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Status of research on detection and control of mycotoxins in food grains in Bangladesh
M. A. Bakr.
Senior Scientific
Officer
Bangladesh Agril. Res. Institute
Regional Agril Res. Station, Ishurdi, Pabna
Mycotoxins are chemically unrelated groups of fungal metabolises characterized by their ability to induce a toxic response in human and animals when food or feed containing them is consumed. Mycotoxins are produced by a number of genera of fungi, however, only the aflatoxins produced by the genus Aspergillus have drawn great attention almost universally. Bangladesh is situated in the monsoon belt of tropical region. Hot and humid climate of the country is very conducive for growth, development and multiplication of microflora. The frequent heavy rainfall and floods during "Kharif" and occassional early shower during late Rabi seasons expose cereals pulses and oilseeds to fungal invasion and accumulation of mycotoxins and damage in storage.
Bangladesh has to feed a population of around 105 million (BBS 1989) with a deficit production of 2-3 million tonnes foodgrains. The national food balanced of the country per capita-day during 1987-88 was 52.1 gm of protein, 46.1 gm of fats producing 2078 calories of energy from sources like cereals, pulses, oilseeds, tuber and root crops, meat, egg, milk, animal & vegetable products, (BBS-1989). In this paper we shall, however, discuss the assessment of microflora associated with the different food grains, level of mycotoxins elaborated by them and the attempt made to prevent the food stuff from mycotoxins contamination.
AFLATOXIN PRODUCING FUNGI
Around 20 aflatoxins have been reported to be produced by fungi in storage. Most of them are produced by imperfect fungi like Aspergillus flavus, A parasiticus, A wentii, A niger A ochraceous, A oryzee, and species of Penicillium like P citrinum, P puberulum and P frequentans and species of Rhizopus. But systematic study of Wilson, et al (1968) revealed that only two species of Aspergillus viz. A flavus and A parasitic us produced aflatoxins. Although besides aflatoxins other mycotoxins like cyclopiazonic acid and sterigmatocystin are also produced by A flavus (Luck et al. 1978, Burkhardt and Forgaes, 1968).
MYCOFLORA OF FOOD GRAINS
Bangladesh is a tropical country with hot and humid climate which makes a suitable environment for the growth of moulds. Moreover, during the harvesting time of paddy in Kharif season frequent shower occurs which hinder the proper solar drying. Similarly at maturity of pulses and oilseeds crops also starts post winter rain making them vulnerable to mould and other pathogenic fungal attack. Studies were conducted to assess the fungal flora associated with different foodgrains. With paddy varieties Mia et. al (1986) reported 19 fungi belonging to 17 genera which includes moulds as well as seed-borne pathogens.
Jonarder et al (1980) has studied the incidence of fungi in both raw and parboiled paddy of Bangladesh. Their results are given in Table 2-3.
It should be noted that during parboiling the contaminating fungal flora are destroyed and the husks are opened. The drying and handling of the parboiled paddy are carried out on the hardened earth floor; this results in fungal recontamination of the paddy.
A total of 14 fungal species belonging to 12 genera were recorded on wheat seeds stored under farmers conditions (BARI, 1984, 1968). Fungi like species of Aspergillus, F!hizopus, Penicillium
Cladosporium and Alternaria were more prevalent than the others, The incidence of Alternaria, Cladosporium, Fusarium and Curvulaia were higher in the prestored condition which gradually decreased with the increase of stored period while species of Aspergillus, Penicillium and Phizopus increased during storage. In a similar study Basak et al. (1987) reported 14 fungail species belonging to 8 genera (Table 1). In another more recent survey on wheat seed storage mould in Government food storage and ration shops in Dhaka and Joydebpur, it was reported that all the seed samples were infected/infested with storage fungi like species of Aspergillus, Pericillium, Rhizopus and bacteria. Among the storage moulds Aspergillus spp. was the highest (47 to 100%) followed by Rhizopus spp. around 30% and Penicillium 20% (Goswami, unpublished). Mould infestation of seeds of other cereals like maize, millets and kaon were also studied by different workers and reported the similar type of incidence.
Regarding pulses and oilseeds, reports on seed micoflora infestation are also available. In a survey of storage fungi of pulses seed at farmers level storage, 25 fungal species belonging to 21 genera were identified in 6 pulses (BARI, 1984). Here also the field fungi like the species Alternaria Fusarium and Curvularia observed gradually decrease in population with the increase stored period. On the other hand the storage fungi like the species of Aspergillus, Penicillium, Rhizopus and Cladosporium increased with increase of storage period.
In another study on the prevalence of seed mycoflora on mustard seeds 18 different fungal species representing 10 genera were identified (BARI, 1986). The genera are Alternaria, Aspergillus, Chaetomium, Cladospori.um, Curvularia, Fusarium, Penicillium, Drechslera, Rhizopus and Trichothecium out of them the prevalence of Aspergillus was next only to Alternaria. The population by Aspergillus also increased with the increase of storage time.
In a similar study on storage mould of peanut under farmers storage condition 16 fungal species representing 13 genera were recorded (BARI, 1988), out of which prevalence of A. flavus was the highest (68%)
AFLATOXIN PROBLEM IN DIFFERENT FOOD GRAINS
The four main aflatoxins are aflatoxic B1, B2, G1 and G2. The letters so assigned are due to the blue and green fluorescence they produce when irradiated with UV light and the subscripts 1 and 2 according to their polarities in TLC plates. When aflatoxin B1 is ingested some transformation occur and secondary new aflatoxin M1 and M2 having same acute toxicity as B1 are produced which are generally found in cow's milk (Coker, 1979).
Aflatoxin is a particularly noxious compound because at acute dose levels it induces liver lesions and death when administered to a number of animals and at chronic levels it produces liver tumours (Bainton, et al. 1980).
1. Aflatoxin in rice
A preliminary study on the aflatoxin content of rice from Bangladesh was carried out in the University of Dhaka in 1968, the results are given in Table 4.
Although no definitive conclusion can be drawn from this data the results indicate that aflatoxin contamination occurs in rice produced in Bangladesh.
A more recent study by Hug (1980) of rice samples from Bangladesh, as a part of a broader project "Rice in Bangladesh Appropriate Technology for the Intra-Village Post-Harvest System" organised by the Institute of Development Studies, University of Sussex, Brighton, UK, in collaboration with the Bangladesh Council of Scientific and Industrial Research has shown a high incidence of aflatoxin in rice and paddy.
2. Aflatoxin in other cereals
Table 5 reveals that other cereal products in Bangladesh, like wheat, maize, kaoin and chcena, are also susceptible to aflatoxin contamination.
A sample of damaged selected wheat has been found to contain a level of 5 ppm-kg. This is a cause for concern and indicates further studies on wheat.
3 Aflatoxin in pulses
A preliminary survey for aflatoxin contamination in pulses was carried out at the University of Dhaka and BCSIR. The results are given in Table 6.
Although some pulses are not susceptible to aflatoxin contamination, a thorough investigation of pulses grown in Bangladesh is needed to establish those that are affected by this compound. Chickpea and Lathyrus are the two most important pulses in Bangladesh which are badly affected by aflatoxin.
4. Aflatoxin in oilseeds and oilcakes
The aflatoxin content of some of the oilseeds and oilcakes produced in Bangladesh are given in Table 7.
It is to be noted that three of the important oilseeds, groundnut, cottonseed and leutil are affected by aflatoxin. Further work on these commodities is required.
PREVENTION FROM MYCOTOXINS
The moisture content of the grains as well as the type of containers play an important role in enhancing the growth and multiplication of mycoflora. Higher moisture content encourages the incidence of moulds while the air tight containers reduces them. Studies were therefore, undertaken to prevent the growth of seed mycoflora by manipulating moisture content of seed and designing suitable storage containers to be used in the farmers level. With six pulses like lentil, chickpea, blackgram, mungbean field pea and lathyrus stored in containers like earthen "motka", gunny bag, kerosin tin, plastic bag, bamboo "dol" etc. showed that kerosine tin and plastic bags were more suitable than those of the other containers (BARI, 1985).
In another study conducted at Plant Pathology Division, BARI with peanut seed stored in eight different types of storage containers like tin containers, gunny bag, gunny bag with potythene lining, polythene bag, earthen motka, earthen motka with polythene lining inside, earthen motka with outside coaltar coating and bamboo "dole" with polythene lining inside. It was found that this earthen motka which farmers are using could safely be used giving a polythene lining inside or allowing a coaltar coating outside (BARI, 1988).
Regarding moisture content in grains studies were also conducted at BARI with chickpea as well as maize seeds maintaining moisture level 10, 12, 14 and 16%. It was found to have bacteria and Rhizopus sp at 10% moisture content of maize seed. In overall observation it was noted that the incidence of mycoflora was encouragingly less in samples stored at 10% moisture content which increased gradually at higher percentage of moisture (Table 8). With chickpea seed stored at 8, 12 and 16% moisture cement also found to have the lowest incidence of mould at 8% (Table 9).
It seems from the above discussion that the foodgrains can be provented from aflatoxin by preventing the growth of the mould by proper and sufficient drying and storing them in suitable containers.
CONCLUSION
Evidence reported herein makes it quite conspicuous that the foodgrains of Bangladesh are not free from mycotoxin. Research work done to prevent them from toxins is quite meagre. Work on detection and quantification of mycotoxin are in progress at BCSIR under the financial support of Ministry of Science and Technology and at the Nutrition Institute at the University of Dhaka. Some isolated projects are working under Ministry of Agriculture by financial support from different agencies like IDRC, DANIDA etc. But this works is still to come out as conclusive recommendation. It is, therefore, essential to look into ways and means of prevention and control of foodgrains from mycotoxins.
REFERENCES
BARI. 1986. Bangladesh Agril. Res. Inst. P.Path.Div. Annual Rept. 1985-86.
BARI.1987. Bangladesh Agril. Rese. Inst. P.Path.Div. Annual Rept. 1986-87.
BARI. 1988. Bangladesh Agril. Res. Inst. P.Path.Div.Annual Rept. 1987-88 PP.
Bainton, S.J; Coker, R.D; Jones, B.D.Morley, E.M; Naglar, M.J. and Turner R.L. 1980. Mycotoxin Training Manual. Tropical products Institute, London.
Basak, A.B. Karim, M.R., Hoque, M.N. and A.P. Biswas 1987. Studies on the fungi associated with different varieties of wheat seed grown in Bangladesh seed research 15 (1): 71-73.
BBS.1984. Statistical pocket Book of Bangladesh. Bangladesh Bureau of Statistics. pp 61.
Burkhar aft, H.J. and J. Forgacs. 1968. O-methyl sterigmatocystin, a new metabolise from Aspergillus flavus. Tet. 24: 717.
Coker, B.D. 1979. Aflatoxin: past, present and future.Trop Sci. 21 (3):143.
Huq. M.S. 1980. Estimation and evaluation of aflatoxin in rice of Bangladesh. Paper presented at the post production workshop of foodgrains organized by the Univ. of Sussex and BCSIR.
Joarder, G.K. and M. Khatun, 1980, Studies on the incidence of fungal flora in rice of Bangladesh. Paper presented at the post production workshop on foodgrains orgainzed by the Univ. Sussex and BCSIR.
Luck, K.C, Kobble, B. and J.M. Townsend. 1971 Production of cyclopiazonic acid by Aspergillus flavus J. gen. Microbiol. 59:119-124.
Mia, M.A.T; Sharma, N.R. and A Miah. 1986. Incidence of grains discolouration in some modern rice varieties Bangladesh J.Plant Pathology 2 (1): 75-77.
Wilson, B.J; Campbell, T.C; Hayes, A.W. and R.T. Hanlin 1968 Investigation of reported aflatoxin production by fungi out side the Aspergillus flavus group. Appl. Microlol. 16:1817-1821.