Beer bitterness

M Verzele, D de Keukeleire. Chemistry and analysis of hop and beer bitter acids. London Elsevier, 1991. 

Calamus has been used for centuries to treat gastrointestinal distress, including colic in children, and as a sedative. A rhizome infusion is used to treat fevers and dyspepsia, and chewing the rhizome is recommended to clear the voice, relieve dyspepsia, aid digestion, and remove tobacco odor from the breath. The powdered rhizome is used as a cooking spice, as is calamus oil, which is responsible for the plant s odor and taste. In the U.S., calamus was once used to flavor tooth powders, beer, bitters, and various tonics, although it is no longer used today. 

Chromatography methods combined with NMR have been used for the analysis of hop acids and beer bitter acids.378 379 Supercritical fluid chromatography has been used in the analysis of ascorbigens in Brassicas, but in this case the NMR was not on line with the chromatography system.380... 

Beer flavor. Primary odor and taste substances from malt and hops determine the type of beer bitter substances (see humulone) and aroma substances of hops characterize Pilsener beer, a relatively high content of Furaneol provides the caramel note of dark beer. The most important aroma substances in light full beer are... 

Of the individual iso-a-acids, isohumulone is concentrated into the foam to a greater extent than isocohumulone and unhopped beer bittered (21 0 BU) with isohumulone had a better head retention (2 = 132) than that bittered with isocohumulone (2 = 115) [146]. Addition of iso-a-acids to beer increases head retention but, in particular, the iso-a-acids are responsible for foam adhesion, cling or lacing. Unhopped beer does not show these effects. Traces of heavy metal ions (iron, cobalt, nickel and copper) also improve the head retention of beers but only in the presence of iso-a-acids [139]. 

De Keukeleire, D., J. Vindevogel, R. Szucs, and P. Sandra, The history and analytical chemistry of beer bitter acids, Trends Anal. Chem., 11, 275-280 (1992). 

Much time and effort was devoted to the Index at the end of the book. It is expected that this index will serve as a reference library-dictionary to hops, hop and beer bitter acids chemistry and analysis. 

In the brewery, hop is boiled with wort at a pH vaiue around 5.5. In these conditions the hop alpha acids are poorly soluble (1), but in the process they are transformed into the iso-alpha acids, which are better soluble in the wort medium (2). Consequently, only traces of alpha acids remain in beer (3) (see 5.3.). The iso-alpha acids are the hop derivatives, which contribute mainly to the beer bitter taste. In this Chapter the chemistry of the isohumulones, which are the most important iso-alpha acids, is discussed. The chemistry of the other iso-alpha acids is practically identical to that of the isohumulones. As humulone is readily available, it is easier to study the isohumulones than the other iso-alpha acids. Therefore, the isohumulones are the best known iso-alpha acids. 

Investigation of the beer bitter components with counter-current distribution (CCD) revealed a mixture of mainly three analogues, corresponding to the hop alpha acids... 

The beta acids are readily oxidized and some oxidation products, such as the hulupones (see 13.1.1.), contribute partially to the beer bitterness. It is generally accepted that In old hops, which have the same bittering potential as fresh hops, the oxidation of the beta acids compensates partly for the decrease In alpha acids content (12, 27-35). The ratio of the alpha acids to the beta acids varies between 1 and 4. This ratio increases with hop growth, but remains fairly constant for ripe hops. The index depends very much on the hop variety, but the significance has been debated. It is however a fact that so-called high-quality hops contain about equal quantities of alpha acids and beta acids. 

If ever research on hop and beer bitter acids is taken up again seriously and systematically, we believe that extraction of large beer volumes and modern LC techniques for separation and identification of the compounds (in fact, such as the Japanese authors have been doing, but with the techniques available at that time), is a promising way to go (see also Chapter 18). 

The evolution of hop chemistry over the last 100 years has paralleled the development of modern chemistry. New chemical tools or insights were rapidly and successfully applied. As an example, NMR spectrometry allowed the elucidation of the structural aspects of hops chemistry in the years 1960-1970. Today, modern liquid chromatography has an increasing impact on the field. Analysis of hops and beer bitter acids has always been associated with the evolution of separation techniques. Therefore, recent developments in high resolution chromatographic techniques is of great value. It is not our intention to provide an exhaustive overview of all known methods for the analysis of hops and beer bitter acids rather the current status and future outlook will be emphasized. 

The general composition of hops has been mentioned before. A more detailed survey was given by Pfenniger et al. in Brewing Science (1). The chemistry and compounds involved in the following overview have been discussed thoroughly in preceding chapters. The presentation of analytical methods concerns only hops and beer bitter compounds, in particular the alpha acids and the iso-alpha acids. Alpha and iso-alpha acids are found in a number of products and preparations. 

For commercial purposes it is useful to standardize the alpha acids content of extracts to 15, 30 or 60 %. Glucose syrup has been used as diluent, but phase demixing, when occurring, may be masked by the dark colour of the extract paste. This has been the cause of great difficulties. Water extracts of the hop extract residue (phenolics, salts, coloured matter, etc.) are also used for standardization purposes. Extracts of various content can also be mixed in the correct proportions to achieve the required result. It is recommended to check the real alpha acids value of an extract, because an error of only 10-20 % can have disastrous effects on the level of beer bitterness. 

Another recent contribution concerns the Micro-LC analysis of hop and beer bitter acids (36) carried out on packed fused silica columns of the same internal diameter as used in GC. This form of chromatography has a number of advantages, such as the absence of metal in frits and even columns. The procedure will be discussed further in more detail. Our latest contribution focuses on solving the trace metal problem (37). This was achieved successfully, leading to alpha acids and iso-alpha acids analysis methods which are probably for the first time correct and reproducible. These methods are fully developed in the appropriate sections in the next chapters. 

Commercial hop extracts are analytically evaluated on the basis of the major alpha acids content as revealed by LC. This obviously excludes the iso-alpha acids, the minor alpha acids and the deoxy-alpha acids which are all present in an ethanol hop extract and which can all contribute to some extent to the beer bittering capacity of the extract. The total amount of these compounds, expressed as peak area in Fig.115 of the corresponding peaks 6,7,8,9,10,11,14,17,18,19, is 7.3 %. With specific absorptivities (absorbances for 1% solutions) at 270 nm of 295 for the iso-alpha acids, 200 for the alpha acids and 300 for the deoxy-alpha acids the total extract content for these peaks is 5.6 %. 

Data showing the difference in metal sensitivity for different hop and beer bitter acids are presented in Table 18. The increasing metal sensitivity in the series deoxyhumulone, colupulone, humulone, trans isohumulone is revealed. These results agree with the observation that RoSil-C18 for "Hop acids" and Nucleosil-C18 for "Hop acids" are satisfactory for LC analysis of alpha acids, but not for iso-alpha acids. Other commercial columns, newer or older columns may of course give figures more or less different from those shown in Table 18. Calibration and the use of alpha acids of known composition as ES may get around the trace metal problem, but clearly this is not the way to go. Trace metal activity of columns will vary by production variation, with column age and with duration of the chromatographic run. Special columns must be used and EDTA must be added to the eluent as shown in this section. 

Table 18. Relative peak area (against 100 % for RoSil-Hop) for some hop and beer bitter acids on various reversed phase LC columns, with and without H3PO4 in the eluent (acetonitrile water 70 30).

The described procedure uses conventionally sized columns with an i.d. of 4.6 mm. A miniaturized procedure using capillary columns may soon become more popular. Such a Micro-LC procedure for the analysis of hop and beer bitter acids was described by us in 1987 (15). 

The problem of the trace metal sensitivity of hop and beer bitter acids has been discussed extensively before in Chapter 16. A clear separation in this discussion between alpha and iso-alpha acids is not possible. Therefore it seems appropriate to repeat some of the material already mentioned. ... 

This does not mean however that all problems in hop chemistry and hop utilization in the brewery have been solved. Far from it Even the simple and still fundamental question What is a good-quality hop " asked of us by a local brewer in Gent (Andre Van der Stricht) who initiated our research in 1945, has not yet received a definitive answer. Many more issues about hop and beer bitter principles could be tackled by fundamental research, some of which may be considered to be important enough to warrant the effort. 

One of the major problems for the brewery technologists is the origin and the control of the so-called harsh bitterness in some beers. Bitterness is not evaluated in the same way on all surfaces in the mouth. Lips, tongue, cheek interior and back of the mouth can distinguish subtle differences. A harsh clinging bitterness in the back of the mouth, practically in the throat is evaluated very negatively, but its origin is still unknown. Clearly, hop-derived compounds are responsible. The search for these compounds, their quantification, identity and control should be a major objective for future research. 

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