Graduate Thesis Or Dissertation
 

Bitterness modifying properties of hop polyphenols

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/3x816q11t

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  • This study examined whether polyphenols contribute a harsh bitterness to beer, and how polyphenols may otherwise modify beer bitterness in the presence of iso-a-acids. The experimental portions of this project were divided into effectively two segments. The first section of the study encompassed groundwork polyphenol extraction development, and the examination of the extract in beer in a series of sensory studies. The polyphenol extract was produced from spent Galena hop material and added to a low bitterness base-beer at three levels (+0, +100, +200ppm) along with iso-α-acids from a pre-isomerized Galena hop extract at two levels (+0, +10ppm) in order to examine the impact on bitterness character by timeintensity (Tl) and free-choice profiling (FCP) methods, using eleven trained panelists. Principal Components Analysis of the Tl results showed that the panelists separated the samples with the higher levels of polyphenols and isoa- acids from those with lower levels of both based on bitterness duration, maximum intensity, and area under the curve. Samples 10-100 and 10-200 (+10ppm iso-a-acids and +100 and +200ppm total polyphenols) were significantly more bitter and had longer duration than the others. A significant effect was found for both iso-α-acids and polyphenols as well as an interaction in terms of peak intensity and area under the curve (p<0.001). Similarly, the results from the FCP easily separated the treatments by bitterness, lingering bitterness, and astringency. Panelists divided each of the treatments into six distinct groups based on bitterness intensity, 5 groups based on duration, and into 2-3 groups based on the remaining consensus descriptors (mouthfeel, astringency, and bitterness come-up time). Samples high in polyphenols were frequently given higher intensities for "harsh," "medicinal," and "metallic" descriptors. The second portion of the experiment involved scaling up the extraction volumes and solving issues regarding the quality of the extract, which was then followed by some experimental brews using spent hop material. The main issues regarding the quality of the extract were the production of haze and the presence of humulinones (oxidized α-acids), which were present in the raw materials and have the potential to confound bitterness sensory data. Attempts to solve the haze problem included boiling the extract with an addition of wheat gliadin in an attempt to remove many of the polyphenols that have a tendency to promote haze formation. While there were promising results, the issue has not been effectively solved. A number of attempts were executed to remove humulinones from the extract, including some chromatographic techniques aimed at using a particular solvent polarity to affect the rates at which the polyphenols and humulinones moved through the chromatographic resins. Isocratic elutions at ethanol concentrations ranging from 20%-90% showed little effectiveness in separating these compounds, and gradient elutions, which showed minor efficacies, are difficult to adapt to large preparative Solid-Phase Extraction columns. The conclusion of the research saw the production of experimental beer, which was produced to examine potential varietal differences in the quality of bitterness produced from hop polyphenols. Two beers were brewed with very high dosings of spent hops using two hop varieties, Millennium and Australian Topaz. In future research, these beers will be blended to an appropriate polyphenol level using an unhopped lager of the same recipe. These treatments will be presented to a trained sensory panel for descriptive profiling. Overall, it was found that samples high in polyphenols had intense and long-lasting bitterness, and that many of the trained panelists found these samples to have metallic, harsh, and medicinal characters. Astringency was also affected by the level of hop polyphenols, as samples which were higher in polyphenols elicited higher astringent sensations for the panelists. There was little, if any, effect on the time-to-maximum bitterness intensity.
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