One of the critical challenges in developing countries is to ensure food and nutrition security whilst ensuring long-term sustainable development. This is mainly because of change in climate, high rate of population growth, low productivity of agriculture and high postharvest loss of harvested crops.

According to Zorya  et  al.  (2011),  sub-Sahara  African (SSA) countries significantly experience about 4 billion USD loss each year due to poor after harvest handling of grains mainly during storage. In SSA countries, grains are stored in traditional storage structures made up of different local materials. Due to limitations of the structures, grains loss due to insect pest is significant. Costa (2014)  estimated  a  59.5%  loss  of  maize  after 3 months of storage, and on average, 64.5% damage was reported within three to six months of storage in South-West part of Ethiopia (Sori and Ayana, 2012). Storage related loss of wheat in Bangladesh was estimated at 41.7% of total harvest (Bala et al., 2010), whereas in Guatemala, it was 45% of total stored maize (IAICA, 2013).

Farmers commonly use different types of preventive and curative insecticides to control impacts of insect pests’ damages on stored grains. However, in developed nations, agricultural products can be stored under modified (MA) or controlled atmosphere (CA) environment to control stored grain pests without use of insecticides. As reported in Donahaye and Navarro (2000), MA/CA is a non-toxic and environmentally sound storage technology to control impact of pests in stored products.

Infusion of CO₂ from pressurized cylinder, on site generation and infusion of nitrogen gas, use of ozone and combustible gases are major active modification methods. In passive modification process, living organisms (aerobic fungi, insects and grains) in air tight storage structure consume O₂ and are reduced to lower level while building up CO₂ (White and Jayas, 1991). Such a modified environment kills insect pests, mites and limit proliferation and impact of aerobic fungi (Weinberg et al., 2008). Modern active modification methods are feasible for large-scale commercial farms and volume of grains. However, the principle can be equally applicable at small-scale level as long as low cost alternative approaches are available. Combustible gases from gas burners are one of onsite modification of MA through burning of hydrocarbon fuel. Combustion of propane and butane yields approximately 13 and 15% CO₂, respectively (Navarro et al., 1995). However, creation of high concentration of CO₂ and CO environment is feasible through burning and infusion of locally available biomass materials in hermetic storage structures.

Generated smoke from combustion process  can  serve as means to get rid of ambient air from interstellar space of stored grains. Under hermetic condition, residual oxygen in the smoke and interstellar space is gradually depleted through passive process. With recent development and availability of flexible plastic materials like Polyvinyl chloride (PVC), different size hermetic storage structures can be constructed and combined with infusion of smoke to create a MA (Navarro, 2010). Therefore, this study aimed to characterize and identify locally available biomass materials as economical smoke source to generate relatively high concentrations of CO and CO₂ to create a MA environment in stored grain.

 

Description of the study area

This survey study was conducted in three districts (Omo Nada, Keressa and Seka Chekoressa) of Jimma Zone, South West part of Ethiopia to identify potential smoke source materials. Geographically Omo Nada (07° 17’ - 07°38’ N latitude and 37° 00 - 37° 28’ E longitude), Keressa (07° 5’ - 08° 00’ N latitude and 36° 46’ - 37° 14’ E longitude) and Seka Chekoressa (07° 20’ - 7° 45 ’ N latitude and 36° 29' - 36° 50' E longitude) districts are located on an altitude of 1500-2500, 1500-2660 and 1580-2560 m.a.s.l respectively. The annual average rainfall in Omo Nada, Keressa and Seka Chekorsa are 1880, 1500 and 1400 mm with average annual temperature of 16 - 27°C, 16 - 26°C and 18 - 28°C respectively.

Exploration and identification of smoke source materials

Focus group discussion (FGD) with farmers and key informant interview (KII) was used in selecting potential smoke source materials. Using pre-tested checklist, discussions were made with selected village (3 per district) farmers (4 male and 4 female) and key informants (2 male and 3 female). Recommended smoke sources were from any biomass material that could be used by farmers as energy source (e.g. woods, leaves, straw and cow dung) to cook foods or be used for other purposes. Availability of source materials in local area, cost and other cultural and religious issues were also considered during the discussions.

Preparation of smoke source materials

Identified smoke source materials chopped to small size (5-6 cm) were bulked, homogenized and sundried for 5 days. Moisture content was determined using oven (Leicester, LE67 5FT, England) drying method at 70°C for 72 h. Figure 1 shows identified and selected smoke source materials used. Bulk density of samples was also determined using gravimetric method.

 

 

Experimental set up to characterize smoke source materials

A laboratory study was conducted to characterize smokes from source materials in terms of CO_2, CO, NO (Nitrogen Oxide) composition and O₂ depletion rate during burning in enclosed metal box. Change in gases (CO_2, CO, NO, O₂) concentrations in smoke was recorded in regular time interval. For characterization purpose, sample burning box was constructed using stainless steel metal having a total volume of 0.125 m³ (0.5 m x 0.5 m x 0.5 m) and net void volume of 0.119 m³. Small metal box (0.10 m x 0.05 m x 0.05 m) for burning samples using electric oven (1500 W) served as heat source maintaining heating level at medium capacity. Characterization of smoke was conducted in two subsequent steps using 20 and 40 g samples for screening and verification steps respectively. Smoke uniformly mixed in burning chamber (Figure 2) with small fan and cooled in rubber tube immersed in cold water (10-15^oC) before it passes through two smoke filters. Eventually sucked smoke was characterized in terms of composition of CO (% volume), CO₂ (% volume), O₂ (% volume) and NO (ppm volume) using gas analyzer (Saxon Junkalor, INFRALYEL, Germany). Figure 2 shows the experimental set up of sample burning and measurement units to characterize smokes in terms of the gases composition.

 

 

Experimental design and data analysis

One factor factorial experiment in complete randomized design (CRD) was used to characterize homogenized smokes in enclosed metal box. Samples measurements was replicated three times and mean values presented with standard error of mean. Data analysis was conducted with one-way analysis of variance (ANOVA) using Minitab computer statistical software program (version 16) and Tukeys’ studentized range test (HSD) at 5% level of significance to separate means when smoke sources in their gases composition were found significant.

 

 

 

 

Variation in moisture content and bulk density of samples

Table 1 shows moisture content and bulk density of smoke source materials before burning. Based upon nature of the materials, moisture content varies from 4.2 to 7%. ML  and FUCH had the lowest moisture content of 4.2%, followed by OAB, CD, MC, MS and OAW. As indicated in Figure 3 for selected biomasses (MS, MC, OAW and FUCH) of 40 g samples, biomass with lower bulk density (MS) exhibited faster rate of burning with lowest temperature rise (Figure 3) regardless of moisture content of the biomasses.

 

 

 

Combustion of biomasses occurred through three different but subsequent stages of drying, devolatalization and char burning. Drying phase correlated with migration of moisture as the higher the moisture the lower the rate. However, as reported in Orang and Tran (2015), only biomass samples having more than 30% showed a significant effect on rate of burning with insignificant effect on devolatalization and char burning. Shen et al. (2013) reported a strong positive correlation  of  emission factors with moisture content of wood during combustion. However, in this study, due to less difference (< 3%) in moisture content, a significant difference in emission factors may not be expected. Due to less moisture content and variation in chemical composition of biomass samples in this study, the bulk density could determine rate of burning and temperature rise than moisture content.

Variation in CO production and accumulation

In the study, smokes from different biomass materials showed significant (p<0.05) difference in terms of generation  of  CO.  As  indicated   in  Figure  4,  for  20 g samples, MS, CD and ML results in lower CO generation and accumulation as compared to biomasses having relatively higher bulk density (FUCH, OAW and MC). Even though MS showed faster rate of generation of CO for the first 6 min, with burning of the sample, a faster decline in concentration was observed. However, production and accumulation of CO for OAW and FUCH increased with an increase in burning time. After 6 min of burning, more than 1% (per volume) CO concentration was recorded for most of the biomasses except ML, CD and MS.

 

 

In subsequent work, a verification study was conducted by increasing samples size to 40 g for four selected samples (MS, MC, OAW and FUCH). MS was included in the list due to its fast CO generation capacity. It was observed that there was more than 1.5% (per volume) CO concentration after 8 min of burning (Table 2). However, unlike in 20 g sample, MS smoke for 40 g sample results in more than 2% (per volume) concentration of CO after the same sample burning time. Figure 5 presents percent increase in CO concentration by doubling samples mass. The highest percent increase is  for   MS   (more   than   50%  increase  after 10 min  of burning), followed by OAW (>30%), MC and FUCH (>20%). This gives an opportunity to increase CO concentration and accumulation through increasing of sample size for better modification of storage environment against insect pests.

 

 

 

Jayas and Jeyamkondan (2002) indicated that modification of stored grain environment using modified atmosphere, involves the alteration of the natural storage gases such as CO₂, O₂ and N₂ to render the atmosphere in the stores lethal to insect pests. In addition to these gases, CO can modify gases composition and a relatively high CO percentage is poison to human and other creatures under enclosed and less ventilated condition.

When lethal concentration of CO is considered, exposure to 4.000 ppm (0.4% per volume) concentration for 30 min could cause an immediate death to human being (Lefaux, 1968). According to the National Institute for Occupational Safety and Health, USA (NIOSH) (2018), of the immediately dangerous to Life or Health (IDLH) concentration is 1200 ppm (0.012% per volume). However, smokes from MS, OAW and FUCH consisted of more than 100 times concentration under an enclosed condition. If  this  high  concentration  is  strictly controlled and properly infused in stored grain, it will critically help to control impact of stored grain insect pests. However, it is hoped that literature data in this regard be compared with this study results.

Variation in CO₂ production and accumulation

Figure 6 shows increase in formation and accumulation of CO₂ during burning of different biomasses (20 g sample size). MS showed fast burning and CO₂ formation for the first 6 min and declined with time. ML and CD show low level of CO₂ production and accumulation when compared with other samples. This might be associated with low bulk density of the three samples (MS, ML and CD) as compared to others (Table 1). OAW, OAB, MC and FUCH exhibited slow rate of burning as compared to samples that exhibited a continuous accumulation of smoke and CO₂ with time (Figure 6).

 

 

 

Increase in CO₂ concentration as well as in sample size might be associated with production of more volume of smoke with fast depletion of O₂ in an enclosed system. Likewise,  regarding  CO,  it   is  understood that  with  an increase in sample size there is an opportunity to produce a MA having 10% CO₂ concentration using MS smoke.

Figure 7 shows percent increase gained in CO₂ concentration due to doubling of sample mass. After 12 min of burning, more than 40% increase in CO₂ concentration could be achieved when MS is used as a source. However, for other samples (MC, OAW and FUCH), doubling samples size contributed only less than 20% increase in CO₂ accumulation.

 

 

Carbon dioxide is one of the gases commonly used to modify grain storage atmosphere. As a method for insect control in bulk commodities, MA systems increase CO₂ or decrease O₂ atmospheres, or a combination of both was used (Donahaye and Navarro, 2000). In modern MA conditions, the gases composition in storage structures is modified through injection of CO₂ gas (active modification) with the balance of air. However, application of the same method in developing countries is not feasible and accessible. As an alternative method for traditional small or medium grain storage hermetic structures, smoke from different biomasses can be used as a means to generate relatively high concentration CO₂ and CO.

Increase in CO₂ concentration with an increase in sample size might be associated with production of more volume of smoke with fast depletion of O₂ in an enclosed system. Similarly, for CO, it is understood that with an increase in sample size there is an opportunity to produce a MA having 10% CO₂ concentration using MS smoke as a medium to inject CO₂. There may be a synergetic lethal effect on storage insect pests due to combined action of high concentration of CO and CO₂ in a given MA environment.

Study indicates that grain MA above 10% CO₂ concentration assists spiracles of insect pests permanently open and results in insects death from dehydration, enhance CO₂ toxicity through tracheae, acidification of hemolymph of insects leading to membrane damage and death of insects (Nico­las and Sillans, 1989). Elevated but sublethal CO₂ levels for prolonged periods can have deleterious effects on insect development, growth and reproduc­tion (White and Jayas, 2003; Nicolas and Sillans, 1989). In higher temperature areas (associated with high ambient temperature of low land areas in Africa), even low levels of CO₂ (7.5 - 19.2%) for prolonged periods sharply increase adult insects mortality (White et al., 1995).

Variation in O₂ consumption and depletion during samples burning

As indicated in Figure 8, MS and FUCH show fast depletion of oxygen (under enclosed system in sample burning chamber) for the first 6 min; but when burning time-increased, concentration of O₂ slightly increased for MS. However, burning of other biomass materials resulted in a consistent decline in O₂ concentration and the highest depletion is for FUCH sample (Figure 8).

 

 

Even though FUCH showed a better O₂ depletion during burning of 20 g sample, MS of 40 g exhibited equivalent performance like CH. As indicated in Table 2, percent residual oxygen left after 18 min of burning were less than 10% for MS and FUCH. Therefore, apart from its potential to produce relatively high concentration of CO and CO₂, MS is also enabled to minimize residual O₂.

Figure 9 shows percent decrease in O₂ concentration in a smoke burning of 40 g selected biomasses. The study showed that it is possible to achieve more than 30% decrease in O₂ concentration for MS after 12 min of burning for 40 g sample.

 

 

The availability of sufficient amount of oxygen in stored grain is a critical factor in terms of supporting the survival and reproduction of insect pests. Depletion in O₂ concentration with simultaneous accumulation of CO and CO₂ from a smoke source and removal of ambient air from stored grain can modify gases composition and influence reproduction and survival of insect pests negatively. With time, grains, moulds and insects will consume the residual O₂ in hermetic storage structure with production  of  more  CO₂  passive  process to create hostile atmosphere to the pests (Moreno-Martinez et al., 2000). The combined effect of high CO and CO₂ concentrations together with low O₂ percentage at the beginning of hermetic storage could enhance mortality of storage insect pest (Annis and Morton, 1997) and minimize or avoid storage related losses of grains.

Formation and accumulation of nitric oxide (NO)

Results  in   Figure   10   indicate  that  smoke  from  OAB contributed to accumulation of high concentration of NO as compared to other smoke sources. This might be associated with relative high presence of nitrogen containing compounds like protein in the bark than other sources. The lowest concentrations were from CD and FUCH, which might be due to loss of nitrogen emanating from fermentation of CD and burning of wood during charcoal making. MS demonstrated more or less reduced level of NO like other biomasses with less impact on the environment unlike that of OAB (Figures 10 and 11, Table 2).

 

 

 

In combustion system, NO is formed in presence of high concentration of nitrogen and oxygen at high temperature. Emission of oxides of nitrogen (NOx) in the air is primarily in the form of NO, and once formed; NO oxidized rapidly to NO₂ in the presence of ozone (US EPA, 2018). These two forms of NOx (nitrogen mono (NO) and dioxides (NO₂) are the two major pollutants of air (World Bank Group, 1998).

From insect pests control point of view, NO plays diverse physiological processes like reproduction, locomotion, learning and memory (Müller, 1997; Davies, 2000). It is also used as a potent fumigant against various life stages of insect pests (Liu, 2013). However, due to its high reactive nature with oxygen, it forms NO₂, and hence such benefits achieved under ultra low oxygen concentration, which is very unlikely to get NO benefits under this study conditions. As indicated in Table 2, by doubling sample size (40 g), accumulation of NO increased as expected. MC and OAW showed significant increase as compared to MS and OAW.

Additional properties of biomass materials

Table 3 shows ranked values of additional observed characteristics of smokes and smoke source materials. MS selected as a biomass contribute to fast rate of burning (+++), smoke with less smell (+), wide availability in the farm (+++), no or low cost for purchase and preparation (+) as well as it has no effect on deforestation (0) unlike that of OAW and FUCH.

Rate of burning of biomass is an important factor for quickly generating smoke to be infused in storage structures. It was observed that MS had fast rate (+++) of burning as compared to other sources. Smoke smell is another factor which may be associated with alternation of sensorial properties of grains. Smoke with bad smell could impart undesirable taste, aroma and flavor to gain flour. In this regard, smokes from all four selected materials resulted in less smell as compared to smokes from CD and ML (data not indicated in Table 2). Availability and cost are the two major factors for small-scale farmers to use smoke sources in creating MA. In this regard, MS is easily available and abundant after harvest with no additional cost to use. Commonly, the stalk functions as a source of fuel to cook family food on a daily basis. In addition to this, the use of MS as smoke source material imposes no effect on deforestation unlike that of Olia africana wood and charcoal.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Food and nutrition insecurity is apparent in most of sub Sahara Africa counties due to low productivity and climate impacts. High after harvest losses of crops further aggravate existing gaps in the countries. Among multiple postharvest loss causes, losses imposed due to storage insect pests is significant. This coupled with traditional storage structures and lack of improved insect pest control methods. Recent storage technologies associated with hermitic storage structures are also either  expensive or are not suitable for storing crops like maize with cob and sorghum head in study zone. Use of modern CA or MA storage structures with infusion of CO₂ gases from commercial sources is not available, feasible and compatible with traditional storage structures and methods. However, use of smoke having high concentration CO and CO₂ could enable modification of gases composition in stored grain environment. In addition to this, infused smoke could enable expulsion of ambient air at high O₂ percentage. In this study, chopped and dried maize stalk identified as a good source of CO and CO₂ for small-scale farmers with sufficient availability, attracts no cost and impact on forest. Infusion of the smoke enables accumulation of high concentrations of CO and CO₂ with low O₂ percentage. Under hermitic condition, actively modified environment gets further support by passive modification with depletion of O₂ and accumulation of more and more CO₂ to hinder activity and reproduction of insect pests. As additional recommendation, the inner layer of traditional storage structures are better lined with air proof locally available plastic (polyvinyl chloride) to create hermetic and maintain MA environment. Creation of efficient MA can be further enhanced by infusing significant volume of smoke using efficient smoke infusion pump.

 

The authors have not declared any conflict of interests.

 

The authors appreciate Volkswagen Stiftung funding initiative "Knowledge for Tomorrow – Cooperative Research Projects in Sub-Saharan Africa", for the financial support received, Konstanz University of Applied Sciences, Germany, and Jimma University, Ethiopia for providing the laboratory facilities to conduct this study.