This image shows the essential elements of the storage ecosystem, the boundaries of which are the storage container (bag, drum, silo, warehouse or whatever).
In our work, we focus our attention on the product we aim to conserve - typically living grain or seeds in a dormant state.
The other living factors of the ecosystem, insects and molds, feed on the grain causing damage and losses, and our objective is to reduce the presence and development of these organisms to zero. Thieves such as rodents and birds, also visit the ecosystem, but are not actually part of it.
To achieve this objective we must control or manipulate the abiotic components of the storage ecosystem, namely: temperature, humidity and atmospheric composition
This brief overview of the major components of the ecosystem and the inter-action between them will reveal the logic behind our research activities:
1. The stored product - This is the component we wish to preserve. Consequently we want to minimize the effect of any factor adversely affecting its conservation. Grain seeds and many other stored-products are living organisms in a state of dormancy that can remain unchanged for prolonged periods. However, dead grain rapidly loses its nutritive value, while non-dormant (germinating) grain cannot be stored. The grain remains viable and dormant because it is dry. High moisture permits the development of microorganisms that tend to kill the grain, while at very high moisture contents the grain begins to germinate.
2. The storage structure - This forms the boundaries of the ecosystem and is a pre-determined fixed component. The materials and nature of its composition and its design are important in determining the extent to which external factors (both biotic and abiotic) affect the system. The structure should protect the product from external environmental factors such as rain and ground water; it should minimize the influence of environmental temperature and humidity and serve as a barrier to the ingress of insects rodents birds and thieves.
3. Temperature - has little direct influence on grain condition, but greatly influences the development of insects and microflora and so it indirectly affects conservation. The grain temperature is only slowly influenced by environmental temperature due to its low thermal conductivity. Therefore the main influence is due to seasonal fluctuations and is more pronounced in small storage structures exposed to the sun.
4. Humidity - is present in the air surrounding the grains, and is influenced by the moisture of the grains, with which it reaches an equilibrium. Its greatest influence is on molds which begin to develop at humidities above 70%. Humidity of the storage ecosystem is only influenced by the fluctuations in environmental humidity when a free exchange of air is possible through the storage structure fabric (such as in cribs and open bins). However, convection currents can move humid air in the grain bulk from one region to another, thereby transfering moisture - frquently to the upper grain layers.
5. Atmospheric composition - Air comprises about 50% of the volume of the storage structure, being present in spaces between the grains and sometimes as head-space above the grain. When there is free movement between air inside and outside the store, the composition of the atmosphere is constant and contains about 21% oxygen and 0.03% carbon dioxide. However, if the storage structure is sealed to restrict or completely prevent movement of air between the grain bulk and the surrounding atmosphere, then the living grain, insects and microorganisms through their respiration, may strongly alter the atmospheric composition of the ecosystem, reducing the oxygen and increasing the carbon dioxide concentration.
6. Insects - Stored-product insects consist of a group of some 250 species (beetles and moths) characterized by their small size enabling them to penetrate the interstices of the grain bulk, by their cosmopolitan distribution, and by their non-descriminating feeding habits. Some 6 species are the major pests and several of them attack crops in the field, thereby entering the ecosystem at the moment of loading the grain into storage. These insects are a major biotic factor causing losses in both weight and quality. Their rates of development and population increase are strongly influenced by temperature of the grain bulk, and their metabolic activity in turn produces heat and moisture which then affect all biotic components of the ecosystem. Their development is suppressed and even controlled when the intergranular atmosphere is rich in carbon dioxide and is oxygen deficient.
7. Micro-organisms - a group name for molds, yeasts and bacteria. They are universally present on the grain, but are inactive at humidities favorable to storage. Dry living grains have protective mechanisms against microorganisms, but when moisture of the grain rises above a critical level, molds begin to develop killing the grain and causing qualitative changes including the production of mycotoxins under certain circumstances. The molds are strongly influenced by the non-living components of the ecosystem. High temperatures and humidities favor their development, different molds having different optima for activity. Since both molds and insects release heat and moisture by their metabolism, they may produce temperature gradients within the ecosystem. This in turn creates convection currents through the grain bulk, carrying warm moist air from the heating region to cooler regions where the moisture is deposited as the air cools. Such areas of condensation favor the development of bacteria and may even cause the grain to germinate. As with insects, mold development is suppressed when the storage atmosphere is strongly modified, though control of molds cannot be obtained by this process and some of the microorganisms develop under anaerobic conditions.
8. Foreign matter - (Chaff, stalks, grain dust, sand, earth, stones etc). This is an abiotic component of the ecosystem being either mineral, or originating from dead parts of plants. Its effects on the ecosystem are many: Chaff and grain dust tend to absorb moisture more rapidly than grain and present a more suitable substrate for mold development than whole grains. Also many insects unable to penetrate sound grain are able to develop well on this material. All the small particled material tends to block the interstitial air spaces. If this prevents insect penetration it may be favorable. Conversely, it may prevent the application of control measures that rely on the penetration of cool air to prevent the bulk from heating, or the penetration of toxic gases throughout the grain bulk.