What Is Citral? Structure, Key Functions & Applications Explained
Citral (C10H16O) is a pale yellow liquid, and it has a strong lemon-like odour. It is also called 3,7-dimethyl-2, c-octadienal. Citral can not be mixed well with water, which means it is insoluble in water, but it is soluble in ethyl odour (ethanol), Mineral oil, and diethyl ether. It is mainly used in the manufacturing of perfumes and other smelling chemicals. Moreover, citral is a combination or mixture of two different aldehydes with different structures and the same molecular formula. Citral is known for its acceptable, distinct, and lemon-like pleasant smell. Mehylionene and lonone are made from citral and used in perfumery. It can be synthesized from myrcene, and ionene can be converted into synthetic Vitamin A.
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Properties of Citral
Two compounds are double bond isomers.
There are two types of citrals, citral A is E-isomer, and citral B is Z-isomer. Isomer geraniol has a lemon-like smell, and isomer neral has a less intense and sweeter lemon-like smell.
Physical Properties of Citral
Citral is a clear yellow-coloured fluid with a lemon-like fragrance.
The density of citral is 0.9 g/cm³.
Less dense than water.
Insoluble in water.
The melting point of citral is <-10°C
Citral is not persistent to alkanes and strong acid
When heated to decomposition, it emits acrid smoke and irritating fumes.
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Citral Uses
Citral is commonly used in beverages, foods, cosmetics, and other products. Its possible carcinogenic effects were investigated because of its versatile use in many products like citral oil and citral good scents. Moreover, in the investigation, two rats – one male and one female were used. Male mice were subjected to microencapsulated pure citral (500-4000 ppm) in food for 14 weeks or two years. Well, researchers could not find any proven data of carcinogenic activity of pure citral in mice and rats (According to anonymous data).
Well, citral is used as the flavouring agent and natural additive ingredient in foods, beverages, and cosmetics. Citral has properties of the acidic environment and chemical instability; hence its application domains are limited. Well, degradation of citral has been an industrial issue for many years, and it is a challenging task for ten years. In addition, the ways of citral degradation retarding are also some of the reasons for the limited use of o citral. Decreasing temperature, removing oxygen, and neutralizing pH are some ways of the citral degradation retarding process. The antioxidation process can prevent citral degradation, but this process is not available for non-commercially extensive extraction and costly processes. Moreover, antioxidants are not always suited as some antioxidants may add undesirable colour and taste to food products. Citral is also used as ingredients of citral sigma products that are very popular.
Citral Oil
Citral is a natural substance that can be obtained from plant oils. Lemongrass oil contains 75 to 80 % citral that may be isolated through the distillation process. Some other natural sources include verbena oil and citronella oil. Citral can be obtained from myrcene. It is generally found in the peel of the orange. Because of their intense aroma and flavour are used in beverages, perfumes, cosmetics, and food. Essential oils that present citral shows antifungal, antimicrobial, antiparasitic features that make it a natural preservative.
Citral Medicinal Uses
Citral has beautiful features like incense, anti-inflammatory, antibacterial, etc. It can also be used as a bug repellent. Here are some properties described that add values of citral to be used in medicines.
Antibacterial: Citral has strong antimicrobial properties in addition to fragrance. A study of Letters In Applied Microbiology declared that citral shows antimicrobial activity against gram-negative and gram-positive bacteria and fungus.
Anti-Inflammatory: Citral may also possess anti-inflammatory properties. A 2017 study on mice found that citral showed a significant decrease in TNF-α (Tumor Necrosis Factor-alpha) levels in tests demonstrating anti-inflammatory activity.
Bug-Repellent: Citral is one of the active components of citronella oil, a common ingredient used in insect repellents and citronella candles. Citronella has been registered as a gentle, plant-based insect repellent in the United States since 1948. Some evidence suggests that citral and the other active compounds in citronella interfere with mosquito olfactory receptors.
FAQs on Citral: Properties, Uses, and Natural Sources
1. What is citral and what are its main chemical properties?
Citral is a naturally occurring acyclic monoterpene aldehyde. It is not a single compound but a mixture of two geometric isomers: geranial (citral a) and neral (citral b). Key properties of citral include:
Appearance: It is a pale yellow liquid with a strong, characteristic lemon-like odour.
Formula: Its chemical formula is C₁₀H₁₆O.
Solubility: It is insoluble in water but highly soluble in organic solvents like ethanol, ether, and mineral oil.
Reactivity: As an aldehyde, it can be easily oxidised to form geranic acid or reduced to form geraniol.
2. What are the most common natural sources of citral?
Citral is predominantly found in the essential oils of several plants, which are responsible for their distinct citrus scent. The concentration varies significantly by source. The primary natural sources include:
Lemon Myrtle: Contains the highest concentration, typically 90–98%.
Lemongrass: A major commercial source, with oil containing 65–85% citral.
Litsea Cubeba (May Chang): Its oil contains about 70–85% citral.
Lemon Verbena: Contains around 30–35% citral.
Lemon and Lime: Found in smaller quantities in the peels, typically ranging from 2–9%.
3. What are the primary uses of citral in commercial industries?
Due to its strong lemon aroma and chemical properties, citral has several important commercial applications. Its primary uses are:
Fragrance Industry: It is a key ingredient in perfumes, colognes, soaps, lotions, and detergents to impart a fresh lemon scent.
Food and Beverage Industry: Used as a flavouring agent in soft drinks, candies, baked goods, and dairy products.
Chemical Synthesis: It serves as a vital precursor for synthesising other important chemicals, including ionones (used for violet fragrances) and, most notably, Vitamin A.
Antimicrobial Agent: Citral exhibits antifungal and antimicrobial properties, making it useful as a natural preservative and in some sanitary products.
4. How do the two isomers of citral, geranial and neral, differ from each other?
Geranial and neral are geometric isomers (or stereoisomers) of citral. While they share the same chemical formula and connectivity, they differ in their spatial arrangement around a carbon-carbon double bond. This structural difference leads to distinct properties:
Geranial (Citral a): This is the E-isomer (trans). It is generally described as having a stronger, more intense lemon aroma.
Neral (Citral b): This is the Z-isomer (cis). It has a less intense but often described as a sweeter and finer lemon aroma compared to geranial.
This difference in geometry, although subtle, affects how each molecule interacts with olfactory receptors, resulting in their slightly different scents and reactivity.
5. Why is citral considered a key compound in the synthesis of Vitamin A?
Citral is a crucial starting material in the industrial synthesis of Vitamin A (retinol). The process leverages citral's specific carbon skeleton and functional group. The synthesis pathway generally begins by reacting citral with acetone to form pseudoionone. This intermediate is then cyclised to create β-ionone, which contains the characteristic ring structure found in Vitamin A. From β-ionone, the carbon chain is systematically extended through further chemical reactions to build the complete structure of Vitamin A. Citral's availability from natural sources like lemongrass makes this a commercially viable route.
6. Is citral safe for human consumption and topical use?
Citral is generally considered safe, but its safety depends on the application. For consumption, it is listed as Generally Recognized as Safe (GRAS) by regulatory bodies like the U.S. FDA when used as a food flavouring agent. For topical use in cosmetics and perfumes, citral is a known dermal sensitiser. This means it can cause allergic skin reactions in some susceptible individuals. To mitigate this risk, organisations like the International Fragrance Association (IFRA) have established strict concentration limits for its use in products that are applied to the skin.
7. What specific features of the citral molecule are responsible for its characteristic lemon-like aroma?
The distinct lemon-like smell of citral is a result of the combination of two key molecular features: its functional group and its overall molecular shape. The presence of the aldehyde group (-CHO) is crucial for its odoriferous properties. This group, along with the specific length and double-bond positions of its 10-carbon terpenoid backbone, creates a unique molecular structure that fits precisely into specific olfactory receptors in our nasal passages. This interaction triggers a neural signal that the brain interprets as the scent of lemon.
