The Sweet Smell Of Chemical Success

The class of compounds that often has a pleasant fruity aroma is called esters. Esters are derivatives of fatty acids and higher alcohols or ethanol. They are found in fruits and their peels, wine, spices, floral scents, perfumes, and essential oils. Esters are the most important group of strawberry aroma compounds, covering 90% of the total number of volatiles in ripe strawberry fruit. Esters are also important as they are responsible for the flavor characteristic of citrus fruits.

Esters, which are the most important group of strawberry aroma compounds

Esters are organic compounds derived from carboxylic acids. They are formed from the reaction of two reactants: alcohols and acids. In esters, the hydrogen in the carboxyl group of a carboxylic acid is replaced by a hydrocarbon group, which could be a methyl, ethyl, or phenyl group.

Esters are an important group of aroma compounds, with a characteristic pleasant, fruity fragrance. They are commonly found in essential oils and pheromones, and are used extensively in the flavour and fragrance industry. Esters are responsible for the aroma of many fruits, including strawberries.

Esters are formed through a process called esterification, which involves treating a carboxylic acid with an alcohol in the presence of a dehydrating agent. The term 'ester' was introduced in the first half of the 19th century by German chemist Leopold Gmelin, who probably derived it from the German 'Essigäther', meaning 'acetic ether'.

Esters derived from carboxylic acids and alcohols contain a carbonyl group, C=O, which is a divalent group at the C atom, giving rise to 120° C-C-O and O-C-O angles. Esters are more polar than ethers but less polar than alcohols. They can participate in hydrogen bonds as hydrogen bond acceptors but cannot act as hydrogen bond donors. This ability to participate in hydrogen bonding gives esters some water solubility.

Esters are generally identified by gas chromatography, taking advantage of their volatility. They are also characterised by IR spectra, which feature an intense sharp band in the range 1730-1750 cm-1 assigned to νC=O.

Esters are widely used in industry, with several billion kilograms of polyesters produced annually. They are also used in the production of perfumes, essential oils, and pheromones. In addition, esters have applications in automotive and aviation engineering, as well as in refrigeration and cosmetics.

Higher alcohols, which are important character-impact aroma compounds

Higher alcohols, also known as fusel alcohols, are alcohols with more than two carbon atoms. They are formed by yeast amino acid metabolism via the Ehrlich pathway. Higher alcohols are substrates for acetate ester production, a reaction catalysed by yeast alcohol acetyltransferases. Many acetate esters are associated with 'fruity' and 'floral' aromas in wine. For example, 2-phenylethyl acetate (2-PEA) is associated with 'rose', 'fruity', and 'honey' aromas.

Higher alcohols are important flavour compounds of spirits, but there are maximum limits since higher concentrations are considered toxicologically relevant. The International Agency for Research on Cancer (IARC) has classified ethanol in alcoholic beverages as carcinogenic for humans. In contrast, higher alcohols are generally regarded as important flavour compounds, so European legislation even demands minimum contents in certain spirits.

Polyfunctional thiols, which are extremely potent odorants

Polyfunctional thiols are powerful odorants found in many foods and drinks. They are characterised by their low molecular weight and high vapour pressure, which allows them to be readily dispersed in the air and detected by the human olfactory system.

Polyfunctional thiols are particularly notable for their contribution to the aroma of wine. They are found in wines made from several Vitis vinifera grape varieties, including Sauvignon blanc, Muscat, Scheurebe, Riesling, and Petite Arvine. During the winemaking process, non-volatile cysteine-S-conjugates are catalysed by yeast β-lyase activity, which converts them into volatile polyfunctional thiols. These compounds are responsible for the varietal aromas associated with different wines. For example, 3-sulfanylhexan-1-ol and 4-sulfanyl-4-methylpentan-2-one impart a grapefruit-like aroma, while 3-methyl-2-butene-1-thiol has a skunky-like smell.

In addition to wine, polyfunctional thiols are also found in hops and contribute to the varietal aroma of beer. The Cascade hop variety, for instance, exhibits a high potential for 3-sulfanylhexan-1-ol, while the Tomahawk and Nelson Sauvin cultivars are known for their bound 3-methyl-2-butene-1-thiol, 3-sulfanylpentan-1-ol, and 4-sulfanyl-4-methylpentan-2-one.

The presence of polyfunctional thiols in food and drinks can significantly impact the sensory profile and consumer acceptance. These compounds are often released during processing, such as fermentation, lipid oxidation, and thermal reactions, enhancing the aroma and flavour of the final product.

Lactones, which are key aroma components in fruits such as peach and nectarine

Lactones are a class of aroma compounds that are key components in the aroma of fruits such as peaches and nectarines. They are cyclic esters formed by the condensation of an acid and an alcohol. They are characterised by a generally fruity, creamy or oily aroma and have a low odour threshold, meaning that they can be detected in small concentrations. Lactones are widely distributed in foods and beverages, and their extraction from natural products can be very expensive.

Lactones are produced by the biotransformation or bioconversion of fatty acids, and in most cases, yeasts are the biological agent used. The main lactones of interest are γ- and δ-decalactones, γ-octalactone and γ-dodecalactone. γ-decalactone is particularly important as it has an intense peach flavour and aroma. It is produced by the biotransformation of ricinoleic acid, which is a major component of castor oil.

Γ-decalactone is generally used in the flavour industry due to its peachy odour and low detection threshold. It is produced by the yeast Yarrowia lipolytica, which has been genetically engineered to increase production. The biotechnological production of γ-decalactone has been optimised through the use of different substrates, culture conditions and extraction techniques.

Terpenoids, which are the largest class of plant secondary metabolites

Terpenoids, also known as isoprenoids, are a class of naturally occurring organic chemicals derived from the 5-carbon compound isoprene and its derivatives, which are called terpenes. Terpenoids are modified terpenes, where methyl groups have been moved or removed, or oxygen atoms have been added. They are the largest class of plant secondary metabolites, representing about 60% of known natural products.

Terpenoids are highly diverse, with a range of applications in medicine, flavour, fragrance, nutraceuticals, and pharmaceuticals. They are commonly used in traditional herbal remedies and play a role in plant defence, protecting them against insects, herbivores, and fungal diseases. They are also essential for plant growth and development.

Terpenoids are differentiated from one another by their basic skeleton and functional groups. They are classified based on the number of isoprene units they contain, with common terpenoids including mono(2 isoprene units), sesqui(3 units), di(4 units), tri(6 units), tetra(8 units), penta(10 units), and triterpenoids (more than 10 units).

Terpenoids are also classified according to the type and number of cyclic structures they contain: linear, acyclic, monocyclic, bicyclic, tricyclic, tetracyclic, pentacyclic, or macrocyclic. Well-known terpenoids include citral, menthol, camphor, ginkgolide, bilobalide, and the cannabinoids found in cannabis.

Terpenoids have a wide range of pharmacological activities, including anti-viral, anti-bacterial, anti-malarial, anti-inflammatory, hypoglycemic, and anti-cancer properties. They are of particular interest to medicinal chemists due to their substantial pharmacological bioactivity.

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