Protein: Difference between revisions

To delve further into the chemistry, many proteins contain "thiol" groups, each made up of a sulfur and hydrogen (–SH) branch from an amino acid (especially cysteine) in the protein or polypeptide. These thiol groups can bond to each other, forming a "disulfide bridge" by linking the sulfur atoms together, removing the hydrogen atoms. This bonding occurs very rapidly under oxidative conditions; oxygen in the mash oxidizes the free thiols, creating links between molecules, thereby forming aggregates.<ref name=lund/> Certain enzymes in grain catalyze the oxidation of thiols, and these may include sulphydryl oxidase, glutathione oxidase, glutathione peroxidase and phospholipid-hydroperoxide glutathione peroxidase (but neither peroxidases nor lipoxygenases).<ref name=stephenson>Stephenson WH, Biawa JP, Miracle RE, Bamforth CW. [https://onlinelibrary.wiley.com/doi/pdf/10.1002/j.2050-0416.2003.tb00168.x Laboratory-scale studies of the impact of oxygen on mashing.] ''J Inst Brew.'' 2003;109(3):273–283.</ref> It's interesting that thiol oxidation is actually used in many scientific studies as a measure of wort oxidation, which is of particular interest to many breweries due to its correlation with gel formation.<ref name=celus/> Disulfide bonds may be broken by reducing (oxygen-scavenging) compounds such as [[sulfite]], resulting in the regeneration of the original protein thiol groups. Regenerated thiol groups can participate in new reaction cycles with reactive oxygen species (ROS) and, taken as a whole, act as catalysts for the removal of ROS by sulfite.<ref name=lundmn>Lund MN, Andersen ML. [https://www.tandfonline.com/doi/abs/10.1094/ASBCJ-2011-0620-01 Detection of Thiol Groups in Beer and Their Correlation with Oxidative Stability.] ''J Am Soc Brew Chem.'' 2011;69(3):163–169.</ref> This antioxidant mechanism includes [[lipid transfer protein]] 1 (LTP1) as an important thiol-containing protein. See [[Oxidation]] for more information.

Flavor stability is the ability of the beverage to resist changes to flavor due to oxidation and other problematic forces). It is significantly influenced by the proteins present in the beer.

Protein thiols, which are derived from cysteine residues in proteins (as discussed above), possess antioxidative capacity in beer and wort.<ref name=lund>Lund MN, Lametsch R, Sørensen MB. [https://onlinelibrary.wiley.com/doi/pdf/10.1002/jib.155 Increased protein–thiol solubilization in sweet wort by addition of proteases during mashing.] ''J Inst Brew.'' 2014;120(4):467–473.</ref> The concentration of free thiols correlates with the oxidative stability of beer because thiols remove reactive oxygen species (ROS).<ref name=lundm>Lund MN, Petersen MA, Andersen ML, Lunde C. [https://www.tandfonline.com/doi/abs/10.1094/ASBCJ-2015-0602-01 Effect of protease treatment during mashing on protein-derived thiol content and flavor stability of beer during storage.] ''J Am Soc Brew Chem.'' 2015;73(3):287–295.</ref> The major contribution to the protein thiol concentration in beer is believed to be mainly from LTP1 because it is rich in cysteine and is a major component of beer protein.<ref name=lund/><ref name=lundm/> A number of other proteins have also been identified in beer that contain several cysteine residues, so they could also contribute significantly to the thiol concentration in the beer.<ref name=lund/> In fact, beer LTP1 has been shown to scavenge one of the dominating radical compounds in beer, the 1-hydroxyethyl radical, at a rate similar to other reactive compounds in beer such as hop bitter acids.<ref name=lundm/> The free thiol concentration diminishes as the beer or wort is exposed to oxygen.<ref name=lund/> As discussed above, thiols can serve as antioxidants during mashing. However, even if the wort is oxidized during mashing, the disulfides are reduced during fermentation, thus converting them to active antioxidants in the beer.<ref name=lund/> Of course, oxygen-limited packaging is also extremely important to prevent degradation of beer flavor.