Introduction

  1. The allegation of an increasing amount of nitrite in refrigerated cooked vegetables has been circulating intermittently among the media and social media platforms.
  2. Nitrate is naturally present in the environment (e.g. soil and water) and plants (e.g. vegetables). It is also used as a fertiliser. Nitrate and nitrite compounds can be used as food preservatives in cheese products and cured meat.
  3. Nitrate levels vary significantly in vegetables. In general, leafy vegetables have higher nitrate contents than root vegetables (e.g. carrot and potato), bulb vegetables (e.g. onion), fruiting vegetables (e.g. cucumber and tomato) or legume vegetables (e.g. snow pea).
  4. A study by the Centre for Food Safety has revealed that among the leafy vegetables, amaranth (莧菜), green Chinese cabbage (小棠菜), Pak-choi (白菜), and spinach (菠菜) have higher levels of nitrate. Beetroot (紅菜頭), a root vegetable, also has a high level of nitrate.
  5. Nitrate is relatively non‐toxic by itself, but its metabolite, nitrite can lower the ability of blood to carry oxygen in humans.
  6. Nitrate can be converted to nitrite by:
    • an enzyme (known as nitrate reductase) naturally present in vegetables; or
    • bacteria.
  7. In the cells of fresh vegetables, the enzyme and nitrate are kept separate from each other in different positions. The levels of nitrate are relatively high while the nitrite contents are low. When fresh vegetables become senescent or are damaged (e.g. pureed or mashed), the enzyme is released and is able to convert nitrate to nitrite.
  8. When vegetables are cooked, the enzyme is denatured and is no longer able to convert nitrate to nitrite. However, bacteria, present in the air and utensils, may contaminate cooked vegetables, converting nitrate to nitrite.
  9. Processing and cooking can affect nitrate and nitrite contents in vegetables:
    • Washing and peeling can reduce the nitrate contents in vegetables;
    • Pureeing fresh vegetables facilitates the conversion of nitrate to nitrite;
    • Boiling allows passing of some nitrate from the vegetables to the surrounding water, reducing the nitrate contents of vegetables.
  10. Storage temperature has a significant effect on the nitrate and nitrite contents of cooked vegetables because low temperature reduces the growth of bacteria and their activities of converting nitrate to nitrite, delaying the formation of nitrite.
    • At frozen temperature (-18℃), the conversion of nitrate to nitrite stops.
    • At refrigerated temperature (0-4℃), the conversion rate of nitrate to nitrite is low.
  11. When cooked vegetables are stored overnight (i.e. 24 hours) in a refrigerator, their nitrite contents do not increase.

Safety and Public Health Significance

  1. Human exposure to nitrate is mainly through the consumption of vegetables while exposure to nitrite is mainly from the conversion of nitrate in the body.
  2. In the body, nitrite can oxidise haemoglobin in red blood cells to methaemoglobin that is unable to bind and transport oxygen to body tissues. Excessive methaemoglobin results in methaemoglobinaemia. Symptoms of methaemoglobinaemia include shortness of breath, cyanosis, headache, fatigue, dizziness, etc. Nitrite-induced methaemoglobinaemia is rarely seen in healthy individuals.
  3. Nitrite may also react with certain amines and amides in the body and produces nitrosamines that cause cancer in animals. In 2010, the International Agency for Research on Cancer (IARC) of the World Health Organization (WHO) concluded that ingested nitrate or nitrite under conditions that result in endogenous nitrosation is probably carcinogenic to humans. However, available evidence does not support the conclusion that nitrate and nitrite intake from the diet is associated with increased cancer risk.
  4. Studies have shown that the beneficial effects of eating vegetables and fruit outweigh the potential risks to human health from exposure to nitrate through vegetables. Vegetables and fruit are important components of a healthy and balanced diet, and, if consumed daily in sufficient amounts, could prevent certain chronic diseases such as heart diseases, hypertension, cerebrovascular diseases, diabetes and certain cancers.

Tips to keep leftovers (including cooked vegetables)

  1. Prepare food in suitable amounts to reduce the amount of leftovers.
  2. Refrigerate leftover food within 2 hours after cooking and eat them as soon as possible. To ensure food safety, the World Health Organization and some relevant authorities recommend that leftovers should not be stored in the refrigerator for longer than 3 days.
  3. Thoroughly reheat leftovers until the core temperature reaches 75℃ before eating. If leftovers cannot be eaten in one go, only take out an appropriate amount of the leftovers from the refrigerator for reheating. Leftovers should not be reheated more than once.
  4. Do not consume cooked food if they have been held at room temperature for more than 4 hours.

Risk Assessment Section
Centre for Food Safety
June 2022