Part 9 – Acetic acid
==================================================
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 H2O (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+.
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.
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.
