SORBIC ACID: STRUCTURE, PROPERTIES, USES, REACTIONS - CHEMISTRY - 2025

The sorbic acid is a solid organic compound whose chemical formula is C ₆ H ₈ O ₂ or CH ₃ - (CH) ₄ -CO ₂ H. monocarboxylic acid is an alpha, beta-unsaturated and also is said to be a fatty acid poly -unsaturated. It is also known as hexadieneic acid. It is a carboxylic acid with two C = C double bonds.

It is naturally present in mountain ash berries, rowan berries (Sorbus aucuparia), and magnolia vine berries. Sorbic acid has antimicrobial properties, which is why it is widely used as an additive in food products to prevent the proliferation of fungi, yeasts and bacteria.

Sorbic acid Author: Marilú Stea

It is generally used as a potassium, sodium or calcium sorbate. Its form of action against microorganisms involves infiltration of cell walls and inhibition of certain important enzymes, sometimes causing damage to genetic material.

However, there are microbes that become resistant to its action by remaining in a dormant state that is activated when conditions are favorable again. Sorbic acid and sorbates are not considered toxic to humans and animals. However, its safety is still under study.

The products of these reactions have been studied to determine their potential harm to humans, and some of them have been found to be safe and others to be mutagenic and genotoxic.

Because it is a polyunsaturated compound, it tends to react with nucleophilic compounds present in food, such as nitrites, sulphites and amines.

Structure

Sorbic acid has a linear skeleton of 6 carbon atoms in which there are 2 C = C carbon-carbon double bonds or bonds and a carboxyl group –COOH.

The C = C double bonds are found on the second and fourth carbon (C-2 and C-4) and are in their trans - trans form.

Structure of sorbic acid where the trans-trans arrangement of the C = C double bonds is observed. Capaccio. Source: Wikipedia Commons.

Nomenclature

- Sorbic acid

- 2,4-hexadieneic acid

- (2E, 4E) -hexa-2,4-dienoic acid

- trans, trans -2,4-hexadieneic acid

- 2-propenylacrylic acid.

Properties

Physical state

Colorless or white crystalline solid. It crystallizes from water or alcohol in the form of needles.

Sorbic acid powder. indiamart. Source: Wikipedia Commons.

Molecular weight

112.13 g / mol

Melting point

134.5 ºC (begins to sublimate above 60 ºC)

Boiling point

228 ºC, boils with decomposition.

Flashpoint

127 ºC (closed cup method).

Density

1.2 g / cm ³

Solubility

Weakly soluble in water, 1.56 g / L at 20 ºC. Soluble in ethanol. Very soluble in ether.

Dissociation constant

pK _a = 4.76 at 25 ° C

Chemical properties

Being a di-unsaturated compound it tends to undergo auto-oxidation in the presence of oxygen. However, in its dry crystalline solid form it is very stable.

Its auto-oxidation in aqueous solution depends on many factors, including pH. At low pH (acidic) it tends to oxidize more easily than at high pH (alkaline), which seems to be because at alkaline pH it converts to its sorbate ion, which is less prone to oxidation.

Its system of conjugated double bonds (that is, bonds that share electrons) means that it can react with many nucleophilic agents (atoms that have excess electrons so they look for an atom with few electrons).

Among such nucleophilic agents, the R-SH thiols, sulfites SO ₃ ^2-, nitrites NO ₂ ^- and the amines R-NH _{2 stand out}.

Other properties

It is volatile in the presence of vapor without decomposing.

Its flavor is slightly acidic and astringent. It has an almost imperceptible odor.

The administration of ointments or topical medications that contain sorbic acid can cause skin contact allergies in some sensitive individuals.

Applying sorbic acid directly to the skin causes severe irritation.

Use in the food industry

Sorbic acid is an antimicrobial agent effective against a broad spectrum of microorganisms, such as fungi, bacteria, and yeasts, and has low toxicity to humans and animals, which is why it is used as a preservative for many edibles or food products.

It is an inhibitor of the growth of microbes. The amount or concentration of sorbic acid to use depends on the type of food, its pH, the bacteria or fungus to be fought, and the regulations imposed by the authorities regarding food and safety.

Sorbic acid is mostly used as potassium, sodium or calcium sorbate, which are more soluble in aqueous medium. Some sources report that its action is more efficient in an acid medium and that it works better than benzoate (another food preservative).

According to certain studies, sorbic acid is metabolized in the human body like other fatty acids, transforming into CO ₂ and water. For this reason it does not accumulate in the body.

The US Food and Drug Administration, or FDA, has classified it as "Generally Recognized as Safe" or GRAS (Generally Recognized as Safe).

It is widely used as a preservative in fruit juices, wines and other beverages, margarine, dairy products such as fresh cheese, sauces, pickles, fish, desserts, and bakery products, among many other foods.

Commercial drink that probably contains sorbates. Author: Miguel Andrade. Source: Unsplash

Its mechanism of action in microbes is estimated to be based on the inhibition of enzymes of the carbohydrate and citric acid cycle. It deactivates such enzymes by forming bonds with their –SH groups.

On the other hand, it affects the electrochemical potential of the cell membranes of microorganisms and infiltrates them, exerting its action. In some cases it even interferes with the genetic material (DNA and RNA) of bacteria.

Sorbic acid reactions in food

Various compounds can be found naturally in food, among them the amines R-NH ₂, and others are added as additives, such as nitrites NO ₂ ^- and sulfites SO ₃ ^2-.

Amines

Sorbic acid and potassium sorbate can undergo nucleophilic addition reactions with simple amines, generating cyclic structures of the dihydropyridone type.

These structures are formed by the double addition of amines to the sorbate double bonds, followed by cyclization with dehydration and loss of an amine. Such reactions can occur even under mild conditions such as those encountered during food processing (50-80 ° C).

Nitrites

Nitrite salts NO ₂ ^- are added to some foods to inhibit the growth of some bacteria such as Clostridium botulinum, a bacillus that generates neurotoxins and causes botulism disease.

Sorbic acid reacts with nitrites to form 1,4-dinitro-2-methylpyrrole and ethylnitrolic acid. This occurs under conditions similar to those of the human gastric tract.

Foods that probably contain nitrites. Author: Robert Doetsch. Source: Pixabay.

Sulphites

Sulfites are found in some foods or beverages for their preservative, antimicrobial, and antifungal properties. One of these drinks is wine.

Sorbic acid and sorbates react with these sulfites.

Side effects of its intake

According to sources consulted, the levels of sorbic acid or potassium sorbate used in food are between 100 and 2000 mg / L of the food.

The acceptable daily intake for humans has been set for several years at 25 mg / Kg of individual weight.

Although sorbic acid and sorbates have been in use for many years and are considered non-toxic, their complete harmlessness to humans has not yet been fully established. At present, many medical-scientific studies are being carried out to clarify the subject.

Effects on animals

The level of toxicity of sorbic acid and sorbates towards mammals is very low. Studies have been conducted over a long exposure time and even with intakes as large as 10% of the diet, with no negative results.

Some pregnant rabbits suffered severe stomach irritation, decreased food consumption, increased miscarriages, and death. But the effects were attributed to sorbic acid killing the natural flora in the intestines, leading to malnutrition.

In experiments with pregnant rats this did not occur, but weight loss was observed during pregnancy.

No tumor formation or any type of carcinogenic activity of these products has been found in the animals studied.

Its low toxicity can be explained because the mammalian body rapidly metabolizes it in the same way as other fatty acids, converting it into CO ₂ and water.

Effects on the immune system

The same thing happens in humans, sorbic acid is metabolized and does not accumulate in the body.

However, some studies have found that sorbic acid can negatively regulate certain biochemical pathways that are related to the functioning of the immune system.

Sorbic acid significantly suppresses tryptophan breakdown and neopterin production in cells of the immune system, such as monocytes. Both effects involve suppression of the Th1-type immune response.

This means that sorbic acid and sorbates decrease a certain type of mechanism in the human body's immune system.

Although the tests were performed with sorbic acid concentrations that can hardly be achieved in the blood system, it is likely that high concentrations are reached in the gastrointestinal tract.

Some sources report that it can cause hives in children or sensitive people, which could be related to the immune system.

Mutagenic effects of the products of their reactions

Various studies have shown that sorbic acid and sorbates are non-mutagenic and non-clastogenic (does not cause chromosomal breaks) in animals, but the potential to cause cancer in humans has not been evaluated.

Some researchers studied the possibility that cyclic and linear compounds formed by reaction between amines in food and sorbic acid could cause mutagenesis (generate mutations in the DNA of cells) and genotoxicity (cause damage to genes).

However, in tests carried out with human cell samples and with Salmonella typhimurium colonies, it was found that the reaction products between sorbic acid or sorbates and amines do not appear to be genotoxic or mutagenic, although their safety cannot be confirmed.

On the other hand, the reaction products between sorbic acid with nitrites, 1,4-dinitro-2-methylpyrrole and ethylnitrolic acid are mutagenic. These are formed under the acidic conditions found in the human stomach (pH between 2 and 4.2).

Additionally, it has been shown that wine containing SO ₂ and sorbic acid can produce mutagenesis.

Cheap wine that probably contains sulphites and sorbic acid. Shalom. Source: Wikipedia Commons.

The oxidation products that are formed during storage and under the action of heat have also been found to be genotoxic and possess cellular transformation capabilities.

All these aspects are still being studied.

Potential cell aging effect

Potassium sorbate causes a dramatic increase in the formation of reactive oxygen species in the cells of microorganisms, which can generate free radicals.

Such species can damage mitochondrial DNA, leading to aging and cell death. This occurs in microbes subject to the action of sorbate.

However, in humans this has not been fully clarified.

Uncontrolled aspect of its use against microbes

Recently (2019), some researchers found that sorbic acid can promote the induction of a BVNC state in bacteria.

The BVNC state (or Viable Non-Cultivable Bacteria) is defined as cells that enter a non-culturable state in response to stress situations, maintaining a reduced metabolism, high levels of ATP and preserving cell integrity, such as the content of chromosomes. and the cell membrane.

This means that they reduce your respiration, decrease the transport of nutrients, restrict the manufacture of certain molecules, store energy and keep your cell wall and its contents in good condition.

By using routine laboratory screening procedures, BVNC contamination can be underestimated, which can lead to the presence of pathogens in food.

Cells in the BVNC state can return to the culturable state when there are favorable conditions, such as the presence of certain nutrients.

Sorbic acid can induce BVNC status even at concentrations lower than those used in food preservation.

Other uses

Due to its antimicrobial action, sorbic acid is also used in cosmetics, pharmaceuticals and for the preservation of tobacco. It is also added to the packaging material of food products.

It is an intermediate product to produce plasticizers and lubricants. It allows to increase the shine of certain alkyd coatings. It is used to improve the grinding of certain gums.

References

1. Winkler, C. et al. (2006). Food preservatives sodium sulfite and sorbic acid suppress mitogen-stimulated peripheral blood mononuclear cells. Food and Chemical Toxicology 44 (2006) 2003-2007. Recovered from sciencedirect.com.
2. Pérez-Prior, MT (2008). Reactivity of Some Products Formed by the Reaction of Sorbic Acid with Sodium Nitrite: Decoomposition of 1,4-dinitro-2-methylpyrrole and Ethylnitrolic Acid. J. Agric. Food Chem. 2008,56, 11824-11829. Recovered from pubs.acs.org.
3. Ferrand, C. et al. (1998). Sorbic acid-amine function interactions. Food Additives and Contaminants, 1998, Vol. 15, No. 4, 487-493. Recovered from ncbi.nlm.nih.gov.
4. Ferrand, C. et al. (2000). Genotoxicity Study of Reaction Products of Sorbic Acid. J. Agric. Food Chem. 2000, 48, 3605-3610. Recovered from pubs.acs.org.
5. US National Library of Medicine. (2019). Sorbic acid. Recovered from: pubchem.ncbi.nlm.nih.gov
6. Ogane, H. et al. (2019). Low-concentration Sorbic Acid Promotes the Induction of Escherichia coli into a Viable but Nonculturable State. Biocontrol Science, 2019, Vol. 14, No. 1, 67-71. Recovered from jstage.jst.go.jp.
7. Unknown author. (1998). Fat as or in Food. Sorbic acid. In Fats and Oils Handbook. Recovered from sciencedirect.com.
8. Soltoft-Jensen, J. and Hansen, F. (2005). New Chemical and Biochemical Hurdles. In Emerging Technologies for Food Processing. Recovered from sciencedirect.com.
9. Pizzorno, JE (2016). Urticaria. In The Clinician's Handbook of Natural Medicine (Third Edition). Recovered from sciencedirect.com.
10. Piper JD and Piper PW (2017). Benzoate and Sorbate Salts: A Systematic Review of the Potential Hazards of These Invaluable Preservatives and the Expanding Spectrum of Clinical Uses for Sodium Benzoate. Comprehensive Reviews in Food Science and Food Safety. Vol. 16, 2017. Recovered from onlinelibrary.wiley.com.