Microorganisms are ubiquitously present in terrestrial ecosystems from which they are disseminated and contaminate plant communities. The ripening seed is no exception and is contaminated by a wide range of bacteria, yeasts, and filamentous fungi via the air, insects, rain splash, equipment, and agronomic practices. When seeds/cereal grains are harvested they thus carry a wide range of microbial contaminants. Postharvest treatment of such seeds and the prevailing environmental factors are key determinants of the impact that they may have on the quality of the seeds, including germinability. It is important to remember that both the harvested seeds/grains and contaminating microorganisms are alive and respiring slowly under dry, safe storage conditions.
Poor postharvest management can lead to rapid deterioration in quality characteristics and severely decrease germinability and nutritional value of the seeds. Microbial activity can cause undesirable effects in grains including discoloration, contribute to heating and losses in dry matter through the utilization of carbohydrates as energy sources, degrade lipids and proteins or alter their digestibility, produce volatile metabolites giving off-odors, cause loss of germination and baking and malting quality, affect use as animal feed or as seed, and filamentous fungal spoilage organisms may also produce mycotoxins that are highly toxic or carcinogenic or cause feed refusal and emesis (Christensen 1973). The spores of some fungi can also cause respiratory disease hazards to exposed workers (Lacey and Crook 1988).
Estimated losses of seeds, especially staple cereal grains in store from all causes varies widely. They may amount to 10% worldwide (Anon 1979) but can reach 50% in tropical regions (Hall 1970). Vassan (1980) estimated losses of high moisture paddy in southern India exceeded 15-25% in only 9 days, while Rohani et al. (1985) found storage losses of paddy in West Malaysia of only 1%.
Deterioration of grain by microorganisms is determined by several factors which can be classified into four main groups: intrinsic factors (those which depend on the characteristic of the growth substrate), extrinsic factors (those imposed from the outside), processing factors (those resulting from the agronomic practices and food processing, which primarily modify the composition of the microflora), and implicit factors (those depending on the particular dominant microbial flora that initially develop in response to the intrinsic, processing, and extrinsic factors) (Sinha 1995). Figure 1 summarizes the factors affecting fungal colonization of the grain.
Wallace and Sinha (1981) in the 1970s were the first to consider stored seeds as a man-made ecosystem which needed to be examined in a more holistic and ecological manner to enable a proper understanding of the processes occurring and to improve postharvest management of stored seeds of all types. This in many respects enabled prevention strategies to be developed and implemented to avoid microbial and pest infestation from damaging seeds. Generally, since most seeds are stored dry, bacteria seldom cause biodeterioration problems. At intermediate moisture content levels fungal spoilage and pests are of major concern. This Chapter will endeavor to examine some of these important abiotic and biotic factors and their interactions that determine whether deterioration will occur and the dominant fungal species which may be involved. This is important as the fungal community structure influences the type of deterioration and whether mycotoxins are produced. Cereal grains particularly wheat, barley, maize, and rice are used as examples of seed systems, since they represent the key staple seeds worldwide.
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