Controlled atmosphere storage of brown cowpea under nitrogen

Abstract

Laboratory studies were conducted to investigate the efficacy of the long-term controlled atmosphere storage of brown cowpeas under nitrogen in Kano, Nigeria, where hot dry weather prevails, typical for a semi-arid zone. The 30-month storage trial consisted of the application of nitrogen in an experimental 0.65 m 3 mini-silo containing half a tonne of the commodity. The storage in nitrogen completely suppressed insect infestation, and grain quality was maintained during the period. There was a significant difference (P < 0.05) in the population of insects in the cowpea stored under nitrogen, and the control. The initial insect population of 15 adults of C. maculatus, and one adult of T. castenium per kg of cowpea died after 12 months of storage under nitrogen. They however, increased in the control to 1288, and 54 adults per kg respectively before their eventual death. Free fatty acid contents increased from 2.60% to 6.51% under nitrogen, but increased rapidly to 58.60% in the control cowpea. This was attributed to the action of insect activities, mould infection and water activities in the control cowpeas. Viability dropped slightly from 89.75% to 72.84% in nitrogen, but dropped rapidly to 0.00% in the control. The palatability of cowpea did not change adversely in nitrogen, whereas it became very poor in the air-stored commodity. The germinability of cowpea seeds stored in the inert atmosphere silos was maintained above 85% after 12 months of storage, which shows that the technology is effective for seeds storage.

Full text

Croat. J. Food Sci. Technol. (2017) 9 (2) 102 - 107
*Corresponding author: omodaramikeresearch@gmail.com, omodarama@nspri-ng.org
Controlled atmosphere storage of brown cowpea under nitrogen
F. A. Babarinsa1, O. N. Ndam2, M. A. Omodara1*
1Nigerian Stored Products Research Institute, Postharvest Engineering Research Department, Km 3, Asa-Dam Road, P.M. B.
1489, Ilorin, 240001, Ilorin Kwara State , Nigeria
2Plateau State Collage of Agriculture, Garkawa, Plateau State, Nigeria
original scientific paper
DOI: 10.17508/CJFST.2017.9.2.03
Summary
Laboratory studies were conducted to investigate the efficacy of the long-term controlled atmosphere storage of brown
cowpeas under nitrogen in Kano, Nigeria, where hot dry weather prevails, typical for a semi-arid zone. The 30-month storage
trial consisted of the application of nitrogen in an experimental 0.65 m3 mini-silo containing half a tonne of the commodity.
The storage in nitrogen completely suppressed insect infestation, and grain quality was maintained during the period. There
was a significant difference (P < 0.05) in the population of insects in the cowpea stored under nitrogen, and the control. The
initial insect population of 15 adults of C. maculatus, and one adult of T. castenium per kg of cowpea died after 12 months of
storage under nitrogen. They however, increased in the control to 1288, and 54 adults per kg respectively before their eventual
death. Free fatty acid contents increased from 2.60% to 6.51% under nitrogen, but increased rapidly to 58.60% in the control
cowpea. This was attributed to the action of insect activities, mould infection and water activities in the control cowpeas.
Viability dropped slightly from 89.75% to 72.84% in nitrogen, but dropped rapidly to 0.00% in the control. The palatability of
cowpea did not change adversely in nitrogen, whereas it became very poor in the air-stored commodity. The germinability of
cowpea seeds stored in the inert atmosphere silos was maintained above 85% after 12 months of storage, which shows that the
technology is effective for seeds storage.
Keywords: storage, brown cowpea, nitrogen, insect mortality, nutritional quality
Introduction
Cowpea (Vigna unguiculata L. Walp) is one of the
major leguminous vegetable crops in Nigeria and West
African countries providing more than half the plant
protein in the human diet. In Nigeria, cowpea is
commonly consumed in the form of bean pudding,
bean cake, baked beans, fried beans and bean soup
(Ola Salawu et al., 2014). This crop is prone to the
heavy post-harvest damage by Callosobruchus
maculates F., which is one of the most destructive
insect pests of cowpea and legume crops in Nigeria.
This insect starts its infestation in the field before the
harvest, and continues on the dried, stored seeds
through several generations per year (Hashem and
Risha, 2000). This insect causes severe losses in seeds.
The damage is a result of the larvae feeding inside the
seed causing weight losses, reduced seed quality and
poor seed viability (Ofuya, 1994).
Effective crop storage is an important aspect of the
attainment of food security. According to Adejumo
and Raji (2007), a major problem regarding the
attainment of food security in Nigeria is an inefficient
food preservation and storage. Cowpea, which is one
the staple food in Nigeria consumed by the entire
populace, is not exempted from this problem. The
general approach to the control of cowpea insect pests
is the use of chemicals, especially phosphine, which is
commonly used for fumigation of grains across the
country. This method has, however, proven ineffective
due to the peculiar nature of the insect pest problems
with cowpea in Nigeria. Resistance of several insect
pests to this fumigant, especially in grain storage
management, has also been reported (Opit et al., 2012).
Chemical residues, which are the traces of a chemical
or its breakdown products that remain in or on the
treated produce after a particular time, are also a major
drawback to the use of chemicals as effective insect
pest control (Hashem and Sharaf El- Din, 2000). These
synthetic products are not only expensive, but may be
unavailable at the time of a critical need, and also pose
health hazards to man and livestock. Reports of deaths
arising from the consumption of beans have also been
documented in the country, because people used the
banned substances on the stored beans, which are later
sold to the innocent consumers.
In Nigeria, the use of chemicals for the control of
insect pest is not properly regulated, and as such, it has
been prone to the abuse and misuse by farmers, grain
handlers and grain merchants. The most worrisome
consequence of this was that the European Union has
recently placed a ban on the beans export from Nigeria
to any of their countries. The rejected beans were
found to contain between 0.03 mg/kg to 4.6 mg/kg of

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103
dichlorvos pesticide, and the acceptable maximum
residue limit is 0.01 mg/kg. This chemical residue
constitutes a danger to human and animal health
(Premium Times, 2016).
Natural methods for the protection of grains against
cowpea beetle, involving the use of plant materials,
have also been studied with promising results
(Oparaeke and Dike, 2005 and Ilesammi and Gungula,
2016). However, the limited scale of application of
this potent material has made it relatively unpopular
among grain merchants.
The Controlled atmospheres (CA) storage has been
demonstrated as a safe residue–free alternative to
chemical (fumigants and protectants) for controlling
stored products insect pests (Carli et al., 2010).
Controlled atmosphere has been considered a
promising method for pest control in grain and other
commodities. The following two principal types of
atmosphere can be used for the controlled atmosphere
storage and disinfestations of grains; low O2, and high
CO2 atmospheres. This environment kills insects and
mite pests and prevents aerobic fungi from growing
(Mitcham et al., 2006). However, the studies in
Nigeria have shown that an atmosphere of high N2 is
most preferable (Agboola, 2012).
Inert Atmosphere silos (a form of a control atmosphere
storage technology) has proven to be an effective
storage structure for grains in tropical countries due to
its huge technical advantages over the conventional
metal silos. The superiority of grains stored in metal
silo filled with nitrogen over grains protected by
Phostoxin fumigant in silos was demonstrated by
Williams et al. (1980), Adesuyi et al. (1980) and
Sowunmi et al. (1982). These researchers tested the
system of using nitrogen to preserve grains at Ibadan,
Nigeria, at 24.8 to 30.3 °C ambient temperature and
72.0 to 80.9% relative humidity. They observed that
storage in nitrogen atmosphere protected grain in
metal silos from the insect damage, and the quality of
the grain was maintained throughout the period of the
trial. White and Jayas (2003) have also observed that
the controlled atmosphere of nitrogen is used as a
periodic treatment to control pests (insects and mites)
in stored grain or, less frequently, as a long-term
storage environment to prevent pest occurrence. This
system of storage eliminated the general problem due
to moisture condensation within conventional metal
silos. They also demonstrated that the nitrogen
atmosphere in the grain silo did not affect the viability
of the grains.
This study was designed to investigate the application
of the controlled atmosphere storage system of brown
cowpea in the semi-arid zone of Nigeria for the effect
of different ecological zone (as in northern part of
Nigeria) on the biological and organoleptic quality of
the stored cowpea. Cowpea was chosen because it is a
commonly grown legume in the area where this study
was conducted. It is also easily attacked by insect pests
in storage with a chemical control proven to be
ineffective in conventional silos; hence the efficacy of
this system can easily be demonstrated.
Materials and methods
Description of silo and the collection of sample
materials
An airtight mini-silo of volumetric capacity 0.65 m3,
used in Kano for this study, was installed under
shade, as suggested in earlier research (Adesuyi et al.,
1980). The mini-silo is provided with three sampling
points located at the bottom, middle and top positions
of the metal silo. A dial bi-metallic thermometer
probe was also fitted midway of the silo with a
sensing probe penetrating to the central axis.
Nitrogen was supplied to the metal silo through a
nitrogen distribution system consisting of a pressure
cylinder and a gas flow instrument panel in such a
way that nitrogen could flow from the silo
downwards through the stored grain. A pressure
relief valve was fitted at the base of the silo.
The cowpea used in this study was purchased dry and
had no insecticide protection. All precautions
regarding silo loading with grains, regularity of
sampling from the silo and purging with nitrogen
were followed. For control, cowpea was stored in air
in a metal drum of the capacity of 180 liters kept
under shade in the proximity of the mini-silo. Control
samples were drawn from the bottom, middle and top
of the drum content with a compartmentalized
sampler, and were mixed for the analysis.
Seed damage and weight loss
The inert atmosphere silos containing the seed stored
under N2 was observed at 12 months intervals up to
30 months of storage. After completion of each
storage period, the seal of the silo was opened and
observations on the insect damage (%), weight loss
(%), moisture content (%), and germination (%) were
recorded. The number of the damaged seeds in each
replication was counted after taking a random sample
of 100 g seeds and converted to per cent insect
infestation. The seed moisture content of the
treatments was determined by the oven method. The
weight loss due to the insect infestation was
calculated by deducting the final weight from the
initial weight, and converted to per cent weight loss.
Seed germination was measured using the standard
paper towel method as per the ISTA rules (1999).

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Quality assessment
Oxygen concentration within the silo was checked
with the Taylor Servomex oxygen analyzer, type OA
272 fortnightly, and after every purge following grain
sampling. The stored grains were sampled from both
the silo and the control for the analyses initially
during loading, after 12 months and subsequently at
6-month intervals. The samples were subjected to the
following tests: insect infestation count, insect
damage, proximate composition, organoleptic
evaluation and viability of the seed. These tests were
carried out with standard methods as described by
Williams et al. (1980) and AOAC (2005). Viability
tests of stored cowpeas were carried out using fifty
seeds per replicate, germinated in the sterilized soil in
the laboratory. The organoleptic evaluation was
carried out on the traditional beans dish prepared by
cooking the stored cowpeas in water and other
ingredients were added. This was performed initially,
and at the end of the experiment by staff of the
Nigerian Stored Products Research Institute, Kano.
Statistical Analysis
The analysis of variance was obtained using SPSS
statistical software. The Duncan’s Multiple Range
Test (DMRT) was used for testing the significance of
means in all conducted experiments.
Results and discussion
Temperature, relative humidity and moisture content
of cowpea inside the silo
Temperatures recorded in the silo were in the range of
21 °C (in the night) to 35 °C (in the day) with a mean of
28.5 °C. The ambient temperature ranged from 14 °C to
43 °C (day) with a mean of 33.2 °C. The ambient relative
humidity ranged from 19 to 100%, with a mean of
44.3%. The interstitial atmosphere within the silo was
maintained below 0.1% oxygen by purges after grain or
insect samplings. This was in agreement with
temperature data reported by Ajayi et al., 2016 which
showed that the temperature variations in the inert
atmosphere silos (using nitrogen) were minimal.
Moisture contents of brown cowpeas stored under
nitrogen, and in air for 30 months are shown in Table
1. After the storage period in nitrogen, the initial
moisture content 9.66% of cowpeas reduced slightly to
8.65%. On the other hand, the control increased
steadily to 19.25%. This showed that the moisture
content of grains in the inert atmosphere was
maintained below the safe level throughout the storage
period, whereas those stored in the control (air storage)
have steadily exceeded their safe moisture level. The
technology was able to maintain the moisture content
of the grains, thus preventing spoilage of the grains
through other means other than the activities of
insects, such as mould growth and seed rot.
Grain damage and insect infestation
The initial damage of cowpeas averaged 3.3%
(Table 2), and the insect population was low (Table 3).
The level of the grain damage increased rapidly in the
control. The cowpea showed initial infestation with
Callosobruchus maculatus (F), 15 living adults per kg
of grain weight and Tribolium castaneum (Herbst),
1 living adult per kg of grain. There was a significant
difference in the insect population during the storage
period. All insects were recorded dead after 12 months
of storage in nitrogen, whereas a living insect was found
in the control. This is due to the lack of oxygen, which
is a major reason for insect mortality, because feeding
activities of the insects generally reduce as oxygen level
reduces; it increases the level of acidity in the form of
lactic acid and causes poisoning, which is in agreement
with the reports of Can et al. (2012). Although similar
observation was made in the control with all
C. maculatus found dead, there was an enormous
increase of insect population to 1288 per kg before their
eventual death. An increase in the population of T.
castaneum to 54 adults per kg was also observed in the
control, and they eventually died after 12 months. The
moisture increase in the control might have made the
environment lethal to insects. These results were in
agreement with those by Mbata and Reichmuth (1996).
Table 1. Variation in the moisture content of cowpea during storage
Storage medium
Moisture content of stored cowpea (%) in months
0
12
18
24
30
Inert Atmosphere Silo(Nitrogen)
Top
9.66
8.67
7.76
8.00
8.12
Middle
9.66
8.63
7.71
8.89
8.78
Bottom
9.66
8.66
7.83
8.07
8.02
Mean
9.66
8.65
7.77
8.65
8.31
Ambient (Air)
9.66
14.92
12.55
19.25
20.04

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Table 2. Insect damage of brown cowpea stored under nitrogen and in air for 30 months
Storage medium
Insect damage (%) in months
0
12
18
24
30
Inert Atmosphere Silo(Nitrogen)
3.3
3.1
3.4
3.2
3.3
Ambient (Air)
3. 1
88.0
100.0
100.0
100.0
Insect count on 1kg samples
Table 3. Insect infestation of brown cowpea stored under nitrogen and in air for 30 months
Storage period
Storage medium
Population of insect species per 1kg of grain weight
T. castaneum
C. maculates
Parasite
Total
0
Nitrogen
0
15A, 25a
0
15A, 25a
Air
1A
0
0
1A
12
Nitrogen
0
1a
1a
2a
Air
54A
1288a
0
1342a
24
Nitrogen
0
3a
0
3a
Air
16a
916a
0
932a
30
Nitrogen
0
1a
0
1a
Air
4a
408a
0
408a
Legend: A = Living insects; a = Dead insects
Proximate composition of stored cowpea
As shown in Table 4, there was an increase from 24.68 to
35.20%. In the crude protein content of cowpeas stored
under nitrogen. The crude protein content of cowpea
stored in air increased more rapidly to 48.41%. This
might have been due to the effect of N2 in decreasing the
insect infestation in the beans stored under N2.This
observed increase in the control is only apparent, as
explained by Shahetal et al. (2009) and Nikolova and
Georgieva (2015) who indicated that the analytical
results of the crude protein of the infected grain would be
inflated by the contribution of the excretory product of
insect nitrogen metabolism to total nitrogen (N2) content.
In particular, the previous works on the insect infestation
of cowpea reported by Sowunmi (1980) and Sheata et al.
(2009) showed that poorer quality protein increases in
infested cowpea, thus making the crude protein content
of infested cowpeas to be greater than the uninfested. The
free fatty acid (FFA) content of cowpea increased from
2.67% (initial value) to 6.51% in 30 months, while the
control (air stored) cowpea increased rapidly to 58.60%.
The observed increase in the fatty acid of cowpea stored
in air (hence infested) can be attributed to the action of
insect activities, mould infection, water activity and other
biodeteriogens. There was a slight decline in the oil
content of cowpea from 1.98% to 1.44% stored under
nitrogen, but the control showed a rapid rise to 2.35%.
Table 4. Variations in proximate composition of stored brown cowpea
Storage
medium
Proximate
composition (%)
Storage period (months)
0
12
18
24
30
Nitrogen
Crude protein
24.68
31.86
32.12
36.33
35.20
Free fatty acid
2.67
6.62
6.38
6.40
6.51
Oil content
1.98
1.31
1.28
1.47
1.44
Air
Crude protein
24.68
37.49
44.20
49.08
48.41
Free fatty acid
2.67
40.48
48.18
55.65
58.60
Oil content
1.98
2.84
2.44
2.26
2.35
Table 5. Variations in weight loss, viability and palatability of stored brown cowpea
Storage medium
Parameters
Storage period (months)
0
12
18
24
30
Nitrogen
Weight loss(%)
0
3.13
3.52
3.51
3.51
Viability (%)
89.75
86.4
71.70
72.84
71.64
Palatability
Good
n.a.d*
n.a.d
n.a.d
n.a.d
Air
Weight loss(%)
0
35.40
51.74
56.81
56.78
Viability (%)
89.75
11.32
0.00
0.00
0.00
Palatability
Good
v.p**
v.p
v.p
v.p
Legend: * n.a.d = not adversely affected; ** v.p = very poor

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Germinability of stored cowpea
There was a slight loss of germinability of cowpea from
89.75 to 71.64% after 30 months of storage under
nitrogen, while a much higher loss of viability to 0.00%
was observed in the control set-up (Table 5). The low
percentage germination observed in the control in this
study could be due to the insect infestation which might
have damaged seed embryo, thus rendering the seeds
implantable. Similar findings have been reported by
Guptal et al. (2014) who found that paddy seed can be
stored safely with 11% moisture content at least up to 12
months under modified atmosphere (up to 80% CO2)
without much reduction in the seed viability. Jayas and
Jeyamkondan (2002) have also concluded in their study
that modified atmosphere does not cause any detrimental
effects on the functional characteristics of grains, and
helps in maintaining seed germination and viability. This
is an indication that the inert atmosphere silos can be
used for seed storage provided the conditions of the
system can be maintained at an optimum level and with
the storage period not exceeding 12 months.
Sensory properties of stored cowpea
The palatability of cowpea stored in nitrogen did not
change adversely in the 30 months storage period.
However, the palatability of the air stored commodity
became very poor (Table 5).
Conclusions
This present work has demonstrated that nitrogen can
preserve cowpeas of moisture content of 10% or less,
stored in airtight containers for up to thirty (30)
months without adverse effect on the nutritional
qualities. The technology is effective in controlling
insect infestation and maintaining the moisture
content of cowpea during storage. The technology
can also be used for the storage of seeds without loss
of viability for a period of 6 months. Because of the
poisonous nature of the degraded materials from
Phostoxin tablets and the high cost of chemical
control of cowpea insect pests coupled with the
associated challenges of mis-use and their abuse, the
use of Inert Atmosphere storage technology is
recommended to farmers and grain merchants in
Nigeria as a safe and economically viable alternative.
Acknowledgments
The authors are grateful to Nigerian Stored Products
Research Institute for funding this research.
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Received: February 13, 2017
Accepted: May 12, 2017