The Western Brewer Journal – 1912
THE FLAVORING AND AROMATIC CONSTITUENTS OF BEER
by C. J. Lintner
The flavoring and aromatic constituents of beer can be classified in three groups, according to their origin viz., those originating in the malt or barley, the hops, and the yeast (including fermentation). Those in barley are situated in the husk; and one special group of carbohydrates, the pentosans, was long suspected of affecting the flavor of beer unfavorably, the idea having probably arisen from the fact that these substances, on decomposition, yield furfural. The pentosans, however, are tasteless, like, all highly complex carbohydrates.
The flavoring constituents of barley husk are mainly soluble in ether and alcohol, but nothing definite can be stated as to their chemical nature, except that, like the hop resins (which they resemble in bitter flavor), they are probably allied to the aliphatic terpenes. The husks also contain astringent substances, including tannin; and substances of the latter character influence the color of the barley causing, for instance, the dark color of the weathered barley – by a process of oxidation with the assistance of oxydases. These substances are extracted in the steep, to an extent varying with the chemical composition and temperature of the steeping liquor; so that, in the case of barleys very rich in tannins, the first liquor often acquires a dark, almost red, color. The influence of the character of the steeping liquor on the flavor of the beer by the elimination of these flavoring constituents has already been discussed by Seyffert.
Aeration and warm steeping liquor has a very energetic action on the husk constituents; and, whilst the removal of dust and superficial germs is desirable, it may be questioned whether extraction of husk constituents may not be pushed too far. It is possible that they are not altogether undesirable additions to beer in the extremely dilute state in which they are present therein; and, in fact, Seyffert’s observations seem to confirm this view – at least to a certain extent, and for pale beers. Hence, the extent to which these modern methods are employed in practical steeping will advisedly depend on the character of the barley in respect of color and texture of husk.
Our knowledge of the minor constituents of the barley-corn is still defective, owing to the very minute quantities in which some of them are present, especially the aromatic substances. One of these is the ethereal oil, recently isolated by Geys, in the author’s laboratory, both from the husks direct and from the ethereal extract, the quantity obtained, however, being too small for close examination. It has a very intense, agreeable smell of honey, and probably, in association with other substances, has an essential influence on the aroma of sound barley. Cumarin, the aromatic substance of woodruff and meadow hay, also seems to form one of the aromatic constituents of barley. Additional flavoring and aromatic substances made their appearance during malting, even in the uncured malt. The cause of the cucumber aroma, so distinctive of healthy germination, is unknown, but is probably an extremely volatile ethereal oil, since it disappears almost completely when the malt dries in the air. Green malt tastes like beans, and is more or less sweet, the flavor being probably caused – apart from ethereal oil and sugar by the amino acids formed from the hordein and other proteins during germination. E. Fischer has found that the flavor of the amino acids depends to some extent on their structure, and can in many instances be employed as a means of differentiation. Thus, α-aminoacetic acid, glycocoll, and other α-amino acids of the aliphatic series, have a sweet taste; whereas, in the β-amino acids this sweetness is less apparent, giving place to a bitter after taste. In the dibasic amino acids the acid flavor becomes more prominent, aspartic acid tasting like tartaric acid. The hydrolysis of proteins by hydrochloric acid furnishes, inter alia, decomposition products tasting like meat extract, and recalling the products appearing in dried yeast. The sweet taste of long – grown green malt is specially due to the saccharose stored up in the tissues of the embryo.
The flavoring substances produced in germination are of some importance to pale malts and beers; whereas, in dark beers the predominating influence is exerted by the new products formed in curing the malt. The curing of pale malt is mainly directed to the elimination of undesired crude flavoring and aromatic constituents, rather than to the production of new ones. The aroma of malt originates in the action of heat on certain soluble constituents of the green malt, the production of aroma and color proceeding hand in hand. The sensitiveness of these substances to heat was observed by the author in preparing diastase from aqueous extract of green malt. The filtrate from the alcoholic precipitate containing the diastase, when concentrated and evaporated to dryness at 100 ° C., gave a pulverulent-brown residue with a strong smell of bread. This, however, may indicate decomposition that has been pushed too far; whereas the fine malt aroma appears during an early stage of the curing process and is formed of readily alterable sugars, especially lævulose, and probably fission products of proteins, the saccharose of the malt remaining unchanged.
The aromatic substances of malt are very stable and seem to pass into the beer without alteration. There does not appear to be any great probability of new aromatic substances being formed in the preparation of wort; but flavoring substances-maltose and the dextrins most nearly allied thereto-are produced during the conversion of starch in mashing. It is difficult to say how far the mineral ingredients in the brewing liquor affect the flavor of the un-hopped wort, but at any rate their state of extreme dilution robs them of any special importance. It is interesting to note that the characteristic and somewhat insipid flavor of the sweet wort is not destroyed, but only masked, by hopping; since, if the bitter principles be extracted from beer with benzol, and the acids and aromatics produced during fermentation be removed by ether, the characteristic wort flavor reappears on the expulsion of the solvents by evaporation.
A special part in the production of dark beers is played by the added color malt, which has an effect on the flavor as well as on the color to an extent depending on the character and quantity of the adjunct. This color malt contains empyreumatic products recalling roasted coffee and these products do little harm when a moderate amount of color malt is used in making beer from dark malt, though they may be injurious when medium or pale malt is used with the color malt. These constituents of color malt are produced mainly from the sugar and starch of the malt, though also from resinous and fatty matters in the husk, these latter being probably the cause of the disagreeable empyreumatic smell. Their production may be diminished by thoroughly washing the barley, in the same way as the corresponding treatment of the coffee bean has been found to improve the flavor developed by roasting.
Hopping has a considerable influence on the flavor and aroma of the wort and beer. Owing to its aromatic and bitter principles, the hop plays the part of an aromatic that cannot be replaced by anything else so far as the aromatic constituents are concerned, though the bitter principles might be replaced by those of the bitter aloe the latter were not poisonous. The migration of the hop constituents into the wort during the hopping process a highly complicated operation, and one that has not yet been fully elucidated, there being in question a number of very complex substances that are by no means readily soluble in water. Thus, the hop oil consists of at least five different bodies, the chief of them (humulin) being a sesquiterpene. This volatilises with steam, and is thus largely dissipated in hopping, though sufficient is still left behind to enable the benzene extract from 100 gms, pale beer to fill a large room with the odor of hops. The quantity is imponderably small, and probably too insignificant to affect the flavor — which is no disadvantage, since ponderable quantities of hop oil give the beer a disagreeably smoky taste, whilst larger amounts destroy the “head.” The bitter flavor of the hop is due to two crystalline substances of acid character and complex composition, neither glucosides nor alkaloids, but non- nitrogenous, and probably forming derivatives of aliphatic terpenes. They are termed α and β lupulinic acid, but should preferably be known as humulone and lupulinic acid, on account of their divergent constitution.
The bitter principles do not dissolve readily in water, but first most undergo changes, embodying oxidation and leading to the formation of amorphous resinous substances. They are readily soluble in alcohol, from which they are precipitable, by water, as partially crystalline soft resins. Their compounds with lime, magnesia, and alkalis are soluble in water, and the formation of these salts may sometimes be assisted by the composition of the brewing liquor, which may then appreciably influence the flavor of the beer, since dissolved substances alone act upon the organs of taste. The lupulinic acid is insoluble in wort and water when air is excluded; but humulone is partially soluble, as experiment has shown. This behavior of humulone confirms Remy’s observation that hops of high bittering power seem to contain more humulone (α acid) than such as are weaker in this respect. Bungener established, twenty-five years ago, that pure lupulinic acid does not become soluble in water until oxidized, and it might be possible in practice to obtain more complete utilization of the hop bitter by rousing the wort during the hopping process. Hop resins are also found in a state of colloidal solution in the beer, but whether they contribute to the flavor seems doubtful.
A not inconsiderable portion of the bitter principles dissolved in hopping is reprecipitated – in part by the coagulation of the proteins on the cooler, and especially by the gluten in the head thrown up during fermentation. Schönfeld has recently shown that the deposition of hop resins during storage is influenced by the acidity of the beer — an observation agreeing with that of Bungener on the behavior of the bitter resin when dissolved in water. The amount of bitter principle remaining in the beer is small; and probably not even the whole is concerned in the production of flavor, so that there is no regularly recurring relation between the content of bitter principles in hops and beer, or even between the amount of bitter principle in beer and the flavor of same. It may happen that hops with a low content of bitter principles have a more intense bitter taste than those with a larger percentage, and it is therefore difficult to form an accurate estimation of the saving in hops effected by special treatment. For this reason – apart from the defects of the analytical methods — the valuation of hops on the basis of their percentage of bitter principles should be accepted with reserve.
With regard to the physiological action of hop bitter principles, a Dr. Engelmann, some years back, brought out a pamphlet in which these bitter principles were classed as alkaloids, their action on the nervous system being ranked with that of opium; whereas, as a matter of fact, no alkaloid has been identified in any part of the hop cone except the seeds — which latter are rarely present in any but American hops; and, even if they were, the process of hopping beer does not present conditions suitable for the extraction of alkaloids. On the other hand, Dresser and Farkas have found that whilst α and β acids are poisonous when introduced directly into the arterial circulation, they are inert when taken in through the stomach, even in large doses. The resins resulting from the oxidation of these acids have totally lost their toxic properties.
The flavoring constituents of beer wort undergo extensive changes during fermentation, the sugar being transformed into alcohol and carbonic acid gas, a portion of the hop resins precipitated, and new, metabolic products introduced by the action of the yeast. Little is known about these substances, their amount being relatively minute but light has already been thrown upon the matter by the researches of Ehrlich in the chemical processes occurring during the metabolism of proteins in plants. Succinic acid and glycerol, which always occur and probably have some effect on the flavor of beer, were hitherto regarded as products of the decomposition and transformation of sugar; but Ehrlich has shown this view to be inaccurate in respect of succinic acid, which he finds to be derived from glutanic acid (furnished by the decomposition of hordein), the nitrogen of this acid being used by the yeast in the reconstruction of organic protein, whilst the residual matter is oxidized to succinic acid. According to this worker, the amino acids (of which no less than 18 of different structure have been determined in the protein molecule), undergo alcoholic fermentation with formation of alcohol of the next lower series (containing one less atom of carbon); and as the alcohols are presumably accompanied by aldehydes and condensation products, there will be no lack of substances contributing to the aroma and flavor of alcoholic beverages.
The amino acids are derived from the hydrolysis of barley proteins during germination and from the autolysis of yeast protein on the death of the yeast cells. Ehrlich assumes that the protein of each race of yeast differs in composition, and that this difference accounts for the racial characteristics and enzyme content; so that each race of yeast, when employed to ferment sugar or worts in low amino-acid nitrogen, produces its own peculiar type of fusel oil. This is more particularly apparent in wine making than in beer, especially in bottom-fermentation beer where the number of different races of yeast is smaller. Among the alcohols originating in amino acids, special interest attaches to tyrosol (from tyrosin) which has been identified by Ehrlich in Berlin beer; and the author has also detected it in Munich beer, so that it is probably a regular constituent. Whilst the amount present is normally so minute that the decidedly bitter flavor is probably inappreciable in comparison with that of hop bitters, it may account for the bitterness developed in beer by disease yeasts, such as Sacch. Pastorianus I. Tyrosin is formed in large quantities when auto-digestion commences in yeast, and it may be that Pastorianus yeast furnishes this product readily, the conversion into tyrosol then being effected by the culture yeast still predominating. Elimination of the dead cells by carefully washing the pitching yeast will enable these undesirable flavoring sub stances to be got rid of.
No accurate information is yet available on the origin of the glycerol, but the researches of Seifert and Reisch have put sugar out of court in this connection, the glycerol being regarded as a metabolic product of yeast.
The so-called “palate-fulness” is a property which should never be lacking in good beer. The substances to which it is due are in themselves tasteless like the proteids in colloidal solution, or have very little flavor, like the lower dextrins. They act in virtue of the condition in which they are distributed in the liquid, as relatively large particles in a highly distended state. They stand in a certain relation to the flavoring substances, owing to absorptive capacity for the latter, especially the proteids and hop bitter principles. Emslander has shown that the hop resins stabilize the coagulable proteids of beer, and the author’s experience, in connection with alcohol precipitations in beer, go to show that the absorption combination between protein and hop resin is very stable. These precipitations were performed for the purpose of recovering dextrin; and on taking up the dextrin with water an insoluble residue of protein and hop resin was left. Since the hop resin is readily soluble in alcohol, it should have remained in solution when the dextrin was precipitated had it not been prevented by the absorptive influence of the protein. It is probable that such absorption compounds may be the cause of the protracted sensation of flavor in the case of certain beers.
Finally, the carbonic acid gas in beer has a decided influence on the flavor. Itself practically devoid of flavor it imparts a kind of prickling and refreshing effect to beer and various other beverages. The prickling sensation is perceived by the tongue, and is due to the tiny bubbles of escaping gas. The functions of the carbonic acid also extend to the complete appreciation of the fine flavoring constituents and the blending of the same into perfect harmony.