Apricot and Plum Drying Studies using Tray and Solar Dryer

International Journal of Emerging Technology and Advanced Engineering

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Apricot and Plum Drying Studies using Tray and Solar Dryer

Anbu Clemensis Johnson

, Khulood Rashid Al Souli

Department of Mechanical and Industrial Engineering, Caledonian College of Engineering, Muscat, Oman

Abstract—Drying studies were carried out on Apricot and Plum using traditional tray drier and table-top solar dryer. The fruits were dried at 40, 50 and 60 °C with air flow velocity of 0.11, 0.4 and 0.17 m/s. Drying constant was calculated using the Page equation and it was found to vary from 0.26 to 0.5 hr-1 _{for apricot and 0.26 to 0.43 hr}-1 _{for plum. The results}

of diffusion coefficient varied between 9.89×10-11 m2/s and 2.91×10-10 _m2_{/s for apricot and 8.13×10}-11 _m2_{/s and}

2.72×10-10 m2/s for plum. The activation energy was determined to be 46.61 kJ/mol and 52.10 kJ/mol for apricot and plum respectively. Solar drying experiment showed that the drying rate was greater for plum than apricot.

Keywords— Solar drying, tray drying, equilibrium moisture content, moisture diffusivity, activation energy

I. INTRODUCTION

Drying is a process of reducing the moisture content of any material. This process is predominantly used in the food industry for perishable items such as fruits, vegetables, fish to name a few for preservation. The advantages of reducing the moisture content of food products are, increased shelf life, reduction in weight, minimizing packaging requirements and lower shipping costs (Sabarez et al., 1997).

Industrial scale drying of fruits is energy intensive and time consuming process. Problems faced in drying whole fruits are the presence of waxy layer on the surface of the fruits which controls the moisture diffusion (Doymaz and Ismail, 2011). This problem is significantly reduced by cutting the fruits into thin slices. Drying process is accomplished in the industry using conventional dryers or traditional solar dryers. The purpose of using solar drying is to minimize the amount of energy for removing maximum amount of moisture from the product. This is an environmentally friendly method; however there are some disadvantages such as: contamination with insect, soil, dust and sand particles if exposed to sun without proper protection (Aghbashlo et al., 2008).

Solar dryers used in drying food products are broadly classified into three. The first type is the natural convection solar dryers also known as (passive dryers). Air flow mechanisms are designed is ways such that moist air is allowed to escape the unit. The second type is the forced convection dryers also known as (active dryers).

In these types of dryers a blower is used to force the moist air from the system. The third type of solar dryer falls in the category of hybrid solar dryers. This category of dryers tends to combine the concepts of natural and forced convection with energy storage units (Fudholi et al., 2010).

Apricot and plum are found in different parts of the globe. Apricot found in the supermarket in Oman is imported from Jordan and various varieties of plum from Spain and USA. Indigenously apricot (Prunus armeniaca

L) and plum (Prunus domestica L) are grown at high altitudes in Jabal Akhdar in Oman (Oases of Oman, 2015). Drying models are used to analyse the drying curves from the laboratory experiments under controlled conditions which give moisture transport information and calculation of diffusion coefficient. Moisture diffusion in food materials has been described by researches using the Fick’s second law and it has been estimated for sweet cherries to be between 5.683×10-10 and 1.544×10-9 m2/s, milky mushroom between 8.76 to 16.5×10-9 m2/s and pomegranate between 3.1 to 52.6 ×10-9 m2/s (Doymaz and Ismail, 2011, Arumuganathan et al., 2009, Yilmaz et al., 2015). Activation energy is the energy required for removing moisture has been reported by researchers using Arrhenius type relationship for date palm to range between 35.7–44.02 kJ/mol, for soybean 31 kJ/mol and for peach 42.53 kJ/mol (Falade and Abbo, 2007, Irigoyen and Giner 2014, Zhu and Shen, 2014).

The main objectives of the research work are to study the kinetic parameters for apricot and plum using the page empirical equation, determine diffusion coefficient, activation energy and to design and build a natural convection table-top solar dryer.

II. MATERIALS AND METHODS

2.1. Tray dryer experimental setup

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Solar dryer was constructed using wood. Carefully cut wood slabs were used in the fabrication. The front side had the dimension 60×30 cm and the sides 30×30 cm. Mirrors of the dimension 30×8 cm were arranged at the top of wood structure. At the centre of the wooden structure wooden box of the dimension 20×30×10 cm was placed. The box was covered using black leather paper for absorbing the solar radiation. Holes were punched on the paper for air circulation. The schematic of the design is shown in Figure 1.

Figure 1. Schematic view of solar dryer design.

1.5 ± 0.3 g of apricot and plum slices were placed inside wooden box, so that the mirrors reflect the light onto the wooden box with the fruits. A thermocouple was placed inside the chamber to monitor the temperature. Temperature readings were collected at an interval of one hour. Wood sticks and metal wires were utilised for suspending the fruits. The experiment was conducted from 8 am to 6 pm. The picture of the chamber with fruits is shown in Figure 2.

Figure 2. Drying of fruit slices inside the chamber of the solar dryer.

2.3. Drying modeling 2.3.1. Data analysis

The moisture ratio (MR) of the fruits were determined using the following relation:

i t

M M

M R (1)

Where Mt and Mi are moisture content at time t (kg/kg), and initial moisture content (kg/kg). The drying curves were fitted using Page moisture ratio thin-layer drying model that is widely used for food and biological materials. These models are generally derived by simplifying the series solution of Fick’s second law. Page’s equation is given as follows:





Kt -exp

MR (2)

Where K is the drying constant (1/hrs), t is the time in hours, N is the product constant giving the degree of nonlinearity of the drying curve.

2.3.2. Apparent diffusion coefficient

The change in moisture concentration in one dimension can be represented by Fick’s second law of diffusion (Crank, 1975).

2 a 2

i t 2

h t D 4 π M M 8 π

ln _

    

 (3)

Where, Da is the diffusion coefficient in m2/s, t in minutes and h is the thickness of the sample being dried. 2.3.3. Activation energy

The effect of temperature on the apparent diffusion coefficient was assumed to follow an Arrhenius type relationship (Math et al., 2004; Geankoplis, 1972; Rice and Gamble, 1989). The equation is given as,

E/RT) exp( D

D_a  ₀  (4)

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III. RESULTS AND DISCUSSION

3.1. Effect of temperature on drying

The result of drying apricot and plum slices at 40, 50 and 60 ºC with air flow velocity of 0.17 m/s are shown in Figures 3(a) and (b). The initial moisture content of apricot and plum was 1.16 ± 0.13 and 1.25 ± 0.61 kg water/kg dry basis respectively. The figures show the variation of moisture ratio of fruits with time. As time progresses, moisture ratio decreases until the moisture in the fruits reach equilibrium moisture content. During the initial drying process the decrease in moisture of the fruits is drastic which plateaus as it reaches equilibrium moisture content. At higher temperature the moisture driving force increases causing an increase in the speed of evaporation and moisture diffusivity. Overlapping of the curves observed at 40 and 50 ºC is due to experimental anomaly.

3.2. Empirical Modelling

Page’s equation is used to describe the drying behaviour of apricot and plum slices. Figures 4(a) and (b) shows page equation plots at 60 ºC with air flow velocity of 0.17 m/s. Linear plots were obtained to determine product constant and drying constant. Drying constant is influenced by air temperature and the exposed area (El-Beltagy et al., 2007). Table 1 shows Page’s parameter of dried apricot and plum at 40, 50 and 60 ºC with air flow velocity of 0.17 m/s.

Figure 3(a). Drying curve of apricot slices at air flow velocity of 0.17 m/s.

Figure 3(b). Drying curve of plum at air flow velocity of 0.17 m/s.

From Table 1 it can be seen that the values of K for apricot and plum show a close resemblance. This can imply the similarity in the diffusion process. The values of R2 for dried apricot and plum slices were in the range of 0.97-0.99. It can be seen that with increase in temperature the values of K increases which can be attributed to the driving force due to mass and heat transfer.

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60 ºC and air flow velocity of 0.17 m/s.

Table 1.

Page’s parameter of fruits at air flow velocity of 0.17 m/s.

3.3. Calculation of moisture diffusion coefficient

The moisture diffusion coefficients of apricot and plum slices at 40, 50 and 60 ºC were calculated by using equation (3) and the values are presented in Table 2.

Table 2.

Moisture diffusion coefficient of apricot and plum with air flow velocity of 0.17 m/s.

It can be seen that with increasing temperature the moisture diffusivity increases. Similar phenomenon has been observed by other researchers (Mirzaee et al., 2009, Math et al., 2004).

This is attributed to the formation of moisture gradient in the sample with increasing temperature. It can also be observed that the values of the coefficients are relatively close.

3.4. Activation energy

Activation energy of apricot and plum slices are determined by using equation (4). With the air flow velocity of 0.17 m/s the values were 46.61 kJ/mol and 52.10 kJ/mol for apricot and plum respectively. The values

of activation energy are between the range of 12.7-110 kJ/mol for most of food materials according to

Zogazas (1996). In previous study, the values of activation energy of apricot at different air velocity were in the range of 29.35-33.78 kJ/mol (Mirzaee et al., 2009). In the case of normal and treated plum the values have been reported to be 24.83 and 21.23 kJ/mol (Sacilik et al., 2006).

3.5. Solar dryer

The initial moisture content of apricot and plum was 1.98 ± 0.13 and 2.48 ± 0.23 kg water/kg dry basis respectively. The results of solar drying of apricot and plum are shown in Figures 5 in terms of moisture ratio. With time the weight of fruits decreased with increase in temperature of the wooden box. Drying experiment was started in the morning from 8 am until sundown at 6 pm and the maximum temperature reached was 39 ºC. It was observed that when the sunlight was focused vertically on the fruit slices, the drying rate increased. It can be seen that the drying rate of apricot was similar to plum until three hours. This proximity deviates with increase in time and the drying rate in greater for plum compared to apricot.

Figure 5. Relationship between moisture ratio and time for apricot and plum dried using solar direr.

Fruits Temp. Page’s parameter

Apricot

(ºC) K (hr-1) N R2

40 0.26 1.03 0.99

50 0.30 1.09 0.97

60 0.50 1.25 0.99

Plum

40 0.26 0.92 0.99

50 0.29 1.12 0.99

60 0.43 1.34 0.99

Fruit Diffusion Coefficient (m2/s)

40 ºC 50 ºC 60 ºC

Apricot 9.89×10-11 1.48×10-10 2.91×10-10

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IV. CONCLUSION

Drying experiments were conducted using tray and table-top solar drier for apricot and plum. Kinetic study was carried out for the results obtained from the tray dryer. Page equation was used to determine the kinetic parameters. The value of K ranged between 0.26 h-1 and 0.50 h-1 for apricot and between 0.26 h-1 and 0.43 h-1 for plum. The value of diffusion coefficient at 60 °C with air flow velocity of 0.17 m/s for apricot and plum was 2.91×10-10 m2/s and 2.72×10-10 m2/s respectively. The activation energy was determined to be 146.61 kJ/mol and 52.10 kJ/mol for apricot and plum. Table-top solar drier was designed and fabricated and drying studies were conducted for apricot and plum and it was determined that the drying rate was greater for plum.

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