Philip C Wietstock, Technische Universität Berlin, Fachgebiet Brauwesen, Berlin, Berlin, Germany
Co-author(s): Thomas Shellhammer, Department of Food Science and Technology, Oregon State University, Corvallis, OR, USA
ABSTRACT: Determining a beer’s bitterness is an important parameter for beer quality. The international bitterness units (IBU) method, the archived ASBC iso-alpha-acids (IAA) method, and examination of hop bitter acids concentration using high performance liquid chromatography (HPLC) are prone to operator error, are time-consuming, and require expensive equipment, respectively. In this study, a solid phase extraction technique was modified for the spectrometric determination of beer bitter units (SBU). Using 30 different commercially available beers, the new SBU method was compared to the IBU method and the archived ASBC IAA method. The same data were correlated with hop acid concentrations as determined using HPLC and sensory bitterness as perceived by a taste panel of 11 trained panelists. The method’s repeatability and reproducibility were examined in a collaborative study (six collaborating labs) with three sample pairs differing in hop bitter acid concentration and level of dry-hopping. The repeatability and reproducibility was then calculated according to ASBC Methods of Analysis. Plotting all data from IBU, IAA, and SBU against iso-alpha-acid concentration as measured via HPLC yielded the highest coefficient of determination for the IAA method (R2 = 0.97), followed by the SBU method (R2 = 0.96) and the IBU method (R2 = 0.90). Sensory bitterness data displayed the highest linearity with the new SBU method (R2 = 0.93) compared with data from the IBU method (R2 = 0.90) and IAA method (R2 = 0.90). Variance analysis indicated that the SBU method had the lowest variance (Var = 226) followed by the IAA method (Var = 286) and the IBU method (Var = 371). Repeatability and reproducibility coefficients of variation for the SBU method ranged from 1.12 to 3.32% and 4.61 to 15.56%, respectively. Taking all the data together, there is an indication that the SBU method is a repeatable and precise measurement for determining both concentration of iso-alpha-acids and sensory bitterness. This new method features elimination of trimethylpentane and is relatively simple to execute using solid phase extraction media.
Philip Wietstock is a scientific assistant at the Technische Universität Berlin, Germany. After graduating from his biotechnology studies with a diploma in engineering from the Technische Universität Berlin (2009), he worked for one year as an intern at the Department of Food Science and Technology at Oregon State University, Corvallis, OR. In 2011, he transferred to his present position, where he is working on his dissertation which focuses on the investigation of the influence of hops on oxidative beer stability.