Food Safety Focus (223rd Issue, February 2025) – Article 2
Consuming Vegetables – Maximising the Benefits while Minimising the Risks
Reported by Arthur YAU, Scientific Officer,
Risk Communication Section, Centre for Food Safety
A balanced diet should include vegetables, as they are a great source of fibre and essential nutrients, including vitamins and minerals. People are encouraged to eat more fruits and vegetables, which helps to lower mortality rates and protects against non-communicable diseases such as heart disease, stroke, colorectal cancer, type 2 diabetes and obesity. The World Health Organization (WHO) recommends that adults consume at least 400 grams of fruits and vegetables daily. Locally, the Department of Health gives similar advice. To maximise the benefits of vegetable consumption, the public is urged to follow the health tips below. We will also take this opportunity to dispel misconceptions about preparing and storing vegetables.
Figure: Salad and stir-fried vegetables
Are Salads Suitable for Everyone?
Vegetable salads are popular among health-conscious individuals since they are easy to prepare and can be enjoyed chilled. Nevertheless, salads are considered high-risk food as they are composed of a variety of raw ingredients without cooking and are ready to eat. While some nutrients (e.g. water soluble nutrients like vitamin C) in raw vegetables are preserved better by not cooking them, this also means that any pathogens present remain viable. Additionally, the extended period of chilled storage for pre-packaged salads especially increases the risk of Listeria monocytogenes proliferation if contamination occurs. Therefore, susceptible individuals — such as pregnant women, the elderly and those with weakened immune systems — should avoid salads and uncooked vegetables. The public can refer to the CFS guidelines on salads for preparing salads safely.
Vegetables: Effects of Cooking and Better Ways to Cook them?
While boiling vegetables is known to cause a considerable loss of water-soluble nutrients (e.g., vitamin C and folates) to the water, there are still some ways to minimise such loss. Indeed, the nutrient content of vegetables can be influenced by a variety of factors, including the types of vegetables, the nutrients involved, and the methods of preparation and cooking. Research shows that the shorter the cooking time and the less cooking liquid used, the more water-soluble nutrients are retained in vegetables. For instance, stir-frying vegetables after blanching or steaming retains water-soluble vitamins better than other cooking methods, while also reducing the formation of the process contaminant acrylamide. On the other hand, some studies suggest that cooking can lead to either a small loss or even an increase in fat-soluble nutrients (e.g., beta-carotene and vitamin K) compared with uncooked vegetables, as cooking may make certain fat-soluble nutrients that are not readily absorbed and utilised by body become bioavailable.
Besides, cooking can eliminate certain natural toxins present in certain vegetables. For example, boiling can remove cyanide-containing compounds in bamboo shoots, and cooking with moist heat after soaking overnight can reduce the levels of phytohaemagglutinin (a type of lectin) in kidney beans to a safe level. By removing phytohaemagglutinin, the interference with the absorption of minerals such as calcium, iron, phosphorus and zinc is also reduced. Lastly, cooking can effectively destroy pathogens that may come from the natural environment or cross-contamination (e.g., Salmonella, E. coli).
Is it Safe to Bring Cooked Vegetables for Lunch the Next Day?
Nowadays, some people discard leftover vegetables from their meals because they worry about rising nitrite levels overnight due to enzyme or bacterial activity. However, an earlier CFS study's findings demonstrated that nitrite levels of cooked vegetables do not increase after storing overnight (i.e., for 24 hours) in a refrigerator, as the conversion rate of nitrate to nitrite is low at temperatures between 0 and 4°C. To minimise nitrite levels in vegetables, handle them properly (e.g., store vegetables in the refrigerator, wash or peel before cooking, and cook soon after chopping or mashing). Indeed, the European Food Safety Authority has stated that the benefits of eating fruits and vegetables outweigh any potential health risks from nitrate exposure through these foods. If you plan to store cooked food overnight (e.g., for lunchboxes), it is advisable only pack the amount you intend to store right after cooking to prevent bacterial contamination. Reheat thoroughly before consumption.
Conclusion
Understanding how to prepare and handle vegetables is vital for maximizing health benefits of vegetable consumption. Susceptible individuals should avoid high-risk foods like raw vegetables, while others should prepare and store vegetables properly before consumption. With proper knowledge, consumers can reap the benefits of vegetable consumption while minimizing health risks.
