Lesson 2 - Beer Foam

Many methods for foam measurement have been devised over the years. These methods largely measure foam stability; however, each has its limitations, and none truly mimic the consumer experience of pouring a beer and enjoying the visual appeal of a good foam head. Simulating beer-pours tend to be inconsistent, and generating a foam is done so in a non-natural way.

The current legacy methods are:

Sigma value

The sigma value method is adapted from Helm (1933) and approved by the ASBC. It involves pouring a beer into a cylindrical separating funnel and removing the drained beer after 30 seconds. After a further time of 225-230 seconds (t), the beer that has drained is removed and measured in mL (b). The remaining foam is collapsed with alcohol, and that volume is measured net of the alcohol (c). The sigma value is calculated according to the formula developed by Ross and Clark (1939).

S = t/[2.303*log (b+c)/2]

Sigma values of more than 105 seconds would be a ‘good’ foam and those < 95 seconds, a ‘poor’ foam.

Foam flashing method

Another ASBC approved method is where beer is forced under pressure through a small orifice to collect 200 mL foam. After 90 seconds, the volume of beer collected is measured (B₁). The remaining foam is collapsed, and its net volume measured. The drained beer (B₁) is compared to the total beer contained in the 200 mL foam (B₂)

Foam Value Units (FVU) = 200*(B₂-B₁)/B₂

Rudin head retention value

The IoB adopted a modified Rudin (1957) method. The apparatus consists of a 350 mm jacketed tube of 27 mm diameter with a sintered disc in its base. The tube is marked at 50, 75, 100 and 325 mm levels. A degassed beer sample is added to the 100 mm mark and foamed with CO₂ up to the 325 mm level. As the foam drains, the time taken for the rising beer foam interface to travel between the 50 and 75 mm levels is equivalent to the foam half-life.