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CLOSE THIS BOOKManual on the Prevention of Post-harvest Grain Losses (GTZ)
6. Fungi and mycotoxins
VIEW THE DOCUMENT(introduction...)
VIEW THE DOCUMENT6.1 Storage fungi
VIEW THE DOCUMENT6.2 Mycotoxins
VIEW THE DOCUMENT6.3 Further literature

Manual on the Prevention of Post-harvest Grain Losses (GTZ)

6. Fungi and mycotoxins

Damage caused by fungi is often neglected until it has reached an advanced stage. Fungi do not only cause direct losses but also can threaten the health of both man and animals by producing poisons, so called mycotoxins, which are contaminating food and feed.

Storage fungi require a relative humidity of at least 65% (or a water activity of aw = 0.65) which is equivalent to an equilibrium moisture content of 13% in cereal grain. They grow at temperatures of between 10°C and 40°C (see section 2.2,5), Every species of fungus has its own optimum climatic requirements.

6.1 Storage fungi

Fungi found in stored food can be devided in two groups, the "field fungi" and the "storage fungi". in some cases a sharp distinction is not possible as fungi growth may start in the field and during storage. The original source of fungi is in any case in the field. Store fungi include above all species of Aspergillus, Fusarium and Penicillium. The growth of fungi in storage is governed by the following factors:

· composition of nutrients in the grain
· moisture and temperature conditions
· biotic factors like competition or the presence of stored product insects.

Storage fungi are much more frequent in lots infested by stored product insects, because insects generate moisture and distribute fungi spores in the commodity.

The following table shows the minimum moisture contents required in grains for the growth of some important storage fungi.

Fungus species

Minimum moisture content in grain

Aspergillus restrictus

13.5%

A. glaucus

14%

A. candidus

15%

A. ochraceus

15%

A. flavus

18%

Fusarium spp.

18 - 19%

Penicillium spp.

16.5 -19 %

Fungus development can occur if:

· grain is stored without having been sufficiently dried
· grain has been damaged during harvest, handling, threshing or drying
· the moisture content of the stored produce increases during the storage

· as a result of moisture being absorbed from the air
· as a result of condensation (see section 2.2.3)
· in "hot spots" (see section 2.2.3)
· because of penetration of water (leakage)

The following damage can be caused by storage fungi:

- Loss of nutrients
- Discoloration of grain
- Reduction in germination ability
- Caking of grains
- Increase in the temperature of the stored goods up to spontaneous combustion
- Mouldy smell and taste
- Production of mycotoxins
- Creation of environmental for the development of special insect species (= indicator for low grain quality)

Attention must be paid to the following in order to avoid damage by fungus:

- Dry the produce as quickly and evenly as possible after harvesting
- Prevent grain damage during harvest, handling, threshing or drying -Keep the store cool and dry
- Prevent condensation (keep temperatures in the store as constant as possible) - Carry out regular controls
- Prevent moisture absorption as a result of incorrect ventilation or water entering the store.
- Avoid development of high insect population (= "hot spots")
- Arrange redrying of parts of the stack with unacceptable high moisture content

Scientific research has confirmed fungistatic effects of some of the plants used traditionally by farmers in Africa to protect stored grain against mould. An extract of dried fruits of Xylopia aethiopica (Annonaceae) and dry seeds of the pepper Piper guineense was even able to completely prevent development of Aspergillus flavus. For practical fungus control purposes, however, these effects do not seem reliable enough.

6.2 Mycotoxins

Mycotoxins are metabolic substances which are produced by various fungi remain in the stored produce as residues. Mycotoxins can be found in the stored produce as soon as 24 hours after infestation with fungus. The optimum climatic conditions for the growth of fungi and the formation of mycotoxins are often not identical and dependent on various unidentified factors. Therefore mycotoxin contamination can only be stated with certainty by means of laboratory examinations.

In the following table some important mycotoxin producing fungi are listed together with affected commodities:

Fungus species

Commodities affected

Alternaria alternate

rice, sorghum, soybeans

A. Iongissima

rice, sorghum

A. padwickii

rice

Aspergillus flavus

cashews, copra, maize, groundnuts, sorghum, soybeans

Fusarium moniliforme

maize, sorghum, soybeans

F. semitectum

maize

Penicillium citrinum

sorghum, soybeans

Among the mycotoxins identified since the first discovery of aflatoxins over 30 years ago five are of special importance in agriculture:

- aflatoxin (aflatoxin B1 is the most toxic of all known fungal metabolites)
- deoxynivalenol (probably the most widely distributed mycotoxin in food)
- zearalenone (an oestrogen analogue which interferes with female mammalian hormones)
- fumonisin (a very common contaminant of maize-based food and feed)
- ochratoxin (occurring mainly in Europe and other moderate temperate areas).

Mycotoxins are highly poisonous to both humans and animals. If eaten, they lead to diseases known as mycotoxicoses or may cause cancer. In the following table an overview is given over mycotoxins, the fungi producing them, commodities affected and health hazards to man and animals:

Mycotoxin and toxin-producing fungi

Commodities

Health hazards

Aflatoxin

maize,

carcinogenic, liver damage and

(Aspergillus flavus,

groundnuts,

other adverse effects in

A. parasiticus)

oilseeds

humans, poultry, pigs and cattle




Deoxynivalenol

wheat,

acute human toxicosis, internal

(Fusarium graminearum

maize,

disorders and decreased growth

and related species)

barley

in pigs and other effects




Citrinin

cereals

kidney diseases in humans and

Penicillium spp.)


pigs




Fumonisin


suspected to cause human oesophageal

(Fusarium moniliforme

maize

cancer, diseases of

and related species)


equines, pigs and chicks




Ochratoxin

barley

cancerogenic, kidney damage

(Penicillium verrucosum

wheat

and other adverse effects in

Aspergillus ochraceous)


pigs and poultry




Zearalenone

maize,

possible human carcinogen,

(Fusarium graminearum

wheat

influence on pig production

Commodities with a particular high risk of aflatoxin production are maize, rice, cashew nuts, copra, groundnuts and most other commodities with a high fat content.

Health hazards for domestic animals are well documented in many instances since the famous aflatoxin-caused Turkey X disease which killed some 100 000 turkey poults in Great Britain in 1960. Clear evidence for association of mycotoxins and human diseases, however, have only been recorded for aflatoxin, Fusarium toxins, ochratoxin A and other rare cases. This fact is due to methodological difficulties and does by no means reflect a minor risk for humans as compared to animals.

In consequence of the high toxicity and cangerogenic action of aflatoxins about 60 countries have issued regulations concerning aflatoxin contamination of food and feed. In industrialized countries aflatoxin limits (maximum residue limits = MRL) generally are fixed as follows:

Commodity

Aflatoxin limits (mg/kg)

human food

5 to 30

baby food

5 to 20

feeds for dairy and young cattle

5 to 20

feedstuffs for pigs and poultry

10 to 30

feedstuffs for beef cattle, sheep and goats

20 to 300

The toxicity of mycotoxins is reflected in the extremely low maximum residue limits. As an example, the MRL's of Malathion and Aflatoxin B1 for human food ate given in mg per kg of grain

- Malathion 5 - 30 mg/kg
- Aflatoxin B1 0.005 mg/kg

This means that the maximum residue limit of Aflatoxin B1 is 1.000 to 6.000 times less than that of Malathion.

Mycotoxins are highly stable and cannot be destroyed by boiling, pressing or processing. This means that infested produce has to be destroyed. The problem cannot be dealt with by mixing contaminated produce with healthy grain or by feeding it to animals, as the toxins will be accumulated in their body and later consumed by people in form of milk or meat.

Note: Mycotoxins can only be avoided by preventing the growth of fungi.

6.3 Further literature

ANONYMOUS (1992)
Fungi and Mycotoxins in Stored Products, ACIAR Proceedings No. 36,
Canberra

CHRISTENSEN, C.M. & R.A. MERONUCK (1986)
Quality Maintenance in Stored Grains and Seeds, University of Minnesota Press, Minneapolis, 138 pp.

HIGHLEY, E., E.J. WRIGHT, H. J. BANKS & B.R. CHAMP, ed. (1994)
Stored Product Protection. Proceedings of the 6th International Working Conference on Stored-product Protection, CAB international, Canberra, volume 2, pages 969- 1083

MULTON, J.L., ed. (1988)
Preservation and Storage of Grains, Seeds and their By-Products, Paris, 1095 pp.

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