The Centre for Food Safety advises blanching is more effective in reducing nitrates in vegetables than soaking.

Unveiling the findings of its latest study today, the centre said the level of nitrate and nitrite in vegetables consumed locally is unlikely to pose an immediate health risk to diners. It examined the level of nitrate and nitrite in vegetables available in Hong Kong and assessed the health risks from dietary exposure to them. It also studied the effects of preparation and cooking methods on nitrate levels.

The study found nitrate can be reduced more effectively by blanching than soaking. Experiment results showed nitrate concentrations in Chinese flowering cabbage, Chinese spinach and celery dropped by up to 31% when blanched for three minutes.

The centre said nitrate and nitrite occur naturally in the environment and in people. They are also used as fertilisers and food additives. All types of vegetables contain nitrate which itself is 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, or Blue Baby Syndrome, a rare condition in which haemoglobin cannot carry oxygen to body tissues, causing the skin and lips to turn blue. Infants are more susceptible to it, which has raised public concern about nitrate and nitrite in vegetables.

The centre collected more than 700 samples of 73 types of commonly consumed vegetables in local wet markets and supermarkets for analysis, including:

* leafy vegetables like petiole Chinese cabbage, Chinese flowering cabbage, Chinese lettuce, Chinese kale;

* brassica vegetables like cauliflower and broccoli;

* stalk and stem vegetables like asparagus, celery and soybean sprouts;

* bulb vegetables like Chinese chives, garlic bulbs and onions;

* legumes like snow peas and sugar snap peas;

* root and tuber vegetables like carrot, lotus root and potato;

* fruiting vegetables like bitter gourd, cucumber, eggplant, tomato and bell pepper; and,

* mushrooms.

More than 80% of the samples had a nitrate level of less than 2,000mg per kilogram.

There were variations in the mean concentrations of nitrate in different vegetable groups. The mean concentrations in descending order were leafy vegetables (2,100mg/kg), root and tuber vegetables (720mg/kg), and 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.

Nitrite concentration was generally low, with less than 1mg/kg on average.

The centre said the test results match those on the Mainland and abroad.

The centre urged consumers to maintain a balanced diet with two servings of fruit and three servings of vegetables a day, eating a wide variety of vegetables.

Although certain leafy vegetables are high in nitrate they are also rich in many essential nutrients and should be eaten regularly. A variety of low nitrate-containing vegetables, such as fruiting and legume vegetables, should also be included.

It said people should handle and cook vegetables properly, keeping them refrigerated when not cooked immediately. Vegetables should be cooked soon after chopping or mashing, and washed and peeled before cooking. Blanch high-nitrate-containing vegetables and discard the cooking water before consumption.