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Study results on nitrate and nitrite in vegetables released

The findings of a study conducted by the Centre for Food Safety (CFS) revealed that the levels of nitrate and nitrite in vegetables consumed locally were unlikely to pose any immediate health risk to the general population. Moreover, blanching was found to be more effective than soaking in reducing nitrate in vegetables.

A CFS spokesman said today (July 30) that people should maintain a balanced diet with two servings of fruit and three servings of vegetables every day and eat a wide variety of vegetables including leafy vegetables, brassica vegetables, root and tuber vegetables, fruiting vegetables, legume vegetables, etc.

The study aimed to examine the levels of nitrate and nitrite in vegetables available in Hong Kong and assess the associated health risks posed to people through dietary exposure to nitrate and nitrite from vegetables. It also explored the effects of preparation and cooking methods on nitrate levels in vegetables.

The spokesman said nitrate and nitrite occur naturally in the environment and can be produced endogenously in humans. They are also used as fertilisers and food additives in processed foods. All types of vegetables contained nitrate which itself was relatively non-toxic. Nitrite can be produced by bacteria from nitrate naturally present in vegetables due to improper handling and storage of food. Consumption of a high level of nitrite can cause methaemoglobinaemia (also known as Blue Baby Syndrome, a rare condition under which haemoglobin is unable to carry oxygen to body tissues and causes skin and lips to turn blue) and young infants are more susceptible to it. This raises public concern about the levels of nitrate and nitrite in vegetables.

The CFS collected more than 700 samples of 73 types of commonly consumed vegetables in local wet markets and supermarkets for analysis. They included:

1) Leafy vegetables: such as petiole Chinese cabbage, Chinese flowering cabbage, Chinese lettuce, Chinese kale;
2) Brassica vegetables: including cauliflower and broccoli;
3) Stalk and stem vegetables: asparagus, celery, soybean sprouts and the like;
4) Bulb vegetables: including Chinese chives, garlic bulb and onion;
5) Legume vegetables: such as snow pea and sugar snap pea;
6) Root and tuber vegetables: carrot, lotus root, potato and the like;
7) Fruiting vegetables (Cucurbits): including bitter gourd, cucumber and hairy gourd;
8) Fruiting vegetables (Mushrooms): such as shiitake mushroom, white button mushroom, chicken-leg mushroom and oyster mushroom; and
9) Fruiting vegetables (Other than Cucurbits and Mushrooms): including eggplant, tomato and bell pepper.

Results showed that over 80% of the samples had a nitrate level of less than 2,000 milligrammes (mg) per kilogramme (kg).

There were variations in the mean concentrations of nitrate in different groups of vegetables. The mean concentrations in descending order were leafy vegetables (2,100mg/kg), root and tuber vegetables (720mg/kg), fruiting and legume vegetables (from 14mg/kg to 370mg/kg). Three types of leafy vegetables, namely Chinese spinach, Shanghai cabbage and petiole Chinese cabbage, contained relatively high levels of nitrate, all with a mean concentration exceeding 3,500mg/kg.

On the other hand, the nitrite concentrations of the vegetables were generally low, with less than 1mg/kg on average. "The test results are generally in line with those found on the Mainland and other countries," the spokesman said.

The study also found that nitrate can be reduced more effectively by blanching than soaking. Experiment results showed that the nitrate concentrations in Chinese flowering cabbage, Chinese spinach and celery reduced significantly by 12 to 31% when they were blanched for one to three minutes.

Vegetables are essential to human health since they are good sources of vitamins, minerals and biologically active substances. The World Health Organisation (WHO) recommends intake of a minimum of 400 grammes of fruits and vegetables per day for the prevention of chronic diseases. The Department of Health in Hong Kong also promotes the consumption of at least two servings of fruit and three servings of vegetables every day (2 plus 3 a day) as part of a balanced diet for optimal health. The spokesman advised the public to reduce the nitrate exposure while maintaining the recommended intake of vegetables.

"Although certain leafy vegetables are relatively high in nitrate, they are also rich in many essential nutrients. Therefore, leafy vegetables should be maintained in our diet. A variety of low nitrate-containing vegetables such as fruiting vegetables and legume vegetables should also be included in our diet," the spokesman said.

"People should also handle and cook vegetables properly - keep vegetables under refrigeration if they are not cooked immediately; cook vegetables soon after chopping or mashing; wash and peel vegetables before cooking; blanch high-nitrate-containing vegetables and discard the cooking water before consumption."

The WHO recommends exclusive breastfeeding for infants up to six months of age with appropriate complementary foods afterwards. Infant foods such as vegetable puree and vegetable congee should be prepared for immediate use. If storage is needed, they should be kept in the freezer (at or below -18 degree Celsius) to avoid accumulation of nitrite due to contamination of the food by bacteria.

The CFS also advised the food trade to obtain vegetables from reliable sources and maintain proper records to enable source tracing when required. They should store vegetables in either the refrigerator or cool and dry places to avoid accumulation of nitrite due to spoilage of the vegetables.

Report Summary

Ends/Friday, July 30, 2010
Issued at HKT 14:31

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Last revision date:30-07-2010