Weezy
Well-Known Member
Hop Bitter Acid Isomerization and Degradation Kinetics in a Model Wort-Boiling System
Of the factors tested (glucose at 100 Plato, maltose at 100 Plato, calcium at 100 ppm, and pH ranging from 4.8 to 6.0) none were shown to affect the rate of production of iso-alpha acids. While pH had a marked effect on the concentrations of alpha acids as measured, the differences may be attributed to solubility issues (as the solubility limit was approached and exceeded with decreasing pH) and did not appear to affect the rate of iso-alpha production.
Kinetic Modeling of Hop Acids during Wort Boiling;Yarong Huang, Johannes Tippmann, and Thomas Becker; International Journal of Bioscience, Biochemistry and Bioinformatics, Vol. 3, No. 1, January 2013
Interesting paper on the effects of pH, temp and time vs. the creation of alpha acids. Math heavy, but they're suggesting higher pH (6.5 even) requires less energy and time to complete the reactions. He seems to suggest that pre-isomerization, as in prior to and separate from the wort production, could be carried out in a controlled solution, in a production brewery. Here's a non-math blurb on pH:
The experimentally determined Arrhenius equations and activation energies derived therefrom for total iso-alpha-acids with different pH values, are summarized in Table I. The results obtained indicated that raising pH value from 4.5 to 5.5 and from 5.5 to 6.5 decreased the reaction energy fast 20 kJ/mol. Clarke noted that the iso-alpha-acids can be hydrolyzed or converted to humulinic acid with aqueous alkali. In iso-alpha-acids, splitting of the isohexenoyl side chain at C4 prefers to at a high pH value.
In this paper, the kinetics of degradation of iso-alpha-acids has been studied through boiling experiments in an aqueous buffer model system as a function of time, temperature and pH value. The free energy of activation and the reaction rate constant (k) of the degradation of total and individual iso-alpha-acids were calculated at four pH levels. The determination of the reactions order was run by a program compiled in Matlab R2007a language, which is a powerful software for numerical calculation. By raising the pH value of the reaction mixture increased the degradation of iso-alpha-acids. By raising the boiling temperature decreased the influence of pH value on the degradation of iso-alpha-acids significantly.
I can't find a link to this one but I have the PDF:
A Kinetic Study on the Isomerization of Hop Alpha-Acids,Journal of Agricultural and Food Chemistry, 2008
This is another interesting one with correlations to temperature, from 80C-100C and the formation and destruction of acids during the boil (higher the temp, the faster the isomerization). Some info:
Hop R-acids and the essential hop oils are at the origin of key flavor attributes of beer, namely, typical beer bitterness and hoppy aroma. With respect to conventional hopping, a most important chemical conversion occurs during wort boiling, namely, the thermal isomerization of the hop R-acids to the bitter tasting iso-R-acids via an acyloin-type ring contraction.
Conversion of each R-acid results in two epimeric iso-Racids, distinguished as a trans- and cis-iso-R-acid. Consequently, six major iso-R-acids are present in beer, i.e., the trans- and cis-isomers of isocohumulone, isohumulone, and isoadhumulone. Unfortunately, upon wort boiling, the isomerization yield of R-acids into iso-R-acids is invariably low (at most 50-60%) and also subject to variations, even from brew to brew. At the origin of the poor R-acid isomerization are the limited solubility of R-acids in wort, incomplete isomerization during the boil, and depletion of R-acids and iso-R-acids because of adsorption on the trub being formed. Furthermore, factors such as pH, wort gravity, hopping rate, hop product(s) used, presence of divalent cations, duration and temperature of the boil, and the degree of dispersal of the R-acids upon addition of hops all have important influences on the R-acid isomerization yield and final utilization (310). Final overall R-acid utilization is related to the beer, and this value is still significantly lower than the isomerization yield, amounting to only 30-40% or even as low as 10-20% (11). This is due to further losses of iso-R-acids postwort boiling, i.e., further losses during wort clarification, fermentation, maturation, and beer filtration.
R-acids is alpha-acids not copy/pasting well.
Of the factors tested (glucose at 100 Plato, maltose at 100 Plato, calcium at 100 ppm, and pH ranging from 4.8 to 6.0) none were shown to affect the rate of production of iso-alpha acids. While pH had a marked effect on the concentrations of alpha acids as measured, the differences may be attributed to solubility issues (as the solubility limit was approached and exceeded with decreasing pH) and did not appear to affect the rate of iso-alpha production.
Kinetic Modeling of Hop Acids during Wort Boiling;Yarong Huang, Johannes Tippmann, and Thomas Becker; International Journal of Bioscience, Biochemistry and Bioinformatics, Vol. 3, No. 1, January 2013
Interesting paper on the effects of pH, temp and time vs. the creation of alpha acids. Math heavy, but they're suggesting higher pH (6.5 even) requires less energy and time to complete the reactions. He seems to suggest that pre-isomerization, as in prior to and separate from the wort production, could be carried out in a controlled solution, in a production brewery. Here's a non-math blurb on pH:
The experimentally determined Arrhenius equations and activation energies derived therefrom for total iso-alpha-acids with different pH values, are summarized in Table I. The results obtained indicated that raising pH value from 4.5 to 5.5 and from 5.5 to 6.5 decreased the reaction energy fast 20 kJ/mol. Clarke noted that the iso-alpha-acids can be hydrolyzed or converted to humulinic acid with aqueous alkali. In iso-alpha-acids, splitting of the isohexenoyl side chain at C4 prefers to at a high pH value.
In this paper, the kinetics of degradation of iso-alpha-acids has been studied through boiling experiments in an aqueous buffer model system as a function of time, temperature and pH value. The free energy of activation and the reaction rate constant (k) of the degradation of total and individual iso-alpha-acids were calculated at four pH levels. The determination of the reactions order was run by a program compiled in Matlab R2007a language, which is a powerful software for numerical calculation. By raising the pH value of the reaction mixture increased the degradation of iso-alpha-acids. By raising the boiling temperature decreased the influence of pH value on the degradation of iso-alpha-acids significantly.
I can't find a link to this one but I have the PDF:
A Kinetic Study on the Isomerization of Hop Alpha-Acids,Journal of Agricultural and Food Chemistry, 2008
This is another interesting one with correlations to temperature, from 80C-100C and the formation and destruction of acids during the boil (higher the temp, the faster the isomerization). Some info:
Hop R-acids and the essential hop oils are at the origin of key flavor attributes of beer, namely, typical beer bitterness and hoppy aroma. With respect to conventional hopping, a most important chemical conversion occurs during wort boiling, namely, the thermal isomerization of the hop R-acids to the bitter tasting iso-R-acids via an acyloin-type ring contraction.
Conversion of each R-acid results in two epimeric iso-Racids, distinguished as a trans- and cis-iso-R-acid. Consequently, six major iso-R-acids are present in beer, i.e., the trans- and cis-isomers of isocohumulone, isohumulone, and isoadhumulone. Unfortunately, upon wort boiling, the isomerization yield of R-acids into iso-R-acids is invariably low (at most 50-60%) and also subject to variations, even from brew to brew. At the origin of the poor R-acid isomerization are the limited solubility of R-acids in wort, incomplete isomerization during the boil, and depletion of R-acids and iso-R-acids because of adsorption on the trub being formed. Furthermore, factors such as pH, wort gravity, hopping rate, hop product(s) used, presence of divalent cations, duration and temperature of the boil, and the degree of dispersal of the R-acids upon addition of hops all have important influences on the R-acid isomerization yield and final utilization (310). Final overall R-acid utilization is related to the beer, and this value is still significantly lower than the isomerization yield, amounting to only 30-40% or even as low as 10-20% (11). This is due to further losses of iso-R-acids postwort boiling, i.e., further losses during wort clarification, fermentation, maturation, and beer filtration.
R-acids is alpha-acids not copy/pasting well.
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