Part 6 - Higher alcohols

Part 6 - Higher alcohols

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v  Alcohols and Higher alcohols (or Fusel alcohols)

Alcohols are carbon compounds containing one (or more) hydroxyl groups (-OH). According to the number of carbons, they are given a name of their own, with suffix –ol.

Higher alcohols (also called fusel alcohols) are alcohols containing 3 carbons or more. Unlike ethanol, higher alcohols are not derived from glucose metabolism.  They are formed as by-products from  amino acid decomposition  and their concentration range in wine is between 100 – 500 mg/L. 

* An alcohol containing three or more hydroxyl groups (e.g. glycerol with 3 -OH groups) is called a polyol (poly = many)

v Substrates for these higher alcohols are <1> α-amino acids and <2> α-keto acids

<1>  Amino acids are carbon compounds containing an amino group (NH₂) and a carboxylic acid group (COOH).

At moderate pH values (e.g. in human body) the COOH can release a H to NH₂ to make them NH₃ and COO ^– (see images below). In an acidic environment, like wine, they remain generally as NH₂ and COOH.

α-Amino-acids are Amino acids where the amino group (NH₂) is attached to the C-atoms after the COOH-group (see Part 5, Fatty acids C-atoms numbering). α-Amino acids are in the must or added to it by the wine-maker. The yeasts need a lot of α-amino acids to build new membranes and proteins. Proteins are made from 20 different α-amino acids.

  

<2> Keto acids are acids with a keto-group (C=O). Keto acids where the keto-group (C=O) is situated right behind the COOH-group are called α-keto acids . There are 3 α-keto acids in yeast.

   

v  Higher alcohol formation

The α-amino acids composition in the must is not necessarily similar to the needs of the yeast cell. When there is a shortage of specific amino acids, yeasts can make them by exchanging the NH₂-CH-group of α-amino acids and the C=O group of α-keto acids.This is called transamination, and is catalysed by the enzymes called transaminases or aminotransferases. When an α-amino acid is changed to an α-keto acid, then decarboxylated and reduced, higher alcohol is formed (see image below:  Ehrlich Pathway for higher alcohol). Note that, both alcohol and higher alcohol are formed by ‘reduction’ at the last step.

  

v  Main higher alcohols in wine

Quantitatively, the main higher alcohols in wine are Isobutanol (4C), Isoamylacohol or Isopentylalcohol (5C) and Phenylethanol (6C),  which are formed from α-amino acids valine, leucine, isoleucine and fenylalanine.

v  Stuck and sluggish fermentation increases higher alcohol formation

Higher alcohol formation has the same purpose as glycerol formation, namely to regenerate NAD+  in order to compensate for the NAD+ deficit caused by a stuck and sluggish fermentation.

A high content of higher alcohols gives the wine a burning, sharp, "foamy" character, to the detriment of the finesse.

v  Higher alcohosl and wine aroma

In small amounts, higher alcohols can contribute to the aromas of wine. Higher alcohols can bind with acetic acids to form esters, which are responsible for that well known pear and banana flavor,  especially in many  industrially produced young white wines  (more about esters in Part7 – Esters).

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P.S.

 - What are esters?

- How are they formed ?

- What  impact can they have on wine ?

Till the next post!