Factors affecting the growth of microorganisms in food

Introduction

Foods of plant and animal origin play an important role in microbial growth. The ability of microorganisms to grow or multiply in such food depends on the food environment. The intrinsic, extrinsic factors, implicit factors of food, and various food processing methods respectively play a role in microbial growth. These factors determine the microbial growth in food and the specific metabolic pathways they use to generate energy and metabolic by-products.

A. Intrinsic factors

The internal self-parameters of plant and animal tissues are referred to as intrinsic factors. These factors include:

1. pH

In general, molds and yeasts can grow at lower pH compared to bacteria and Gram-negative bacteria are more sensitive to low pH than Gram-positive bacteria. The pH range of growth for molds is 1.5 to 9.0; for yeasts 2.0 to 8.5; for Gram-positive bacteria 4.0 to 8.5; and for Gram-negative bacteria 4.5 to 9.0. Based on the pH ranges, microorganisms can be grouped as:

1. Neutrophiles grow best at a pH range of 5 to 8.   
2. Acidophiles grow best at a pH below 5.5.
3. Alkaliphiles grow best at a pH above 8.5.

Based on pH, foods can be grouped as

2. Water activity (aw)

Water activity (aw) is the amount of water available for biological functions which can be reduced by an osmotic effect. Water in food is made available in various ways such as

1. Solutes and ions tie up water in the solution
2. Hydrophilic colloids
3. The water of crystallization or hydration

Microorganisms need water in an available form to grow in the food. The water activity of food can be expressed by the ratio of the water vapor pressure of the food to that of pure water at the same temperature. It ranges is >0 to <1, because no food can have a water activity of either 0 or 1.

Various foods and their water activities are:

• The bacteria require a higher amount of water activity for growth compared to fungi. The bacteria do not grow below the level of 0.91 and the molds can grow as low as 0.80.
• The gram-negative bacteria are more sensitive to low water activity than gram positives. 

Based on water activity range, microorganisms can be grouped as

1. Halotolerant that can grow in the presence of high concentrations of salt
2. Osmotolerant that can grow in the presence of high concentrations of   unionized organic compounds such as sugars
3. Aerotolerant that can grow on dry foods.

Some microorganisms and their required range of the aw are:

3. Oxidation-reduction potential (Eh)

The oxidation-reduction or redox potential of a substance is defined as a measurement of a transfer of electrons between atoms or molecules. The oxidation-reduction potential is usually written as Eh and measured in terms of millivolts (mV). The redox potential of food depends on the

1. pH of the food
2. Availability of oxygen (physical state, packaging)
3. Poising capacity or buffering capacity
4. Food composition (such as protein, ascorbic acid, reducing sugars)

Various food and their Eh range:

The Eh range at which different groups of microorganisms can grow are as follows:

1. Aerobes that can grow best at +500 to +300 mV such as Molds, yeasts, Bacillus, Pseudomonas, Moraxella, and Micrococcus.
2. Facultative anaerobes that can grow best at +300 to +100 mV such as the lactic acid bacteria and those in the family Enterobacteriaceae.
3. Anaerobes that can grow best at +100 to –250 mV or lower such as Clostridium spp.

4. Nutrient content

• The microorganisms need nutrients such as proteins, carbohydrates, sulfur, phosphorus, vitamin, lipids, water, energy, nitrogen, and minerals for their growth and to carry out metabolic functions.
• Food is the best source of nutrition for microbial growth.  
• Microorganisms that are commonly found in food vary greatly in nutrient requirements and those which can utilize available nutrition in abundant amount are usually predominate in foods.
• The simple carbohydrates and amino acids are utilized first, followed by the more complex forms of these nutrients. 
• The nutritional requirements of Gram-positive bacteria are higher compared to yeasts and then followed Gram-negative. Molds have the lowest nutrient requirements.

5. Presence of antimicrobial constituents

• Various foods have inherent antimicrobial components that prevent the microbial attack of various foodborne pathogens 
• The food of plant and animal products contains antimicrobial components.

6. Biological structures

• Some foods have a biological structure that prevents microbial entry.
• The natural covering protects from damage and reduces the chance of microbial spoilage.
• Structures such as the outer coverings on fruits, the shells of nuts, and shells of eggs, meat has fascia, and skin that prevents the entry of foodborne pathogens and spoilage microorganisms.

B. Extrinsic factors

The factors which are controlled through external conditions are referred to as extrinsic factors. These factors include:

1. Temperature of storage

The enzymatic reactions and microbial growth are affected by the environmental temperature. The growth temperatures for yeasts and molds demonstrate a broad range of 10–35° C.

Based on temperature, bacteria can be grouped as

1. Psychrotrophs are cold-tolerant and ubiquitous microorganisms that can grow in a temperature range of 0–20°C. These include Pseudomonas spp and Enterococcus spp.
2. Mesophiles are microorganisms that can grow between 25°C and 40°C, with an optimum growth temperature close to 37°C. These include species of Salmonella, Staphylococcus, Clostridium, Shigella, and Bacillus.
3. Thermophiles are microorganisms that grow at a high temperature above 45 °C,  with an optimum growth temperature between 50°C and 70°C. These include the species of Bacillus, Clostridium, and Geobacillus.

2. Relative Humidity

• The relative humidity is the amount of moisture in the atmosphere or food environment.
• Relative humidity can influence the water activity (aw) level on the food and hence can influence the growth of microorganisms. 
• For example, dry grains stored in an environment with high humidity will take up water and undergo mold spoilage.

3. Presence and Concentration of gases in the environment

• Gases such as Carbon dioxide (CO₂), ozone (O₃), and oxygen (O₂) have a direct toxic effect that can inhibit growth and proliferation. 
• Ozone (O₃), and oxygen (O₂) are highly toxic to anaerobic bacteria and carbon dioxide is effective against obligate aerobes.

C. Implicit factors

• The third factor that is important in determining the nature of microbial growth in food is referred to as implicit factors.
• This factor defines the properties of the organisms themselves and how they respond to such environments. 
• Microorganisms can inhibit or stimulate the growth of one another.
• Some various actions and reactions that may be harmful or beneficial for microorganisms are predation, parasitism, commensalism, amensalism, allotropy, asymbiosis, and neutrality.
• The organisms may produce substances that are either inhibitory or lethal to other organisms such as antibiotics, bacteriocins, hydrogen peroxide, and organic acids.

D. Food Processing factors

• During the processing of foods, microorganisms can be subjected to various physical or chemical stresses.
• Such processing factors are heat, freezing, drying, osmotic effects, irradiation, and various chemicals.
• Heating helps to reduce the microbial level in food by damaging the cytoplasmic membrane, alters metabolic and enzymatic activities.
• Freezing reduces the growth of microbes by exhibiting the inhibitory effect of reduced pH and increase of Aw.
• Drying reduces microbial growth as it causes metabolic injuries that impair the proliferation of the cells.

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