Investigation into the Thin Layer Drying Models of Nigerian Popcorn Varieties

Leonardo Electronic Journal of Practices and Technologies
Issue 13, July-December 2008

ISSN 1583-1078
p. 47-62



Investigation into the Thin Layer Drying Models of Nigerian Popcorn
Varieties

Taiwo ADEMILUYI, E.O. OBOHO, and Michael OWUDOGU
Department of Chemical/Petrochemical Engineering,
Rivers State University of Science
and Technology, Port Harcourt, Nigeria

E-mail:ademuluyi@yahoo.com

Abstract
Drying kinetics of three popcorn varieties (Pin, Deep and Light yellow) was
investigated. The popcorn kernels initially conditioned to 25% moisture
content were dried in a bench scale rotary drier to 14% moisture content at
various air flow rates (0.83, 1.397, 2.79 m/s) and temperatures (50oC, 60oC,
70oC and 80oC). Falling rate drying period was observed for the three popcorn
varieties with Pin popcorn having the highest drying rate. Eight drying models
were used to determine the thin layer drying kinetics. The Page equation, Two
term model, Modified Page were found the best to describe the thin layer
drying of Pin, Light yellow and Deep yellow popcorn, but the newly
proposed model serve as a general model which best describe the drying
kinetics of all the three popcorn varieties, with high correlation coefficient.
The drying behaviour of the three kernels was found to be largely dependent
on changes in temperature and air velocity
Keywords
Popcorn varieties, drying kinetics, mathematical models, thin layer




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Investigation into the Thin Layer Drying Models of Nigerian Popcorn Varieties
Taiwo ADEMILUYI, OBOHO E.O, and Michael OWUDOGU

Introduction


The most important factor influencing the economic value of popcorn is popping
volume i.e., the volume of popped corn produced from a given weight of unpopped kernels.
Processors may reject popcorn that does not meet specified minimum popping volume.
Popping volume is affected somewhat by harvesting and handling practices, and by the
moisture history of the popcorn prior to popping; but the primary factor is the moisture
content of the kernels when popped. Studies have shown that maximum popping volume is
produced at moistures ranging from 13.0 to 14.5%, with 13.5% being optimum. Data also
indicate that popcorn must be initially dried to at least 13.5% moisture before it attains
maximum popping volume. After that, moisture can increase to 15% without significantly
decreasing popping volume. Over dried popcorn (11% or below) can be rewetted to 13.5%
moisture, but it will not recover the maximum popping volume it had on initial dry down to
13.5%. Popcorn mechanically harvested on the ear at 20-25% moisture content must be dried
promptly to prevent mold growth in storage [1]

In Nigeria, popcorn is sun dried below 14% moisture to enable long storage of grain
but this creates a lot of work for the processors, as the popcorn has to be rewetted before
popping. During this rewetting process the moisture content of the popcorn may be more or
less than 14%. Hence it is necessary to understand the drying characteristics of Nigerian
popcorn varieties so that they can be conditioned, dried to 14% moisture content and
packaged properly like the foreign type before selling to the processor.

Most of the drying studies on popcorn was carried out using thin layer drying models
for example Ross and White, 1972; White et al., 1981. and Morel, ,1984 reported that the
exponential thin layer model and page was adequate for predicting popcorn drying rates and
establishing the effect of drying air conditions on the fully exposed drying characteristics of
popcorn. Tran et al (1999) applied the two-compartment model to thin layer drying of two
Australian popcorn varieties and obtained a correlation coefficient of 0.9858 and standard
error of 0.034, assuming a single linear temperature dependence of the drying constants.

The effect of ethyl oleate on the drying rates for corn was investigated in a pilot plant
air-dryer by Doymaz and Mehmet. (2003). Corn was air dried without treatment or after
dipping in a cold solution of ethyl oleate. The shorter drying times and best quality dried
product were obtained with corn kernels dipped in the solution of ethyl oleate. The single
48

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Leonardo Electronic Journal of Practices and Technologies
Issue 13, July-December 2008

ISSN 1583-1078
p. 47-62

exponential equation and the Page equation were used to determine the thin-layer drying
characteristics. Both the equations fitted well to the experimental data. The Page equation was
found to better describe the thin-layer drying of corn than the single exponential equation.

Togrul and Pehlivan (2002) developed a mathematical model of solar drying of
apricots in thin layer. Drying curves obtained from the data were fitted to 14 mathematical
models and the effects of drying air temperature, velocity and relative humidity on the model
constants and coefficients were evaluated by the multiple regression and compared to
previously given models. The logarithmic drying model was found to satisfactorily describe
the solar drying curve of apricots with a correlation coefficient (r) of 0.994.

No work on drying kinetics of Nigerian popcorn varieties was found; hence the
objective of this work is to study the thin layer drying characteristics of three Nigerian
popcorn varieties. The data obtained would be fitted into 7 generally accepted thin layer
drying models and a newly proposed model.




Materials and Methods

Experiment

Popcorn varieties: yellow Pin (rice shaped), Deep yellow, Light yellow (pearl shaped)
were obtained from Ahmadu Bello University (ABU), Zaria in Kaduna state, Nigeria. These
varieties upon harvesting were sun-dried and stored in bags before they were purchased.
These local popcorn varieties upon reception contain about 12.8 – 13.8% moisture. The
kernels were cleaned manually to remove all foreign matter and broken kernels. The popcorn
kernels were conditioned to 25% moisture content by adding a calculated quantity of water,
mixing thoroughly and then sealing in separate polyethylene bags. The samples were kept at
10°C in a refrigerator for 7days for the moisture to distribute uniformly throughout the
samples. Before each test, the samples were taken out of refrigerator and allowed to warm up
to room temperature. Moisture content of the popcorn kernels was determined with the aid of
Delm Horst grain moisture detector (G7) per pass through the rotary dryer.

40g of popcorn sample with 25% moisture content were weighed with electronic
balance and dried using a bench scale rotary dryer at drum speed of 12 rev per min. The
drying tests were carried out for each popcorn sample at various drying temperature ranging
49

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Investigation into the Thin Layer Drying Models of Nigerian Popcorn Varieties
Taiwo ADEMILUYI, OBOHO E.O, and Michael OWUDOGU

from 50oC, 60oC, 70oC and 80oC and air velocities 0.83, 1.397, and 2.79 m/s. The system was
allowed to run till the drying temperature stabilized before the popcorn was charged into the
dryer. The samples were allowed to cool in desiccators after each run before final weighing.
The drying experiments continued for each sample until the final moisture content of 14% wet
basis was obtained.



Thin layer drying models

In drying of thin-layers of agricultural crops, the Page equation has been used
extensively. The equation is empirical, and is given in equation (1)[2]
M ? M
MR
e
=
=
(

Exp ?kt n )





(1)
M ? M
o
e
where: MR = Moisture ratio; Mo = initial moisture content (% db); Me = equilibrium moisture
content (% db); M = moisture content at time t (% db); t = drying time (hr); k and n = drying
constants.

Moisture ratio data obtained at air temperatures of 50 - 80oC for each variety were
fitted into 7 generally accepted drying models and a newly proposed model (Taiwo Modified
Page model), as shown in Table 1. Regression analyses were done using the Datafit 8.2
(Oakdale Engineering statistical software, USA). The coefficient of correlation (r) was one of
the primary criteria for selecting the best equation to define the thin layer drying curves of
each popcorn variety. Equilibrium moisture content (Me) of 14% (dry basis) was used; this
value has been accepted as the final moisture for packaging these varieties before popping [3].

Table 1: Mathematical models given by various authors for the drying curves
Model No.
Name
Model equation
References
1 Newton
MR=
exp(-kt)
[4]
2
Page
MR = exp(-ktn) [5]
3
Modified Page
MR = exp(-(kt)n) [6]
4
Henderson and Pabis
MR = a·exp(-kt)
[7]
5
Logarithmic
MR = a·exp(-kt) + c
[8]
6
Two-term
MR= a·exp(-kot) + b·exp (-k1t) [9]
7
Wang and Singh
MR = 1 + at + bt2 [10]
8
Newly proposed model
MR = a·exp(-(kt)n) [11]



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Leonardo Electronic Journal of Practices and Technologies
Issue 13, July-December 2008

ISSN 1583-1078
p. 47-62

Results and Discussion



Effect of Moisture Content and Drying Time on Drying Rates.

The variations of drying rate with moisture for the three popcorn varieties are shown
in Figs 1 and 2. Drying rate decreases for all the samples with decrease in moisture content.
Falling rate drying period was exhibited by the varieties. This means that the movement of
water is basically diffusion controlled as reported by McCabe et al, (1987).

0.03
d/min)
0.025
ate
pin
0.02
deep yellow
light yellow
0.015
g evapor
0.01
a
te (
0.005
ying r
0
Dr
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
Moisture content (g water/ g dry solid)

Figure 1. Rate of drying curve for different popcorn varieties at 70°C, 12 rev/min, 1.397m/s

0.03
0.025
0.02
Pin
0.015
Deep yellow
0.01
light yellow
aporated/min)
ev 0.005
Drying rate (g water
0
0
5
10
15
20
25
30
35
Drying time (min)

Figure 2. The changes with time of the drying rate of popcorn varieties at 70°C, 12 rev/min,
1.397m/s
51

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Investigation into the Thin Layer Drying Models of Nigerian Popcorn Varieties
Taiwo ADEMILUYI, OBOHO E.O, and Michael OWUDOGU

The moisture content decreases with drying time for all the varieties, but the drying
times of each kernel samples were not the same. This same behaviour was exhibited during
the drying of apricot [12] and Australian popcorn varieties [13]
Pin popcorn had the shortest drying time (19 mins) while light yellow popcorn has a longer
drying time of 30 mins, to reach a final moisture of 14% wet basis as shown in Fig. 3.

26
24
22
20
pin
18
light yellow
deep yellow
16
ure content (wet basis) 14
12
% Moist 10
0
5
10
15
20
25
30
35
Drying time (mins)

Figure 3. Drying curve of different popcorn at 70oC, 12 rev/min, 1.397m/s



Effect of Air Flow Rates and the Temperature on the Drying Rates
From Figs 4 - 6, the moisture content decreases as temperature increases for all the
varieties. The drying rate also increases with temperature. Pin popcorn dried faster than deep
yellow and light yellow popcorn at all temperatures (i.e. 50oC, 60oC, 70oC and 80oC). The
porosity of the varieties reported by Taiwo et al., (2006), revealed that Pin popcorn had a
porosity of 42.88% while Light and Deep yellow had 38.80% and 31.50% respectively. Pin
popcorn seems to be more porous hence moisture will be released faster from its pores than
other varieties.
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Leonardo Electronic Journal of Practices and Technologies
Issue 13, July-December 2008

ISSN 1583-1078
p. 47-62

i
s
)
26
24
et bas
50C
w
22
60C
20
70C
ontent (
18
80C
e
C
16
tur
ois
14
M
%
12
10
0
10
20
30
40
50
60
Drying time (mins)

Fig 4. Drying Curve for Pin Popcorn at different temperatures.

) 26
s
is 24
50C
t ba 22
60C
70C
t (we 20
n
80C
te
n 18
o
C 16
ture 14
is
o 12
M
% 10
0
10
20
30
40
50
60
70
80
Drying time (mins)

Figure 5. Drying curve for deep yellow popcorn at different temperatures.



53

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Investigation into the Thin Layer Drying Models of Nigerian Popcorn Varieties
Taiwo ADEMILUYI, OBOHO E.O, and Michael OWUDOGU

26
a
s
i
s
)
24
50C
e
t
b 22
60C
t (w 20
70C
t
e
n
n
80C
18
o
C 16
re
tu
is 14
o
M 12
% 10
0
10
20
30
40
50
60
70
80
Drying time (mins)

Figure 6. Drying Curve for Light Yellow Popcorn at different temperatures.


At different air velocities, the moisture content also decreased with time as the air
velocity increased as shown in Fig 7. The changes occurring in the moisture content of
popcorn by varying the air flow rates at constant air temperature seem more pronounced than
those observed by varying the air temperatures at constant air flow rates. This dominance of
air flow rates over the temperature is related to the transport of water vapour to the main air
stream. Increasing airflow rates reduces the thickness of the boundary layer between airflows
and the surface of the popcorn kernels. A similar behaviour was reported for apricot and fruit
drying [12]
26
a
s
i
s
)
24
e
t
B
22
0.83m/s
t (W
1.397m/s
20
n
2.79m/s
18
onte
c
16
14
ture
is
o
12
M
10
%
0
20
40
60
80
100
120
Drying time (mins)

Figure 7. Effect of air velocity on the drying behaviour of Deep yellow popcorn
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Leonardo Electronic Journal of Practices and Technologies
Issue 13, July-December 2008

ISSN 1583-1078
p. 47-62



Modeling of Drying Curve

The falling rate drying behaviour exhibited by the popcorn varieties reveals that the
Newly proposed model and other acceptable general drying models listed in the Table 1 can
be used to model the drying curve for the three popcorn varieties. The experimental results
were used to calculate the moisture ratio. Equilibrium moisture content of 14% was used in
the calculation because this is the best moisture content popcorn can be dried before
packaging. The highest popping volume is obtained at this moisture content as stated earlier.
From Figs 8 - 10, the moisture ratio decreased as temperature increases and approaches zero
as it reaches the equilibrium moisture content. Tran et al, 1999 reported a similar result for
Australian popcorn.

1.2
1
50C
0.8
tio
60C
ra 0.6
70C
ture
is
o
80C
0.4
M
0.2
0
0
10
20
30
40
50
60
Drying time (mins)

Figure 8. Moisture ratio versus drying time for Pin popcorn

Table 2- 4 shows the regression analysis of the experimental results for the popcorn
varieties using eight models. The model results (Table 2) from regression analysis of the
experimental data for Pin popcorn showed that the drying behaviour of Pin popcorn can be
modeled with Two term, Page, modified Page or the newly proposed model with a coefficient
of multiple determination of 0.996.
55

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Investigation into the Thin Layer Drying Models of Nigerian Popcorn Varieties
Taiwo ADEMILUYI, OBOHO E.O, and Michael OWUDOGU

1.2
1
0.8
tio
50C
ra
0.6
60C
ture
is
70C
o
0.4
M
80C
0.2
0
0
20
40
60
80
Drying time (mins)

Figure 9. Moisture ratio versus drying time for Deep yellow popcorn

1.2
1
0.8
50C
60C
0.6
70C
oisture ratio
80C
M 0.4
0.2
0
0
10
20
30
40
50
60
70
80
Drying times (mins)

Figure 10. Moisture ratio versus drying time for Light yellow popcorn


Moisture ratio obtained from experiment, Page model and Newly proposed model for
Pin popcorn is presented in Fig 11. Doymaz & Mehmet [14], obtained similar result for corn.
Wang and Singh model has the least r2. The logarithmic model was not applicable for
predicting the drying behaviour of the popcorn varieties. The results were obtained using
moisture content results of four passes through the rotary dryer for good comparison between
popcorn varieties.
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Document Outline

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