Drying kinetics and quality aspects of seasonal vegetables dried in a tray dryer

Introduction

Bitter gourd (Momordica charantia L.) is a member of the cucurbit family. It is considered to be native to Tropical africa. It is cultivated widely in many tropical and subtropical regions of the world and is frequently used in India and orient as a foodstuff and a medicinal plant (Habicht et al., 2011) [3] . The major states contributing to its production and area under cultivation are Uttar Pradesh, Tamil Nadu, Maharashtra, Andra Pradesh, Karnataka, Odisha, west Bengal and Gujarat. Spine gourd (Momordica dioica L.) is a cucurbitaceous, dioecious perennial vegetable and it fetches very good price in the market. In tropical regions of South America, Africa and Asia (India, Bangladesh, Pakistan & China), commonly known by different names i.e. small bitter gourd, bristly balsma pear, teasle gourd, prickly carolaho, kantola, jungle kerala etc. (Thiruvengadam et al., 2016) [12] . It is indigenous to India and other parts of Asia and in India it is grown in Andra Pradesh, Assam, Meghalaya, West Bengal, Uttar Pradesh, Bihar, Maharashtra, Madhya Pradesh, Gujarat and Andaman Islands. West Bengal and Karnataka are two major Indian states that grow spine gourd commercially (Talukdar and Hossain, 2014) [11] . In the Andrapradesh, production was 2.94 lakh tons and Area was 48.23 thousand hectares. Sweet potato (Ipomoea batatas L.) is one of the five most important food crop in developing countries. It is one of the crops relatively easy to grow even in poor, dry soil. Tuber crops achieve levels of importance as great as cereal grains in providing the greater part of people's daily caloric needs in the tropics. The sweet potato shows significant nutritional contributions to the diet and has high caloric value per cultivated area (Bouwkamp et al. 1985) [2] . In the Andhra Pradesh, production was 14.91 metric tons and area was 497 hectares.

Material and Methods

Determination of drying kinetics of osmotically dehydrated products during tray drying

The common method in the drying of food products is tray drying. In order to verify the influence of the drying process on the vitamin C content in dried products, some authors studied some conventional processes using tray drying method. Different kinds of fruits and vegetables and different cultivars have been studied by many researchers. Apples, tomato, mango, apricot, plum, fig, peppers, potato, brinjal, onions and others are examples of fruits and vegetables whose drying behaviour have been studied and their vitamin C retention has been determined There is very meagre data published on osmotic dehydration of the spine gourd, bitter gourd and sweet potato, hence by conducting preliminary experiments the parameters were selected and were classified in two major categories as a) independent parameters such as thickness of samples 5-10 mm, temperatures 40 °C, 50 °C and 60 °C and Immersion time 1 hour, 2 hour and 3 hour. b) dependent parameters such as colour in L a b values, ascorbic acid in milligrams per gram and rehydration ratio. The osmosed bitter gourd spine gourd and sweet potato slices were dried in hot air dryer. This experiment was conducted on the laboratory using hot air tray dryer at different temperatures 40 °C, 50 °C and 60 °C. 100 g osmosed samples were weighed and spread uniformly on upper tray. During drying spine gourd, bitter gourd and sweet potato were weighed initially at 30 minutes interval for first hour and at 1hour intervals for next two hours until completion of experiment.

Quality Evaluation

Quality of the osmo-convectively dehydrated product was evaluated by using colour, rehydration characteristics and sensory evaluation as described below.

Colour

Colour is one of the most important qualities for acceptance of a product, reflects sensation to the human eye. Colour is important to consumer as a mean of identification, as a method of judging quality and for its basic aesthetic value. Dried products are usually darker in colour, but darker colour does not mean better quality. Too dark may imply that the product is over dried. The advantage is that this parameter can be visually determined for assessing dryness quality. A laboratory colourimeter was used. Colour of the dried spine gourd, bitter gourd and sweet potato slices was measured using a Hunter Lab Colorimeter (Model CFLX/DIFF, CFLX-45). A cylindrical glass sample cup (63.5 mm in diameterx40 mm height) was placed at the light port (31.75 mm diameter). The instrument was initially calibrated with a black as well as with standard white plate supplied with the equipment. The 3-dimensional scale L*, a* and b* is used in a Hunter Lab Colourimeter. The L* is the lightness coefficient, ranging from 0 (black) to 100 (white) on a vertical axis. The a* is purple-red (positive a* value) and blue-green (negative a* value) on a horizontal axis. A second horizontal axis is b*, that represent yellow (positive b* value) or blue (negative b* value) colour. This 3D colour system can be seen in Plate 3.5. The values of L*, a* and b* can be converted to Chroma (C*) values, analogous to colour saturation or intensity (Mc Guire, 1992) [5] .

Ascorbic acid was estimated by titrimetric method using 2, 6-dichlorophenol-indophenol dye (Ranganna, 1995) [9] . One gram of bitter gourd, spine gourd and sweet potato samples were taken and ground well and using 10 ml of 4% oxalic acid solution was pipetted out in to a conical flask and titrated against dye. The dye factor, i.e., mg of ascorbic acid per ml of the dye was calculated. The value of ascorbic acid was estimated by titrating the sample solution against the dye for the appearance of a persistent pale pink colour.

Ascorbic acid content = Titrate value X dye factor X volume made Volume taken X weight of sample X100 = 0.5 X V2 X50 V1 X 5ml X weight of sample X100

V 1 = volume of dye occupied by the working standard V 2 = volume of dye occupied by samples

Rehydration characteristics

Every dried product cannot be consumed directly. Most need to be rehydrated by soaking in water prior to consumption. There are several factors affecting rehydration, such as the soaking period, temperature of the water and the rehydration capacity of the product.

The rehydration capacity can be influenced by the drying process. Drying processes that change product composition to a lesser extent are supposed to offer better rehydration characteristics of finished product. Due to drying process collapsing of cellular structure and shrinkage may eventually affect the rehydration characteristics. For rehydration capacity, pre weighed samples were soaked in ample amount of water for 5 hours at room temperature. This steeping is known as re-hydration. The ratio of mass of re-hydrated and dehydrated samples can be used to find following re-hydration characteristics (Pokharkar, 1994) [8]: [6] also observed more moisture removal at initial stages while conducting drying experiments on pomegranate aril. Similar trend was also observed by Ranjith et al. (2023) [10] for dragon fruit slices. The optimization of dehydration process for bitter gourd was done. Bitter gourd slices were dried in a cabinet tray dryer at temperatures 50 °C, 60 °C and 70 °C.

Temperature for drying was optimized on the basis of parameters such as drying rate, drying time required, moisture in final product, yield of powder and sensory quality by Kusat et al. (2021) [4]

Effect of average drying time on drying rate

To study the effect of average drying time on drying rate of Bitter gourd, spine gourd and sweet potato slices, a graph was plotted by taking average drying time on X-axis and drying rate on Y-axis (Fig. 2 [10] , and jackfruit slices (Mithun and Kaleemullah, 2019) [17] .

Colour (L-values)

Colour is often used as an indication of quality and freshness for food products. Hence it has become important for food processors to be able to evaluate and grade their products based on colour.

Colour values measured using a colour flex Hunter lab colorimeter, were relative to the absolute values of perfect reflecting diffuser as measured under the same geometric conditions (ASTM method). Observations were taken at room temperature 30. [13] . The L value of fresh sapota slices was about 34.68. The L-values of osmo-convectively dried sapota sample at various experimental conditions were ranged between 52.04 and 61.61. As the temperature increased Lvalue of colour increased that means sample became lighter in colour from 50 °C to 70 °C. which may be due to discolouring the sample slightly because of elevated temperature. The drying air temperature 60 °C was found better and recorded significantly higher colour (L-value = 59.27).

Rehydration characteristics (RR)

The reconstitution qualities of Bitter gourd, spine gourd and sweet potato samples were determined by conducting rehydration tests as described in material methods. The dehydrated products were immersed in distilled water at 40 °C, 50 °C and 60 °C and the mass of the products after five hour was measured.

From the table 1, 2 & 3. It was clear that the maximum rehydration ratio was obtained 2.03, 2.24 and 2.13 at 60 °C when compared to other temperatures of 40 °C and 50 °C. for bitter gourd slices immersed in distilled water for 5 h. The rehydration ratio was increased as the temperature increased from 40 °C to 60 °C. Similar results were quoted by Vikaskumar et al., 2014 [13] . The rehydration ratio of the osmotically dehydrated sapota slices at different temperatures ranges from 3.64 to 5.05.

Conclusion and Discussion

The samples blanched in boiling water and dehydrated at 60 °C were observed most acceptable in terms of rehydration property and nutritional quality. seasonal vegetables (bitter gourd, spine gourd and sweet potato) slices dehydrated in tray dryer at 60 °C were of the best quality followed by tray dryer at drying temperature of 40 and 50 °C. It was observed that the moisture content of the product was reduced with drying time and no constant rate period was observed in tray drying. There was a variation in drying time from 420 to 900 min for the range of drying air temperatures (40-60 °C) taken for study. Minimum drying time was observed for high air temperature (60 °C) and maximum time was recorded for low air temperature (40 °C) for all air velocities. Quality of dried product in respect to colour was superior. The osmo-tray dried samples were found more acceptable after tray drying at 60 °C.

References

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