Introduction

  1. There have been occasional reports on the abuse of formaldehyde as bleaching agents and preservatives in food.
  2. In Hong Kong , formaldehyde is not permitted for food use.
  3. The Centre for Food Safety (CFS) has collected some Bombay-duck and noodlefish samples for testing. Results found that Bombay-duck samples contained formaldehyde at levels compatible with natural occurrence. There is no evidence of use of formaldehyde in the Bombay-duck samples. However, in the absence of dimethylamine, formaldehyde was detected (170-570 ppm) in some noodlefish. It was believed that formaldehyde has been added as a preservative after the noodlefish were caught, or during transportation or storage.

Nature of Formaldehyde

  1. Formaldehyde is a chemical commonly used in industry for the manufacture of plastic resins that can be used in wood, paper and textile industry. Formalin, which is a solution of about 37% formaldehyde, serves as disinfectant and preservative for household products.
  2. Formaldehyde is sometimes added inappropriately in food processing for its preservative and bleaching effects. The common incriminated food items are soya bean sticks, mung bean vermicelli and hydrated food such as tripe, chicken paws, etc.
  3. However, this chemical also occurs naturally in the environment. As a metabolic intermediate, formaldehyde is present at low levels in most living organisms. Formaldehyde can be found naturally in food up to the levels of 300 to 400 mg/kg, including fruits and vegetables (e.g. pear, apple, green onion), meats, fish (e.g., Bombay-duck, cod fish), crustacean and dried mushroom, etc ( Appendix).
  4. In some seafood species such as Bombay-duck, formaldehyde is a natural breakdown product of a chemical known as trimethylamine oxide (TMAO) that exists in their bodies. TMAO breaks down into formaldehyde and dimethylamine in equal parts after the death of the marine organisms. The level of formaldehyde can accumulate in certain marine fish during frozen storage and crustacea after death. Its levels were reported to be up to 400 mg/kg in Bombay-duck after cold-storage. The detection of dimethylamine in Bombay-duck was used to distinguish whether formaldehyde had been added deliberately.
  5. For noodlefish, the situation is rather different. In the absence of dimethylamine, the detection of formaldehyde (170 to 570 mg/kg) in some noodlefish samples indicated that formaldehyde might have been added as a preservative after the noodlefish were caught, or during transportation or storage.
  6. Ingestion of a small amount of formaldehyde is unlikely to cause any acute effect. Acute toxicity after ingestion of large amount can cause severe abdominal pain, vomiting, coma, renal injury and possible death.
  7. The main health concern of formaldehyde is its cancer causing potential. The International Agency for Research on Cancer (IARC) considered that there was sufficient evidence for carcinogenicity in humans upon occupational exposure via inhalation. On the other hand, WHO in 2005 when setting its Drinking Water Guidelines considered that there was no definitive evidence for carcinogenicity upon ingestion.

Control Measures

  1. Food for sale in Hong Kong must be fit for human consumption as stipulated in the Public Health and Municipal Services Ordinance, Cap. 132. The use of formaldehyde in food contravenes the Preservatives in Food Regulation and is liable to a maximum fine of HK$50,000 and imprisonment for 6 months.
  2. For foods containing natural formaldehyde, there is no international consensus on their reference levels.
  3. The testing of formaldehyde in food is included in our food surveillance programme. Follow-up action will be undertaken when there is suspected abuse of formaldehyde in foods.

Advice to the Public

  1. Patronise reliable food premises and food retailers.
  2. Choose only fish that are fresh and avoid those with unusual smell; and avoid buying noodlefish that are stiff (formaldehyde could stiffen flesh of fish).
  3. Wash and cook food products thoroughly as formaldehyde is water soluble and could dissipate upon heating.
  4. Take a balanced diet so as to avoid excessive intake of food chemicals from a small range of food items.

Advice to the Trade

  1. Be cautious about the origins from which food products are sourced, and should only obtain them from reliable sources.
  2. Do not add formaldehyde in food.
  3. Maintain a proper cold chain to ensure that fish and fish products are kept safely throughout processes including storage, transportation and display for sale.

Risk Assessment Section
Centre for Food Safety
January 2009

Examples of Foods Known to Contain Naturally Occurring Formaldehyde

 I. Fruits & Vegetables

<
Food type Level (mg/kg)
>Apple 6.3 – 22.3
Apricot 9.5
Banana 16.3
Beetroot 35
Bulb vegetables (e.g. onion) 11.0
Cabbage 5.3
Carrot 6.7 – 10
Cauliflower 26.9
Cucumber 2.3 – 3.7
Grape 22.4
Green Onion 13.3 – 26.3
Kohlrabi 31
Pear 38.7 – 60
Plum 11.2
Potato 19.5
Spinach 3.3 – 7.3
Tomato 5.7 – 13.3
Water-melon 9.2
White Radish 3.7 – 4.4
Shiitake mushroom (dried) 100 – 406
Shiitake mushroom (raw) 6 – 54.4

II. Meat and meat products

Food type Level (mg/kg)
Beef 4.6
Pig 5.8 – 20
Sheep 8
Poultry 2.5 – 5.7
Processed meat products
(including ham and sausages)		 < 20.7
Liver paste < 11.9

Examples of Foods Known to Contain Naturally Occurring Formaldehyde (Continued)

 III. Dairy products

Food type Level (mg/kg)
Goat's Milk 1
Cow's Milk < 3.3
Cheese < 3.3

IV. Seafood

Food type Level (mg/kg)
Cod 4.6 – 34
Shrimp (raw) 1 – 2.4
Squid 1.8
Fish ball 6.8
Crustacean 1 – 98
Bombay-duck < 140

Formaldehyde was also reported in studies to develop after death in marine fish and crustaceans and accumulate during the frozen storage of some fish species. Its levels can be as high as 400 mg/kg in Bombay-duck after cold storage.

V. Others

Food type Level (mg/kg)
Alcoholic beverage 0.02 – 3.8
Soft drinks 8.7
Brewed coffee 3.4 – 4.5
Instant coffee 10 – 16
Syrup <1 – 1.54

Sources: From World Health Organization and others.