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Publication Title | Chemistry of Hop Aroma in Beer

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Chemistry of Hop Aroma in Beer’

Val E. Peacock’ and Max L. Deinzer, Department of Agricultural Chemistry, Oregon State University, Corvallis 97331

ABSTRACT

Three beers were analyzed by gas chromatography/mass spectrometry for hop-derived flavor components. Hop ether, karahana ether, linalool, g e r a n i o l , h u m u l o l , h u m u l a d i e n o n e , h u m u l e n o l 11, a n d h u m u l e n e e p o x i d e s I, 11,andIIIareamongthecompoundsidentified inbeerthatarebelievedto influence beer flavor. These humulene oxidation products probably contribute to the traditional “kettle-hop” flavor/aroma of beer, but geraniol and linalool contribute to a floral flavor note that is distinctly different from the kettle-hop aroma/taste. The humulene oxidation products, the main one of which is humulene epoxide 11, increased in concentration with hop storage.

Key words: Aroma. Beer, Geraniol, Hops, Humulene. Linalool, Taste

The major components of hop oil, terpene and sesquiterpene hydrocarbons, are rarely found in beer (9,13) and are not considered responsible for hoppy flavors in beer. However, the oxidation products of these hydrocarbons, which make up 5-10% of typical hop oil, ure commonly found in beer and are therefore much more likely to be responsible for hoppy flavors.

Oxidation products of humulene, one of the major sesquiterpene hydrocarbons, have been the focus of much attention in recent years as a possible source of the traditional “noble-hop”or “kettle- hop”aroma of beers brewed with “aroma hops.”These compounds are in much higher concentration in the humulene-rich traditional “aroma” varieties than in other hops (6). Also, these compounds are formed by oxidation of humulene during hop storage (14), which may be related to the fact that aging of hops before use improves hop aroma properties.

The earliest mention of humulene oxidation products in beer was by Shimazu et al (12) in 1974. They found humuladienone in a variety of beers at levels of 34-72 pg/L and reported a sensory threshold of 100pg/ L for the compound. Sandra and Verzele (1 1) reported looking for humuladienone in beer and finding none. They claimed to have a sensitivity limit of 1 pg/ L. Tressl et al(l3) reported a humuladienone level of 10 p g / L in beer.

Both Tressl et a1 (13) and Peacock et a1 (9) reported relatively large amounts of other humulene oxidation products as well as many other hop-derived compounds in beer. Tressl et a1 (13) reported humulenol I1 at a concentration of 1,150 pg/L, and Peacocketal,whofounditat250-500 pg/L,speculatedthatitisa hop flavor contributor with a sensory threshold of 500 pg/L (9). Tressl et a1(13) reported large amounts of humulol(220 pg/ L) and humulene epoxide I ( 125 pg/ L) and a lesser amount (40 pg/ L) of humulene epoxide I1 in a “hoppy” German beer. Peacock et a1 (9) found lesser amounts of humulol and humulene epoxide 1 in various American beers and noted that the beers brewed with the traditional “aroma” varieties contained much more of these compounds (especially humulenol 11) than the beers brewed with other hops.

Tress1et a13put more emphasis on the flavor of the hop-derived carotenoids P-damascenone and p-ionone, which they found in beer at 30 and 3 pg/ L, respectively. They reported their sensory thresholds in water to be 0.009 and 0.007 pg/L, respectively.

Meilgaard (3) reported the threshold of p-ionone to be 1.3 pg/ L in beer, so these carotenoids may well have an effect on beer flavor.

‘Presented at the 47th Annual Meeting, Miami, FL, May 1981.

*Present address: Philip Morris Research Center, Beverage Products Research and

Annual Meeting, Portland, OR, April 1979.

0361-0470/81/04013606/$03.00/0

a1981 American Society of Brewing Chemists, Inc.

Tressl et a1 also speculated that the bicyclic terpenoids hop ether and karahana ether may play a part in beer hop flavor. They reported 35and 60pg/ L, respectively, of these compoundsin beer3 and 5 pg/ L thresholds for both compounds in water.

Linalool has been found in beer by Micketts and Lindsay (5), Tressl et a1 (13), and Peacock et a1 (9). All three groups have speculated that it may be a flavor contributor to beer. Peacock et a1 (8) found large amounts of geraniol and geranyl isobutyrate in some beers and claimed that these compounds, with linalool, are responsible for a floral flavor note in these beers.

EXPERIMENTAL

Beer Analyses

The three beers were a commercial Austrian beer, a premium American beer, and a pilot brew prepared by a commercial American brewer.

The following basic work-up procedure was used for all three beers. Two liters of beer was mixed with 2 kg of Celite 545 until the entire mass had a powdery consistency. The beer-saturated Celite was placed in a large (13 X 40-cm) liquid chromatography (LC) column. (Only a third of the Celite mixture could fit in the column at one time.) Each of the thirds of Celite was eluted with 2 L of freshly distilled methylene chloride, using a total of 6 L. The eluent was dried over anhydroussodium sulfate and reduced in volume at reduced pressure to a mass of about 0.5 g. This residue contained a large percent of free fatty acids and phenolic compounds (maltol and isomaltol) that interfered with the gas chromatography (GC) analysis. The following procedures were used to clean up the sample and quantify the volatiles of interest.

Beer 1. The residue was dissolved in I50ml of ether and extracted twice with 150 ml of 10% Na2CO3. The ether was then dried over anhydrous MgS04 and reduced in volume at reduced pressure to a mass of about 1 g. This sample was further concentrated under a stream of N2 until it no longer smelled of ether (about 10min). The remaining material was weighed (0.158 8). One microliter of this sample was injected into the gas chromatograph and all the resulting peaks were quantified. Assuming that 100%of the sample was detected by the instrument, each compound was quantified by multiplying the mass of the entire sample (0.158 g) by the area of its peak in the chromatogram and dividing by the sum of the peak areas of the entire sample. Some nonvolatile residue remained in the injection port of the gas chromatograph, which would cause some error in these calculations.

Beer2.Thissamplewasanalyzedidenticallytobeer 1exceptthat it was extracted with 10% NaOH instead of with Na2C03. The sample residue had a mass of 0.090 g.

Beer3. Before beer 3 was analyzed, 1.O ml of 0.1%naphthalene in 100%ethanol was pipetted into the 2 L of beer, resulting in a 500 pg/L concentration of the compound as a standard. The Celite procedure was followed as usual. The resulting residue was cleaned up by the following procedure. An LC column (2.5 cm i.d.) was filled with 25 cm of Fischer adsorption alumina, then 2.5 cm of Na2CO3, and then 2.5 cm of anhydrous Na2S04. The column was wetted with hexane and the sample poured onto the top of the column. The column was eluted with 100 ml of hexane and then 200 ml of ether. The combined hexane and ether fractions were reduced in volume at reduced pressure to 0.1-0.2 ml and analyzed by GC and GC/mass spectrometry (MS) as such. The compounds reported were quantified by direct comparison with the naphthalene standard. This avoided the previous problem of the

Development, P.O. Box 26583, Richmond, VA 23261.

’R. Tressl, F. Fendesack, and H. Koppler. Paper presented at ASBC 45th

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