Factors Affecting Hop Bitter Acid Isomerization Kinetics in a Model Wort Boiling System

Potential impact of medium characteristics on the isomerisation of hop α-acids in wort and buffer model systems

“…However, in aqueous buffer medium, our results indicate that isomerisation yields obtained at the lower pH values (pH 4.8 up to pH 5.5) remain similar. Thus, when boiling in buffer, a change of the pH in the range of 4.8-5.5 does not really influence a-acids isomerisation yields, which is in agreement with previously published data by Malowicki and Shellhammer (2006).…”

“…5.0) the solubility of humulone is only about 60 mg/l at 100°C (Benitez et al, 1997; see also Spetsig (1955) and Malowicki and Shellhammer (2006)). In addition to solubility problems, the efficiency of the isomerisation reaction in the kettle is highly influenced by temperature, vigour and length of boil, pH of wort, specific gravity, hopping rate, hop product(s) added and point(s) of addition, kettle design, and adsorption of bittering principles on the protein break (Benitez et al, 1997;Irwin, Murray, & Thompson, 1985;Malowicki & Shellhammer, 2005, 2006Narziss, 1992;Verzele, 1965). A significant part of the hop and beer bitter acids, i.e., a-acids and iso-a-acids, respectively, will be removed with the hot trub during wort clarification and, further on, also during fermentation/maturation, and final beer filtration (Benitez et al, 1997;Laws, McGuinness, & Rennie, 1972).…”

Degradation of Iso-α-Acids During Wort Boiling

“…One of the main reasons is the poor solubility of the unisomerised humulones, as determined by Kolbach 29 and Spetsig 49 . Solubility of α-acids can be improved by using an increased pH value, which may result in an accelerated isomerisation reaction, although the isomerisation reaction itself is not affected by changes in pH 33 . Additionally, boiling time, boiling temperature, original gravity, type of hop product and hop variety used are all factors that influence the yield of iso-α acids 13,[33][34]41 .…”

“…Solubility of α-acids can be improved by using an increased pH value, which may result in an accelerated isomerisation reaction, although the isomerisation reaction itself is not affected by changes in pH 33 . Additionally, boiling time, boiling temperature, original gravity, type of hop product and hop variety used are all factors that influence the yield of iso-α acids 13,[33][34]41 . Prolonged boiling time, a higher temperature and a higher concentration of α-acids in the dosed product have all shown a positive effect 11,17,18 .…”

“…Recent research has also shown that there are some degradation reactions which are specific for trans-isohumulones 22,23 . These reactions form substances with a lingering and a harsh bitter taste 33 .…”