24th June 2022 – (Hong Kong) The Centre for Food Safety (CFS) of the Food and Environmental Hygiene Department today released the study results on the changes in nitrite contents of cooked vegetables after storage. The study results showed that nitrite was not detected in the cooked vegetable samples after they had been stored in the refrigerator overnight.
A spokesman for the CFS said, “As there have been rumours about the rapid increase of nitrite contents in cooked vegetables that have been stored overnight in the refrigerator, the CFS therefore conducted the study and collected from retail outlets samples of five vegetables (amaranth, pak choi, flowering white cabbage, Chinese lettuce and zucchini) that are commonly consumed. Their nitrite contents were measured before and after cooking. After cooking, each type of cooked vegetable was divided into two groups and stored separately at room temperature and refrigerated temperature (0 to 4 degrees Celsius). Their nitrite levels were measured at 6, 12, 24, 36, 48 and 72 hours after cooking.
The study results showed that the nitrite contents did not increase after the cooked vegetables had been stored in the refrigerator overnight, and the nitrite contents still remained low when the cooked vegetables had been stored in the refrigerator for three days. Storage temperature is the main factor affecting the increase of nitrite contents in cooked vegetables. Refrigerated temperature inactivates bacteria and delays nitrite formation.
The spokesman added, “Fresh vegetables absorb nitrate from the soil for the production of proteins for growth. Nitrate in cooked vegetables can be converted to nitrite by the bacteria in the environment. Nitrite can lower the ability of blood to carry oxygen and may also produce nitrosamines, chemicals that cause cancer in animals, in the body. However, currently available scientific evidence does not support the conclusion that nitrate and nitrite intake from the diet is associated with cancer risk in humans. The World Health Organisation advises five daily servings of vegetables and fruit to promote health and minimise the risk of non-communicable diseases (NCDs). Vegetables provide antioxidants, phytochemicals, vitamins, minerals, dietary fibre and plant proteins. These compounds could act synergistically to prevent or slow down the onset of some chronic NCDs.”