Factors affecting the growth of microorganisms in food
Food is essential for human survival and well-being, but it can also be a source of microbial contamination and spoilage. Microorganisms are ubiquitous in nature and can be found in various types of food, such as fruits, vegetables, meat, dairy products, grains, and beverages. Some of these microorganisms are beneficial for food production, preservation, and quality, such as those involved in fermentation, probiotics, and enzymes. However, some microorganisms are harmful for food safety and health, such as those that cause foodborne diseases, food poisoning, and food deterioration.
The growth of microorganisms in food is influenced by many factors that can be classified into four categories: intrinsic, extrinsic, implicit, and processing factors. Intrinsic factors are the inherent properties of the food itself, such as pH, water activity, oxidation-reduction potential, nutrient content, antimicrobial constituents, and biological structures. Extrinsic factors are the environmental conditions surrounding the food, such as temperature, relative humidity, and gas composition. Implicit factors are the characteristics of the microorganisms themselves, such as their interactions with other microorganisms and with the food matrix. Processing factors are the treatments applied to the food during production, storage, distribution, and preparation, such as heating, cooling, drying, irradiation, and chemical additives.
These factors affect the growth of microorganisms in food in different ways. Some factors may enhance or inhibit the growth of certain types of microorganisms, while others may have no effect or a variable effect depending on the situation. Some factors may act synergistically or antagonistically with other factors to influence the growth of microorganisms in food. Therefore, it is important to understand how these factors affect the growth of microorganisms in food and how they can be controlled or manipulated to ensure food quality and safety.
In this article, we will discuss each of these factors in detail and provide examples of how they affect the growth of microorganisms in different types of food. We will also provide some tips on how to prevent or reduce microbial growth in food by modifying these factors. By the end of this article, you will have a better understanding of the factors affecting the growth of microorganisms in food and how to use them to your advantage.
pH is a measure of the acidity or alkalinity of a solution, which affects the growth and survival of microorganisms. Different microorganisms have different ranges of pH tolerance and optimum pH for their metabolic activities. In general, molds and yeasts can grow at lower pH than bacteria, and Gram-negative bacteria are more sensitive to low pH than Gram-positive bacteria.
Based on the pH ranges, microorganisms can be grouped as :
- Neutrophiles grow best at a pH range of 5 to 8. Most bacteria that cause food spoilage and foodborne diseases belong to this group, such as Salmonella, Staphylococcus, Escherichia coli, and Bacillus.
- Acidophiles grow best at a pH below 5.5. Some molds and yeasts that cause food spoilage are acidophiles, such as Aspergillus niger, Saccharomyces rouxii, and Mucor.
- Alkaliphiles grow best at a pH above 8.5. Some bacteria that produce alkaline fermentation products are alkaliphiles, such as Clostridium botulinum, Pseudomonas aeruginosa, and Vibrio cholerae.
Based on pH, foods can be grouped as :
- Highly acidic foods have a pH below 3.7. These foods are usually preserved by adding acids, such as vinegar or lemon juice, or by natural fermentation, such as sauerkraut and pickles. These foods are resistant to most bacteria, but may support the growth of some acid-tolerant molds and yeasts.
- Acidic foods have a pH between 3.7 and 4.6. These foods include fruits, juices, jams, honey, and some cheeses. These foods are susceptible to spoilage by molds and yeasts, but can inhibit the growth of most bacteria.
- Medium acidic foods have a pH between 4.6 and 5.3. These foods include vegetables, meat products, dairy products, and some cereals. These foods are prone to spoilage by both bacteria and fungi, and require proper storage and processing to prevent microbial growth.
- Low acidic foods have a pH above 5.3. These foods include eggs, milk, fish, poultry, and legumes. These foods are highly perishable and can support the growth of a wide range of microorganisms, including pathogens.
The pH of food can be influenced by various factors, such as the natural composition of the food, the addition of acids or bases during processing or preparation, the microbial activity that produces acids or bases as metabolic by-products, and the environmental conditions that affect the chemical reactions in food . Therefore, controlling the pH of food is an important strategy to prevent or reduce microbial growth in food.
The oxidation-reduction or redox potential of a substance is defined as a measurement of the transfer of electrons between atoms or molecules. The redox potential is usually written as Eh and measured in terms of millivolts (mV). The redox potential of food depends on the:
- pH of the food
- Availability of oxygen (physical state, packaging)
- Poising capacity or buffering capacity
- Food composition (such as protein, ascorbic acid, reducing sugars)
Positive values for Eh indicate oxidizing conditions, whereas negative values indicate reducing conditions. Oxidoreduction reactions are the basic principle of energy generation in biological systems in which energy-rich compounds are oxidized stepwise. Dioxygen tension and redox potential influence the growth and survival of microorganisms.
Based on Eh range, microorganisms can be grouped as:
- Aerobes that can grow best at +500 to +300 mV such as molds, yeasts, Bacillus, Pseudomonas, Moraxella, and Micrococcus.
- Facultative anaerobes that can grow best at +300 to +100 mV such as the lactic acid bacteria and those in the family Enterobacteriaceae.
- Anaerobes that can grow best at +100 to –250 mV or lower such as Clostridium spp.
The redox potential of food can affect the type and rate of microbial growth. For example, foods with high Eh values such as fresh fruits and vegetables are more prone to spoilage by aerobic microorganisms, whereas foods with low Eh values such as meat and cheese are more susceptible to anaerobic microorganisms. The redox potential of food can also be altered by processing methods such as heating, freezing, drying, irradiation, or adding chemicals that can change the oxidation state of food components. Therefore, controlling the redox potential of food is an important strategy to prevent microbial spoilage and ensure food safety and quality.
Microorganisms need nutrients such as proteins, carbohydrates, lipids, vitamins, minerals, and water for their growth and metabolic functions. Food is the best source of nutrition for microbial growth, and different microorganisms vary greatly in their nutrient requirements and preferences.
Some microorganisms can utilize simple sugars and amino acids more readily than complex carbohydrates and proteins, while others can degrade complex molecules into simpler ones. Some microorganisms can synthesize their own vitamins and amino acids, while others depend on external sources. Some microorganisms can oxidize reduced carbon, nitrogen, and sulfur compounds present in food and contribute to the biogeochemical cycling.
The nutrient content of food can affect the type and number of microorganisms that can grow in it. Generally, foods with high protein and moisture content, such as meat, milk, and eggs, are more susceptible to microbial spoilage and contamination than foods with low protein and moisture content, such as cereals, nuts, and dried fruits.
The nutrient content of food can also influence the production of secondary metabolites by microorganisms, such as organic acids, alcohols, gases, flavors, pigments, toxins, and enzymes. These metabolites can have beneficial or detrimental effects on the food quality and safety. For example, some metabolites can enhance the sensory attributes of fermented foods, such as yogurt, cheese, bread, wine, and beer. Some metabolites can act as natural preservatives by inhibiting the growth of other microorganisms or degrading harmful substances. Some metabolites can cause food spoilage by altering the color, texture, odor, or taste of food. Some metabolites can pose health risks by producing toxins or allergens that can affect human health.
Therefore, the nutrient content of food is an important intrinsic factor that affects the growth of microorganisms in food systems.
Some foods contain natural or added substances that have antimicrobial properties, meaning they can inhibit or kill microorganisms. These substances can be classified into three categories: organic acids, phenolic compounds, and proteins or peptides.
Organic acids are weak acids that can lower the pH of food and create an unfavorable environment for microbial growth. Some examples of organic acids are acetic acid (found in vinegar), citric acid (found in citrus fruits), lactic acid (produced by fermentation of milk or vegetables), and sorbic acid (used as a preservative in cheese, bread, and beverages). Organic acids can affect the membrane permeability, enzyme activity, and metabolic pathways of microorganisms.
Phenolic compounds are aromatic compounds that have one or more hydroxyl groups attached to a benzene ring. They have antioxidant and antimicrobial effects due to their ability to interact with proteins, lipids, and nucleic acids of microorganisms. Some examples of phenolic compounds are benzoic acid (used as a preservative in jams, sauces, and soft drinks), eugenol (found in cloves), thymol (found in thyme), and capsaicin (found in chili peppers). Phenolic compounds can disrupt the cell membrane, inhibit enzymes, and damage DNA of microorganisms.
Proteins or peptides are chains of amino acids that have various biological functions. Some proteins or peptides have antimicrobial activity due to their ability to bind to specific receptors, form pores, or degrade cell walls of microorganisms. Some examples of proteins or peptides are lysozyme (found in egg white and saliva), lactoferrin (found in milk and colostrum), lactoperoxidase (found in milk and saliva), and nisin (produced by lactic acid bacteria and used as a preservative in dairy products). Proteins or peptides can interfere with the nutrient uptake, oxidative stress, and cell division of microorganisms.
The presence of antimicrobial constituents in food can help to prevent or delay the spoilage and contamination of food by microorganisms. However, some factors can affect the effectiveness of these substances, such as the concentration, solubility, stability, pH, temperature, water activity, and interactions with other food components or microorganisms. Therefore, it is important to consider these factors when using antimicrobial constituents in food processing or preservation.
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 prevent the entry of foodborne pathogens and spoilage microorganisms.
The intestinal epithelial layer is another example of a biological structure that forms a major barrier that separates our body from the external environment. It prevents the invasion of harmful microorganisms and maintains a symbiotic relationship with commensal bacteria. The tight junction proteins, such as occludin, claudins, and zonula occludens, are crucial for the maintenance of epithelial barrier integrity. The disruption of this barrier can lead to various diseases, such as intestinal inflammatory disorders, autoimmune diseases, and metabolic disorders.
Therefore, biological structures play an important role in protecting food and our body from microbial contamination and infection.
One of the most important factors that affect the growth of microorganisms in food is the temperature of storage. Temperature influences the metabolic and enzymatic activities of microorganisms, as well as their survival and proliferation. Different microorganisms have different temperature ranges and optima for growth, and they can be classified based on their temperature preferences :
- Psychrophiles are microorganisms that can grow at low temperatures below 15°C, with an optimum growth temperature between 0°C and 10°C. These include some species of Pseudomonas, Listeria, and Vibrio.
- Psychrotrophs are cold-tolerant and ubiquitous microorganisms that can grow in a temperature range of 0–20°C, with an optimum growth temperature between 15°C and 30°C. These include Pseudomonas spp., Enterococcus spp., Bacillus spp., Clostridium spp., and some strains of Escherichia coli and Salmonella.
- Mesophiles are microorganisms that can grow between 25°C and 40°C, with an optimum growth temperature close to 37°C. These include most species of Salmonella, Staphylococcus, Clostridium, Shigella, and Bacillus.
- Thermophiles are microorganisms that grow at high temperatures above 45 °C, with an optimum growth temperature between 50°C and 70°C. These include some species of Bacillus, Clostridium, and Geobacillus.
- Hyperthermophiles are microorganisms that grow at extremely high temperatures above 80 °C, with an optimum growth temperature above 90°C. These include some species of Archaea, such as Pyrococcus and Thermococcus.
The temperature of storage can be manipulated to prevent or slow down the growth of microorganisms in food. There are two main methods of temperature control: refrigeration and freezing .
Refrigeration is the process of storing food at low temperatures above freezing point (0°C), usually between 1°C and 5°C. Refrigeration slows down the growth of most foodborne microorganisms, especially mesophilic bacteria, by reducing their metabolic and enzymatic activities. Refrigeration also inhibits the activity of enzymes and chemical reactions that can cause food spoilage. However, refrigeration does not kill microorganisms or stop their growth completely. Some psychrotrophic and psychrophilic bacteria, such as Listeria monocytogenes and Yersinia enterocolitica, can still grow at refrigeration temperatures and cause foodborne illness. Therefore, refrigerated foods should be consumed within a certain time limit or discarded.
Freezing is the process of storing food at sub-zero temperatures below freezing point (0°C), usually between -18°C and -40°C. Freezing prevents the growth of most foodborne microorganisms by forming ice crystals that damage their cell membranes and reduce their water activity. Freezing also stops the activity of enzymes and chemical reactions that can cause food spoilage. However, freezing does not kill all microorganisms or eliminate their toxins. Some bacteria, such as Clostridium botulinum and Staphylococcus aureus, can survive freezing and produce toxins when thawed. Therefore, frozen foods should be handled properly before and after thawing to avoid cross-contamination and re-growth of microorganisms.
In summary, temperature is a critical factor that affects the growth of microorganisms in food. Different microorganisms have different temperature ranges and optima for growth, and they can be classified based on their temperature preferences. The temperature of storage can be manipulated to prevent or slow down the growth of microorganisms in food by using refrigeration or freezing methods. However, neither refrigeration nor freezing can kill all microorganisms or eliminate their toxins. Therefore, proper handling and hygiene practices are also essential to ensure food safety.
Relative humidity (RH) is the amount of moisture in the atmosphere or food environment. RH can influence the water activity (aw) level on the food and hence can influence the growth of microorganisms.
Microorganisms require a certain level of moisture to grow and multiply. The majority require RH of 60 percent or more, though some can survive and multiply in >20 percent RH. Thus, decreasing temperature and moisture (RH), creates a less hospitable environment for microorganisms to grow.
However, some microorganisms can adapt to low RH conditions by producing spores, pigments, or extracellular polysaccharides that protect them from desiccation. Some microorganisms can also grow on dry foods by utilizing the water vapor in the air.
The effect of RH on microbial growth also depends on other factors such as temperature, ventilation, and food composition. For example, dry grains stored in an environment with high humidity will take up water and undergo mold spoilage. On the other hand, increased ventilation and reduced humidity can significantly decrease bacterial growth at different temperatures.
Therefore, controlling RH is an important factor in preventing microbial spoilage and ensuring food safety. The optimal RH for food storage depends on the type of food, its aw level, and its susceptibility to microbial growth. Generally, foods with low aw should be stored at low RH to prevent moisture uptake and mold growth, while foods with high aw should be stored at high RH to prevent moisture loss and quality deterioration.
Gases such as oxygen (O2), carbon dioxide (CO2), nitrogen (N2), ozone (O3), and carbon monoxide (CO) can have direct or indirect effects on the growth and survival of microorganisms in food . The effects of gases depend on the type of microorganism, the type of food, and the environmental conditions.
- Oxygen (O2) is essential for aerobic microorganisms that use it as a terminal electron acceptor in their metabolism. However, oxygen can also be harmful to anaerobic microorganisms that cannot tolerate it and may produce toxic by-products such as hydrogen peroxide and superoxide radicals. Oxygen can also cause oxidative damage to food components such as lipids, proteins, and vitamins. Therefore, reducing the oxygen level in food packaging can inhibit the growth of aerobic spoilage microorganisms and extend the shelf life of food. This can be achieved by vacuum packaging or modified atmosphere packaging (MAP) using inert gases such as nitrogen or carbon dioxide .
- Carbon dioxide (CO2) is a by-product of microbial fermentation and can also be used as a packaging gas in MAP. Carbon dioxide can inhibit the growth of some aerobic microorganisms by lowering the pH, disrupting membrane permeability, and interfering with enzyme activity. However, some microorganisms can adapt to high CO2 levels and may even use it as a carbon source. The inhibitory effect of CO2 depends on its concentration, temperature, pH, water activity, and the presence of other gases .
- Nitrogen (N2) is an inert gas that does not affect microbial growth directly but can be used to displace oxygen in MAP. Nitrogen can also prevent oxidative rancidity and discoloration of food products.
- Ozone (O3) is a highly reactive gas that can kill microorganisms by oxidizing their cell components. Ozone can also degrade pesticides, hormones, and other contaminants in food. However, ozone can also damage food quality by causing off-flavors, browning, and loss of nutrients. Ozone is unstable and decomposes quickly to oxygen, so it needs to be generated on-site and applied continuously or intermittently .
- Carbon monoxide (CO) is a colorless, odorless gas that can bind to hemoglobin and myoglobin in meat and fish products, giving them a bright red color and enhancing their appearance. CO can also inhibit the growth of some aerobic microorganisms by reducing the availability of oxygen. However, CO can also mask the spoilage signs of meat and fish products, posing a potential health risk to consumers. Therefore, the use of CO in food packaging is controversial and regulated in different countries .
In conclusion, gases can have various effects on microbial growth in food, depending on their type, concentration, and interaction with other factors. Gases can be used as a preservation method to control microbial growth in food, but they also have some limitations and risks that need to be considered.
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