Know thy enemy - microbiological hazards in RTE foods - Part 1

By Rika Le Roux Kemp on 03 February 2017

Ready to Eat (RTE) food can be defined as food products which are in a form that is edible without additional preparation to achieve food safety. The final product may receive additional preparation for palatability or aesthetic, epicurean, gastronomic or culinary purposes but NOT to make it safe to eat. It thus calls for focussed control during the production/processing of such products to maintain the required level of food safety to protect the end consumer. Rika highlights the need for this extreme caution in part 1 of a three part series on ready to eat foods.

Microbiological hazards of ready-to-eat products and meals

RTE foods are just that – ready to eat - there is no further processing by the consumer as there would be in the case of ready to cook products. Thus the major concern is the microbiological hazards of RTE foods. It is essential to ensure these hazards are ruled out.

So which micro-organisms are of importance? Please note that information on food poisoning incidence occurrence in South Africa and the possible causes thereof are not available from reliable sources. We thus  refer to statistics available in the United Kingdom and European Union.


Microorganisms that should not be detected in RTE foods

(You don’t want to find these in your products) 

  1. Campylobacter species: This is the most common cause of bacterial gastrointestinal infections in the UK.  The consumption of low numbers of Campylobacter in food is sufficient to cause infection. Campylobacter species are unable to grow in food, they are killed by heat and a reduction in numbers has also been observed following freezing of contaminated foodstuffs. Cross-contamination of ready-to-eat foods in the food preparation environment is a common route of transmission.
  2. Escherichia coli O157 and other verocytotoxin-producing coli (VTEC) – not your common garden variety E.coli: The most important Escherichia coli from a food safety perspective are the verocytotoxin-producing E. coli (VTEC). It is estimated that VTEC infection is the cause of approximately 70% of the cases of renal failure in children. Most infections in the United Kingdom are due to a single serotype of VTEC, i.e. O157, but other serotypes have been associated with sporadic cases of illness or outbreaks of foodborne disease. In continental Europe and Australia, infections from a broader range of VTEC serotypes are reported. The other serotypes of VTEC that have been associated most frequently with disease in humans include O26, O103, O111 and O145.
  3. Salmonella species: Salmonella infection is caused by ingestion of viable bacteria. The infectious dose for Salmonella species is usually quite large. However, data from outbreaks has shown that consumption of low numbers of Salmonella in food together with the mode of delivery of the bacterium to the gastrointestinal tract can cause infection and this is particularly evident with high fat / low water activity foods, such as chocolate, fermented meats, cheese and snacks, in which the organism can survive for long periods of time. Application of good hygiene and temperature and time control during food preparation is also important to prevent cross-contamination and multiplication in foods or ingredients that are able to support its growth.
  4. Shigella species: Shigellosis is caused by ingestion of viable bacteria and in contrast to the most common foodborne pathogenic agents, shigellosis is exclusively a human disease. The majority of cases in the UK are acquired as a result of person-to-person spread and occasionally by eating food contaminated by infected food handlers, or through the consumption of vegetable or fruit crops irrigated with untreated water or contaminated by infected crop workers.
  1. Vibrio cholera: Cholera is caused by the ingestion of viable Vibrio cholerae.  Cholera is an extremely virulent disease that affects both children and adults. Case fatality rates of 1-10% have been reported. Foodborne transmission occurs through consumption of crops cultivated in, or irrigated by untreated water: through washing or handling foods which receive no further processing; or by the consumption of raw or undercooked seafood.  


Microorganisms that present a low risk when detected but growth thereof during shelf life can increase the hazard:


  1. Bacillus cereus: Large numbers of Bacillus cereus are needed to cause illness. It is particularly associated with farinaceous products such as rice and pasta dishes as well as meat products, soups, vegetables, puddings and sauces. The Bacillus cereus  spores may survive the cooking process; hence people are frequently exposed to low numbers of cereus through food without becoming ill. Minimum growth temperatures for B. cereus vary between 4°C and 12°C with an upper limit of around 50°C although some psychrotrophic strains occur.
  2. Bacillus species (other pathogenic Bacillus): Symptoms are similar to those from cereus and illness follows the consumption of a wide variety of poorly stored cooked foods containing large numbers of Bacillus (105 to 109 cfu/g or more) and includes food prepared from poultry, meat, vegetables, and farinaceous products such as rice and bread.  Spices and spice products (which might be added as garnish or seasoning) such as pepper and curry paste often carry a significantly high load of Bacillus species, usually in the spore form. If high levels of Bacillus spp. are found in RTE foods, the possibility that spices such as pepper have been added after the main cooking process, for example to the egg mayonnaise or mashed potato topping, should be investigated.
  3. Clostridium perfringens: Clostridium perfringens is found in the gut and thus indicates faecal contamination although spores commonly occur in the environment. Control is achieved by preventing spore germination and growth in food and rapid cooling (refer to the requirements of SABS10156), adequate cold storage and adequate reheating of food are of paramount importance. perfringens will grow between 15°C and 52°C with virtually no growth below 12°C. C. perfringens in a RTE food is unsatisfactory and indicates poor processing, particularly during cooling.
  4. Listeria monocytogenes: Foodborne listeriosis is a relatively rare but serious disease with high fatality rates (20–30%) compared with other foodborne microbial pathogens, such as Salmonella. The disease largely affects specific segments of the population who have increased susceptibilities. Basically, monocytogenes is an opportunistic pathogen that most often affects those with a severe underlying disease or condition (e.g. immunosuppression, HIV/AIDS, chronic conditions such as cirrhosis that impair the immune system); pregnant women; unborn or newly delivered infants; and the elderly.  L. monocytogenes is widely dispersed in the environment and foods. However, it was not until several large, common-source outbreaks of listeriosis occurred in North America and Europe during the 1980s that the significance of foods as the primary route of transmission for human exposure to L. monocytogenes was recognized. An important factor in foodborne listeriosis is that the pathogen can grow to significant numbers at refrigeration temperatures when given sufficient time. Despite the fact that a wide variety of foods may be contaminated with L. monocytogenes, outbreaks and sporadic cases of listeriosis are predominately associated with RTE foods.   
    L. monocytogenes must be absent in ready to eat food intended for consumption by infants or for special medical purposes. RTE foods must not exceed the limit of 100 cfu/g for L. monocytogenes at any point during their shelf life.
  5. Staphylococcus aureus and other coagulase-positive staphylococci: Illness due to Staphylococcus aureus is caused by enterotoxins which are preformed in food. Only some aureus contain enterotoxin genes and therefore have the potential to cause food poisoning. Adequate cooking will kill the bacterium, however some protection is afforded in dry, high-fat and high-salt foods. Staphylococcal enterotoxins are heat-stable and can survive some normal cooking processes including boiling, hence active toxin can be present in the absence of viable organisms. Most coagulase-positive staphylococci grow between 7°C and 48°C with no growth at refrigeration temperatures. Many people carry S. aureus and contamination of foods after processing by food handlers can occur. Toxin production starts at 10°C and storage of foods below this temperature should prevent its development. In foods such as ripened cheeses and fermented meat products, S. aureus levels are highest 2–3 days after initial production and may reduce significantly during storage.
  6. Vibrio parahaemolyticus: Vibrio parahaemolyticus is a marine bacterium found in coastal and estuarine waters. It is a rare cause of illness in the UK and is most frequently associated with the ingestion of live Vibrio parahaemolyticus in uncooked seafood or ingestion of foods cross-contaminated with seafood. Growth has been reported between 14°C and 40°C and therefore does not occur in seafood stored at proper refrigeration temperatures. It is interesting to note that freezing does not destroy the organism, but it may be killed by most heat treatments.


In the next article we look at HOW to control these hazards practically.


Need to know what is growing in your products? Check out our laboratory service providers in our supplier directory