ARID ZONE JOURNAL OF ENGINEERING, TECHNOLOGY & ENVIRONMENT
AZOJETE, March, 2019. Vol. 15(1):17-24

Published by the Faculty of Engineering, University of Maiduguri, Maiduguri, Nigeria.
Print ISSN: 1596-2490, Electronic ISSN: 2545-5818

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17

ORIGINAL RESEARCH ARTICLE

MOISTURE-DEPENDENT THERMAL PROPERTIES OF COCOYAM CORMELS

B. O. Oyefeso¹* and A. O. Raji
(1Department of Agricultural and Environmental Engineering, University of Ibadan, Ibadan, Nigeria.)

*Corresponding Author: oyefesobabatunde@gmail.com

ARTICLE INFORMATION

Submitted 28 March, 2018
Revised 15 December, 2018
Accepted 19 December, 2018

Keywords:
Cocoyam cormels
thermal conductivity
thermal diffusivity
specific heat
moisture content

ABSTRACT
Cocoyam (Xanthosoma sagittifolium) has served as a cheap
carbohydrate source for many people in the developing countries with
prospects in the production of animal feed, lager beer, and its starch as
binding agent in drug formulation. Drying is an important unit
operation in cocoyam processing and there appears to be lack of
information on the thermal properties which are significant in
designing heat treatment systems such as dryers for the cormels. This
study therefore, investigated some thermal properties of X.
sagittifolium cormels as affected by moisture content. Thermal
properties namely thermal conductivity, diffusivity and specific heat of
white- and pink-fleshed cocoyam cormels were investigated at
moisture contents within the ranges of 130 to 395% and 85 to 200%
(dry basis), respectively. The selected thermal properties of the cormels
were determined with the aid of a thermal properties analyser which
has a dual needle probe and uses transient line heat source method.
Thermal conductivity, diffusivity and specific heat were within the
ranges of 0.454 to 0.562 Wm-1K-1, 1.62×10-7 to 2.32×10-7 m2s-1,
1.836to 4.029 kJkg-1K-1, respectively for white-fleshed cocoyam and
0.489 to 0.601 Wm-1K-1, 1.79×10-7 to 2.65×10-7 m2s-1, 1.615 to 2.991
kJkg-1K-1, respectively for pink-fleshed cormels. Analysis of variance
showed that there were significant differences between the measured
properties at different moisture content levels. Regression models were
established between the investigated thermal properties and moisture
content.

© 2019 Faculty of Engineering, University of Maiduguri, Nigeria. All rights reserved.

1.0 Introduction
Cocoyam, Xanthosoma sagittifolium, is cultivated for various applications. It ranks third in
importance after cassava and yam although it is superior to them in terms of the nutritional
value (Onyeka, 2014). The cormel has a lot of potentials which include its utilization for
production of lager beer (Onwuka and Eneh, 1998)), as binding agents in paracetamol tablet
formulation (Odeku et al., 2005) and as carbohydrate source of juvenile African catfish (Aderolu
et al., 2009). Itis also used traditionally as soup thickeners (Enwere, 1998).
It is processed before consumption and used essentially in the same way as yam (Odebunmi et
al., 2007). It undergoes several heat treatment processes such as boiling (cooking), drying, frying,
smoking and refrigeration to produce its various end products such as chips, flakes, flour and
fufu. The knowledge of its thermal properties, such as thermal conductivity, thermal diffusivity
and specific heat, is necessary in designing heat treatment systems for processing of the cormels;

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Oyefeso and Raji: Moisture-Dependent Thermal Properties of Cocoyam Cormels. AZOJETE, 15(1):17-24. ISSN 1596-
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to foster a sound understanding of the behaviour of the crop when exposed to heat treatments
such as cooling, drying, freezing, frying and smoking (Yang et al., 2002); in modelling of heat
transfer and temperature distribution during processing (Zhu et al., 2007); in the simulation of
heat treatment processes involved in the conversion of cocoyam cormels into cocoyam flour
which is one of its important final products.
Thermal conductivity, thermal diffusivity and specific heat have been determined for cassava
root, yam tuber and plantain fruit (Njie et al., 1998), cumin seed (Singh and Goswami, 2000), shea
nut kernel (Aviara and Haque, 2001), borage seeds (Yang et al., 2002), berberis fruit (Aghbashlo
et al., 2008), guna seed (Aviara et al., 2008), roselle seeds (Bamgboye and Adejumo, 2010) and
doum palm fruit (Aremu and Fadele, 2010). However, there appears to be scarcity of information
on the thermal properties of cocoyam. The objectives of this study include the determination of
thermal conductivity, thermal diffusivity and specific heat of white- and pink-fleshed cocoyam
(Xanthosoma sagittifolium (L.) Schott) cormels and the investigation of the effect of moisture
content on the selected thermal properties.
2. Materials and Methods
Fresh white- and pink-fleshed cormels were obtained from Ogunmakin market near Ibadan city,
Ogunmakin town, Ogun State, South Western Nigeria. Fresh agricultural products are usually
brought in large quantities to the market from neighbouring farms. The cormels were cleaned,
peeled manually and cut into the cylindrical shape of 22 mm diameter and 33 mm height with
the aid of a cylindrical borer for uniformity and reduce the probable effect of variation in shape
and size. The dimensions of the samples were similar to that of a standard verification block
provided with the equipment used in determining the thermal properties. The initial average
moisture content of the cocoyam cormels was determined by the air oven method as described
by Aghbashlo et al. (2008). This involved keeping cocoyam sample of known weight in an oven
maintained at a temperature of 105±2°C until the differences between three consecutive
weights were within 0.05g. The quantities of moisture removed from the samples were obtained
and the moisture contents obtained according to Equations 1 and 2.

MCwb =
Mw
Mwp

× 100 (1)

MCdb =
MCwb

100−MCwb
× 100 (2)

where:
MCdb = % moisture content (dry basis)
MCwb = % moisture content (wet basis)
Mw = mass of moisture removed from the cormel (kg)
Mdp = mass of fresh cocoyam cormel (kg)
The thermal properties of white-fleshed cocoyam cormels were investigated at 130, 180, 250,
320 and 395% average moisture content levels (db) and pink-fleshed cocoyam cormels at 85,
105, 125, 150 and 200% average moisture content levels (db). The desired moisture contents
were obtained by conditioning the samples as described by Olaniyan and Oje (2002). Selected
thermal properties of the cocoyam cormels namely thermal conductivity, thermal diffusivity and
specific heat, were determined with the aid of KD2 Pro Thermal Properties Analyser (Decagon
Devices Inc., USA) which has a dual needle probe and uses the transient line heat source method
in its operation. One of the needles has a heater while the second one has a temperature sensor.

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The dual needles (30mm long, 1.3 mm in diameter and spaced at 6mm apart) were inserted into
the cocoyam sample. Pilot holes were made near the centre of the sample to ensure that the
needles have correct spacing. The needles were allowed to equilibrate to the surrounding before
beginning a reading and good thermal contact was ensured between the sensors and the
cocoyam sample by fitting the sample tightly into the hole to ensure accurate measurements.
After inserting the needles into the cocoyam sample, both the needles and sample were allowed
to attain temperature equilibrium before starting the measurements. Current was passed
through the needle with a heater and the heat was applied for a minute and 30 seconds were
allowed for temperature equilibration before heating starts. Sixty temperature readings were
taken at 15 seconds intervals. Analysis of the probe temperature was used to determine the
thermal conductivity of the cocoyam samples (Njie et al., 1998) while the volumetric specific heat
was obtained from an algorithm developed based on relationships established between thermal
conductivity and specific heat. Thermal diffusivity was determined according to Equation 3. The
measured thermal conductivity, diffusivity and specific heat were read directly from the menu
display and properly documented for further analysis. The measurements were taken by allowing
15 minutes interval between readings to allow the temperatures to re-equilibrate. All the
measurements were carried out in triplicates and the average values were recorded. The
experimental set up for thermal properties measurement was as presented in Figure 1.

μ = k
ρCp

(3)

where: μ = thermal diffusivity m2s−1

k = thermal conductivity Wm−1K−1

ρ = density of cormel (kgm−3)
Cp = specific heat (kJkg−1K−1)
Analysis of variance (ANOVA) was carried out to determine the effect of moisture content on the
measured thermal properties.

Figure 1: Experimental set up for thermal properties measurement

Menu display
needle probe

Dual needle probe

Cocoyam sample

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Oyefeso and Raji: Moisture-Dependent Thermal Properties of Cocoyam Cormels. AZOJETE, 15(1):17-24. ISSN 1596-
2490; e-ISSN 2545-5818, www.azojete.com.ng

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3. Results and Discussion
The thermal conductivity, specific heat and thermal diffusivity of the white- and pink-fleshed
cocoyam cormels are as presented in Figures 2 to 4.

Figure 2: Thermal conductivity of cocoyam cormels

Figure 3: Specific heat of cocoyam cormels

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Arid Zone Journal of Engineering, Technology and Environment, March, 2019; Vol. 15(1):17-24. ISSN 1596-2490;
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Corresponding author’s e-mail address: gbengailori@unilag.edu.ng

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Figure 4: Thermal diffusivity of cocoyam cormels

Thermal conductivities of white-f and pink-fleshed cocoyam cormels increased with increasing
moisture content (Figure 2). A similar trend has been reported for potato (Donsi et al., 1996) and
cassava root and yam tuber (Njie et al., 1998).The thermal conductivity of the white-fleshed
cormels were within the ranges of 0.454 to 0.562 Wm-1K-1 while that of the pink-fleshed cormels
were within the ranges of 0.489 to 0.601 Wm-1K-1 for the range of moisture content considered.
The relationships between the thermal conductivity of the cormels and moisture content could
be represented by Equations 4 and 5.

White: k = 0.0003M + 0.4762 (R² = 0.886) (4)
Pink: k = -1E-06M2 + 0.0009M + 0.395 (R² = 0.923) (5)

The increase in thermal conductivity with increasing moisture content could be attributed to the
high thermal conductivity of water in the cormel which made it possible for heat energy to be
easily conducted through its tissues. This implies that low heat energy input will be required to
uniformly distribute heat (as in drying) through the cocoyam cormels at high moisture content
(Taiwo et al., 1996; Cansee et al., 2008). Similar trends were observed for shea nut kernel (Aviara
and Haque, 2001) and borage seeds (Yang et al., 2002).
The relationships between specific heat and thermal diffusivity of white-f and pink-fleshed
cocoyam cormels and moisture content could be represented by Equations 6 to 9.
Pink: Cp = 3E-06M3 - 0.0015M2 + 0.1977M - 5.999 (R² = 0.771) (6)
White: Cp = 2E-07M3 - 0.0002M2 + 0.0432M - 0.4359 (R² = 0.670) (7)
White: µ = -3E-06M3 + 0.0013M2 - 0.1806M + 9.7461 (R² = 0.943) (8)
Pink: µ = -1.35E-07M3 + 1.02E-04M2 - 2.23E-02M + 3.32E+00 (R² = 0.953) (9)
The relationships were non-linear and sinusoidal in nature since the specific heat and thermal
diffusivity increased and decreased within the range of moisture content considered for both
varieties. This sinusoidal variation in specific heat and thermal diffusivity of the white-fleshed and
pink-fleshed cocoyam could be attributed to the non-homogeneous nature of the cormel as a
bio-material and structural arrangement of its tissues (Aremu and Fadele, 2010). Similar
sinusoidal trend was observed for the thermal diffusivity of doum palm fruit (Aremu and Fadele,
2010) while a linear decreasing trend was observed for whole guna seed and kernel (Aviara et al.,

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Oyefeso and Raji: Moisture-Dependent Thermal Properties of Cocoyam Cormels. AZOJETE, 15(1):17-24. ISSN 1596-
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2008). Taiwo et al., (1996) observed an increase in specific heat of ground and hydrated cowpea
up to a point followed by a decrease with subsequent increase in moisture content while only a
linear decreasing trend was observed for roselle seed (Bamgboye and Adejumo, 2010).The
specific heat and thermal diffusivity of the white-fleshed cormels were found to be within the
ranges of 1.836 to 4.029 kJkg-1K-1 and 1.62  10-7 to 2.32  10-7m2s-1, respectively while those
of the pink-fleshed cormels were within the ranges of 1.615 to 2.991 kJkg-1K-1 and 1.79  10-7 to
2.65 10-7m2s-1, respectively. The thermal conductivity and specific heat of the cormels
compared well with the range of values obtained for other agricultural materials such as doum
palm fruit, berberis fruit, plantain, cassava root and yam tuber (Aremu and Fadele, 2010;
Aghbashlo et al., 2008; Njie et al., 1998).
The ANOVA results of the effect of moisture content on the selected thermal properties are as
presented in Tables 1 and 2. The statistical analysis showed that the effect of variation in
moisture content was significant (p<0.05) on all the measured thermal properties. This clearly
indicated that the response of the cocoyam cormels as biological materials to heat treatments is
dependent on the amount of moisture present in the cormels. This could serve as a guide in
determining the amount of energy required to properly condition cocoyam cormels at a given
moisture content during heat treatments and in storage. Therefore, knowledge of thermal
properties investigated with respect to moisture content is applicable in the optimization of
thermal processing systems and serve as an essential guide in the design of heat treatment
equipment such as a dryer for cocoyam cormels.

Table 1: Effect of moisture content on thermal properties of white-fleshed cocoyam cormels
Source Model Sum of

Squares
Degree of
freedom (Df)

Mean
Square

F-
value

Level of
Significance

Thermal
conductivity

Regression 0.002 1 0.002 10.014 0.051

Residual 0.001 3 0.000
Total 0.002 4

Thermal
diffusivity

Regression 0.000 1 0.000 6.360 0.086

Residual 0.000 3 0.000
Total 0.000 4

Specific heat Regression 0.173 1 0.173 0.829 0.430
Residual 0.627 3 0.209
Total 0.800 4

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Table 2: Effect of moisture content on thermal properties of pink-fleshed cocoyam cormels
Source Model Sum of

Squares
Degree of
freedom (Df)

Mean
Square

F-
value

Level of
Significance

Thermal
conductivity

Regression 0.000 1 0.000 7.737 0.069

Residual 0.000 3 0.000
Total 0.001 4

Thermal
diffusivity

Regression 0.000 1 0.000 1.551 0.301

Residual 0.000 3 0.000
Total 0.000 4

Specific heat Regression 0.317 1 0.317 5.910 0.093
Residual 0.161 3 0.054
Total 0.478 4

4. Conclusions
Thermal properties of white- and pink-fleshed cocoyam (X. sagittifolium) cormels were examined
in the study. The results showed that:
Thermal conductivity of the cormels increased with increasing moisture content and the values
were within the ranges of 0.454 to 0.562 Wm-1K-1 and 0.489 to 0.601 Wm-1K-1, for white- and
pink-fleshed cormels respectively; Thermal diffusivity of white-f and pink-fleshed cormels were
within the ranges of 1.62×10-7 to 2.32×10-7 m2s-1 and 1.79×10-7 to 2.65×10-7 m2s-1, respectively;
Specific heat of white-f and pink-fleshed cormels were within the ranges of1.836 to 4.029 kJkg-
1K-1 and 1.615 to 2.991 kJkg-1K-1, respectively; All the investigated properties were dependent on
the level of moisture present in the cormel. Data on the thermal properties investigated with
respect to variation in moisture content is applicable in the optimization of thermal processing
systems and as an essential guide in the design of heat treatment equipment for cocoyam
cormels.

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2490; e-ISSN 2545-5818, www.azojete.com.ng

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