is the adhesion of beer foam to the side of a glass during beer consumption. It is frequently known as “lacing.” While most drinkers perceive cling to be a desirable attribute of beer, indicative of high quality, there are some who are put off by the phenomenon, as they feel it has a characteristic of dirtiness. Its occurrence depends on the cross-reaction of hydrophobic polypeptides with bitter acids, with the involvement of metal ions that act as bridges. It is a time-dependent interaction and the resultant “solidification” that leads to the foam being in an adherent state takes between 30 seconds and 2 minutes to occur. Cling increases in proportion to beer bitterness and is especially strong when the reduced bitter compounds (“light-stable hop extract”) that some brewers use to prevent “skunking” are used. The pattern of lacing also depends on the mode of bittering, with conventional iso-alpha acids offering a finer and less coarse appearance. See iso-alpha acids. Cling is inhibited by shorter chain fatty acids.

Cling depends on the glass surface being clean and free from lipid or detergent and can be assessed by optic instruments that scan the surface of the beer glass after beer has been removed and assess the proportion of the surface that is covered by foam. Alternatively, the Lacing Index procedure evaluates the total amount of cling by recovering the foam from the glass in water and measuring the absorbance of ultraviolet light by the resultant solution. While both of these methods correlate with the total amount of clinging foam, they do not quantify the aesthetic appeal of the lacing pattern, which remains in the eye of the beholder.