References: The perfect microbiological match
Andy Muirhead, ALS Microbiology, explains how to choose the most appropriate microbiological specification for your specific product.
1. The rationale for food micro testing can be split into three main objectives:
- Testing for pathogens
- Testing for spoilage organisms
- Testing for organisms which are trending markers
Here is a list of the most commonly requested organisms when testing for pathogens, spoilage and indicator tests:
- Salmonella – A natural gut inhabitant of many of the animals used in food production. Has the ability to persist in low water activity, low pH environments so tends to be included in most specifications, even foods that are going to be heat treated before consumption
- STEC E. coli – The Shiga Toxin coli tests (a set of six tests performed by PCR) can only be done by a few specialist labs. The test for the most common organism in this group is E. coli 0157. Relevant to test in raw meat (especially beef, venison, boar), but there have been outbreaks associated with salad products and unpasteurised cheese
- Listeria monocytogenes – Its relevance in raw meat is questionable as we are likely to find it, so it tends only to be included in specifications for Ready to Eat (RTE) products. The test will also detect the non-pathogenic members of the group
- Campylobacter – Will be present in raw poultry but can still be monitored to assess prevalence. Is easily killed by cooking, so not usually included in the specifications of cooked products unless there are concerns regarding potential cross contamination
- Vibrio parahaemolyticus – This salt tolerant organism can be found in shellfish so is included in specifications for these types of products
- Coagulase Positive Staphylococci – These organisms produce a heat stable toxin when growing on the food. The organisms are carried on the skin of humans and animals, so this test is included in many specifications for different food matrices – especially anything where the manufacturing process involves manual handling.
- Bacillus cereus – This spore-producing organism is common in dried products such as cereals and rice and is usually included in the specifications for these types of products
- Clostridium perfringens – This spore-forming anaerobic organism is present in the intestines of humans and animals. It is therefore relevant to test for this bacterium in any foods containing raw meats, especially if they are packed under modified atmospheres or vac packed. Even if the samples are not vac packed, they may still be present in high numbers if the manufacturing process facilitates anaerobic conditions. For example, large volumes leading to poor oxygen penetration.
- Yeasts and moulds – These two common spoilage organisms are both tested for by a single test, so they nearly always appear on specifications together. Typically, they grow in products rich in carbohydrates and they can grow in low water activity and acidic environments. Spoilage by yeasts and moulds is usually seen more in longer shelf life products, so testing for these organisms in short shelf life refrigerated products is not always necessary
- Pseudomonas spp – This group of organisms is responsible for organoleptic changes to a range of products from raw meats (especially fish and poultry) and fresh produce. It is an obligate aerobe so does not always grow well in the TVC test which uses the pour plate technique
- Lactic Acid Bacteria (LAB) – As the name suggest, this group of bacteria can grow in acidic environments so tend to cause spoilage in fruit juices and marinades.
- Total Viable Count (TVC) – Nearly all specifications will include the TVC test as it often gives useful information on the production process and the overall quality of the food. However, not all of the organisms which may be included in a specification will be able to grow in this pour plate technique and in the temperature and incubation time carried out in the test. Some food matrices (many types of fresh produce) have naturally high TVC counts so the test is not always appropriate, and it should never be carried out on fermented products (for example cheese, yoghurt, salami) as it will pick up any surviving starter culture fermenting organisms
- E. coli, Coliforms, Enterobacteriaceae – The vast majority of coli are not pathogenic, so can be included in this group of trending organisms. Coliforms fall within the Enterobacteriaceae group, so testing for Enterobacteriaceae casts the net wider. Therefore, this test is widely used as a trending marker in the food industry. However, most Coliform bacteria ferment lactose, meaning this test is still commonly used for dairy products. Like with the TVC test, some food matrices (particularly salads) will naturally contain high levels of Enterobacteriaceae so for these products E. coli tends to be applied as the trending tool
- Listeria spp can also be used as an indicator of environmental hygiene as species other than Listeria monocytogenes can be found in harbourage points in certain manufacturing environments and can indicate issues with hygiene and environmental control.
2. The EU 2073/2005 regulation on the microbiological criteria for foodstuffs outlines the legislative requirements, and there are helpful publications such as the IFST Microbiological Criteria for Foods and PHE (HPA) Guidelines for ready to eat foods, but in general…
- Pathogens – For Salmonella, STEC, Vibrio, Campylobacter. Criteria is always absence or ‘not detected’ usually after testing 25g of sample. The criteria for monocytogenes are slightly more complex as they depend on whether the organism is capable of growing in the food. If the manufacturer has evidence that Listeria cannot grow, then the ‘satisfactory’ criteria is <100cfu/g
- Coagulase Positive Staphylococci – levels of >10,000 in RTE foods are considered unacceptable, with a target level of <20cfu/g generally regarded as satisfactory
- Bacillus cereus – levels of >100,000 in RTE foods are considered unacceptable, with a target level of <1,000cfu/g generally regarded as satisfactory
- Clostridium perfringens – levels of >10,000 in RTE foods are considered unacceptable, with a target level of <10cfu/g generally regarded as satisfactory.
3. The following is a typical specification, including the most commonly requested tests:
- TVC – Remember not always applicable, especially for fermented products and fresh produce as levels will be naturally high
- Enterobacteriaceae – Not suitable for fresh produce as levels may be naturally high
- coli – Included in most specifications. A reliable indicator of faecal contamination
- STEC or E. coli 0157 – Raw meats (beef/venison), fresh produce
- Coagulase Positive Staphylococci – Especially when products have been handled
- B cereus – Particularly, if there is cereal or rice in the product
- Cl perfringens – Specifically if the product is of meat origin and is vac packed or in MAP
- Salmonella – Included in most specifications
- Listeria monocytogenes – Only for Ready to Eat products
- Yeasts & Moulds – For low water activity, low pH foods. Not always applicable for proteinaceous/ short shelf life foods
- Lactic Acid Bacteria – Can be included in acidic foods and long shelf life dairy products
- Pseudomonas – Often included in raw poultry/fish specifications to check for potential spoilage
- Campylobacter – Used in raw poultry and sometimes cooked (to check for cross contamination) poultry specifications
- Vibrio parahaemolyticus – Included in specifications for fish and marine shellfish.