Naturally Occurring and Processed Inhibitory Substances in Food

Every food you eat – whether it’s a fresh apple, a glass of milk, or a jar of honey – contains substances that naturally fight off microbes. These inhibitory substances are one of the key reasons many foods remain safe to consume even without refrigeration or chemical preservatives. Some of these antimicrobials are built into the food by nature, some are produced by friendly microorganisms living in or on the food, and others are created during processing. Understanding how these substances work is essential for anyone studying food safety, food preservation, or food product development.

What are inhibitory substances in food? 🔗

Inhibitory substances are chemical compounds present in food that can slow down, prevent, or completely stop the growth of bacteria, yeasts, moulds, and other microorganisms. They function as a natural defence system, protecting the food from spoilage and pathogenic contamination. These substances can be broadly categorised into three groups: naturally occurring antimicrobials found in plants and animal tissues, microbially produced inhibitors generated by beneficial microorganisms, and processing-derived compounds that form during cooking, heating, or fermentation.

The growing consumer preference for clean-label and chemical-free food products has made these natural antimicrobials highly relevant. Research published in Frontiers in Microbiology confirms that essential oils, enzymes, bacteriocins, chitosans, and organic acids are well-suited as preservatives and could serve as valid alternatives to synthetic food additives.

Naturally occurring antimicrobials from plants 🔗

Plants are the largest source of naturally occurring inhibitory substances in food. Herbs, spices, fruits, and vegetables all contain compounds that can restrict microbial growth. These compounds are mainly secondary metabolites – chemicals that plants produce not for basic growth, but for defence against insects, animals, and microorganisms.

Essential oils 🔗

Essential oils extracted from herbs and spices are among the most studied natural antimicrobials. Oils from oregano, thyme, cinnamon, clove, rosemary, and basil are particularly effective. Their antimicrobial action comes from phenolic compounds like thymol, carvacrol, eugenol, and cinnamaldehyde. These compounds disrupt microbial cell membranes, causing the contents to leak out and the cell to die.

For example, oregano essential oil has been shown to be effective against Pseudomonas species and coliforms in ground beef, while thyme oil shows broad-spectrum activity against both bacteria and fungi. As IntechOpen notes, essential oils in meat products can reduce the chances of foodborne diseases and also slow down lipid oxidation.

However, there is a practical limitation. Many essential oils have a strong flavour and aroma, which can alter the taste of the food product. This means their use in commercial food preservation is often restricted to products where such flavours are acceptable, or they must be used in very low concentrations.

Organic acids 🔗

Many fruits and vegetables naturally contain organic acids such as citric acid (in citrus fruits), malic acid (in apples), tartaric acid (in grapes), and benzoic acid (in cranberries). These acids lower the pH of the food environment, making it unfavourable for the growth of most bacteria. In their undissociated form, organic acids can penetrate microbial cell walls and interfere with internal cellular processes.

Sorbic acid and propionic acid, though now often added as preservatives, also occur naturally in certain foods and have been used for decades to control mould and bacterial growth in baked goods, dairy products, and beverages.

Phenolic compounds and other plant metabolites 🔗

Beyond essential oils, plants produce a wide range of phenolic compounds including flavonoids, tannins, alkaloids, and saponins. These secondary metabolites have documented antimicrobial and antioxidant properties. They work primarily by disrupting cell walls and interfering with the metabolic processes of microbial cells. Grape seed extract, for instance, has shown synergistic antibacterial effects when combined with other antimicrobials against Listeria monocytogenes.

Naturally occurring antimicrobials from animal tissues 🔗

Animal-derived foods, especially milk and eggs, contain several powerful antimicrobial proteins and enzymes. These are part of the animal’s innate immune defence system, and they continue to function even after the food is harvested or collected.

Lysozyme 🔗

Lysozyme is one of the most well-known antimicrobial enzymes in food science. It is found abundantly in egg whites, human and cow milk, tears, and saliva. Lysozyme works by breaking down peptidoglycan, a key structural component of bacterial cell walls. Specifically, it cleaves the bond between N-acetylmuramic acid and N-acetylglucosamine, causing the bacterial cell to rupture and die.

Lysozyme is particularly effective against Gram-positive bacteria because their cell walls have a thick peptidoglycan layer that is directly accessible. It has been commercially used under the name Inovapure for extending the shelf life of products like cheese, raw and processed meats, and other dairy items. It is classified as GRAS (Generally Recognized As Safe) by the US FDA for direct addition in foods.

Lactoperoxidase system 🔗

The lactoperoxidase system (LPS) is a naturally occurring antimicrobial system in milk. It consists of three components: the enzyme lactoperoxidase, thiocyanate (SCN⁻), and hydrogen peroxide (H₂O₂). When these three come together, the system produces reactive oxidised intermediates that have strong bactericidal effects against both Gram-positive and Gram-negative bacteria.

According to research on the lactoperoxidase system, activating the LPS by adding small amounts of hydrogen peroxide and thiocyanate to raw milk can extend its shelf life under refrigeration. The LPS also works synergistically with lactoferrin and lysozyme, enhancing the overall antimicrobial defence in milk.

Lactoferrin 🔗

Lactoferrin is an iron-binding glycoprotein found in milk, especially in colostrum. It inhibits microbial growth by sequestering iron, which is an essential nutrient for many bacteria. Without access to sufficient iron, bacteria cannot replicate and eventually die. Lactoferrin also has direct bactericidal activity – it can bind to the surface of bacterial cells and disrupt their outer membranes.

Microbially produced inhibitory substances 🔗

Certain beneficial microorganisms that live in or on food produce their own antimicrobial compounds. This is often referred to as biopreservation – using the natural competitive mechanisms of safe microbes to suppress harmful ones. Lactic acid bacteria (LAB) are the most important group in this category.

Organic acids produced by microorganisms 🔗

Lactic acid bacteria produce lactic acid, acetic acid, and other organic acids as byproducts of fermentation. These acids lower the pH of the surrounding environment, directly inhibiting the growth of both Gram-positive and Gram-negative spoilage and pathogenic bacteria. This is the fundamental principle behind fermented foods like yoghurt, sauerkraut, kimchi, and pickles – the acid-producing bacteria create an environment hostile to harmful microbes.

LAB also produce hydrogen peroxide, diacetyl, and ethanol, all of which have additional antimicrobial effects. The combined action of these metabolites makes fermented foods remarkably resistant to spoilage.

Bacteriocins 🔗

Bacteriocins are ribosomally synthesised antimicrobial peptides produced by bacteria, primarily by LAB. They can kill or inhibit the growth of closely related or even distantly related bacterial species without harming the producing organism itself. Research published in PMC describes bacteriocins as promising natural biopreservatives due to their broad antimicrobial activity against Gram-positive bacteria and foodborne pathogens.

Nisin is the most commercially significant bacteriocin. It is produced by Lactococcus lactis and was the first bacteriocin approved by the US FDA for use in food. Nisin is effective against a wide range of Gram-positive organisms, including dangerous pathogens like Listeria monocytogenes, Staphylococcus aureus, Bacillus cereus, and Clostridium botulinum. It is also particularly effective against bacterial spores. Nisin works by disrupting the membrane potential and pH gradient of target cells, and it is commonly used in processed cheese, meats, and beverages at concentrations of about 1-25 ppm.

Other commercially relevant bacteriocins include pediocin PA-1 (effective against Listeria species in meat and vegetable products) and natamycin (used primarily against fungi in cheese and baked goods). A review in Bioscience Reports notes that Micocin®, a bacteriocin mixture produced by Carnobacterium maltaromaticum, is also effective against Clostridium botulinum and Listeria monocytogenes.

Reuterin 🔗

Reuterin is a broad-spectrum antimicrobial compound produced from glycerol by certain strains of Lactobacillus reuteri. It is effective against a wide range of both pathogenic and spoilage microorganisms, making it another valuable tool in the biopreservation arsenal.

Inhibitory substances developed during food processing 🔗

When food is heated, fermented, smoked, or otherwise processed, new chemical compounds can form that have antimicrobial properties. These are not present in the raw food but develop as a result of chemical reactions during processing.

Furfural and hydroxymethylfurfural (HMF) 🔗

One of the best-known examples of processing-derived inhibitory compounds is furfural. It forms during the caramelisation of sugars – a process where sugars are heated to high temperatures without the involvement of amino acids. Furfural is produced when sugars like glucose and fructose undergo dehydration, primarily through intermediate compounds like 3-deoxyglucosone.

A systematic review on furanic compounds explains that both caramelisation and the Maillard reaction (where sugars react with amino acids during heating) generate furanic compounds including furfural and 5-hydroxymethylfurfural (HMF). These compounds contribute to the brown colour, flavour, and aroma of heat-processed foods. They also have antimicrobial properties that can help protect foods from microbial spoilage.

However, it is important to note that excessive levels of HMF may pose food safety concerns. HMF can be metabolised into potentially harmful compounds, so its levels in processed foods are monitored. The formation of furfural and HMF depends on factors like the type of sugar present, temperature, pH, and the presence of amino acids.

Maillard reaction products 🔗

The Maillard reaction – a non-enzymatic browning reaction between reducing sugars and amino acids – produces a complex mixture of compounds during food processing. Besides furfural and HMF, these include pyrazines, melanoidins, and various dicarbonyl compounds. Melanoidins, the brown-coloured polymers formed in the later stages of the Maillard reaction, have been shown to possess antimicrobial and antioxidant properties. They are commonly found in bread crusts, roasted coffee, dark beer, and baked goods.

Smoke compounds 🔗

Wood smoke, used in the traditional preservation of meats and fish, contains a range of antimicrobial compounds including formaldehyde, acetic acid, phenolic compounds, and various organic acids. These compounds work together to inhibit microbial growth on the surface of smoked foods, significantly extending their shelf life. Liquid smoke, a commercially available product, concentrates these antimicrobial components for use in modern food processing.

Combining inhibitory substances for better preservation 🔗

In practice, a single antimicrobial substance is often not enough to fully protect a food product. This is why the food industry increasingly relies on what is known as hurdle technology – using multiple preservation strategies together so that their combined effect is greater than any individual method alone.

For instance, combining bacteriocins with essential oils or organic acids has been shown to produce synergistic antibacterial effects. Nisin combined with carvacrol (from oregano oil) shows enhanced activity against Listeria monocytogenes. Similarly, combining natural antimicrobial enzymes like lysozyme with modified atmosphere packaging and refrigeration can dramatically extend the shelf life of perishable dairy and meat products.

This combinatorial approach allows food manufacturers to use lower concentrations of each antimicrobial – reducing any negative impact on taste and texture – while still achieving effective microbial control.

Why inhibitory substances matter for food safety 🔗

The importance of inhibitory substances goes far beyond academic interest. They are directly relevant to food safety, food quality, and public health. Published reviews on natural antimicrobials emphasise that these compounds not only prevent the growth of undesirable microorganisms but also improve consumer confidence in food safety by reducing dependence on synthetic chemical additives.

As the global population grows and the food supply chain becomes more complex, the risk of foodborne illness increases. Natural and processing-derived inhibitory substances offer a practical, safe, and consumer-friendly way to manage this risk. They are especially valuable in developing countries and in situations where cold chain infrastructure is limited.

What do you think? Which category of inhibitory substances – plant-derived, animal-derived, microbial, or processing-derived – do you think holds the greatest potential for revolutionising food preservation in the future? And how might the challenge of maintaining food taste and quality while using natural antimicrobials be addressed in commercial food production?

References

1. https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2012.00287/full
2. https://www.intechopen.com/chapters/56718
3. https://pmc.ncbi.nlm.nih.gov/articles/PMC9572377/
4. https://en.wikipedia.org/wiki/Lactoperoxidase
5. https://pmc.ncbi.nlm.nih.gov/articles/PMC9637989/
6. https://en.wikipedia.org/wiki/Nisin
7. https://portlandpress.com/bioscirep/article/45/04/277/235956/Toward-safer-and-sustainable-food-preservation-a
8. https://pmc.ncbi.nlm.nih.gov/articles/PMC12484514/
9. https://www.mdpi.com/2304-8158/12/16/3128
10. https://www.sciencedirect.com/science/article/abs/pii/S0956713521002048