An important characteristic in beers is the ability of the beer to retain a nice foamy head for a long period of time. Commercial brewers go to great lengths to improve head retention by a variety of additives. However homebrewers also have access to quite a few ingredients and additives that can help your foam last until the last drop.
Foam is the result of CO2 bubbles rising through the beer. These bubbles attach themselves to substances in the beer and form a skin around the bubble. Obviously the more CO2, the more bubbles, but the goal of the brewer is not bubbles but stability of the head. As foam collapeses, evaporating bubbles tend to solidify the beer near the surface, allowing more beer to be poured with less foaming after about a few minutes have passed. Head stability depends on the presence of substances with low surface tension in the beer which can form stable elastic bubbles. The two primary contributors to head retention are certain high molecular weight proteins and isohumulones (alpha acids from hops). Therefore beers with more proteins that are highly hopped will have a higher head retention.
Methods for Improving Head Retention
We will explore the following possibilities:
- The use of body and head enhancing malts such as crystal, wheat, or carafoam
- The altering of the mash schedule to enhance head retaining proteins
- The use of heading agents - additives that enhance head retention
- Addition of high alpha hops - which will increase bitterness, but also increas isohumulones that enhance head retention
- Limiting the use of household soaps on drinking glasses and homebrew equipment
- The use of a nitrogen and CO2 mix for carbonation and serving
- The shape of the glass used to serve the beer
Head Enhancing Malts
The inclusion of proteins and dextrines enhance the body and head retention of finished beer. Unfortunately when used to excess, proteins and dextrines can interact with tannins and reduce clarity and promote cloudiness, so a proper balance must be struck. Crystal malts to include the light Carapils and Carafoam, and caramel malts. These are the most common body and foam enhancing additives that enhance head retention primarily by adding dextrines and other complex proteins. The overuse of such malts can result in proteins reacting with tannins to create a chill haze. Similarly, other grains high in protein such as flaked barley and wheat can be used to enhance head retention, though again at the cost of clarity.
Since head retention depends on the level of high molecular weight proteins, any step in the mash that breaks down proteins is undesirable. For example, a protein rest in the 50-60 C (122-140 F) range would not be desirable. To improve head retention you would generally favor a full bodied, higher temperature mash, with main conversion in the 158 F (70 C) range, and avoid intermediate protein rests.
Homebrew shops sell a variety of additives, usually under the generic title heading agent. Some are intended to be added at bottling time, while others need to be added at the end of the boil. Follow the instructions included with the agent to determine what is required. Many heading agents are derived from an enzyme called pepsin that is derived from pork. Other popular heading agents include iron salts, gums, and alginates. All heading agents will alter the flavor of the beer, in general making the character softer. In general, heading agents are not necessary for homebrews that are made from 100% malted barley and wheat. Heading agents are more commonly used in commercial beers that have high rice and corn content, lacking the necessary proteins of an all-malt beer.
As mentioned in the introduction, isohumulones which are a form of alpha acid also will enhance the head retention of beer. Alpha acid is the primary bittering agent in hops. Therefore highly hopped beers will have better head retention. Obviously overall malt-bitterness balance is still required, but one can use higher levels of hops, particularly in darker full bodied beers to enhance head retention.
Limit Use of Household Soaps
Household soaps such as common dish soap and dishwashing soap have a significant detrimental effect on head retention in beer. You should not use household soaps on either your brewing equipment or your main bar drinkware. Detergent washed glasses in particular will quickly reduce the head on even a well constructed beer. Instead use a beer-friendly cleaning agent from your local homebrew supplier.
A Nitrogen Mix
Some beers, most notably Guiness Irish Stout, are carbonated and poured with a mix of nitrogen and carbon dioxide. CO2 is relatively soluable in beer, and therefore does not promote the formation of gas bubbles as well as non-soluable gasses. Nitrogen dissolves less easily in beer, and provides a better base for forming a stable head. However, nitrogen alters the perceived character of the beer, and use of pure nitrogen would result in an unacceptable mouthfeel and carbonation. A mix, therefore, is always used. The mix varies depending on the style of beer - a low carbonation stout might be served with a mix of 25% carbon dioxide and 75% nitrogen, while ales and lagers might include more CO2 - perhaps 60% CO2 and 40% nitrogen. Low carbon dioxide mixes (25/75) can be applied by mixing the gases in the cylinder, but higher mixes generally require two separate tanks - one of CO2 and one of nitrogen. A high precision blending device either at the tap (i.e. a stout tap) or inline are needed to blend the two gasses for dispensing.
Shape of Serving Glass
The shape of the glass is also a determining factor in both head formation and head retention. A tall narrow glass enhances the formation and retention of the head, while short wide glasses do not. This is the reason many Bavarian wheat beers and Pilsners are served in tall narrow glasses. Use the proper glass for the style of beer you are pouring to enhance the overall presentation.