Saturday, 15 May 2010

Flavour Compound of the Week - Ethanoic Acid

If you’re really posh you will pay £20 for your ethanoic acid in wine bottles from a bloke called Tarquinii wearing an apron and drizzle it on your salad. If you are common you’ll shake it on your chips. Ethanoic acid is otherwise known as acetic acid and is what makes vinegar, vinegar. You no doubt will be able to see from this week’s 3D space station at the top that ethanoic acid is ethanol (alcohol) with an added oxygen atom. So if you oxidise (add oxygen to) alcohol you get ethanoic acid. This reaction happens naturally in air but is very slow. Certain bacteria and wild yeast have enzymes which speed up this process. Ethanoic acid will always be present in beer because it exists in equilibrium with ethanol. In fresh beer the concentration will be well below its flavour threshold. It is thought however to contribute to the crispness of beer to a degree by making it more acidic.

The bacteria which are associated with ethanoic acid production in beer are grouped together and called acetic acid bacteria. Two common examples are Acetomonas and Acetobacter. The latter is discriminated from the former by motility through the use of peritricous flagellation (I tried that once but didn’t like it). The most important thing about acetic acid bacteria in terms of brewing is that they are obligate aerobes which means they cannot grow in the absence of oxygen. Yeast use up all the oxygen in beer at the first stage of fermentation so unless you have the misfortune to buy beer from a pub where they serve you beer from casks which have been left open for 7 days, you are unlikely to find them problematic.

Ethanoic has a sour smell and sour sharp taste. The characteristic sourness of Lambics is principally due to a combination of ethanoic and lactic acids although a number of other organic acids are involved.


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