Dynamic Evolution of Bitterness Units in Beer Worts: Modeling and Concerns
AbstractBeer 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.
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