About 95% of a standard beer is made of water. Depending on its source and composition the pH should therefore be around 7 (neutral) or slightly above. We know however that the average pH of a beer is much lower at around 4 to 4,5, and even less for sour beers.
So what causes this drop? The first drop happens when malt and hot water are mixed together during mashing. Malt contains phosphates which bind to water’s calcium and magnesium and precipitate out of solution and release hydrogen ions and thus increase in acidity.
Dark malts are also more acidic and therefore contribute to a greater pH drop during mashing. Water composition is normally adjusted depending on the malts used in the recipe to target a mash pH around 5.2/5.6.
During fermentation, yeast will metabolise wort sugars and malt amino acids though a complex enzymatic process. This will result in the secretion of over a hundred organic and fatty acids.
The main organic acids will contribute to the beer acidic taste and its aroma bouquet, mainly lactic, citric, pyruvic, succinic and acetic acids. The result is a final pH drop from 5.2/5.6. to 4.2-4.4.
This final acidity lands at a level which our human brain has evolved to consider safe to consume and therefore particularly enjoys!
Sour beers are unsurprisingly even more acidic with an acidification that will differ depending on the technic and preferred microorganism. Their flavours can vary drastically and will be determined by the dominant acid and the way the fermentation took place.
Acetic acid is responsible for strong acidity and cider/vinegar like flavours. It is generally produced by Acetobacter or sometimes Brettanomyces and Lactobacillus depending on strains and conditions, mainly oxygen exposure. This is why it is mostly found in barrel aged and spontaneous fermented beers.
Lactobacillus and pediococcus are the main responsible for most popular souring methods. They produce lactic acid which is considered a weak acid and softer in flavour than acetic.
With a volume of these acids much higher in sour beers than in regular saccharomyces fermented beers, the pH will finish as low as 3 to 3.5.
In summary, the drop in beer pH is due to the interaction between malt and water during mashing, as well as the different acids produced by the yeast during fermentation.
Organic acids, such as lactic acid and acetic acid, contribute to the characteristic acid taste of beer. In sour beers, the pH is even lower due to the greater volume of these acids. This acidity is appreciated by consumers and contributes to the diversity of beer flavours.
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