Aflatoxin Contamination and Soils Control: Comparison
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Please note this is a comparison between Version 2 by Catherine Yang and Version 1 by Xue Wang.

Aflatoxins are potent carcinogenic compounds, mainly produced by fungi species of the genus Aspergillus in the soil. Because of their stability, they are difficult to remove completely, even under extreme conditions. Aflatoxin contamination is one of the main causes of safety in peanuts, maize, wheat and other agricultural products. Aflatoxin contamination originates from the soil. 

  • Aspergillus flavus
  • aflatoxin
  • soil
  • aflatoxin contamination

1. Introduction

Aflatoxin is produced by Aspergillus flavus, which is classified as a Group 1 carcinogen by the International Agency for Research on Cancer (IARC). It is one of the most toxic compounds known, acting mainly on human and animal liver tissues, capable of inducing cancer of the liver (primarily), as well as the pancreas, kidney, bladder and other organs. Aflatoxin may also lead to malnutrition, immunosuppression, and other pathologies with mutagenic, hepatotoxic, and nephrotoxic outcomes [1,2][1][2]. Aflatoxin mainly contaminates grain and oil crops, feed, nuts, Chinese herbs and other crops, and then contaminates meat, eggs, milk and other by-products after being ingested by animals. The import and export of agricultural and sideline products all over the world have strict limits on aflatoxin, thus restricting industrial development and export trade. Aflatoxin contamination not only causes huge economic losses to food crops, but also has a negative impact on the health of consumers. According to the data from the Food and Agriculture Organization of the United Nations (FAO), about 25% of crops worldwide are contaminated with moulds and their toxins each year, while about 2% of agricultural products lose their value due to excessive toxin contamination [3], such as the 100,000 turkey deaths that first occurred in the 1960s in the UK, and the high annual economic losses caused by aflatoxin contamination in peanuts in Georgia, USA [4].
There have been many cases of human and animal mass poisonings caused by aflatoxin contamination of agricultural products and foodstuffs all over the world. Aflatoxin is highly toxic to the liver and central nervous system of humans and animals. It can cause acute poisoning or even death in humans and animals when ingested in large amounts at one time and can be teratogenic, mutagenic, and carcinogenic when ingested in small doses over a long period of time [5,6][5][6]. According to the IARC, about 500 million people in the developing world alone are still at risk of aflatoxin exposure [7]. The European Union, one of the economies with the best food safety management systems today [8], has strict limits for fungi toxin contamination in food and feed, and China also has strict limits for aflatoxin B1 in food (Table 1). The Chinese GB 2761-2017 “National Standard for Food Safety Limits for Mycotoxins in Food” requires that the maximum limits for aflatoxin B1 (AFB1) in different cereal products range between 5–20 µg/kg, while the maximum limit for AFB1 and aflatoxin M1 (AFM1) in special dietary foods is 0.5 µg/kg and should not be detected in infant diets [9].
Table 1.
Acceptable limits of aflatoxin in crops in several countries.
Countries Crops Limits (µg/kg)
China cereals 20
USA maize 5
EU cereals 4
Japan cereals 0
India peanuts, maize 5
Therefore, research on the prevention, control and detoxification of aflatoxins in food and feed has become one of the most important aspects of food safety and has attracted widespread attention. In order to prevent and control aflatoxin contamination from the source of crop production, improve the quality and safety of agricultural products in China, and ensure consumer safety and healthy development of the agricultural industry, the source, nature and contamination pathways of aflatoxin, and the current effective methods to deal with aflatoxin in crop production were summarized in this paper. It is expected that the emerging new technologies for aflatoxins control in soils will be widely used in crop production.

2. Aspergillus flavus in Agricultural Soils

There are an estimated 7000 species of fungi that inhabit the soil [10]. Luo et al. [11] studied the rhizosphere soil fungi community composition of camellia and explored the correlation between rhizosphere soil fungi and soil environmental factors, concluding that camellia diseases could be prevented by regulating soil environmental factors. Wu et al. [12] studied the fungi community structure in the rhizosphere soil of Rehmannia varieties and found that changes in the number of some common fungi pathogens such as A. flavus and Aspergillus niger might be the cause of soilborne diseases in the soil, which suggests that the Rehmannia root system had a certain plastic ability to the number, composition and species of fungi in rhizosphere soil.
So far, there are few reports on soil fungi of grain and oil crops. Due to the limitation of separation and detection technology in soil, only species suitable for artificial environments can be isolated from soil, so it cannot fully reflect the real soil colony environment. Fungi isolated from soil can be cultured. Only propagules capable of growing and sporing on the isolated medium used can be detected, and only about 17% of known fungi species can be successfully grown in the culture at present [13].
A single fungus may produce multiple mycotoxins, and a toxin may also be produced by multiple fungi; there are over 150 species of fungi that can produce one or more of 300 potential mycotoxins. As fungal growth is geographically specific, the predominant mycotoxins vary from region to region, e.g., in subtropical and tropical regions, agricultural products and feed are mainly contaminated with aflatoxins and certain ochratoxins. A. flavus was first used by LINK in 1809 as a generic term for saprophytic moulds in soils [14]. It has a wide range of hosts and has been reported in agriculture on maize, rice, wheat, cottonseed, peanuts and nuts, with peanuts and maize being the most affected [15,16][15][16]. The aflatoxin-producing fungi in the soil are diverse, and the distribution characteristics of different toxin-producing A. flavus occur differently. So far, the infestation pathways, effects and field distribution characteristics of A. flavus as the source of aflatoxin production in soil have not been systematically studied.
Aspergillus flavus is widely present in soil. According to the data from FAO, A. flavus is one of the most important contaminating fungi of cereals worldwide [4]. The optimum growth temperature for A. flavus ranges from 12 °C to 34 °C, while the optimum toxicity-producing temperature ranges from 20 °C to 30 °C, within 45 °C latitude [17,18][17][18]. A. flavus is mostly distributed in the soils of the Yangtze River basin and has the greatest risk of contamination [19]. Studies on the distribution of soil microbial flora and toxin contamination have also been carried out all over the world. Soil type is also related to aflatoxin pollution to some extent. According to different soil, the distribution of microbial flora is different, and the degree of mycotoxin pollution is also different, so targeted prevention and control measures can be taken. Wei et al. [20] found the existence of non-toxic and toxin-producing A. flavus in peanut soil. Based on the distribution characteristics of the strains, the risk of toxin contamination in different producing areas of China was evaluated. Zhang et al. [21] studied the genetic characteristics of A. flavus in peanut soil, providing a technical basis for the later screening of non-toxic strains and the development of aflatoxin biocontrol fungi.
In recent years, only the isolation and screening of aflatoxin in peanut root soil has been reported. Yang et al. [22] predicted the aflatoxin contamination of postpartum peanuts based on the number of aflatoxin colonies in the soil of four peanut-producing areas in China, so as to ensure the prevention and control of aflatoxin in peanuts in the later period. Zhang et al. [19], Zhu et al. [23] studied the distribution, toxin production and aflatoxin infection of A. flavus in the soil in the main peanut-producing areas of China, which provided a theoretical basis for the establishment of a model for the prevention and control of aflatoxin in China. According to the analysis of aflatoxin and its virulence in 11 producing areas of China, the Yangtze River Basin has the largest distribution of aflatoxin and the greatest risk of aflatoxin pollution. Because of the unique climatic conditions and geographical environment of the Yangtze River Basin, Hubei province has also become the largest peanut production area in China. Zhu et al. [24] also studied the distribution and toxic characteristics of aflatoxin in the soil of typical peanut growing areas in Hubei Province, providing a theoretical basis for the establishment of the early warning and prevention model of aflatoxin pollution of peanuts in Hubei Province. Zhang et al. [25] first discussed the relationship between soil types and A. flavus colonies in the peanut production area of Xiangyang, Hubei province. This work suggested that the number of A. flavus groups in the clay loam was higher and the virulence was higher than that in the sandy loam, while the sandy loam had a smaller distribution density and infection risk of A. flavus under appropriate irrigation conditions. The results of this study have important guiding significance for field fertilization, irrigation, A. flavus control and other agronomic management in the local peanut planting process.

References

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