MATERIALS AND METHODS

Indian varieties of garlic bulbs were obtained from the local market (Kanpur). Sartorious electronic top loading balance (Model-1213 MP, Sartorius GMBH, Gottingen, Germany) was used for weighing the sample. Yorco hot air steriliser (oven) [Model-YS1431, Yorco Sales Pvt. Ltd., New Delhi, India],: "Radarange' Microwave oven [Model RG5-145, Amana Refrigeration Inc. Amana IOWA, U.S.A.), Fluidised bed dryer Belt type)[SMST, SM SCIENTECH, Calcutta, India] and freeze dryer (http://www.go-dove.com/event-15883/Abbott-Laboratories-Ltd-Tablet-and-Capsule-Production and-Test-Equip/lot-41/CHRIST-ALPHA-2-4-Laboratory-Freeze-Dryer-Gefriertrockner) were used for drying operations. Glass petriplates were used for drying of cloves in various types of driers.

Selection: The garlic bulbs were sorted and selected for the following characteristics like uniform shape and size, full matured with no greenness without sprouts and firm texture. The selected bulbs were cleaned to remove dust/earth, chaff, dry stalk and any foreign materials.

Separation: The clean bulbs were manually separated into cloves and winnowed to remove dust and dirt. The cloves were graded for their size and only medium sized cloves were taken in the experiment.

Peeling: The outer skin of the cloves was removed off by hand while taking care of the surface of clove to remain undamaged. Whole cloves were used for all the drying operations except freeze-drying. In the latter case, the cloves were into 2-3 mm thick slices to increase the surface area for quick drying. In each case, 100 g of cloves/slices were taken for the drying operations.

Chemical characteristics
Moisture determination: Moisture content in 15 untreated clove samples was estimated as per the standard procedure of ISI 1797 (1973) (Methods of sampling and test for spices and condiments) by Entrainment.

Volatile content determination: The volatile oils in 15 untreated and each treated (after drying) samples were determined by IS standard method as in IS 1797-1973(ISI 1797, 1973).

Sensory evaluation: The sensory analysis of untreated and treated cloves/slices was carried out by a trained panel of 5 members on 5 point scale from excellent to poor. Each time a control sample was used for comparison of samples.

Drying methods
Hot air drying: One hundred gram of garlic cloves were spread evenly in a petridish and kept in the oven for 1, 2 and 3 h at 45, 50, 55, 60, 70 and 75 °C. After each drying period samples were taken out of the oven and put into a dessicator and after sometime, the loss in weight was measured as the moisture loss. The samples were evaluated by a sensory method as earlier and analyzed statistically.

Microwave heating: One hundred grams of cloves were placed in a petridish and dried in microwave oven for 8, 14 and 20 sec. After each drying period, sample were taken out off the microwave oven and put into a desiccator till the constant weight was obtained. The samples were evaluated by a sensory method as describe earlier and analyzed statistically.

Fluidized bed drying: One hundred grams of cloves were dried in the fluidized bed dryer to form a bed for 1 and 2 h at 50, 55, 60, 65 and 70°C. The air velocity was maintained constant throughout the experiment. After each drying period samples were taken out of drier and put into a desiccator, till a constant weight was obtained. The samples were evaluated by sensory methods as described earlier and analyzed statistically.

Freeze drying : Whole garlic cloves (peeled), pricked garlic cloves and slices of garlic cloves each in 50 g quantity were taken into the dish of freeze dryer (Model Alpha 2-4) and dried as per the procedure described in the operating manual under the following conditions: Pre freezing temperature-30°C Main drying vacuum-0.370 mbar. The final drying was done till the product attained 20°C temperature.

Statistical analysis: Statistical analysis was carried out as described by Steel and Torrie (1980).

RESULTS AND DISCUSSION

Chemical characteristics
Moisture and volatile oil content: Moisture content and Volatile content in Control Sample are presented in Table 2. The mean moisture content and volatile oil content in untreated 15 garlic cloves samples found to be 61.2 and 0.22%, respectively. The same was used for the comparison purpose. The result shows that the data obtained by the method were quite reproducible as indicated by the very low value of coefficient.

Table 2:	Moisture content and volatile content in control sample

Table 3:	Effect of hot air drying on moisture loss and sensory quality of garlic cloves
Means in the same column followed by different letter differ significantly (p<0.05), df = 4, Small letter denotes the effect of time at same temp. Whereas capital letter denotes the effect of temp. at the same time

Effect of various drying processes on the quality of garlic cloves
Hot air drying: Moisture removal in garlic clove by hot air drying is found to be quite effective but reduced the quality to a greater extent. At the same temperature a significant change has been observed upon increasing the duration of time. This was found to be true for all the drying temperatures studied. On the other hand on increasing the drying temperature for the same drying period, the -moisture loss did not increase significantly up to 60°C (for 1 h 55°(2 h) and 50°C for 3 h). Afterwards highly significant changes were observed and higher moisture losses were observed at 75°C with respective different periods same as the results by Abano et al. (2011).

Very small changes in sensory attributes were observed at the lower temperatures of drying but quite significant changes were at higher temperatures. Except the sample dried at 70 and 75°C, the change in appearance was found almost insignificant. The deterioration in textural qualities as well as in aroma has been started just after increasing the temperature from 50°C and the trend was similar up to 75°C) (Rasouli et al., 2011; Sacilik and Unal, 2005; Fernando et al., 2008). By comparing the moisture loss and overall sensory quality score with respect to temperature for a constant drying period of 3 h, it was observed that 60°C temperature was judged optimum for drying of garlic cloves with higher moisture loss and minimum damage to overall quality. The effect of Hot air drying on moisture loss and sensory quality of garlic cloves is given in Table 3.

Microwave heating: The garlic cloves were subjected to microwave heating for 8, 14 and 20 sec exposure however in 20 sec treatment the tips of the cloves were burnt and therefore the rest of the sensory attributes were not accessed (Rao et al., 2007).

Table 4:	Effect of microwave heating on moisture loss and sensory quality of garlic cloves
Means in the same column followed by different letter differ significantly (p<0.05), df = 4

Table 5:	Effect of fluidized bed drying on moisture loss and sensory quality of garlic cloves
Means of the same column followed by different letter differ significantly (p<0.05), df = 4

The moisture loss increased from 4.12 to 14.1% on the increase of exposure to microwave for 8 to 20 sec as expected. After exposure the skins of cloves was found to be shrunken and the cloves were less hard than the normal ones. The aroma was slightly lost in 8 sec treatment but 14 excusers caused slightly more loss due to more evaporation of volatiles (Murthy and Prasad, 2005; Figiel et al., 2010).

The overall quality score decreased from 18.5 to 16.9 for 8 and 14 sec treatments respectively but the 14 sec treatment was found suitable due to more moisture loss and hence optimum. The effect of Microwave drying on moisture loss and sensory quality of garlic cloves is given in Table 4.

Fluidized bed drying: Fluidized bed drying greatly affected the loss of moisture at the higher temperature whereas did not show any significant change at low temperature range (50-55°C) for 1 h but significantly increase for 2 h treatment as shown in Table 4. It is clearly evident that for 1 h treatment the maximum increase occurred between 60 and 65°C. On the other hand maximum difference (from 2.6 to 4.9) for 2 h treatment was observed for the range of 55-60°C than in that of 50-55°C. Analyzing the sensory quality not much change was observed at 50°C for 1 h treatment. The pattern is followed up to 65°C for which there was no significant change, however the quality greatly reduced at 70°C. Treatment for 2 h greatly reduced the quality while increasing the temperature as the garlic cloves were shrunken and became soft and soggy probably due to oozing out of the volatile oil from the interior. Taking into account optimum sensory score as well as higher moisture loss, the treatment at 55°C for 2 h can be judged optimum with 17.1% moisture and 14.2 overall quality score. The effect of Fluidized bed drying on moisture loss and sensory quality of garlic cloves is given in Table 5.

Freeze-drying: The effect of freeze-drying on various garlic samples is given in Table 6. Whole garlic cloves were having 14% moisture (moisture loss 47%) at the end of drying probably due to impermeability of the garlic skin and hence the restricted mobility of the moisture from the interior of the garlic.

Table 6:	Effect of freeze drying on moisture loss and sensory quality of whole, pricked and sliced garlic

Table 7:	Effect of various drying processes on the moisture loss, sensory quality and volatile retention of garlic cloves

This is especially true of vegetables whose external cover makes removal of water difficult (Condori et al., 2001; Sablani et al., 2007). Also some undesirable changes were observed in these cloves like shedding of the skin, light brown and sticky. On the other hand the pricked garlic cloves and slices could be dried for the final moisture content of 2.2 and 2.2%, respectively. However, the dried slices showed better sensory quality score (19) than that of pricked cloves quality (14). The reason for inferior quality of pricked garlic cloves lied in the stickiness due to oil droplets coming out of the cloves.

Volatile oil retention: The volatile oil retention was found to be maximum (93%) in freeze dried samples followed by (75%) in microwave dried (14 sec), (65%) in fluidized bed dried (55°C for 2 h) and then (60%) in hot air dried (60°C for 3 h) samples (Kubra et al., 2012). The effect of various drying processes on the moisture loss, sensory quality and volatile retention of garlic cloves is given in Table 7.

CONCLUSION

An overview of the prime methods applied hitherto to garlic cloves has been shown in Table 3. As expected, the freeze dried sample had maximum moisture loss of 58.8% with superior quality score (19) followed by fluidized bed drying, oven drying and microwave heating having moisture losses of 17.1, 13.3 and 9.7%, respectively. The overall quality score among the above three were higher in oven drying method. However, the low processing time of 2 h in fluidized bed drying makes it superior than that of oven drying having 3 h treatment. These results suggest that the fluidized bed drying can successfully be combined with freeze drying to obtain dehydrated garlic with maximum moisture loss and desirable qualities.