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Hermetic Storage of Dry Grains and Other Durable Agricultural Products
Factors Affecting Insect Mortality in Hermetic Storage
If we can seal the storage ecosystem to prevent air from entering or leaving it, the respiratory metabolism of insects, molds and the grain itself will lower the oxygen content and raise the carbon dioxide content of the intergranular atmosphere to a level where aerobic respiration is no longer possible. This is the principle behind hermetic storage. However, although it sounds simple in theory, in practice it is much more complex. The following brief historical overview will explain much, and link you to our own developments in this field, including summaries of our research studies.
The important role of low O₂ concentration rather than high CO₂ in causing mortality of stored-product insects in hermetic storage was demonstrated by Bailey (1965). Only later was the importance of the synergistic effect of concomitant O₂ depletion and CO₂ accumulation for insect control clearly demonstrated (Calderon and Navarro, 1979; 1980). These synergistic and combined effects are essential for successful insect control, as shown by studies of the effects of incomplete air-tightness upon insect populations (Oxley and Wickenden, 1963; Burrell, 1968). Furthermore the lower the grain moisture content (m.c.) and corresponding inter-granular humidity, the higher the mortality, due to the desiccation effect on insects caused by low O₂ (Navarro, 1978), or elevated CO₂ concentrations (Navarro and Calderon, 1973). The influence of temperature on insect respiration implies that in warm climates O₂ intake by insects is very intensive. Conversely in temperate climates, insect metabolism is much slower, depletion of O₂ may be lower than its ingress, and insect control may not be achieved. This led Burrell (1980) to postulate that for light infestations of cool grain, residual populations would provide an inoculum for reinfestation after the grain is removed from hermetic storage.
Read more on the history of hermetic storage and its applications.
Hermetic Storage as a Present & Future Alternative
The need for alternative methods of prevention and control of insect infestations in stored products has become acute over the last few years. This is because conventional measures using insecticides are being questioned by environmental agencies and pressure-groups, and the choice of available permissible materials is decreasing. Of the two remaining fumigants in general use, the phase-out of methyl bromide due to its destructive effect on ozone in the stratosphere is aslready under way. This is coupled with mounting evidence of development of insect resistance to phosphine indicating that even phosphine may not be economically effective in years to come. The use of modern and safer acceptable technologies such as aeration, refrigerated aeration and modified atmospheres are still expensive and require adequate infrastructure. In sharp contrast to the use of chemicals, hermetic storage is an environmentally friendly technology, involving no hazard to the storage operators, consumers or non-target organisms, and as such, its application is beginning to enjoy a high level of consumer acceptance.
Our accumulated experience of hermetic storage using several types of flexible liners for above-ground storage, in-the-open, under tropical and subtropical conditions is summarised in the following observations:
Structural durability
The use of PVC-based sheeting without mesh reinforcement produces a material of suitable strength and elasticity for storing grain. This material was formulated to have a high resistance to solar UV irradiation. Rodent penetration has been recorded on only exceptional occasions involving minor damage. Our hypothesis that rodents find it difficult to gain a tooth-hold on the smooth surface has been corroborated by laboratory studies using wild-caught roof rats and house mice (unpublished data).
Liners have been used continuously for over 10 years, and though they have lost some plasticity, permeability to gases decreases as the plasticisers evaporate. This characteristic renders the liners more effective with time in retaining gas concentrations, e.g., for 0.83 mm PVC, the initial permeability (expresed throughout as a measure given at a gradient from 21% O2) decreased from 87 to 50 ml O2/m2/day after 4 years of exposure under Mediterranean climate (unpublished data).
Insect control
At a liner thickness of 0.83 mm and a gas permeability level of 87 ml O2/m2/day, there is a possibility of insect survival close to the grain-liner interface in small enclosures where the surface area:volume ratio is relatively high. This is especially so at the top layer of the structures where moisture content tends to be higher than the remaining parts of the bulk. However, after the minimum O2 concentration is reached, survival is usually well below one insect/kg, and would require multiple sampling to detect a single insect (Navarro et al., 1984; 1993). This residual infestation is more of a problem on return to aerobic conditions and the commodity should be consumed without additional prolonged storage. This residual infestation is less serious than the danger of reinfestation by insects from the surroundings under storage by conventional methods. Recent findings in the Philippines with milled rice in 150 ton capacity Volcani cubes, indicate that oxygen concentrations of less than 1% can be obtained, at which, all insects perish after a very short exposure period.
Moisture migration
Diurnal temperature fluctuations, accentuated by solar radiation on liners, followed by rapid cooling at night, cause successive moistening and drying cycles at the upper grain surface. This may result in gradual moisture accumulation, particularly during the transient seasons between summer and winter when temperature fluctuations are greatest, so that initially dry grain may rise to above critical levels enabling limited microfloral spoilage to occur. This is particularly accentuated along the peaks of bunkers where warm air rising on convection currents tends to concentrate the moisture condensation in confined areas. For bunkers of 12,000 to 15,000 tonnes capacity constructed in recent years, the condensation phenomenon has been alleviated and almost eliminated by levelling the peaked apex (with a ridge of less than 2 m) to a slightly convex, wide apex of bunker cross-section (with a ridge of more than 6 m) that is just sufficient to permit rain-water run-off. This configuration appears to enable the dispersal of moisture migration over a much larger area. Differences in the intensity of moisture increase between bunker peaks with narrow ridges and peaked apices, and apices with a broad ridge has been demonstrated.
For dry grain kept in "storage cocoons" in subtropical climates, moisture migration is not a pronounced phenomenon. However, for corn or paddy stored in the tropics, moisture migration is accentuated particularly when there are large diurnal temperature fluctuations. For this purpose, our first solution to moisture migration was to place an insulating layer between the liner and the upper layer of bagged grain. This consisted of a layer of bagged agricultural wastes such as rice hulls, or straw, or if these weree not available a "felt-fibre" layer with insulating properties was used. Recently we have shown that by spreading a reflective awning of aluminium coated webbed PP fibres over the cocoons we are able to reduce temperature gradients and moisture migration to a minimum. For more - visit the GrainPro Cocoon page.
