Technical Session 04: Hops II Session
Leif A Garbe, TU Berlin / VLB Berlin
Co-author(s): Nils Rettberg, TU Berlin/VLB Berlin, Germany
ABSTRACT: Hop essential oils and their oxidation products
are of special interest for brewing science and quality control as well
as for practical brewers. They are volatile, chemical reactive, and
usually are present in very low concentrations in the final product. In
hops, they are embedded in complex matrices that hinder rapid analysis
and make trace analysis a challenging discipline. Various sample cleanup
and target isolation strategies have been established. The more intense
the sample cleanup procedures are, the more sources of analyte losses
and errors have to be considered in quantification and calibration. In
trace analysis, external calibration is not reasonable, and the quality
of internal standard assays strongly depends on the availability and
properties of the internal standard. Stable isotope dilution assay
(SIDA) is a special type of an internal standard assay. In SIDA standard
and analyte are isotopologues, thus they are as nearly identical as
possible. Their chemical and physical properties match, and their
chromatographic and mass spectrometric characteristics are very alike.
This paper highlights the advantages and challenges of SIDA in trace
analysis on hop oils. The important hop oil terpene hydrocarbons and
terpene alcohols myrcene, linalool, terpineol, nerol, geraniol, and
farnesol, as well as caryophyllene and humulene, are not commercially
available as stable isotope labeled standards. Therefore, their chemical
synthesis as isotopologues carrying Deuterium (hydrogen-2), oxygen-18,
or carbon-13 is of crucial necessity for SIDA. We have performed lab
synthesis of these terpene compounds. Using SIDA, precise and valid
quantification even at low concentrations from the complex beer matrix,
raw hops, and any intermediate is strongly simplified. In the presented
paper, a short introduction to SIDA and SIM-MS methods are given. The
major focus of the paper deals with data from hop oil analysis via
conventional and SIDA methods, respectively. One disadvantage of SIDA is
the necessary instrumentation—chromatography coupled to mass
spectrometry. However, SIDA is also proposed as a reference method in
evaluation of routine assays preformed on cost-effective non-MS
equipment like GC-FID analysis.
Leif-Alexander Garbe is
professor for biochemical and technical analysis at the Berlin Institute
of Technology (TUB). Additionally, he chairs the Department for Special
Analyses at the Research and Teaching Institute for Brewing in Berlin
(VLB). Leif graduated in 1996 from TUB with a diploma in chemistry. Then
he worked as a researcher and teacher at VLB and TUB. He supervised
biotechnology and brewing students and performed several research
projects in brewing and life sciences. He finished his Ph.D. thesis in
April 2002 on the “Metabolism of Hydroxy-Fatty Acids in Yeasts,” and his
habilitation thesis in 2009 on “The Biochemistry of Oxidized Lipids:
Analytical Characterization of Bioactive Metabolites” at TUB. Today
Leif’s research interests focus on mass spectrometry, NMR, trace
analysis, biotransformation, isotope dilution technique, and Maillard
reaction of peptides/proteins.
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