Introduction

Tomato (Lycopersicon esculantum L.) is one of the most widely consumed fresh vegetables in the world. Tomatoes are rich source of polyphenols (10-50 mg kg-1), lycopene (60-90 mg kg-1) and small quantities of vitamin E (5-20 mg kg-1) and also a nutritionally recognized vegetable for their vitamin C content, with an average tomato supplying about 40% of the adult United States Recommended Daily Allowances (RDA) of 60 mg [6].

Dehydration offers a unique challenge to preserve the fruits and vegetables for a longer time. The removal of moisture being one of the important factor which will be accomplished in a manner that will be least detrimental to the product quality due to the structural configuration of the fruit and vegetable. Tomato (Lycopersicum esculantum L.) is highly perishable with a limited shelf life at ambient conditions thus creating glut during production season and becomes scanty during off-season. Thus, there exists a need to develop suitable technology for processing and preservation of this valuable product which will not only check losses but can also generate additional revenue for the country [3].The quality of the tomato depends on many parameters like tomato variety, total soluble solid content of the fresh product, size of the tomato segments and air temperature, thus there exists a need to use specific drying methods to dry tomatoes while retaining maximum quality parameters. Spray drying and convection drying using solar or mechanical systems has been used for many years for drying of tomatoes [14, 2, 8, 9, 4, 16]. Traditional sun-drying is a slow process compared with other drying methods and quality losses may result from high moisture content, colour degradation by browning, microbial growth [11].

Presently, there are few published studies comparing the single or mixed effects of calcium chloride and sodium metabisulfite dipping treatments on quality parameters of cabinet-dried tomatoes. Hence, the objective of this study was to evaluate the effects of different pre-treatments and drying methods on the water activity, dehydration and rehydration ratio.

Materials and Methods

Two varieties of fresh tomato (Shalimar I and Punjab Chuhra) were selected for the present study. Fruits were sorted and washed with water to remove dirt and soil and finally they were cut into slices of 15mm thickness. Following pre-treatment methods were applied to tomatoes before drying:

T₁ : Whole tomatoes were dipped in 2% ethyl oleate + 4%  potassium carbonate solution for one minute and the 1% ascorbic acid +1% citric acid dipping solution was applied to sliced tomato samples for 2 minutes.

T₂: Whole tomatoes were dipped in 2% ethyl oleate + 4%  potassium carbonate solution for one minute and then 2% sodium metabisulfite dipping solution was applied to sliced tomato slices for 2 minutes.

T₃: Tomato slices were treated with 1% calcium chloride + 0.25% sodium chloride solution for 2 minutes.

T₄: Tomato slices were treated with 1% sodium chloride solution for 2 minutes.

T₀ Control: Non- pretreated samples were used as control samples.

The pretreated samples were dried by following drying methods namely:

Sun drying (SD)

 Perforated sample trays were used in sun drying experiments. During the sun drying of tomato slices, the air temperature and relative humidity were determined by using thermometer and hygrometer. The air temperature and relative humidity was recorded as 26-32^oC and 33- 44%, respectively. Open sun drying experiments were done between 10:00 and 05:00.

Solar tunnel drying (ST)

 Solar tunnel drier (Fig.1) was constructed using high density polyethene (HDPE). Sample was placed on trays specially designed for the solar tunnel drier. During solar tunnel drying, the air temperature and relative humidity was recorded as 30-37^oC and 39-45%, respectively using thermometer and hygrometer respectively.

Figure 1: Solar Tunnel Drier Click here to View figure

Cabinet drying (C

Cabinet drying was carried out in cabinet drier which was designed and constructed in Ludhiana, India with model no NSW -154. Three different drying temperatures, 45^oC (D₁), 55^oC (D₂) and 65^oC (D₃) were used to dry the product.

Dehydration Ratio

Known weight of samples was dried and the weight of dried sample was recorded [15]. Dehydration ratio was calculated using equation:

Rehydration Ratio

Dried sample weighing 5 g was placed in 500ml beaker containing 150 ml boiled distilled water. Beaker was covered with watch glass and continued to boil for 20 minutes. Then sample was transferred into a glass funnel covered with coarsely porous Whatman No. 4 filter paper. After filtration, sample was removed from funnel and weighed immediately [15]. Rehydration ratio was calculated using equation:

Water Activity

Water activity of fresh and dried samples was determined by using water activity meter (PRE AQUA LAB, Water activity analyzer, SN: PRE000197).

Statistical Analysis

Experimental data was subjected to the statistical analysis following analytical procedures as described by [5]. The data collected was subjected to statistical analysis using statistical software “STATISTICA-AG” from Stat Soft (USA) licensed to FOA, SKUAST-Kashmir, Wadura campus.

Results and discussion

Water activity (a_w)

The effect of pre-treatments and drying methods on water activity of dried sample is depicted in Table 1. At the completion of drying process, the water activity of open sun dried samples was significantly higher than solar tunnel dried and cabinet dried samples in all treatments. The water activity recorded in open sun dried samples without any pre-treatment was maximum 0.64 in variety Shalimar I followed by Punjab Chuhra with a water activity of 0.63, whereas, the water activity of pre-treated samples varied from 0.38 to 0.60. Minimum water activity (0.38) was recorded in samples which were cabinet dried at 65^oC using T₄ as pre-treatment solution in Punjab Chuhra followed by 0.39 in samples pretreated with same T₄ solution in Shalimar I at same temperature. Irrespective of drying methods and varieties the untreated samples possessed over all mean water activity (0.54) compared to lower values (0.46 to 0.51) in treated samples. Minimum water activity 0.46 was recorded in samples pre-treated with T₄ and T₃ solution respectively. Irrespective of pre-treatments and drying methods the variety Shalimar I and Punjab Chuhra recorded over all mean water activity 0.48 and 0.48 respectively. Irrespective of pre-treatments and varieties the open sun dried sample possessed over all mean water activity of 0.54 compared to lower value 0.46 in cabinet drying at 55^oC.

Table 1: Effect of pre-treatments and drying methods on the water activity (a_w) of the dried tomato slices

CD (p≤0.05)

T: 0.035      D: 0.035          T×D: 0.079    V: NS

T×V: 0.050       D×V: 0.050       T×D×V: 0.011

Where T₀= Control, T₁= 2% ethyl oleate + 4% potassium carbonate (1 min) + 1% ascorbic acid + 1% citric acid (2 min), T₂= 2% ethyl oleate + 4% potassium carbonate (1 min) + 2% sodium metabisulfite (2 min), T₃=1% calcium chloride + 0.25% sodium chloride (2min) & T₄= 1% sodium chloride (2 min)

SD= Sun Drying, ST= Tunnel Drying, D₁= 45^oC, D₂=55^oC & D₃= 65^oC.

In this study, it was observed that due to various drying methods and pretreatments, the value of water activity decreased from initial of 0.64 to final of 0.38. Lowest water activity in cabinet dried samples could be due to the efficient and quick removal of water from tomato slices because of uniform heat transfer compared to sun drying. Similar results were reported by [10] in green leafy and yellow succulent vegetables upon drying and subsequent ambient storage.

Dehydration ratio

The effect of pre-treatments and drying methods on water activity of dried sample is depicted in Table 2. The dehydration ratio recorded in open sun dried samples without any pre-treatment was minimum 20.00 in variety Punjab Chuhra followed by Shalimar I with the dehydration ratio of 22.51, whereas, the dehydration ratio of pre-treated samples varied from 17.24 to 26.31. Maximum dehydration ratio (33.33) was recorded in samples which were cabinet dried at 55^oC using T₁ as pre-treatment solution in Shalimar I followed by 26.31 in samples pretreated with T₂ solution in Punjab Chuhra at drying temperature (65^oC). Irrespective of drying methods and varieties the untreated samples possessed over all mean dehydration ratio (20.32) compared to higher values (20.55 to 24.03) in treated samples. Minimum dehydration ratio (20.55 and 21.48) was recorded in samples pre-treated with T₄ and T₃ solution respectively. Irrespective of pre-treatments and drying methods the variety Shalimar I and Punjab Chuhra recorded over all mean dehydration ratio 21.43 and 22.01 respectively. Irrespective of pre-treatments and varieties the open sun dried sample possessed over all mean dehydration ratio of 20.01 compared to higher value 24.61 in cabinet drying at 65^oC.

Table 2: Effect of pre-treatments and drying methods on the dehydration ratio of the dried tomato slices

CD (p≤0.05)

T: 0.013     D: 0.013          T×D: 0.029     V: 0.082

T×V: 0.018       D×V: 0.018        T×D×V: 0.041

Where T₀= Control, T₁= 2% ethyl oleate + 4% potassium carbonate (1 min) + 1% ascorbic acid + 1% citric acid (2 min), T₂= 2% ethyl oleate + 4% potassium carbonate (1 min) + 2% sodium metabisulfite (2 min), T₃=1% calcium chloride + 0.25% sodium chloride (2min) & T₄= 1% sodium chloride (2 min)

SD= Sun Drying, ST= Tunnel Drying, D₁= 45^oC, D₂=55^oC & D₃= 65^oC.

In this study, it was observed that higher dehydration ratio in cabinet dried samples could be attributed to efficient removal of water more quickly. The results are in conformity with the observations of [7, 13].

The dehydration ratio was found to be lowest in NaCl treated samples as NaCl being a osmotic agent, leached the juice into the medium. Further Sodium metabisulfite and calcium appears to maintain the structural integrity of the cell walls. Similar observations were recorded by [1].

Rehydration ratio

The effect of pre-treatments and drying methods on water activity of dried sample is depicted in Table 3. The rehydration ratio recorded in open sun dried samples without any pre-treatment was minimum 1.33 in variety Punjab Chuhra followed by Shalimar I with the rehydration ratio of 1.83, whereas, the rehydration ratio of pre-treated samples varied from 1.11 to 2.98. Maximum rehydration ratio (2.98) was recorded in samples which were cabinet dried at 55^oC using T₂ as pre-treatment solution in Punjab Chuhra followed by 2.97 in samples pretreated with T₁ solution in Shalimar I at drying temperature (65^oC). Irrespective of drying methods and varieties the untreated samples possessed over all mean rehydration ratio (1.86) compared to higher values (1.90 to 2.05) in treated samples. Minimum dehydration ratio (1.90 and 1.99) was recorded in samples pre-treated with T₁ and T₂ solution respectively. Irrespective of pre-treatments and drying methods the variety Shalimar I and Punjab Chuhra recorded over all mean rehydration ratio 2.00 and 1.89 respectively. Irrespective of pre-treatments and varieties the open sun dried sample possessed over all mean rehydration ratio of 1.85 compared to higher value 2.69 in cabinet drying at 65^oC.

Table 3: Effect of pre-treatments and drying methods on the rehydration ratio of the dried tomato slices

CD (p≤0.05)

T: 0.016     D: 0.016          T×D: 0.036      V: 0.010

T×V: 0.022       D×V: 0.022       T×D×V: 0.051

Where T₀= Control, T₁= 2% ethyl oleate + 4% potassium carbonate (1 min) + 1% ascorbic acid + 1% citric acid (2 min), T₂= 2% ethyl oleate + 4% potassium carbonate (1 min) + 2% sodium metabisulfite (2 min), T₃=1% calcium chloride + 0.25% sodium chloride (2min) & T₄= 1% sodium chloride (2 min)

SD= Sun Drying, ST= Tunnel Drying, D₁= 45^oC, D₂=55^oC & D₃= 65^oC.

In this study, it was observed that maximum rehydration ratio in cabinet dried samples has been attributed to uniform and efficient heat transfer and quick removal of water which leads to less textural changes during dehydration which subsequently offered higher rehydration ratio of final product [12].

Further results revealed that the effectiveness of sodium metabisulfite on the textural qualities of tomato resulted in the best rehydration properties and showed a higher value. Similar results were observed by [11].

Conflict of interest

There is no conflict of interest.

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