Determine the Mass Transfer Characteristic of Thermal Drying and Osmotic Dehydration of Pumpkin and Bitter Leaf


Determine the Mass Transfer Characteristic of Thermal Drying and Osmotic Dehydration of Pumpkin and Bitter Leaf


The experiment was carried out and three osmotic solutions were prepared, “hypertonic solution 60% concentrated, hypotonic solution 40% concentrated and isotonic solution 50% concentrated” and oven drying was also carried in the course of the experiment, the effect of sodium chloride (osmotic agent) concentration, temperature and immersion time on overall mass transfer coefficient, effective diffusivity, drying rate weight loss and shrinkage ratio on oven drying and osmotic dehydration of Pumpkin and bitter leaf. Results showed that both pumpkin and bitter leaf had a highest mass transfer coefficient in oven drying at highest temperature of 80oc, for pumpkin leaf 0.149(m/min), bitter leaf was 0.149(m/min), the results were also obtain for osmotic dehydration at 80Oc at highest concentration (Hypertonic solution) had the highest mass transfer coefficient, for pumpkin leaf was 0.015(m/min) for bitter leaf was obtained to 0.032(m/min). For osmotic dehydration, both samples weight loss percent (WL%) for hypertonic solution at 800C for 90mins had the highest weight loss percent, but the shrinkage ratio decrease with increase in time.The mass transfer during oven drying of pumpkin and bitter leaf was described using Fickian equation of diffusion with drying taking place in the falling rate period, the effective moisture diffusivity value showed temperature dependence on both samples. Effective diffusivity values were also determined for oven drying at different temperature, and the values increases as the temperature increases, for pumpkin and bitter leaf at 600C, 700C and 800C, the effective diffusivities were 1.0E-09, 1.87106E-09 and 2.1843E-09m2/min and 8.52966E-10, 1.00015E-9 and 2.45308E-08m2/min.




Fruits and vegetables contribute a crucial source of nutrients in daily human diet, the world fruit production is estimated to be 434.7 million metric tones and vegetables 90.0 million metric tones. India is the second largest fruits and vegetable producer and its annual production is 44 million metric tones from an area of 3, 949, 000 haduring 2000-2002 (Srivastava& Kumar, 2002). Fruits and vegetables losses in the developing countries are considerably high. In India, post harvest losses of fruits and vegetables are estimated as more than 25 percent. Many processing techniques can be employed to preserve fruits and vegetables by drying and dehydration is one of the most important operations that are widely practiced because of considerable saving in packaging, storageetc

Vegetables contain nearly 70% to 95% of moisture which make them highly perishable. If this moisture is reduced to some extent, bulk transportation of the final product can be made to other parts of the country where it is not available. Also the shelf life of the product is increased. Conventionally sun drying and hot air drying is used to dry and preserve the product. This produced discolored and shrieked products which were of not interest to patronage. When osmotic dehydration is used prior to drying steps it is evident that it conserves energy and reduces the heat damage to the product in terms of color, flavor and aromaetc

Osmotic dehydration aims at extending life of food by removing water without phase transition [Kowalska and Lenart 2001, Matuska et al. 2006]. The process is carried out by immersing the raw material in a hypertonic solution (solution with high concentration of sugars, sodium chloride, etc.).

Osmotic dehydration is used for foods with a tissue structure, such as shredded fruit and vegetables [Torreggiani 1993]. The method is based on the natural phenomenon of osmosis through cell membranes of biological material [Shi and Le Maguer 2002]. In the osmotic dehydration the process of water flow to the outside of food material takes place, and entering of substances dissolved in a hypertonic solution into the product. Since the cell membrane is not perfectly selective, the solutes present in cells (organic acids, sugars, minerals, fragrances, and colorants) can pass with water into the hypertonic solution [Derossi et al. 2008].

1.2 Mass transfer phenomena during osmotic dehydration

There are three major types of counter current mass transfer in osmotic concentration process (Karthiayani, 2004;Tiwari, 2005) (Figure 2).

1. Important water out flow from product to solution.

2. A solute transfer, from the solution to the product; it makes thus possible to introduce the desired amount of an active principle, a preservative agent, any solute or nutritional interest, or a sensoryquality improvement of the product.

3. Leaching out of products own solutes (sugar, organic acids, minerals, vitamins etc.), which is quantitatively negligible when compares with the first two types of transfer, but essential with regard to the composition of final product/

Drying of fruits and vegetables such as pumpkin, fruit pepper and bitter leaves e.t.c is one the most time and energy consuming process in the modern food industry [sunjka et al 20004]. However these fruit and vegetable are usually in short supply during dry season because they are perishable crops which deteriorate within a few days afterharvest (which occur mainly in rainy season). Preserving these crops in their fresh state for months has been a problem that is yet unsolved [famurewa et al 2006; Agarryetal 2006]. Drying processes play an important role in the preservation of agricultural products. They are defined as a process of moisture removal due to simultaneous heart and mass transfer in which energy must be supplied [waewsak et al 2006]


The general condition include the following:

1) Heat is transfer by evaporation of liquid or moisture from the surface of the solid

2)Mass is equally transferred


Most of the work done on this subject have been on the change that occur in the nutritional properties and mass transfer at different temperature, immersion time and concentration of the solution


This works tends to do proper investigation on the drying rate and mass transfer characteristic in different osmotic solution and thermal drying of vegetable to bring it to the awareness of the public, this work tends to bring out the importance of choosing the best optimum temperature and best osmotic solution for thermal drying and osmotic dehydration

1.6 AIM

The aim of this research work is to investigate the mass transfer characteristic of pumpkin and Bitter leaf


The following objectives were use to achieved the aim.

  1. To investigate the effect of temperature on drying rate of pumpkin and bitter leaf

2. To investigate the effect of osmotic concentration of pumpkin and Bitter leaf

3. To investigate the effect of time for drying rate and osmotic dehydration for Pumpkin and Bitter leaf

4. To determine the optimum condition of thermal drying and osmotic dehydration.


This is necessary to determine the suitability of the drying process that will help in mass transfer, and it also provide a suitable method that will help reduce crop loss in Nigeria.


The method used in this work include;

1). Osmotic solution (hypertonic, Isotonic and hypotonic solution) using NaCL as the osmotic agent , were prepared at different temperature interval the solution was stirred until complete dissolution, the samples were pumpkin and bitter leaf

2). Samples of pumpkin and bitter leaves were weight and spread on a metal tray which was then place in a laboratory oven. The drying was carried out at different temperature of 600C, 700C and 800C

3). The effective diffusivity was determine by using fickian equation

The scope of this research is limited to the study of mass transfer coefficient during thermal drying and osmotic dehydration of pumpkin and Bitter leaf

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