1. Extraction and isomerization rates of α-acids for bittering beer
Cindy Spangler and Dr. Paul Hooker
Department of Chemistry, Westminster College, Salt Lake City, Utah 84105
Introduction
Method
Results
Results continued
For over 400 years hops have been added to beer in order to impart a distinctive bitterness that complements the sweetness of the malt, to act as a preservative, and to provide a flowery, herbal, or citrus aroma. The final bitterness of a brew, often the defining feature of a particular beer style, is determined by the quantity of α- acids extracted during a protracted boil of the wort. Alpha acids are organic compounds found in relatively high concentrations in the glands of the hop cones, humulus lupulus. The bittering capacity of a particular hop is usually quoted as an alpha acid percentage, whereas the bitterness of the beer is quoted as the iso-alpha acid content, as the alpha acids undergo an isomerization reaction during the boil.
In this study, the rate of extraction of alpha acids and their subsequent isomerization has been investigated. Our goal is to determine what other factors influence this crucial process, such as wort gravity, and initial alpha acid content of the hops.
Columbian pellet hops (14.1% alpha acids) were purchased from a local brewing shop. Ten separate beakers were filled with equal amounts of water and equal quantities of hops were added to the boiling water. In intervals of 5 to 10 minutes, samples were collected. Figure 1 shows the process of analysis conducted.
HPLC Chromatographs for Iso-alpha Acids
Hops boiled in water and collected at 5 to 10 minute time intervals for an hour
Tube shaken for 30 minutes
0.1 M HCl added and shaken for 20 minutes
Centrifuge tube for three minutes
Pipet ether level to plastic vial, diluted 10x
Sample filtered for HPLC analysis
Hops filtered from wort for alpha acid analysis
Dried hops and transferred to centrifuge tube
Methanol and diethylether added
Wort collected, iso-alpha acid analysis
Adjust pH to 2.5 using phosphoric acid
Prep SPE C8 column and filter 20 mL wort
Collect final aliquot and measured mass
Filter sample for HPLC analysis
5 minute boil
30 minute boil
Graphs of isomerization rates (top) and alpha acid extraction (bottom)
Conclusion
The concentration of alpha acids and iso-alpha acids does show a negative correlation through the boil, i.e., as the alpha acids are extracted from the hops, the concentration of iso-alpha acids increases in solution; however the results do not demonstrate the consistency needed in order to investigate other factors affecting the extraction and isomerization rates of alpha acids. Therefore, we intend to simplify the experiment by using an iso-alpha standard to find the percent recovery after extraction and to investigate whether iso-alpha acids degrade in solution during the boil. After our analytical procedure has been refined and verified, we will analyze the affects of gravity and hops alpha acid content.
50 minute boil
HPLC Chromatographs for Alpha Acids
Procedures used to extract alpha acids from hops and iso-alpha acids from wort.
Columbian hops
International calibration standards for HPLC analysis of isomerized α-acids were obtained from the American Society of Brewing Chemists (ASBC). Alpha acid calibration standards were obtained from the European Brewing Convention. All standards were prepared using the procedures provided by the manufacturer.
Samples were filtered and ran on a Buck Scientific BLC-20 isocratic liquid chromatographic machine with a 20 µL injection loop using an EC 250/4 Nucleosil C18 Hop column. Absorbance was measured at 254 nm for alpha acids and 325 nm for iso-alpha acids.
Acknowledgements
De Keukeleire, Denis. “Fundamentals of beer and hop chemistry.” Quimica
Nova, 23(1) (2000):
Keukeleire, J., Ooms, G., and Heyerick, A. “Formation and
Accumulation of alpha-acids, beta-acids, desmethylxanthohumol, and Xanthohumol during Flowering of Hops.” J. Agri. Food Chem Jun 18;51,
Malowicki, M. and Shellhammer, T. “Isomerization and
degradation kinetics of hop (Humulus lupulus) acids in a model wort-boiling system.” J. Agric. Food Chem Jun 1;53(11):
Moir, Michael. “Hops-A Millennium Review.” J. Am. Soc. Brew. Chem.I
58(4): , 2000
20 minute boil
Clockwise: Leaf hops, pellet hops, and plugs Fresh hop cones on plant
50 minute boil