The Drying Characteristics of Amaranth Leaves under Greenhouse Type Solar Dryer and Open

Ph. D. Scholar, Department of Agricultural Engineering and Food Technology, College of Agriculture, S.V.P. University of Agriculture & Technology, Meerut-250110 (U.P.) INDIA. Professor & Ex-HOD, Department of Agricultural Engineering and Food Technology, College of Agriculture, S.V.P. University of Agriculture & Technology, Meerut-250110 (U.P.) INDIA. Assistant Professor, Department of Agricultural Engineering and Food Technology, College of Agriculture, S.V.P. University of Agriculture & Technology, Meerut-250110 (U.P.) INDIA. Division of Farm Machinery and Post Harvest Technology, Indian Grassland and Fodder Research Institute, Jhansi (U.P.).


INTRODUCTION
Amaranthus plant is a popularly grown leaf vegetable in tropical and subtropical regions of the world including Africa, India, Bangladesh, Sri Lanka and the Caribbean.It is also grown as leaf vegetable through South-East Asia and Latin America.Leafy vegetable have gained commercial importance and form an essential part of diet, providing vitamins and micronutrients.As a result of their high moisture and short shelf life, there is the need to process them (Driscoll, 2004) that can store for longer periods (Jayaraman and Das Gupta, 2006) and so are available all year round (Mudambi et al., 2006).Drying present one of the most effective methods of food preservation.Grain amaranth is produced commercially in the United States in wet and dry areas.Potential advantage of the amaranthus is the chemical composition which is source of plant protein and rich source of vitamins and minerals.The world shortage of animal protein particularly in developing countries in Africa has necessitated investigations of several novel nutritional materials for possible incorporation in to animal feeds (particularly poultry) as replacements for the expensive conventional sources such as fish meal, groundnut cake and soybeans (Akonor, 2012).In Uttar Pradesh, fresh amaranth leaves are abundantly available during summer and rainy seasons but has very short shelf life even under refrigerated conditions.This leads to a market scarcity and a sharp rise in price during lean period.Fresh amaranth leaves are perishable in nature and require immediate processing or preservation.During peak period, a considerable amount of the produce is wasted due to lack of proper post harvest processing techniques.Therefore, there is a need for dehydrated leafy vegetables so that their availability to the consumers can be extended during off season.
Limited scientific information on drying of leafy vegetables under solar dryers are available.As we know that the degree of greenness is important in determining the final quality of thermally processed green vegetables which gets their colour from chlorophyll pigments.The degradation of naturally occurring colour pigments in the food during thermal processing is a major problem.In all green vegetables , the change in colour from bright green to dull olive -green or olive -green colour is due to the conversion of chlorophyll to their respective pheophytins and further breakdown of products such as pheophorbides and chlorines, which is undesirable to the consumer (Schwartz et al., 1983).Although with the advent of high temperature short time processing methods, it has become possible to reduce the destruction of pigment in processing steps (Tan and Francis, 1962).Hence, medium temperature drying is much suitable to achieve the quality of dehydrated product.The drying is influenced by processing conditions, sample treatment and composition (Ahmed et al., 2001).

MATERIALS AND METHODS
This study was conducted in Meerut district during the year 2012-2013.The levels of each variables were selected on the basis of the earlier research work and trial experiments.Chemical treatments were carried out by dipping the sample for 15 min in the solution of 0.1% magnesium chloride, 0.1% sodium bicarbonate and 2% potassium metabisulphite for 15 min in water at room temperature.Here, the ratio of amaranth leaves to pretreatment mixture was taken as 1:5 (w/w) (Kaur et al. 2006).Blanching in boiling water in the ratio of 1:5 (leaves:water) containing 3% sodium bisulphate as pretreatment for 2 min was done.Untreated amaranth leaves were dried as control samples.Pretreated samples were exposed to three levels of loading density viz; 1.5, 2.0 and 2.5 kg/m 2 .Then the leaves were weighed and loaded in perforated stainless steel tray and subjected to thin layer drying under greenhouse type solar dryer and in the open sun.Experiments were also conducted to study the effect of drying conditions on quality viz; sensory characteristics and rehydration characteristics.

RESULTS AND DISCUSSION
Experiments were also conducted for control samples (untreated) of amaranth leaves.The samples were dried from initial moisture content of 655.35 to 727.12% db to a final moisture content of 5.381 to 5.79% db.Drying started at 10:00 AM and terminated at 6:00 PM.The loaded trays were weighted at every 30 minutes until the end of the drying period.After 6:00 PM, the samples were collected and kept in air tight plastic covers to induce uniform moisture distribution in amaranth leaves.They were spread again on the trays in the next morning and the process was repeated until the final moisture content was reached.Weighing of the loaded tray took 15 sec for each sample while removing the tray from the greenhouse type solar dryer as well as from open sun.Weight of the samples was taken at an interval of 30 minutes during the experiment.The weight loss was converted in corresponding moisture loss for calculating the moisture (d.b).The calculated moisture content (d.b) was used as basic data for further analysis.In order to describe the thin layer drying of amaranth leaves, temperature and relative humidity were measured in the ambient air and the greenhouse type solar dryer.
Rehydration characteristics of dehydrated samples were studied in terms of rehydration ratio for various drying condition.The effect of drying methods (greenhouse type solar dryer and open sun drying), loading density and pretreatments on rehydration characteristics were studied.The sensory evaluation for overall acceptability was also studied to optimize the drying conditions.The results obtained are discussed below:

Drying characteristics
The moisture losses after every 30 min were considered as base data for the determination of moisture content, moisture ratio and drying rate.The experimental data of the drying behaviour of amaranth leaves in relation to moisture content, moisture ratio and drying rate are given in table 1.The samples were dried up to the final moisture content level of 5.381 to 5.79%.ANOVA (balanced design) was done to examine the effect of drying condition.The moisture content moisture ratio and drying rate at different drying time and for the treated and untreated samples were compared under selected drying methods.

Moisture content
The data for moisture content is given in table 1. which shows that for all experiments, the initial moisture content prior to drying were observed in the range of 655.35 to 727.12% (d.b).The treated and untreated samples were subjected to drying methods and loading density.The change in moisture content with drying time for treated and untreated sample are shown in Fig. 1 to 6, exhibiting a non linear decrease of moisture with drying time.Drying of amaranth leaves shows a sample heat up period where there was little drying.This was due to low temperatures at the beginning of the drying process.Fig. 1 to 6 indicate a very short constant drying rate period (30-60 min) as was observed in the drying of most perishable food products (Hallstrom et al., 2006) and a long falling rate period.Moisture decreased rapidly as drying chamber warmed up, and then slowed down considerably.Initially, material surface was saturated with water and therefore, with increase in air temperature, faster drying took place after 1 hour and upto 150 min.In the falling rate period, the material surface was no longer saturated with water and drying was controlled by diffusion of moisture from the interior of the material to the surface.For example, table 1. shows the drying behavior of amaranth leaves under greenhouse type solar dryer ( GSD ) and loading density of 1.5 kg/m 2 for chemically treated samples.As expected, the drying time varied with drying method and loading density.The drying time ranged from 450 min (chemically treated at 1.5 and 2.0 kg/m 2 under greenhouse type solar dryer) to 660 min (boiling water treated 2.5 kg/m 2 under open sun), being generally lower at higher drying temperatures.It was observed that drying time increases with loading density e.g.under open sun drying (540 min at 1.5 kg/m 2 , 570 min at 2.0 kg/m 2 and 630 min at 2.5 kg/m 2 for chemically treated samples).It was observed in case of greenhouse type solar dryer that chemically treated samples took lesser time (450 min at 1.5 kg/m 2 ) as compared to untreated samples (480 min at 1.5 kg/m 2 ).Similar trend was also observed in case of open sun drying.The final moisture content varied from 5.381% to 5.79% (d.b).The decrease in drying rate with the period of drying was non-linear.It was observed that the drying rate was higher at higher temperature (i.e.under greenhouse type solar dryer), as normally expected.

Overall drying rate
The overall rate of drying was calculated using the formula mentioned under section.Overall drying rate varied from 1.043 to 1.526% d.b. /min (Table 1.) for the total range of variable under study.Normally, it can be expected that the overall drying rate should be higher at higher temperature from 30 0 C (open sun) to 44 0 C (greenhouse type solar dryer), which is reflected in the result present in table 1. Overall drying rate increased with increase in average temperature.It is seen that the overall drying rate decreased with increase in loading density in each drying methods.
It is also seen that the overall drying rate was slightly lower for boiling water blanched and untreated sample than for chemically treated at almost all experimental samples.From an examination of data in Table 1., it is obvious that the amaranth treated chemically, dried faster than the others, as chemicals used in chemical treatment resulted into more expansion of amaranth pores caused faster heat and mass transfer between amaranth surface and air, and therefore, increased drying rate.
The ANOVA was carried out for analyzing effect of drying method, loading density and treatment on overall drying rate and it was found that the effect of both drying method and loading density was significant at 5% level of significant as Fc is more than Ftab and the effect of treatment was insignificant at 5% level of significance.ANOVA is present in Table 2.

SUMMARY AND CONCLOSIONS
Thermal performance evaluation of greenhouse type solar dryer revealed that the temperature inside the dryer goes maximum upto 52 0 C during the day time, and it is 18 0 C more than that of ambient.The temperature range of 31 0 C to 52 0 C was observed from 10AM to 6PM under the greenhouse type solar dryer, which is sufficient for the drying of vegetables.The ambient temperature was varied within 24 0 C to 34 0 C during day time.Average drying temperature in the dryer and in the open sun was 44 0 C and 30 0 C, respectively whereas the average relative humidity was about 37% and 42% respectively.The average relative humidity inside the dryer was found lesser due to ventilation.Because of the lower value of relative humidity inside the dryer than that of the ambient air for most of the day, the air in the dryer has significantly higher drying potentials than the ambient air.Total drying time considerably reduced with the increase in drying air temperature from 30 0 C under open sun drying to 44 0 C under greenhouse type solar dryer.Chemically treated samples dried under greenhouse type solar dryer took lesser drying time (7. 5 to 8 hrs) than blanched and untreated samples under both the drying methods.The drying time for chemically dipped samples dried under greenhouse solar dryer was about 1 hrs shorter than that of the blanched samples dried under greenhouse type solar dryer.While drying time for chemically dipped samples dried under greenhouse type solar dryer was about 3 hrs shorter than that of the blanched samples dried under open sun.On the basis of experimental results and data analysis the following conclusion could be drawn: Drying rate curves revealed the preserve of a short constant rate period followed by more falling rate period.Drying rate should a positive correlation with temperature while it showed a deceasing trend with increasing loading density.The average drying rate increased with increase in temperature and decreased with increase in time and loading density.Major drying took place in falling rate period except some accelerating period initially.Total drying time considerably reduced with the increase in drying air temperature from 30 0 C under open sun drying to 44 0 C under greenhouse type solar drying conditions.Average drying temperature in the greenhouse type solar dryer and in the open sun were 44 0 C and 30 0 C, respectively whereas the relative humidity was about 37% and 42% respectively.Chemically treated samples dried under greenhouse type solar dryer took average drying time of 7.666 hrs which was 3 hrs lesser than drying time of blanched samples under open sun drying.The chemically blanched samples took less time in dehydration as compared to blanched and untreated leaves.The initial moisture contents of amaranth leaves were found in the range between 655.35 -727.12% (d.b).The amaranth samples took 450 to 660 minutes drying time depending upon drying temperature, drying methods and pretreatments.It was observed that total moisture loss increased with increase in drying temperature and decrease with decrease in drying temperature.Quality Parameters of the final dehydrated amaranth leaves including final moisture content, Rehydration characteristics and sensory were determined.Final moisture content observed in all the experiments ranged from 5.381to 5.750% (d.b).The rehydration ratio of dried amaranth leaves was more under greenhouse type solar dryer as compared to open sun drying.The rehydration ratio for chemically treated samples were almost more than blanched and untreated samples for both drying conditions except under greenhouse type solar dryer at 2.5 kg/m 2 loading density.The product quality in terms of rehydration characteristics and sensory evaluation was found to be most acceptable when amaranth leaves treated in the solution of 0.1% Mgcl2 + 0.1% NaHCO3 + 2% KMS, were dried under greenhouse type solar dryer.greenhouse type solar dryer of amaranth were better than the traditional open sun in respect in preserving the colour, flavour taste and overall acceptability.

Fig. 1
Fig. 1 Variation of moisture content with time under greenhouse type solar dryer for chemically treated samples