Beer Froth Characteristics
When you pour a beer, you’ll notice a head of froth forms at the top. The characteristics of the froth depend on two factors: the conditioning the beer has received, and the cereal grains in the grain bill of the beer's recipe.
Beers with primarily nitrogen conditioning have a white head of froth. Beers with primarily carbonation conditioning have a head color that depends on the cereal grains in the grain bill.
Unmalted grain and base malts (less than 10°L) contribute to a white froth. Specialty grains and malt contribute to a darker head of froth. Interestingly, a dark-colored beer can have a light-colored froth. The possible colors of beer froth can be divided into 10 subjective nomenclature categories.
The characteristics of beer froth depend on the conditioning the beer has received, during the cellaring stage of brewing. The conditioning step dissolves gas bubbles into green beer as it ages. The gas bubbles can be carbon dioxide (CO2) or nitrogen (N2).
Beer can be conditioned using natural carbonation (CO2). This is done by allowing yeast to ferment sugars and keeping the system closed, so the carbonation builds pressure. Beer can also be conditioned using artificial carbonation. This is done by forcing pressurized carbonation into the conditioning container.
Carbonation conditioning provides a more coarse head of froth. Its head retention can vary from good to poor.
Beer can be conditioned using artificial nitrogen (N2). This is done by forcing pressurized nitrogen into the conditioning container.
Nitrogen conditioning provides a smooth, creamy, white head of froth. It has great head retention for two reasons. First, nitrogen bubbles are smaller than carbonation ones, and do not collapse as easily. Second, nitrogen is a higher fraction of ambient air (~78%), so it does not try to leave the beer as quickly. Nitrogen does not stay dissolved very well, so beers with nitrogen conditioning create a thick head right away, but since the head lasts so long, this characteristic is fine.
The term head creation defines how well a specific beer releases bubbles to form a head of froth. This is simply how quickly or steadily a beer releases nitrogen or carbonation. Head creation depends on a few factors.
If the beer is allowed to warm up, head creation is improved. If the beer is swirled or shaken, head creation is improved. Heat, motion, and even light are forms of energy that break the molecular bonds, so dissolved beer gases can release and ascend to the head.
If the glassware has nucleation sites, head creation is improved. Nucleation sites make dissolved beer gases need less energy release and ascend.
The term head retention defines how well a specific beer keeps its head of froth once formed. This characteristic depends on a few factors.
If the walls of the glassware slope outward, head retention is improved. The outward slope supports the bubbles, so they collapse more slowly.
If the beer recipe creates dense amino acids, the head retention is improved. Amino acids are found cereal grains, hops, and yeast. Dense amino acids coat the bubbles, so they collapse more slowly. If fats or oils are present on the glassware, head retention is worsened.
If the glassware has nucleation sites, head retention is improved. Nucleation sites create smaller bubbles than natural heterogeneous nucleation does, and the smaller bubbles collapse more slowly.