O-9. The effects of pH on beer haze, beer foam, and astringency perception

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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