Post Harvested Stored Wheat Grain: Fungal Infestation and Nutrition
Post Harvested Stored Wheat Grain: Fungal Infestation and Nutrition
The wheat grain production in any country varies from year to year and hence the grains should be stored strategically from years of overproduction for the use in year of under production. Also grain must be stored for several other reasons such as point of production is not the point of consumption and the time of production is not the time of consumption. Grain quality after harvest is influenced by a wide variety of biotic and abiotic factors and has been studied as a stored grain ecosystem. Stored grains can have losses in both quantity and quality. Losses occur when the grain is attacked by microorganisms and other organisms including insects, mites, rodents and birds (Neetirajan et al., 2007). The wheat grains come in association with the fungi from the time of grain maturity and also at the time of storage. Some of these fungi are in intimate association and are present as dormant mycelium under the pericarp or dormant spores on the surface of the kernel. However, there are a number of fungi which are only superficially associated with stored grains. The association of fungi with cereal grains starts from the field itself. Shortly after the grain reaches to maximum size, the lemma and palea protecting it are pushed apart exposing the grain to infection by fungi (Machacek and Greaney, 1938) and their extensive studies has been carried out in the laboratory on these aspects (Sankaran et al., 1975; Sankaran et al., 1976). Poor post harvest management can lead to rapid deterioration in grain quality, severely decreasing the germinability and nutritional value of stored grains. Mould growth in grains may cause deleterious changes in addition to the formation of mycotoxins. Many spoilage fungi cause loss of germination in seed grains, discolouration and darkening of the grains, reduction in protein content, musty odours and changes in fatty acid profiles and other constituents of the grains. Mould development may also reported to be encouraged the mite and insect infestation (Wicklow, 1995). Fungal activity can cause undesirable effects in grains including discolouration, contribute to heating and losses in nutritional value, produce off-odours, losses in germinability, deterioration in baking and milling quality, and can result in contamination by mycotoxins (Hocking, 2003).
It has been mostly observed that most of the fungi which contaminate the grain in the field are different from the one in storage, the former being called the "field fungi" and the latter "storage fungi". A number of the fungi have been found to enter the subepidermal region (below the pericarp) of the grain (Oxley and Jones, 1944). A sizable and definite fungal flora has been reported which is favoured by the microecological conditions prevailing around the grains and their milling fractions (Lacey and Magan, 1991). The activity of mycotoxigenic fungi in stored grain must be examined in the contest of the ecosystem as a whole in order to understand the dominanance of certain species under certain environmental condition and also overall conditions in stored grains are not in steady state and thus the dynamics of the system will depend on time (Magan et al., 2003). The storage fungi belong to the most ubiquitous mould genera, Aspergillus and Penicillium with some of their species predominating over others, they can grow at low partial pressures of oxygen and at low temperatures (Lacey and Magan, 1991). Mould development during storage can be controlled or prevented by ensuring that grain is adequately dry at intake. Further protection can be provided by preventing the development of temperature and moisture gradients by cooling and/or aeration of the grain. Protection from insect infestation will also help to prevent mould development in stored grains, both in bulk or bag storages (Hocking, 2003).
A number of studies have been reported on the loss of dry matter content and visible moulding of stored wheat grains (Magan et al., 2004). This could be considered as a subjective index of the safe storability of grain. There are problems with the use of visible moulding as a criterion of deterioration (Lacey et al., 1997; Hocking, 2003). Wheat quality loss has been measured and models developed based on germination rates, visible mould growth or respiration of grain and microorganisms (Fleurat-Lessard 2002). The microscopic growth may be a more effective measurement of initial colonization than visible moulding. Some attempts have also been made to relate dry matter losses to actual calorific losses due to the activity of mycotoxigenic moulds.
References:
Fleurat-Lessard, F. (2002). Qualitative reasoning and integrated management of the quality of stored grain: a promising new approach. J. Stored Product Res. 38: 191-218.
Hocking, A.D. (2003).Stored grain in Australia 2003. Proceedings of the Australian Postharvest Technical Conference, Canberra. CSIRO Stored Grain Research Laboratory, Canberra. pp. 55-58.
Lacey, J., Hamer, A. and Magan, N. (1997). Respiration of wheat grain in different environments. (E.J. Donahaye and S.Navaro eds.). Proc. Int. Symp. on Controlled Atmospheres and Fumigation (CAF), Caspit Press, Jerusalem, Israel. pp. 113-122.
Lacey, L. and Magan, N.(1991). Fungi in cereal grains: their occurrence and water and temperature relationships. In: Cereal Grain: Mycotoxins, Fungi and Quality in Drying and Storage (J. Chelkowski, ed.). Elsevier, AmsterdamLondon New YorkTokyo. pp. 77-118.
Machacek, J.E. and Greaney, F.J. (1938). The block point of kernel smudge disease of cereals. Can. J. Res. 16: 84-113.
Magan, N., Hope, R., Cairns, V. and Aldred, D. (2003). Post-harvest fungal ecology: impact of fungal growth and mycotoxin accumulation in stored grain. European J. Plant Pathol. 109: 723-730.
Magan, N., Sanchis, V. and Aldred, D. (2004). Role of spoilage fungi in seed deterioration. Chapter 28, In: Fungal Biotechnology in Agricultural, Food and Environmetal Applications (D.K.Aurora, ed.). Marcell Dekker. pp. 311-323.
Neethirajan, S., Karunakaran, S., Jayas, D.S. and White, N.D.G. (2007). Detection techniques for stored-product insects in grain. Int. Fd. Control 18: 157-162.
Oxley, T.A. and Jones, J.D. (1944). Apparent respiration of wheat grain and its relation to a fungal mycelium beneath the epidermis. Nature, London 154-826.
Sankaran, R., Sehgal, D. D., Parihar, D. B. and Nath, H. (1976). Cereal grain fungi and grain quality. Def. Sci. 26: 161.
Sankaran, R., Mattada, R. R. and Thangamani. (1975). Mycoflora and changes of wheat seed quality during storage. Itzd. Fd. Packer 29: 1.
Wicklow, D. T. (1995). The mycology of stored grain: an ecological perspective. In: Stored grain ecosystems (D. S. Jayas, N. D. G. White and W. E. Muir, eds.). New York, Marcel Dekker, pp. 197-249.
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