Dynamic Evolution of Bitterness Units in Beer Worts: Modeling and Concerns
Keywords:iso-alpha acids, spectrophotometer, IBU, mathematical model, beer, bitterness
Beer bitterness is reported in International Bitterness Units (IBU), which is the concentration (in ppm) of iso-alpha-acids in the product. Such acids result from the isomerization of alpha-acids originally found in hops which is carried out by boiling the beer wort. This concentration can be measured by HPLC, which led, in the past, to some empirical predictions for the effects of boil time and wort density on the dynamic evolution of IBU in barley worts. Since HPLC is an expensive and time onerous procedure, the organizations devoted to the standardization of procedures in the brewing industry established protocols that indirectly evaluate the IBU by spectrophotometry. This work investigated the dynamic evolution of IBU values as a function of the wort density when evaluated by this standard procedure, which involves the extraction with iso-octane and the UV absorbance measurement in a spectrophotometer. Three barley worts were prepared with dry malt extract (DME) having specific gravities (SG) of 1.030, 1.040 and 1.050. These worts and a fourth sample constituted of distilled water (SG = 1.000) had their pH buffered in 5.2 and were boiled under atmospheric pressure. During the boiling step, Czech Saaz hop pellets were added according to a time schedule (1, 3, 5, 10, 25, 40 and 60 minutes). All reactions were conducted in small (500 ml) aluminum reactors with triplicates for each isomerization time. Finally, the IBU of the samples were measured and used to fit non-linear empirical models for the IBU evolution in the wort, based on a maximum likelihood statistical criterion along with deterministic optimization methods. Results indicated that IBU values, evaluated by the standard procedure, rise to their final value within a few minutes, instead of one hour, which is traditionally assumed.
. F. Vriesekoop, M. Krahl, B. Hucker, G. Menz. “Bacteria in brewing: The good, the bad and the ugly”. Institute of Brewing & Distilling, 118, 335-345, 2012
. H. O. Askew. “Changes in hop α-acids concentrations on heating in aqueous solutions and unhopped worts”. J. Inst. Brew., 70, 503-513, 1964.
. B. Jaskula, G. Aerts, L. D. Cooman. “Hop alpha-acids isomerization and utilization: an experimental review”. Cerevisia, 35, 57-70, 2010.
. K. Sakamoto, W. N. Konings. “Beer spoilage bacteria and hop resistance”. International Journal of Food Microbiology, 89, 105-124. 2003.
. I. Caballero, C. A. Blanco, M. Porras. “Iso-α-acids, bitterness and loss of beer quality during storage”. Trends in Food Science & Technology. 26, 21-30. 2012
. J. Christensen, A. M. Ladefoged, L. Norgaard. “Rapid Determination of Bitterness in Beer Using Fluorescence Spectroscopy and Chemometrics”. Journal of The Institute of Brewing, 111 (1), 3-10, 2005.
. G. A. Howard, L. R. Bishop, P. A. Martin, J. R. Hudson, J. S. Pierce “Recommended Methods of Analysis. Institute of Brewing Analysis Committee”, J. Inst. Brew., 77, 181-226, 1971.
. D. Nibumona, C. A. Blanco, I. Caballero, A. Rojas, C. Andrés-Iglesias. “An approximate shelf life prediction of elaborated lager beer in terms of degradation of its iso-α-acids”. Journal of Food Engineering, 116, 138-143, 2013.
. B. Jaskula, P. Kafarski, G. Aerts, L. De Cooman. “A Kinetic Study on the Isomerization of Hop α-Acids”. J. Agric. Food Chem. 56 (15), 6408-6415, 2008.
. M. G. Malowicki, T. H. Shellhammer. “Isomerization and degradation kinetics of hop (humulus lupulus) acids in a model wort-boiling system”, J. Agric. Food Chem. 53, 4434-4439, 2005.
. B. Jaskula, G. Aerts, L. D. Cooman. “Potencial Impacts of medium characteristics on the isomerization of hop α-acids in the wort and buffer model systems”. Food Chemistry, 123, 1219-1226, 2010.
. M. G. Malowicki, T. H. Shellhammer. “Factors Affecting Hop Bitter Acid Isomerization Kinetics in a Model Wort Boiling System”. J. Am. Soc. Brew. Chem, 64 (1), 29-32, 2006.
. O. Oladokun, K. Smart, D. Cook. “An improved HPLC method for single-run analysis of the spectrum of hop bittering compounds usually encountered in beers”. J. Inst. Brew. 122, 11-20, 2016.
. B. Jaskula, K. Goiris, G. De Rouck, G., G. Aerts, L. De Cooman. “Enhanced Quantitative Extraction and HPLC Determination of Hop and Beer Acids”, Journal of The Institute of Brewing, 113 (4), 381-390, 2007.
. B. Jaskula, E. Syryn, K. Goiris, G. De Rouck, F. Van Opstaele, J. De Clippeleer, G. Aerts, L. De Cooman, “Hopping technology in relation to beer bitterness consistency and flavour stability”. J. Am. Soc. Brew. Chem. 65 (1), 38–46, 2007.
. H. O. Askew. “Changes in concentration of α and β acids and of iso-compounds on heating extracts of hops in aqueous solutions and unhopped worts”, J. Inst. Brew., 71, 10-20, 1965.
. K. Huvaere, M. L. Andersen, L. H. Skibsted, A. Heyerick, D. De Keukeleire. “Photooxidative Degradation of Beer Bittering Principles: A Key Step of the Route to Lightstruck Flavor Formation in Beer”, Journal of Agricultural and Food Chemistry, 53 (5), 1489-1494, 2005.
. C. A. Blanco, D. Nimubona, I. Caballero. “Prediction of the ageing of commercial lager beer during storage based on the degradation of iso-α-acids”, J Sci Food Agric. 94, 1988-1993, 2014.
. F. Gunst, M. Verzele. “On The Sunstruck Flavor of Beer”, J. Inst. Brew., 84, 291-292, 1978.
. S. Kappler, M. Krahl, C. Geissinger, T. Becker, M. Krottenthaler. “Degradation of Iso-α-Acids During Wort Boiling”. Journal of The Institute of Brewing, 116 (4), 332-338, 2010.
. M. Kowaka, E. Kokubo. “Composition of Bitter Substances of Hops and Characteristics of Beer Bitterness”. ASBC Journal, 35 (1), 16-21, 1976.
. D. Intelmann, O. Demmer, N. Desmer, T. Hofmann. “18O Stable Isotope Labeling, Quantitative Model Experiments, and Molecular Dynamics Simulation Studies on the Trans-Specific Degradation of the Bitter Tasting Iso-α-acids of Beer”, J. Agric. Food Chem. 57, 11014-11023, 2009.
. G. Vanhoenacker, D. De Keukeleire, P. Sandra. “Analysis of iso-α-acids and reduced iso-α-acids in beer by direct injection and liquid chromatography with ultraviolet absorbance detection or with mass spectrometry”. Journal of Chromatography A, 1035(1), 53–61, 2004.
. E. Polshin, A. Rudnitskaya, D. Kirsanov, A. Legin, D. Saison, F. Delvaux, F. R. Delvaux, B. M. Nicolai, J. Lammertyn. “Electronic tongue as a screening tool for rapid analysis of beer”. Talanta, 81(1-2), 88–94, 2010.
. L. S. Calado, A. L. F. Lacerda, S. B. Fiaux, L. A. Sphaier, V. N. H. Silva, F. C. Peixoto. “Low-cost fluorescence-based method for beer bitterness measurement”. Journal of Food Engineering, 262, 9-12, 2019.
. G. A. Howard. “Estimation of the bitterness of beer”. J. Inst. Brew., 74, 249-251, 1968.
. O. Oladokun, A. Tarrega, S. James, K. Smart, J. Hort, D. Cook. “The impact of hop bitter acid and polyphenol profiles on the perceived bitterness of beer”, Food Chemistry, 205, 212–220, 2016.
. I. Nascimento, L. Calado, M. E. Duncan, B. Trindade, L. Sphaier V. N. H. Silva, F. C. Peixoto. “Interference in beer bitterness measurements caused by polymer tubes”, Journal of Food Engineering, 275, 1-4, 2020.
. J. Philpott, D. M. Taylor, D. R. Williams. “Critical Assessment of Factors Affecting the Accuracy of the IoB Bitterness Method”. Journal of the American Society of Brewing Chemists, 55 (3), 103-106, 1997.
. ASBC “Method of Analysis. American Society of Brewing Chemists. α- and β-Acids in Hops and Hop Pellets”. The Society. St Paul, MN, U.S.A, 2011.
. M. A. A. Kappel, F. C. Peixoto, G. M. Platt, R. P. Domingos, I. N. Bastos. “A study of equivalent electrical circuit fitting to electrochemical impedance using a stochastic method”, Applied Soft Computing. 50, 183-193, 2017.
. T. F. Edgar, D. M. Himmelblau, L. S. Lasdon. “Optimization of Chemical Processes”, McGraw-Hill, Nova York, N. Y., U.S.A., 2001.
How to Cite
Authors who submit papers with this journal agree to the following terms.