Odorants in beer

The evaluation of only one GC run has an important drawback. Since it is not possible to exactly evaluate the intensity of the odor during sniffing, the results cannot be used to decide whether a compound is a key odorant in beer flavor or contributes little to the overall odor. Furthermore, the number of the compounds detected depends on accidental factors, e.g., the amount of die food used for the isolation of the volatile fraction or the degree of the concentration. 

Volatile aldehydes, and (E)-2-nonenal in particular, had already been identified as the cause of "rancid odors" in beer (7,8). These substances result firom the oxidation of unsaturated fatty acids. The direct precursor of (E)-2-nonenal and others carbonyl components is linoleic acid (Ci8 2 A 9,12) (9). Volatile aldehydes may be derived firom fatty acids in various manners. Chemical auto-oxidative factors would seem to provide the most likely explanation for the presence of these components in oak stave wood after seasoning in the open air. On the other hand, enzymatic factors may explain the presence of ese components while the tree is still standing or immediately after it has been cut. Additional research is necessary to pinpoint the exact formation and accumulation mechanisms of these molecules in the wood. 

D Komarek. Key odorants in beer—influence of storage on the flavor stability. PhD dissertation, Technical University of Munich, Garching, Germany, 2001. 

Figure 3. Flavor dilution (FD) chromatogram of the neutral/basic odorants in a pale lager beer. The numbering follows Figure 2. RI retention index on a silicone SE-54 GC stationary phase [adapted from ref. 18],...

Figure 4. Key odorants in pale lager beer. The numbering follows Figs. 2 and 3. Odorants with roman numbers were identified in the acidic fraction (FD-factor).

Comparison of odor activity values of important odorants in pale and dark beer [18]... 

Comparison of odor thresholds of important beer odorants in water and alcohol-free beer [P Schieberle, unpublished results) ... 

Sensory thresholds (in beer) and odor descriptors. a 5-Hydroxymethyl-2-furfuraI. h 5-Ethoxymethyl 2-furfural c Minimum. d Maximum. e Mean value. 

The odor is described as sweet-spicy, warm, and slightly caramellic (Arctander, 1967). The flavor threshold in beer is 20 ppm (Meilgaard, 1975). 

Fritsch, H., Schieberle, P. (2005). Identification based on quantitative measurements and aroma recombination of the character impact odorants in a Bavarian PUsner-type beer. Journal of Agricultural and Food Chemistry, 53, 7544—7551. http //dx.doi.org/10.1021/jf051167k. 

Produces acid, acetic and lactic, esters (ethyl acetate fruity odor), and cloudiness in beer. 

Callemien D, Dasnoy S, Collin S (2006) Identification of a stale-beer-like odorant in extracts of naturally aged beer. J Agric Food Chem 54 1409-1413... 

Buettner, A., A. Beer, C. Hannig, M. Settles, P. Schieberie, Quantitation of the in-mouth release of heteroatomic odorants, in Heteroatomic Aroma Compounds, T.A. Reineccius, G.A. Reineccius, Eds., Amer. Chem. Soc., Washington, D.C., 2002, p. 296. 

Some reaction systems, which have been described in the patent literature for the production of meat aromas, regard thiamine as precursor. 3-Methyl-2-butene-l-thiol is one of the roast odorants of coffee (cf. 21.1.3.3.7) and can cause on off-flavor in beer (cf. Table 5.5). In general, only very small amounts are formed which are still aroma active on account of the very low odor threshold (Table 5.21). The formation of the thiol is explained by the fact that the 3-methyl-2-butene radical is formed from terpenes by photolysis (beer) or under the drastic conditions of the roasting process (coffee). This radical then meets a SH -radical formed from cysteine under these conditions. In the case of beer, humulons (cf. 20.1.2.3.2) are under discussion as the source of the alkyl radical. In coffee 3-methyl-2-butene-l-ol (prenyl alcohol) is also a possible precursor, which yields the thiol after water elimination and hydrogen sulfide addition. 

Tertiary thiols (Table 5.35) are some of the most intensive aroma substances. They have a fruity odor at the very low concentrations in which they occur in foods. With increasing concentration, they smell of cat urine and are called catty odorants. Tertiary thiols have been detected in some fruits, olive oil, wine (Scheurebe) and roasted coffee (Table 5.35). They make important contributions to the aroma and are possibly formed by the addition of hydrogen sulfide to metabolites of isoprene metabolism. In beer. 

Table 20.7. Odorants in lager beer and alcohol-free beer...

Secondary and tertiary amines are formed from precursors other than amino acids. Dimethylamine results from degradation of choline (which is present in some phospholipids), some alkaloids (e.g. in beer it is produced from gramine (see 10-198) present in germinating barley grains and also in non-enzymatic browning reactions from methylamine and formaldehyde or by decarboxylation of sarcosine. Trimethylamine, together with dimethylamine, methylamine and ammonia, is an odorous compound of fish and other aquatic animals. It is formed by reduction of the sensory indifferent trimethylamine oxide (trimethylaminoxide, 8-143) in tissues post mortem. 

Sensory experiments have shown that the odor of this compound was similar after appropriate dilution, but not identical, when evaluated in comparison to the sunstruck flavor in beer by a trained sensory panel [5]. It was, therefore, assumed that, besides MBT, further odorants contributed to the flavor difference observed in beer during exposure to light [6]. However, this assumption has not yet been confirmed by sensory studies and/or quantitative data. Since its detection by means of headspace gas chromatography was not successful [5], numerous methods have been proposed for MBT enrichment and quantation [7-10], and concentrations... 

In summary, the results indicate that 3-methyl-2-butene-l-thiol may be a key odorant in certain types of beer that contributes to their typical aroma. The data suggest that the tolerable amount for a positive aroma contribution is about 0.2pg/L, but this has to be proved by further sensory experiments. 

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