Primary versus secondary gushing and assay procedures used to assess malt/beer gushing potential.

In light of the concern over the contamination of the 1993 and 1994 barley crops with atypical levels of molds (and possibly the 1995 crop as well), a literature search was conducted in order to better understand the phenomenon of gushing. Primary gushing, resulting from the use of mold infected malt, is caused by the presence of high numbers of Type III micro-bubbles. These stabilized bubbles are believed to be the nucleation sites responsible for violent CO 2 release and gushing. They consist of surface active materials forming a solid-condensed pellicle around CO 2 gas, effectively insulating the CO 2 from the bulk of the liquid. It is speculated the surface active material is a protein, produced as either a mold metabolite, or an atypical malt constituent induced by an interaction between mold and barley during malting. Agitation is an essential prerequisite for the formation of primary gushing and symptoms can be temporarily cured or masked by repasteurization or chilling, or aggravated by various other processing parameters (e.g., the use of foam enhancing hop extracts, overcarbonation, etc.). Prevention is only assured by the avoidance of weathered barley and malt. Secondary gushing is unrelated to malt issues and is caused by Type I (solid particles) or Type II (gas residues sorbed on a solid support) particles acting as nuclei. Typically these result from contamination of packaging materials with foreign particulate matter, filtration breakthroughs, abnormal haze development during beer aging, etc. Normally primary gushing involves the majority of packaged beer from specific lots while secondary gushing is more sporadic in occurrence. Methods to assay and predict the gushing potential of malt and beer are also reviewed.