O-15
The effect of pH on beer stabilization
Presenter: Karl Siebert, Dept. of Food Science & Technology, Cornell University, Geneva, NY, USA
Co-Author: P.Y. Lynn, Dept. of Food Science & Technology, Cornell University, Geneva, NY, USA

The interaction of haze-active (HA) proteins with HA polyphenols has been shown to have a striking dependency on pH. In model systems, the same amounts of protein and polyphenol produced about seven times as much haze at a pH slightly above 4 than at pH 3. The amount of haze formed as pH increased further above the haze maximum declined steadily. The haze intensity was interpreted as indicating the strength of the protein-polyphenol interaction. The nature of polyphenol – polyvinylpyrrolidone (PVP) interaction appears to resemble that of protein-polyphenol interaction and might show a similar pH effect. If so, insoluble PVP (PVPP) effectiveness could well be different for beers of different pHs in the typical beer range (pH 3.9 – 4.6), and this might account for some of the observed differences in adsorbent effectiveness in different beers. The haze-active (HA) polyphenol tannic acid was combined with soluble PVP (PVP K90) in alcoholic (5% v/v) buffer solutions of different pH. After incubation at 25ºC, the turbidity was determined. The results indicated that there were indeed substantial differences in the amounts of haze formed at pHs within the beer range, with a maximum near pH 4. This indicates that PVP – polyphenol interaction strength does indeed vary with pH. Experiments in which unchillproofed beer was adjusted in pH and then treated with PVPP were carried out. The HA polyphenol content was assessed before and after treatment. Since it has been demonstrated that both silica and polyphenols bind to the proline residues in HA polypeptides, it is conceivable that the interaction of proline with silanol OH groups is similar to that with polyphenol OH groups. If so, they might be affected by pH in a similar way. At least one literature report indicates that the efficacy of chillproofing beer with silica is influenced by beer pH, and is maximal at pH 4.3 - 4.4. That is close to the pH optimum for protein-polyphenol interaction. It was of interest to investigate this in greater detail, as it could account for some of the observed differences in silica effectiveness with different beers. Experiments in which unchillproofed beer was adjusted in pH and then treated with silica were carried out. The HA protein content was assessed before and after treatment.

Karl Siebert received a Ph.D. in biochemistry from Penn State in 1970. He 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 chairman and now has a predominantly research appointment. Dr. Siebert served on ASBC technical subcommittees and was a member and chairman of the Technical Committee. He is serving his second stint on the JASBC Editorial Board (1980-1992; 1996-). He is active as a consultant in the beverage industry.