Aroma Alcohol: The Science Behind The Scent

Aromatic alcohols, also known as aryl-alcohols, are a class of chemical compounds containing a hydroxyl group bonded directly to an aromatic hydrocarbon group. They are produced by the yeast Candida albicans and are found in beer.

Aromatic alcohols are also associated with the flavour of alcoholic beverages. The presence of alcohol (ethanol) in drinks determines which aromas are easier to detect. For example, the more alcohol there is in a drink, the more hydrophilic aromas will escape to the headspace. Conversely, the higher the volume of water, the more hydrophobic aromas will move into the headspace.

The aroma compounds in alcoholic beverages are influenced by the raw materials used, such as malt and hops, or formed by yeast during fermentation. These compounds include higher alcohols, esters, polyfunctional thiols, lactones, furanones, and terpenoids.

How is aroma affected by alcohol?

The term "aroma" refers to the smell of something, which is detected by our noses or mouths. Humans can differentiate between up to 10,000 different odors, although we can't identify them all. This is because of their "detection threshold", which is the minimum concentration of an aroma that must be present for us to perceive it.

"Aroma" is made up of one or more aroma molecules, which are volatile and reach our sense of smell via "headspace", the air space directly above the surface of liquids.

Alcohol (ethanol) has partially hydrophobic properties. This means that it affects the detection of different aromas. Hydrophobic aroma molecules are water-averse and tend to flee when surrounded by water molecules, exiting into the headspace where they are easier to detect. On the other hand, hydrophilic aroma molecules are attracted to water molecules and prefer to remain in liquids.

The proportion of liquids—water versus alcohol—determines which aromas are easier to detect than others. The more alcohol there is in a drink, the more hydrophilic aromas will escape to the headspace. Conversely, the higher the volume of water, the more hydrophobic aromas will escape from the liquid and move into the headspace.

For example, non-alcoholic Gewürzstraminer contains more floral aromas than its alcoholic version. The juice's fruitiness is also more pronounced. This is because, in the absence of alcohol, the Gewürzstraminer's floral molecules escape into the headspace and cause the wine to smell more flowery.

Therefore, the presence of alcohol in a drink affects the aroma by influencing the behavior of different aroma molecules and determining which aromas are easier to detect.

What are the different types of aroma compounds?

Aroma compounds, also known as odorants, aromas, fragrances or flavourings, are chemical compounds that have a smell or odour. These compounds are small, with molecular weights of less than 310. They are highly volatile and can be detected by our olfactory system.

Aroma compounds can be found naturally in various foods, such as fruits, wine, spices, floral scents, perfumes, and essential oils. They are also used to improve the flavour and increase the appeal of products in the food service industry.

There are two main types of aroma compounds: organic and inorganic.

Organic Aroma Compounds

Organic aroma compounds fall into several categories, including:

• Esters
• Terpenes
• Amines
• Aromatics
• Aldehydes
• Alcohols
• Thiols
• Ketones
• Lactones

Some examples of organic aroma compounds include:

• Fruits, pineapple, and apple
• Fruity, pear, and banana
• Rose geranium and lemongrass
• Lavender, coriander, and sweet basil
• Caraway or spearmint
• Dill, caraway, and spearmint
• Feces and orange blossoms

Inorganic Aroma Compounds

Inorganic compounds with low molecular weights that can bind olfactory receptors are also considered aroma compounds. Examples of inorganic aroma compounds include:

• Hydrogen sulfide (H2S), which has a distinctive rotten egg scent
• Elemental chlorine gas (Cl2), which has an acrid smell
• Ammonia (NH3)

Aroma Compounds in Alcoholic Beverages

Alcoholic beverages, such as wine and spirits, contain hydrophobic aroma molecules that are detectable by our sense of smell. The proportion of water to alcohol in a drink determines which aromas are easier to detect. Alcoholic drinks with a higher proportion of alcohol will have more hydrophilic aromas, while those with a higher volume of water will have more hydrophobic aromas.

For example, non-alcoholic Gewürzstraminer wine contains more floral and fruity aromas than its alcoholic version, as the floral molecules are able to escape into the headspace and be detected by our sense of smell.

How does aroma travel to our sense of smell?

Aroma, or odour, is made up of molecules that are detected by our sense of smell. Humans can distinguish between up to 10,000 different odours, or even 1 trillion according to some studies. However, the human language does not have distinct words for each of these scents, and so much of our scent detection happens without us being fully aware.

Our sense of smell is unique in that it does not head straight to the thalamus, the brain structure associated with consciousness. Instead, it first arrives at the olfactory bulb, which passes information to the hippocampus, the part of the brain that controls our mood, emotions, and memory. This is why certain smells can trigger vivid, emotional memories.

Scents are detected through our noses (orthonasal) and through the mouth (retronasal). When we swallow, aromas travel up through the back of our throats and into the nasal passage. This is why wine experts aerate their wine, to detect more of the wine's volatile aroma molecules.

The proportion of liquids in a solution—water versus alcohol, for example—determines which aromas are easier to detect. Hydrophobic aroma molecules are water-averse and tend to flee to the headspace where they are easier to detect. Conversely, hydrophilic aroma molecules are attracted to water molecules and prefer to remain in liquids.

How does yeast contribute to aroma?

Yeast plays a significant role in the aroma of alcoholic beverages, particularly wine. Yeast has a complex metabolism and does much more than simply convert sugar into ethanol. It also produces other metabolites and acts on other substrates in the juice and wine, which affects aromas, flavours, mouthfeel, and colour.

The major odour-active components (OAC) formed during fermentation are volatile substances like esters, higher alcohols, and carbonyl compounds. These compounds are formed from the secondary metabolism of the yeast. The role of the key parameters during fermentation, such as temperature, yeast assimilable nitrogen (YAN), and suspended solids, is vital for the final organoleptic characteristics of wine.

Yeast selection matters, and different strains of yeast can have different effects on the aroma of the final product. For example, yeast can break the bonds between binders and aroma moieties to release beautiful aromas. This is especially important for wines that do not have monoterpenols (pleasant-smelling compounds) in their grape skins, such as Muscats, Riesling, Gewürztraminer, and Viognier.

The cell walls of yeast also contain polysaccharides, which can add layers of complex aromas and flavours and enhance mouthfeel as yeast cells die and cell walls break down, a biological process known as autolysis. During autolysis, the cell also releases aroma compounds, flavour agents, and peptides that may contribute sweetness.

In addition to the above, yeast can also influence the wine's colour. Anthocyanins, which have no aroma or flavour, can condense with yeast metabolites, such as acetaldehyde and pyruvic acid, to form more stable pigments known as pyranoanthocyanins, which may impact colour and colour stability.

How does water content affect aroma?

Water content has a significant impact on the aroma of a substance. The hydrophobic aroma molecules are water-averse, and they tend to flee when surrounded by water molecules, exiting into the headspace where they are easier to detect with our sense of smell. On the other hand, hydrophilic aroma molecules have an affinity for water molecules and prefer to remain in liquids.

The proportion of water to alcohol in a drink determines which aromas are easier to detect. The more alcohol there is in a drink, the more hydrophilic aromas will escape into the headspace. Conversely, a higher volume of water will result in more hydrophobic aromas escaping from the liquid and moving into the headspace.

For example, non-alcoholic Gewürzstraminer contains more floral and fruity aromas than its alcoholic version. The higher water content causes the wine's floral molecules to escape into the headspace, making the wine smell more flowery.

Water content also affects the aroma of foods. For instance, the flavor of pea protein isolates was found to change significantly when stored under different water activities and temperatures. Similarly, the taste of mineral water is influenced by its mineral content, which varies depending on its source. High levels of calcium and magnesium in hard water can negatively impact cooking by increasing the boiling point and affecting the taste and texture of ingredients.

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