaCl solution for 30minutes and the drying was carried out in microwave at 5 powers. To determine the moisture loss during experiments, gooseberries samples were taken from the microwave at various time intervals and weighed in digital balance. The weighed samples were again dried up in the microwave until no mass change was seen. The data was collected at various time intervals and noted for further calculation of moisture ratio, final moisture content (FMC) and rate of drying.

2.4 Analysis of drying data:

The experimental data obtained were fitted to the well known drying models. The moisture ratio is given as follows:

MR = (M - Me) / (M 0 - Me ) (1)

where MR is the dimensionless moisture ratio,

M is the moisture at any time, M 0 is the initial moisture content in kg H2o/kg wet mass. However MR is simplified to M/M0 instead of Equ.(1) due to the continuous fluctuation of relative humidity of the drying air during their drying process (Diamente & munro,1993).

The rate constants and coefficients of models were estimated using a non linear regression procedure and the statistical validity of models were evaluated. Determination of R2 (correlation of determination), RMSE and chi square (χ2) is also done. RMSE gives the deviation between the predicted and experimental values and it is required to reach zero. The R2 also gives the ability of the model and its highest value is 1 or near to it. The highest R2 values, lowest χ2 and RMSE values gives the best fit to the theoretical model. The thin layer drying models given by various workers for drying curves is given in the Table 1.

Table 1 Mathematical models applied to drying curves

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The curves were fitted to these drying models and the statistical parameters were known using MATLAB 7 program. R2 was the primary comparison criteria for selecting the best model to fit the models to the experimental data. Chi square can be calculated using Equ.(2) and RMSE can be calculated using the Equ.(3).

Chi square can be calculated as:

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RMSE can be calculated as:

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3.Results and discussion

3.1 Estimation of parameters and comparison of drying curve models

Drying of Indian gooseberries slices started with an initial moisture content of 87.01% (w.b) and continued until complete moisture was removed. The model coefficient and constant were calculated using the non linear regression method. The statistical analysis results applied to these models at different microwave powers are given in the Table 2.

Table 2. Estimated Parameters of untreated and osmotic-pretreated Indian gooseberries

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Table 2 list the rate constants and coefficients for Midilli et al drying model for thin layer microwave drying of Indian gooseberries. Midilli et al model gave excellent fit to the experimental data for R2. Also, the values for χ2 and RMSE obtained from these models were less than obtained from other models. It can be seen from the values that there was a good conformity between experimental and predicted moisture ratios.

3.2 Determination of effective diffusivities

The effective diffusivity of the samples is estimated by using Fick’s second diffusion model. The solution of Fick’s second law in slab geometry, with the assumption of moisture migration being by diffusion, negligible shrinkage, constant diffusion coefficients and temperature was as follows(crank,1975).

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