Pu’er tea, a beverage with a history spanning over three thousand years, has recently become the center of a heated debate. The controversy was ignited when popular science writer Fang Zhouzi published an article suggesting that Pu’er tea could be carcinogenic. In response, the Yunnan Pu’er Tea Association swiftly issued a statement to clarify the facts and announced its intention to pursue legal action to clear Pu’er tea’s name.
So, does the beloved Pu’er tea really pose a cancer risk? This article will explore the question from a scientific standpoint, using experimental data to shed light on the issue.
What is Pu’er Tea?
Pu’er tea is a unique type of fermented tea made from the sun-dried leaves of the Yunnan large-leaf tea varietal (Camellia sinensis var. assamica), which is grown in specific regions of China’s Yunnan province. The processed leaves are sold as both loose-leaf and compressed tea.
Pu’er is primarily categorized into Raw Pu’er (Sheng Cha) and Ripe Pu’er (Shu Cha). Raw Pu’er is made from fresh tea leaves that are aged naturally over time without undergoing an accelerated fermentation process. Ripe Pu’er, on the other hand, is created through a process of artificial, accelerated fermentation known as “wo dui” (渥堆) or pile fermentation.
Based on the degree of fermentation, Pu’er can also be classified into categories such as non-fermented (Pu’er green tea), lightly fermented (Pu’er oolong, yellow, and white tea), fully fermented (Pu’er black tea), and post-fermented (Pu’er compressed tea, dark tea).
The two processes that define the type and quality of Pu’er are fermentation and aging. Different conditions of temperature, humidity, and ventilation during these stages result in varying degrees of fermentation. Post-fermented teas, particularly Ripe Pu’er, are especially popular and widely known.
Pu’er tea is often harvested from old, high-yielding tea trees, which contributes to its rich mineral content. Studies from several Chinese institutions suggest that drinking Pu’er tea may help lower blood pressure and improve circulation, with some reports also citing benefits for digestion and weight loss. Of course, these health benefits are closely tied to the quality of the tea.
The preceding paragraphs briefly introduce Pu’er’s definition, classification, and production. The recent cancer controversy stems directly from its production methods. The core of the debate lies in the potential safety risks during Pu’er’s fermentation and aging processes—specifically, the likelihood of mold contamination and the severity of its consequences.
Does Pu’er Tea Contain Carcinogens?
Naturally fermented and post-fermented teas require storage at room temperature (20-30°C) for fermentation, followed by years of aging. When the aging process is properly controlled, it yields superior-quality Pu’er. However, improper storage can easily lead to the growth of harmful microorganisms like mold, resulting in a final product that could be detrimental to human health.
So, does Pu’er tea itself contain carcinogens?
No, Pu’er tea itself is not carcinogenic. The cancer risk arises from improper storage that leads to contamination and excessive levels of aflatoxin. Aflatoxin is the real culprit.
Fang Zhouzi referenced a study conducted in 2011 by researchers at the Guangzhou Center for Disease Control and Prevention. They tested 70 wet-stored Pu’er tea samples from a Guangzhou tea market and found that all samples contained aflatoxin B1. Among them, 8 samples (11.43%) exceeded the safety limit of 5 micrograms per kilogram [1].
However, the study’s authors also pointed out that the tested samples were low-priced products that had been aged under “wet storage” conditions.
Why use wet storage? This method, which involves aging tea in a high-humidity environment, causes the leaves to darken more quickly, mimicking the appearance of genuinely aged Pu’er. It is essentially a cost-cutting shortcut. The image below is a screenshot from an interview with Zhu Wei of the Guangzhou CDC.
Aflatoxin (AF) is a toxic metabolite produced by the fungi Aspergillus flavus and Aspergillus parasiticus. These molds can thrive on grains and nuts if they are not dried properly, producing aflatoxins under suitable conditions of temperature, pH, and humidity.
Caption: Corn and peanuts contaminated with Aspergillus flavus.
Aflatoxin is extremely hazardous to health and is a primary cause of liver cancer. There are at least 13 types of aflatoxin, with B1 being the most toxic. Other toxic variants include B2, G1, G2, M1, and M2.
Its carcinogenic mechanism involves the body ingesting the toxin, which is then activated by cytochrome P450 enzymes in the liver. This activated form binds to DNA, forming adducts that disrupt DNA replication and can trigger the onset of cancer.
The potent carcinogenicity of aflatoxin is well-documented. In animal studies, all rats fed a diet containing 15 µg/kg of aflatoxin B1 developed signs of liver cancer [2]. The International Agency for Research on Cancer (IARC) classifies it as a Group 1 carcinogen. Based on acute toxicity studies in animals, the median lethal dose (LD50) is estimated to be 18.2 µg/kg for B1, 84.8 µg/kg for B2, 39.2 µg/kg for G1, and 172.5 µg/kg for G2. While about 20% of ingested aflatoxin can be excreted through urine, high doses can cause acute poisoning with symptoms like jaundice and ascites. Chronic, low-level exposure can lead to progressive liver damage and induce liver cancer. For individuals with Hepatitis B, exposure significantly increases the risk of developing liver cancer.
Therefore, aflatoxin is one of the most potent naturally occurring toxins found in food.
Caption: A magnified view of Aspergillus flavus fungi.
Most alarmingly, aflatoxin is not destroyed by ordinary cooking temperatures. It can only be broken down by UV light exposure in the presence of oxygen or by treatment with strong acids (pH < 3) or bases (pH > 10). This stability makes it particularly dangerous, as typical home cooking methods are ineffective at neutralizing it.
If Pu’er tea is stored in a warm, humid environment, it becomes highly susceptible to contamination by Aspergillus flavus. Drinking moldy Pu’er tea would naturally present a cancer risk.
In other words, the potential for Pu’er tea to cause cancer is entirely dependent on whether it has been contaminated by mold.
How Susceptible Is Pu’er Tea to Mold Contamination?
As a Pu’er enthusiast, my first thought upon hearing about this controversy was that this topic has indeed been explored in serious scientific research. Let’s examine a scientific paper [3] to understand the complex relationship between Pu’er tea and Aspergillus flavus.
As mentioned earlier, the “wo dui” pile fermentation is a critical step in producing post-fermented Pu’er tea. This process relies on a beneficial fungus called Aspergillus niger. Although its name is similar to Aspergillus flavus, its properties are vastly different. Aspergillus niger is a cornerstone of the fermentation industry, vital for making soy sauce, vinegar, tea, wine, and producing enzymes and organic acids.
The “wo dui” process takes 22-24 days, during which Aspergillus niger ferments the Pu’er tea and produces organic acids.
What would happen if the tea were contaminated with Aspergillus flavus during this process? Would the aflatoxin levels become dangerously high?
In one experiment, two strains of Aspergillus flavus—strain 1 (YM 31880 from ATCC) and strain 2 (YM 31882 from IM-CAS)—were inoculated into Pu’er tea. The tea was then cultured for 22 days to mimic the “wo dui” fermentation process.
Caption: Aspergillus flavus in a culture medium.
Both strains of the harmful mold grew rapidly during the first 8 days of fermentation, peaking on the 8th day before declining. By day 22, Aspergillus flavus was undetectable in the tea inoculated with strain 1 and only present in small amounts in the tea with strain 2. Crucially, no aflatoxin was detected in either group. It’s worth noting that only about 10% of naturally occurring Aspergillus flavus strains are capable of producing toxins.
This suggests that even if a product is contaminated with the mold, it may not necessarily produce toxins, as production depends on environmental conditions.
The experiment demonstrates that the nutritional composition and environment of Pu’er tea during fermentation are not conducive to the growth of Aspergillus flavus or its production of aflatoxin. By the later stages of fermentation, the environment becomes so unfavorable that the mold cannot thrive.
Caption: Pu’er tea leaves inoculated with Aspergillus flavus.
The study also compared the competitive growth between Aspergillus flavus and Aspergillus niger.
As noted, Aspergillus niger is the beneficial fungus in Pu’er post-fermentation. Both fungi grew rapidly until day 8. Afterward, both declined, but Aspergillus niger became the dominant species, outcompeting Aspergillus flavus. This indicates a competitive relationship between the two microorganisms.
The research showed that Aspergillus niger not only inhibits the growth and toxin production of Aspergillus flavus but can also help degrade any aflatoxin that might be present.
Furthermore, the experiment included a control group that was not artificially inoculated with mold. Throughout the entire fermentation process, this group showed no signs of natural contamination by Aspergillus flavus.
The study’s conclusions were:
- During the “wo dui” post-fermentation process, Pu’er tea is not prone to contamination by Aspergillus flavus or the production of its toxins.
- Post-fermented dark tea can become contaminated with Aspergillus flavus and potentially produce aflatoxin if stored in an environment with high humidity, high temperature, and poor ventilation. If tea develops a bitter or musty taste, it should not be consumed.
As an essential part of Chinese tea culture, Pu’er tea has seen a surge in popularity in recent years. Research into this tea should not only focus on its benefits but also delve deeper into its safety and hygiene.
Our advice is to purchase high-quality, vacuum-sealed products from reputable dealers. Be cautious about buying loose-leaf Pu’er tea sold in unclean environments. Don’t compromise your health to save a little money.
Putting Risk in Perspective: Food Safety and Common Sense
After reading this article, you can likely form your own judgment about the Pu’er cancer scare. The Pu’er controversy, like many issues in food safety, highlights the tension between potential risk and general safety. While it is potentially possible for low-quality Pu’er to develop aflatoxin during production and aging, the cancer risk from properly produced and inspected Pu’er tea is negligible.
Consider many of the foods we regularly enjoy: cured meats like sausages and ham, fermented foods like stinky tofu, preserved eggs, and pickles, and even grilled or fried foods. The list of potential carcinogens that can be produced during their creation is long and varied.
Yet, they remain cherished delicacies on our tables, satisfying discerning palates worldwide. The modern food industry works tirelessly to minimize harmful contaminants in food production, and an overreaction of abstaining from food out of fear is not productive. If we label all Pu’er as carcinogenic without distinction, what food could ever be considered absolutely safe?
In the face of potential risks, we tend to trust the professionalism of the food industry and the comprehensive food safety standards and regulations enforced by the state. This allows us to view these foods as generally safe for consumption. Isolated incidents of poor practice should not be used to discredit the progress and efforts of an entire industry. Instead, we can advocate for greater government investment in perfecting food safety legislation and oversight.
Just as improper Pu’er fermentation can create carcinogens, the notion that “farm-fresh” or “homemade” is inherently safer can be misleading. Small-scale, decentralized production often means lower standards and less oversight. Large-scale, standardized industrial production is the most reliable path to improving food safety for everyone.
References
[1] Chen Jianling, Li Wenxue, Yang Guangyu, et al. A survey of mycotoxin contamination in Pu’er tea from a tea market in Guangzhou [J]. Carcinogenesis, Teratogenesis & Mutagenesis, 2011, (1).
[2] “Mycotoxins (Aflatoxins) (PDF)”. Life and Health, No. 8. Tokyo Metropolitan Institute of Public Health. pp. 1-6 (August 2005).
[3] Li Yali, Kang Guanhong, Yang Liyuan, et al. Study on Aflatoxin Production by Exogenous Inoculation of Aspergillus flavus during Pu-erh Tea Fermentation. Journal of Tea Science, 2014.
[4] Zhu Jinguo, Wen Guohua, Zhang Yunbei, et al. Investigation and analysis of mold contamination in tea and its related factors. Journal of Inspection and Quarantine, 2003, 13(4).