Introduction

 In alcoholic beverages, there are aroma compounds produced by the plant that are directly imparted into the beverage or released from its glycosidically bound form by enzymatic cleavage. These compounds were discussed last month in Wine Aroma Part 1. There are also “yeast-derived” flavors that are produced by the significant degradation of macromolecules through metabolism or catabolism and the de novo synthesis of aroma compounds.
  • Metabolism: Describes all chemical reactions involved in maintaining the living state of our cells.
  • Catabolism: A type of metabolism that is responsible for breaking complex molecules into smaller molecules.
  • De novo synthesis: Macromolecules made from smaller molecules rather than degradation of macromolecules into smaller ones.  

In this article, the following were insightful literature reviews that we referenced and highly recommend reading in their entirety:

[1] Styger, G., Prior, B., & Bauer, F. F. (2011). Wine flavor and aroma. Journal of Industrial Microbiology and Biotechnology, 38(9), 1145. Retrieved from: www.researchgate.net/publication/51518412_Wine_flavor_and_aroma

[2] Ronald Jackson’s Wine Science, our favorite Wine Textbook, which can be purchased at www.elsevier.com/books/wine-science/jackson/978-0-12-816118-0

[15] Robinson, A.L., Boss, P.K., Solomon, P.S., Trengove, R.D., Heymann, H. and Ebeler, S.E., 2014. Origins of grape and wine aroma. Part 1. Chemical components and viticultural impacts. American Journal of Enology and Viticulture, 65(1), pp.1-24. Retrieved from: https://www.ajevonline.org/content/65/1/1

[22] de-la-Fuente-Blanco, A., & Ferreira, V. (2020). Gas Chromatography Olfactometry (GC-O) for the (Semi)Quantitative Screening of Wine Aroma. Foods, 9(12), 1892. www.mdpi.com/2304-8158/9/12/1892

​[31] Lambrechts, M. G., & Pretorius, I. S. (2000, August). Yeast and its importance to wine aroma - A Review. South African Journal of Enology and Viticulture. Retrieved August 23, 2022, from https://www.journals.ac.za/index.php/sajev/
​article/view/3560
[50] Swiegers, J. H., Bartowsky, E. J., Henschke, P. A., & Pretorius, I. (2005). Yeast and bacterial modulation of wine aroma and flavour. Australian Journal of grape and wine research, 11(2), 139-173. Retrieved from: www.researchgate.net/publication/280765129_Yeast_and_bacterial_modulation_of_wine_aroma_and_flavour

Amino Acids and the Ehrlich Pathway

​The primary yeast-derived aroma compounds are fatty acids, higher alcohols, and esters. All these classes of aroma compounds are influenced to some extent directly or indirectly by the degradation of amino acids through the Ehrlich Pathway. In this process the nonpolar branched-chain amino acids: valine, leucine, and isoleucine are broken down. In this process:
Compounds involved
Amino acids [3] which are composed of: Amino group + α-keto acid
α-keto acid [3]:
Carboxylic acid group + ketone group

Transamination reaction
In this reaction Styger et al (2011) notes:
  • The amino group from an amino acid is transferred from one molecule to another. [5] In this case, the amino group is transferred from a branched-chain amino acid to α-ketoglutarate thereby forming α-keto acid and glutamate.
  • The reaction is catalyzed by mitochondrial and cytosolic branched-chain amino acid aminotransferases (BCAATases) enzymes, encoded by the BAT1 and BAT2 genes.
  • Yeast can also synthesize a-keto acids through the anabolic pathway, from glucose via pyruvate.

The amino acid content of a grape is influenced by factors including grape cultivar and grape growing conditions of climate and vintange variation. This in turn impacts its derivative compounds including fatty acids, higher alcohols and esters
for an insigtful study on this concept: Louw, L., Tredoux, A. G. J., Van Rensburg, P., Kidd, M., Naes, T., & Nieuwoudt, H. (2010). Fermentation-derived aroma compounds in varietal young wines from South Africa. https://doi.org/10.21548/
31-2-1418

​Ehrlich Pathway [6]
  • The α-keto acid is then decarboxylated (removal of a carboxyl group) by decarboxylase enzymes which forms a fusel aldehyde.
  • Then, depending on the redox status of the yeast
  • If oxygen is present the fusel aldehyde will be oxidized into a fusel acid/short-chain fatty acid.
  • If oxygen is not present, the fusel aldehyde will be reduced into a fusel alcohol.

For a great video of the Ehrlich pathway:
Still Behind The Bench. (2022, February 21). Ep 031- Fusel Oils (Fusel alcohols) and the Ehrlich Pathway. YouTube. Retrieved September 23, 2022, from www.youtube.com/watch?v=bNmfhwneuuo

Esterification: The formation of esters by a reaction between a carboxylic acid (of which fatty acids are a type) and an alcohol or phenol (Sarens et al 2006).

Yeast strain and species and their underlying genetics influence the rates of reaction due to the expression of the enzymes that catalyze these reactions. Detailed insight into the modulation ester and higher alcohol expression through genetic variation which goes beyond the scope of this article:
Cordente, A.G., Curtin, C.D., Varela, C. et al. Flavour-active wine yeasts. Appl Microbiol Biotechnol 96, 601–618 (2012). https://doi.org/10.1007/s00253-012-4370-z

Amino Acids ​in Grapes and Wine [50]
Amino Acid type is influenced by:
  • Grape cultivar
  • Grape maturity
  • Amino Acid quantity in the wine/must
Grape cultivar
  • The level of solids is impacted by clarification/flotation/racking.
  • Skin contact/maceration

Fatty Acids

​Fatty acids in wine can be produced by both yeasts and by grapes. They are a type of carboxylic acid that is composed of a:
Carboxyl group: (C(=O)OH)

Aliphatic chain, which is either
  • Saturated in which the carbon groups are joined by single bonds
  • Unsaturated in which the carbon groups are joined by double bonds

Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, from 4 to 28.[7]

General Influence
Impact on Aroma
Fatty acids can have an aroma though they are typically found in wine below aroma threshholds. However, they are more influential as a precursor to other aromatic compounds. Their impact on the wine often depens on if the fatty acids are produced by yeast or produced by grapes. For example, Schwab et al (2008) notes [8]:

Compounds derived from grape fatty acid include:
Compounds derived from yeast fatty acid include:
  • Esters
  • Aldehydes

Impact yeast development
Phospholipids are a major component of the lipid bilayer of the cell membrane in eukaryotes (like animals and yeast). They are composed of two hydrophobic "tails" derived from fatty acids that are commonly joined together by glycerol or another alcohol. In the cell membrane of eukaryotes, the lipid class sterol (cholesterol for example) is also interspersed among the phospholipids as it provides two-dimensional fluidity as well as mechanical strength against rupture. [9]

​Types of Fatty Acids in Wine[10]
Saturated and Unsaturated Fatty Acid
Saturated fatty acids: Fatty acids with only single bonds along the carbon chain.
​Unsaturated fatty acids (UFAs)
  • Unsaturated fatty acids have one or more double covalent bonds along the carbon chain.
  • improves membrane integrity and fluidity. This increases ethanol tolerance, the activity of membrane-associated enzymes and transporters including ATPase, and general amino acid permease. 
  • The main unsaturated fatty acids produced by Saccharomyces cerevisiae are saturated and mono-unsaturated fatty acids of 16- and 18-carbon compounds and in particular Palmitic acid (16:0) and Stearic acid (18:0). [12] Short-chain fatty acids: <6 carbons (C2-C5)
Formation
Developed from the degradation of amino acids via the Elrich pathway. For more insight into the mechanisms of the pathway see the section “Higher Alcohols”.

Notable short-chain higher alcohols include: [14]
Acetic Acid (C2)
  • Aroma: Vinegar
  • Aroma Threshold: 200,000 μG/L
•Formation:
  • Aldehyde: Acetaldehyde
  • Alcohol: Ethanol
  • A metabolic intermediate in the synthesis of acetyl-CoA from pyruvic acid. [15]

​Butyric Acid (C4)
  • Aroma: Vomit, cheese
  • Aroma Threshold: 173 μG/L [15]

Isobutyric Acid/
2-Methylpropanoic Acid (C4)
  • Aroma: Rancid, cheese
  • A Brettanomyces bruxellensis spoilage marker which may be capable of masking its volatile phenols. [15]
  • Aroma Threshold: 2300 μG [15]
•Ehrlich Pathway [1]
  • Amino Acid: Valine
  • a-Keto acid: a-Ketoisovalerate
  • Aldehyde: Isobutyraldehyde
  • Higher Alcohol: Isobutanol

Isovaleric Acid/3-Methylbutyric acid (C5)
•Aroma: Sweat, rancid

A Brettanomyces bruxellensis spoilage marker which may be capable of masking its volatile phenols. [15]
•Aroma Threshold: 33μG
•Ehrlich Pathway [1]
  • Amino Acid: Leucine
  • a-Keto acid: a-Ketoisocaproate
  • Aldehyde: Isovaleraldehyde
  • Higher Alcohol: Isoamyl alcohol
For an insightful study on fatty acid synthesis by yeast:
Lambrechts, M. G., & Pretorius, I. S. (2000, August). Yeast and its importance to wine aroma - A Review. South African Journal of Enology and Viticulture. Retrieved August 23, 2022, from www.journals.ac.za/index.php/sajev/article/view/3560
Medium-chain saturated fatty acids (MCFA): 6-12 carbons
Production mechanism
Medium chain and long chain fatty acids are synthesized by yeast via de novo synthesis. This is in contrast to short-chain fatty acids which are produced by the degradation of amino acids. In this process:
  • Acetyl-CoA is produced via glycolysis. For more insight into glycolysis: www.hawaiibevguide.com/a-guide-to-wine-fermentation-yeast
  • Acetyl-CoA is used to synthesize fatty acids by the type I fatty acid synthase (FAS) complex.[16] This complex, according to Lomakin et al (2007), is a highly integrated multienzyme. [17]
  • For more on the development of fatty acids:
  • •Schweizer E, Hofmann J. Microbial type I fatty acid synthases (FAS): major players in a network of cellular FAS systems. Microbiol Mol Biol Rev. 2004 Sep;68(3):501-17, table of contents. https://doi.org/10.1128/MMBR.68.3.501-517.2004

Influenced by:
 Yeast strain [18] [15] 
Oxygen
•Trace amounts of oxygen help yeast biosynthesize sterols and long-chain fatty acids. For more on the influence of oxygen see: www.hawaiibevguide.com/wine-fermentation-environment.html#physical-factors
•Anaerobic conditions, though originally thought to be inhibitory to yeast as noted by Edwards et al (1990), were found by Bardi et al. (1999) to be symptomatic. Styger et al (2011) notes that cell growth is instead arrested by the lack of oxygen and fatty acid biosynthesis increases in such conditions.

Grape Amino acid content
Boss et al (2015) studied the influence of the addition of free fatty acids, their methyl esters or acyl-carnitine and acyl-amino acid conjugates and found that: [20]
  • Β-alanine (an amino acid) concentration, within the ranges found in grapes, increases the synthesis of some MCFAs:
  • Ethyl hexanoate, octanoate and decanoate and their MCFA counterparts
  • Ethyl and phenylethyl acetates.
  • Isoamyl acetate did not increase because of limited isoamyl alcohol concentrations.
  • The addition of concentrations above that found in grapes had little to no impact nor do other amino acids involved in CoA biosynthesis.

Must insoluble solids
  • Over clarification of must can result in higher levels of hexanoic (C6), octanoic (C8), and decanoic (C10) acids than wines that completed fermentation.
  • The addition of insoluble grape solids and/or yeast hulls resulted in lower levels of medium-chain fatty acids while stimulating alcoholic fermentation and delayed MLF. [18]

Temperature
  • In red wine Restrepo et al (2019) [8] noted:
  • The greatest concentrations were produced under normal fermentation conditions of 28°C, pH 3.5, and aeration at the beginning of fermentation.
  • Concentration decreased if the temperature
  • Dropped below 24°C
  • Increased above 32°C. Additionally, this caused the medium-chain fatty acid to long-chain fatty acid (MCFA/LCUFA) ratio to increase therefore the authors recommend keeping fermentations below 28°C to avoid stuck fermentations.
Phase of fermentation and aging
Yunoki et al 2004 studied medium chain fatty acids in wine and found:[21]
  • During red wine fermentation
  • Capric acid (10:0) was newly detected.
  • Lauric acid and myristic acid increased.
  • Stearic acid (18:0) and other saturated fatty acids increased.
•After alcoholic fermentation
  • Concentration decreases significantly with a notable decrease in polyunsaturated fatty acids.
  • These fatty acids may be used by yeast during alcoholic fermentation as a carbon source or membrane lipids.
  • During Malolactic Fermentation

Fatty acid compositions remained essentially the same as after alcoholic fermentation with linoleic and palmitic acids generally being predominant.
  • Aging
Long-term aging decreases fatty acid content, particularly that of polyunsaturated fatty acids, possibly as a result of oxidation.

Significant MCFA acids in Wine [14]
Hexanoic Acid (C6)
  • Aroma: Sweat
  • Aroma Threshold: 420 μG/L

Octanoic acid (C8))
  • Aroma: Sweat, cheese
  • Aroma Threshold: 500 μG/L

Decanoic acid (C10)
  • Aroma: Rancid, fat
  • Aroma Threshold: 1000 μG/L

Long chain unsaturated fatty acids
(LCUFA): (>12 carbons)
Long chain fatty acids do not contribute directly to the aroma of wine because they are too large and nonvolatile.

​Yeast, however, requires long chain unsaturated fatty acids for survival. Robinson et al. (2014) notes those of particular importance are: Palmitic acid (C16) and Stearic acid (C18).
In grapes, medium and long chain fatty acids make up the primary class of lipids. Yunoki et al (2004) found in red grapes:

The most common fatty acids:
  • Palmitic acid (16:0): 29.9-47.0%
  • Lauric (12:0): 8.4-15.5%
  • Myristic acids (14:0): 12.6-22.9%
  • Oleic acid (18:1): 4.2-13.4%
  • Linoleic acid and α-linolenic acid (18:2): 3.2-14.1%

This influences hexenol and hexanal production, which in turn can influence thiol production. For more insight into the impact of linoleic acid on wine: hawaiibevguide.com/wine-aroma-compounds-1
Casu, F., Pinu, F. R., Fedrizzi, B., Greenwood, D. R., & Villas-Boas, S. G. (2016). The effect of linoleic acid on the Sauvignon blanc fermentation by different wine yeast strains. FEMS Yeast Research, 16(5), fow050. https://doi.org/10.1093/femsyr/fow050
  • The quantity of fatty acids in grapes and their must varies from grape to grape and is influenced by:
  • Production method
  • Cultivation site
  • Grape seed inclusion
  • Grape Cultivar

Fusel Alcohols/ Fusel Oils/ Higher Alcohols

Alcohols with more than two carbon atoms are commonly called higher or fusel alcohol. This is compared to ethanol which only has one carbon atom.
Influence on Wine
Higher fusel alcohols are created by the decarboxylation of amino acids in the Ehrlich pathway. For more on the particulars of this production mechanism see the section “Ehrlich pathway”.

Direct Influence
  • Aroma: Strong, pungent
  • The most abundant higher alcohols in wine: 1-propanol, isobutanol, isoamyl alcohol and 2-phenylethanol
  • Low concentrations (0.3 g/liter or less): Generally add an aspect of complexity to the bouquet.
  • Higher levels: Increasingly overpower the fragrance.
  • In distilled beverages, fusel alcohols provide a distinctive aromatic character.
  • In Port Wine (porto), the brandy added provides a fusel character.

Secondary Influence
Esterification. During fermentation and aging, esters form when organic acids react with higher alcohols. For more see the section: “Esters”.

Influenced by [2] [50]
Beyond amino acid concentration and makeup, winemaking factors that impact higher alcohol include:

Yeast species and strain as
  • Some species of Saccharomyces cerevisiae produce higher levels of fusel alcohol.
  • Non-Saccharomyces yeast like Pichia fermentans produces higher concentrations of fusel alcohols.
  • Additional higher alcohols may come from the metabolic activity of spoilage yeasts and bacteria. [2]

Ethanol concentration
Fermentation temperature
pH
Oxygen presence
The concentration of grape and yeast insoluble solids. The addition of yeast hulls (yeast ghosts),was found to decrease fusel alcohol concentration. [18]

Types include
Primary Alcohols/ Aliphatic alcohols:
Straight, branched, or cyclic (ring) structures

n-propanol/1-propanol
  • Aroma: Alcohol, ripe fruit [22]
  • Amino Acid: Threonine

Isoamyl alcohol/3-methyl-butan-1-ol
  • Aroma: Foot odor, solvent, sharp [22]
  • Concentration in wine: 45-490 mg/L [31]
  • Amino Acid: Leucine [1]

Isobutanol/2-methylpropan-1-ol/ Isobutyl Alcohol
  • Aroma: Fusel, alcohol, fruity [22]
  • Concentration in wine: 40-140 mg/L [31]
  • Amino Acid: Valine [1]

Active amyl alcohol/2-Methyl-1-butanol
  • Aroma: Alcoholic, harsh, marzipan [1]
  • Concentration in wine: 15-150 mg/L [31]
  • Amino Acid Isoleucine [1]

Aromatic alcohols:
Contain a cyclic (ring) structure
2-phenylethyl alcohol/

2-Phenylethanol
  • Aroma: Rose, floral [22]
  • Amino Acid: Phenylalanine [1]

Tyrosol
  • Aroma: Sweet, floral [22]
  • Amino Acid: Phenylalanine through the Ehrlich pathway and Tyrosine though conversion via the plant pathway. [24]

Non-Ehrlich Pathway Alcohols

Glycerol [25]
Influence on wine
Aroma: Slightly sweet taste
Typical concentrations:
  • •In dry wine: 4 -10 g/L
  • •In botrytized late harvest wine: <20 g/L is not uncommon

Though frequently suggested to improve mouthfeel and viscosity, experiments have yet to show a correlation between the concentrations at which glycerol is normally found in wine and the attribute making the impact probably not perceived or at least over-emphasized. [26] Noble and Bursick, 1984 found that concentrations of >5 g/L in dry wine is required for sweetness impact and 26 g/liter is required for mouthfeel impact. [27]

Influenced by:
Primary
  • Grape ripeness.
  • Wine type as red wine has higher glycerol than white wine. [50]
  • Botrytis cinerea infection as the fungus produces significant amounts as a result of metabolism.

Secondary
  • Yeast production, as it is the most abundant yeast byproduct after ethanol and carbon dioxide. It forms by the reduction of dihydroxyacetone phosphate by the glycerol-3-phosphate dehydrogenase enzyme, consuming NADH and forming glycerol-3-phosphate. [28]
  • Microbial flora on grape berries and cellar equipment, as some spoilage bacteria metabolize glycerol. [2]
  • Fermentation conditions include pH, temperature, nitrogen source and the yeast strain.
1-octen-3-ol/octenol
Aroma:
  • Dust, toasted, citrus, and mushroom [22]
  • Sweet earthy, with a strong herbaceous note reminiscent of lavender-lavandin, rose and hay. [29]
  • Influenced by: Derived from the enzymatic breakdown of

linoleic acid by Botrytis cinerea. [2]
2,3-butanediol

(2,3-butylene glycol)
Aroma: Buytter, lactic [22]

Influenced by:
Carbonic maceration [30]
For insight into the formation pathway: Ng, C., Jung, My., Lee, J. et al. Production of 2,3-butanediol in Saccharomyces cerevisiae by in silico aided metabolic engineering. Microb Cell Fact 11, 68 (2012). https://doi.org/10.1186/1475-2859-11-68

Sugar alcohols [2]
Influence on Wine
  • The sensory and enological significance of the conversion to its respective sugar is unknown.
  • Polyols and sugar alcohols may influence the body of the wine.

Influenced by
  • Higher concentrations: Typically caused by fungal infection in the vineyard or bacterial growth in the wine.
  • Sugar alcohols can be oxidized by some acetic acid bacteria to the respective sugars.

Types Include: Alditol, arabitol, erythritol, mannitol, myo-inositol, and sorbitol, are commonly found in small amounts in wine.

Esters

Influence on wine
Esters generally provide alcoholic beverages with fruit aromas and are particularly important for white wines and both red and white wine intended to be consumed young. Additionally, they can interact with each other and with non-ester compounds thereby altering the beverage’s aroma by amplification, suppression, or creating a completely different aroma. [2] While there are many classes of esters, the two primary classes that are relevant to the aroma of alcoholic beverages are Acetate esters and Fatty Acid Ethyl Esters (also known as ethyl esters).
The reason yeast synthesizes esters is unknown, however, it is hypothesized that volatile aromatic esters attract insects and other animals to fermenting fruit thereby spreading yeast, and longer chain fatty acid esters play a role in membrane fluidity. For an insightful literature review on the topic:
Saerens, S. M., Delvaux, F. R., Verstrepen, K. J., & Thevelein, J. M. (2010). Production and biological function of volatile esters in Saccharomyces cerevisiae. Microbial biotechnology, 3(2), 165-177. https://doi.org/10.1111/j.1751-7915.2009.00106.x

This research team of Dr Sophie Saerens, Dr. Kevin Verstrepen, Dr. Freddy Delvaux and, Dr. Johan Thevelein have done an insightful series of studies on ethyl esters which we have used for this section. The series also includes:
Saerens, S. M. G., Delvaux, F., Verstrepen, K. J., Van Dijck, P., Thevelein, J. M., & Delvaux, F. R. (2008). Parameters affecting ethyl ester production by Saccharomyces cerevisiae during fermentation. Applied and environmental microbiology, 74(2), 454-461. https://doi.org/10.1128/AEM.01616-07
Saerens, S. M., Verstrepen, K. J., Van Laere, S. D., Voet, A. R., Van Dijck, P., Delvaux, F. R., & Thevelein, J. M. (2006). The Saccharomyces cerevisiae EHT1 and EEB1 genes encode novel enzymes with medium-chain fatty acid ethyl ester synthesis and hydrolysis capacity. Journal of Biological Chemistry, 281(7), 4446-4456. ps://doi.org/10.1074/jbc.M512028200
We also suggest reading Dr. Caren Coetzee’s 2 part series on esters:
[32] Coetzee, C. (2020, September 29). Esters 101 - part 2. Sauvignon Blanc South Africa. Retrieved August 23, 2022, from https://sauvignonblanc.com/esters-101-part-2/
[36] Coetzee, C. (2020, September 29). ESTERS 101 - PART 1. Sauvignon Blanc South Africa. Retrieved September 23, 2022, from https://sauvignonblanc.com/esters-101-part-1/

Influenced by
Esters, while synthesized in grapes, are rarely of sensory significance. [2] However, the grape’s chemical composition indirectly impacts ester formation as it provides ester precursors that are used by yeast in different metabolic processes.

Enzyme-free formation by the equilibrium reaction between an alcohol and an acid is too slow to account for the large amounts of esters normally found in wine. [31]
Yeast metabolism is the primary formation mechanism of esters. [31] This mechanism has different metabolic pathways depending on the type of ester. However they are all catalyzed by the enzyme coenzyme A (CoA) which reacts with an alcohol to form an ester.

​Yeast metabolism is the primary formation mechanism of esters. [31] This mechanism has different metabolic pathways depending on the type of ester. However they are all catalyzed by the enzyme coenzyme A (CoA) which reacts with an alcohol to form an ester.
Clarification and SO2
  • Addition to ultra-filtered grape juice resulted in equal or higher levels of the major esters compared to wines made with insoluble grape solids. [18]
  • Both low SO2 levels and juice clarification support ester synthesis and retention. [2]

Temperature
  • Low fermentation temperatures (10 ºC)
  • Favor the synthesis of fruit esters, such as isoamyl, isobutyl and hexyl acetates [2]
  • Higher temperatures (15–20 ºC)
  • Promote the production of higher-molecular-weight esters including ethyl octanoate, ethyl decanoate, and phenethyl acetate. [2]
  • Higher temperatures can favor ester hydrolysis. However, some winemakers believe the number of esters obtained from fermenting at a higher temperature can create an increase in esters as it is enough to offset the losses caused by estery hydrolysis. [32]

Oxygen
  • Both acetate and ethyl esters dramatically decrease in concentration by aeration, though ethyl hexanoate is an exception. [16]
  • Intercellular grape fermentation (carbonic maceration) produced by the absence of oxygen can influence ester formation as it produces:
  • Shikimic acid which then degrades to cinnamic acids and further into benzaldehyde, vinylbenzene and ethyl cinnamate.
  • Enzyme-produced aroma compounds include ethyl and methyl vanillate, ethyl 9-decenoate, and 1-octanol.
  • Acetaldehyde and acetic acid [33]
  • For more on carbonic maceration:
  • www.hawaiibevguide.com/wine-prefermentation
            .html#maceration

Sur Lie maturation [1]
  • Decreased the concentrations of volatile compounds imparting a fruity aroma and increasing long-chain alcohols and volatile fatty acids.
  • Can remove some of the unpleasant wine volatile phenols due to its biosorbent qualities.

Aging
  • Can increase some esters due to chemical esterification. [1]
  • Generally leads to a loss of varietal aromas and the formation of new aromas characteristic of older wines or atypical aromas associated with wine deterioration. [1]

Methods to preserve the esters during aging include:
  • Low storage temperatures which reduce acid hydrolysis. [19]
  • Oxygen mitigation through sufficient antioxidant protection like with SO2.
  • For more on wine storage: www.hawaiibevguide
            .com/wine-bottling.html
  • Esters
Types of Esters
Acetate esters (Fruit Esters)
Acetic acid + Alcohol (mainly fusel alcohol)

Influenced by [34]
Acetate esters are catalyzed by
  • Alcohol acetyltransferases I and II coded by the genes Atf1p and Atf2p are sulfhydryl enzymes produced by Saccharomyces cerivisae.
  • Isoamyl alcohol acetyltransferase
  • Ethanol acetyltransferase

Formed when [35]
  • Alcohol acetyltransferase, reacts with Acetyl-CoA and various higher alcohols, to synthesize acetate esters through acetyltransferase activities.
  • These activities are strongly repressed under aerobic conditions and by the addition of unsaturated fatty acids.

Repressed by (the primary limiting factor)
The concentration of the acetyltransferase enzymes expressed by the genes ATF1 and ATF2 is the limiting factor in acetate ester formation. [36] It has been found that this is impacted by [34]:
  • Oxygen
  • Unsaturated fatty acid concentration

While new acetate esters can be formed, there is a net loss due to degradation during maturation and storage/aging caused by:
  • Esterase enzymes which are coded by the gene IAH1, and are expressed in varying quantities depending on yeast strain. [37] These enzymes hydrolyze a multitude of esters and in particular hexyl acetate, ethyl acetate, and 2-phenylethyl acetate and isoamyl acetate. [38]
  • Acid hydrolysis which reduces acetate esters back to their equilibrium constant components of alcohols and acetic acid. [32]

Mitigation strategies include:
  • Juice clarification and must aeration to reduce esterase enzymes.
  • Low storage temperatures to minimize reaction kinetics.

Types include
Ethyl acetate
(both an acetate and ethyl ester)

Influence on Wine
  • Aroma: Pineapple, fruity, solvent, balsamic [44]
  • Aroma threshold: 12264 μg/L [14]
  • Typical Concentration in “normal wine”: 30–60 mg/liter [39]
  • Above 150 mg/liter: Undesirable nail-polish remover, sour-vinegary, aroma. Jackson in Wine Science notes that excessive levels are typically associated with grape, must, or wine contamination with acetic acid bacteria as the bacteria directly synthesize ethyl acetate and also produce acetic acid that can react nonenzymatically with ethanol to form ethyl acetate.

Mechanism of formation
Condensation of: Acetic acid + ethanol
As acetyl-CoA/acetic acid and ethanol are the most abundant acids and alcohols present in the fermentation, ethyl acetate is normally the most abundant esther. [31]

​Isoamyl acetate (3-methylbutyl acetate)
  • Aroma: Banana-like (the synthetic form is used in banana-flavored candy)
Mechanism of formation:
  • Condensation of: Acetic acid + Isoamyl alcohol
  • Derived from the amino acid: Leucine. [40]
  • Concentration in wine: 0.03–8.1 mg/L
  • Aroma threshold: 30 μg/L [14]

Isobutyl acetate [32]
  • Aroma: Fruity, floral
  • Condensation of: isobutyl alcohol + acetic acid
  • Derived from the amino acid: Valine [1]

2- phenylethyl acetate [32]
  • Aroma: Honey, fruity, rose, floral
  • Concentration in wine: 0.01–4.5 mg/L [31]
Influenced by:
  • Condensation of: Phenethyl alcohol + acetic acid
  • Derived from the amino acid: Phenylalanine [1]

Benzyl acetate [32]
  • Aroma: Apple, jasmine
  • Condensation of: Benzyl alcohol + Acetic acid

Hexyl acetate
  • Aroma: Apple, cherry, pear, floral
  • Condensation of: (hexan-1-ol or hexanal) + Acetic Acid [41] [42]

Medium-chain fatty acid ethyl esters aka Ethyl esters
(Ethanol + medium-chain to a fatty acid)
Influence on wine
  • Typically has weaker odors than acetate esters to the extent that they can be aromatically insignificant or provide minor aromatic contributions. [32]
  • Aroma shifts as the length of the hydrocarbon chain of the acid increases, going from being fruity to soap-like and, finally, lard-like with C16 and C18 fatty acids. [2]

Influenced by
Mechanism
•Catalyzed by: Acyl-CoA: ethanol O-acyltransferase enzymes EEB1, which is the main enzyme, and EHT1 which plays a minor role. [16]
•Process: The acyl-CoA condenses with ethanol. [34]
Factors that influence the formation, according to Saerens et al. (2008) include:
•Fatty acids concentration is the major limiting factor of ethyl ester production and not the expression of enzymes

EEB1 and EHT1.
  • The Hydrostatic pressure of the fermentation tank as its increase is attributed to the increase in dissolved carbon dioxide in the fermentation medium.
  • Temperature increases can increase ethyl ester concentration as it influences “the thermodynamic equilibrium of ester solubility in cellular lipids and the aqueous medium”. This is notable as ethyl esters excretion into the fermenting medium decreases drastically with increasing chain length, whereas acetate esters are easily excretable by yeast.
Lactic Acid Bacteria, and in particular Oenococcus oeni, can produce carboxylic acids during malolactic fermentation which are then esterified into their corresponding acid. Styger et al (2011) notes:
•
Lactic acid is produced through the degradation of malic acid. The corresponding ester: Ethyl lactate
•
Acetate is produced through the catabolization of acetaldehyde into ethanol and acetate. The corresponding acid: Ethyl acetate
•
Citric acid is metabolized into carbonyl or acetonic compounds, including diacetyl, acetoin, and 2,3-butanediol which have a buttery flavor.
•
Methionine, an amino acid, is metabolized into sulfur-containing compounds like methanthiol, methyl disulfide, and methionol 3-(methylsulfanyl) propionic acid. For more on these compounds: www.hawaiibevguide.com/wine-faults.html#volatile-sulfur-compounds
•
Other esters associated with malolactic fermentation include ethyl hexanoate and ethyl octanoate.
•
For more on the impact of Oenococcus oeni’s impact on esters: Sumby, K. M. (2013). Molecular and biochemical characterization of esterases from oenococcus oeni and their potential for application in wine (Doctoral dissertation).
https://hdl.handle.net/2440/83646
During aging ethyl esters of fatty acids diminish more slowly as they are thought to be in chemical equilibrium in young wines and the hydrolysis that occurs during storage happens relatively slowl

​3-isopropyl-2-methoxypyrazine (IPMP)
Influence on Wine
Aroma: Peas, green/herbaceous, peanuts (and burnt peanut butter at high concentrations), vegetal, canned green bean, dead leaves, fresh mushroom, musty, earthy

Perception threshold:[19]
  • White wine: 0.32 - 1 ng/L
  • Red wine: 1-6 ng/L
Typical concentration in wine
  • White Wine: 0.3 ng/L
  • Red Wine: < 27 ng/L

Influenced by:
  • Generally found at the highest levels in stems of grapevines, with lower levels found in the grape seeds and skins.
  • Harmonia axyridis aka the Multicolored Asian Lady Beetle being included in the wine (Pickering et al 2005). This is known as ladybug taint, and can be problematic at 200-400 ladybugs per ton of fruit.
Other pyrazines
Isopropyl and sec-butyl methoxypyrazines typically occur at concentrations just at or below their perception thresholds [2].

C6 Compounds [13]
(Characterized by a 6 carbon structure)
3-Hexen-1-ol aka hexenol (an alcohol) and hexanal (an aldehyde)
Influence on wine
Aroma:
  • Grassy to herbaceous aroma associated with Grenache, Sauvignon blanc, and wines made from immature grapes. [2]. This can be positive or negative depending on the wine style.
  • As hexenal is one of the major contributors to the aroma of freshly damaged green leaves and helps, along with methoxypyrazines, to provide the typical “grassy” character of Sauvignon Blanc [14].

Levels
  • Perception threshold in white wine: 400 μg/L
Typical levels
  • Gewurtztraminer: 75 μg/L
  • Falanghina and Macabeo (white wine): 600 μg/L
  • Young Red Wine: 650 μg/L
  • Aged Red Wine: 800 μg/L

Influenced by
Primary:
Mechanism of formation[14] [15]
  • Formed from C18 grape polyunsaturated fatty acids that primarily originate from membrane lipids. These fatty acids include linoleic acid and α-linolenic acid.
  • The C18 polyunsaturated fatty acids, when damaged from fungal infection, mechanical harvesting, and/or crushing, are degraded by enzymes to form compounds that have anti-fungal/anti-yeast properties.
  • The enzyme lipoxygenase and/or hydroperoxide lyase degrades the C18 polyunsaturated fat to 3-hexenal.
  • The enzyme alcohol dehydrogenase degrades 3-hexenal to

3-hexen-1-ol.
  • The concentrations of the precursor polyunsaturated fatty acids differ by cultivar.
  • Quantities are highest at pre-veraison as linolenic acid decreases with ripening.

Secondary
Maceration time and increase in temperature increase 3-hexen-1-ol formation.
Oxygen is needed for formation.

Tertiary
Stable during storage with a slow decline after 210 days but no impact from different SO2 levels.
Types include
Ethyl butyrate/Ethyl butanoate
  • Aroma: Lactic, strawberry, sweet [22]
  • Condensation of:

Ethanol + Butyric acid [43]
Ethyl lactate
  • Aroma: Lactic, buttery, fruity [43]
  • Condensation of:

Lactic Acid + Ethanol [45]
Ethyl hexanoate
  • Aroma: Anise, fruit, ester [22]
  • Condensation of:

Hexanoic acid + Ethanol [46]
Ethyl octanoate
  • Aroma: Fruit, ester, sweet [22]
  • Condensation of:

Octanoic acid + Ethanol [47]
Ethyl decanoate
  • Aroma: Grape [14]
  • Condensation of:

Decanoic acid + Ethanol [48]
Diethyl succinate
  • Aroma: Fruity, melon [44]
  • Condensation of:

Succinic acid + Ethanol
Ethyl 2-methyl butanoate
  • Aroma: Fruit, sweet, strawberry, anise [22]
  • Condensation of: 2-methylbutyric acid + Ethanol [49]
  • Derived from the amino acid: Isoleucine [1]
  • Concentration in wine: 0–0.9 mg/L [31]
Ethyl isovalerate/Ethyl 3-methylbutyrate [50]
  • Aroma: Fruit, sweet anise [22]
  • Condensation of: Isovaleric acid + Ethanol
  • Derived from the amino acid: Leucine [1]
  • Concentration in wine: 0–0.7 mg/L[31]

​Other amino acid metabolic pathways
(for flavor and aroma compounds) [1]
Aspartic acid
  • Oxalo-acetate is formed by deamination (removal of the amino group).
  • Oxalo-acetate can be catabolized into: acetoin, diacetyl, and 2,3-butanediol by bacteria but it is not known whether any yeast strains can complete this reaction. [1]

Threonine
Can be converted into acetaldehyde, and further into ethanol or acetic acid.

Cysteine
Can form odor-impacting compounds through Maillard Reaction. In this reaction, amino and carbonyl groups form new compounds.

Methionine
  • Methanethiol is formed when methionine is catabolized and demethylated.
  • Methanethiol can be further converted to undesirable sulfur compounds like hydrogen sulfide (H2S), and thioesters/​sulfhydryls. [50] For more on Methanethiol see https://www.hawaiibevguide.
​           com/wine-faults.html#volatile-sulfur-compounds

Resources and Suggested Reading

​1.Styger, G., Prior, B., & Bauer, F. F. (2011). Wine flavor and aroma. Journal of Industrial Microbiology and Biotechnology, 38(9), 1145. Retrieved from:
www.researchgate.net/publication/51518412_Wine_flavor_and_aroma

2.Ronald, S. J. (2019). Wine science: principles and applications. ELSEVIER ACADEMIC PRESS.

3.Wikipedia contributors. (2022, July 20). Amino acid. In Wikipedia, The Free Encyclopedia. Retrieved 12:39, August 21, 2022, from https://en.wikipedia.org/w/index.php?title=Amino_acid&oldid=1099291473

4.Wikipedia contributors. (2022, July 21). Keto acid. In Wikipedia, The Free Encyclopedia. Retrieved 12:39, August 21, 2022, from https://en.wikipedia.org/w/index.php?title=Keto_acid&oldid=1099475940

5.Wikipedia contributors. (2022, May 17). Transamination. In Wikipedia, The Free Encyclopedia. Retrieved 15:42, September 20, 2022, from https://en.wikipedia.org/w/index.php?title=Transamination&oldid=1088273136

6. Hazelwood, L. A., Daran, J. M., Van Maris, A. J., Pronk, J. T., & Dickinson, J. R. (2008). The Ehrlich pathway for fusel alcohol production: a century of research on Saccharomyces cerevisiae metabolism. Applied and environmental microbiology, 74(8), 2259-2266. https://doi.org/10.1128/AEM.02625-07

7. Wikipedia contributors. (2022, September 3). Fatty acid. In Wikipedia, The Free Encyclopedia. Retrieved 17:20, September 22, 2022, from https://en.wikipedia.org/w/index.php?title=Fatty_acid&oldid=1108215379

8. Schwab, W., Davidovich-Rikanati, R., & Lewinsohn, E. (2008). Biosynthesis of plant-derived flavor compounds. The plant journal, 54(4), 712-732. https://doi.org/10.1111/j.1365-313X.2008.03446.x

9. Wikipedia contributors. (2022, August 26). Phospholipid. In Wikipedia, The Free Encyclopedia. Retrieved 12:30, September 19, 2022, from https://en.wikipedia.org/w/index.php?title=Phospholipid&oldid=1106695068

​10. Restrepo, S., Espinoza, L., Ceballos, A., & Urtubia, A. (2019). Production of fatty acids during alcoholic wine fermentation under selected temperature and aeration conditions. American Journal of Enology and Viticulture, 70(2), 169-176. Retrieved from: https://www.ajevonline.org/content/70/2/169
11. Duan, L.L., Shi, Y., Jiang, R., Yang, Q., Wang, Y.Q., Liu, P.T., Duan, C.Q., & Yan, G.L.. (2015). Effects of adding unsaturated fatty acids on fatty acid composition of saccharomyces cerevisiae and major volatile compounds in wine. South African Journal of Enology and Viticulture, 36(2), 285-295. Retrieved September 24, 2022, from http://www.scielo.org.za/scielo.php?script=sci
_arttext&pid=S2224-79042015000200001&lng=en&tlng=es.

12. Yazawa, H., Iwahashi, H., Kamisaka, Y., Kimura, K., & Uemura, H. (2009). Production of polyunsaturated fatty acids in yeast Saccharomyces cerevisiae and its relation to alkaline pH tolerance. Yeast, 26(3), 167-184. Retrieved from: www.academia.edu/29839980/Improvement_of_polyunsaturated_fatty_acids_synthesis_by_the_coexpression_of_CYB5_with_desaturase_genes_in_Saccharomyces_cerevisiae

13. Liu P, Ivanova-Petropulos V, Duan C, Yan G. Effect of Unsaturated Fatty Acids on Intra-Metabolites and Aroma Compounds of Saccharomyces cerevisiae in Wine Fermentation. Foods. 2021; 10(2):277. https://doi.org/10.3390/foods10020277

14. Francis, L. (2013, August). Fermentation-derived aroma compounds and grape-derived monoterpenes. The Australian Wine Research Institute. Retrieved September 22, 2022, from www.awri.com.au/wp-content/
uploads/2013/08/francis-W07-AWITC15.pdf

15. Robinson, A.L., Boss, P.K., Solomon, P.S., Trengove, R.D., Heymann, H. and Ebeler, S.E., 2014. Origins of grape and wine aroma. Part 1. Chemical components and viticultural impacts. American Journal of Enology and Viticulture, 65(1), pp.1-24. Retrieved from: https://www.ajevonline.org/content/65/1/1

16. Saerens, S. M. G., Delvaux, F., Verstrepen, K. J., Van Dijck, P., Thevelein, J. M., & Delvaux, F. R. (2008). Parameters affecting ethyl ester production by Saccharomyces cerevisiae during fermentation. Applied and environmental microbiology, 74(2), 454-461. https://doi.org/10.1128/AEM.01616-07

17. Lomakin, I. B., Xiong, Y., & Steitz, T. A. (2007). The crystal structure of yeast fatty acid synthase, a cellular machine with eight active sites working together. Cell, 129(2), 319-332. https://doi.org/10.1016/j.cell.2007.03.013

18. Edwards, C.G., Beelman, R.B., Bartley, C.E. and McConnell, A.L., 1990. Production of decanoic acid and other volatile compounds and the growth of yeast and malolactic bacteria during vinification. American Journal of Enology and Viticulture, 41(1), pp.48-56. For: https://www.ajevonline.org/content/ajev/41/1/48.full.pdf

19. Bardi, L., Cocito, C., & Marzona, M. (1999). Saccharomyces cerevisiae cell fatty acid composition and release during fermentation without aeration and in absence of exogenous lipids. International journal of food microbiology, 47(1-2), 133-140. Retrieved from: https://www.academia.edu/34108797/Saccharomyces_cerevisiae_cell_fatty_acid_composition_and_release_during_fermentation_without_aeration_and_in_absence_of_exogenous_lipids

Continued Resources and Suggested Reading

20. Boss PK, Pearce AD, Zhao Y, Nicholson EL, Dennis EG, Jeffery DW. Potential Grape-Derived Contributions to Volatile Ester Concentrations in Wine. Molecules. 2015; 20(5):7845-7873. https://doi.org/10.3390/molecules2
0057845

21. Yunoki, K., Tanji, M., Murakami, Y., Yasui, Y., Hirose, S., & Ohnishi, M. (2004). Fatty acid compositions of commercial red wines. Bioscience, biotechnology, and biochemistry, 68(12), 2623-2626. https://doi.org/10.1271/bbb.68.2623

22. de-la-Fuente-Blanco, A., & Ferreira, V. (2020). Gas Chromatography Olfactometry (GC-O) for the (Semi)Quantitative Screening of Wine Aroma. Foods, 9(12), 1892. www.mdpi.com/2304-8158/9/12/1892

23. Chen, E. C. H. (1978). The relative contribution of Ehrlich and biosynthetic pathways to the formation of fusel alcohols. Journal of the American Society of Brewing Chemists, 36(1), 39-43. https://doi.org/10.1094/ASBCJ-36-0039

24. Guo, W., Huang, Q., Liu, H., Hou, S., Niu, S., Jiang, Y., ... & Fang, X. (2019). Rational engineering of chorismate-related pathways in Saccharomyces cerevisiae for improving tyrosol production. Frontiers in bioengineering and biotechnology, 7, 152. https://doi.org/10.3389/
fbioe.2019.00152

25. Carlson, C. (2004). Glycerol | Waterhouse Lab. Waterhouse Lab at UC Davis. Retrieved August 23, 2022, from https://waterhouse.ucdavis.edu/whats-in-wine/glycerol

26. Coetzee, C. (2022, March 30). Glycerol – does it really contribute to mouthfeel? Sauvignon Blanc South Africa. Retrieved August 23, 2022, from https://sauvignonblanc.com/glycerol-does-it-really-contribute-to-mouthfeel/

27. Noble, A. C., & Bursick, G. F. (1984). The contribution of glycerol to perceived viscosity and sweetness in white wine. American Journal of Enology and Viticulture, 35(2), 110-112. https://www.ajevonline.org/content/35/2/110

28. C, K., & Saccharomyces Genome Database. (2007, December 12). Saccharomyces cerevisiae glycerol biosynthesis. Yeast Pathways. Retrieved September 23, 2022, from https://pathway.yeastgenome.org/YEAST/
NEW-IMAGE?type=PATHWAY&object=PWY3O-48

29. U.S. National Library of Medicine. (n.d.). 1-octen-3-ol. National Center for Biotechnology Information. PubChem Compound Database. Retrieved August 23, 2022, from https://pubchem.ncbi.nlm.nih.gov/compound/1-octen-3-ol
Yang, D. Y., Kakuda, Y., & Subden, R. E. (2006). Higher alcohols, diacetyl, acetoin and 2, 3-butanediol biosynthesis in grapes undergoing carbonic maceration. Food research international, 39(1), 112-116. https://doi.org/10.1016/
j.foodres.2005.06.007
31.
Lambrechts, M. G., & Pretorius, I. S. (2000, August). Yeast and its importance to wine aroma - A Review. South African Journal of Enology and Viticulture. Retrieved August 23, 2022, from www.journals.ac.za/index.php/sajev/article/
view/3560

32. Coetzee, C. (2020, September 29). Esters 101 - part 2. Sauvignon Blanc South Africa. Retrieved August 23, 2022, from https://sauvignonblanc.com/esters-101-part-2/

33. Santamaría, P., González-Arenzana, L., Escribano-Viana, R., Garijo, P., Sanz, S., & Gutiérrez, A. R. (2022). Influence of the temperature and the origin of CO2 (anaerobiosis methodology) on the intracellular fermentation of wines made by carbonic maceration. Journal of the Science of Food and Agriculture. https://onlinelibrary.wiley.com/
doi/full/10.1002/jsfa.12078

34. Saerens, S. M., Delvaux, F. R., Verstrepen, K. J., & Thevelein, J. M. (2010). Production and biological function of volatile esters in Saccharomyces cerevisiae. Microbial biotechnology, 3(2), 165-177. https://doi.org/10.1111
/j.1751-7915.2009.00106.x

35. Lilly, M., Lambrechts, M. G., & Pretorius, I. S. (2000). Effect of increased yeast alcohol acetyltransferase activity on flavor profiles of wine and distillates. Applied and environmental microbiology, 66(2), 744-753. https://doi.org/10.1128/AEM.66.2.744-753.2000

36. Verstrepen, Kevin J., Stijn DM Van Laere, Bart MP Vanderhaegen, Guy Derdelinckx, Jean-Pierre Dufour, Isak S. Pretorius, Joris Winderickx, Johan M. Thevelein, and Freddy R. Delvaux. "Expression levels of the yeast alcohol acetyltransferase genes ATF1, Lg-ATF1, and ATF2 control the formation of a broad range of volatile esters." Applied and environmental microbiology 69, no. 9 (2003): 5228-5237. https://doi.org/10.1128/AEM.69.9.5228-5237.2003

​37. Coetzee, C. (2020, September 29). ESTERS 101 - PART 1. Sauvignon Blanc South Africa. Retrieved September 23, 2022, from https://sauvignonblanc.com/esters-101-part-1/

38. Lilly, M., Bauer, F. F., Lambrechts, M. G., Swiegers, J. H., Cozzolino, D., & Pretorius, I. S. (2006). The effect of increased yeast alcohol acetyltransferase and esterase activity on the flavour profiles of wine and distillates. Yeast, 23(9), 641-659. http://academic.sun.ac.za/wine_
biotechnology/RESEARCH/PDF/2006_05_Lilly_Yeast.pdf

Continued Resources and Suggested Reading

39. Australian Wine Research Institute. (n.d.). Wine flavours, faults and taints. The Australian Wine Research Institute. Retrieved September 23, 2022, from /www.awri.com.au/industry_support/winemaking_resources/sensory_assessment/recognition-of-wine-faults-and-taints/wine_faults/

40. Dickinson, J. & Lanterman, Margaret & Danner, Dean & Pearson, Bruce & Sanz, Pascual & Harrison, Scott & Hewlins, Michael. (1997). A 13C Nuclear Magnetic Resonance Investigation of the Metabolism of Leucine to Isoamyl Alcohol in Saccharomyces cerevisiae. The Journal of biological chemistry. 272. 26871-8. 10.1074/jbc.272.43.26871.

41. Dennis, E. G., Keyzers, R. A., Kalua, C. M., Maffei, S. M., Nicholson, E. L., & Boss, P. K. (2012). Grape contribution to wine aroma: Production of hexyl acetate, octyl acetate, and benzyl acetate during yeast fermentation is dependent upon precursors in the must. Journal of Agricultural and Food Chemistry, 60(10), 2638-2646. https://doi.org/10.1021/jf2042517

42. National Center for Biotechnology Information (2022). PubChem Compound Summary for CID 8908, Hexyl acetate. Retrieved September 6, 2022 from https://pubchem.ncbi
.nlm.nih.gov/compound/8908.

43. National Center for Biotechnology Information (2022). PubChem Compound Summary for CID 7762, Ethyl butyrate. Retrieved September 6, 2022, from https://pubchem.ncbi.nlm.nih.gov/compound/7762.

44. Peinado, R. A., Moreno, J., Bueno, J. E., Moreno, J. A., & Mauricio, J. C. (2004). Comparative study of aromatic compounds in two young white wines subjected to pre-fermentative cryomaceration. Food Chemistry, 84(4), 585-590. Retrieved from: www.researchgate.net/publication/2
34144864_Comparative_study_of_aromatic_compounds_in_two_young_white_wines_subjected_to_pre-fermentative_cryomaceration
45. National Center for Biotechnology Information (2022). PubChem Compound Summary for CID 7344, Ethyl lactate. Retrieved September 6, 2022 from https://pubchem.
ncbi.nlm.nih.gov/compound/Ethyl-lactate

46. National Center for Biotechnology Information (2022). PubChem Compound Summary for CID 31265, Ethyl hexanoate. Retrieved September 6, 2022 from https://pubchem.ncbi.nlm.nih.gov/compound/Ethyl-hexanoate.

47. National Center for Biotechnology Information (2022). PubChem Compound Summary for CID 7799, Ethyl octanoate. Retrieved September 6, 2022 from https://pubchem.ncbi.nlm.nih.gov/compound/7799

48. National Center for Biotechnology Information (2022). PubChem Compound Summary for CID 8048, Ethyl decanoate. Retrieved September 6, 2022 from https://pubchem.ncbi.nlm.nih.gov/compound/8048

49. National Center for Biotechnology Information (2022). PubChem Compound Summary for CID 24020, Ethyl 2-methylbutyrate. Retrieved September 6, 2022 from https://pubchem.ncbi.nlm.nih.gov/compound/Ethyl-2-methylbutyrate.

50. National Center for Biotechnology Information (2022). PubChem Compound Summary for CID 7945, Ethyl isovalerate. Retrieved September 6, 2022 from https://pubchem.ncbi.nlm.nih.gov/compound/7945.

51. Perpete, P., Duthoit, O., De Maeyer, S., Imray, L., Lawton, A. I., Stavropoulos, K. E., ... & Richard Dickinson, J. (2006). Methionine catabolism in Saccharomyces cerevisiae. FEMS yeast research, 6(1), 48-56.
PUBLISHED: OCTOBER 2022