Nutritionist > Poultry
Mycotoxin formation during storage
Factors mostly influencing fungal growth on plants and hence mycotoxin production are free water, temperature and the time of colonisation during storage.
Fusarium mycotoxins (zearalenone, trichothecenes, fumonisin, etc) are produced mainly during the field (cultivation) phase. Aspergillus and Penicillium mycotoxins (aflatoxin, ochratoxin, etc) are produced, for the most part, during storage.
Contrary to what happens during cultivation aflatoxin synthesis during storage can take place in tropical and sub-tropical conditions.
Principal factors influencing mycotoxin production are:
- Intrinsic factors, connected to the fungal stock
- The power of generating toxins which can vary inside each stock from 1 to 103-104
- The fungal species which determine the category of mycotoxin produced
- The initial contamination level which influences the amount of mycotoxins produced (the more moulds, the greater the potential for more mycotoxins)
- Extrinsic factors, i.e. environmental conditions. These factors determine fungal growth and therefore mycotoxin production
- Chemical, physico-chemical and physical factors such as humidity, free water, temperature, the substrate, the gas composition (atmosphere) and the mechanical damages to the caryopsis
- Biological factors, such as insects, either as vectors of fungal spores or as vectors responsible for mechanical damage to the caryopsis, favouring the entry of moulds; the microbiological flora, and the competition between mycological strains; the plant stress (drought); the resistance of the layer, either as genetic strength or caryopsis integrity.
The ideal growth conditions are summarised below:
Environmental humidity or free water
The most interesting benchmark is free water. Fungal colonisation of feedstuffs is more frequent when the bacterial load at free water levels is lower than 0.85. This not because fungi cannot grow at higher levels, but rather because bacteria are strongly competitive and become the predominant microflora at values between 0.85 and 1.00, and in particular at levels over 0.90-0.93.With a level of free water between 0.85 and 0.93 only some bacteria can rapidly increase in number (lactic ones and cocci in particular) so the invasion of moulds and yeasts predominates.
According to the behavioural differences related to water availability, fungi species can be classified as follows:
- Hydric (i.e.Epicoccum nigrum, Trichothecium roseum, Mucor circinelloides): The spore can only germinate at free water levels above 0.90 (the optimal growth takes place at 1.00).
- Medium level (i.e. Alternaria tenuissima, Cladosporium cladosporioides, Penicillium cyclopium): The spores germinate at free water levels between 0.80 and 0.90. The best and optimal growth takes place at 0.95-1.00.
- Xerofile (i.e. Aspergillus repens, Aspergillus restrictus, Aspergillus versicolor): The spores germinate at free water levels less than 0.80. The best and optimum growth takes place at 0.95.
Temperature
The ideal temperature for mould development is between 15 and 30°C, with optimal values of 20-25°C. Some species such as Cladosporium herbarium have an apparent growth at -6°C. Others, such as some species of Penicillium can develop in frozen fish at -20°C. Within the literature some spores (Rhizopus nigricans, Mucor mucedo, Aspergillus niger, Aspergillus glaucus) are reported to persist after the immersion for 77 hours in liquid hydrogen at -253°C and for 492 hours in liquid air at -190°C.
The colony does not develop at high temperatures, except for Aspergillus fumigatus, which can visibly contaminate the first respiratory tract at body temperature. At 35-40°C Monilia sitophila, a typical contaminant of bread, can still survive. Other species, for example Bortytis cinerea, can persist at refrigeration temperatures and can still replicate at 5°C.
pH and Oxygen
The development of fungal colonies takes place at pH values between 4 and 8. However some moulds can grow at lower or higher values, modifying the acidity of the medium, during the development of the colony.
Moulds are generally aerobic organisms that develop above the surface of the medium. Some species can however develop in deep mediums, such as Stachybotrys, or in liquid mediums, with a low oxygen rate, assuming a jelly appearance, or even in a modified atmosphere, with CO2 and N2.
Toxin production
The environmental conditions and the moisture content influence the production of mycotoxins. Toxin production is improved by levels of free water of 0.90. Aspergillus flavus can start to produce aflatoxin at 0.83. A. ochraceus needs at least 0.97 to produce ochratoxins.
The humidity of the sub stratus is the principal factor to be considered in the prevention of contamination and its control has become fundamental, for example, in feed production.
Aspergillus flavus easily produces aflatoxins at approximately 25°C. Under 10°C the toxin production has never been demonstrated. Fusarium tricinctum can produce T2 toxin at temperatures between 1 and 4°C, to a maximum of 15°C. Aspergillus ochraceus produces ochratoxin from 20 to 30°C, but never below 12°C. The same mycotoxin is produced by Penicillium viridicatum between 4 and 31°C
Thus it is difficult to define temperature values to control mycotoxin production, except aflatoxin, which is never produced under 10°C, even in deeply mouldy conditions.
Oxygen concentration and the acidity of substratum are not relevant for mycotoxin production.
An important factor to be taken into consideration is the type of substrate. Vegetal substrates improve mycotoxin production, more than the animal ones and those of animal origin.
In particular the presence of starch seems to help mycotoxin generation. Moreover the presence of zinc, only referred to the aflatoxin production. Cereals, oleaginous seeds and dried fruit are the foods most frequently contaminated by aflatoxins. The more frequent products contaminated are maize, peanuts and cotton seeds.Fruit and juices are the principal carriers of patulin, and cereals of ochratoxin.
