Frozen steamed buns contain aflatoxin? Can we still eat them?

Recently, a short video claiming that "Zhejiang University research found that frozen steamed buns cannot be eaten, and will grow aflatoxin if frozen for more than two days" quickly became popular and became a hot search on multiple platforms, causing widespread attention and panic.

Image source: Internet

So, does freezing steamed buns for two days really cause aflatoxin to grow? Is it really not edible?

Let me start with the conclusion: First, from the perspective of current scientific research progress, this statement over-exaggerates the possible risks of frozen steamed bread. Second, after searching, none of the self-media that published this news listed the exact information of the research, including the specific paper title, research team, etc. As a report involving scientific research, this is not credible.

Let’s analyze it in detail.

It is difficult for frozen steamed buns to mold, let alone grow aflatoxin. Although everyone is afraid of aflatoxin, the production of this toxin is not as easy as everyone thinks. On the one hand, Aspergillus flavus is a large family with many species, and only some specific strains of Aspergillus flavus can produce aflatoxin. On the other hand, it is actually difficult for frozen steamed buns that are properly stored to grow aflatoxin, because Aspergillus flavus is very "picky". It does not like the frozen environment in the refrigerator, nor does it like to "eat" steamed buns. Specifically:

The first is the environment. Aflatoxin production requires specific conditions, including two important factors: temperature and humidity. Aflatoxin can grow in an environment of 10 to 45 degrees Celsius, and the most suitable temperature is around 30 degrees Celsius, while the freezer in the refrigerator is -18 degrees Celsius, which is too cold for it to grow happily; the most suitable humidity for Aflatoxin is about 85%, while the freezer in the refrigerator is too dry.

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Some friends may ask, even if there is no aflatoxin, will other molds make frozen steamed buns moldy? Don't worry, for most molds, the suitable growth temperature is 10~40℃, and the temperature of the refrigerator freezer is -18℃ under normal circumstances. Therefore, as long as a steamed bun is not moldy when placed in the refrigerator freezer, the possibility of mold in just 2 days of correct freezing is very small.

Secondly, Aspergillus flavus is also very picky about "food". Aspergillus flavus needs nutrients to grow and produce toxins. The nutrients it likes are mainly found in rice, corn, peanuts and other grains and oils and their products. Aspergillus flavus does not like other nutrients. Therefore, sometimes the nutritional conditions are not suitable, even if Aspergillus flavus grows reluctantly, it may not synthesize toxins.

In addition, according to the results of global food safety risk assessment, aflatoxin mainly contaminates foods such as grains, nuts and seeds, and milk. Among them, moldy corn and peanuts are the most serious and common contamination. This is mainly because grains and other crops in the field before harvest are infected by toxin-producing fungi such as aflatoxin and produce toxins. Therefore, moldy grains and nuts are easily contaminated by aflatoxin. As for the presence of aflatoxin in milk, it is actually because cows eat contaminated feed, and then aflatoxin enters the milk.

From this point of view, the possibility of frozen steamed bread growing Aspergillus flavus and producing aflatoxin is almost negligible, and there has been no aflatoxin poisoning caused by frozen steamed bread contaminated by Aspergillus flavus. Therefore, since no authoritative papers and details have been seen so far, it cannot be ruled out that this is a distorted report and false news taken out of context.

Stay away from aflatoxin and remember these two principles

The “injustice” of frozen steamed buns has been resolved, but the harm of aflatoxin cannot be ignored. Therefore, I will spend a little more time here to share with you the knowledge about aflatoxin.

Aflatoxin comes from Aspergillus flavus, which is a kind of mold, a saprophytic fungus widely distributed in nature, which can parasitize on grains, food and feed to grow and reproduce. Aspergillus flavus itself is not toxic, but the aflatoxins it produces are toxic.

Aflatoxins are secondary metabolites produced by Aspergillus flavus and Aspergillus parasiticus during their growth. Common ones include aflatoxin B1, B2, G1, G2, M1, M2, etc., among which aflatoxin B1 is the most common. Aflatoxins have good thermal stability and are not easily decomposed by conventional cooking and heating methods.

The harm of aflatoxin to humans and animals is mainly reflected in the immunosuppressive properties of aflatoxin, as well as its mutagenic, carcinogenic and teratogenic properties. The World Health Organization (WHO)'s cancer research institute has classified aflatoxin B1 as a Class I carcinogen, which mainly increases the risk of liver cancer. So how can we properly prevent the risks brought by aflatoxin? Just remember these two points:

1. The main source of aflatoxin is moldy grains and nuts, such as moldy rice, corn, peanuts, etc. When storing rice, corn, etc. that you usually buy, you should pay attention to mold prevention, and try not to buy native peanut oil.

2. Aflatoxin tastes very bitter. If you taste the bitterness when eating moldy peanuts, nuts and other foods, spit them out immediately and rinse your mouth.

To sum up, aflatoxin does harm our health, but properly stored frozen steamed bread is not suitable for the growth of Aspergillus flavus, and will not produce aflatoxin, so you can still buy and eat it with confidence.

Next, I will share with you some details about the storage of frozen steamed buns, which can help you eat more healthily. Here are some suggestions for you:

1. Try to avoid repeated thawing and freezing. Repeated thawing and freezing will make the steamed buns taste worse and there is also a risk of breeding microorganisms.

2. Do not leave it at room temperature for too long. Eat the leftover steamed buns as soon as possible, or keep them in the freezer. Do not leave them outdoors, as the outdoor temperature is high and they will be contaminated by microorganisms.

References

[1] Wu Fengqi, Yue Zhenfeng, Zhang Yi, et al. Research progress in analytical methods for major mycotoxins in food[J]. Chromatography, 2020, 38(7): 759-767.

[2] Sun Zhongqing, Lin Ying, Mi Ye, et al. Research progress in the prevention and control of mold and mycotoxins in nuts and seeds[J]. Chinese Journal of Food Hygiene, 2023, 35(05): 795-800. DOI: 10.13590/j.cjfh.2023.05.027.

[3] Zhang Muchen, Zheng Nan, Wang Jiaqi. Research progress on aflatoxin B1 contamination in food[J]. Food Science, 2018, 39(07): 312-320.

[4] Wang Jun, Liu Xiumei. Assessment of dietary exposure to aflatoxin in the Chinese population[J]. Chinese Journal of Food Hygiene, 2007, (03): 238-240. DOI: 10.13590/j.cjfh.2007.03.013.

[5] Yang Xue, Gao Yanan, Wang Jiaqi, et al. Analysis of mycotoxins and their exposure risks in Asia, Europe and the United States [J]. Food Industry Science and Technology, 2020, 41(5): 311-318.

Author: Ruan Guangfeng, Deputy Director of Kexin Food and Health Information Exchange Center

Review丨Zhang Yu, researcher at the Chinese Center for Disease Control and Prevention, national health science expert

Han Hongwei, Researcher at the National Food Safety Risk Assessment Center