Fungal hydrophobins as predictors of the gushing activity of barley and malt

Hydrophobins are small, moderately hydrophobic proteins produced by filamentous fungi. Hydrophobins have been found on the cell walls of hyphae and on spore surfaces, but they can also be secreted into the culture medium. The characteristic feature of these surface-active proteins is their ability to self-assemble at hydrophilic/hydrophobic interfaces forming amphipathic membranes. This property allows hydrophobins to fulfil a broad spectrum of functions in fungal growth and development. It has been established that hydrophobins are able to stabilize air bubbles and oil droplets in water. Our studies have indi-cated that hydrophobins also act as gushing factors of beer. As reported earlier, we were able to isolate hydrophobins from mycelium and culture broths of gushing active fungi. The proteins were purified by reverse-phase chromatography. Their ability to cause gushing was studied by adding pure protein into beer bottles and by rocking the bottles for three days. After opening the bottles, the amount of overfoaming beer was measured. Already very small amounts of hydrophobins, at the level of micrograms, were found to cause gush ing. Polyclonal antibodies against the hydrophobin of Fusarium poae were raised in rabbits. An immunological ELISA-method for detection of hydrophobins in barley and malt was developed. Malt samples were analyzed with the developed ELISA method, and the results were com-pared to the gushing test, where an aqueous extract of malt was added to bottled beer and the bottles were rocked for three days, as well as to the deoxynivalenol (DON) content of the samples. The results of the hydrophobin-ELISA test were observed to correlate with the gushing potential of the malt. According to data obtained from gushing positive and negative samples, a tentative gushing limit absorbance value, which can be used for prediction of the gushing activity of barley and malt, was set. Correlation was not found between the gushing potential and the DON content of malt or between the amount of hydrophobins and DON in malt. We also measured hydrophobin levels at different stages of a lab-scale mashing process. We observed that hydrophobins ended up in the wort, although they were also detected in spent grains and trub. The hydrophobin-ELISA method has also been applied to detection of hydrophobins in different beverages and raw materials.