Increasing Global Food Security and Reducing Post-Harvest Loss with Hermetic Storage of Grains

Cereal grains account for nearly half of the calories consumed by humans worldwide, and as such, form a key component in global food security. Documented grain post-harvest loss estimates worldwide differ by grain and by region, but typically range from about 2-10% and in extreme cases can be more than 50% (Lipton 1982; Greely 1986; Boxall 1998; Appiah et al. 2011). This comprises not only physical weight loss such as spillage, but also qualitative losses such as insect or heat damaged kernels, and insect infested and mycotoxin contaminated grain unfit for consumption or sale. Grain losses negatively impact growers as well as grain handlers and processors, and this impact is felt most acutely in less developed countries. Developing tools to reduce the amount of grain lost after harvest is an important strategy to fight hunger and poverty, and increase global food and nutrition security.

Grain is harvested only once or twice per year but is utilized year round, making storage a critical link in the grain processing value chain. Maintaining grain quality during the storage period is highly important. Factors that contribute to grain quality deterioration include fungi, rodents, birds, and certain species of insects that feed on grain. To maintain grain quality during storage, many solutions are available including aeration, moving (i.e., “turning”) grain to kill insects by impact, applying contact pesticides to grain or storage structure surfaces, heating or cooling grain, fumigation, controlled atmosphere treatments (CA), and biomodified atmosphere (i.e., hermetic) storage.

Hermetic storage is an ancient technology used to control insect infestation in grain and preserve its nutrient quality and caloric value. In ancient times, agricultural societies stored their surplus grain in containers or structures that kept the grain safe from the elements and restricted the entry of rodents, birds and insects. Well-sealed structures also prevent air exchange between the inside and outside of the storage space (Calderon 1990; Reed 1992; Sigaut 1980). By preventing air ingress to the grain, the aerobic respiration of grain, insects and fungi decreases O₂ and increases CO₂ in the interstitial space in the grain mass. It has been shown that CO₂ generated from aerobic respiration alone does not increase to levels high enough to kill insects, and it is the lower O₂ level that is the key factor in halting insect development. In large scale hermetic storage, it may take weeks or months for O₂ levels to decrease by respiration to levels lethal to stored product insects (1-2%), especially when infestation is low. As a result, practical commodity management may require combining hermetic storage with additional control methods such as artificial CO₂ or nitrogen injection or fumigation (De Lima 1990). Historically, cereal grains were stored hermetically in underground pits lined with clay or in small containers such as gourds or clay pots, but today technology exists that allows large amounts of grain to be stored hermetically. Large bulk silos and warehouses have been sealed to a high degree, most notably in Australia (Banks and Ripp 1983; Newman 1990).

Making large structures completely gastight is a challenging and costly prospect. In permanent structures, allowing for a small amount of air exchange between the interior and exterior of the structure (i.e., pressure venting) is a practical necessity because of potential structural damage due to internal and external pressure fluctuations. Airtightness standards have been thoroughly researched, and the half-life pressure decay test remains the simplest and most effective test for gas tightness (Navarro and Zettler 2001). Hermetic storage has been accomplished successfully on large scales in the form of underground pits covered with flexible roofs in Cyprus and Argentina (Navarro et al. 1994).

On a smaller scale, hermetic storage has been used by smallholder and subsistence farmers for many years in the form of clay containers plastered with dung and straw, containers such as gourds, and underground cellars. More recently, reclaimed metal and plastic drums and other sealable containers have been used successfully (Navarro et al. 1994). Modern hermetic storage technologies such as triple plastic bags (Purdue Improved Crop Storage, or PICS), multi-layer plastic bags with an oxygen limiting barrier layer (GrainPro SuperGrainbags™) and bag stacks enclosed in sealable plastic liners (GrainPro Cocoons™) may be more affordable for smallholder farmers or village level cooperatives than constructing large-scale hermetic storage structures. These technologies are increasingly available in developing countries, and can provide a sustainable and affordable solution to the prevention and reduction of post-harvest loss, and thus increase global food and nutrition security.

For medium scale grain storage, silo bags which were originally developed for anaerobic storage of chopped forages, have been adapted for bulk grain storage first in Argentina in the early 2000s and from there adopted into many countries around the world (Bartosik et al. 2013). For the past few years about 40% of grain (35-40 million MT) produced in Argentina is being stored in silo bags. Silo bags are a key hermetic storage technology that reduces post-harvest losses while providing an economic alternative for farmers to gain control over marketing their stored grain between crop harvests.

Silo bags consist of three layers of polyethylene totaling 250 microns thick. The outer layer is white to reflect solar radiation, and the inner layer is black to block sunlight. A typical silo bag is about 60 m long, 3 m in diameter, and can store about 200 metric tonnes (MT) of wheat, maize, and soybeans (180 MT of sunflower), though they can be smaller by simply using less of the bag. When the bag is properly sealed, the silo bag is watertight and has a high degree of gas tightness to CO₂ and O₂.

The ground upon which a silo bag will be placed should be level and should not accumulate standing water during rains. It also must be free of any objects that could puncture the plastic (i.e., field stubble, sharp rocks, or branches) because even very small holes will compromise the hermetic seal, allowing moisture and insects to enter the grain mass. Because a high level of CO₂ and low level of O₂ are the means by which insect and fungal activity is controlled in the silo bag, holes or leaks will permit air exchange with the outside and will confound the desired internal atmosphere.

Grain is loaded and unloaded from a silo bag using special equipment that can be attached to a PTO-driven tractor. The silo bags and equipment to load and unload the grain are relatively inexpensive compared to permanent grain storage structures and the associated grain handling equipment. One 2009 estimate was about $50,000 USD for the loading and unloading equipment, and about $600 per silo bag (Young et al. 2009). Equipment is becoming increasingly available as dealers and service suppliers pick up on the trend in countries that have adopted silo bag usage.

Damp grain should not be stored in silo bags. In warm temperatures, higher moisture grain creates an interstitial equilibrium relative humidity (RH) above 65-70% which provides a good environment for fungi to develop. While it has been shown that low oxygen environments can prevent the proliferation of certain species of fungi present in grain (Adler et al. 2000), trials with damp grain stored in silo bags showed that fungi develop to the detriment of grain quality (Castellari 2010). It is therefore best to store grain at the recommended safe storage moisture content when placed into silo bags.

The world grain industry will face many challenges in the next decades due to a growing population and associated rapid increase in demand for animal protein. For instance, many of the insecticides and fumigants once approved for use in raw commodities, such as methyl bromide, have not been reissued governmental approval in several countries because of environmental toxicity, harmful residues in the grain, and worker safety concerns (Bell 2000; Haritos et al. 2006; Donahaye 2000). In the early 1980s, the Environmental Protection Agency of the United States approved CO₂ and N2 for disinfesting raw and processed agricultural commodities, opening the door for more widespread use of CA treatments and hermetic storage in the U.S.

In addition, consumers’ health concerns about harmful chemical residues in foodstuffs, as well as the environmental impact of such chemicals are putting pressure on companies to use nonchemical control methods to protect their stored grain. The organic food sector is growing, and uses hermetic storage technology to aid disinfestation of grains and other foods like nuts, fruits, and vegetables (Dilley 1990).

Riots over high food prices in Africa in 2007 renewed concerns over food security and a consistent supply of grain (Hodges and Stathers 2013). Producers who lack the capital or credit necessary to obtain permanent bulk grain storage and handling equipment are often forced to sell grain at low prices. Silo bags allow farmers to safely store surplus grain themselves and market it at favorable prices later. This has revolutionized the grain industry in Argentina during the past decade. While farmers used to depend on custom harvest crews transferring grain from the combine into trucks owned by traders hauling grain directly from the field to the country or export elevator, custom harvest crews now fill farmer-owned silo bags and place them along the sides of fields. Now farmers decide when they want to sell and deliver their grain instead of traders dictating the marketing of grain.

Insect resistance to grain fumigants, especially phosphine, is a growing global problem. The number of resistant insect populations has increased significantly in the last decade. Opit et al. (2012) found R. dominica and T. castaneum populations in Oklahoma with strong resistance to phosphine. The resistant populations were 1,519 and 119 times more resistant to phosphine than susceptible strains, respectively. In Western Australia two strains of T. castaneum were found with strong resistance to phosphine, and frequency of weak resistance to phosphine was an average 48% across all species (Newman 2010). High levels of phosphine resistance have been identified in Africa and India (Bell 2002). The key to combating phosphine resistance is to ensure that grain is fumigated in a well-sealed storage structure at the proper concentration and for the correct length of time. This helps prevent gas leakage and ensure a complete kill of insect pests. Cardoso et al. (2009) showed that silo bags provide a complete seal and are suitable for phosphine fumigation. In their field experiments, aluminum phosphide tablets to provide concentrations of 3 and 6 g/m3 were inserted into two silo bags filled, respectively, with wheat. Both silo bags maintained a concentration of above 200 ppm for five days, except near the closure ends of the bags where the concentration was only about 125 ppm. Fumigation resulted in 100% mortality of S. oryzae in bioassay tubes, compared to 13 to 33% mortality in the control silo bag. It is recommended for silo bag fumigation that the ends of the bag be heat-sealed or sealed between two wood slats and nailed shut, rather than simply folded and placed under a heavy object. In some cases, increasing the initial dose or reapplying the dose at the bag end would help ensure an effective phosphine concentration.

In North America, when the size of grain harvests exceed available storage capacity, grain handling facilities often store excess grain temporarily in large ground piles. These grain piles may be covered with tarps but quality loss will likely occur due to moisture ingress from leaks or by condensation, especially in the top layer of grain. In addition to tarps, there is a significant amount of equipment needed to safely store grain in piles, including aeration ducting and fans, wood or concrete sidewalls, grain throwers or other conveyors, and unloading equipment. One advantage of grain piles is that they can store hundreds of thousands of metric tonnes of grain, whereas silo bags can only store up to about 200 MT per bag. Research on silo bags has primarily been undertaken in South America and Australia. In North America, large hermetically sealed bags have historically been used to store silage, but since the early 2000s silo bags have been used to store grain. However, until 2010 there were no North American scientific studies published that characterize the atmosphere inside silo bags and the effect of silo bag storage on grain quality. The few studies that have been undertaken are reviewed here.

Four silo bags filled with yellow dent maize and soybeans in Mississippi in October 2010 and January 2011 were stored for 20 and 16 weeks, respectively (Ward and Davis 2012). Each silo bag was approximately 7.6 m long by 3.7 m wide by 1.4 m tall when filled with grain. Ward and Davis (2013) designed a monitoring array to measure the temperature and RH in the cross section of each silo bag (Figure 1). Thermocouples and RH sensors were attached every 30.5 cm to two 2.4 m long food-grade high-density polyethylene C-channel. The sensors were calibrated and wired into data loggers. The arrays were inserted into the silo bags crosswise, and the bags were patched with duct tape and extra silo bag material after installation. Grain samples were taken from next to each linear array using a sampling probe. These sensor arrays were utilized to study the effects of the silo bag internal environment on maize and soybean quality. Grain temperature, interstitial RH, and moisture content (wet basis) were measured. Ambient temperature and RH, solar radiation, wind speed and direction, and precipitation data were also recorded.

 

Figure 1. Cross section of silo bag with temperature and relative humidity monitoring array (from Ward and Davis 2013).

 

They defined the peripheral region of the grain mass as being between 30.5 - 45.7 cm from the surface of each bag based on grain temperature (V5, H7-H8) tracking ambient temperature closely. In comparison, temperatures in the center (V3) and at the bottom (V1) of the bags were almost unaffected by daily temperature fluctuations (Figure 2). However, the average bag temperature tracked the weekly mean ambient temperature which was the same pattern observed in other studies (Chelladurai et al. 2011; Bartosik et al. 2008). 

Figure 2. Ambient and internal temperatures at selected positions inside maize silobag (from Ward and Davis 2011).

There was no significant difference in moisture content of maize during the storage period (Table 1). On average, it increased by only 0.2 percentage points, from 14.1% to 14.3% MCwb, across the four silo bags although in two bags the average moisture content increased by 0.3-0.4 points. Condensation in the peripheral region is likely when the ambient temperature is cooler than the grain temperature. However, grain temperature inside the bags did not drop below the dew point temperature (calculated based on measured temperature and RH at several locations), so condensation should not have occurred. The observed moisture increases were likely due to internal moisture movement within the silo bags and/or small holes that allowed water to enter the bags. Total kernel damage remained essentially unchanged and no heat damaged kernels were observed. Thus, maize was stored safely in silo bags without quality loss for 20 weeks. 

Table 1. Initial and final maize factors after 20 weeks of silobag storage (from Ward and Davis 2012).

Soybean quality decreased over the storage period. Mean moisture content in the four bags increased from 10.4% to 12.1% MCwb, and total kernel damage increased from 2.2% to 5.9% during storage (Table 2). This resulted in the drop of one U.S. quality grade. The moisture increase was more substantial compared to maize. The observed increase was likely due to moisture movement within the silo bags coupled with condensation of moisture within the upper peripheral layer. There were also likely small holes that allowed for moisture ingress. Ward and Davis (2012) suggested that during silo bag storage, the quality of cereal crops is more readily maintained than oilseeds unless initial moisture content is lowered further. 

Table 2. Initial and final soybean quality factors after 16 weeks of silobag storage (from Ward and Davis 2012).

Chelladurai et al. (2011) studied silo bag storage of canola seed and its effect on seed quality parameters including free fatty acid value (FAV) (mg KOH/100g dry seeds) and germination rate (%). Gas samples were analyzed using a gas chromatograph to determine CO₂ concentration. Canola seed at 8%, 10% and 14% MCwb were studied over a 32-week period beginning in October 2010. Three bags, each containing approximately 20 MT of canola seed were used at each MC. Thermocouples were used to monitor grain temperature and gas samples were taken using 60 ml syringes. Grain samples were taken every two weeks at 0.15 m, 0.8 m and 1.3 m from the top of the bag. Moisture content of grain samples was determined using a hot air oven.

In the high moisture (14% MCwb) bag, germination dropped from over 90% to below 50% within the first 16 weeks of storage (Figure 3). In the 10% MCwb bag, seed in the bottom and middle of the bag maintained a germination rate of about 90% over 32 weeks, but in the top of the bag it dropped to about 55%. In the 8% MCwb bag germination stayed above 90% during the entire storage period.

After one month of storage CO₂ levels peaked at about 20%, 9% and 4% for the 14%, 10% and 8% MCwb bags, respectively. At the end of 8 months of storage the CO₂ levels were about 2.5%, 1.2% and 1.1% for the same bags (Figure 4). This pattern of CO₂ levels was also observed by Ochandio et al. (2010) who hypothesized that accidental moisture ingress may have caused a localized increase of biological activity which then decreased as the moisture equilibrated throughout the grain mass. This lowered the moisture in the localized region and thus also the biological activity and subsequent CO₂ concentration. This would explain the initial increase in CO₂, but the decrease in overall CO₂ may also be understood by the fact that grain readily absorbs CO₂ from the air (De Lima 1990), and this may be the reason for the decrease in CO₂. Another explanation for freshly harvested grain such as canola seed is initial respiration of the “green” seed kernels due enzyme activity in the seed coat and germ that continues for several days after the seed kernel is removed from the living plant. Between 12-16 weeks of storage, CO₂ dropped below 5% in all bags, which would not be sufficient to control stored grain insects. 

Figure 3. Germination rate for high, medium and low moisture canola seed stored for 32 weeks in silobags (from Chelladurai et al. 2011).

In the dry commodity (8 and 10% MCwb) there was no significant quality loss, while in high moisture canola (14%) FAV increased from 25.2 to 41.2 mg KOH/100 g dry seed (Figure 5). They concluded that canola seeds can be stored safely at low moisture contents (8-10% MCwb) for up to 28 weeks, while higher moisture seeds should only be stored a short time (perhaps less than 4 weeks) because of the observed loss in germination for high moisture canola.

A study performed by Kansas State University investigated silo bag storage of wheat (Subramanyam et al. 2012). It considered key grain quality parameters including test weight, protein content (%), falling number, kernel weight, kernel diameter, kernel moisture, total fungal counts (cfm/g), and mycotoxin contamination (aflatoxin, fumonisin, and vomitoxin), in addition to grain temperature, interstitial CO₂ levels, and RH within the grain mass. Insect bioassays with R. dominica were also performed to determine the efficacy of silo bag storage on this common pest. Wheat was loaded into four silo bags each approximately 20 m long by 3 m in diameter with a capacity of approximately 50 MT. Airtightness was checked using a pressure decay test (400 to 200 Pa). Internal silo bag temperature and RH were recorded with HOBO data loggers placed at the top and bottom of the silo bags at both ends. Bioassay chambers were constructed from PVC pipe 0.3 m long and 9.0 cm in diameter with 200 micron mesh covering the ends. They were filled 80% with grain and then 30 unsexed adult insects were added. Six bioassay tubes were placed in two silo bags (at the top and bottom of three sampling points) and mortality was recorded after two and four months. 

Figure 4. Interstitial carbon dioxide concentration in high, medium, and low moisture canola (from Chelladurai et al. 2011).

 

Figure 5. Free fatty acid values (FAV) for high, medium, and low moisture canola stored 28 in silobags (from Chelladurai et al. 2011).

CO₂ levels did not increase sufficiently high to control insect and fungal development (Table 3), presumably due a combination of imperfect sealing of the bag ends, damage to the bags caused by rodents and Cadelle beetle larvae that allowed CO₂ to escape, and the fact that dry wheat has a low respiration rate to begin with. 

Table 3. Interstitial carbon dioxide concentration in wheat stored in silobags (from Subramanyam et al. 2012).

Bioassays in the silo bags saw a 10-fold increase in adult R. dominica while the control bioassays in the laboratory saw a 140-fold increase over four months; both were significant increases (Table 4). The decreased development in the silo bags was probably due to the onset of cooler temperatures given that CO₂ levels did not increase sufficiently high to control insect life stages.

Fungal counts were significantly higher in the bottom of the silo bags in each month sampled, ranging from 218.8 (± 10.9) to 257.5 (± 12.5) cfu/g compared with 127.5 (± 4.3) to 173.8 (±5.5) cfu/g in samples taken from the top (Table 5). Counts at the same depth increased over the storage period but the increases were not significant. Sampling time and sampling depth were significant for fumonisin and vomitoxin, with the highest levels at the bottom of the bags, likely due to a larger amount of broken kernels, fines, and infected kernels compared to the top. Aflatoxin level did not vary by storage time but was significantly higher at the bottom layer after four months of storage.

Another explanation for higher fungal and mycotoxin counts in the bottom layer may be that moisture movement in the silo bag caused condensation in the bottom of the bag given that dry wheat was bagged during the summer months and ground temperature would have been consistently cooler than the temperature in the upper periphery grain mass which fluctuated with ambient temperature. The average grain temperature decreased as storage continued into the cooler fall months. The reverse occurs during the spring period when the average grain temperature increases as storage continues into the warmer spring months. Moisture movement reverses and condensation may occur in the upper periphery grain mass which can lead to increase in moisture content and fungal growth, which would cause a decrease in quality parameters.

Wheat quality tests are shown in Table 6. Test weight, protein content, kernel weight and diameter were not significantly affected by storage time. Protein content, falling number, and kernel moisture were significantly different among storage times. However, wheat quality did not change appreciably. 

Table 4. Adults of R. dominica bioassays in silobags and in a growth chamber (from Subramanyam et al. 2012).

 

Table 5. Fungal counts (cfu/g) in wheat stored in silobags (from Subramanyam et al. 2012).

 

Table 6. Wheat quality parameters by month (from Subramanyam et al. 2012)

An economic analysis by Texas A&M Extension studied the relative costs of traditional on-farm and off-farm storage versus silo bag grain storage, assuming a $160/MT price for maize and a 6- month storage period (Young et al. 2009). On-farm storage cost estimates assumed storage capacity of 2000 MT with metal silos amortized over 20 years and overhead and operating costs typical of the Coastal Bend and Upper Gulf Coast in Texas, USA. Off-farm storage costs were typical of those charged by commercial grain handling facilities in the area. Costs for silo bag storage assumed 2000 MT handled per year with bags used once and related equipment depreciated over 10 years. The estimated handling and storage costs for off-farm, on-farm and silo bag storage were $29.04/MT, $13.24/MT and $9.00/MT, respectively. In this case storing grain in silo bags had a cost advantage over on-farm storage by 32% and off-farm storage by 69%. This would give producers and commercial grain managers a technically feasible and economically viable solution for short-term storage (1-6 months), especially in years of bumper crops or during transportation (i.e., rail car and barge shipments) shortages. 

Silo bags for grain storage were introduced in the U.S. in 2007 but are only now gaining more popularity due to large harvests, relatively low grain prices, and in some areas of the country, lack of storage space. The four crop (maize, soybean, wheat, canola seed) studies performed in North America demonstrated that cereal grains and oilseeds can be safely stored in silo bags for up to six months and even longer provided that grain is sufficiently dry when it is initially stored. This is consistent with previous silo bag studies conducted in Argentina, Australia and other countries. Close monitoring of the grain is necessary due to the somewhat fragile bag material. If there are tears or holes in a silo bag, the hermetic storage environment is compromised. Furthermore, the holes will allow water ingress and raise grain moisture content and spoilage potential. In all cases where spoiled grain was found in silo bags, it was in areas where holes or tears in the bag allowed moisture to enter.

Currently, the U.S. rail system is bottle-necked with competing cargoes of oil, shale, containerized consumer goods, and grain. Transporting grain to export terminals is taking longer due to space competition for rail cars and barges. Silo bags are an inexpensive and flexible method for producers and commercial grain managers to store grain. A cost analysis indicated that silo bag storage was 32% - 69% less expensive than traditional on- and off-farm storage. While silo bags allow farmers to market grain when prices are more favorable, it may at the same time expose them to more risk. Silo bag stored grain must be monitored closely, and the ground specially prepared before bags are placed and filled. Producers and commercial grain managers are finding that silo bags can be an economical buffer to ease temporary storage shortages.

Overall, silo bags are an effective and proven hermetic storage technology that is expected to substantially contribute to reducing post-harvest loss and increasing global food and nutrition security 

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