POST HARVEST MANAGEMENT
- A Review of the Post-harvest Management Course
held at University of Botswana from 28th May - 4th June, 2001

Post harvest management
The current African crisis of food production is partly caused by the avoidable losses incurred during the storage of produce. Post harvest food losses are losses of stored products such as cereals, cereal products and grains after harvesting. The stages at which post harvest losses occur include, during drying, threshing, cleaning, sorting, storage, processing and distribution [1]. Losses in storage arise from insects, microorganisms, rodents or birds or a combination of the same [1].

Post harvest losses and lack of market opportunities are critical constraints within poor people’s livelihoods. There is need therefore for Governments, NGOs, the private sector and civil society overseas to train personnel in the prevention of post harvest losses.

To understand post harvest management here in Africa, let us look at some of the ways in which grains are handled after harvesting.

Preparation of seed grains for storage is one of the most important steps in handling stored products. The time and method of harvesting and subsequent handling of the crop are relevant to possible deterioration in store [2]. Immature seeds will deteriorate faster compared to mature grains, unless treated further. This is because in immature grains, enzymes are still active and the moisture content of the seeds is high. It is therefore very important that dry grains are maintained at low moisture content [1]. Different grains have variable limits of moisture during storage. One way of getting appropriate moisture content in our grains is through drying.

Natural drying of grains is primarily concerned with the removal of capillary water and occasionally free surface water if the threshed seed has been wetted by rain or dew [3]. There are three distinct phases of drying :

Phase 1- this is where surface of the drying material remains completely wet.
Phase 2- this is where there is decrease in the surface area that remains wetted, that is, the drying rate decrease is directly proportional to the fraction of the surface that remains wet.
Phase 3- this occurs when the surface is completely dry.

There are several factors that affect the drying rates of stored grains. These are: temperature of the drying air, initial moisture content of the grain, and relative humidity in the drying air [3].

Different traditional drying and storing systems are used in Africa. These traditional systems rely on the use of natural air [3]. Depending on the storage system, some of the crops are allowed to dry while standing in the field.

Solar drying involves direct exposure of grains to the sun. This can be achieved by using solar driers [3]. In artificial drying, forced ventilation with heated air is used to dry the grains. These include continuous flow driers, batch driers, on-floor driers and in-bin driers.

As mentioned earlier on, amongst the factors that lead to deterioration of grains are microorganisms. One of the major problematic microorganisms in storage is the mold (yeasts and the fungi) [2]. The major types of losses caused by fungi in storage include, decrease in viability of the grains, discoloration of part or all kernel or seed, heating or mustiness, various biochemical changes, loss in weight and production of toxins that can be detrimental to man and domesticated animals [4,5,6].

I will dwell on these mycotoxins, but first let us look at the relationship between moisture content and fungal growth. Safe moisture content, which is the minimum moisture content of grains that precludes mold growth, is an important factor to look at when drying grains. In maize for example, the safe moisture content is 12 – 13.5%, while in peanuts it is 7% [2,7].

Several fungi are known to produce mycotoxins. These fungi are divided in three groups: fungi growing on living plant material, fungi growing on stored plant material, and fungi growing on decayed plant material [2, 7].

Fungi growing on living plant material include: Aspergillus flavus, Claviceps purperea, Fusarium graminearium, Fusarium moniliformine, Helminthosporium bisespatum, Rizoctonia leguminicola, and Scerotina sclerotiorum [8,9]. Fungi growing in stored plant material include, Aspergillus flavus, A. chevalieri, A. clavatus, A. fumigatus, A. ochraceous, A. parasiticus, A. rubrum, A. versicolar, Chaetomium globosum, Fusarium graminaerium, F. moniliformine, F. nivale, F. tricinctum, Penicillium islandicum, P. citreoviride, P. citrinum, P. expansum, P. palitans, P. puberulum, P. roquifortii, P. rubrum, P.rugulosum, P. uriticae, P. verrucosum var. cyclopium, and P. verrucosum var. verrucosum [ 7,8,10]. Fungi growing in decaying plant material include: Alternaria longipes, Chaetomium globosum, Cladosporium sp., Dendrodochium toxicum, Fusarium graminaerium, F, sporotrichoides, Myrothecium verrucaria, Pericornia minutissima, Pithomyceschartarum, Stachybotrys atra, Trichoderma viride, and Tricothecium roseum [7].

Several factors have been attributed to mycotoxin production by these fungi. These include: physical, chemical and biological factors. These factors have been divided into field, harvest and drying and storage [2,10,11].

How and where do mycotoxins occur?
Mycotoxin contamination starts from right in the field up to distribution and processing. The flow diagram below shows some of the five different stages in which mycotoxin occurrence can occur.

With food shortages in Africa occuring due to natural disasters such as droughts and floods, it is really important to know how to prevent post harvest losses.

Africa, let us unite in this fight against food losses and preserve our produce to save our population, for the biggest problem facing Africa today is the lack of correlation between food production and our population growth. With this, DISASTER is imminent!

Figure 1 Five different stages in which mycotoxin can occur

REFERENCES

1. Simpanya MF, Allotey J and S Mpuchane Insect and Mycoflora Interactions in Maize Flour. African Journal of Food and Nutritional Sciences (AJFNS). 2001; 1: 3-8.

2. FAO. Food and Agriculture Organization Training in Mycotoxins Analysis. Food Nutrition Paper 14/10. FAO, Rome, Italy. 1990.

3. Odamtten GT Training Course on Components Essential for Safe Effective Post Harvest Management. A Post Harvest Management Course at University of Botswana in Collaboration with Peanut Collaborative Research Support Program (CRSP), University of Georgia USA. May 27- June 2nd 2001.

4. Kuiper-Goodman T, Scott PM and H Watanabe Risk Assessment of Mycotoxin, Zearalenone. Regul. Toxicol. Pharmacol. 1987; 7: 253-306.

5. Miller JD Fungi and Mycotoxins in Grain: Implications for Stored Product Research. J. Stored Prod. Res. 1995; 31: 1-16.

6. Pestka JJ, Azcona-Olivera JI, Plattner RD, Minervini F, Doko MD and A Visconti Comparative Assessment of Fumonisin in Grain based Foods by ELISA, GC-MS, and HPLC. J. Food Prot. 1994; 57: 162-172.

7. Wilson DM and D Abramson Mycotoxins in Storage of Cereal Grains and Their Products. American Association of Cereal Chemists. St Paul, Minnesota. 1992; 341-391

8. Pitt JI and AD Hocking Fungi and Food Spoilage. 2nd Ed. Blackie Academic and Professional, London, UK. 1997.

9. Samson RA and ES van Reenen-Hoekstra Introduction to Food-borne Fungi, 3rd Ed. Centralbureau voor Schimmelcultures Baarn, The Netherlands. 1988.

10. Twiddy DR Volatiles as Indicators of Fungal Growth on Cereal Grains. Trop. Sci. 1994; 34: 416-428.

11. Clevstrom G Studies on the Fungal Flora of Plants and Feed and the Influence of Formic Acid on Growth and Aflatoxin Production in Aspergillus flavus. PhD Thesis. Swedish University of Agricultural Sciences, Department of Microbiology. 1986; 15-19.