The Use of Organic Acids and Esters as Food Preservatives

Food spoilage is one of the biggest challenges in the food industry. Every year, a significant portion of food produced globally goes to waste because of microbial contamination – bacteria, molds, and yeasts breaking down food before it reaches consumers. To combat this, food scientists rely heavily on organic acids and esters as chemical preservatives. These compounds are naturally present in many foods, yet they are also added intentionally to inhibit microbial growth and extend the shelf life of perishable products. Among the most widely used are benzoic acid, sorbic acid, and propionic acid – each with distinct properties, applications, and mechanisms of action.

How do organic acids work as preservatives? 🔗

Organic acids are weak acids characterised by the general structure R-COOH. Their preservative action depends on a key principle: the undissociated (non-ionised) form of the acid is responsible for antimicrobial activity. At low pH, a greater proportion of the acid remains undissociated, which makes it more lipophilic and allows it to pass through microbial cell membranes easily.

Once inside the cell, the acid enters a more neutral environment (pH 6-7 in the cytoplasm), where it dissociates into protons and anions. This release of protons lowers the internal pH of the microorganism, disrupting critical metabolic processes. The cell is forced to spend energy pumping out the acid anions, which raises its ADP/ATP ratio and ultimately slows or halts growth. In simple terms, organic acids exhaust the microbe’s energy reserves while also interfering with enzyme function and nutrient transport.

Because the undissociated form is the active agent, pH plays a critical role in determining the effectiveness of organic acid preservatives. This is why these preservatives are most commonly used in acidic or mildly acidic food products.

Benzoic acid: the go-to preservative for acidic foods 🔗

Benzoic acid (C₆H₅COOH) is one of the oldest and most commonly used chemical preservatives in the food industry. It was, in fact, the first chemical preservative approved for use in foods by the U.S. FDA. It occurs naturally in many fruits, particularly cranberries, prunes, and plums, and is also produced as an intermediate metabolite by various plants.

How benzoic acid inhibits microbial growth 🔗

Benzoic acid works by penetrating microbial cell membranes in its undissociated form. Once inside the cell, it dissociates and releases protons, causing intracellular acidification. This disrupts essential enzymatic functions and impairs the cell’s ability to transport nutrients. Research has also shown that benzoic acid acts as a membrane-perturbing agent, affecting membrane organisation and potentially impairing mitochondrial function in yeasts.

Benzoic acid has a pKa of 4.19, meaning it is most effective at pH levels below 4.5. As the pH rises above this range, a larger proportion of the acid becomes dissociated and unable to penetrate cell membranes. This limits its use to acidic food systems.

Common applications of benzoic acid 🔗

In practice, benzoic acid is rarely used in its free acid form because of its relatively low water solubility (only about 0.3 g per 100 mL at room temperature). Instead, its sodium salt – sodium benzoate – is preferred because it dissolves much more readily in water. Once dissolved in an acidic food product, sodium benzoate converts back to the active benzoic acid form.

Typical food products preserved with benzoic acid or sodium benzoate include fruit juices, carbonated beverages, pickles, jams, salad dressings, and condiments. The use of benzoic acid as a preservative is widespread across the beverage industry in particular, where naturally acidic conditions make it highly effective.

Regulatory agencies around the world set strict limits on its use. For example, the WHO has established guidelines permitting no more than 5 mg/kg of benzoic acid and sodium benzoate in certain applications.

Sorbic acid: the versatile mold and yeast fighter 🔗

Sorbic acid (CH₃-CH=CH-CH=CH-COOH) is a six-carbon unsaturated fatty acid that was first isolated in 1859 from the berries of the mountain ash tree (Sorbus aucuparia). Today, it is commercially synthesised through a condensation reaction between crotonaldehyde and ketene, though it still occurs naturally in certain berries and fruits.

Mechanism of action 🔗

Sorbic acid works by inhibiting key enzymes in the carbohydrate and citric acid metabolic cycles of microorganisms. Specifically, it forms covalent bonds with sulfhydryl (SH) groups on these enzymes, inactivating them. It also disrupts cell membrane function by interacting with membrane phospholipids. The net effect is that microorganisms cannot generate enough energy to grow and reproduce.

Sorbic acid has a pKa of 4.76, and like other weak acid preservatives, it is most effective in its undissociated form at low pH. However, a key advantage of sorbic acid is that it retains some antimicrobial activity even at pH values above 6.0, making it more versatile than benzoic acid or propionic acid in near-neutral food systems.

Applications in the food industry 🔗

Sorbic acid is particularly effective against molds and yeasts, though it also inhibits many bacteria. It is widely used in cheese, wine, baked goods, dried fruits, processed meats, fruit juices, and yoghurt. Because sorbic acid itself has limited water solubility (about 0.16 g per 100 mL at 20°C), the food industry commonly uses its potassium salt – potassium sorbate (E202) – which is far more soluble.

Both sorbic acid and potassium sorbate are classified as Generally Recognised as Safe (GRAS) by the U.S. FDA. The Joint FAO/WHO Expert Committee on Food Additives (JECFA) has set an acceptable daily intake (ADI) of 25 mg per kg of body weight for sorbic acid, which is higher than for most other preservatives – reflecting its strong safety profile.

Another practical advantage is that sorbic acid is essentially odourless and tasteless at typical usage concentrations (0.025% to 0.10%), so it does not alter the sensory qualities of food.

Propionic acid: the bread protector 🔗

Propionic acid (CH₃CH₂COOH), also called propanoic acid, is a short-chain fatty acid that occurs naturally in small quantities in milk and at higher concentrations in dairy products like yoghurt and Swiss cheese (where it is produced during bacterial fermentation). It is classified as GRAS by the U.S. FDA for use as both an antimicrobial agent and a flavouring agent, with no limitation other than good manufacturing practice.

How propionic acid prevents mold 🔗

Propionic acid is primarily fungistatic – meaning it inhibits mold growth rather than killing mold outright. It acts by preventing the cellular uptake of substrate molecules that microorganisms need for growth. Like other organic acids, its undissociated form penetrates microbial cell membranes and disrupts internal metabolic functions.

One notable feature is that propionic acid remains largely undissociated at low pH, which allows it to penetrate fungal cell membranes more effectively than acetic or lactic acid. Studies have found it to be one of the most economical and effective organic acids for controlling mold across a range of applications.

Where propionic acid is used 🔗

The primary application of propionic acid is in baked goods – bread, cakes, pastries, and other flour-based products – where mold growth is a common cause of spoilage. Its calcium and sodium salts (calcium propionate and sodium propionate) are the forms typically added to these products. Calcium propionate, for instance, is a standard ingredient in commercial bread production worldwide.

Beyond baked goods, propionic acid is extensively used as a mold inhibitor in animal feed, particularly for grain preservation during storage. Application rates depend on the moisture content of the grain and storage conditions.

Esters as food preservatives: the role of parabens 🔗

While organic acids dominate as food preservatives, certain esters also play a significant role. The most important group of esters used in food preservation is the parabens – alkyl esters of para-hydroxybenzoic acid (p-hydroxybenzoic acid). The most commonly used parabens in food include methylparaben, ethylparaben, and propylparaben.

How parabens work 🔗

Parabens are effective against a broad range of fungi and gram-positive bacteria, though they are less effective against gram-negative bacteria. Their antimicrobial activity increases as the alkyl chain length increases (from methyl to butyl), but water solubility decreases at the same time. This trade-off means that combinations of two or more parabens are often used to achieve synergistic effects.

A major advantage of parabens over free organic acids is their effectiveness across a wider pH range (3-8). Unlike benzoic or sorbic acid, whose activity drops sharply as pH rises, parabens remain active even in near-neutral conditions. This makes them suitable for a broader variety of food products.

Applications and regulatory status 🔗

Parabens are used in a range of processed foods, including frozen dairy products, baked goods, fruit juices, pickled vegetables, sauces, and certain beverages. The European Food Safety Authority (EFSA) has established a group ADI of 0-10 mg/kg body weight per day for methyl and ethyl parabens and their sodium salts. In the United States, the FDA sets maximum application levels at 0.1% for methyl and propyl esters of p-hydroxybenzoic acid.

It is worth noting that propylparaben has faced additional scrutiny in recent years due to studies showing effects on reproductive parameters in juvenile rats, and EFSA has excluded it from the group ADI for methyl and ethyl parabens.

Comparing the three major organic acid preservatives 🔗

Each of the three primary organic acid preservatives fills a distinct niche in food preservation:

Benzoic acid is best suited for highly acidic liquid foods like beverages, juices, and pickles. Its effectiveness drops significantly above pH 4.5, so it is not ideal for neutral or mildly acidic foods.

Sorbic acid is the most versatile of the three. It is effective against molds, yeasts, and many bacteria, works across a wider pH range than benzoic or propionic acid, and has minimal impact on food taste or smell. This makes it the preservative of choice for dairy products, wines, and a wide range of processed foods.

Propionic acid is more specialised. Its primary strength lies in preventing mold in baked goods and grain products, where it is used almost universally in commercial production.

In many food products, combinations of these preservatives are used to achieve broader antimicrobial coverage. For example, a fruit beverage might contain both sodium benzoate and potassium sorbate to target bacteria, molds, and yeasts simultaneously.

Safety and regulatory considerations 🔗

All three organic acids – benzoic, sorbic, and propionic – along with parabens, have been extensively evaluated by international regulatory bodies including the WHO, the U.S. FDA, and EFSA. They carry GRAS status in the United States and are approved for food use across the European Union, where they are assigned E-numbers (E200 for sorbic acid, E210 for benzoic acid, E280 for propionic acid).

Sorbic acid is often considered the safest among common preservatives. It is metabolised in the body through normal fatty acid oxidation pathways (beta-oxidation), breaking down into carbon dioxide and water without accumulating in the body. Propionic acid similarly occurs as a normal metabolic intermediate and is handled through standard biochemical pathways. Benzoic acid is conjugated with glycine in the liver to form hippuric acid, which is excreted in urine.

Despite their strong safety records, the use of these preservatives is strictly regulated by maximum permitted levels in specific food categories. Exceeding these levels is not only a regulatory violation but can also cause adverse sensory effects in food.

Practical tips for understanding preservative labels 🔗

When checking food labels, you may not always see the names “benzoic acid” or “sorbic acid” listed directly. Instead, look for their salt forms, which are more commonly used in manufacturing. Sodium benzoate (E211) replaces benzoic acid in most liquid foods. Potassium sorbate (E202) is the standard stand-in for sorbic acid. Calcium propionate (E282) is the form most often added to bread and baked goods.

Understanding these names and their E-numbers helps you identify what preservatives are in your food and why they have been added.

What do you think? Given that organic acid preservatives like sorbic acid are metabolised harmlessly in the body, do you believe consumer concerns about “chemical preservatives” in food are justified? And as food science advances, could natural fermentation-derived organic acids eventually replace synthetically produced ones entirely?

References

1. https://foodsafety.institute/food-fundamentals-chemistry/role-of-preservatives-in-food-safety/
2. https://pmc.ncbi.nlm.nih.gov/articles/PMC492424/
3. https://www.sciencedirect.com/topics/chemical-engineering/sorbic-acid
4. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/sorbic-acid
5. https://www.ams.usda.gov/sites/default/files/media/2023Technical_Report_Potassium_Sorbate_Crops.pdf
6. https://www.ams.usda.gov/sites/default/files/media/Prop%20acid%20report.pdf
7. https://www.mdpi.com/2071-1050/17/8/3434
8. https://www.sciencedirect.com/science/article/abs/pii/S0278691505000530
9. https://www.efsa.europa.eu/en/news/efsa-advises-safety-paraben-usage-food
10. https://www.who.int/
11. https://www.fda.gov/
12. https://www.efsa.europa.eu/