A 'system' may be viewed as a set of interacting components, where the interactions are just as important as the components themselves (after Open University, 1987). A 'systems' approach to the control of mycotoxins utilises (Coker, 1997) conceptual models of interactions between, and within, commodity, spoilage, mycotoxin, and control subsystems. Within a system, the sub-systems can freely interact; in other words, activity within one subsystem can influence events in one or more other sub-systems.
A better understanding of both the interactions and the components associated with these systems will assist in understanding the aetiology of mycotoxin production, and in formulating appropriate interventions for the control of mycotoxins and mycotoxicoses.
THE COMMODITY SYSTEM
Any commodity system is composed of numerous interacting technical and socio-economic 'processes' including, for example, pest and disease control, harvesting, drying, processing, marketing, credit and pricing policies and cultural issues, to name but a few. A generalised, simplified commodity system is represented in Figure 1 where selected processes are represented as interacting subsystems.
Figure 1 The Commodity System
At any point within the commodity system, the condition of the commodity is determined by a complex milieu involving a multitude of interactions between the crop, the macro- and micro-environment and a variety of biological, chemical, physical and socio-economic factors. A change within any one process will invariably bring about changes in one or more of the other processes. Action taken before harvest to control pest damage and/or increase production (e.g selection of varieties, timing of harvest) can have a significant impact on the post-harvest quality of the commodity. Hybrid white maize, for example, has much higher yields than traditional varieties but has poor on-farm storage characteristics. Similarly, since it is very rare for a single commodity system to exist in isolation within a given agro-climatic region, it should be remembered that activities within one system can significantly effect events in other systems. Given the finite resources of farmers, for example, an increase in the importance of one commodity will frequently lead to the allocation of less resources towards the care of other commodities.
THE SPOILAGE SYSTEM
Biodeterioration is the net result of numerous interacting spoilage agents which may be broadly described as biological, chemical, physical, macro-environmental and micro-environmental (Figure 2). However, the relative impact of these agents will often be largely determined by the nature and extent of human intervention.
Figure 2 The Spoilage System
The factors which primarily contribute to bioterioration (including mould growth) within an ecosystem, are moisture, temperature and pests. Moulds can grow over a wide range of temperatures and, in general, the rate of mould growth will decrease with decreasing temperature and available water. In grains, moulds utilise intergranular water vapour, the concentration of which is determined by the state of the equilibrium between free water within the grain (the grain moisture content) and water in the vapour phase immediately surrounding the granular particle. The intergranular water concentration is described either in terms of the equilibrium relative humidity (ERH, %) or water activity (aw). The latter describes the ratio of the vapour pressure of water in the grain to that of pure water at the same temperature and pressure, whilst the ERH is equivalent to the water activity expressed as a percentage. For a given moisture content, different grains afford a variety of water activities and, consequently, support differing rates and type of mould growth. Typical water activities which are necessary for mould growth range from 0.70 to 0.99, the water activity, and the propensity for mould growth increasing with temperature. Maize, for example, can be relatively safely stored for one year at a moisture level of 15 per cent and a temperature of 15oC. However, the same maize stored at 30oC will be substantially damaged by moulds within three months.
Insects and mites (arthropods) can also make a significant contribution towards the biodeterioration of grain because of the physical damage and nutrient losses caused by their activity, and also because of their complex interaction with moulds and mycotoxins. The metabolic activity of insects and mites causes an increase in both the moisture content and temperature of the infested grain. Arthropods also act as carriers of mould spores and their faecal material can be utilised as a food source by moulds. Furthermore, moulds can provide food for insects and mites but, in some case, may also act as pathogens.
Another important factor that can affect mould growth is the proportion of broken kernels in a consignment of grain. Broken kernels, caused by general handling and/or insect damage, are predisposed to mould invasion of the exposed endosperm.
Mould growth is also regulated by the proportions of oxygen, nitrogen and carbon dioxide in the inter-granular atmosphere. Many moulds will grow at very low oxygen concentrations; a halving of linear growth, for example, will only be achieved if the oxygen content is reduced to less than 0.14 per cent. Interactions between the gases and the prevailing water activity also influence mould growth.
The interactions described above, within granular ecosystems, will support the growth of a succession of micro-organisms, including toxigenic moulds, as the nutrient availability and microenvironment changes with time. In the field, grains are predominantly contaminated by those moulds requiring high water activities (at least 0.88) for growth, whereas stored grains will support moulds which grow at lower moisture levels.
It is well recognised that the main factors which influence the production of mycotoxins are water activity and temperature. However, given the complexity of the ecosystems supporting the production of mycotoxins, the conditions under which toxigenic moulds produce mycotoxins are still poorly defined; and have recently been comprehensively reviewed (ICMSF, 1996).
THE MYCOTOXIN SYSTEM
The Mycotoxin System (Figure 3) may be considered in terms of three interacting subsystems: metabolism & toxicology; health & productivity; and wealth. After exposure (by ingestion, inhalation or skin contact), the toxicity of a mycotoxin is determined by a sequence of events (metabolism) involving the administration, absorption, transformation, pharmacokinetics, molecular interactions, distribution, and excretion of the toxin and its metabolites. In turn, the toxicity of a mycotoxin will be manifested by its effect on the health and productivity of crops, humans and animals; and, these effects will influence the production of wealth associated with human endeavour and agricultural and livestock products.
Figure 3 The Mycotoxin System