Protein: Difference between revisions

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Flavor stability is the ability of the beverage to resist changes to flavor, mainly due to oxidation. Proteins in the beer have a significant influence on flavor stability.

Protein thiols, which are present on 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 comes mainly from LTP1 because it is rich in cysteine and is a major component of beer protein.<ref name=lund/><ref name=lundm/><ref name=wumj>Wu MJ, Clarke FM, Rogers PJ, et al. [https://www.mdpi.com/1422-0067/12/9/6089/pdf Identification of a protein with antioxidant activity that is important for the protection against beer ageing.] ''Int J Mol Sci.'' 2011;12(9):6089–6103.</ref> 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/> Thiols will react with almost all of the reactive oxygen species, which makes them especially important as far as beer aging is concerned.<ref name=wu/> 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 particular, yeast thioredoxin (TRX) is another thiol-rich protein suggested to provide defense against ROS.<ref name=wu/>

Protein thiols, which are present on 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><ref name=wumj/> 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 comes mainly from LTP1 because it is rich in cysteine and is a major component of beer protein.<ref name=lund/><ref name=lundm/><ref name=wumj>Wu MJ, Clarke FM, Rogers PJ, et al. [https://www.mdpi.com/1422-0067/12/9/6089/pdf Identification of a protein with antioxidant activity that is important for the protection against beer ageing.] ''Int J Mol Sci.'' 2011;12(9):6089–6103.</ref> 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/> Thiols will react with almost all of the reactive oxygen species, which makes them especially important as far as beer aging is concerned.<ref name=wu/> 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 particular, yeast thioredoxin (TRX) is another thiol-rich protein suggested to provide defense against ROS.<ref name=wu/>

Note that the free thiol concentration diminishes as they are exposed to oxygen, removing the antioxidative ability of LTP1 and the other cysteine-rich proteins.<ref name=lund/><ref name=lundm/><ref name=wu>Wu MJ, Rogers PJ, Clarke FM. [https://onlinelibrary.wiley.com/doi/pdf/10.1002/jib.17 125th anniversary review: The role of proteins in beer redox stability.] ''J Inst Brew.'' 2012;118(1):1–11.</ref> However, even if the wort is fully oxidized during mashing, the disulfides (bound thiols) are subsequently reduced during fermentation, thus converting them to active free thiol antioxidants in the beer.<ref name=lund/><ref name=lundm/><ref name=wu/> Of course, oxygen-limited packaging is also extremely important to prevent degradation of beer flavor.

Note that the free thiol concentration diminishes as they are exposed to oxygen, removing the antioxidative ability of LTP1 and the other cysteine-rich proteins.<ref name=lund/><ref name=lundm/><ref name=wu>Wu MJ, Rogers PJ, Clarke FM. [https://onlinelibrary.wiley.com/doi/pdf/10.1002/jib.17 125th anniversary review: The role of proteins in beer redox stability.] ''J Inst Brew.'' 2012;118(1):1–11.</ref> However, even if the wort is fully oxidized during mashing, the disulfides (bound thiols) are subsequently reduced during fermentation, thus converting them to active free thiol antioxidants in the beer.<ref name=lund/><ref name=lundm/><ref name=wu/> Of course, oxygen-limited packaging is also extremely important to prevent degradation of beer flavor. Beers with higher levels of free thiols taste better (i.e. less oxidized).<ref name=wumj/>

Reducing agents such as sulfite can re-activate oxidized thiols,<ref name=wu/><ref name=lundm/> although adding sulfite at packaging is ~~generally~~ not something we recommend~~. See [[Packaging]]~~.

Reducing agents such as sulfite can re-activate oxidized thiols,<ref name=wu/><ref name=lundm/> although adding sulfite at packaging is not something we recommend.

Science FYI! A possible working mechanism for its ROS-scavenging ability is proposed: LTP thiol(s) is oxidized to the sulfenic acid by oxidants such as H2O2, which results in the destruction of a peroxide molecule in 1:1 stoichiometry. The free thiol can be recovered by two sequential reactions (reactions 2 and 3). The reaction 2 generates a disulfide (LTP-SSR) through reaction with a small molecule (HS-R) such as yeast thioredoxin. The reaction 3 uses sulfite or phenolic compounds to generate free thiol from the disulfide for the next round elimination of ROS.<ref name=wumj/>

Similar to sparkling wine, yeast-derived proteins can also positively affect the flavor stability of beer, even though massive yeast autolysis can generate off-flavors in beer.<ref name=picariello/>

This high content of thiol cysteines in the protein is the basis for its radical scavenging and antioxidant activities. However, native barley LTP1 would not have antioxidant activity because all its thiol groups are occupied in the formation of disulfide bonds. The labeling of LTP1 thiols in beer demonstrated that the disulfide bonds in the native LTP1 were disrupted and linearised, most likely due to denaturing steps of malting, wort boiling and brewing. These free thiols were maintained during brewing and in packaged beer by a variety of factors. One of them could be the glycation of glycine and lysine residues with sugars such as glucose and xylose via the Maillard reaction [21]. The foam stabilising property of LTP1 has also been attributed to its glycosylation [19]. A possible working mechanism for its ROS-scavenging ability is proposed: LTP thiol(s) is oxidised to the sulfenic acid by oxidants such as H2O2, which results in the destruction of a peroxide molecule in 1:1 stoichiometry. The free thiol can be recovered by two sequential reactions (reactions 2 and 3). The reaction 2 generates a disulfide (LTP-SSR) through reaction with a small molecule (HS-R) such as yeast thioredoxin. The reaction 3 uses sulfite or phenolic compounds to generate free thiol from the disulfide for the next round elimination of ROS.<ref name=wumj/> Beers with higher levels of free thiols taste better.

Thiols were found to be irreversibly oxidized during storage at both 35 and 22°C, showing that they are, in fact, reactive in beer, but also that they are not consumed at the same rate as sulfite. Further, sulfite is more vulnerable to a higher storage temperature than thiols. Protease-treated beer was found to develop a fruity aged or vinous character and an increased formation of furfural through Maillard reactions during storage, whereas the control beer developed a more papery character due to increased formation of linear aldehydes through oxidation. Therefore, in order to obtain increased flavor stability of lager beer during storage, it is necessary to further improve the protease treatment to avoid elevated concentrations of free amino acids and Maillard reactions during storage.<ref name=lundm/>

Similar to sparkling wine, yeast-derived proteins can also affect the flavor stability of beer ~~(Guido et al., 2004)~~, even though massive yeast autolysis can generate off-flavors in beer.<ref name=picariello/>

===Foam===