Yeast Aromas: Primary Scents And Their Sources

Yeast cells produce a wide range of aroma compounds during fermentation, including higher alcohols, carbonyl compounds, phenolic compounds, fatty acid derivatives and sulfur compounds. These compounds are often vital for product quality.

The primary fermentation metabolites are ethanol, CO2, acetaldehyde and acetic acid. Ethanol is the most important compound produced by yeast and is the reason why early civilisations developed fermentation methods.

Yeast cells also produce secondary metabolites, including various higher alcohols, carbonyls, phenolic compounds, fatty acid derivatives and sulfur compounds. Many of these secondary metabolites are volatile and have pungent aromas.

The Ehrlich pathway is perhaps the most well-characterised biochemical pathway in yeast aroma production. This pathway modifies assimilated amino acids, the major source of nitrogen in many fermentation processes.

The three-step Ehrlich pathway modifies amino acids, harvesting and utilising the essential nitrogen as needed and, in turn, producing an array of fragrant and distinct aroma compounds.

The aroma compounds produced by yeast can be classified as higher alcohols, esters, aldehydes, ketones, and volatile sulfur compounds.

Higher alcohols can impart a much-desired effect on the product's flavour despite their higher sensory threshold, which can differ several orders of magnitude compared to their corresponding acetate esters. The major fusel alcohols found in alcoholic beverages are 1-propanol, 1-butanol, isobutanol, 2-phenylethanol and isoamyl alcohol.

Esters are formed by a condensation reaction between acetyl/acyl-CoA and an alcohol. The use of acetyl-CoA or acyl-CoA divides esters into two different categories, acetate esters and fatty acid ethyl esters, respectively. The small size and lipophilic nature of acetate esters allow them to readily diffuse from the cytoplasm into the extracellular medium whereas the longer hydrocarbon tails of fatty acid ethyl esters reduce their capacity to diffuse across the membrane. Therefore, acetate esters impart significantly more influence over flavour and fragrance than the fatty acid counterparts.

Vicinal diketones can provide a pleasant nutty, toasty and toffee-like flavour in fermented foods and beverages, most notably beer, wine and dairy products, but are considered off-flavours when present in high concentrations, changing their sensory perception to 'buttery' or 'rancid'.

Aldehydes and organic acids are another important aroma compound. Acetaldehyde is the central intermediate between pyruvate and ethanol but it is also an important aroma compound. It is quantitatively the most abundant aldehyde in most fermented products including apple juice and spirits, beer, cider and perry, wine, cheese, yoghurt and ripened butter.

Sulfur compounds are most relevant in beer, wine and cheese-making industries. While sulfur compounds are considered to be positive in some cases, such as in lager beers and whiskey, they are considered to be negative in others, such as in wine.

Amino acid metabolism is responsible for numerous aroma compounds including higher alcohols and esters.

Amino acid metabolites include vicinal diketones, higher alcohols, esters, sulfur-containing compounds and phenolic compounds.

Yeast produces higher alcohols, which can contribute to the flavour of the product

Yeast produces higher alcohols through the Ehrlich pathway, which is an amino acid catabolism pathway. Higher alcohols can be produced from catabolism of branched-chain amino acids, such as leucine, isoleucine, and valine. Higher alcohols can impart a much-desired effect on the product's flavour despite their higher sensory threshold, which can differ several orders of magnitude compared to their corresponding acetate esters. The major fusel alcohols found in alcoholic beverages are 1-propanol, 1-butanol, isobutanol, 2-phenylethanol, and isoamyl alcohol.

Yeast produces volatile fatty acids, which can be responsible for defects in wine

Yeast produces a wide range of compounds, including volatile fatty acids, during fermentation. These compounds are often vital for product quality. Volatile fatty acids, such as acetic acid, can be responsible for defects in wine.

Yeast produces esters, which add fruity and flowery notes to beer

Yeast is an essential ingredient in beer, and its esters are responsible for the beverage's fruity and flowery notes. Yeast produces a wide range of aroma compounds, including higher alcohols, esters, polyfunctional thiols, and terpenoids. These compounds contribute to the complex and diverse flavour and fragrance profiles of beer.

Terpenoids, for instance, are responsible for the floral, citrus, and honey notes often found in beer. They are derived primarily from hops, which pass on organoleptic properties to the beer. Monoterpenoids and sesquiterpenoids are particularly valued for their robust sensory qualities. Linalool and α-terpineol, for instance, contribute to the floral notes in beer, and their odours are perceptible even at low concentrations. Geraniol, another type of monoterpenoid, is known for its rose-like citrus odour.

The production of esters, which are responsible for fruity notes in beer, is closely linked to the metabolism of amino acids by yeast. Pyruvate, a key intermediate in yeast metabolism, feeds into the anabolism of amino acids, leading to the production of esters. This process also results in the formation of higher alcohols and sulfur-containing compounds, all of which contribute to the overall aroma profile of the beer.

The specific aroma compounds produced by yeast can be influenced by various factors, including the yeast strain, fermentation conditions, and the presence of other compounds in the wort or beer. For example, the use of hop pellets during fermentation can lead to an increase in monoterpenoid levels due to the release of terpenoids from glycosidically conjugated flavour-inactive precursors. Additionally, new strategies in genetic modification have helped enhance the production of certain aroma compounds. By optimising the upstream isoprenoid pathway flux and introducing plant-originated terpenoid synthases, brewers can improve the fruity and floral notes in beer.

While genetically modified brewing yeasts have improved the consistency and intensity of fruity and floral notes in beer, they have not gained widespread acceptance among consumers. Traditional brewing methods and naturally occurring aroma compounds remain essential to many beer enthusiasts.

Yeast produces aldehydes, which can have a pleasant fruity aroma when the concentration is low, but a grassy off-flavour when it is high

Aldehydes are one of the most important aroma compounds in the food and beverage industry, as they contribute to the quality of fermented products. They are synthesised by S. cerevisiae during fermentation and contribute highly desirable aromas, such as fruitiness, to fermented beverages. However, when the concentration of aldehydes is high, they produce green and grassy off-flavours. Acetaldehyde is the major aldehyde that constitutes over 90% of the total aldehyde content of wine. The low concentrations of acetaldehyde give a pleasant fruity aroma. However, when its concentration is high, it produces green and grassy off-flavours. Acetaldehyde has a direct effect on the aromatic profile of fermented beverages.

Yeast produces ketones, which can be responsible for defects in wine

Yeast can produce ketones, which can be responsible for defects in wine. Ketones are produced during the metabolism of alkenes and ketones by Candida maltosa and related yeasts. The degradation of ketones in yeasts is not well understood, but it is known that they can be either oxygenated to esters or reduced to the corresponding alcohol.

The ketone metabolism in yeasts is comparable to that of bacteria. In bacteria, a subterminal degradation of alkanes to ketones and their further metabolization has been described, which always involved Baeyer-Villiger monooxygenases (BVMOs).

In the context of wine, ketones are considered defects. Raspberry ketone, for example, is the primary aroma compound found in raspberries and is a valuable flavoring agent. The economic incentives for the production of raspberry ketone, combined with the very poor yields from plant tissue, make this compound an excellent target for heterologous production in synthetically engineered microbial strains.

In summary, yeast produces ketones, which can be responsible for defects in wine. The degradation of ketones in yeasts is not well understood, but comparable to that of bacteria.

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