ARID ZONE JOURNAL OF ENGINEERING, TECHNOLOGY & ENVIRONMENT
AZOJETE, September 2019. Vol. 15(3) 714-724

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

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714

ORIGINAL RESEARCH ARTICLE

EFFECT OF MOISTURE CONTENT VARIATION ON THERMO-PHYSICAL PROPERTIES OF
BROWN VARIETY TIGERNUT (Cyperus esculentus)

D. D. Usman1*, E. O. Adanu2, B. G. Jahun1 and K. Ibrahim1
(1Department of Agricultural and Bioresource Engineering, Abubakar Tafawa Balewa University,

P. M. B. 0248 Bauchi, Nigeria.
2Department of Agriculture, Federal College of Education (Technical), Gombe, Nigeria)

* Corresponding author’s email address: ddusman@atbu.edu.ng

ARTICLE INFORMATION

Submitted 22 November, 2018
Revised 7 March, 2019
Accepted 10 March, 2019

Keywords:
Tigernut
thermal conductivity
thermal diffusivity
brown variety
thermophysical Properties

ABSTRACT
Knowledge of thermophysical properties is essential to the food
processing industries. The properties are useful in modeling of thermal
behavior of seeds during processing operations such as frying, baking,
boiling fermentation and drying. To this end, the thermo physical
properties (bulk density, porosity, surface area, thermal conductivity,
thermal diffusivity and specific heat capacity) of brown variety tigernut
(Cyperus esculentus) were investigated as a function of moisture
content. In this study, thermal conductivity and specific heat capacity
were determined by transient state heat transfer (line heat source) and
mixture methods respectively, whereas, thermal diffusivity, bulk density,
porosity and surface area were determined using formula methods. The
evaluation was conducted at 5 moisture levels (20, 25, 30, 35 and 40%
w.b) with 5 replications. The thermal conductivity and diffusivity
increased from 0.03Wm-1k-1 to 0.13Wm-1k-1 and from 7.24 x 10-7m2s-1
to 11.06 x 10-7m2s-1 as the moisture content increased from 20 to 40%
respectively. The specific heat increased from 10.55 Jkg-1K-1 to 18.75
Jkg-1K-1, while the bulk density and surface area showed an increase
from 421.40 kg/m3 to 639.80 kg/m3 and from 197.26 mm2 to 413.21
mm2 as moisture content increased. However, porosity decreased in
value from 61.84 to 32.33% with an increase in moisture. Moisture
content variation was statistically significant (p<0.05) on all properties.
The obtained results will find use in the selection of suitable methods
for processing of tigernut products, in a quality assessment of optimal
modes of technological processes, and in the development of modern
processing and handling equipment.

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

1.0 Introduction
Knowledge of thermo-physical properties is of primary importance to the food industry. This
information is required to make proper design of food processing equipment such as tanks,
pumps, pipes, chillers and evaporators (de Moura et al., 1998). The thermal conductivity of
materials can be influenced by a number of factors such as the moisture content of the material,
porosity and fiber orientation of the material (Mohsenin, 1990; Stroshine and Hamann, 1994).
Several researchers including Rahman (1993) and Yang, et al. (2002) have emphasized the need
for thermal properties of foods in general and seafood in particular. Thermal properties are

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useful in modeling thermal behavior of seeds during thermal processing operations such as
drying, boiling, baking and frying (Alagusundaram et al., 1991; Yang et al., 2002). The primary
thermal properties of food and agricultural products are specific heat capacity, thermal
conductivity and thermal diffusivity. Specific heat is the property needed in the estimation of the
amount of energy required to change the temperature of a product, while thermal conductivity
and thermal diffusivity are involved in the determination of the rate of heat transfer for efficient
process and equipment design. (Aviara et al., 2003). Knowledge of thermo-physical properties is
a basic condition for describing material’s behavior during food processing. There is, however,
dearth of information on the thermo-physical properties of tiger-nut which has limited its
utilization in the food industry in Nigeria.

Tiger-nut (Cyperus esculentus var. sativus), an emergent grass- like plant (Figure 1) belonging to
the sedge family, is a cosmopolitan perennial crop of the same genus as the papyrus plant
commonly found in seasonally flooded wetlands (Vilmorin, 2010). It is widely distributed in the
temperate zones within South Europe as its probable origin, and has become naturalized in
Ghana, Nigeria and Sierra Leone (Anorn, 1992). Tiger- nut (Figure 2) is one of the earliest
domesticated crops and found abundantly in Nigeria as either fresh, semi-dried or dried form in
local markets and consumed as snacks in its uncooked form. Tiger nuts are grossly under-
utilized due in part to paucity of information on their nutritional potential (Rita, 2009). Tigernut
grows mainly in the middle belt and northern regions of Nigeria. It is known in Nigeria as‘Ayaya
in Hausa, ‘Imumu’ in Yoruba and ‘Akiausa’ in Igbo, and three varieties (black, brown and yellow)
are cultivated (Osagie and Eka, 1998). Among these, only two varieties, yellow and brown are
readily available in the market. The yellow variety is preferred over others because of its inherent
properties like its large size, attractive colour and fleshier nature. The yellow variety also yields
more milk, contains lower fat, higher protein and less anti-nutritional factors especially
polyphenols (Okafor et al., 2003; Belewu and Abodunrin, 2005; Ebringa, 2007). The tubers
contain up to 30% of non-drying oil which is used in cooking, soap making, component of
beauty product, medicinally as enemas and oral medications (Shaker et al., 2009; Muhammad et
al., 2011).

Figure 1: Tigernut Plant Figure 2: Tigernut Tuber

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Usman, et al: Effect of moisture content variation on thermo-physical properties of brown variety tigernut (Cyperus
esculentus). AZOJETE, 15(3):714-724. ISSN 1596-2490; e-ISSN 2545-5818, www.azojete.com.ng

Corresponding author’s e-mail address: ddusman@atbu.edu.ng 716

The selected thermo-physical properties (specific heat capacity, thermal conductivity, thermal
diffusivity, bulk density, porosity and surface area) for this present study are important in many
problems associated with the design of machines and the analysis of the behavior of the product
during agricultural process operations such as handling, planting, harvesting, threshing, cleaning,
sorting and drying. Solutions to problems in these processes involve knowledge of these
engineering properties (Irtwange, 2000; Varnamkhasti et al., 2008; Tavakoli et al., 2009).
Temperature and moisture present the most important parameters having clear relation to the
character of physical, chemical and physiological processes in biological agricultural materials.
Physical processes, above all heat and moisture transport, affect the intensity of physiological
processes, nominally of breath, germination, microorganisms progress and many others and in
consequence determine the final quality of food sources. The understanding of the thermal
properties of food and their responses to process conditions is necessary not only because they
affect physical treatment received during processing but also because they are the commonest
indicators of other properties and qualities. Hence, the main objective of this study is to
determine the effect of moisture content variation on the thermo-physical properties of brown
variety tiger-nut.

2. Material and Methods
2.1 Sample Preparation
The brown variety of tigernut used for this study was obtained at a local market in Bauchi,
Bauchi State, Nigeria. The tubers were cleaned manually to remove all foreign matters such as
dirt, stones, immature and broken seeds. The initial moisture content of the samples was
determined by oven drying method at 103 ºC for 48 h, as used by Sacilik et al. (2003). Selected
samples were moistened with a calculated quantity of distilled water and conditioned to raise
their moisture content to the desired different levels. Equation (1) was used to calculate the
quantity of distilled water used according to Coskun et al. (2006).

Q = Wi (mi−mf)
100−mf

(1)

Where: Q = mass of added water (kg), Wi is the initial mass of the sample (kg), mi is the initial
moisture content of the sample (%,) and mf is the final moisture content of the sample (%,).
Five levels of moisture contents were used for the brown variety, 20, 25, 30, 35 and 40% (w.b),
which were achieved by oven drying at 72 °C (recording mc at every 15 min interval). The
samples were then stored in airtight polythene and kept at 5 °C in a refrigerator for a week to
achieve uniform moisture distribution within them.

2.2 Determination of Bulk Density
Bulk density is the ratio of the mass of a sample of seed to total volume (Shafiee et al., 2009).
Bulk density for all the samples were determined by filling an empty 300 mL beaker with tiger-
nut seeds and weighing it. The weight of the seeds was obtained by deducting the weight of the
empty beaker from the weight of the seed-filled beaker. To achieve uniformity in bulk density,
the beaker was tapped 10 times for the seeds to consolidate in the beaker (Ahmadi et al., 2009).
The beaker was filled with seeds dropped from a 15 cm height and a sharp–edged, flat metal file
was used to remove excess seeds to level the surface at the top of the graduated beaker
(Nalbandi et al., 2010). Bulk density was calculated using Equation (2).

ρb = m/v (2)

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Where, ρb, bulk density, (kg/m3), m, mass of sample bulk, (kg) and v, volume occupied by seed
bulk, (m3).This was replicated five times and the mean calculated for each sample.

2.3 Determination of Porosity
The porosity (ε) values were calculated from the values of true density and bulk density using the
relationship in Equation (3) as reported by Bup et al. (2013).

Porosity (ε) = (1 - ��
��
) x 100 (3)

Where ρb, bulk density (kg m-3), ρt, true or particle density (kg m-3)

2.4 Determination of Surface area
The surface area (Sa) was determined by analogy using Equations (4) and (5) as reported by
Balami et al. (2014).

Sa = ����
��−�

(4)

d = (b c) 0.5 (5)
Where, a, major diameter (mm), b, intermediate diameter (mm), c, minor diameter, (mm) and d,
geometric mean diameter, mm)

2.5 Determination of Specific Heat Capacity
The method of mixtures has been the most common technique reported in the literature for
measuring the specific heat of agricultural and food materials (Singh and Goswani, 2000; Nouri
Jangi et al., 2011). For the determination of specific heat in this study, the method of mixtures
was used. Sample of brown variety tiger nut of known mass, temperature and moisture content
was dropped into a copper calorimeter containing water of known mass and temperature. The
calorimeter was well insulated so as to prevent heat loss to the room in which the experiment
was performed. The mixture was stirred continuously using a glass rod stirrer. The equilibrium
(final) temperature was noted and the specific heat determined using Equation (6) as used by
Aviara and Haque (2001).

�䓨 = �e�er�i�i �ti− t݁r�� �
Ms � TertR −t䓨�

(6)

where: CC, CS and CW are the specific heats of calorimeter, sample and water, respectively (J kg-1
K-1), MC, MS and MW are the masses of calorimeter, sample and water (kg), respectively, R is the
rate of temperature fall of the mixture after equilibrium (K s-1), Te is the equilibrium temperature
of the sample and water mixture (K), TS, and TW are the initial temperatures of sample and water,
respectively (K), and t is the time taken for the sample and water mixture to come to equilibrium
(s). The term tR accounts for the heat of hydration and heat exchange with the surroundings.

2.6 Determination of Thermal Conductivity
The thermal conductivity of the seeds was determined at five different moisture levels, using the
transient line heat source method assembled in a thermal conductivity probe. The probe was
passed through the center of the seed and a fall in temperature occurred. A fall in temperature
continued until a constant temperature was obtained. A rise in temperature occurred after a

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Usman, et al: Effect of moisture content variation on thermo-physical properties of brown variety tigernut (Cyperus
esculentus). AZOJETE, 15(3):714-724. ISSN 1596-2490; e-ISSN 2545-5818, www.azojete.com.ng

Corresponding author’s e-mail address: ddusman@atbu.edu.ng 718

constant temperature period and at this point, the time was taken against the temperature. The
thermal conductivity was calculated from Equations (7) and (8) as reported by Sadiku and
Bamgboye (2014):

k = ��� (�2−�0)/ (�1−�0)/4� (t2−t1) (7)

� Q = �2� (8)

Where: k, thermal conductivity of sample (W/m°C), Q, power dissipation by heater wire (W/m), t1,
time since probe heater was energized (s), t2, time since probe heater was energized (s), t0, time
correction factor (s), T1, temperature of probe thermocouple at time t1 (°C) and T2, temperature
of probe thermocouple at time t2 (°C), I, electric current in A and R, electric resistance (ohm/m).

2.7 Determination of Thermal Diffusivity
The thermal diffusivity was calculated using experimental values of specific heat, thermal
conductivity and bulk density using Equation (9) as reported by Vijay (2013).

∝ = �/�e� (9)

Where: α is the thermal diffusivity (m2 s-2), k is the thermal conductivity (Wm-1°C-1), ρ is the bulk
density (kg/m3) and Cp is the specific heat (kJkg-1 °C-1)

2.8 Statistical Analysis
The data generated were analyzed using the Analysis of variance (ANOVA) to determine which
treatment effect was significant or not and Tukey’s studentized range (p < 0.05) was used to
determine significant difference between means using statistical analysis system (SAS9.2) 2010.

3.0 Results and Discussion
3.1 Results the values of physical and thermal properties of tigernut at varying moisture
The mean values and standard deviations of bulk density, porosity, surface area, specific heat,
thermal conductivity and thermal diffusivity variation with moisture content of brown variety
tigernut is shown in table 1. It is observed that values of the properties increases as moisture
increases for the properties with the exception of porosity, which decreases as moisture
increases.
Table 1: Thermophysical values at varying moisture content of Brown variety tigernut
Property Unit Moisture Content (%)

20 25 30 35 40
Bulk Density kg/m3 421.4d±2.61 475.2c±0.45 564.8b±1.10 620a±0.71 639.8a±1.1

0
Porosity % 61.84h±3.16 52.27g±2.46 48.52f±1.83 33.47e±2.3

9
32.33e±1.2
2

Surface Area mm2 197.26l±28.68 211.07k±10.73 221.93k±26.9
8

298.21j±27.
24

413.21i±34.
07

Specific Heat
Capacity

kJkg-1 °C-1 10.55o±0.18 12.37o±0.15 14.39n±0.17 15.92n±0.6
1

18.75m±0.4
5

Thermal
Conductivity

Wm-1°C-1 0.032e±0.00 0.056d±0.00 0.081c±0.00 0.106b±0.0
0

0.133a±0.0
0

Thermal
Diffusivity

x10-8 m2
s-2

7.24g±0.13 9.45f±0.43 9.94f±0.34 10.59f±0.48 11.06f±0.36

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Values in the same row followed by different letters are significantly different (p<0.05).

Table 2 is the analysis of variance on the variation of moisture on the physical and thermal of
brown variety tigernut.
Table 2: ANOVA on variation of moisture content on thermal properties of Brown tigernut

Source Df SS MS F- Value Pr > F
Properties (BV) 5 6020489.673 1204097.935 9922.34** < .0001
Replication 4 349.557 87.389 0.72ns 0.5799
Moisture Content (MC) 4 93925.097 23481.274 193.5** < .0001
Properties* MC 20 247091.85 12354.592 101.81** < .0001

**highly significant, ns = not significant

3.2 Discussion
3.2.1 Effect of moisture content on bulk density
Table 1 presents the variation of bulk density with moisture content for brown variety tigernut.
The bulk density of brown variety tigernut increased linearly from 421.4 to 639.8 kgm-3 as
moisture content increased from 20 to 40% as presented in table 2.The increase in the bulk
density was due to increase in the mass of tigernut with a gain in moisture which was higher
than the volumetric expansion of the bulk (Pradhar et al., 2008). The effect of seed moisture
content on bulk density was statistically significant (p<0.05). Similar increasing bulk density with
variation in moisture content was reported by Baryeh and Mangope (2002) for QP – 38 variety
pigeon pea, Kingsley et al., (2006) for dried pomegranate seeds, Nikoobin et al. (2009) for
chickpea seeds and Oyerinde and Olalusi (2013) for two varieties of tigernut.

3.2.2 Effect of moisture content on porosity
Porosity of materials usually is dependent on the bulk as well as the true densities (Oyerinde and
Olalusi, 2013). The porosity of the brown variety tigernut decreased from 61.84 to 32.33% as
moisture content varied from 20 to 40%. Similar trend was reported by Oyerinde and Olalusi
(2013) for two variety tiger-nut. This decrease may be due to the changes in the mass and
density value of the tiger-nut as it absorbed water. Higher porosity at low moisture content
indicates that high number of tigernut tubers can be stored at lower moisture content than at
high moisture content due to an increase in the cohesion of the cell structure of the tigernut as
moisture increases (Abano and Amoah, 2011). Table 2 presents the variation of porosity with
moisture content for the brown variety, and the effect of moisture on porosity was statistically
significant (p< 0.05) except between 25 and 35%moisture content.

3.2.3 Effect of moisture content on surface area
The effect of moisture content on the surface area of the tigernut is presented in Table 2. The
surface area increased linearly from 197.26 to 413.21 mm2 as the moisture content varied from
20 to 40%. The values for surface area were statistically significant (p < 0.05) from 30 to 40%,
however, a reverse trend was reported for black variety tigernut by Abano and Amoah (2011).
Hence, the results may be due to the different shape assumption of the tuber and this suggests
that it’s advisable to store the tubers at lower moisture content when the surface area is lowest
for the tuber to present less surface area for moisture absorption during storage (Abano and
Amoah, 2011).

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Usman, et al: Effect of moisture content variation on thermo-physical properties of brown variety tigernut (Cyperus
esculentus). AZOJETE, 15(3):714-724. ISSN 1596-2490; e-ISSN 2545-5818, www.azojete.com.ng

Corresponding author’s e-mail address: ddusman@atbu.edu.ng 720

3.2.4 Effect of moisture content on specific heat capacity
Specific heat capacity is needed in estimating the amount of heat energy required to change the
temperature of material by 1°C (Sadiku and Bamgboye, 2014). The specific heat capacity of
brown variety tigernut increased from 10.55 to 18.75 kJkg-1 °C-1 as moisture content increased
from 20 to 40% and the result (Table 2) was not statistically significant (p > 0.05). Similar trend
was reported for Parkia seeds (Sadiku and Bamgboye, 2014) which indicated an increase from
2.74 to 4.38 kJkg-1 °C-1 for a moisture increase of 5.9 to 28.2%. The increasing trend in specific
heat capacity with moisture content correlates with work done by other researchers on Cumin
seeds (Singh and Goswani, 2000), Lentil seeds (Tang, et, al., 1991), Potato (Wang and Breman,
1993) and Pistachio nuts (Razavi and Taghizadeh, 2007), all of which shows similar trend with
brown variety tigernut. The increased specific heat capacity with moisture is due to high specific
heat of water compound to dry material (Sadiku and Bamgboye, 2014).

3.2.5 Effect of moisture content on thermal conductivity
Thermal conductivity is the possibility of transmitting heat within seeds in bulk. Hence, the
thermal conductivity of brown variety tigernut increased from 0.032 to 0.133 Wm-1°C-1 as
moisture increased from 20 to 40%, this indicates that heat transmission within the tigernut is
less when dried and more if wet (Sadiku and Bamgboye, 2014). The increase in thermal
conductivity as moisture increased was because the increased moisture, increased the bulk
thermal conductivity as moisture has higher thermal conductivity than that of air (Vijay, 2013).
Similar results were reported for borage seed by Yang et al. (2002), soya bean pod by Mohsen et
al. (2013), coriander seed (Vijay, 2013) and three varieties of melon (Isa et al., 2014). The result
for thermal conductivity of brown variety tigernut is in agreement with the findings of Sadiku
and Bamgboye (2014) for Parkia seeds. The effect of moisture content on the thermal
conductivity of tigernut was presented in Table 1, though the moisture effect was not statistically
significant (p > 0.05). The relationship between thermal conductivity and moisture content of
tigernut was linear (as shown in Table 1).

3.2.6 Effect of moisture content on thermal diffusivity
The variation of thermal diffusivity with moisture content is presented in Table 2. Thermal
diffusivity of tigernut increased from 7.24 x 10-8 m2 s-2 to 11.06 x 10-8 m2 s-2 as the moisture
content increased from 20 to 40%. The increase in thermal diffusivity at different moisture
content may be due to the fact that the value of bulk density decreased at these moisture levels
(Singh and Goswani, 2000). Bamgboye and Adejumo (2010) also reported similar results for
Roselle seeds. The results for the thermal diffusivity of brown variety tigernut is in agreement
with the findings of Aviara and Haque (2001), that thermal diffusivity of sheanut kernel increased
with moisture content, but disagreed with the findings of Subramanian and Viswanathan (2003),
for minor millet grain and flour, Singh and Goswani (2000), for cumin seeds and Aremu and
Fedele (2010), whose obtained values for doum palm fruits showed an inverse relationship with
moisture content. This ascending and descending trend of relationship between moisture
content and thermal diffusivity is because the magnitude of thermal diffusivity depend on the
combined effect of thermal conductivity, bulk density and specific heat capacity (Yang et al.,
2002).

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3.2.7 Analysis of Variance (ANOVA)
Analysis of variance (ANOVA) was conducted to ascertain the significant effect of moisture
content variation on the thermophysical properties (Specific heat capacity, Thermal conductivity,
Thermal diffusivity, Bulk density, porosity and Surface area) of brown variety tigernut as shown in
Table 2. The results of the ANOVA showed that there were highly significant differences (F =
9922.34, p = 0.0001 < 0.05) among the values obtained for the properties of brown variety
tigernut. This indicates that the values of the properties were highly influenced by moisture
content. The effect of replication from the ANOVA was not significant (F = 0.72, p = 0.599 >
0.05), this may be due to minimized error in the performance of the experiment in the laboratory.
The variation in moisture content was highly significant (F = 193.5, p = 0.0001 < 0.05), this
means as moisture increased, the values of thermophysical properties either increased or
decreased. However, the interaction between the properties and moisture content indicate a
highly significant difference (F = 101.81, p = 0.0001 < 0.05). The interaction effect shows that the
relationship between tigernut properties and moisture content depends on the amount or
variation of the moisture content.

4. Conclusion
Thermophysical properties of brown variety tigernut (Cyperus esculentus) were determined for
the typical ranges of moisture content. The results showed the significant variations in thermo-
physical properties values with changing moisture content. All the properties increased as the
moisture level increased, with the exception of porosity, which reduced as moisture increased.
The bulk density and surface area increased from 421.40 kg/m3 to 639.80 kg/m3 and from
197.26 mm2 to 413.21 mm2 as moisture content increased. However, porosity decreased from
61.84 to 32.33% with an increase in moisture. The specific heat increased from 10.55 Jkg-1K-1 to
18.75 Jkg-1K-1, while the thermal conductivity and diffusivity increased from 0.03 Wm-1K-1 to
0.13 Wm-1K-1 and from 7.24 x 10-7m2s-1 to 11.06 x 10-7m2s-1 as the moisture content increased
from 20 to 40% respectively.

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esculentus). AZOJETE, 15(3):714-724. ISSN 1596-2490; e-ISSN 2545-5818, www.azojete.com.ng

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