Presenter: Karl J. Siebert, Cornell University, Geneva, NY
Beer haze and foam and astringency perception are all strongly
influenced by pH. Beer haze formation is mainly due to
interactions between barley hordein, a protein rich in proline,
and polyphenols. Haze-active polyphenols bridge the protein
molecules together, resulting in complexes; these can grow
to become suspended insoluble particles, at which point they
scatter light and result in turbidity. Astringency perception
is also mainly due to precipitation of proline-rich proteins
(normally present in saliva and providing lubrication of oral
surfaces) by polyphenols. Proteins generally have the least
solubility in water near their isoelectric points, where the net
charge on the protein is 0. Beer haze formation is greatest
slightly above pH 4 and is much weaker at both higher and
lower pHs. However, the isoelectric point of barley hordein
is considerably higher than 4. The greatest precipitation of
salivary proline-rich proteins by polyphenols also occurs
slightly above pH 4, and these proteins also have fairly high
isoelectric points. So, it appears that the pH at which the
strongest protein-polyphenol interaction occurs has something
fundamental to do with the nature of the interaction between
proline-rich proteins and polyphenols, rather than protein
precipitation per se. Beer foam is well known to involve one
or more barley albumen proteins forming complexes with
iso-alpha-acids. Studies with both beer and a model system
showed greater foam formation with higher pH within the beer
range. The foam protein–iso-alpha-acid interaction mechanism
appears not to be due to ionic bonding, and in fact, the net
charge on the beer albumens associated with foam decreases
with increasing pH. Both the haze and foam interactions
appear to result not from ionic bonding but from hydrogen or
hydrophobic bonding or a combination of the two.
Karl Siebert received a Ph.D. degree in biochemistry from
Penn State in 1970. He then joined the Stroh Brewery Company
in Detroit, MI, where he spent 18 years and held positions
from research associate to director of research. In 1990, Karl
joined Cornell University as professor of biochemistry in the
Department of Food Science and Technology. He served five
years as department chair and now is involved in research,
teaching, and extension. Karl is active as a consultant in
beverage technology and chemometrics. He twice received
MBAA Outstanding Paper 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. Karl was named an honorary
professor of the Moscow (Russia) State Academy of Food
Processing in 1996, and in 1999 he received the ASBC Award
of Distinction. He received the MBAA Award of Merit in 2011.
He is currently a member of the ASBC Journal Editorial Board.
Karl’s research interests involve foam and haze in beverages,
perception of astringency and other flavors, application of
chemometric methods in food science, and assessment of
microbiological risk.
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