A Multicomponent UV Analysis of α- and β-Acids in Hops

Abstract

b S Supporting Information T his lab uses humulones (α-acids) and lupulones (β-acids) present in hops, a component of beer, to explore and explain several different chemical and mathematical concepts. The hop system is educationally engaging because it can be integrated into the curriculum at many different levels. In the nonmajors chemistry lab this experiment can be used to introduce the concept of an absolute method that does not require calibration versus a relative method that does. If a minor interfering species is ignored, the hop analysis can also be used at the general chemistry level to introduce the standard two-component analysis using high school algebra. At the chemistry major's analytical chemistry level the same lab can be used as a multicomponent analysis that includes an interfering species, and the mathematics behind the multicomponent analysis can be used as a bridge to the method of Gaussian elimination and matrix algebra to explain how systems of several equations and unknowns can be solved. At all levels a comparison of this method with the original, industry standard analysis method 1 can be used to illustrate a more "green" methodology in analytical methods development. This lab also can be used to expose students to units of specific absorptivity as well as the more often used unit of molar absorptivity. Finally, as a lab with a direct practical application to a real-world problem, a problem that will allow a student to interact with a brewer on a professional level, we predict that this exercise is one that the students will remember for years.Most analytical chemistry texts dealing with spectroscopy discuss the BeerÀLambert law,where A is the absorbance at a particular wavelength, ε is the absorption coefficient, l is the path length, and c is the concentration for a single absorbing species. In a two-component system the absorptions of the individual components add, soTo find the concentration of both components, the absorbance of the solution at two wavelengths must be obtained, and a system of two equations and two unknowns set up.Initially, the mixture of α-and β-acids present in a hop sample appears to be a simple two-component system that can be analyzed by standard techniques presented in most texts on UVÀvisible spectroscopic analysis. However, more careful analysis reveals that this system is a complex system containing at least two analytes and an additional interfering species.Hops, the inflorescence (cone) of the Humulus lupulus plant, contain both α-acids (humulones) and β-acids (lupulones), the major chemical components of which are shown in Figure 1. The quantity of α-and β-acids varies with the type of hop used, 2,3 how the hop was processed, 4 how it was stored, and how long it has ABSTRACT: A method is presented for the determination of α-and β-acids (humulones and lupulones) in a hops sample using a multicomponent UV spectroscopic analysis of a methanolic hop extract. When compared with standard methods, this lab can be considered "greener" because it uses smaller...

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“…The method used was adapted from Alderton et al (1954) and Egts et al (2012). Specifically, in a 25 mL volumetric flask, 50 µL of the filtrate was added to a methanolic solution of NaOH (0.5 mL of 6M NaOH in 250 mL of methanol) and the complete spectrum (520 to 210 nm) was recorded against a solution of methanol in methanolic NaOH (50 µL:25 mL) as a blanc.…”

“…where A 355 , A 325 , and A 275 stand for the absorbance of the three analytical wavelengths and C α , C β , and C comp3 stand for the concentrations (in mg/L) of the α-acids, β-acids, and the third component, respectively (Egts et al, 2012).…”

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“…It is also possible to narrow the focus to the α-acids or the iso-α-acids responsible for the bitter flavor in beer. Experiments suitable for lower level chemistry courses use UV−vis spectroscopy to quantify iso-α-acids and calculate international bitterness units (IBUs) for bottled beer 19 or to quantify αand β-acids in hop extracts 20 while an HPLC method, more appropriate for quantitative analysis, can be used to build skills in quality assurance and method validation. 21 In contrast, the topic can also be broadened.…”