Silica Gel is a beer stabilizer that reduces the level of haze that can form in finished beer. It works by removing the small proteins or polypeptides that react with polyphenols, which are the basis for the most common type of beer haze. The use of silica gel as an adsorptive beer stabilizer was first introduced in the 1960s. The two types commonly used in brewing are hydrogel and xerogel.
A difference in production makes for different handling characteristics of these materials but leaves the function of silica gel unchanged. Silica gel adsorbs those hydrophilic, haze-active proteins with a molecular weight of approximately 40,000 kDa and larger, while not interacting with those hydrophobic proteins that promote positive foam development with an approximate molecular weight of 10,000 to 20,000 kDa.
Silica gel is mixed in water prior to use and normally is dosed into unfiltered beer just prior to filtration. Because it is not soluble in beer, the silica gel itself is also removed by the filter, along with the proteins. Its mode of action is very fast, requiring just a few minutes to adsorb proteins.
The main difference between hydrogel and xerogel is that hydrogel has a higher moisture content, usually 60% to 70%, needs greater mixing time to adequately disperse, and is less expensive. Xerogel disperses readily in water, but it is very dusty when handled because of its low moisture content of about 5% and has been known to negatively affect filter flow. Another feature of xerogel is that it does not readily dissolve in a caustic solution, which can have an impact on regenerable filter systems.
Bibliography
O’Rourke, T. Colloidal stabilization of beer. The Brewer International 2 (2002): 23–25.