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Dried Edible Fungi Rich in Dietary Fibre but Not a Major Source of Collagen Soak with Care Before Cooking to Avoid Risk of Bongkrekic Acid Poisoning

  • 2023.02.15

Dried edible fungi, including black fungus (also known as cloud ear fungus), white back black fungus, and snow fungus (also known as white fungus), are popular ingredients. The first two are hailed as “vascular cleansers”, while snow fungus is reputed to be “bird's nest for common people”. The Consumer Council tested the nutritional values of 28 samples of prepackaged edible fungi on the market, including black fungus, white back black fungus and snow fungus. All samples were found to be rich in nutrients, especially dietary fibre. Regarding food safety, over 40% (12 samples) were found to contain at least 1 type of pesticide, while all contained metallic contaminants though unlikely to cause any significant health risk. The Council cautions that although dried edible fungi are highly nutritious, consumers should pay attention to the proper preparation methods of these ingredients, and avoid soaking them for too long to prevent bongkrekic acid poisoning.

From August to September 2022, the Consumer Council sourced 10 samples of black fungus, 9 of white back black fungus and 9 snow fungus from major retail outlets, with price ranging from $12 to $99 per pack, equivalent to a unit price of $7.9 to $130.3 per 100g. Test items included soluble and insoluble fibre content, as well as the content of 3 minerals, namely calcium, iron, and potassium. Food safety tests included the content of 6 metallic contaminants (total arsenic, cadmium, chromium, lead, total mercury, and antimony), pesticide residues and 2 preservatives, including benzoic acid and sulphur dioxide. Among the black fungus and white back black fungus samples, those with the lowest price per 100g scored 4.5 to 5 points for pesticides, metallic contaminants and overall rating, reflecting that affordable products can also be of good quality.

Among the 3 types of fungi tested, black fungus was found to have the highest average iron content. Iron is an essential element for making haemoglobin, and consuming iron-rich foods could help prevent iron deficiency anaemia. White back black fungus had the highest average insoluble fibre content, fitting consumers with general concern about intestinal health. Snow fungus was found to have the highest average soluble fibre content, making it suitable for consumers concerned about blood cholesterol and blood sugar levels.

White Back Black Fungus Highest in Average Dietary Fibre Content

There are 2 types of dietary fibre: soluble and insoluble. Soluble fibre helps lower blood cholesterol and stabilise blood sugar, which can help control coronary heart disease and diabetes, while insoluble fibre promotes bowel movement and reduces the risk of carcinogenic substances accumulating in the bowels, preventing the development or deterioration of intestinal cancer, constipation and haemorrhoids. Edible fungi are a source of dietary fibre. According to the Centre for Food Safety’s (CFS) Technical Guidance Notes on Nutrition Labelling and Nutrition Claims, solid foods containing no less than 6g of soluble or insoluble fibre per 100g are classified as “high” soluble or insoluble fibre foods.

The category with the highest total dietary fibre content per 100g on average was white back black fungus with 70.7g, while black fungus and snow fungus had comparable total dietary fibre content of 58.3g and 58.6g respectively. Taking the white back black fungus sample with the highest total dietary fibre content (74.4g per 100g) as an example, a serving of 10g (about 1 bowl cooked, 250ml to 300ml capacity per bowl) would provide about 30% of the recommended minimum daily intake of dietary fibre for adolescents and adults.

In terms of soluble fibre, the content per 100g varied drastically among samples, from 0g to 18.4g, with snow fungus being the highest on average. All snow fungus samples met the definition of “high” soluble fibre with an average of 11.1g, while that of the white back black fungus group was 5.6g on average. The lowest soluble fibre content was found in the black fungus group, with only 3.4g on average, while 5 samples were not detected with any soluble fibre at all. Consumers concerned about blood sugar and cardiovascular health may consider eating an appropriate amount of snow fungus, as soluble fibre could help lower blood cholesterol levels. In addition, all 3 types of edible dried fungi contained anticoagulants which could also help prevent blood clots.

The insoluble fibre content per 100g ranged from 40.9g to 68.6g among samples, all “high” in insoluble fibre. White back black fungus was the highest with an average of 65.1g, followed by black fungus (54.9g) and the lowest was snow fungus with an average of 47.5g. Insoluble fibre could expand stool volume, stimulate peristalsis of colon muscles, and has water-absorbing, bulking, and colon lubricating effects. Individuals suffering from constipation could alleviate the condition by consuming appropriate amounts of white back black fungus.

Black Fungus Highest in Average Calcium and Iron Content

Minerals are essential nutrients required by the body in small amounts to promote growth, development and normal bodily functions. Calcium strengthens bones, iron is the main element in the production of haemoglobin, and potassium is responsible for maintaining electrolyte balance and proper cell function.

Based on the content per 100g, black fungus was the richest in calcium and iron among the 3 types of fungi samples. The calcium content of black fungus samples was 443mg on average, which was over 3.6 times and 10 times higher than that of white back black fungus and snow fungus. Consuming 10g of dried black fungus (about 1 bowl when cooked) would provide an average of 44mg of calcium, about the same as 1/6 to 1/8 cup of low-fat milk (1 cup is about 240 ml). Apart from calcium, black fungus samples were also highest in iron content with 16.2mg on average, while white back black fungus and snow fungus samples only contained an average of 4.6mg and 0.2mg of iron respectively. As for potassium, snow fungus samples had the highest average of 2,908mg, which was over 2 times higher than black fungus (898mg) and 5 times higher than white back black fungus (450mg).

Over 40% Found to Contain at Least 1 Type of Pesticide

All Samples Detected with Metallic Contaminants

Although pesticides and metallic contaminants were found in the samples, the amount detected was small and as consumers tend to consume a limited serving size of edible fungi each time, the overall risk is not significant.

Over 40% of the samples (12 samples) were found to contain at least 1 type of pesticide, of which 2 black fungus samples and 2 white back black fungus samples each contained 1 pesticide residue, but by using a conversion factor to extrapolate residues in their fresh state, estimates were below the limits for fresh edible fungi under the Hong Kong Pesticide Residues in Food Regulation (“Pesticide Regulation”). However, among the snow fungus samples, apart from 1 sample with no pesticide detected, 1 to 5 types of pesticide residues were detected in the remaining 8 samples. 1 sample labelled “organic” was found with residues of 4 types of pesticides, of which the level of Abamectin B1a was 0.29mg/kg, and after applying the conversion factor to estimate the residue level in its fresh state, would reach the maximum residue limit (MRL) of the Pesticide Regulation. However, for a 60kg adult, assuming no other dietary exposure to the pesticide in question, would need to consume 207g of the sample (about 21 bowls cooked) per day to pose a health risk based on the Acceptable Daily Intake (ADI) of 0-0.001mg/kg body weight set out by the Joint Food and Agriculture Organization (FAO)/World Health Organization (WHO) Meeting on Pesticide Residues (JMPR). Nevertheless, the Council urges the industry to conduct regular product tests to ensure pesticide residues in their products comply with the relevant regulations to protect the health of consumers, while at the same time, engage raw material suppliers with good agricultural practices, prioritise physical control measures against pests and diseases, and reduce the use of chemical measures such as pesticides, to minimise risks to humans and the ecology.

As for metallic contaminants, total arsenic and lead were detected in all samples, cadmium was detected in all samples of black fungus and snow fungus, while chromium was also detected in the former, but all the levels were well below the upper limit of relevant regulations and should not pose any adverse health effects under normal consumption conditions. Consumers are reminded to wash the fungi thoroughly before consumption to remove surface impurities (including metallic contaminants) and to maintain a balanced diet to avoid excessive intake of certain metallic contaminants due to picky eating.

Sulphur dioxide was also detected in 2 samples and although the levels did not exceed the permitted upper limit stipulated in the Preservatives in Food Regulations, the information was not included in the list of ingredients in accordance with the Food and Drugs (Composition and Labelling) Regulations.

The safety test results of the dried edible fungi samples were sent to CFS for follow-up.

Avoid Soaking for Too Long

Possible Risk of Bongkrekic Acid Poisoning

Dried edible fungi are usually soaked before cooking. Consumers should note that soaking at room temperature should not be too long, usually no more than 2 hours, otherwise it should take place in a refrigerator. This is because soaked snow fungus, black fungus and white back black fungus are more susceptible to contamination by the bacterium Burkholderia cocovenenans, which may produce toxic bongkrekic acid. This toxin, once produced, cannot be destroyed by washing or cooking and may cause abdominal pain and vomiting, or in severe cases, cause deranged liver function and even death. Leftovers should be stored in a refrigerator at 4°C or below within 2 hours after cooking to slow down bacterial growth and delay formation of nitrite. Food should be thoroughly reheated to a core temperature of 75°C before serving and should not be reheated more than once to ensure food safety. Discard any leftovers stored in a refrigerator for more than 3 days.

Dried Edible Fungi Not a Major Source of Collagen

Dried edible fungi are often reputed to be rich in collagen, and snow fungus, also hailed as the “bird’s nest for common people”, is widely believed to have skin-nourishing effects. According to the Hong Kong Nutrition Association, collagen is mainly found in food products from animals, whereas black fungus, white back fungus and snow fungus are not main sources of collagen. The thick and creamy consistency of soups or desserts made from dried edible fungi is due to soluble fibre dissolved into water during cooking. There is also insufficient evidence to show that collagen consumption is effective in replenishing collagen in the face. However, some animal studies have shown that polysaccharides contained in snow fungus could improve skin ageing in mice, and some studies have shown that the polysaccharides contained in snow fungus have hydrating properties, but these studies have rarely been tested on human skin.

Recommendations on consuming and purchasing dried edible fungi:

  • Buy from reputable shops. Do not pick wild fungi for food;
  • Dried edible fungi are susceptible to moisture and mould and should be stored in airtight containers in a cool, dry place away from direct sunlight;
  • Of the 3 types of fungi, black fungus is richer in calcium, and moderate sun exposure can help the skin produce vitamin D, which assists calcium absorption;
  • Black fungus and white back black fungus are sources of iron. Eating foods rich in vitamin C can also help iron absorption. Beverages such as tea and coffee can hinder iron absorption and should be avoided between 2 hours before and 2 hours after a meal;
  • For individuals with reduced kidney function resulting in occasional high potassium levels, only small amounts of edible fungi should be consumed occasionally as a side dish, and consult a doctor or registered dietitian for advice. Consumers with suspected or confirmed cardiovascular disease should seek medical advice and medicate as directed, rather than relying solely on the consumption of dried edible fungi as treatment.


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