Drying Kinetics of Green Bitter Gourd (Momordica charantia L.) Slices in a Fluidized Bed Dryer

Contents

1.INTRODUCTION

2.REVIEW OF PAST RESEARCH

3.EXPERIMENTAL PROCEDURE

4.RESULTS OF RESEARCH
4.1 Moisture ratio

5. SUMMARY AND CONCLUSIONS

6.BIBLIOGRAPHY

APPENDIX

ABSTRACT

1.INTRODUCTION

Vegetables are an important part of human diet. They provide protein, carbohydrates, mineral elements, vitamins and bulk which along with some cereals and other foods constitute the essentials of a balanced diet. India is the largest producer of vegetables in the world, next only to China with an annual Production of 72.83 million tones from 5.63 million ha with an average productivity of 13 t/ha ( Negi and Mitra L. 1999). Being perishable in nature about 25% is lost annually (Alam A. 1999). In northern part of the State some vegetables are consumed in dried form during winter months. Of these bitter gourd is cherished very much because of its characteristic taste. Bitter gourds (Momordica charantia L.) belong to ‘cucurbitacceae’ family and are commonly called as bitter gourd, papailla, karalla, balsam apple and balsam pear etc. in different part of the world. Most of the cucurbitaceous vegetables including bitter gourd are usually cultivated in relatively small areas for local consumption and do not enter the production statistics in a significant way. In India the gourds group of vegetables occupy an area of 4,48,000 ha with production of 45,70,000 t (Sharma and Pandey, 2003). Tamil Nadu, Uttar Pradesh, Maharashtra, Kerala and Karnataka are the important bitter gourd producing states. In Karnataka the crop is cultivated over an area of 10.75 ha with a production of 7,943 t and productivity of 7.36 t/ha. It is usually consumed as the whole plant while some people may consume only its fruit and seeds. All the part of the plant, including the fruit, tastes very bitter. The presence of momordicines in bitter gourd is responsible for its bitter taste. Bitter gourd has several uses. The fruits are used as vegetables in many ways and quite commonly used in cooked, fried and stuffed forms. The fruits are also pickled, canned and dehydrated. Every part of the plant is used medicinally. The fruits are cooling, digestive, laxative, antipyretic and its administration uses biliousness, blood diseases, rheumatism and asthma. The leaf is used internally as laxative and as ointment for sources. It is claimed that the fruit powder is used for healing wounds, leprous and malignant ulcers. It is reported for its usefulness in snakebites. The roots have abortifacient activity. In Ayurveda, the juice of fresh leaves is prescribed for diabetes. It has good nutritional value with 2.1 g of protein, 4.2 g of carbohydrates 1.8 mg iron, 20 mg of calcium, 55 mg of phosphorus, 210 mg of vitamin A and 88 mg of vitamin C per 100 g of edible portion.

Bitter gourd contains an array of biologically active plant chemicals including triterpenes, protein and steroids. Bitter gourd have extremely low amount of calories but have a lot of valuable nutrients. It is the first resource of vitamins B1, B2 and B3, C, folic acid, magnesium, phosphorous, zinc, and manganese and include high national fiber. It is loaded with iron, has double the amount of beta-carotene to broccoli, double the amount of calcium to spinach and the the double the amount of potassium to banana. It consists of about 88.42% water, 3.2% protein content, 5.62% total ash content and ascorbic acid is11.1g/1000g. the quantity of reducing sugar, non reducing sugar and total sugar is 3.45%, 0.35 and 3.75% respectively (Kulkarni et al, 2005). Bitter gourd or karalla is a popular vegetable Indo-Pakistan subcontinent. In India, bitter gourd stored in the the dried form. The indigenous practice is to dry the sliced vegetable in sun during October-November and used up to the month of May. However, the quality of the dried product is rather poor. Drying is an important unit operation in post harvest processing of agriculture produce and dates back to the beginning of civilization. The purpose of drying is to remove moisture to a certain level which is good enough to avoid microbial growth and slow down action of enzymes, leading to an extension of self-life while maintaining product quality. Characterization of drying is of paramount importance as it determines drying time and control measure can be taken to obtain an energy efficient process that produces quality product. Several researchers have used Page’s model, exponential model and logarithmic model to characterize the drying process.

Drying food makes more concentrated in from then food preserved in other ways. They are less costly to produce, store and transport then canned or preserved foods. However, blanching of vegetable prior to drying is requiring protecting their color. Texture and nutrients and to inactive harmful enzymes. Lal et, al 1986, reported blanching of bitter gourd in boiling water for 7 to 8 minutes and dried at 66° to 71°C for 7 to 9 hours Blanching has several advantage as it reducing drying time; inactivates the enzymes’ that bring undesirable change in food product, expulse air from the tissue and better retain minerals and acid. However no systematic work has been reported regarding drying of bitter gourd and various drying parameters affecting its drying characteristics and quality attributes. Keeping in view the above mentioned fact and figures, the present study was undertaken with the objectives to study the effect of various parameter (temperature, air velocity and sample weight) on drying kinetics of green bitter gourd slice, to evaluate models (Page’s, Exponential and Logarithmic) for drying data, to study quality characteristic of dried product and to optimize the process parameter of dried product.

2.REVIEW OF PAST RESEARCH

The available literature on green bitter gourd, its nutritional and therapeutic importance, its processing, drying concept, various studies on drying and effect of various factor on hot air drying of bitter gourd. In this chapter an attempt has been made to review the literature of the past research work relevant to study, which is presented under section as follows.

The Moinordica charontja L., Cucurbita Ceae, is known variously as bitter gourd, balsam pear, bitter melon, bitter cucumber, and African cucumber. Although it has many culinary uses, especially in south, southeast and east Asia, it is also grown as an ornamental and is used extensively in folk medicine. Fruits, flowers, and young shoots are also used as flavoring agents in various Asian dishes. Young Momordica shoots and leaves are also cooked and eaten as leafy vegetables, and leaf and fruit extracts are used in the preparation of tea . Unlike other cucurbitaceous vegetables, the bitter fruit flavor of M. chorantia is considered desirable for consumption, and 'thus bitter flavor has been selected during domestication.The bitterness of most cucurbits is mainly due to cucurbitacins . The bitterness of bitter gourd is due to the cucurbitacin like alkaloid momordicine and triterpene glycosides (momordicoside K and L ). These compounds lack the oxygen function at C-il that characterizes "true" and are the bitterest compounds in the plant kingdom.

Most of the cucurbitaceous vegetables including bittergourd are usually cultivated in relatively small areas for local consumption and do not enter the production statistics in a significant way. The genus Momordica comprises nearly 23 species in Africa alone. Region of eastern Indian and southern China are the centers of domestication. The crop is extensively grown in China, Japan, India, Malaysia, South East Asia, tropical Africa and South and North America. Nevertheless in India, it is estimated that the gourds group of vegetables occupy an area of 1,16,939 hectares with production of 14,28 296 t having productivity 0f 12.21 t/ha. Tamil Nadu, Uttar Pradesh, Maharashtra, Kerala and Karnataka are the important bitter gourd producing states (Anonymous 2001).

Bitter gourd fruits are a good source of carbohydrates, proteins, vitamins, and minerals.

Table 2.1 Nutritive value of bitter gourd

(Source: Gopalan et al., (1985). Nutritive values of Indian foods.National Institute of Nutrition, ICMR, Hyderabad.)

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Bitter gourd has been used for centuries in the ancient traditional medicine of India, China, Africa, and Latin America. Bitter gourd extracts possess antioxidant, antimicrobial, antiviral, antihepatotoxic and antiulcerogenic properties while also having the ability to lower blood sugar (Wealth of India 1976). These medical activities are attributed to an array of biologically active plant chemicals, including t.riterpenes, pisteins and steroids. Ethno-medical reports of M. churuntici indicate that it is used in folkloric medicine for treatment of various ulcers, diabetes, and infections. While the root decoctions have abortifacient properties, leaf and stem decoctions are used in treatment of dysentery, rheumatism, and gout . In addition, juice of M. charantia drawn directly from fruit traditionally has been used for medicinal purposes world- wide. Likewise, the extracted juice from leaf, fruit and even whole plant are routinely used for treatment of wounds, infections, parasites (e.g., worms), measles, hepatitis, and fevers..

Drying is an industrial preservation method in which moisture content of the fruits and vegetables is decreased by heated air to minimize biochemical, chemical and microbiological deterioration (Doymaz and Pala, 2003). Drying is a process of simultaneous heat and moisture transfer. It is a process in which water is removed to halt or slow sown the growth of spoilage microorganisms as well as the occurrence of chemical reaction. Drying is usually defined as the removal of moisture until equilibrium with the environment, while the removal of moisture to a very low moisture content, nearly bone-dry condition is called dehydration (Stuchly and Stuchly, 1983).

Fluidized bed drying is an advanced drying method which is comparatively better and produce better quality product than obtained by conventional hot air drying method. While. Fluidized bed drying is well-established method in chemical industry; it is yet to find wide application in food Filka and canudas (1970) and white.(1983). The technique has not been investigated adequately to develop rational design of industrial fluid bed dryer. The published information on the technique with specific application in the food industry is scant and scattered. Earlier reviews are those of Venecek et al. (1965) and Romanovs (1972) on fluid bed dryer and their design and Gupta and maumdar (1983) on recent development in fluidized bed drying and has discussed the theory of drying in fluidized bed drying including details of incipient fluidization gas velocity, two phase theory of fluidization, selection of gas velocity and heat transfer in fluidized particles.

The diffusion models are often too complex in practical application as well as time consuming. The simplified rational equations do not describe the drying process of biological materials accurately, over the entire moisture content range encountered in the drying process. This is due to simplicity of the boundary conditions, the assumptions of constant diffusivity and irregularities in shape and size of biological materials. To overcome the problems associated with the rational models, empirical thin layer drying equations have been an era of major interest in dryer designing and computer simulations.

For thin layer drying, the equation representing movement of the moisture during the falling rate period is based on Newton’s Law which refers to heating or cooling of solids, as suggested by (Huckill, 1947). A number of investigators, (Henderson and Pabis, 1961; Kanawade, 1990; Mishra, 1991; Burande, 1992 and Faisal, 2003) have applied the above relationship for describing thin layer behaviour of food grains as well as vegetables.

The drying is a simultaneous heat and mass transfer process. The drying process depends on compositional and non compositional factors of the material. Composition of material affects the thermo-physical properties which play an important role in heat and mass transfer in the matter. Non-compositional factors, namely, drying air temperature, air velocity, sample size, pre-treatment also affect the drying process. The factors which are responsible to influence the drying are discussed as below.

Thermal conductivity, specific heat and density depend on the chemical composition and physical structure of the material. Therefore, these properties vary from material to material. Moreover, they also change during drying process as the composition of solid matrix-change. It is reported that the carrot pieces took 5 h as against 7 h for potato under same drying conditions and initial dimensions of the sample to reach a final moisture content of 0.06 (Van Arsdel, 1963).

Size and shape of sample has a definite effect on dehydration characteristics of material. But, generally sample sizes are selected on the basis of end use of product after further processing.

Sahu (2007) studied the effect of loading density (2.5, 3.0 and 3.5 kg/m2) on the product quality and found that 3.5 kg/m2 loading density was most acceptable in terms of chlorophyll, ascorbic acid, rehydration characteristics, sensory evaluation and storage.

Silva and Almeida (2008) studied the drying kinetics of coriander and found that leaves dried earlier as compare to leaves with stem, and were acceptable in terms of colour and quality.

Most of the research workers (Kaur et al., 2006; and Sagar et al., 2006) have taken the sample size range from 0.60 to 2.5 kg to study the dehydration characteristics of green leaves.

In case of agricultural products, the pretreatments are necessary before processing in order to retain colour, inactivate enzymes and/or enhance rate process. The ultimate aim of pretreatment is to improve quality of final product and reduce processing cost. The pretreatments differ from product to product.

Shrivastav and Nath (1985) studied the drying of fresh and brined cauliflower. They blanched the cauliflower cloves, snow-ball for 6 min in boiling water, 5% NaCl and 0.05% citric acid, O.25% EDTA and 1% citric acid, 0.75% sodium metabisulphite and 0.25% sodium sulphite. They observed that dried product obtained from raw cauliflower was very dark and poor rehydration ratio. Blanching alone in water did not improve colour of dried product. However, they observed that use of 0.75% sodium metabisulphite and 0.25% sodium sulphite mixture reduces the enzymatic browning to 0.3372.

Pawar et al. (1988) subjected 2-3 mm thick onion slices to a pretreatment of 0.2 and 0.25% KMS for 5 min prior to solar drying and air drying. They reported that sulphitation with 0.25% KMS gave the best product in terms of firmness as well retention of color and pungency. They also stated that sulphitation increased energy absorption by the product leading to the elevated product temperature and shorter drying period, exhibited less browning compared to the corresponding controls, retarded the loss of ascorbic acid by preventing its oxidation, exhibited higher rehydration ratio than the control, increased drying ratio, decreased slightly total content in the final product than in control.

Negi and Roy (2001) subjected the leaves of savoy beet, amaranth and fenugreek to different blanching and drying treatment to establish the retention of β-carotene, ascorbic acid and chlorophyll. Water followed by potassium metabisulphite (KMS) dip was found most suitable for blanching and selected for subsequent drying and concluded that low temperature dryer had least drastic effect on β-carotene, ascorbic acid and chlorophyll content of the processed product.

Ahmed et al. (2006) studied drying characteristics and quality attributes of coriander leaves at selected drying air temperatures. Water blanching at 80oC for three minutes resulted in greater chlorophyll content and rehydration capacity were found to be maximum when the blanched leaves were dried at 45oC.

Pandre et al., (2011) studied the drying kinetics of fenugreek under different blanching and drying treatment. The samples were pre-treated with steaming, 0.1% sodium bicarbonate, 0.5% citric acid and 0.5% sodium metabisulphate. The pre-treatment of steaming, 0.1% sodium bicarbonate and 0.5% citric acid showed moderate rate for moisture removal. But 0.5% sodium metabisulphate respond more better to evaporation of moisture compared to all other samples while least moisture removal was observed in control sample i.e. without any pre-treatment. However, they observed that treatment with 0.5% sodium meta bi-sulphate is ideal for solar drying at 55oC.

Blanching of fruits and vegetables is principally followed to inactivate the enzymes responsible for enzymatic and oxidative browning. Hot water, steam and chemical blanching is done. The loss of nutrients takes place during blanching.

Blanching also improved the rehydration ratio, color and other organoleptic properties (Kanwade, 1990). Blanched material dries more rapidly than unbalanced at least on the high moisture range (Van Arsdel, 1963).

The drying process greatly depends on the tempe­rature used for drying of agricultural products. Various studies revealed that the increase in drying air temperature increased drying rate significantly and thus reduced drying time. However, the temperature could be increased to a certain extent beyond which deterioration in quality took place.

Kanwade, 1990 carried out air drying of button mushroom at 24 to 800C with pre-treatments of chlorine dipping of whole mushroom and sulphiting of different degree to cut mushrooms. They concluded that the best product ( in terms of flavor ,color ,storage stability and bacterial population) was obtained when pretreated mushrooms were dried in two stages , using lower temperature of 24 to 320C at first stage and higher temperature of 770C in the shorter finishing stage.

Many research workers studied the effect of drying air velocity on drying time. In general, at higher velocities higher drying rates were obtained which reduced the drying time.

Mulet et al. (1987) studied the effect of air flow rate on carrot drying. The influence of air flow rate on the kinetics of drying 10 mm carrot cubes is presented. For this geometry kinetic equations are available, for the falling rate drying period. Drying air flow rates of 1000, 2000, 2500, 3000, 4000, 5000, 6000, 8000 and 9000 kg/m2h were employed. It was found that for flow rates above 6000 kg/m2h the value of D/r2 remained almost constant, thus indicating that when the air flow rate was higher it had no influence on the drying rate.

Demirel and Turhan (2003) during the study of drying of banana in cabinet dryer had reported that the mass transfer resistance on banana surface did not control the drying at average air velocity 3.3 m/sec.

Generally, drying of foods is characterized by two separate phases: the constant rate and falling rate periods. For a high moisture food, prior to drying, the surface of the food is saturated with water. The drying rate is thus constant for drying period of time until the migration of moisture to the surface is not sufficient to keep it in a saturated state, assuming the composition of the drying air does not change. The constant rate period ends and the moisture content at this point is referred to as the critical moisture content. The falling rate period then starts, and the drying rate falls monotonically to the end of the process. The drying rate during the falling rate period is caused by the concentration gradient of moisture inside the food matrix. The internal moisture movement results from a number of mechanisms such as liquid diffusion, capillary flow, flows due to shrinkage and pressure gradients (Lyderson, 1983).

Monroy et al. (1986) conducted studies on drying of olive and grape pomacce constituents on a fluidized bed system based on their aerodynamics behavior. Separation kinetics was explained trough an empirical modification of the lava equation. They proposed that during the first stage of operation particals entrainment is controlled by the drying rate and during second stage by mechanical effect. Laboratory experiment were conduct to obtained to data on equilibrium moisture content . thin layer drying equation were devloped for deffated olive flesh and grape seed.

Hawldar et al. (1991) studied the drying characteristics of tomatoes under various operating conditions. Experiments were conducted with different temperature (40, 50, 60, 70 and 800C) and air flow velocities (0.4, 0.7, 1.0, 1.4 and 1.5 m/s) to determine the drying characteristics of tomato shrinkage was observed and the effect was taken into account in the basic diffusion model through the use of power low expression that related experimental data yielded co-relations between the temperature and air velocity. An equation was also developed to estimate the drying time to reach particular moisture level.

Mishra (1991) studied the drying behavior of Potato cubes at various air drying temperature range of 40-800C and air velocity range of 100 to 180 m/min. On the basis of experimental results, he concluded that Potato cubes should be dried at air temperature of 700C or higher to obtain final moisture content below 12% (db). Page’s model can predict drying behavior of Potato cubes with moderate errors, whereas, exponential model results in large prediction errors.

Hassan and Hobani (2000) studied thin layer drying rates for dates at different temperature (70, 80 and 900C) and evaluated for tree drying models namely, exponential, page’s and diffusion models. They concluded that page’s models fitted the data best

Faisal (2003) studied the drying behavior of cauliflower under single layer drying condition in relation to drying air temperatures ranging from 43 to 760C and air velocities 0.47 to 6.5 m/s maintaining sample size from 0.5 to 2.5 kg using flow through hot air dryer. On the basis of experimental results, he concluded that the cauliflower should be dried at air temperature of 600C, 3.5 m/s air velocity and 1.7 kg sample size for both pretreated samples.

Mudgal (2009) studied the drying kinetics of bittergourd (Momordica charantia L) slices were i) blanched for 3 min in boiling water, ii) blanched for 3 min in 5% salt solution, and iii) dipped in 15% salt solution for 30 min. The samples dehydrated in mechanical tray dryer were found to be more acceptable as compared to sun dried and solar dried products. Exhaustive dehydration trails were conducted using mechanical tray dryer in three replications at 60, 70, 80 and 90̊ C drying air temperatures. The dried bittergourd slices were packed in different types of containers for predicting their shelf life. Bittergourd slices blanched in boiling water were of the best quality followed by 5% salt solution at drying temperature of 60 and 70̊ C. The samples blanched in boiling water and dehydrated at 60̊ C were observed most acceptable in terms of rehydration property and nutritional quality, It also proved to be superior in colour retention up to 60 days of storage. The result showed that the rehydrated bittergourd slices could very well be utilized for substituting the fresh product during off-season.

. Kulkarni et al (2005) concluded that dehydrated bitter gourd rings with pretreatment (brine blanched) i.e. 5% salt solution and 90̊C for 2 minutes was found to be the best for preparation as its improved color and soft texture, even though it reduced bitterness and impart salty taste. This pretreatment has got better sensory quality as compared to all other pretreatments.

Lidhoo and khar (2007) dried the bitter gourd slices after scraping and blanching using heated air at 50, 60, 70, 80 and 90̊C to about 5% moisture content. They have found that minimum discoloration of dried bitter gourd slices occurred at drying air temperature of 60̊C. Discoloration distinctly intensified when bitter gourd was dried at temperature below or above 60̊C.

Jadav (2010) studied the solar cabinet drying of bitter gourd slices (6-7 mm thick with two variables, viz., blanching time (1-5 min) and potassium metabisulfite (KMS) (0.2-0.5 %). After the pretreatments, the bitter gourd slices were dried up to 6 % (Approx.) moisture content (d.b) in solar cabinet dryer at air temperature, relative humidity and air velocity in the range 38 to 62°C, 45 to 55 % and 0.9 to 1 m/s, respectively. The responses taken were chlorophyll retention (mg/100g of dried sample) and texture (hardness) of dehydrated bitter gourd slices. The optimum process condition was found at 4.24 min blanching time and 0.49 % KMS which resulted in 20.15 mg/100g chlorophyll retention and 340.55 g hardness. Some selected quality attributes of solar cabinet dried bitter gourd slices pretreated with the optimized combination of blanching time and KMS were compared with the quality of freeze and open sun dried samples and it was found that quality of solar cabinet dried product was falling in between open sun and freeze dried products. Also, the drying data of bitter gourd slices obtained during solar cabinet, freeze and open sun drying were analyzed and drying constants as well as moisture diffusivity were calculated.

Hiremath (2010) investigated simple technologies for drying of bitter gourd that can be adopted by the farmers at field level. The results of the study indicated that osmotic dehydrated. The results of the study indicated that osmotic dehydrated products dried under electric drier were found acceptable. Maximum recovery of 11.884 per cent, maximum rehydration ratio of 5.762, reconstitute ability ratio of 0.683 were recorded in slices treated with three per cent brine + 0.1 per cent potassium metabisulphite, while highest chlorophyll content of 20.28 mg per 100 g was observed in slices treated with 0.5 per cent potassium metabisulphite, whereas minimum time for drying was recorded in untreated control.

For the evaluation of colour grades, RGB index is utilized. Though, Photoshop can display colour in 3 different models: RGB (red, green, blue), HSI (Hue, Sturation, Intensity) and L, a, b. RGB is used for displaying colours on computer monitors, HSI is suitable for finding the ripeness of fruits and vegetables and L, a, b is used in measuring colours in food research.

L, a, b is an international standard for colour measurements adopted by the commission international d’ Eclairage (CIE) in 1976. This colour model is designed to use to create or output the image, L is the luminance or lightness component, which ranges from 0-100. a (from green to red) and b (from blue to yellow) are the two chromatic components, which range from ±120. The principles of colour measurement can be found elsewhere, and there are other colour models used in food research.

Spyridone et al. (2000) developed a versatile and inexpensive technique for measuring the colour of foods. A combination of digital camera, computer and graphics software provides a less expensive and more versatile way to determine colour of food surfaces than traditional colour measuring instruments. They used the Adobe Photoshop software to analyze the image for colour of the bottom surface of micro waved pizza. The software can display colour in L, a, b the preferred colour model for food research. They found that this technique is versatile affordable easy to use and can be applied to many other food besides pizza.

Nisha et al. (2004) worked on the effect of salt on the degradation of visual green colour in spinach puree over a temperature range of 50-120oC as well as under conditions of normal pan cooking, pressure cooking and a newly developed and patented fuel efficient eco-cooker. The temperature dependence of degradation was adequately modelled by Arrhenius equation. The results obtained indicate a protective effect of salt on the degradation of visual green colour.

Ahmed et al. (2006) investigated the effect of selected pre-treatments on drying characteristics and colour of green chilli. Blanching was carried out in hot water at 95oC for 5 min. Pre-treatments resulted in increased drying rate, while pre-treating blanched chilli in 1% lye solution containing 0.25% magnesium carbonate retained maximum colour of the product. The drying kinetics revealed that drying took place under the falling rate period and Page’s model described well the drying behaviour of green chilli.

Tan and Francis (2006) studied the effect of processing temperature on pigments and colour of spinach. The method, developed for calorimetric studies, involved extraction of the spinach with acetone and chromatographic separation on a sugar-starch (70:30) column. The method gave recovery values of 95-98% for a wide range of pigment mixtures, and a coefficient of variation of approximately 1.5% for reproducibility on the same extract. The major reasons for the change in colour of the spinach puree upon processing were, first the degradation of chlorophyll-a to pheophytin-a, and second the degradation of chlorophyll-b to pheophytin-b.

Usuba et al. (2008) in this research work colour evaluation of pupae samples was determined using a spectrophotometer which measures three parameters: lightness (L), redness (a) and yellowness (b). In terms of desired silkworm pupae colour properties higher (L), higher (a) and higher (b) are preferred.

Lakshmi and Vimla (2000) worked on green leafy vegetables. Amaranth, curry leaves, gogu and mint leaves were blanched before drying after establishing the conditions for blanching including blanching time, temperature and blanching treatment solution. Drying was done by sun and cabinet-drying. The nutritive values of the powders were determined. It was found that in spite of considerable losses in vitamins, green leafy vegetable powders retained good amounts of proteins, fiber and minerals (Ca, Mg and Fe) and fair amounts of ascorbic acid and β-carotene.

Gupta et al. (2008) studied the effect of different blanching treatment on retention of ascorbic acid of green leafy vegetables. Spinach were blanched for 1, 2, 4 min at 80, 90 and 98oC in water and chemical media, steamed for 5 and 10 min with and without chemical treatment and microwaved for 1 and 1.5 min. Retention of ascorbic acid was reduced as time and temperature of blanching increased. Stem blanched for 5 min had a higher level of ascorbic acid than conventional blanched samples. Chemical blanching was proved to be advantageous.

The rehydration characteristic of a dried product is widely used as a quality index. Rehydration is a complex process and indicates the physical and chemical change caused by drying and treatment preceding dehydration (Lewicki, 1998).

Srivastava (1985) found that incorporation of sugar in blanched pieces considerably improved rehydration characteristic of dehydrated cauliflower with coefficient of rehydration (79.9%).

Maskan (2000) examined the effect on rehydration charactritistic of different drying regimes (convection (60̊ C at 1.45 m/s); microwave (350,490 and 7000 W power) and convection followed by microwave (at 350 W, 4.3 mm thick sample) finish drying. They observed that, the rehydration of sample dried with microwave finish method had the highest value (about 79 kg water absorbed/100kg of dried sample)

Kar et al. (2003) studied the effect of different drying methods (microwave assisted convective drying, microwave for finishing drying, convective drying, freeze drying, microwave drying) on rehydration characteristic of dried banana slices. Rehydration ration varied from 1.693 for microwave drying to 3.416 for freeze drying. The rehydration ration for freeze drying was, however, significantly different from those for the other method for drying. It terms of percentage regain in initial weight is only 36.8% for microwave drying. 74% for microwave assisted convection drying and 95% for freeze drying.

Jayaraman et al. (2006) reported that short time high temperature drying treatment in vegetables improves the rehydration characteristic, namely, as rehydration ration coefficient of rehydration. There was also a reduction in drying time and reconstitution time for such a treatment.

Based on the literature search, it can be remarked that some attempts have been made to develop dried bitter gourd slices. However, very limited work was reported on optimization of process parameter of hot air drying of bitter gourd slices. Drying process can be modelled using empirical equation. Numerical optimization technique can be used for optimizing process parameters for drying process.

3.EXPERIMENTAL PROCEDURE

Experiment were conducted to study the drying kinetics of green bitter gourd slices at five level of temperature five level of sample weight and three level of air velocity. Bitter gourd slice were dried in hot air dryer. Moisture loss data were recorded periodically at an interval of five minutes for the first forty minutes and consequently fifteen minutes intervals. The data were analyzed to study drying characteristics of a bitter gourd. Each experiment was replicated three times. After drying to safe moisture content about till the constant weight of dried bitter gourd was packed in double polythene bag.

The dark green bitter gourd slices (Plate 3.1) without any defect like uniform medium size and dark green color free from infestation, blemish, dirt or mud were procured from the local market of the Pantnagar (U.S. nagar), Uttarakhand for the proposed study. Uniformity of the raw material was maintained on the basis of the uniform size (2 mm diameter), and shape.

The fresh bitter gourd of uniform size was sorted out for the experiments. They were washed thoroughly under tap water. Trimming of pointed ends was done by using stainless steel knife. After trimming the pointed ends, than bitter gourd were cut into slice thickness of 2 mm with the help of hand slicer.

The equipments were TNAU fluidized bed dryer. The list and specification of these equipment and the apparatus used are given in Table 3.1.

Slices were treated with Brine blanched i.e. 5 percent Na Cl at 90̊C for 2 min. was found most effective for preparation of dehydrated bitter gourd rings as it improves its color ,texture and also most effective in reducing bitterness and impart salty taste. Based on the past review the standardization of pretreatment was carried out.

Sample weight, drying temperatures and air velocity were taken as independent variables, whereas proximate composition such as moisture content, ash content, rehydration characteristics and color considered as a dependent variables. The full factorial experimental design were adopted. The experimental plan is summarized in the Table 3.2 and 3.3.

The dryer used in the study was developed by the Department of Agricultural processing, TNAU. The dryer utilized as fluidized bed dryer. The dryer consist of a centrifugal blower, holding bin, heating coils, motor and thermostat control. The blower was centrifugal type with a capacity of 32m3/min, run by a 3 hp, 3Φ and 1400 rpm motor through a belt drive system. A sliding shutter was provided at the suction end to control the flow rate of air. The delivery-end of the blower was connected to heater drum. The heater drum was made of mild steel sheets to a diameter of 38 cm and of length of 121 cm. Four number of fin type electrical heaters of 90 cm length were placed inside the drum and connected to main supply through a stem type thermostat. To the other end of heater drum a bend to a height of 60 cm was provided by a 33 cm diameter duct at the top, the drying chamber was placed. The drying chamber of 30 cm diameter and 30 cm height was made of stainless steel. The thermostat is placed at the discharge end of heater drum to sense the temperature of hot air and connected to the main supply to control the power supply to the heater coils. Hot air of 50-750C temperature at a flow rate of 9 to 32m3/min could be obtained in the dryer. The whole assembly was mounted on a suitable frame made of mild steel.

Table 3.1 Specification of experimental equipment/ apparatus

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