Research Journal of Applied Sciences, Engineering and Technology 4(5): 393-397, 2012
ISSN: 2040-7467
© Maxwell Scientific Organization, 2012
Submitted: July 02, 2011
Accepted: September 05, 2011
Published: March 01, 2012
Evaluation of Ventilated Underground Pit Structures for Yam
(Dioscorea Spp) Storage
T.U. Nwakonobi, S.E. Obetta and H. Iorwtsav
Department of Agricultural and Environmental Engineering, University of Agriculture,
Makurdi, Benue State, Nigeria
Abstract: Underground pit structures are commonly employed by farmers for on farm storage of yam
(Dioscorea spp) in Nigeria and other parts of developing world. This traditional storage structure is used
without provision for adequate air exchange or ventilation. This study investigated the performance of yams
stored in underground pit structures provided with air vents. Three experimental pits of similar dimensions were
constructed with Pit 1 installed with one PVC vent; Pit 2 had two PVC vents and Pit 3 without vent which
served as control. 20 Kg weight of yams was loaded in each pit measuring 1.0 m in diameter and 0.65 m in
depth. The results of the study show that a lowest temperature range of 30-38ºC was maintained in pit 1
(improved) temperature range of 34-40ºC in Pit 2 (semi-improved) while temperature in pit 3 range from 3642ºC. The relative humidity obtained were 84, 76 and 70% in pit 1, 2 and 3 respectively. The sprouting indexes
obtained within 8 weeks storage period were 46.2, 53.8 and 76.9 for pit 1, 2 and 3, respectively. The cumulative
weight loss obtained in pit 1, 2 and 3 were 44.8, 69.5 and 79.2%, respectively for the 8 weeks storage period.
Increased in storage period for yams is possible with adequate ventilation in underground pit structure.
Key words: Air-vent, Nigeria, pit air - thermal properties, sprouting index, underground structure, yam storage
Opara, 1999). Weight losses of 33 to 67% after 6 months
storage period have been reported by Coursey (1967).
These losses arise from physical damage and attack by
fungi or bacteria as well as physiological factors
(sprouting, dehydration and respiration). Autolytic
process is reported as the most important factor that
contributes to storage losses in yam (Opara, 1999).
Autolytic processes give rise to chemical or biochemical
changes in the tuber as a result of interaction amongst
parts of the tuber or between it and environment.
Infestation by insects and other arthropods can destroy all
part of the produce and cause the partial or complete
spoilage. Microbiological attack has deleterious effects on
stored produce especially in warm tropical climates.
The three main requirements for yam storage have
been presented to be aeration, reduction of temperature
and regular inspection of produce to remove sprout.
Optimum conditions of 15 or 16ºC at 70 to 80% relative
humidity have been recommended for cured tubers
(Martin, 1984; McGregor, 1987). Kay (1973) reported
that the trifida began to sprout within 3 weeks at a storage
condition of 20 to 29ºC and 46 to 62% relative humidity.
A number of storage structures are used by farmers in
yam producing regions. These include the traditional yam
barn (Ezeike, 1985; Bencini, 1991; Chukwu and Ifenkwe,
2001). The use of the open sided shelves store was
INTRODUCTION
Yam is an important crop in many parts of the
tropical and subtropical regions where it is a major part of
the diet. About 96% of the world production is from
Africa, with Nigeria alone accounting for nearly 71-75%
of the total world production (Opara, 1999; FAO, 2005).
The World annual production was estimated to be 25
million tonnes in 1974 (Martin, 1984). FAO (2007)
reported that the estimated world production of yam is
3.136 million metric tonnes and West Africa accounts for
90-95% of total production (Acquah and Nganje, 1991).
In the south pacific, yam is a significant food crop
accounting for over 20, 8.1 and 4.6% of the total dietary
calorie intake in the kingdom of Tonga, Solomon Islands
and Papua new Guinea, respectively (Opara, 1999). In
addition to its importance as food source, yam also plays
a significant role in the socio cultural lives of some
producing regions, for example, through the famous yam
festivals in West Africa.
Like most tubers, the storage of yam (dioscorea) has
remained a problem among peasant yam producers which
constitute about 70% of farming population in Nigeria.
Losses in tuber weight of about 10 to 12% occurred in the
first 3 months and 30 and 60% after 6 months during
storage in traditional or improved barns (Ezeike, 1993;
Corresponding Author: T.U. Nwakonobi, Department of Agricultural and Environmental Engineering, University of Agriculture,
Makurdi, Benue State, Nigeria
393
Res. J. Appl. Sci. Eng. Technol., 4(5): 393-397, 2012
Fig. 1: Schematic diagram of underground pit storage structure. A-Thermometer; B- Air-vent; C- Cover; D- Ground surface; EUnderground pit; F- Yam tuber; G- Wooden plank
For an underground pit structure, the soil profile
needed is that of free draining sandy soil in higher
elevated area of the land. This study improves on the
traditional pit structure through the provision of chimney
for air escape and evaluates the storage performance in
terms of thermal factors, weight loss and yam sprouting
index.
recommended by the Nigerian stored products research
institute (NISPRI, 1982). This is to enable careful
handling and easy inspection in comparison with tying
tubers to poles, which can cause physical damage and
rotting (Bencini, 1991). Yams are also stored in raised
platforms in shed (Opara, 1999). Other traditional storage
structures included underground structures such as pits,
ditches and clamps (Opara, 1999). The use of
underground storage for other tubers such as cocoyams
(Taro) has been reported by Obetta et al. (2007).
Improved traditional shelter for yam has been reported by
(Ezeike, 1985; Akoroda and Hahn, 1995; Osuji, 1987).
Although the environmental regime for cold storage of
yam have been established, the use of refrigerated storage
is not wide spread (Opara, 1999). Successful use of
gamma radiation to inhibit sprouting has been reported by
several researchers (Adesuji, 1976, 1982; Olaifa, 1994;
Ukpabi et al., 1994). Afolabi et al. (1997) reported that
coating of yam tubers with termitaria soil reduces weight
loss and rotting during storage. Okoedo-Okojie and
Onemolease (2009) reported the major constraints
limiting farmers’ adoption of modern yam storage
technologies in Edo State, Nigeria which include; cost,
non-availability, and ignorance of technology existence.
For yam to keep well in storage there must be use of
sound and healthy tubers, proper curing, combined with
fungicide - treatment, adequate ventilation and protection
from direct sunlight and rain (Opara,1999).
MATERIALS AND METHODS
This study was conducted at the experimental farm of
the Department of Agricultural and Environmental
Engineering, University of Agriculture, Makurdi, Benue
State, Nigeria in 2010.
The material used: Freshly harvested yam tubers that
were free of any form of infestation and physical damage
were obtained from local farmers. Three underground pits
of similar dimensions (1.0 m in diameter and 0.65 m
deep) constructed at the experimental farm of the
Department of Agricultural and Environmental
Engineering, University of Agriculture Makurdi, Benue
state, Nigeria were used for the investigations.
The underground pit structure: Three Pits namely 1, 2
and 3 were dug carefully in an area with a silty-sand soil
profile which allows free drainage. These three circular
pit
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Res. J. Appl. Sci. Eng. Technol., 4(5): 393-397, 2012
Table 3: Sprouting index of yam tubers at various weeks
Sprouting index, % no. of weeks
-------------------------------------------------------------------Pit No.
1 2
3
4
5
6
7
8
Pit 1
0 0
0
0
7.7
15.4
38.5
46.2
Pit 2
0 0
0
7.7
23.1
38.5
46.2
53.8
Pit 3
0 0
7.7
30.8
53.9
61.5
69.2
76.9
8
36
40
38
8
84
76
67
Pit 1
Pit 2
Pit 3
100
Relative humidity (%)
Table 1: The average data per week for pit air thermal properties
Week
1
2
3
4
5
6
7
a: Pit air temperature (ºC) versus time (Weeks)
Pit1
32
34
32
32
34
38
30
Pit 2
34
36
34
36
38
40
36
Pit 3
36
38
36
40
40
42
36
b: Relative humidity (%) versus time (Weeks)
Week
1
2
3
4
5
6
7
Pit 1
55
77
67
76
80
84
82
Pit 2
46
72
61
69
73
74
75
Pit 3
43
64
55
63
66
70
66
Table 2: percentage weight loss for a storage period of 8 weeks
Weight loss (%)
Cumulative weight loss (%)
No. of ------------------------------------------------------------------weeks Pit1
Pit 2
Pit 3
Pit 1
Pit 2
Pit 3
1
1.5
2.0
3.0
1.5
2.0
3.0
2
2.0
3.5
4.0
3.5
5.5
7.0
3
2.6
4.0
7.0
6.1
9.5
11.0
4
4.0
7.5
8.7
10.1
17.0
19.7
5
5.6
10.0
12.2
15.7
27.0
31.9
6
7.1
12.1
14.3
22.8
39.1
46.2
7
9.5
14.2
16.2
32.3
53.3
62.4
8
12.5
16.2
16.8
44.8
69.5
79.2
80
40
20
0
1
2
3
5
4
6
Storage time (weeks)
7
8
7
8
Fig. 2: Relative humidity versus storage time
Tem perature (°C)
storage structures were uniform in size with a radius of
0.5 m and a depth of 0.65 m and were located in an
elevated area to avoid high water table. Each of the pits
was treated with reliable pesticides to guard against
infestation by pests such as termites. Water proof material
was used as wall lining in the pits to prevent moisture
migration into the pits. Pit covers were constructed which
had provision for installation of wet bulbs and dry bulbs
thermometers used for measuring the wet and dry bulb
temperatures of the pits. Pit 1 (improved) was installed
with two PVC vents; Pit 2 (semi-improved) was provided
with one vent while Pit 3 which served as control pit was
without vent. The PVC vents served as mediums for air
exchange and ventilation. Wooden platforms were placed
on the floor of all the three pits where the yam tubers were
placed to avoid direct contact of yam tubers with the soil
to prevent sprouting.
The initial weights of yam tubers before loading into
the pits were taken using a weighing balance, reading to
0.01 g. Each pit was loaded with 13 tubers of yam which
weighed 20 kg. A total of 39 yam tubers weighing 60kg
were stored in these three pits. The experiment set- up for
underground pit storage is as shown in Fig. 1.
These pit structures with yam tubers stored in them
were monitored for eight weeks to evaluate the efficiency
of the underground pit storage. Parameters monitored
were dry bulb temperature and wet bulb temperature.
These two parameters were used to determine the relative
humidity from a psychometric chart. The measured
temperatures (pit air wet-and dry- bulb temperatures) were
collected twice daily; at noon and evening at 6:00 pm
(G.M.T.) for a period of two months. The thermometers
used were installed permanently throughout the storage
period. The average data per week for the pit air thermal
Pit 1
Pit 2
Pit 3
45
40
35
30
25
20
10
5
0
1
2
3
5
4
6
Storage time (weeks)
Fig. 3: Temperature versus storage time
properties for the storage period of 8 weeks was as shown
on Table 1.
Apart from the thermal properties, the weight of the
stored yams was also measured on weekly basis using the
weighing balance to monitor weight change. The
sprouting index of the stored yams was also calculated on
weekly basis using the expression given as:
Sprouting Index, I = (Number of tubers/Totol number
of tubes) × 1005
(1)
The data obtained for weight loss and the cumulative
values are as shown on Table 2 while the sprouting index
values obtained are as shown on Table 3. The sprouting of
tubers was first observed during monitoring at week 5, 4,
and 3 in Pit 1, 2 and 3, respectively.
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Res. J. Appl. Sci. Eng. Technol., 4(5): 393-397, 2012
Cumulative weight loss (%)
90
the 8 weeks of storage. It was however observed that
weight loss in yam tubers stored in underground pit with
no provision for air exchange is enormous, 79.2%
compared with that stored in pits installed with air vents
which is 44.8% at the end of 2 months storage.
The sprouting indices obtained in the three pits were
plotted against storage time as shown in Fig. 5. This was
also observed to be increasing with storage time with the
highest value of 76.9% recorded in pit 3 at 8th week while
the lowest value of 46.2% was recorded in pit 1 at the
same period. The zero values observed in the first four
weeks of storage in pit 1indicate that sprouting only set in
after five weeks of storage while in Pit 2 and 3 it was
noticed at four weeks and three weeks, respectively.
Sprouting activity was less in Pit 1 where the
temperature range was relatively low and highest in Pit 3
where the temperature was highest.
Pit 1
Pit 2
80
70
Pit 3
60
50
40
30
20
10
0
1
2
3
5
4
6
Storage time (weeks)
7
8
Fig. 4: Cumulative weight loss against time
Spouting index (%)
90
Pit 1
Pit 2
Pit 3
80
70
60
50
40
CONCLUSION
The various parameters evaluated in the three pits
were pit air temperatures, relative humidity, percentage
weight loss and the sprouting index. The lowest air
temperature was obtained in pit 1 which had two vents,
followed by pit 2 and highest, pit 3 in that order. This
behavior was attributed to insufficient air flow
particularly the pit 2 and pit 3. The sprouting pattern and
the percentage weight loss were also affected in the same
way with pit 1 maintaining the lowest value, while pit 2
recorded higher values and pit 3 recorded the highest of
the three pits. The variations in the pit air temperature
affected the storage durability of the product as observed
in pit 3 with the highest air temperature.
30
20
10
0
1
2
3
4
5
6
Storage time (weeks)
7
8
Fig. 5: The sprouting pattern of yam tubers in the three pits
RESULTS AND DISCUSSION
The variation of the relative humidity over the
storage duration period for the three pits is shown in
Fig. 3. It was observed that the relative humidity value
obtained was 84% and highest in pit1, 76% in pit 2 and
lowest value of 70% obtained in pit 3. Figure 4 showed
the pit air temperature over the 8 weeks storage duration
for the three pits. The air temperature level attained in pit
3 is capable of causing dehydration of yam tubers as a
result of relatively low generated ambient air vapour
pressure which created higher moisture exchange driving
potential in this pit than pit 2 and pit 3. Soil in
underground pit is always in a moist condition and the air
in close contact is therefore laden with moisture resulting
in its high relative humidity. The variation of pit air
temperature over the storage period is as shown in Fig. 4.
Pit 1 recorded a temperature range of 30 to 38ºC within
the storage periods of 8 weeks. Pit 2 maintained 34 to
40ºC range of temperature while pit 3 recorded the highest
temperature level ranging from 36 to 42ºC within the
same storage duration period.
The percentage weight loss was also observed to be
increasing with the storage time in all the three pits (see
Fig. 5). The total percentage weight loss of 44.8, 69.5 and
79.2% were recorded in Pit 1, 2 and 3, respectively within
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