|
|
| Line 143: |
Line 143: |
|
| |
|
| ===Foam=== | | ===Foam=== |
| Proteins in beer directly contribute to the formation of foam and its stability. See [[Foam]]. | | Proteins in beer directly contribute to the formation of foam and its stability. See [[Foam]] for more information. |
| | |
| In the presence of sulfite, lipid adduction increased significantly the thermal stability of LTP1, whereas glycation has no or only a slight effect on the structural stability. Therefore, whatever LTP1 modification, lipid adduction or glycation, heating and reducing environment act synergistically on LTP unfolding. (Glycation, lipid adduction, and unfolding observed on malting and brewing, in this work and in previous studies (9, 17), should contribute to the transformation of barley LTP1 from a poor foaming to a foam-promoting protein.)<ref name=perrocheau>Perrocheau L, Bakan B, Boivin P, Marion D. [https://pubs.acs.org/doi/abs/10.1021/jf052910b Stability of barley and malt lipid transfer protein 1 (LTP1) toward heating and reducing agents: relationships with the brewing process.] ''J Agric Food Chem.'' 2006;54(8):3108−3113.</ref>
| |
| | |
| The malting and brewing process transforms the barley albumins into foaming proteins. The mechanism involved in these structural maturation has been recently delineated, especially for one of the major beer proteins, LTP1. Actually, the barley LTP1 does not form any foam while the corresponding beer protein display good foaming properties. In beer, LTP1 is a mixture of glycated proteins displaying different unfolding state. Glycation occurs on kilning during the malting process while unfolding occurs on brewing. Heat-induced unfolding of LTP1 is strengthened by the reduction of disulfi de bonds of the protein during the extraction of malt. The disulfide bond reducing mechanism of malt has not been identified yet, but it should involve the thioredoxin and/or glutathione oxido-reducing pathways. The other important modifi cation is the acylation of LTP1. This acylation is catalyzed by two enzymes from the embryo, a 9-lipoxygenase (9-LOX) and an allene oxide synthase (AOS). 9-LOX generates a 9-hydroperoxide from linoleic acid, the major polyunsaturated free fatty acids (FFAs) from cereal seeds while the AOS transforms the hydroperoxide in the corresponding allene oxide. The electrophilic attack of the unstable allene oxide by an acidic side chain of the protein (Asp7) leads to the covalent binding of an alpha-ketol to LTP1. This acylation has been observed in beer for the glycated LTP1. Up to two alpha-ketol can be covalently bound to the barley LTP1. Finally, glycation and acylation increase the amphiphilic character of the protein while unfolding improves spreading of the protein at the air–water interface allowing a better anchoring in the interface through the acyl adducts and formation of protein–protein interaction in the film surrounding gas bubbles.<ref name=didier/>
| |
|
| |
|
| ===Haze=== | | ===Haze=== |
| Proteins can play a significant role in the formation of beer haze. See [[Haze]]. | | Proteins can play a significant role in the formation of beer haze. See [[Haze]] for more information. |
|
| |
|
| ==See also== | | ==See also== |
