Unlike walnuts and pecans, peanuts are grown beneath the soil. And for this reason, they are prone to infections, especially aflatoxins. Aflatoxins are produced by the soil-dwelling fungi (mold) Aspergillus flavus and A. parasiticus. In peanuts and other nuts and grains, it is the most important of the mycotoxins from a human health perspective.

Aflatoxins are quickly and widely absorbed from the gut and are metabolized to toxic epoxides in the liver. These epoxides bind to and harm vital cell components like DNA, RNA, and protein enzymes. The primary clinical outcome of aflatoxin consumption is liver damage in all animal species. According to epidemiological studies, aflatoxins and the hepatitis B virus are cocarcinogens, and the risk of liver cancer is higher in areas where both aflatoxins contamination and hepatitis B are prevalent.

Internationally, aflatoxins in peanuts are required to not exceed 15 μg/kg. This considerably very low MRL (maximum residue limit) ensures that peanuts with high concentrations of aflatoxins cannot be traded. In the United States, the Food and Drug Administration (FDA) has set the allowable limit of aflatoxin levels in peanut products at 20 ppb.

Fortunately, there are several ways to remove aflatoxins in peanuts.

In commercial operations, aflatoxins are regularly removed from food or food ingredients. Solvent extraction methods have been used in most cases. Some foods or food ingredients have been treated with appropriate chemicals to inactivate aflatoxins. Some chemicals, such as ammonia, hydrogen peroxide, and sodium hypochlorite, have been found to be effective at inactivating aflatoxins.

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Let’s discuss further.

WHAT IS AFLATOXIN?

Mycotoxins are toxic secondary metabolites produced by certain fungi. When consumed in large amount or regularly may cause certain cancer. Of all mycotoxins, aflatoxins have unquestionably been the most studies. Consumers have been aware of their existence since 1960, when over 100, 000 turkey poults in England perished after consuming imported peanut from Africa and South America.

Aflatoxin is classified into two types, B1 and G1. Each of them has several subtypes. Aflatoxin B1, produced by A. flavus and A. parasiticus, is thought to be the most potent. In fact, it has been classified as carcinogenic (Group 1) to humans by the International Agency for Research on Cancer (IARC). For more discussion on these types of aflatoxin, refer to this file.

Aflatoxin production has been shown to occur in many food products, which include nuts, dried fruits, and cereals. In general, toxin production is directly related to a mold strain’s growth rate. In microbiological media suitable for mold growth, Asp. flavus strains can produce optimum concentrations of aflatoxin at 91°F (33°C), pH 5.0, and Aw (water activity) of 0.99. Some toxin can be detected within 24 hours under optimal growth conditions.

In peanuts, aflatoxin formation occur primarily during the curing of peanuts after they have been removed from the soil. Hence, it is crucial to perform proper handling and storage to control the growth of aflatoxins. Moisture and temperature during storage should especially be given more attention. They are the two most important factors that affect aflatoxin formation. The presence of aflatoxins in peanuts is obvious if there is discoloration, visible mold growth, and the nuts are shriveled.

Aspergillus spp.-contaminated peanuts. Source

With that being said, here are 5 effective methods that you can do you remove aflatoxins from peanuts.

5 EFFECTIVE METHODS OF REMOVING AFLATOXINS IN PEANUTS

Aflatoxin contamination can occur in the field prior to harvest, during harvesting, or during storage and processing. While certain treatments have been found to reduce aflatoxin formation in peanuts, the most effective way is to avoid it in the first place. This is not always possible. But there are technologies and methods that can prevent contamination that would otherwise occur.

A variety of industrial food-processing methods can also be used to manage the aflatoxins in peanuts. Some of these can also be performed at home. As simple as sorting by color, screening, and lot separation (either with or without blanching) can reduce the toxin levels. In subsistence farming situations, the manual sorting of inferior nuts is also an option. Aflatoxins are thought to be heat stable, but roasting peanuts can reduce B1 aflatoxins by 50–80%.

Sorting and cleaning

Sorting and cleaning is the simple yet effective way for removing aflatoxins from peanuts. This entails removing any discolored, moldy, or damaged and shriveled peanuts from the batch because they are more likely to contain aflatoxin. To remove any dirt or debris, peanuts can be washed in clean water. Although this method is simple and easy to use at home, it may not remove all of the aflatoxin found in peanuts. Hence, this should be combined with another method.

In some practices, water is treated so that the process is more effective. For example, peanuts may be soaked in electrolyzed acidic water. Electrolyzed acidic water has a low pH of around 2.5, and is used as an effective disinfectant in food contact surfaces. One study has found that soaking grains in electrolyzed acidic water for 15 minutes can reduce 80% to 90% of B1 aflatoxin in peanuts.

Roasting

One of the most widely used techniques for removing aflatoxin from peanuts is roasting. High heat causes the aflatoxin molecules in peanuts to disintegrate, rendering them safe to consume. It’s crucial to remember that not all roasting techniques are equally efficient. As already mentioned, aflatoxins are heat stable. Studies say the optimum temperature for the production of aflatoxins is between 81°F (27°C) and 86°F (30°C). But they can withstand heat at normal cooking temperatures.

It has been demonstrated that dry roasting, which involves roasting peanuts without any additional oil, is more efficient at lowering aflatoxin levels than oil roasting. Additionally, to ensure that all aflatoxin molecules are destroyed, peanuts must be thoroughly and evenly roasted. Dry roasting is typically done on a very high heat.

To remove aflatoxins from peanuts properly, ensure to reach a temperature of 320°F (160°C) or higher. This study has shown that at a temperature of 320°F or higher, aflatoxins can be completely degraded. Another study showed that adding sodium chloride and citric acid can substantially improve the effectiveness of the process. If you wish to replicate the process, the authors recommend using up to 5% of citric acid. More than that may adversely affect the taste of peanuts.

Chemical treatment

Commercially, chemical treatment to remove aflatoxins in peanuts is widely practiced. What is involved in this method are common disinfectants or sanitizers in the food industry. One of them is hydrogen peroxide, a strong oxidizer. The peanuts are soaked in a hydrogen peroxide and water solution for a set amount of time before being thoroughly rinsed. One study said that treating peanuts with 0.075% of hydrogen peroxide for 1 minutes can reduce aflatoxin levels by up to 90%. Another good thing is that hydrogen peroxide residue can be removed by drying. Furthermore, it is environmentally friendly since it breakdowns into oxygen and water once exposed to light.

Although this method has been shown to significantly reduce aflatoxin levels in peanuts, it may also have a subtle effect on the taste and quality of the peanuts.

Other chemicals used to remove aflatoxins in peanuts include ammonia, sodium hypochlorite, and benzoyl peroxide.

Irradiation

Irradiation is one of the most effective methods in making foods shelf life stable, in general. But its high cost required makes it less utilized. Irradiation can remove aflatoxins by exposing the peanuts to ionizing radiation to break down the aflatoxin molecules. Most applications of irradiation require a maximum overall average dose of 10 kGy (kilogray).

For the purpose of microbial reduction, the doses range between 1 to 10 kGy. However, the outcome is positively correlated with an increase in the radiation dose. In one study, a 10 kGy dose reduced the toxin levels in peanuts to 58.6. Another study used lower doses of 4, 6, and 8 kGy, which reduced the aflatoxin levels to 7.6%, 17.3%, and 23.25% respectively. Nonetheless, irradiation is a viable option in reducing the aflatoxins in peanuts to below the maximum allowed levels.

While effective, the irradiation process may become less appealing to consumers who are reluctant to consume irradiated foods due to possible exposure radiation. It is crucial to remember that the radiation used to irradiate food is extremely low and has been approved as safe by regulatory bodies such as the Food and Drug Administration (FDA) and World Health Organization (WHO).

Furthermore, the taste, texture, or nutritional value of the peanuts are unaffected by irradiation. Peanuts are rich in proteins and fats. And these macronutrients are barely unaffected by radiation at usual doses.

Biological control

Biological control is a technique for removing aflatoxins from peanuts that involves using naturally occurring predators or competitors of the fungus that causes the toxin. Many factors influence a fungus’s ability to compete for a host. These include the soil type, pH level, water content, mineral available, nitrogen and carbon availability. The soil microbiome, primarily fungi and bacteria, influences fungi’s ability to produce secondary metabolites.

Using a strain of the A. flavus that does not produce aflatoxins is one example of bio-control. Aflatoxin levels in peanuts are decreased when this strain is introduced to the soil where peanuts are grown because it outcompetes the strain that produces the toxin. This 2020 study evaluated 18 non-aflatoxigenic strains of A. flavus. 6 of them reduced the aflatoxin levels produced by the native aflatoxigenic strains by 50%. Although promising, this strategy faces challenges that prevent it from being a clear solution to reducing aflatoxins in peanuts. For example, its biology is not well understood due to the diversity of A. flavus.

An alternative to this is using other species that can effectively affect the fungi from producing aflatoxins.

In this 2018 Korean study, the authors used Aspergillus oryzae M2040, a strain isolated from fermented soybean. 1% inoculation level of the A. Oryzae strain showed that it can effectively displace A. flavus and inhibit aflatoxin production.

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Another example is the bacterium Bacillus subtilis, whose many strains are safe for human consumption. It also produces enzymes that break down aflatoxins. This Japanese study used strains of B. subtilis that proved to be inhibitory on the growth of both A. flavus and A. parasiticus.

References:

Y. Motarjemi, G. Moy, E. Todd (2014). Encyclopedia of Food Safety. Academic Press.

Y. Motarjemi, H. Lelieveld (2014). Food Safety Management: A Practical Guide for the Food Industry. Academic Press

G. Cooper (2018). Food Microbiology. Library Press.

J. Jay, M. Loessner, D. Golden. (2005). Modern Food Microbiology (7th edition). Springer.

B. Ray (2005). Fundamental Food Microbiology (3rd edition). CRC Press.

I. Shaw (2013). Food Safety: The Science of Keeping Food Safe. John Wiley & Sons, Ltd.

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