Fresh whole Aloe Vera leaves, of 32 cm to 52 cm of length, were collected from Herbal Garden, Department of Horticulture of Sardar Vallabhbhai Patel University of Agriculture and Technology, Meerut. The refined sugar was selected as the osmotic agent. The Aloe Vera leaves were thoroughly washed under tap water to remove adhering materials, and then the spikes, placed along their margins, were removed before slicing the leaf. The thick epidermis is carefully separated from the parenchyma using a stainless steel cutter. The fillets were cut into 2.5×2.5×2.5 cm cubes with the help of sharp stainless steel cutter. The cubes were extensively washed with distilled water to remove the exudates from their surfaces. Sugar syrup of desired concentration is prepared by dissolving required amount of sugar in distilled water.

Osmotic dehydration
The prepared samples (Aloe Vera cubes) were weighed for every experiment and immersed in the sugar syrup (43.2, 50, 60, 70, 76.8ºBrix) contained in glass beaker. The beaker were placed inside the constant temperature water bath. The syrup in the beaker was manually stirred at regular intervals to maintain uniform temperature. One beaker was removed from the water bath at designated time and the samples were immediately rinsed in flowing water and placed on tissue paper to remove the surface moisture. The samples were weighed and their moisture contents were determined. Number of experimental runs was decided on the basis of RSM.

Experimental Design
For the optimization of osmotic dehydration by response surface methodology, the experiments were conducted according to Central Composite Rotatable Design (CCRD) with three variables at five levels. The independent variables were osmotic solution temperature, solute concentration, and duration of osmotic dehydration process. The low and high levels of the variables were 50 and 70ºC for osmotic solution temperature, 50 and 70ºBrix for sucrose solution concentration as shown in Table 1. The experiments plan in coded and uncoded form of process variables is given in Table 2.
Table (1):
Independent Process Variable and their Variables.

Table (2):
Experimental Plan for Study of Osmotic Dehydration.

Mass Transfer Kinetics
Water Loss (WL)
Water loss and solid gain will be determined by gravimetric measurement, by assuming that, under the conditions to be use, the solutes initially present in the slices do not diffuse against the solids concentration gradient into the concentrated osmotic solution.

WL %=(initial moisture,g-final moisture,g)/(initial weight of the sample,g) ×100

…(1)

Solid Gain (SG)
The solid gain (SG) will be measured by the following equation as suggested by Lenart & Flink 1984. It is determined by gravimetric measurement.

SG %=(final solids,g-initial solid,g )/(initial weight of the sample,g) ×100

…(2)

Mass Reduction (MR)
The overall exchange in the solid and liquid of the sample do affect the final weight of the sample. The mass reduction can be defined as the net weight loss of the fruit on initial weight basis.

WR=(Wi-W₀)/Wi

…(3)

From equations the solid gain (SG) can be correlated with mass reduction (WR) and water loss (WL) as

SG = WL – WR

Where, WL = water loss (g per 100g mass of sample); SG = solid gain (g per 100g mass of sample); WR = mass reduction; W_è = mass of samples after time è, g; W_i =initial mass of samples, g ; X _è =water content as fraction of mass of samples at time è; X _I = water content as a fraction of initial mass of samples.

Convective Drying
The osmosed and untreated Aloe Vera cubes were dried in conventional dryer. This experiment was carried out in two convective dryers i.e, tray dryer at 50ºC, 60ºC and 70ºC and in hot air dryer at 60ºC, 70ºC and 80ºC.

Mass transport in osmosis
The mass reduction, water loss and solid gain were observed in the range of 57.7026 to 77.5184, 81.9235 to 94.2818, 16.7634 to 24.2209 percent, respectively for 4 hour duration of osmosis at various sugar concentrations, temperature range of osmotic solution and various sugar to gel ratio. The effect of process variables on product responses are shown in Table 3.

Table (3):
Effect of Process Variables on Product Responses.

Mass reduction in osmosis
The mass reduction was different for different condition of syrup temperature and concentration. The mass reduction after osmotic dehydration was found to be in the range of 57.7026 to 77.5184 percent, corresponding to experiments at low level (43.2°Brix, 60°C, 6:1 STGR after 4 hr) and high level (76.8°Brix, 60°C, 6:1 STGR after 4 hr). Mass reduction in osmosis increased with increase in syrup temperature and concentration of solution. While STFR had the negligible effect on mass reduction. At initial stage increasing temperature and sugar concentration of solution raises water loss more than solid gain which causes an increase in mass reduction. Similar results have been reported by Sagar (2001), Eren et al., (2006) and Gracia-Segovia et al., (2010).

Water Loss in Osmosis
The water loss after osmotic dehydration was found to be in the range of 81.9235 to 94.2818 percent, corresponding to studies at low level (43.2°Brix, 60°C, 6:1 STGR after 4 hr) and high level (76.8° Brix, 60°C, 6:1 STGR after 4 hr). The water loss increased with increase in sugar concentration from 43.2 °Brix to 76.8 °Brix. It can be observed that for 60°Brix and 6:1 STGR when syrup temperature increased from 43.2 to 76.8°C, water loss increased from 85.4055 to 91.8976 percent after four hours of osmotic dehydration causing approximately 6.49 percent point increment. Similar results were obtained for 43.2°Brix, 50°Brix, 70°Brix. Water loss in osmosis increased with increase in sugar concentration, this was because an increase in the concentration of sugar solution resulted in osmotic gradient, which increased the driving force for water removal between solution and fruit, and thereby giving higher mass transfer rates. Similar results have been reported by Sagar (2001), Kaleemullah et al., (2002) and Singh et al., (2007).

Solid Gain in Osmosis
The values of solute gain were much lower than the water loss for all the process parameters during osmotic dehydration, because sucrose having larger ionic radius could not diffuse easily through the cell membrane and thus the approach to osmotic equilibrium was achieved primarily by flow of water from cell. It was observed that solid gain goes on decreasing as sugar concentration goes on increasing. The solid gain was 16.7634 % at high level 76.8°Brix, 60 °C and 6:1 STGR after 4 hrs to 24.2209 % at low level 43.2°Brix, 60°C, 6:1 STGR after 4 hrs. The values of solute gain were much lower than the water loss for all the process parameters during osmotic dehydration, because sucrose having larger ionic radius could not diffuse easily through the cell membrane and thus the approach to osmotic equilibrium was achieved primarily by flow of water from cell.

Checking of fitting mode for various responses
The optimization of the osmotic dehydration process was aimed at finding the levels of independent variables, which would give maximum mass reduction, water loss and minimum solid gain. The statistical analysis of the experimental data was performed to observe the effect of various process parameters on measured responses. The results indicate the adequacy of quadratic model for all responses because of high R². The relative effect of each process parameter on individual response was compared from the p-value less than 0.05 indicate model terms are significant. Analysis of variance of fitted model showed that p-value less than 0.05 of sugar concentration and process temperature having the definite effect on mass reduction and water loss while STGR had negligible effect on mass reduction and water loss. In case of solid gain process variables like sugar concentration and process temperature having the definite effect. Interaction of process variables viz. sugar concentration and process temperature and also the quadratic term of sugar concentration having the definite effect on solid gain, whereas STGR had the negligible effect. Similar reports were reported by Singh et al., (2007) for osmotic dehydration of carrot cubes, Eren et al., (2006) for osmotic dehydration of potato.

Mass Reduction
The coefficient of significant of fitted quadratic model was evaluated by using F-test and P-value. The analysis of variance (ANOVA) for mass reduction given in Table 4. Regression model fitted to experiment result of mass reduction showed the P-value for lack of fit as 0.7083 implies the lack of fit was non-significant. The value of R² was found to be 0.9437. Regression analysis showed that mass reduction was significantly (P<0.05) affected by the linear effect of sugar concentration (A) and process temperature (B) whereas it was not significantly (P>0.05) affected by STGR. It implies that increased mass reduction with increase of sugar concentration and process temperature. Fig. 1 indicates an increase in mass reduction with increase in sugar concentration and process temperature.
Table (4):
ANOVA for mass reduction.

Fig. 1: Effect of Process Variables on Mass Reduction

Water loss
The significant of coefficient of fitted quadratic model was evaluated by using F-test and P-value. The analysis of variance (ANOVA) for water loss of quadratic is given in Table 5. Regression model fitted to experimental result of water loss showed the P-value for lack of fit as 0.7474 implies the lack of fit ws non-significant. The value of R² was found to be 0.9428. regression analysis showed that water loss was significantly (P< 0.05) affected by the linear effect of sugar concentration (A) and process temperature (B) whereas it was not significantly (P> 0.05) affected by STGR. Fig. 2 indicates an increase in water loss with increase in sugar concentration and process temperature.

Table (5):
ANOVA for Water Loss.

not significant

Fig. 2: Effect of Process Variables on Water Loss

Solid gain
The significant of coefficient of fitted quadratic model was evaluated by using F-test and P-value. The analysis of variance (ANOVA) for solid gain of quadratic is given in Table 6. Regression model fitted to experimental result of solid gain showed the P-value for lack of fit as 0.5592 implies the lack of fit was non-significant. The value of R² was found to be 0.9542. regression analysis showed that water loss was significantly (P< 0.05) affected by the linear effect of sugar concentration (A) and process temperature (B) and also it was affected by interaction of process variables viz. sugar concentration, process temperature (AB), quadratic term of sugar concentration (A²) and process temperature (B²) whereas it was not significantly (P> 0.05) affected by STGR.

Table (6):
ANOVA for Solid Gain.

not significant

Fig. 3: Effect of Process Variables on Solid Gain

Optimization of osmotic dehydration process for Aloe Vera
Multi response optimization technique was adopted to determine the workable optimum conditions for the osmotic dehydration of Aloe Vera. The main criterion for constraints optimization  was maximum possible mass reduction, maximum possible water loss and solid gain as minimum as possible (Singh et al., 2007). The optimum condition for each process variable along with the predicted values of responses and oVerall desirability are given in table below.

Selected optimization condition
The highest desirability value obtain was 0.903 (Fig. 4). The sugar concentration 70°Brix, process temperature 70°C and STGR 6.07 were predicted as the best condition by RSM design, for osmotic dehydration of Aloe Vera.

Table (7):
Optimum Solutions for Various Process Parameters.

Fig. 4: Effect of Process Variables on Overall Desirability