Higher alcohols
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The formation of higher homologues of ethanol is linked with nitrogen, rather then sugar metabolism. Ester formation is therefore also associated with nitrogen metabolism, since esters of higher alcohols are important contributors to the flavor and aroma of beer. For instance, high levels of assimilable nitrogen in wort, rapid fermentation and quick removal of yeast from beer are important. [5] Most of the higher alcohols are chemically very similar to the amino acids and they are formed either from the amino acids in the wort or during the reactions in which the yeast synthesizes amino acids. Use of large amounts of sugar or low-N adjunct tends to decrease higher alcohol and ester formation. [2] The amount of usable (assimilable) N affects the extent of formation of higher alcohols and their esters.[1]
Amyl alcohols (3-methyl-butanol and 2-methyl-butanol) were abundant and it is reported to be the most quantitatively significant flavour compound, responsible for fruity notes as well as ‘alcoholic’ flavour and aroma. The other aliphatic alcohols are responsible for ‘‘solvent’’ aroma of beer and produce a warm mouthfeel. 2-Phenylethanol is also important in some beers, imparting a rose or floral aroma. The higher alcohols are esters precursors, contributing to acetate esters formation and hence their amount influence the beer flavour.[2]
Melanoidin-Catalyzed Oxidation of Higher Alcohols: Next to ethanol, beer can contain significant amounts of higher alcohols (e.g., 2-methylpropanol, 2-methylbutanol, 3-methylbutanol, 2-phenylethanol). Oxidation of these compounds to their corresponding aldehydes can take place, although not directly by oxygen, but rather by the electron-accepting ability of melanoidins (high molecular weight polymers formed by Maillard reactions). The hydrogen atom of the hydroxyl group of the higher alcohol is transferred to a carbonyl group of the melanoidins. Oxygen facilitates this reaction, as does a lower pH.146 Supplementation of higher alcohols to beer results in higher amounts of the corresponding aldehydes.40 However, their oxidation takes place only in the case of light irradiation, proceeds less readily with increasing molecular weight of the alcohol, and is inhibited by the presence of iso-α-acids and polyphenols.71 Therefore, this pathway is believed to be of lesser importance. As a footnote, melanoidins may also have positive side effects with regard to aldehyde formation, because they appear to inhibit the oxidation of fatty acids and the degradation of bitter acids.146[3]
Higher alcohols are characterized by fusel-like odors, and are generally thought to contribute to the complexity of wine fermentation bouquet. However, when present in very high concentrations they can have a negative impact on wine aroma, mainly because they mask fruity characters.[4]
Elevated presence of malt-derived fatty acids (e.g. from excess trub) increases the concentration of higher alcohols including iso-butanol, iso-propanol, 2-methylbutanol, and 3-methylbutanol.[5]
References[edit]
- ↑ Cvengroschová M, Šepel'ová G, Šmogrovičová D. Effect of mashing-in temperature on free amino nitrogen concentration and foam stability of beer. Monatsschrift Brauwiss. 2003;56(7/8):128–131.
- ↑ Liguori L, De Francesco G, Orilio P, Perretti G, Albanese D. Influence of malt composition on the quality of a top fermented beer. J Food Sci Technol. 2021;58:2295–2303.
- ↑ Baert JJ, De Clippeleer J, Hughes PS, De Cooman L, Aerts G. On the origin of free and bound staling aldehydes in beer. J Agric Food Chem. 2012;60(46):11449–11472.
- ↑ Kelly M. Why, when, and how to measure YAN. Penn State Extension Wine & Grapes University website. 2020. Accessed online March 2024.
- ↑ Gibson BR. 125th anniversary review: improvement of higher gravity brewery fermentation via wort enrichment and supplementation. J Inst Brew. 2011;117(3):268–284.