- Peanuts affected by aflatoxin
Aflatoxin is the name for a group of toxins (carcinogenic compounds) that are produced by two fungi called Aspergillus flavus and Aspergillus parasiticus. These toxins occur naturally and have been found in a wide range of commodities (including peanuts) used for animal and human consumption. Depending on the levels, the toxins can severely affect the liver and they are a known human carcinogen (i.e. causes cancer). In many developing countries aflatoxin is a major health risk to humans and animals due to the high levels of contaminated product consumed. In Australia aflatoxin is not a health risk because of the thorough testing done at various stages along the food chain and the effective technology and procedures employed by agribusinesses to eliminate contaminated product.
On this page:
- Impact on industry
- Risk factors
- Post-harvest drying
- Etiella management
- Monitoring and managing pre-harvest aflatoxin risks
- Management strategies
Impact on industry
The Australian peanut industry has an excellent track record of eliminating aflatoxin-affected peanuts from the food chain to supply a product which is well below the minimum level required of 15 ppb (parts per billion) or 15 µg/kg of kernals. To reduce the health risk even further, the industry is committing considerable resources to preventing or at least minimising the production of aflatoxin on-farm. This will provide even more certainty for consumers that they are eating a safe and healthy product. It will also reduce the high cost to the industry of eliminating any contaminated product. Shellers have a significant amount of resources committed to eliminating contaminated product. They pass on the costs of testing, segregating, blanching and colour sorting the kernels to peanut growers by imposing penalties. The penalties range from $150 to $450/t depending on aflatoxin levels. These costs are threatening the viability of the industry particularly in the dryland areas.
High temperature and moisture are the main factors found to significantly increase pre-harvest aflatoxin risk.
|Soil and air temperatures||Between 22 and 35oC seem to encourage aflatoxin production|
|Kernel moisture||Kernel moisture in a range around 15 to 30% for more than 7-14 days. These conditions can occur pre-harvest when end-of-season drought leads to crop water stress, or post-harvest when wet weather affects windrowing and/or when pods are inadequately dried (see moisture-content graph).|
Poor post-harvest drying methods (in windrows and artificial driers) can result in uneven drying, which can lead to increased aflatoxin contamination in storage. Further, both under- or over-drying can significantly affect seed quality (especially splits, blanchability, off-flavours and seed germinability, in the case of seed programs).
Current research projects are developing cost effective post-harvest drying technologies that minimise aflatoxin risk in storage and improve kernel quality and profitability to peanut growers. Rapid assessments of pod moisture and airflow rates in drying bins are key factors in assessing the efficacy of the drying process. The researchers are also developing low-cost monitoring systems for kernel/pod moisture and finding ways to use the information in the pre- and post-harvest aflatoxin monitoring program.
The etiella grub, similar to the American southern corn rootworm which causes significant yield loss, predisposes pods to A. flavus invasion and subsequent aflatoxin contamination, especially during drying.
Studies are in progress to monitor and manage etiella populations in the field and to develop models to predict the extent of etiella damage in relation to varying stages of pod development and environmental conditions.
Monitoring and managing pre-harvest aflatoxin risk
Most dryland peanuts are grown in rotation with other Aspergillus flavus or A. parasiticus fungi hosts crops such as corn, so there is usually enough inoculum present in the soil to cause infection. Therefore, aflatoxin risk is largely determined by high temperature and the degree of drought stress. Monitoring these factors can assist in timely adoption of appropriate management strategies to reduce aflatoxin risk.
An aflatoxin model incorporated in the Agricultural Production Systems Simulator (APSIM) peanut model has been effectively used at the Department of Agriculture, Fisheries and Forestry (DAFF) to assess in-season risk of aflatoxin on a daily basis.
Daff has implemented Afloman, a web-based decision support system which allows in-season assessment of aflatoxin risk. Farmers can use the aflatoxin-risk prediction model to monitor the in-season risks on their paddock.
Using Afloman as a tool to assess the degree of risk that applies on their farm, peanut growers can make timely decisions about the extent to which they implement the following strategies or practices.
Block selection and preparation
Ideally blocks should have good crop rotation, high nutrient levels (especially calcium), high soil water storage characteristics and a low likelihood of grub or insect damage.
- Disease susceptibility
The newer high oleic varieties, including Middleton and the new ultra-early runner and virginia varieties, have not been tested widely for their aflatoxin susceptibility. However, it is thought that Middleton should have similar tolerance to Streeton, because these two varieties are of similar maturity.
- Ultra-early maturing varieties
Ultra-early maturing varieties (those which can mature by 100-110 days) may play a major role in reducing pre-harvest risk of contamination in regions where severe end-of-season droughts are common. The ultra-early runner type 'Walter' was released in 2008 and newer virginia type and a more disease resistant 'Tingoora' will be released in late 2010 for commercial production during 2010-11.
- Early harvest
In high-aflatoxin risk situations, early harvest usually reduces aflatoxin levels. Although this practice can result in slightly poorer kernel grades in some seasons, the lower aflatoxin penalties and, in some cases, higher yields, mean gross returns are maximised.
- Separating diseased and stressed areas
Stressed and diseased patches are usually the aflatoxin hot spots, which should be harvested and delivered separately. Research at Kingaroy has investigated the potential of using aerial and satellite infra-red remote sensing images to identify these stressed hot spots in peanut crops. The results showed that this technology could offer growers a powerful technique to segregate high-risk peanuts during the harvesting operation and thereby increase profitability by minimising aflatoxin contamination on a field scale (Wright, Lamb & Medway, 2002).
- Inverted windrows
Peanut pods in fully inverted windrows dry faster and pass through the kernel-moisture risk zone more quickly. Inverted windrows also allow earlier thrashing which reduces the risk of the windrow being caught in wet weather and the pods sitting in the risk zone.
- Short periods between cutting and thrashing
The crop in windrows should be thrashed as soon as possible (within three to five days) to minimise the risk of wet weather on windrows. Recent introduction of new thrashers with the capacity is likely to better achieve this, especially for the high-risk years.
- Pre-clean thoroughly
Pre-cleaning not only removes extraneous matter but also small and damaged pods which often have high aflatoxin levels.
- Effective drying
Peanuts should be dried in drying bins immedately to safe moisture (<12.5%) after thrashing and pre-cleaning with a continuous air flow of 200L/sec/m³, at 50 to 65% relative humidity. The ideal maximum depth of peanuts in the drying bin is 2 m. The moisture loss should not exceed half a percent per hour.
- Only store after drying
Peanuts should only be stored or allowed to sit after they have been dried down to below 12.5%.