Fact sheet: aerobic mesophilic count

General information
The aerobic mesophilic count is often also referred to as the "total viable count". It provides information about the number of microorganisms (bacteria, yeasts and molds) that multiply optimally under aerobic conditions within a temperature range between +30 °C and +40 °C.

• More specific information on important microorganisms in foods: see other fact sheets

Origin / Occurrence

• Varies depending on the microorganism: e.g. raw materials, humans, animals, plants, soil, water, air as well as work surfaces and equipment

Significance

• Bacteria are present on almost all foods (exception e.g. sterilized foods such as canned goods)
• Aerobic mesophilic organisms are normal and unavoidable up to a certain level depending on the food
• For raw foods or ingredients of plant or animal origin, the aerobic mesophilic count can be high due to the natural microbial flora. This must be taken into account when assessing test results
• If live microorganisms are intentionally used in foods, the total viable count will be correspondingly high and CANNOT be evaluated as a hygiene or spoilage parameter. This must be taken into account when assessing test results. Examples include yogurt, quark, some cheeses and raw sausages
• Exceedances of assessment criteria can have different causes depending on the food as well as the production and storage conditions

Growth conditions

• Temperature:
  • Optimum: between +30 °C and +40 °C (mesophilic)
  • Minimum: usually +5 to +10 °C (depending on the organism, exceptions even down to 0 °C)
  • In general no growth above +50 °C (exceptions are thermophilic bacteria)
• pH: Generally growth occurs at pH 5.0 to 9.0 (depending on the organism, some can also grow at considerably lower pH values)
• a_w (water activity): most bacteria grow at medium to high water activity of at least 0.88 (in rare cases also at significantly lower a_w values)
• Oxygen requirement: many bacteria can grow with and without oxygen (facultative anaerobes). However, there are also bacteria that can only grow under aerobic or anaerobic conditions.

Inactivation by heating

• Most microorganisms are inactivated at +72 °C for at least 2 minutes (Note: monitor core temperature)
• Thermoresistant bacteria as well as bacterial and fungal spores are more heat resistant (relevant spore-formers include e.g. Bacillus cereus, Clostridium perfringens and Clostridium botulinum)

1. Check the implementation of cleaning and disinfection:
   1. Hygiene plan
   2. Execution of cleaning and disinfection, e.g. dosing of disinfectant, contact time, use of suitable and clean materials and cleaning cloths
2. Avoid moisture residues and biofilms. Ensure thorough drying of equipment, tools and surfaces
3. Train staff regarding potential errors and improvement measures
4. Repeat cleaning and disinfection
5. Verify effectiveness by visual on-site inspections, environmental testing and follow-up analyses

1. Review work organization in the facility
   1. Separate work areas, equipment and utensils for raw and processed foods
   2. Separation of "clean" and "unclean" areas and activities
2. Train staff regarding potential errors and improvement measures
3. Verify effectiveness by visual on-site inspections and follow-up analyses

Errors in personnel hygiene: 
This can lead to transfer of (hazardous) microorganisms to, for example, product-contact utensils or foods.
Possible errors include inadequate hand hygiene or glove changes, deficiencies in personal hygiene, contaminated work clothing or improper behavior when ill (deficiencies in infection control)

1. Train staff regarding potential errors and improvement measures
2. Regular cleaning and disinfection of hands and/or regular changing of gloves
3. Regular change of work clothing. Ensure work clothing is clean
4. Train employees on infection control requirements
5. Verify effectiveness by visual on-site inspections and follow-up analyses

1. Ensure traceability of the cold chain during transport or request documentation from the supplier
2. Check incoming and storage temperatures
3. Monitor cooling temperatures/cold chain during processing, transport and sale
4. If over-aging (or excessive shelf-life) is suspected, perform appropriate storage tests
5. Verify effectiveness by follow-up analyses

Errors in heating, hot-holding or cooling: 
The result is insufficient inactivation of microorganisms due to inadequate heating or strong microbial growth due to too low hot-holding temperatures combined with prolonged cooling phases

1. Check processes, production protocols and documentation
2. Inactivation of most bacteria at a core temperature of +72 °C for at least 2 minutes. Exceptions include heat-stable spores and toxins. In these cases significantly higher temperatures and exposure times are required. Further information can be found in the respective information sheets (fact sheets)
3. Verify correctness of processes and temperatures
   1. In general, hot-holding temperatures above +60 °C are required
   2. Rapid cooling: after 3 hours a core temperature of at least +7 °C must be reached (special caution is required for large quantities)
4. Verify effectiveness by follow-up analyses

1. Check products on receipt for e.g. incoming/transport temperature, shelf-life, packaging, deviations or abnormalities
2. This also applies to goods before use in production. In case of doubt, do not use the food and consult supervisors
3. Review microbiological specifications. If necessary, arrange laboratory testing of suspicious products or request results from the supplier
4. Verify effectiveness by follow-up analyses

1. Check processes, environmental conditions, production protocols and documentation
2. Verify effectiveness by follow-up analyses