vrijdag 15 januari 2016

Part 9 - Acetic acid

Part 9 – Acetic acid
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v Acetic acid (CH3-COOH)  is the main volatile acid in wine. Its presence at high concentrations gives off a vinegar odor and a disagreeable sensation in the mouth. Acetic acid can be produced by <1> yeasts, <2> acetic acid bacteria and <3> lactic acid bacteria.

<1>  Yeasts produce acetic acid in 2 ways : by ethanol oxidation and by acetyl-coA hydrolysis. If there are no problems during alcoholic fermentation, yeasts produce only small quantities of acetic acid (200-300 mg/L), which is far below its detection threshold (800 mg/L).



            

<2>   Acetic acid bacteria (AAB) ,  genus Acetobacter aceti, are aerobic, which means that they need oxygen to grow. In the presence of oxygen, they convert ethanol into  acetic acid. In proper fermentation, the AA-bacterial growth is minimum because all the oxygen is consumed by yeasts. By stuck and sluggish fermentation, they get a chance to develop. They use ethanol for their energy source to grow. The ethanol is oxidized via alcohol dehygogenase (ADH) and aldehyde dehydrogenase (ALDH) into acetic acid which comes in the wine. The removed hydrogen protons (H+) are pumped into the periplasm and come back in the cytoplasm via ATPase to form ATP. The hydrogen electronen are transported by the ubiquinol oxidase (Ox)  and finally captured by oxygen (O) which combines with 2H+  and become H2(see also Part 3 ETC).






<3>   Lactic acid bacteria (LAB)   found in wine belong to three genera: Leuconostoc, Lactobacillus and Pediococcus. They are anaerobic, which means they don’t need oxygen to grow. They grow on sugars and acids by converting them into lactic acid. That’s why they are called lactic acid bacteria. Like yeasts, LAB can also break down glucose in cytoplasm by glycolysis to produce 2 ATP. Yeasts reduce aldehyde to ethanol and LAB  reduce pyruvate to lactic acid. They both do that for the same purpose: to regenerate NAD+.



                              
 Like Acetic acid bacteria, LAB live in the must, but can not grow because of the powerful yeast growth. By stuck and sluggish fermentation, they get chance to develop, especially Lactobacillus and Pediococcus, which are regarded as “bad guys”. They change pyruvate into acetic acid.


          



v Acetic acid  & ethyl acetate

l Acetic acid (threshold 800 mg/L) smells like vinegar. A normal wine contains 200-300 mg/L acetic acid.

l Part of the acetic acid in wine can combine with ethanol to form esters, called ethyl acetate (see Part 7)


l  Ethyl acetate (threshold 150 mg/L) smells like glues and nail polish removers.

l  In wine ethyl acetate is always accompanied by acetic acid. At a value of 150-180 mg/L of ethyl acetate combined with 700-800 mg/L of acetic acid, there is a vinegar sting (piqûre acétique). This is a wine fault, and the cause is bacterial contamination.

l  The legally permitted maximum volatile acid content (e.g. acetic acid, lactic acid and ethyl acetate)  is 1080 mg/L in  
    white wines, 1200 mg/L in red wines and 1500 mg/L in botrytis wines.

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P.S.
In the next post, we'll take a look at the formation of acetaldehyde (or ethanal) and the difference between chemical and biological oxidation.










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