Food Microbiology 1

Food Microbiology

Microbiological Food Safety of Raw Vegetables Intended for Human Consumption


1. For the purpose of this brief, raw vegetables intended for human consumption refer to those vegetables which have not been cooked and will not undergo cooking before consumption. In Hong Kong such vegetables are mainly available in two forms: (1) fresh vegetables (e.g. used as ingredients of salads or coleslaw); and (2) preserved vegetables (e.g. fermented or acidified vegetables).

2. As a major source of vitamins, minerals and dietary fibres, vegetables are considered to be an important food group in maintaining good health. Together with the general view that “fresh” or “natural” is good to health, consumers are buying and eating more and more raw vegetables (e.g. salads, coleslaw, etc.). However, it has also been reported that outbreaks of infectious diseases associated with the consumption of raw vegetables had become more frequent in many developed countries over the last few years.

3. Many vegetables grow low to the ground where they are likely to come in contact with the soil. If the soil is contaminated with improperly treated animal manure as fertilizer or irrigated with contaminated waters, vegetables are also likely to be contaminated. Another cause of contamination may be attributed to the unhygienic handling practices by farm and food factory workers.

4. Consumption of raw vegetables contaminated with harmful microorganisms may result in food poisoning due to the fact that there is no kill step such as heating during preparation that would inactivate the harmful microorganisms.

Food Safety and Public Health Significance

1. Intact/cut/sliced/skinned/shredded fresh produces and sprouts have been reported causing food poisoning outbreaks in many countries during the last decade. Fresh vegetables may harbour a variety of bacterial pathogens (e.g. Salmonella and Listeria monocytogenes), enteric viruses (e.g. Norovirus and Hepatitis A virus) and parasites (e.g. Giardia lamblia and Cryptosporidium parvum).

2. As regards fermented/pickled vegetables such as kimchi, if properly prepared, the bacteriological safety of the food is usually guaranteed through the production of chemicals such as organic acids and ethanol during fermentation, or the use of vinegar, sugar and/or salt during pickling. However, fermented/pickled vegetables using raw ingredients that have been contaminated with infective parasite stages have the potential to cause human infection because fermentation or salting (or acidifying) alone may be insufficient to kill the parasites, although human infections following the ingestion of fermented/pickled vegetables are sparsely reported.

3. Although the proportion of outbreaks of gastrointestinal illness caused by contaminated meat and poultry products remains greater, consumption of raw vegetables is also a cause for concern in relation to public health significance because raw vegetables may be contaminated with a variety of pathogenic microorganisms. Consumption of raw vegetables may result in food poisoning because the harmful microorganisms may remain viable at the moment of consumption due to the absence of a cooking step.

Advice to the Public

1. Susceptible groups (including pregnant women, children, the elderly and people with low immunity) are advised not to eat pre-prepared or pre-packaged salads in general. If salad is wanted, they are advised to prepare their own salad and consume it as soon as possible.

2. For healthy persons who want to consume raw vegetables should take the following precautionary steps to reduce the risk of foodborne illness:

  • Purchase
    • At retail stores, purchase fresh vegetables that are not bruised or damaged.
    • If buying pre-packaged fresh-cut ready-to-eat vegetables (e.g. ingredients for salads), examine the package carefully for any sign of spoilage, check the expiry date and make sure they are refrigerated.
    • Buy only enough vegetables for immediate use or consumption.
    • When bringing fresh vegetables (especially the ready-to-eat items) home, make sure they do not come into contact with unprocessed food items such as meats and their juices to avoid cross-contamination.
  • Washing and preparation
    • Before and after handling fresh vegetables, always wash hands thoroughly with soap. Also, make sure preparation areas are sanitary
    • Discard any rotten vegetables and cut away any bruised or damaged areas. Clean the knife after cutting the damaged areas.
    • Discard the outer leaves of leafy vegetables such as lettuce and cabbage before washing. Wash thoroughly with running water. Use of soaps, special detergents or produce washes is not recommended.
    • Scrub firm produce, such as cucumbers, with a clean produce brush. Improperly washed fresh vegetables can become contaminated during cutting.
    • Use one clean cutting board for ready-to-eat fresh vegetables and a separate one for raw meat, poultry, and seafood. Cutting boards, dishes, utensils, and counter tops should be cleaned and sanitised before and after cutting.
  • Storage
    • Immediately refrigerate any fresh-cut items such as salad for best quality and food safety.
    • Freshly cut, ready-to-eat vegetables should be wrapped or covered and stored above raw meat, poultry or fish to avoid cross contamination.
  • Consumption
    • Take only enough raw vegetables such as salads from the refrigerator for immediate consumption.
    • Prepared raw vegetables should be consumed within 1 to 2 days.
    • Leftover cut vegetables should be discarded if left at room temperature for more than four hours.


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