Anethole

Anethole is the molecule of anise smell and anise taste. It is the main constituent of star anise, aniseed and fennel oils. The international standard for star anise oil asks for between 86 and 93 percent anethole [1]. The oil smells sweet and liquorice-like [2]. The taste is sweet [3]. Anethole hardly dissolves in water and dissolves in alcohol [2]. The cloud that forms when water is added to an aniseed spirit comes from this property [4]. Spirit, beverage, confectionery, bakery and chewing-gum producers buy it for its anise character [5]. In the oil, estragole and anisaldehyde come along with anethole.

Where in star anise, and how much?

The oil is in the dried fruit. Star anise (Illicium verum) is a tree grown in southern China and Vietnam; its star-shaped fruit is dried and sold as a spice [6], [5]. The oil is taken from the dried fruit by water or steam distillation. Dried fruit gave 7.5% oil, and anethole in the oil measured 89.5% [7]. Finely ground fruit gave more oil and higher anethole: 10.2% and 96.6% [8]. Oil taken with liquid CO₂ (carbon dioxide liquefied under pressure) also stayed at 89 to 92% anethole [9]. Fruit oil measured in Vietnam contained 90.1% anethole [10]. China's official pharmacopoeia also counts twigs and leaves as raw material [5]. The standard is for fruit oil only [1]. The certificate states which part the oil came from.

Drying changes the amount of oil. How the fruit is dried decides how much oil can be taken. Fresh fruit was dried by five methods; the best gave volatile oil at 12.3% of dry weight [11].

Stored fruit loses anethole. As the fruit waits, oil evaporates and anethole falls. Closed, dark packaging slows the loss. Dried fruit was stored for one year at room temperature in four packagings; a light-proof sealed bag preserved the volatile oil and the anethole best [12]. Commercial fruit of unknown age, bought on the market, was also measured. In its oil fraction anethole was 47.2%, estragole 14.4% and foeniculin (a minor constituent specific to star anise oil) 8.9% [13]. The difference may come from storage time or from the way the oil was extracted [2].

Aniseed and fennel also carry anethole. In both plants anethole is the main constituent. In aniseed the range is close to star anise; in fennel it is much wider. In 14 European oils taken from aniseed (Pimpinella anisum), anethole varied from 76.9 to 93.7% [14]. In fennel (Foeniculum vulgare) seed, anethole measured 34.8 to 82.0%, with fenchone at 1.6 to 22.8% and estragole at 2.4 to 17.0% alongside [15]. The proportion is read from the measurement, not from the plant's name.

The spice industry is not the only buyer of the fruit. Star anise fruit is the world's main source of shikimic acid, the starting material of the flu drug oseltamivir [6]. During the 2005 bird-flu period, 90% of China's production went into drug manufacture [5].

Anethole, estragole, anisaldehyde: what is the difference?

Values are taken from PubChem. XLogP shows whether a substance prefers oil or water. The higher the value, the more it prefers oil. Anethole, at 3.3, is on the oil side. When anethole dissolved in alcohol was added to water, only 15% of it stayed dissolved in the water, which is 0.1 g per litre [16].

Anethole has two forms. The form in the oil, and the wanted one, is trans-anethole. cis-Anethole is another arrangement of the same molecule, formed by light and heat [2]. In star anise oil, cis-anethole has been reported at 0.1 to 1.7% [2]. In aniseed spirits on the market, cis-anethole measured between 0.8 and 18.7% of the aroma constituents [17]. Fennel seed oil can carry up to 8.6% cis-anethole [15], so part of that range comes from the raw material.

Estragole comes with anethole in the oil. Its share in the smell is small, but in four food groups it has a legal limit, and in those groups it sets the dose of the oil. Estragole (methyl chavicol) is a close relative of anethole. The star anise oil standard puts it at 0.6 to 6.0% [1]. The smell of fennel tea was rebuilt in the laboratory from its constituents; removing estragole did not change the smell [18]. In anise tea too, anethole and anisaldehyde carried most of the smell, and estragole's share was judged negligible [19]. The European Union has put estragole on the list of substances that may not be added to food as such. It also sets upper limits in finished foods to which flavourings are added: 50 mg per kilogram in dairy products, processed fruit and vegetables and fish products, and 10 mg in non-alcoholic beverages [20]. The limit applies to the finished product, not to the oil. A food with no added flavouring, to which only the spice itself has been added, is outside the limit; a food with added oil is within it [20].

Anisaldehyde is part of the anise smell, not a rejection criterion on its own. Anisaldehyde occurs naturally in fresh oil [14] and can also form as anethole degrades [21]. Its smell is part of the anise character. In anise tea, anisaldehyde contributed most to the smell after anethole [19]. The standard asks for 0.1 to 0.5% anisaldehyde in star anise oil. In the same study, five genuine steam-distilled star anise oils averaged 3.1% [1]. In aniseed oils it has measured up to 5.4% [14]. This line is read together with the cis-anethole and anethole lines, not on its own.

Foeniculin is the signature of star anise. Among the three anise oils, foeniculin is found only in star anise oil, at 0.1 to 3.0% in the standard [1].

It is sweet. The sweetness of anise comes from anethole, the bitterness from another minor constituent. An aniseed extract was separated into its constituents and each was tasted. The sweet-tasting substance was anethole; the bitter-tasting one was another minor constituent (pseudoisoeugenyl 2-methylbutyrate) [3].

What is it used for?

Spirits: an aniseed spirit is defined by natural anethole. Anethole is the character of the pastis, sambuca, ouzo and anis family. The European Union requires natural extracts when it defines these spirits. An aniseed-flavoured spirit drink is made only with natural extracts. The source is star anise, anise, fennel or another plant carrying the same principal constituent. Only flavouring preparations and natural flavouring substances may be used. Pastis carries 1.5 to 2 g of anethole per litre, pastis de Marseille 1.9 to 2.1 g per litre. Sambuca contains 1 to 2 g of natural anethole and at least 350 g of sugar per litre [22]. Turkish rakı is distilled with aniseed [23]. In pastis, sambuca, anis and rakı samples on the market, trans-anethole made up 41.2 to 98.0% and estragole 0.1 to 18.0% of the volatile constituents [17]. Fresh sambuca measured 1.2 g of anethole per litre. A mistrà (an unsweetened Italian aniseed spirit) that had waited 11 months in the producer's warehouse was found below the legal minimum at 0.6 g per litre; the authors attributed this to storage loss [21].

Confectionery and bakery: the anise note. The oil is used in confectionery, bakery and chewing-gum flavours [5]. Anethole has been protected by taking it into a sugar capsule (cyclodextrin). Pure anethole in the capsule dissolved better in water and degraded less in light [24]. In a dry environment it hardly left the capsule at all; it was released as humidity and temperature rose [25]. In yeast-leavened products there is an opposite finding. In a laboratory vessel, pure anethole killed baker's yeast at 200 micrograms per millilitre in an airless, sugary environment; in an aerated environment no effect was seen [26].

Beverages: clarity and the estragole calculation. In a water-based beverage anethole makes a cloud. When water is added to pastis, the anethole instantly separates into droplets too small to see, and the drink turns cloudy. The droplets grow very slowly [27]. Anethole dissolved in water stays at the level of 0.1 g per litre [16]; in a clear beverage this limit cuts the dose first. The sugar capsule has increased the water solubility of anethole [24]. In a cloudy beverage the estragole limit cuts the dose; the calculation is below.

Spice blends and meat products: smell and stale taste. The anise note of Chinese five-spice and of meat broths comes from star anise. The spice itself has been tested for suppressing stale taste in meat products. In braised duck leg, 0.2% star anise reduced four stale-smelling compounds from fat deterioration by 15 to 42%. The meat-flavour (umami) value was also calculated highest at this level [28]. In pre-cooked beef kept six days in the refrigerator, 1 to 4% star anise reduced the stale smell. 1% gave the lowest stale-smell value, 3% brought fat deterioration to its lowest, and at 4% deterioration rose again compared with 3%. The dominant smell carrier of the addition was anethole [29].

60 percent or 90 percent: which form?

Essential oils are also sold diluted in an edible oil. For a diluted product, the certificate states whether the proportion refers to the oil itself or to the diluted product.

Why star anise oil rather than a single molecule? For the spirit producer, half of the answer is the law. Aniseed-flavoured spirit drinks, pastis and anis are made only with natural extracts and natural flavouring substances, and in sambuca the anethole must be natural [22]. Synthetic anethole cannot enter these categories. Anethole isolated from the plant counts as a natural flavouring substance [32]. The other half is the label: to write "natural star anise flavouring", at least 95% of the flavouring component must come from that plant [32]. For the confectionery and spice producer, the answer is character. Star anise smells different from anethole itself; structurally similar minor constituents complete the smell [33]. Anisaldehyde and foeniculin come with the oil [1]. The trace of the source also stays in the oil. In the laboratory, an origin test separated natural anethole from synthetic, and the minor constituents told which plant it came from [1].

The choice depends on five questions.

• Which plant character? Star anise oil carries linalool and foeniculin, aniseed oil himachalene, fennel oil fenchone [1]. In aniseed oil, anisaldehyde reaches up to 5.4% [14]. All three smell of anise; the minor constituent chooses the character.
• What is the estragole budget? In non-alcoholic beverages the limit is 10 mg per kilogram [20]. If the oil's estragole is 3.1% (the average of five genuine star anise oils [1]), at most 0.3 g of oil goes into 1 kg of beverage. The figures are examples.
• How much is needed? Pastis requires 1.5 to 2 g of anethole per litre [22]. If the oil carries 86% anethole [1], 1.7 to 2.3 g of oil per litre is needed. Because there will be loss over the shelf life, the calculation is made close to the upper figure, not the lower one; in an unsweetened product the loss is larger [21]. The figures are examples.
• Clear or cloudy? In a product diluted with water, anethole gives a cloud [4]. In pastis this is wanted. In a clear beverage, dissolved anethole is limited to 0.1 g per litre [16].
• Price. From highest to lowest, the price order has been reported as aniseed oil, fennel oil, star anise oil [1]. The comparison is made on the price per kilogram of anethole, not per kilogram of oil. Because anethole in fennel oil stays between 50 and 80% in the standard [1], less anethole may come for the same money.

How is an analysis report read?

Method. The oil is measured by gas chromatography (GC, a method that separates a volatile mixture into its constituents and measures the share of each); the constituents are identified by mass spectrometry. The percentages are usually the share of the constituents relative to each other (area normalisation).

trans and cis are separate lines. If "anethole" is a single line on the certificate, it is not clear which one it is. The standard keeps trans-anethole between 86 and 93% and cis-anethole between 0.1 and 1.0% [1]. cis-Anethole is more toxic than the trans form [2]. If cis-anethole is high on the certificate, the oil may have seen light, or it may have come from the raw material [2], [15].

The minor-constituent lines tell the source. Among the three oils, linalool, alpha-terpineol and foeniculin point to star anise; gamma-himachalene and pseudoisoeugenyl 2-methylbutyrate to aniseed; fenchone to fennel [1]. If an oil carries the markers of two plants together, it has been blended. If no markers are present at all, the oil may have been built from a single molecule; the definite answer comes from an origin test [1]. The estragole line goes into the calculation of the limit in the finished food [20].

Specification and result are different things. The specification is the producer's promise; the measured value is the result for that batch. The technical data sheet (TDS) is the general document; the certificate of analysis (CoA) belongs to the batch.

What shows quality?

Eight of thirty commercial oils turned out adulterated. Some of the oils on the market have been diluted or extended with anethole from another source. Thirty commercial star anise, aniseed and fennel oils were examined. Six proved adulterated by constituent profile, eight when the origin test was added. Four samples contained a cheap diluent (such as triethyl citrate). One star anise oil was 36.5% triethyl citrate. Six samples had anethole added from another plant or from synthesis, two of them diluted at the same time. One aniseed oil showed none of the aniseed markers; instead it showed linalool and menthol, and the origin test showed its anethole was synthetic [1].

Solvent residue. The residue line depends on the solvent. Distilled and CO₂ products contain no organic solvent; the line is read for an ethanol extract. The profile of an ethanol extract differs from the oil, as the row in the table shows [31].

How does it behave in a formulation?

Cold. Anethole freezes even in a cool warehouse. Its melting point is between 20 and 21 °C [21]. China's official pharmacopoeia describes star anise oil as "turning turbid or giving crystals in the cold, clearing on warming" [5]. A container arriving in winter may be solid; warmed and stirred, it returns to its former state. In a spirit, alcohol increases solubility, yet a 40% alcohol, sugar-free test solution gave anethole crystals after three months at 4 °C. Solutions carrying 125 g of sugar per litre or more, and commercial spirits, stayed clear [21].

Light. Light degrades anethole. Unsweetened mistrà and sweetened sambuca were held for 20 minutes under ultraviolet light. Mistrà lost 30 to 40% of its anethole, sambuca 20 to 30%. The sweetened spirit lost less [21]. In a water-diluted, cloudy spirit the conversion by light is slower. Under light, degradation products build up in a container that admits air [34]. Dark glass and a container filled to the top are needed for this reason.

Heat. In warmth the loss speeds up. A 40% alcohol test solution was held for five hours at 60 °C. The sugar-free solution lost close to half of its anethole. The solution with 350 g of sugar per litre lost 25%. At 40 °C the losses were 18 and 10%. No increase in anisaldehyde or cis-anethole was seen on heating [21].

Water. Anethole does not mix with water. When its alcoholic solution is added to water, an emulsion (a cloudy mixture of oil and water) forms by itself [4]. The share that stays dissolved is at the level of 0.1 g per litre [16].

Taste and smell. In fennel tea, anethole carries most of the smell; it is the substance whose amount rises highest above its odour threshold (the lowest amount the nose detects). Anethole and estragole passed into the tea at 10 to 15%. Limonene and pinene stayed below 2%, while fenchone and linalool passed at 26 to 50%, more than anethole [18]. In a water-based product, the character is carried by anethole together with minor constituents such as fenchone and linalool; limonene and pinene stay behind.

Food label. The term "natural star anise flavouring" is used only when at least 95% of the flavouring component comes from star anise. If it comes partly from that source, "natural star anise flavouring with other natural flavourings" is written [32].

Safety data sheet. When working with the extract, the product's safety data sheet (SDS) is followed. The transport and storage class is read from the SDS.