Graduation date: 2007
Hop derived bitter compounds, including alpha-acids, reduced and non-reduced iso-alpha-acids, were evaluated for their contribution to peak bitter intensity in lager beer. Alpha-acids are the precursors to the major bittering components in beer (iso-alpha-acids). Typically, alpha-acids do not survive the brewing process, but if a product is dry-hopped, they may solubilize into the finished beer depending on the system pH, temperature and ethanol content. The impact of alpha-acids on the bitterness of lager beer was investigated using a trained panel and a test with a consumer panel. The trained panel evaluated samples with and without alpha-acids to offer initial analysis on aroma and bitterness intensity, and a triangle test comparing an unhopped lager with and without 14 ppm alpha acids (the solubility limit in beer) was presented to over 100 consumers for evaluation. Both panels found no significant difference between the samples. Furthermore, statistical similarity of the samples with and without alpha-acids was validated. This confirmed that alpha acids contribute negligibly to the overall bitterness of lager beer.
Iso-alpha-acids are a hop derived compound formed from thermally induced isomerization of the alpha-acids. When exposed to ultra-violet light (UV), the iso-alpha-acids are degraded forming off-odors and flavors. Therefore, where UV degradation is of concern, it is important to brewers to find an alternative to iso-alpha-acids. The reduced iso-alpha-acids, rho-iso-alpha-acids, hexahydro-iso-alpha-acids, and tetrahydro-iso-alpha-acids, can be used as a substitute for iso-alpha-acids and provide bitterness and UV stability. The reduced iso-alpha-acids offer varying degrees of change to the temporal bitterness qualities of a beer when compared to iso-alpha-acids.
The relative bitterness relationships of reduced to non-reduced iso-alpha-acids were measured using a time-intensity protocol, in which a trained panel evaluated seven concentrations of each compound in an unhopped lager beer. The peak intensities were identified, and a non-linear dose-response curve, called a change-point model, was fit to the data. Three parameters, a, b, and θ, identified the shape of the model. Panelist’s replicated well but varied in sensitivity to the compounds and how they rated bitter intensity. Per-panelist and panel-wise equi-bitter equations were constructed from the parameters. Statistical analysis was performed to identify differences in bitter impact. Accordingly, rho was significantly less bitter than iso-alpha-acids, and hexahydro-iso-alpha-acids and tetrahydro-iso-alpha-acids were not different significantly in bitter impact over a range of iso concentration. The predicted equal bitter concentrations for each reduced iso-alpha-acids to iso-alpha-acids were validated by a consumer panel at a single concentration of iso-alpha-acids.
In conclusion, each hop compound researched (alpha-acids, iso-alpha-acids, rho-iso-alpha-acids, hexahydro-iso-alpha-acids, and tetrahydro-iso-alpha-acids) differed in the contribution to peak bitter impact of lager beer. Alpha-acids did not contribute significantly to the bitterness of an unhopped lager as validated by a consumer panel. This is particularly important for brewers that dry-hop their beers. And for those brewers wanting to use reduced iso-alpha-acids to replace iso-alpha-acids as a method for eliminating UV degradation, it is important to understand the peak bitter relationship for each of the compounds. By applying the change-point model, the natural variation among panelists was accommodated and compound differences that were not initially quantifiable were revealed and defined.