Program Book - Master Brewers Association of the Americas

O-10
Recent developments in protein-polyphenol haze
KARL SIEBERT (1)
(1) Cornell University, Geneva, NY
Considerable new information about protein-polyphenol haze
has emerged over the last 25 years. Only proteins that contain a
significant amount of proline bind polyphenols and form haze.
Such proteins have finite numbers of sites where polyphenols can
attach. Polyphenols that bind to proteins have at least two hydroxyl
groups on a single aromatic ring. Vicinal triphenols bind proteins
more strongly than vicinal diphenols, which bind more strongly
than m-diphenols. Haze-active (HA) polyphenols have at least
two such rings and can bridge proteins together. ‘Single ended’
polyphenols have only a single binding ring; they can compete with
HA polyphenols for binding sites in proteins. Structurally larger
polyphenols of the same type are more HA. Protein-polyphenol
interaction involves hydrogen bonding and hydrophobic bonding,
but not ionic bonding. The proportion of protein to polyphenol
affects both the amount of haze (light scattering) and the size of the
haze particles. A conceptual model that accounts for this behavior
was developed. Weight ratios of protein to polyphenol near 2:1 and
5:1 correspond to larger haze particles and more light scattering
than higher or lower ratios. The changes seen as the ratio changes
are mainly in the proportions of particles of a few discrete sizes
rather than gradual shifts of a monomodal distribution. The pH has
a profound effect on protein-polyphenol interaction. Maximum light
scattering with the same concentrations of protein and polyphenol
occurs near pH 4 and drops off fairly sharply as pH increases
or decreases. The pH of this haze peak is far from either the
isoelectric point of the protein or the pKa of the polyphenol. Haze
formation has two linear phases. At first little or no haze formation
is apparent. After some time haze starts to increase linearly. The
rate of haze formation appears to be a function of the product of
the concentrations of HA protein and dimeric proanthocyanidins.
Silica, unlike bentonite, is remarkably specific for haze-active
proteins because it binds to the same feature (proline residues) to
which polyphenols attach. Silica virtually ignores proteins that
are low in proline (including many that are foam-active). Silica is
more effective in removing HA protein when the ratio of protein to
polyphenol is high. PVPP is more effective in removing polyphenol
when the ratio of protein to polyphenol is low. Binding of HA
polyphenol to PVPP and HA protein to silica also involves hydrogen
and hydrophobic bonding but not ionic bonding.
Karl Siebert received a Ph.D. degree in biochemistry from Penn State
in 1970. He then joined the Stroh Brewery Company in Detroit,
where he spent 18 years and held positions from research associate
to director of research. In 1990, Dr. Siebert joined Cornell University
as professor of biochemistry in the Department of Food Science and
Technology. He served five years as department chair and now
has a predominantly research commitment. Dr. Siebert is active as
a consultant in beverage technology and chemometrics. He twice
received MBAA Presidential Awards for papers he presented, and
he and his colleague, Penny Lynn, received the ASBC Eric Kneen
Memorial Award (for the best paper published in the Journal of the
American Society of Brewing Chemists in the prior year) three times.
Dr. Siebert was made a honorary professor of the Moscow (Russia)
State Academy of Food Processing in 1996, and in 1999 he received
the ASBC Award of Distinction. He is currently a member of the
ASBC Journal Editorial Board. Dr. Siebert’s research interests involve
foam and haze in beverages, the application of chemometric methods
in food science, and assessment of microbiological risk.
O-11
Diagnosis of causes of foam instability in commercial beers
JONATHAN GOLDBERG (1), Charles Bamforth (1)
(1) University of California, Davis, CA
The cause of foam instability in commercial beers was explored.
Three proposed causes of foam instability in beers were
investigated: that arising from the presence of lipid, that from lipid
hydroperoxides, and that from low molecular weight polypeptides.
To diagnose the cause of foam instability commercial beers were
ultra-filtered (UF). The unfiltered beer, the permeate and the
retentate were then dosed with varying amounts of egg white and
improvement in foam stability was observed. Further investigations
using immobilized bovine serum albumin (BSA) to adsorb lipid from
the original beer and permeate and proteolytic enzymes to hydrolyze
any low molecular weight polypeptides passing through to the
permeate were used to diagnose foam instability arising from lipid
and low molecular weight polypeptides respectively. UF fractions
were tested for the presence of lipid hydroproxides. Data was
correlated with results from the Steinfurth foam stability tester.
Jonathan Goldberg received a B.S. (Hon.) degree in brewing and
distilling from Heriot- Watt University in Edinburgh, Scotland. He
has worked for breweries and distilleries in Canada, the United
Kingdom, and the United States. Presently, he is a graduate student
at the University of California, Davis, in the Department of Food
Science and Technology, working in Professor Charles Bamforth’s
malting and brewing science laboratory. He is a member of the ASBC
and MBAA.
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