Hop varieties

The feasibility and limitations of using multisensor array systems in food and aroma applications will be discussed with an application intended to discriminate hop varieties [149] byway of example... 

The sensor responses generated in a measurement result from physical and/ or chemical interactions between the sensors and the volatile compounds present in the headspace above the measured sample. By using a QMB sensor system with an array of six sensors, good discrimination between three hop varieties can be observed (Fig. 15.10a). In this example only 12 measurements per sample were analysed. The distance between clusters is reduced if the data set is increased to 50 measurements per sample (Fig. 15.10b). 

Fig. 15.10 Discrimination of hop varieties with six quartz crystal microbalance (QMB) sensors with 12 (a) and 50 (b) measurements per sample. N Nugget, S Select, M Magnum, P Perle, T Tradition, B Northern Brewer...

Fig. 15.11 Discrimination of six hop varieties (see Fig. 15.10) by means of metal oxide sensor (MOS) and QMB sensor arrays...

In order to correlate a discrimination to the different varieties, a sensor system that selectively interacts with variety-specific compounds in the headspace is needed. The GC analysis of the essential oil reveals that there are some minor volatile compounds, which can be used for a differentiation of different hop varieties (e.g. Nugget and Tettnanger), as shown in Fig. 15.12. 

By choosing appropriate fragment ions a virtual sensor array based on a mass spectrometer in single ion monitoring mode can be implemented to discriminate the hop varieties (Fig. 15.13) without appreciable interference of the main component myrcene. In contrast to the analyses performed with QMB and MOS sensors, the discrimination obtained with the MS sensor is based on chemical knowledge and not on assumptions. 

Fig. 15.12 Expanded region of a GC chromatogram of the hop varieties Nugget and Tettnanger...

Fig. 15.13 Discrimination of hop varieties with a mass spectrometry based sensor array...

The amount of bitterness that a hop will add to a beer is a function of its alpha acid content, which is often listed as a percentage on the package label. Different homebrewers use different methods of gauging the expected bitterness of the beer. The method that we use is the alpha acid unit (AAU), which is described in Charlie Papazian s book, The Complete Joy of Homebrewing. One AAU is equal to one percent alpha acid per ounce. On this scale, an ounce of hops with 5% alpha content is said to contribute 5 AAUs of bitterness. A recipe that lists hop additions in AAUs is better than one that lists hop additions in ounces because it makes it easier for the brewer to substitute hop varieties. For example, if a recipe calls for 7 AAUs of a hop such as Clusters, the brewer can easily add a bit more of a lower alpha content hop or a bit less of a higher alpha content hop. 

The main purpose of using hops in beer is to add bitterness to the final product. A series of compounds referred to as Ci and B-acids are responsible for this taste (1 ). A secondary, ill defined flavor (flavor referring to smell and taste) is also imparted to beer by brewing with "aroma hops." Not all hop varieties are considered "aroma hops," and there is evidence that the flavors Imparted to beer by different aroma hops are different ( ). There has been considerable controversy in recent years as to the nature and source of this flavor. Researchers have credited ter-pene alcohols (2, ), humulene oxidation products (, ), multi-cyclic terpenoid ethers (6) and carotenoids (.6) as being in part responsible for this flavor. 

The most easily definable hop contribution to beer aroma is a floral flavor note that certain hop varieties (not necessarily the traditional "aroma hop" varieties) impart to beer ( ). Indications are (Table I) that the floral compounds linalool and ger-anlol are responsible for this aroma note. Geranyl isobutyrate, though present in the more floral beers, is probably in too low concentration to have a major effect on beer flavor. a-Terplneol is eliminated from consideration for the same reason. Linalool has been reported in beer at an estimated concentration of 34ppb ( 3) by Lindsay and at a concentration of 470ppb ( 7) by Tressl. 

It has been noted recently (16) that hop aroma quality correlates well with the humulene/caryophyllene (H/C) ratio of the hop oil. This may just be an indirect way of saying high humulene concentration is Important, but the results are of Interest. Some hop varieties with an analysis of their more important constituents along with their (H/C) ratios are presented in Table III. Hallertauer, one of the most valuable aroma hops, has a high (H/C) ratio while Cluster (not considered an aroma hop) has a much lower (H/C) ratio. The other varieties on the list are considered to have aroma properties somewhere between Hallertauer and Cluster. Some other hops with low aroma properties are Galena (H/C = 2.2), Brewer s Gold (1.6) and Talisman (1.3),... 

It will be seen from the foregoing that farmers have to use a wide range of powerful agents (Fig. 12.11) to produce high yields of good quality, undamaged hops. Brewers are worried that residues from these chemicals may persist on the hops at harvest and so find their way into the beer. (The detection of such residues is a difficult analytical problem.) Hop varieties selected for resistance to disease, which require minimal chemical treatment, would be preferred but the breeding of a variety resistant to all the pests and diseases which can attack hops is unlikely. 

Schieberle, P. and M. Steinhaus, Characterization of the odor-active constituents in fresh and processed hops (variety Spalter Select), in Gas Chromatography-Olfactometry The State of the Art, J.V. Leland, P. Schieberle, A. Buettner, T.E. Acree, Eds., Amer. Chem. Soc., Washington, D.C., p. 23, 2001. 

Like any natural product, hops are variable in quality and even hops of the same variety will differ in brewing value from farm to farm and from one year to another. Extraction offers an opportunity to smooth out these variations and to present a product with standardised brewing value. Apart from standardised bitter content, an extract may contain a specified blend of hop varieties or a standard volatile oil content which assists in the achievement of consistent hop flavour and aroma. 

Since the organoleptic differences between hop varieties are almost entirely due to the composition of the volatile oil, an ethanol extract is virtually devoid of varietal character and can really only be considered as a source of bitterness. By contrast, a (liquid) carbon dioxide extract retains most of the aroma and flavour characteristics of the hops from which it derives. It can thus become the starting point for the preparation of a range of products for controlling the bitterness, aroma and flavour of beers. 

The most obvious difference between a liquid CO2 hop extract and one produced under supercritical conditions - colour - is probably the least significant. Depending upon the conditions chosen, the supercritical extract ranges in colour from pale to dark green, the darker colour corresponding to higher temperature and pressure. In the case of a liquid CO2 hop extract, the shade of yellow will vary from one hop variety to another. Both types of extract are significantly darkened in colour if iron contamination is present. 

The basic humulone homologues (8-265) are cohumulone and adhumulone and minority components are prehumulone and posthumulone. Analogously, the main lupulone homologues (8-266) are colupulone, adlupulone, prelupulone and postlupu-lone. These acids are bitter only by name, but in fact they have an indifferent taste and are almost insoluble in water and have pronounced bacteriostatic activity. The approximate composition of these two groups of bitter acids in different hop varieties is shown in Table 8.43. Their content is about 25% of the dry weight of the hop cones. 

Prenylated isoflavones are related to prenylated flavonoids (chal-cones and flavanones or dihydroflavones) located in mature female cones of hops Humulus lupulus, Cannabinaceae), which exhibit oestrogenic and anticarcinogenic effects. Their amount depends on the weather conditions and the hops variety. The most common compound is a chalcone xanthohumol (10-98), which does not exhibit oestrogenic activity. Its content in hop cones is around 1%. Xanthohumol is accompanied by the flavanone isoxanthohumol, which arises from xanthohumol by conversion in an acidic media during heating. Isoxanthohumol is therefore the main prenylated flavonoid in beer. At concentrations of about 10 to 100 times lower. 

In the Northern hemisphere the earliest varieties are picked at the end of August or the beginning of September. About half the weight of the above-ground visible plant consists of cones. Some hop varieties are picked as late as October. In November the vine withers away while the underground root system continues to take up nutrients and water. In the Kashmir valley of India two crops per year can be raised, but the total yield Is not greater than elsewhere (9). 

There exist many hop varieties and the search for newer types with improved characteristics still goes on. Some well known varieties are Northern Brewer, Hallertau, Saaz, Brewers Gold, Bullion and Jakima. Newer varieties are especially studied at Wye College, Ashford, Kent in England (10). 

Any extraction procedure will eliminate a large amount of material which is otherwise boiled with the wort. The alpha acids content of extracts is higher than that of hops. Thus, extraction eradicates many of the differences between hop varieties. This means that brewers, who would not use certain hop varieties as such (estimating that they are of insufficient quality), will use extracts produced from these hop varieties (estimating that the quality of extracts is mainly determined by their alpha acids content and that the origin of the extracted hop is not so important). The practice of years in many breweries shows that this policy is sound. 

Humulone, cohumulone and adhumulone are the main constituents of the hop alpha acids. The percentage of adhumulone in the mixture is fairiy constant, from 10 to 15%. The amounts of humulone and cohumulone are widely variable (20-70%) depending on the hop variety. 

Rigby s view (56) that a high cohumulone content would be a negative quality factor has already been mentioned (Chapter 1). Nowadays, however, hop varieties are available with any desired characteristics (57). 

The evolution of the hop bitter acids during ripening of the hops has been studied in detail (63). The formation of the hop bitter acids starts already in the very early stages of the growth of the hop cone. It appears that the various hop alpha acids are produced at different rates. The percentage of cohumulone in unripe hops is much lower than normal (64), but near the end of the ripening period the ratios become more or less constant for a given hop variety. 

Most of the hop varieties grown on the European continent contain approximately equal amounts of two homologous beta acids, namely lupulone (see 11.1.3.) and colupulone (see 11.1.4.). In most cases lupulone is present in slightly higher percentage. British, American and Australian hops contain in general more colupulone. In fact, there is no main individual beta acid as is the case for the alpha acids. 

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