Diagnostic Key Postfermentation Classes of Off-Character Compounds

Classes of Off-Character Compounds

Bacterial Production of Off Characters

Below is a table of the compounds produced by bacteria in wine and the type of aroma they produce.

LAB = lactic acid bacteria, AAB = acetic acid bacteria, Lb. = Lactobacillus, Oeno = Oenococcus, Pd. = Pediococcus

Off Aroma Compounds Produced by Bacteria in Wine

Bacteria

Compound

Sensory Effect

Reported Threshold

LAB, AAB

Acetic Acid

Vinegar, pungent, sour

0.2 ppt

LAB, AAB

Ethyl acetate

Nail polish remover

7.5 ppm

AAB

Acetaldehyde

Nutty, dry straw

100 ppm

Lb.,  Oeno.

Diacetyl

Butter, nutty, caramel

0.1 to 2 ppm

Lb., Pd.

2-Ethoxy-3,5-hexadiene

Geranium leaves

0.1 ppb

Lb., Oeno.

2-Acetyl-tetrahydropyridine

Mousy

4 to 5 ppb

Lb., Oeno.

2-Ethyltetrahydropyridine

Mousy

2 to 18 ppb

Lb., Oeno.

2-Acetyl-1-pyrroline

Mousy

7 to 8 ppb

Lb., Pd.

Acrolein (+anthocyanin)

Bitter

Pd.

b-D-Glucan

Ropy, viscous, oily

Oeno.

Mannitol

Viscous, sweet

LAB

Skatole (indole)

Fecal

1.7 ppm (1.8)

The production of the first three compounds; acetic acid, ethyl acetate, and acetaldyhyde; results from the metabolism of ethanol by the bacteria.  Ethyl acetate is the result of the reaction of acetic acid and ethanol to form an esther.  Acetic acid is produced by acetic acid bacteria from ethanol in the presence of oxygen.  The production of acetaldehyde is an intermediate step in the production of acetic acid.  Acetic acid in lactic acid bacteria is the result of the metabolism of tartaric and malic acids.  Malic acid is, of course, primarily converted to lactic acid by Oenococcus or Lactobacillus during the malo-lactic fermentation.   A by-product of that conversion is acetic acid.  Tartaric acid can be converted to succinic acid by Lactobacillus, again with the production of some acetic acid.

2,3 butandione (diacetyl) is produced from citric acid by Lactobacillus and Oenococcus.   This compound produces a buttery aroma and is the same chemical used to produce the buttery aroma in microwave popcorn.  We have found that mostLactobacillus casei strains we have isolated form wine produce large amounts of diacetyl.

The mousy off-characters produced by lactic acid bacteria are also produced from ethanol.  These compounds; 2-acetyl-tetrahydropyridine, 2-ethyltetrahydropyridine, and 2-acetyl-1-pyrroline; require the interaction of the ethanol breakdown product acetaldehyde with sugar to produce acety-coA which then interacts with the amino acid ornithine or lysine to produce the off aroma compound.  The perception of these compounds seems to vary significantly among individuals but the level of detection is low for those people who are sensitive to them.  This is a good reason to always have your wine sampled by a number of people in your winery.  Relying only on your own palate can result in your missing some issues with your wines.

The other compounds are found less frequently and only under very specific conditions. 2-Ethoxy-3,5-hexadiene, which produces a geranium aroma, is found only when sorbic acid is used as  a disinfectant.  The sorbic acid interacts with the ethanol and is converted by lactic acid bacteria to produce the off odor compound.  Acrolein is produced by Lactobacillus orPediococcus from glycerol in the wine and interacts with phenolics in the wine to produce an intensely bitter taste.Pediococcus can also produce large amounts of glucans in the wine from glucose which can give the wine a ropy or slippery texture and mouth feel.  Oenococcus can convert fructose to mannitol which imparts a thick sweet character to the wine.

Wild Yeast (Brettanomyces) Production of Off Characters

Most wild yeasts are found early in fermentation and produce off aromas that are transient.  The most notorious yeast producer of off characteristics is Brettanomyces bruxellensis.  Brettanomyces occurs after primary fermentation and can produce a range of off aromas depending upon the growth conditions and substrates available in the wine.  The wild yeasts in early fermentations, especially the Hanseniaspora/Kloeckera species, usually produce high levels of acetic acid, ethyl acetate, and acetaldehyde, resulting in high VA wines.  These problems often resolve during the later stages of fermentation as CO2 is produced and drives off the volatile aroma compounds.  In general these wild yeasts are not considered to be major sources of wine spoilage. Brettanomyces, on the other hand is one of a winemakers worse fears.  It is very resourceful and grows on the substrates left behind by Saccharomyces after the primary fermentation.  It is very resistant to low pH and high alcohol.

Below is a table of the types of compounds that Brettanomyces produces and the aroma off aroma characters they can impart to wine.

Off Aromas Produced by Brettanomyces in Wine

Chemical Type

Odor Impact

Detection Threshold

Ethyl Phenol

Chemical, Band Aid, smoke, burnt, medicinal, spicy

0.14 to 0.62 ppm

Vinyl Phenol

Leather, burnt, metallic, woody

0.1 to 15 ppm

Fatty Acid

Barnyard, sweat, rancid, solvent, sewage

5 ppm

Pyridine

Mousy, rancid tortilla chips, crackers

2 to 18 ppb

Aldehyde

Solvent, burnt rubber, air freshener

1 to 100 ppm

Long Chain Alcohol

Floral, fruit, chemical, furniture polish

0.1 to 50 ppm

Ester

Fruit, floral

0.1 to 100 ppm

The phenolic compounds 4-ethyl phenol (4EP) and 4-ethyl guaicol (4EG) are most often associated with off aroma caused byBrettanomyces.  These compounds are produced from the cinnamic acids, coumaric and ferulic acid, through the sequential reaction of a cinnamate decarboxylase, which produces a vinyl phenol, and a vinyl phenol reductase, which produces an ethyl phenol.  4EP produces a characteristic “Band Aid” aroma that can also be described as creosote or medicinal.  4EG has a smoky aroma that has also be described as bacon or in concert with 4EP as burnt rubber.

IMAGE 4EP

Brettanomyces can also produce other off aromas in wine including leather, horsy, sweat, barnyard, cheesy, rotten, plastic, burnt beans, soy sauce, and mousy.  These aromas are the result of the production of other compounds such as fatty acids, pyridines, aldehydes, and long chain alcohols.  Brettanomyces, like Saccharomyces, can produce positive aromas in the form of esters and fusel alcohols.  However, after primary fermentation the substrates present for Brettanomyces to use are more limited and often lead to the production of off odor compounds instead.  The production of many of these compounds is the result of carboxylic acid metabolism. Fatty acids with ten to twenty carbons are very important to the cells ability to survive under high alcohol conditions.  The short chain, four to ten carbon, carboxylic acids are also notorious for producing foul odors.  These shorter chain acids are used to produce the longer chain fatty acids, 10 to 20 carbon, necessary for alcohol resistance.  Most often associated with Brettanomyces is isovaleric acid that has been described as smelling like dirty feet, sweat or dirty socks.  Isobutyric acid has a cheesy aroma, butyric acid has an odor like rancid butter, and valeric acid’s aroma is described as putrid, sweaty, or rancid.  All of these can be produced by Brettanomyces.

Aldehydes are produced by yeasts from organic acids, such as amino acids or fatty acids, as an intermediate in the pathway to alcohol production.  Both aldehydes and the longer chain alcohols, also called fusel alcohols, can contribute to the aroma of wine.  Some of the longer chain aldehydes and alcohols can have a positive affect on wine aroma but many of them are responsible for an unpleasant “chemical odor” when they occur at higher levels.  The alcohols can be toxic to cells and are often converted to the less toxic and more pleasant smelling ester by a reaction between the alcohol and a carboxylic acid.

The production of a mousy odor by Brettanomyces is a rare phenomenon compared with mousy taint caused by lactic acid bacteria.  The mousy off flavor from Brettanomyces contamination is caused by the compounds 2-acetyl- tetrahydropyridines (ATHP) and 2-ethyltetrahydropyridine (ETHP).  ATHP is produced from L-lysine and is converted, over time, to ETHP.  Both of these compounds produce the mousy character in wine.  This character is often difficult to detect until the wine is taken into the mouth.  This can be a most unpleasant surprise.