Beer lacing is the white foam residue that is left on the side of the glass after the beer has been consumed or once the head has subsided. It is called lacing because it resembles white lace cloth. Lacing is considered a beneficial quality for a beer to have, and typically speaking, a beer with a good head retention will also have good lacing.
Head retention refers to a beer’s ability to retain its foamy head once the beer has been poured. In most styles of beer, a thick foamy head that does not dissipate too quickly is very desirable. The three primary factors that impact a beer’s head are the carbonation level of the beer, residual proteins that form the body of the finished beer, and isomerized humulones pulled from the hops that were added during a beer’s boil. Hops that are added during fermentation or once the beer has cooled below approximately 175° F will not isomerize and will have very little impact on head retention. A stronger or hoppier beer will tend to have better head retention because it will usually have more residual proteins and a greater amount of isomerized humulones.
The most common ways of enhancing a beer’s head retention are to add high alpha acid hops during the boil, utilize grains such as crystal malts or wheat, or add an adjunct such as maltodextrin to your boil. Striking the right balance is a bit of an art, as you do not want to compromise the taste of your beer or risk clarity issues by pushing too hard for good head retention.