2.3 Maize storage problems and their effects on the nutrient content and safety of maize .1 A perfect storage method .1 A perfect storage method
2.3.4 Fungal species that damage maize grain and their effect on maize quality
Under favourable conditions, metabolic activities of several species of fungi produce toxic secondary products known as mycotoxins. In the presence of moisture, temperatures at 20 0C - 40
0C have been reported to be favourable conditions for growth of moulds (FAO, 1985). Therefore, moulds would infect stored maize where conditions in the storage facilities are as indicated above. At 5 0C - 8 0C growth is very slow (Agrios, 2005), below 5 0C fungi become inactive.
Thus, storage temperatures below 5 0C can keep food products safe from fungal infestation.
However, farm households in Africa store maize at ambient temperatures, thus, maintaining storage temperatures below 5 0C is not practical for them.
Much of the mycotoxins contamination of commodities has been associated with inadequate storage technologies, maize kernels being damaged, and climatic conditions such as high humidity, dampness and low temperatures (Williams, 2004; Gourama and Bullerman, 1995).
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Thus, it is recommended that in order to prevent infection by storage fungi, stored maize should be free from injury, insect pests, moulds and dirt prior to storage, and 12 % moisture content of maize or less should be maintained (Kaaya and Khamunyangire, 2010). For preventing growth of moulds in maize, Reed et al. (2007) recommended rapid drying prior to storage for maize which is not dry enough at harvest; whereas Semple et al. (1989) recommended up to 48 hours after harvest as the time limit within which maize which is not dry enough at harvest should be dried depending on drying conditions. Apart from contaminating maize, moulds also reduce its energy content and its viability, which in turn reduces its marketability (Reed et al., 2007).
Maize which is infested by fungi is susceptible to infestation by insect pests as well due to the attraction of the pests to the odour caused by the deterioration of the fungi infested maize (Ako et al. 2003). Thus, maize storage methods that allow growth of moulds also put stored maize at risk of being infested by insect pests. Aspergillus, Penicillium and Fusarium species have been associated with production of mycotoxins that affect both human beings and animals (Sweeney et al., 2000; Akande et al., 2006). These fungal pathogens may start infesting crops before they are ready for harvest (Sweeney et al., 2000), during drying, or during storage (Stack et al., 2003).
Furthermore, Fusarium (Chelkowski et al., 2007), Penicillium and Aspergillus (Stack et al., 2003) species cause ear and kernel rots of maize. A more detailed account of the three ear rots mentioned above is provided in the sections that follow.
2.3.4.1 Penicillium ear rot and associated mycotoxins
Penicillium ear rot is caused by Penicillium species and a maize cob that is infested with Penicillium ear rot would be characterized by blue green powdery growth on the surface, between kernels and in the germ (Stack et al., 2003). On culture, flat cotton-like filamentous structures grow rapidly to form colonies which are initially white in colour and later turn blue green. When observed under the microscope, colonies show septate hyaline hyphae with long conidiophores that branch in a broom-like manner spores that look like glass beads (De Hoog et al., 2000).
Conidia are single-celled and may measure up to 5 µm in diameter (Clinical Microbiology Proficiency Testing (CMPT), 2007). Mycotoxins caused by Penicillium species include ochratoxins which often contaminate decaying vegetation, cereals, peanuts and cotton seeds (Almeida et al., 2010) and patulin which can be found in apples and apple products (FAO, 2004).
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Ochratoxin A, a type of ochratoxins mostly found in food (Lawley, 2007), is produced at 15 - 37
0C (FAO, 2004) in the presence of moisture. Exposure to unacceptable levels of ochratoxins causes kidney problems in animals and human beings (Hayes, 2001, Diekman and Green, 1992).
Ochratoxins may also cause retarded growth to chickens that are raised for slaughter, decreased egg production to laying hens (Diekman and Green, 1992) and may cause cancer of the liver in human beings (Munkvold et al., 2009; Wood et al., 2003). Although there are no international set standards for acceptable levels of ochratoxins, 50 µg per kilogram of maize (µg/kg) is the highest acceptable level for a number of countries (FAO, 2001). Thus, levels above 50 µg/kg ochratoxins in maize may be harmful to the consumers.
2.3.4.2 Fusarium ear rot and associated mycotoxins
Fusarium ear rot is caused by Fusarium species (Chelkowski et al., 2007; Parsons and Munkvold, 2010). Maize kernels that are infested with Fusarium ear rot would be characterized by growth of white cotton-like mycelium which remains white or turns pale pink, pale purple or reddish pink upon maturity (University of Adelaide, 2011; Seifert, 1996). On culture, Fusarium ear rot form hyaline, slightly curved macro conidia which are 2 - 5 µm in diameter, 3-7 septate with an apical cell (Seifert, 1996). Micro conidia are single celled and may be 2 - 3 µm in diameter and 5 - 12 µm long. Mycotoxins that are produced by Fusarium species include fumonisins, zearalenone, diacetoxycirpenol, T-2 and HT-2 toxins, deoxynivalenol, moniliformin, beauvericin and fusaproliferin (Logrieco et al., 2002). While fumonisins are said to be mainly produced by F. moniliforme and F. proliferatum, T-2 toxins are produced by F. tricinctum, F.
equiseti and F. sporotrichioides (Öhlinger et al., 2004) in the presence of moisture and warm temperatures.
There are no known temperatures that favour the production of T-2 toxins (FAO, 2004); whereas fumonisins are produced at 25 - 30 0C (Marin et al., 1995). Fumonisins may cause cancer of the oesophagus, kidney, pancreas, gastrointestinal tract and blood cells problems, and they may interfere with brain, liver, and lungs function (Vercelli and Parker, 2002). The effects of fumonisins on animals and chickens as highlighted by Tardieu et al., (2007) are the same as those of ochratoxins. Consumption of high levels of T-2 toxins are said to cause aleukia, a disease of the alimentary canal, and digestive disorders (Bennet and Klich, 2003) and immunosuppression
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(Tai and Pestka, 1990). In chickens T-2 toxins can cause lesions, low egg production, reduced weight and other diseases similar to those caused by ochratoxins (Sokolović et al., 2008; Tardieu et al., 2007). Currently, there are no set acceptable standards for T-2 toxins in maize. The FDA’s standards for fumonisins in milled maize is four parts per million (4ppm), equivalent to 4mg/kg (Wu, 2004). Thus, commodities that have more than four mg/kg of fumonisins may cause health problems.
2.3.4.3 Aspergillus ear rot and associated mycotoxins
Aspergillus ear rot is caused by different species of Aspergillus, predominantly A. flavus and A.
parasiticus, and like Penicillium species Aspergillus species may be found in the soil and in decaying organic matter (Stack et al., 2003). Maize which is affected by Aspergillus ear rot is characterized by a production of powdery masses of either black, pale yellow, yellow or yellow green spores that turn dark green to brown with maturity {International maize and Wheat Improvement Center(CIMMYT), 1999}. When cultured, the ear-rot forms septate hyaline hyphae and globose fruiting bodies which are flask shaped or round at the end. The phialides may be attached directly to the vesicle or via a sprouting cell, while conidia form chains on the phialides.
They are globose, smooth or slightly rough, and measure 2 - 8 µm in diameter and 2 µm in length (Eltem et al., 2003). Aflatoxins in maize are mainly produced by A. Flavus (Munkvold et al., 2009). However, A. parasiticus, A. nomius, A. ochraceoroceus, A. pseudotamari (Bennet and Klich, 2003) and A. Bombycis (Peterson et al., 2001) also produce aflatoxins in stored cereals such as maize.
Moisture and 15 0C - 43 0C temperatures are factors that favour the production of aflatoxins (FAO and the International Agency for Atomic Energy (IAEA), 2001). Aflatoxins may cause cancer of the liver in human beings, decreased eggs production in chickens (Exarchos and Gentry, 1982; Oliveira et al., 2002), decreased milk production and interference with reproductive efficiency in animals (Cornell University, 2009). Suppression of the immune system in both human beings and animals can also occur (Bennet and Klich, 2003).
Acceptable levels of aflatoxins for human beings are 20 parts per billion (20 ppb) or 20 µg/kg for all types of foods except milk (Munkvold et al., 2009; Wu, 2004). In this case maize meal that has more than 20 ppb of aflatoxins would cause health problems to the consumers. Aflatoxin
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contamination resulting from moulds infestations of food crops has been identified as a major problem in many countries (Siwela et al., 2005), including Sub- Saharan Africa (Ranajit, 2007).
Other toxigenic types of fungi include Phomopsis leptostromiformis which produce phomopsins, (Culvenor et al, 1983), Acremonium lolii which produce lolitrem alkaloids, Acremonium coenophialum which produce ergopeptin alkaloids, Pithomyces chartarum which produce sporidesmins and Clavices purpurea which produce ergot alkaloids (D’Mello, 2000). In April 2004, 125 people in Kenya alone died due to aflatoxin contamination (Lewis et al., 2005). In Tanzania, studies that were conducted on home stored maize obtained from 120 households from Ruvuma, Iringa and Kilimanjaro regions in 2005 revealed the presence of unacceptable levels of aflatoxins (Kimanya et al., 2008). However, there is no indication that studies have been done in Tanzania in general and in Rungwe district in particular to relate specific storage technologies with mycotoxins contamination of commodities.