Biology Reference
In-Depth Information
Low molecular weight polypeptides constituted over 75 % by
weight of the total polypeptide content of beer, as estimated by
peak area integration of size exclusion chromatography [
11
,
24
].
Picariello et al. [
11
] analyzed beer polypeptides without enzyme
digestion using nanoscale capillary liquid chromatography-elec-
trospray tandem mass spectrometry (LC-ESI MS/MS) coupled
with a hybrid quadrupole orthogonal acceleration time of fl ight
mass spectrometer (Q-TOF MS). They identifi ed several C-terminal
fragments of protein Z (Mw < 4,500 Da), small size peptides
derived from LTP1 (Mw < 1,000 Da), and various low molecular
mass peptides (Mw < 1,000 Da) derived from
3-, B-, and
B1-hordeins. In more recent research, Colgrave et al. [
12
] also
analyzed the beer and wort proteins by LC-MS/MS without
enzyme digestion and identifi ed a number of proteins including
γ
-,
C-, B1-, B3-, and D-hordeins and avenin-like protein-a. These two
reports had a high impact in identifying hordein-derived polypep-
tides in beer. C-hordein degradation products were observed in
wort but only trace amounts were found in beer, suggesting that
the majority of these peptides do not survive the brewing and fi l-
tering processes [
12
].
γ
Foam and colloidal haze (clarity) are key factors for the visual
impression of beer. Excellent beer foam gives an impression that
the beer is fresh, pleasing to behold, and adequately carbonated.
Also, foam stability and bubble size infl uence the feel on the lips
and mouth. Beer foam quality is characterized by stability, quan-
tity, lacing, whiteness, creaminess, density, viscosity, and strength
[
25
]. Among these, foam stability is infl uenced by beer proteins.
Several studies have shown that protein Z family (Z4 and Z7) [
26
-
29
] and LTP1 [
30
,
31
] are involved in beer foam stability. Beer
proteins also infl uence beer haze formation. Haze is a serious qual-
ity problem since stale beer displays haze. It has been suggested
that hordein [
32
], which is the most abundant storage protein in
barley grain is a negative factor for colloidal haze formation.
However, factors causing beer foam stability and haze formation
are still controversial. One of the reasons for the poor understand-
ing of protein factors controlling beer quality traits may come from
the poor knowledge of protein species contained in beer samples.
Thus, comprehensive analysis of beer protein by proteomics is
essential to estimate the relationship between beer proteins and
beer quality traits, e.g., foam stability and haze formation.
2.4 Identifi cation
of Proteins Related
to Beer Quality Traits
Protein Z and LTP1 are abundant proteins in beer, and these pro-
teins have been suggested as foam-positive proteins. In particular,
modifi cation of protein Z and LTP1 was analyzed in detail using
proteomics. van Nierop et al. [
31
] compared the 2DE images of
LTP1 among normal, 96, and 105 °C heat treated samples, and
found that heat treatment caused signifi cant modifi cation.
2.4.1 Beer Foam-Related
Proteins
