Extrinsic Parameters Influencing Microbial Growth: Environmental Conditions

Every food product exists within an environment, and that environment plays a decisive role in whether microorganisms thrive or struggle to survive. While the internal characteristics of food (such as pH and water activity) get plenty of attention, extrinsic parameters – the external, environmental conditions surrounding the food – are equally critical. These include the temperature of storage, relative humidity, and the concentration of gases in the storage atmosphere. Together, they form the external line of defence (or vulnerability) in food safety management.

What are extrinsic parameters? 🔗

Extrinsic parameters are environmental factors that exist outside the food itself but directly influence how microorganisms behave on and within it. Unlike intrinsic factors, which are inherent properties of the food (such as its pH or nutrient content), extrinsic factors relate to the conditions of the surrounding storage environment. Even minor fluctuations in these external conditions can shift the microbial balance of a food product, turning a safe item into a spoilage risk or a potential health hazard.

The three primary extrinsic parameters are storage temperature, relative humidity, and the gaseous composition of the environment. Food scientists and processors use these three factors – often in combination – to create conditions that slow or halt microbial multiplication.

Temperature of storage 🔗

Temperature is arguably the single most influential extrinsic factor in determining whether microorganisms grow, remain dormant, or die. Every microorganism has a specific temperature range within which it can multiply, and it also has an optimum temperature at which its growth rate peaks. This relationship between temperature and microbial activity is the foundation of virtually all food storage and preservation practices.

Classification of microorganisms by temperature preference 🔗

Based on the temperatures at which they grow best, microorganisms relevant to food can be broadly classified into the following groups:

Psychrophiles and psychrotrophs are cold-loving or cold-tolerant organisms. Psychrotrophs, which are more commonly encountered in food, can grow at refrigeration temperatures (0-7°C) but typically have optimum growth rates between 10°C and 30°C. Organisms in this category include species of Pseudomonas, Listeria, Yersinia, and Aeromonas. This is why refrigerated foods are not immune to spoilage – psychrotrophic bacteria and molds continue to multiply, albeit slowly, even at cold temperatures.

Mesophiles grow best at moderate temperatures, typically between 20°C and 45°C, with an optimum close to 37°C – human body temperature. Most foodborne pathogens, including Salmonella, Staphylococcus aureus, Clostridium perfringens, and Bacillus cereus, are mesophiles. This group represents the greatest food safety concern because their optimal growth range overlaps with common ambient and food handling temperatures.

Thermophiles prefer high temperatures, generally above 45°C, with optimum growth between 50°C and 70°C. They are mostly spore-forming organisms and become particularly important in the canning and thermal processing of foods, where heat-resistant spores may survive if the process is inadequate.

The danger zone and temperature control 🔗

In food safety practice, the temperature range between approximately 5°C and 60°C is commonly referred to as the danger zone. Within this range, pathogenic and spoilage microorganisms can multiply rapidly. The risk is highest between 20°C and 45°C, where mesophilic bacteria can double their population in as little as 20 minutes under favourable conditions.

Proper refrigeration (below 5°C) and hot-holding (above 60°C) are the two most fundamental strategies for keeping food out of this zone. Freezing at temperatures below -18°C effectively halts microbial growth entirely by converting available water into ice and making it unavailable for biological activity.

It is important to note that the effect of temperature on microbial growth does not operate in isolation. As documented by the Michigan State University food safety resource, the growth rate at any given temperature is also influenced by other factors such as pH, water activity, and the presence of nutrients. For instance, Clostridium perfringens grows significantly slower at pH 5.8 compared to pH 7.2 across a wide temperature range.

Relative humidity of the storage environment 🔗

Relative humidity (RH) refers to the amount of moisture present in the air relative to the maximum moisture the air can hold at a given temperature. It directly influences the water activity at the surface of a food product, and therefore plays a significant role in microbial colonisation.

How humidity promotes or inhibits microbial growth 🔗

When food is stored in a high-humidity environment, the moisture in the surrounding air can be absorbed by the food’s surface, raising its surface water activity to a level that supports microbial growth. Conversely, a low-humidity environment draws moisture out of the food, reducing surface water activity and making conditions less favourable for microorganisms.

A practical example: dry grains stored in a warehouse with high relative humidity will absorb moisture from the air and eventually support mould growth, even if the grains were perfectly dry at the time of storage. Similarly, research has demonstrated that higher humidity significantly increases bacterial multiplication on food surfaces such as fresh meat, especially when combined with poor ventilation and warm temperatures.

The relationship between relative humidity and water activity 🔗

There is a direct mathematical link between relative humidity and water activity. According to the U.S. FDA, the equilibrium relative humidity (ERH) expressed as a percentage equals water activity multiplied by 100 (ERH% = a_w × 100). This means a food stored in an environment with 90% relative humidity will eventually reach an equilibrium water activity of 0.90 at its surface – a level that supports the growth of many spoilage organisms and some pathogens.

This is why the packaging material used for food is so important. Foods with low water activity, such as crackers or dried spices, must be packaged in moisture-impervious materials. If exposed to a humid environment, moisture condensation on the product surface can create localised zones with high enough water activity to support mould or even bacterial growth.

Practical humidity control in food storage 🔗

Different food categories require different humidity levels for optimal storage. Fresh fruits and vegetables generally need higher RH (85-95%) to prevent wilting and dehydration, but this same humidity accelerates microbial growth if temperature is not carefully managed. Meat and dairy products do best at moderate humidity levels (75-85%), while dry goods such as grains, flour, and cereals require low humidity (50-60%) to prevent moisture absorption and spoilage.

Effective humidity control in commercial settings involves a combination of ventilation, dehumidification, and appropriate packaging. As one study noted, increased ventilation alongside reduced humidity is one of the most cost-effective ways to control bacterial growth on stored food, with ventilation contributing even more to bacterial reduction than humidity control alone.

Concentration of gases in the storage environment 🔗

The gaseous atmosphere surrounding food is the third major extrinsic parameter. Microorganisms differ in their oxygen requirements, and this variation provides a powerful tool for controlling microbial growth through atmospheric manipulation.

Oxygen requirements of different microorganisms 🔗

Microorganisms fall into several categories based on how they respond to oxygen:

Aerobic microorganisms require oxygen to grow and metabolise. Most common spoilage organisms, such as Pseudomonas species, are strict aerobes. When oxygen is reduced or removed, these organisms cannot multiply.

Anaerobic microorganisms grow in the absence of oxygen and can be inhibited or killed by its presence. Clostridium botulinum, the organism responsible for botulism, is the most well-known and dangerous anaerobic pathogen found in food.

Facultative anaerobes can grow with or without oxygen, shifting their metabolic pathways depending on availability. Many foodborne pathogens, including Salmonella and E. coli, are facultative anaerobes, making them harder to control through atmospheric manipulation alone.

Microaerophiles, such as Campylobacter jejuni, require low concentrations of oxygen – higher or lower levels inhibit their growth.

Modified atmosphere packaging (MAP) 🔗

One of the most widely used technologies for manipulating the gaseous environment around food is modified atmosphere packaging (MAP). MAP involves replacing the normal air inside a food package with a carefully designed mix of gases – typically carbon dioxide (CO₂), nitrogen (N₂), and sometimes controlled levels of oxygen (O₂).

Carbon dioxide (CO₂) is the primary antimicrobial gas used in MAP. When CO₂ dissolves into the moisture on a food’s surface, it forms carbonic acid, which lowers the surface pH and disrupts the cell membranes of many bacteria. It is particularly effective against Gram-negative aerobic spoilage bacteria like Pseudomonas. A minimum concentration of around 20% CO₂ is generally needed to achieve meaningful preservation effects in most products.

Nitrogen (N₂) is an inert gas used mainly as a filler to displace oxygen and prevent package collapse caused by CO₂ absorption into the food. It does not directly inhibit microbial growth but supports the preservation system by maintaining package structure.

Oxygen (O₂) is either reduced (for most products) or kept at high levels in specific cases. For instance, fresh red meat is often packaged with approximately 70-80% O₂ to maintain its bright red colour (through oxygenation of myoglobin), while most other products benefit from reduced oxygen levels.

Safety considerations with controlled atmospheres 🔗

While MAP can significantly extend shelf life, it also comes with risks. By suppressing aerobic spoilage organisms, MAP can mask the usual signs of spoilage – such as off-odours or slime – while allowing anaerobic or facultative anaerobic pathogens to grow undetected. Organisms like Clostridium botulinum, Listeria monocytogenes, and Salmonella can potentially survive or even thrive in low-oxygen environments.

For this reason, MAP is never used as a standalone preservation method. It must always be combined with other control measures – particularly temperature control (maintaining temperatures below 3°C), low water activity, or appropriate pH levels. This multi-barrier approach is known as hurdle technology, and it remains the gold standard for ensuring the safety of MAP products.

How extrinsic parameters interact with each other 🔗

In real-world food storage and processing, extrinsic parameters rarely act alone. Temperature, relative humidity, and gaseous atmosphere work together – and also interact with intrinsic food properties – to create a combined environment that either supports or suppresses microbial growth.

For example, a food product stored at low temperature (4°C) in a low-humidity environment with a CO₂-enriched atmosphere faces three simultaneous barriers to microbial growth. Each barrier alone might not be sufficient to prevent spoilage or pathogen growth, but their combined effect can be highly effective. This concept of stacking multiple partial barriers is the principle behind hurdle technology, widely adopted in modern food preservation.

Conversely, a failure in any one of these parameters can compromise the others. Temperature abuse during distribution – even briefly – can accelerate microbial growth despite a protective gas atmosphere. Similarly, a breach in packaging integrity can undo the entire benefit of modified atmosphere packaging by allowing outside air to re-enter.

Practical applications in the food industry 🔗

Understanding extrinsic parameters has direct implications for food handling, packaging, distribution, and retail display. Some key applications include:

Cold chain management: Maintaining unbroken refrigeration from production to consumer purchase is the single most important application of temperature control. Even brief temperature excursions into the danger zone can dramatically shorten shelf life and increase food safety risks.

Packaging design: Packaging materials are chosen not just for physical protection but for their ability to control moisture transfer and gas permeability. The oxygen transmission rate (OTR) of a packaging film must be carefully matched to the product’s needs – too permeable and the modified atmosphere is lost; too impermeable and CO₂ can build up excessively or the product may become anaerobic.

Warehouse and retail storage: Commercial storage facilities actively monitor and control temperature and humidity. Dry goods warehouses aim for low RH and stable temperatures, while fresh produce storage rooms balance high humidity (to prevent wilting) with adequate air circulation to prevent moisture accumulation on product surfaces.

HACCP integration: Extrinsic parameter control is a core component of Hazard Analysis and Critical Control Points (HACCP) systems used across the food industry. Monitoring storage temperatures, verifying package integrity, and validating MAP gas concentrations are common critical control points in HACCP plans.

Key takeaways 🔗

Extrinsic parameters – temperature, relative humidity, and gaseous atmosphere – are powerful tools in controlling microbial growth in food. Temperature determines the speed at which organisms multiply, humidity affects surface moisture available for microbial colonisation, and the gas composition of the storage environment determines which types of organisms can survive and proliferate. None of these parameters works in isolation; their effectiveness depends on how well they are managed together and in combination with the intrinsic properties of the food itself.

For anyone involved in food production, storage, or handling, a solid grasp of these environmental factors is essential – not just for extending shelf life, but for preventing foodborne illness.

What do you think? Which extrinsic parameter do you believe is the hardest to control consistently across a food supply chain – and what innovations might help solve that challenge?

References

1. https://link.springer.com/chapter/10.1007/978-3-030-42660-6_1
2. https://www.sciencedirect.com/topics/food-science/microbial-growth-in-food
3. https://microbenotes.com/factors-affecting-the-growth-of-microorganisms-in-food/
4. https://www.canr.msu.edu/smprv/uploads/files/Safe_Practices_for_Food_Processes_Chpt._3_Factors_that_Influence_Microbial_Growth.pdf
5. https://pmc.ncbi.nlm.nih.gov/articles/PMC9691097/
6. https://www.fda.gov/inspections-compliance-enforcement-and-criminal-investigations/inspection-technical-guides/water-activity-aw-foods
7. https://pmc.ncbi.nlm.nih.gov/articles/PMC7089433/
8. https://en.wikipedia.org/wiki/Modified_atmosphere