international journal of computer theory and engineering (ijcte)

Drying Kinetics of Moringa (Moringa oleifera)

Seeds

Ademola K. Aremu University of Ibadan, Department of Agricultural and Environmental Engineering, Nigeria

Email: [email protected]

A. Akintola Oyo state College of Agriculture, Igboora, Oyo State, Nigeria

I. INTRODUCTION

Moringa oleifera is native to some parts of Africa and

Asia and it is the sole genus in the flowering plant family

Moringaceae. All of the parts of the tree can be used in a

variety of ways. Moringa is full of nutrients and vitamins

and is good for both human and animal consumption [1].

Moringa helps to clean dirty water and is a useful source

of medicines [2]. It provides lots of leafy material that is

useful when using alley-cropping systems [3], [4].

Drying is a unit operation which involves the

simultaneous heat and moisture transfer in order to lower

the moisture content of products to a safe level [5]. Foods

are dried commercially, starting either from their natural

state (e.g. vegetables, fruits, milk, spices, and grains) or

Manuscript received January 2, 2016; revised June 18, 2016.

after processing (e.g. instant coffee, whey, soup mixes,

non-dairy creamers) [6]. Drying kinetics is the description of the changes of

moisture content of material during drying [7]. It can be

expressed as a drying curve or drying rate curve. Several

researches have reported on the drying characteristics of

different food products; some of these works include:

“Ref. [7]” evaluated the effects of drying air temperature

on functional properties of dehydrated aloe vera and

proposed a mathematical model to simulate the drying

curves of the product; “Ref. [8]” studied the water

sorption isotherms and drying characteristics of tomato

seeds; “Ref. [9]” studied the kinetics of drumstick leaves

during convective drying; “Ref. [10]” carried out a

research on air-drying and rehydration characteristics of

date palm; “Ref. [11]” investigated the drying kinetics of

grape seeds; “Ref. [12]” studied the thin layer drying of

millet and effect of temperature on its drying

characteristics; “Ref. [13]” studied the effect of slice

thickness and temperature on the drying kinetics of

Mango; “Ref. [14]” reported the kinetics of thin layer

drying of apple. The major observation in all these

researches was that the drying of these products took

place just under the falling rate period and the drying

temperature was the most influent parameter in the

process.

There appear to be no available information on drying

kinetics of Moringa seed. Therefore, the objective of this

research is to investigate the drying kinetics of Moringa

in a mechanical dryer and fit the drying data into three

mathematical models to determine which is most suitable.

Nomenclature

MR moisture ratio

r gas constant (kJ/mol K)

M moisture at any time t during drying

T temperature in (C)

Mi initial moisture content

RMSE root mean square error

Me equilibrium moisture content

K, N, a drying constants

Deff moisture diffusivity coefficient (m2/s)

R2 coefficient of determination

D0 maximum diffusion coefficient (at infinite

temperature)

Journal of Life Sciences and Technologies Vol. 4, No. 1, June 2016

7© 2016 Journal of Life Sciences and Technologiesdoi: 10.18178/jolst.4.1.7-10

Abstract—Drying of Moringa oleifera seed was investigated

at air temperatures of 50℃, 60℃ and 70℃. 200g of the bulk

seeds were dried using a cabinet tray dryer with a fan

blowing at a velocity of 3.5m/s over the heating elements

into a drying chamber with perforated trays. The drying

data were applied to three drying models namely Page,

Lewis and Henderson and Pabis. The model performance

was evaluated by comparing the coefficient of determination

(R2) and the Root Mean Square Error (RMSE) of the

experimental and predicted moisture ratios using non-linear

regression analysis. The R2 and RMSE vary between0.902-

0.999 and 0.053 and 0.372 respectively for the three models

used for prediction. Page model satisfactorily predicted the

drying behaviour of moringa seed with the highest R2 and

lowest RMSE value and also gave best fitting curves. The

effective moisture diffusivity of moringa seed was found to

be 20.1×10-9, 25.1×10-9 and 30.5×10-9m2/s at 50℃, 60℃ and

700C respectively and the activation energy was found to be

19.49kJ/mol. This study showed that the drying of moringa

seeds can be accurately predicted using any of the thin-layer

drying models used. The Page model proved to be an overall

better prediction model for drying of moringa seed. These

instructions give you basic guidelines for preparing camera-

ready papers.

Index Terms—drying kinetics, moringa seeds, cabinet tray

dryer, drying models

Ea activation energy for diffusion (kJ/mol)

II. MATERIALS AND METHODS

Moringa pods were sourced from subsistence farmers

around Ajibode area, a neighbourhood of University of

Ibadan, Nigeria. The pods were harvested in February

when the average temperature was around 27-30℃ and

the relative humidity of the environment was around 70-

80%. The pods were carefully shelled to obtain Moringa

seeds. The seeds were prepared using the method

described by [15] with some modifications to suit

laboratory and experimental conditions. The seeds were

cleaned and soaked in water for 20hours, decanted to

remove chaffs and then kept in moisture tight polyethene

bags and stored inside refrigerator at about 15℃ to allow

for a stable and uniform moisture content of the bulk

seeds. 200g of the stored seeds was brought out and

placed in the laboratory for about 2 hours to attain

ambient conditions before utilized for the experiment.

A. The Drying System

Drying studies were carried out at drying temperatures

of 50℃, 60℃ and 70℃. 200g of the stored sample was

used for each drying experiment and each experiment

was repeated thrice. The weighing was carried out using a

Metra precision weighing balance (0.1-5000g).

Cabinet tray drying was adopted and it was carried out

at the Department of Agricultural and Environmental

Engineering in the University of Ibadan. The cabinet tray

dryer [Plate 1] designed and fabricated at the Department

of Agricultural and Environmental Engineering consists

of a 0.374kW axial flow fan blowing at a velocity of

3.5m/s over the heating elements into a drying chamber

with perforated trays. The dryer casing is lagged with

cushion to give it a compact look. A door was provided to

suite the design for loading and unloading the dryer.

The weight was monitored at interval of 30mins until a

constant weight was reached. The time interval was

chosen based on the drying rate discovered during

preliminary studies.

Plate 1: The Cabinet tray dryer used for experimentation.

B. Drying Analysis

Simplified drying models have been used to quantify

drying kinetics of various grains and some seeds [16].

Three models [(1), (2) and (3)] among the common

models used for drying kinetics in this research are:

Page model:MR= exp(-KtN) (1)

Lewis model:MR=exp(-Kt) (2)

Henderson-Pabis model:MR=a exp(-Kt) (3)

The empirical constants for the thin-layer drying

models were determined experimentally from normalized

drying curves at different temperatures, which were

evaluated based on R2. The form of the normalized Page

Eq. (4) is:

ln(-lnMR)=ln(K)+ Nln(t) (4)

K and N, are determined from the intercept and slope

of the ln (ln(MR)) vs ln(t) curve, respectively. The form

of the normalized Lewis Eq. (5) is:

ln(MR)=-kt+1 (5)

and normalized Henderson–Pabis Eq. (6) is:

ln(MR)=-kt+a (6)

where the drying constants k and a are determined from

the slope and intercept of the ln(MR)vs time curve

respectively. For the Lewis equation, the intercept is set

equal to 1. The equilibrium moisture content (Me) was

obtained by extending the drying time until no

measurable weight loss was observed.

The goodness of fit for each model was evaluated

based on RMSE and R2 values. The best model

describing the drying kinetics of moringa seed was

chosen as the one with the highest R2

value and lowest

RMSE (Doymaz) [17]. The RMSE was calculated using

Eq. (7):

𝑅𝑀𝑆𝐸 = [1

𝑁∑(𝑀𝑅𝑝𝑟𝑒 −𝑀𝑅𝑒𝑥𝑝)

2]

1

2 (7)

where pre and exp represents predicted and experimented

values respectively.

C. Effective Moisture Diffusivity

The determination of diffusivity coefficient was

interpreted using Fick’s second law of spherical bodies as

recorded by [17]. This interpretation was chosen because

the shape of the seeds is almost spherical. The diffusivity

coefficient (Deff) was obtained from the equation of

spherical bodies as shown in Eq. (8):

MR=6/π2exp((-π

2 Deff)/R

2t) (8)

Effective radius (R) is the radius of the seed at the

initial moisture content just before drying commenced.

This was calculated by the equation given by [1] with

equivalent diameter De given by Eq. (9):

De 1+D2+D3 (9)

where R=De/2

D1 = Arithmetic mean diameter = (L1+L2+L3)/3

D2 = Geometric mean diameter =√L1L2L3

D3 =Square mean diameter = √(L1L2+L2L3+L3L1)/3

L1, L2 and L3 are the dimensions in the three

perpendicular axes.

Therefore,

ln(MR)= (-π2 Deff)/R

2 t+ln6/π

2 (10)


8© 2016 Journal of Life Sciences and Technologies

3)/= (D

Moisture diffusivity coefficient (Deff) was calculated

from the slope derived from the linear regression of

ln(MR) against time at temperatures 50℃, 60℃ and 70℃.

D. Activation Energy

Moisture diffusivity coefficient was predicted by the

Arrhenius equation as shown in Eq. (11):

Deff=Do e ((-E

a)/(r(T+273.15)))

(11)

By linearizing the equation, we have:

ln Deff= [-1/(R(T+273.15))] Ea+lnD0 (12)

D0 and Ea were obtained by plotting lnDeff against [-

1/(R (T+273.15))].

Where Ea is the slope and D0 is the intercept.

III. RESULTS AND DISCUSSIONS

A. Equilibrium Moisture Content

It was observed that the equilibrium moisture content

is significantly less than the initial moisture content due

to fluctuating relative humidity during drying, the

equilibrium moisture content was assumed to be 0 g/g dry

solid [18], [19]). This assumption is valid only at the

beginning of drying because as the sample dries the

moisture content approaches the equilibrium. Also, if the

drying curve is not allowed to continue until it reaches

equilibrium, the equilibrium moisture content is measured

to be too high (Doymaz [19]). Table I shows the initial

moisture content and equilibrium moisture content of

moringa seeds for the various drying conditions studied.

From Table I, the initial moisture content of

moringaoleifera ranges between 67.2% and 68.6% while

the equilibrium moisture content was found to be

between 1.56% to 2.92%.

TABLE I. INITIAL MOISTURE CONTENT AND EQUILIBRIUM MOISTURE

CONTENT OF MORINGA SEEDS AT VARIOUS DRYING TEMPERATURES.

Drying

Temperature

Initial Moisture

Content

Final Moisture

Content

50℃ 0.672±0.28 0.0162±0.21

60℃ 0.686±0.21 0.0156±0.07

70℃ 0.678±0.14 0.0292±0.21

B. Drying Curves

Drying was affected by moisture content and water

activity, temperature change, relative humidity and

drying rate.

Figure 1. Result obtained during drying of moringa seeds using

cabinet tray dryer at temper atures 50℃, 60℃ and 70℃.

The drying curve Fig. 1 was plotted between Moisture

Ratio (MR) and drying time (t). The time is in hours and

the moisture ratio has no unit.

C. Modelling and Model Fittings

The model constants and the coefficients from the

result of statistical analysis undertaken by regression on

the three models were given in Table II. Moisture ratio

data obtained from the drying experiment were fitted to

the three thin layer drying models using the non-linear

regression to obtain the RMSE and R2 values. Parameters

of thin layer drying models were determined statistically

by fitting experimental data to the model equations in

order to estimate and select the appropriate model within

the three models selected. The values of coefficient of

determination (R2) and root mean square error values

(RMSE) of the three models at 50℃-70℃ are presented

in Table II. The values R2 and RMSE for these models

vary between 0.902- 0.999 and 0.053 and 0.372

respectively. Page model gave the best prediction with

the highest R2 (0.999) value and lowest RMSE compared

to other models used for prediction.

TABLE II. PARAMETERS OF THE THREE MODELS FOR DRYING

KINETICS OF MORINGA SEEDS.

Air temperature

Models

Parameters 500C 600C 700C

Page k

N

R2

RMSE

1.059

0.604

0.999

0.289

0.881

0.528

0.999

0.221

0.813

0.594

0.999

0.192

Lewis k

R2 RMSE

0.744

0.934 0.053

0.993

0.965 0.263

0.958

0.902 0.235

Henderson-Pabis K a

R2

RMSE

0.618 0.517

0.993

0.356

0.945 0.831

0.986

0.296

0.773 0.475

0.992

0.372

Figure 2. Semi-log plot of moisture ratio for estimation of diffusivity coefficient of moringa seed at temperatures 50oC, 60oC and 70oC.

D. Effective Moisture Diffusivity (Deff)

The effective moisture diffusivity of moringa seed was

found out to be 20.1×10-9

, 25.1×10-9

and 30.5×10-9

m2/s

for 50℃ , 60℃ and 70℃ respectively. The effective



moisture diffusivities for moringa seeds are within a

range of 10-12

-10-8

m2/s reported by [20] for food and

agricultural materials. The effective moisture diffusivity

for moringa seeds increased with increasing temperature.

The effective mean radius for moringa seeds was found

out to be 5.67mm.

Fig. 2 shows the semi-log plot of ln MR against drying

time used to determine Deff.

E. Activation Energy (Ea)

The activation energy Ea was obtained from the slope

of the straight line when lnDeff was plotted against [-

1/(R(T+273.15))] as shown in Fig. 3. Ea was obtained to

be 19.486kJ/mol. The activation energy is within the

range 12.7-110kJ/mol given for most high moisture

agricultural and food products by [20].

Several other researchers reported the values of

activation energy: 51.26kJ/mol for okra [19], 18.03 and

21.47kJ/mol for Castor seed [15], 28.95kJ/mol for mango

[13] The values of effective moisture diffusivity (Deff)

and activation energy (Ea) of moringa seed at

temperatures 50℃, 60℃ and 70℃ is shown in Table III.

-1/(R(T+273.15))×10

-3(1/kJmol

-1)

Figure 3. Semi-log plot of effective moisture diffusivity against [-1/(R (T+273.15))] to determine the activation energy.

TABLE III. MOISTURE DIFFUSIVITY COEFFICIENT AND ACTIVATION

ENERGY OF MORINGA SEED

Temperature (0C) Deff(× 10-9m2/s) Ea (kJ/mol)

50 20.1

60 25.1 19.486

70 30.5

IV. CONCLUSION

This study revealed that the drying of moringa seeds

can be accurately predicted using any of the thin-layer

drying models. The Page model gave the best prediction

model for drying of moringa seed. The moisture transfer

can be described by diffusion, and the temperature

dependence of the effective moisture diffusivities was

shown to follow an Arrhenius relationship.

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[6] R. G. Moreira, “Impingement drying of foods using hot air and

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thickness and temperature on the drying kinetics of mango

(Mangifera indica),” IJRRAS, vol. 15, no. 1, pp. 41-50, 2013. [14] E. Meisami-asl and S. Rafiee, “Mathematical modeling of kinetics

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Ademola K. Aremu (B. Sc’91–M. Sc’94–Ph.D’04–MNIAE’99–

MNSE’00–MASABE’08–R. Engr’01). This author bagged a Bachelor

of Science Degree in 1991, Master of Science Degree in 1994 and Doctor of Philosophy in 2004 in Agricultural Engineering, University of

Ibadan, Ibadan, Nigeria. He became a Member, Nigerian Institute of Agricultural Engineers (NIAE) in 1999, a Member, Nigerian Society of

Engineers (NSE) in 2000, a Member, American Society of Agricultural

and Biological Engineers in 2008, and a Registered Engineer, Council for the Regulation of Engineering in Nigeria (COREN) in 2001.



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