Hops, oil

Bahak gives the following tabic of eharaclers of a selected uumber ol aainples of hop oil —... 

More than 250 chemicals have been identified in hop oils and no doubt more minor products remain to be characterised. Annual world beer sales are currently about 300 billion (130 billion litres). 

Farnesene 69 is a constituent of hops oil and many other oils. a-Farnesene 70 is the structural isomer. Structural representations of a-farnesene and -farne-sene are illustrated in Structure 4.17. 

Humulene 109 (Structure 4.31) is isomeric with caryophyllene. First isolated from hops oil (Humulus lupulus), it is a common constituent of essential oils. 

The volatile hop oil is nearly completely extracted without danger of decomposition owing to the low extraction temperature. 

Hops Oil occurs as a light yellow to green-yellow liquid with a characteristic, aromatic odor. Age darkens the color, and the oil tends to become viscous. It is the volatile oil obtained by steam distillation of the freshly dried membranous cones of the female plants of Humulus lupulus L. or Humulus ameri-canus Nutt. (Fam. Moraceae). It is soluble in most fixed oils and, sometimes with opalescence, in mineral oil. It is practically insoluble in glycerin and in propylene glycol. 

Synthesis ofkarahanaenaue. The ionic reaction of NBS in CCI4 at room temperature with a y-ethylcnic tertiary alcohol leads to an -bromotetrahydrofurane. The reaction has been used in a convenient synthesis of karahanaenone (4), a constituent of hop oil, from linalool (1). Thus reaction of (1) with NBS affords 2-mcthyl-2-vinyl-5-(l-bromo-l-methylethyl)tetrahydrofurane (2) in 85% yield. Dehydrohalogena-tion of (2) with collidine at 110 leads to the allyl vinyl ether (3), which immediately... 

More than 20 years ago Sutherland et a/." first showed that when humulene (72) was treated with N-bromosuccinimide in aqueous acetone it underwent a remarkable transannulation, leading to the tricyclic bromohydrin (113) in 25% yield. Hydrolysis of (113a) then led to tricyclohumuladiol (113b), which was later found as a constituent of hop oil, and two synthetic steps converted (113b) into the cyclononadiene caryophyllene (63 equation 65). 

CAS 503-74-2. (CH3)2CHCH2COOH. Occurs in valerian, hop oil, tobacco, and other plants. 

Naya and Kotake, in an examination of Japanese hop oil, have isolated three humulane-type compounds, viz., humuladienone (161, R = Me), humulenone II (161,R = =CH2), and humulol (162), in addition to the tricyclic diol (163, R = OH), m.p. 207 °C. This diol has already been prepared in two different ways (a) Sutherland et treated humulene (164) with AT-bromosuccini-mide in aqueous acetone and converted the resultant bromohydrin (163, R = Br) to the diol (163, R = OH), m.p. 205—206 °C, by hydrolysis, (b) McKervey and Wright obtained the same diol, m.p. 201—203 °C, by acid-catalysed (20% sulphuric acid) rearrangement of humulene 1,2-epoxide (165), a known natural product. On the basis of these findings and the fact that both caryophyllene (166) and humulene can be derived from the above bromohydrin by two in vitro steps, McKervey and Wright postulated that humulene 1,2-epoxide may be involved in the biosynthesis of the tricyclic diol and caryophyllene. This postulate does not, however, readily accommodate the observed rotations of the relevant... 

Monoterpenes, on the other hand, possess particularly desirable flavor notes. Three of them, (3-myreene (A.45), limonene (A.46) and p-cymene (A.59) are common to coffee, cocoa and tea, and to numerous plant families (3-myrcene for example constitutes 63% of hop oils. It is worth remembering that they particularly contribute to the flavor of spices. In a study of the effluvia of fresh red coffee berries, Mathieu et al. (1996) identified, in two varieties of robusta, a series of mono- and sesquiterpenes, a blend also present in cold pressed oil from pommelos. In fact, the effluvia of red berries attracts the females of the coffee berry borer, a very important pest. Without giving a list of the hydrocarbons identified in this study, let us note that limonene is the most abundant of them, caryophyllene (4,11,11-trimethyl-8-methylenebicyclo[7.2.0]undec-4-ene), followed by humulene (2,6,6,9-tetramethylundeca-1,4,8-triene) and a-pinene (2,6,6-trimethylbicyclo[3.1.1]hept-2-ene), are rather important in two robusta... 

Naturally Occurring Thiophens. - A review on the formation of heterocyclic compounds, including thiophen derivatives, by enzymic and non-enzymic browning in relation to food flavour has been published.Some thiophens have been found as trace constituents in the essential oil from Thymus capitatus " From steam-distilled hop oils, 3-(4-methylpent-3-enyl)thiophen... 

Compound Table I. Cascade Beer Hop Oil Components Found in b c European Cluster Mixture Beer Beer Beer Hallertauer Beer Threshold in Beer... 

Figure 1. Structures of hop oil components. Key I, humulene II, humulene epoxide I III, humulene epoxide II IV, humulol V, humulenol II VI, humula-dienone VII, a-eudesmol VIII, -eudesmol IX, hop ether X, karahana ether XI, p-ionone and XII, j3-damascenone.

This long wort boiling time is Important in that most of the mass (80-90%) of typical hop oil is made up of terpene and sesquiterpene hydrocarbons which are either steam distilled out of the wort, polymerized or oxidized to more water soluble compounds during the process (, 5, 11, 12). As a result, these hydrocarbons are not found in beer (, 5, T) and therefore are not responsible for this flavor. Investigators in this field do agree that this... 

Humuladlenone has been suggested to be important to kettle hop aroma in beer ( ). Concentrations of from 34-72ppb humuladlenone in beer and a sensory threshold of lOOppb for the compound in beer have been reported ( 5). Small amounts of humuladlenone are formed by boiling humulene (a sesquiterpene typically comprising 25-45% of the aroma hop oils) for 90 minutes, and that the humuladlenone concentration of hops Increases with exposure to light and/or air ( ). It has also been reported that the compound gives beer a "hop like" flavor at lOOppb ( ). 

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),... 

These compounds may well have an effect on the hop aroma of beer but if they are responsible for the traditional "kettle hop" aroma, then the concentration of humulene in hops should not be so important. Further, from Table 111 the concentration of hop ether and karahana ether in hop oils does not correlate well with aroma quality. Tressl (17) has noted that the concentrations of these two ethers go up dramatically as hops age. 

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