FIGURE 1. FLOWSHEET OF PROCESS FOR HYDROGENATION OF FRESHLY DISTILLED SPIRITS Vapora of freshly distilled spirits passing from the doubler at 10 to the condensers at 11 are diverted at 1 for treatment. Hydrogen from 12 and recyoled. hydrogen admitted at 2 are mixed with thia stream, which then passes into treater 3 through perforations 14. Treater 3 is provided with eteam coils, 13, a sight glass, and a gage. Catalyst in auapension is forced by the catalyst circulating pump, 4, through spray nozzles, 15. The vapors then psss through baffle tower 6 , provided with a pressure release, 16, and a vacuum release, 17, to the tube condenser, 6 , which is cooled by water circulating from 18 to 19. The cooled liquid and vapor then enter the gas-whisky separator, 7, through tail box 20. The gaswhisky separator is provided with a sight glass and gage. The hydrogen thus recovered is recycled to 2 by mean8 of pump 8, and the whisky passes to the cistern house at 21, being circulated by pump 9.
Hydrogenation of Freshly Distilled Spirits CARROLL A. HOCHWALT, CHARLES ALLEN THOMAS, AND ERNEST C. DYBDAL Thomas & Hochwalt Laboratories, Inc., Dayton, Ohio
E
HISKY is 99.75 per cent ethyl alcohol and
water. The remaining quarter of one per cent is a complex mixture which supplies smoothness, flavor, and potabiliiy to the product. A m%ure of grain alcohol and -n-ateris extremely irritating to the sensory nerves of the mouth; therefore in the final analysis this small fraction of complex substances serves as a palliative for the astringent mixture so that it can pass easily into the stomach. The bouquet and mellowness characteristics of whiskies rest solely in the composition of these minute quantities of organic “impurities” in ethyl alcohol and water. Thus, these impurities, called “congeneric substances” or “congeners,” as well as the traces of constituents responsible for so-called greenness, are the only factors to be considered in any problem dealing with the aging of whisky. The standard analytical methods for the evaluation of alcoholic beverages, while satisfactory for distinguishing the family groups of acids, esters, fusel oils, aldehydes, and solids, are inadequate for the identification of the minute amounts of the individual congeners or members of each family group. For instance, in the acid determination, all the volatile acids are reported as acetic; undoubtedly this is not the case. What a vast difference exists in the odor of acetic and butyric acid! Yet odor or aroma is just as important a factor in judging old whiskies as taste. However, the standard analytical methods have shown the type of compounds present in the congeners to be acids (both volatile and fixed), aldehydes (including furfural), esters, fusel oils, and nonvolatile solids which are resinous in character.
Chemical Changes in Whisky Aging Explanations advanced as to the chemical changes taking place during the aging of whisky in wooden casks have been both varied and contradictory. Concurrence in opinions is left mainly to generalities. Since a small variation in the compounds of the congeners is the only factor which distinguishes bourbon or rye from Scotch or even rum, one cannot take the findings of past workers too seriously without knowing precisely the type and brand of spirits in question. Unfortunately the reports are not always clear in these points. For example, Hewitt (3) stated that aged Scotch whiskies contain only one-third to one-fourth the aldehydes present in the original fresh spirits, whereas Schidrowitz and Kaye (6) observed no perceptible change in the aldehyde content. With American w h i s k i e s 4 e., rye or bourbon-Crampton and Tolman (1) showed that the aged whiskies contain greater quantities of aldehydes than the fresh spirits. However, since these aldehydes are present in such minute quantities, it is difficult to determine just what previous workers have classified as aldehydes. It is assumed that they refer to true aldehydic substances such as acetaldehyde and furfural. In general the ester content of aged whisky is much higher than that of the freshly distilled spirits. Aged spirits also have a higher solid content as a result of their long contact with the wood of the casks. The effect of the cask is an important factor in the flavor of the whisky since different odors and tastes result from aging in the presence of chars or roasted woods of differing characteristics. It is believed that the fusel 1404
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oil content of aged whisky is as high as, and in some cases higher than, that found in freshly distilled spirits. Schidrowitz and Kaye ( 5 ) ,in order to determine the constitution of the 0.25 per cent of secondary products in Scotch whisky, distilled yeast and found amines, pyridine bases, pyrrole bases, phenols, nitriles, mercaptans, thiosulfates, sulfur dioxide, and hydrogen sulfide in the distillate. These compounds, it was cincliyded, should be present in freshly distilled liuuor which is a uroduct of yeast fermentation. In substantiation of this theory, an examination of new pot whiskies revealed the presence of pyrroles, phenols, hydrogen sulfide, nitriles, and sulfurous acid. To these substances were attributed the bad odor and taste of green whiskies, and they believed that upon aging the following chemical reactions occur: The pyrroles polymerize to resins; the phenols interact with aldehydes t o form condensation hroducts; the hydrogen sulfide and sulfurous acid oxidize; and the nitriles change to acids and esters. These changes, it was stated, account for the elimination of unpleasant odor and taste from freshly distilled spirits. The natural aging of spirits can be divided into two operations: (1) the removal of the unpleasant odors and taste, known as greenness, found in the freshly distilled liquors; and ( 2 ) a maturing process, principally a chemical esterification and polymerization, which imparts a mellow taste and body to the liquor. The two operations undoubtedly occur together during the natural aging. However, the rates of operations 1 and 2 are different, the maturing being the faster. At no time under four years can naturally aged whisky be said to be entirely free from green constituents. Actually the maturing constituents tend to suppress the green constituents which may still remain, so that even the palate, the most sensitive test known, fails to detect them. The maturing process is therefore unnecessarily prolonged to mask the greenness. The work reported here pertains only to operation 1. Removal of Green Constituents It is logical to believe that the small traces of unpleasant smelling and tastingmaterials can be changed by a chemically controlled process and that the time which is normally consumed in the natural aging can be greatly reduced. The most numerous of the proposals for “degreening” whisky involve the use of oxidizing agents, especially oxygen, ozone, and air ( 2 ) . Since oxidation is certainlv one of the maior reactions in the natural aging of beverages, many investigators have attempted t o decrease the natural aging period by giving the liquor a preliminary oxidation treatment. In many cases green whisky is improved by being subjected to a mild oxidizing action for relatively long intervals, but it is impractical to prolong the treatment. To shorten the time, a more vigorous oxidation has been proposed, but in such a process there is much danger of forming large proportions of aldehydes and other undesirable by-products which are often more offensive than the original constituents of the distilled spirits. During the research on various means of destroying the unpleasant odor and taste of freshly distilled spirits without obtaining deleterious by-products, reduction was investigated. The belief has persisted that aging is principally an oxidation process and that a chemical oxidation method should produce the same result, but the writers’ results indicate that, if aging is considered as occurring in two operations, oxidation is of benefit only in the second-i. e., the formation of congeners-and that the best method of rapidly destroying unpleasant odors and tastes is the opposite of oxidation-hydrogenation. Here it was found that, with the proper catalyst, a small amount of hydrogen completely eliminates the green or “hog track” odor and taste of freshly distilled spirits. ,
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Although for convenience the process is called “hydrogenistion” and the product is designated here as “hydrogenated whisky,” it is clearly evident from the preceding remarks that only hydrogenation of the undesirable constituents is meant and that strictly speaking the product “hydrogenated whisky” is a misnomer; more correctly the product is a hvdroaen-treated whiskv.
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Hydrogenation Procedure Various methods of catalytic hydrogenation of freshly distilled spirits were investigated. Vapor-phase hydrogenation a t 100’ C. in the presence of a nickel catalyst gave the best results from both the standpoints of ease of plant operation and quality of product. Platinum black or finely divided nickel proved to be rtn effective catalvst in the vapor but, since nickel is - Dhase. quite as active”and much less expensive, it was selected for plant operations. The nickel was obtained by the reduction of nickel carbonate or formate. A more convenient source of nickel, however, is the Raney nickel-aluminum alloy ( 4 ) ; the catalyst is prepared by dissolving the aluminum with sodium hydroxide solution and separating the free active nickel which remains in a finely divided form. After washing the nickel free of sodium hydroxide, it is kept under water ready for use. A relatively small proportion of nickel is effective; about 3 pounds are sufficient for the hydrogenation of 1500 gallons of freshly distilled spirits. The principal units of the hydrogenation plant are a treater and a gas-whisky separator, represented on Figure 1 as 3 arid 7, respectively. The process is conducted as follows: In the distillery the whisky-vapor line is tapped between the doubler and the main condensers; the vapors of the freshly distilled spirits enter the hydrogenation unit a t point 1a t 80” to 90’ C. and contain varying amounts of water. Since this plant was set up in a distillery maintaining batch operations, the proof of the incoming vapors varied with the distillation cycle. Hydrogen from the recycling pump, 8, is mixed with the incoming vapors a t 2 . Fresh hydrogen is admitted a t 2 occasionally to maintain the proper partial pressure of hydrogen in the treater. The mixture of green whisky vapor and hydrogen enters the treater, 3, and is dispersed into small streams by passing through perforations in the inlet tube. The ascending vapors come into intimate contact with the finely divided catalyst in the water spray issuing from the nozzles a t the top of the treater. The water with the catalyst in suspension collects in the bottom of the treater, and steam coils are provided to maintain the water a t such a temperature that the level of the catalyst-water suspension, as noted in the sight glass provided on the side of the treater, remains constant. By this means the temperature of the water is always maintained above the boiling point of the incoming vapor to insure vapor-phase conditions. This allows flexible operation, and thus the variation in proof of the incoming vapors can be disregarded. The catalyst-water suspension is circulated from the bottom of the treater, 3, by the centrifugal pump, 4, to the top where it emerges through the spray nozzles. Six or more spray nozzles controlled by valves are placed in the treater t o permit variation in the quantity of catalyst circulated and the size of the individual spray particles, and thus serve to control the extent of hydrogenation. Some freshly distilled spirits require a greater degree of hydrogenation than others; hence such control is advantageous. The hydrogenation is completed in the treater, and the vapors together with the hydrogen pass through the baffle tower, 5 ; the purpose of the latter is to remove any sprayfor example, catalyst particles-conveyed by the gas stream. The whisky vapors are then condensed in the tube condenser, 6. In order to separate the hydrogen from the whisky and t o
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condition; a partial pressure of one pound per square inch has been found satisfactory. Additional flexibility of operation may be secured by varying the concentration of the catalyst, which is also dependent on these same two factors. In starting this equipment, a short time is required to obtain equilibrium. Usually it requires about 30 to 40 gallons throughput to regulate the temperature, amount of incoming vapor, and the pressure of hydrogen before effective hydrogenation is obtained, yielding a product free of all objectionable odor and taste. The plant illustrated in Figures 2, 3, and 4 has a capacity of 125 gallons per hour and has produced approximately 200,000 gallons of satisfactory product. The inexpensiveness of the vapor-phase hydrogenation process and the fact that the hydrogenation reactions are completed almost instantaneously make the process particularly attractive. I n the liquid-phase hydrogenation of freshly distilled spirits the same type of catalyst is used as in the vapor-phase process. However, it was found that platinum black was preferable to nickel in that it was less soluble in the freshly distilled spirits. Small amounts of dissolved nickel had an adverse effect on the taste of the whisky. In carrying out liquidphase hydrogenation, the catalyst is suspended in the liquid and violently agitated while a stream of hydrogen enters a t the bottom of the vessel. The cata-
wiil depend upon the condition of the green whisiyand the volume of whisky to be hydrogenated. Under most operating
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be exercised to avoid overhydrogenation. since excess hydrogen produces a whisky with a peppery taste. In another method of vapor-phase hydrogenation the nickel is suspended on pumice or other porous inert material, and the whisky vapors are passed over the catalyst in a heated tube. A wider range of operating conditions is obtained in this manner since the whisky vapors may be superheated to from 100Oto 140' C. At temperatures of 120" to 140OC. some esterification also occurs during the hydrogenation. The processes for hydrogenating new whisky are seen from the foregoing to be relatively simple. Operating costs are low since the only variables to control are the activity of the catalyst, the partial pressure of the hydrogen, and the temperature of the catalyst-supporting medium. The cost of the hydrogen is a small item because it is recycled and fresh hydrogen is admitted only to maintain the proper partial pressure. The catalyst, which has a comparatively long life, may be regenerated after it has been poisoned, thus making its total cost small.
Chemical Changes during Hydrogenation The disappearance of the disagreeable odor and taste is the immediate obvious change that occurs on hydrogenation of freshly distilled spirits. The compounds which cause the unpleasant odors and taste known as greenness and which prolong the maturing period are changed by the reducing action to odorless and tasteless bodies. ,4chemical reduction takes place since hydrogen alone has no effect on the removal of these objectionable bodies. Reduction takes place with hydrogen only in the presence of a catalyst such as nickel or platinum. This reaction is analogous to the hydrogenation of oils and fats, in that unpleasant odor and taste inherent in the original oil or fat are removed. It has been definitely established that hydrogenation does not affect the congener content of freshly distilled spirits nor does it aid in quickening the formation of congeners when the whisky is subsequently aged in wooden casks. The following table shows the congener content of a whisky before and after hydrogenation (in grams per 100 liters) :
Solids Acids Esters
Before After Hydrogena- Hydrogenation tion 2 2 13 11 17 16
Before After Hydrogena- Hydrogenation tion .ildehydes 2.5 2.0 0.3 Furfural 0.3 126 121 Fuseloils
The following table shows the acid and ester content (in grams per 100 liters) of two whiskies; both were aged nine months and one had been hydrogenated before aging. Whisky Untreated bourbon Hydrogenated bourbon
rlcids 58 59
Esters 26 30
Both physical and chemical methods have been unsuccessful in detecting any change in the hydrogenated product; for instance, index of refraction, pH values, and unsaturation tests such as bromine numbers show no difference from the original product. Only the empirical tests such as taste and odor, which are extremely sensitive, show the difference of the hydrogenated product. Thorpe (6) has shown that acrolein is present in freshly distilled spirits; the writers believe that this compound or its homologs may account for a part of the undesirable odor and taste. When a trace of acrolein is added to a mixture of 50
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per cent alcohol and water, the mixture acquires an odor and taste somewhat similar to that of green whisky. If this mixture is then hydrogenated, using nickel as a catalyst, the odor and taste of the acrolein immediately disappear. It is possible that any unsaturated aldehydes, acids, or esters in the green whisky are converted t o the corresponding saturated compounds, which, in general, have much less irritating and objectionable odors and tastes. As previously pointed out, Schidrowitz and Kaye ( 5 ) have postulated that the unpleasant odor and taste in freshly distilled spirits are caused by compounds such as pyridine bases pyrroles, nitriles, mercaptans, hydrogen sulfide, and sulfurous, acid. Thus, despite extreme care in choice of grain, mash formula, purity of yeast, etc., some greenness is unavoidable In this work it was found that, when a suspension of yeast, and water is hydrogenated, the characteristic odor disappears! completely and only a faint trace of an amine-like odor re. mains. In the same way all yeasty odors in freshly distilled spirits are readily removed by hydrogenation. The effective.. ness of hydrogen in destroying sloppy odors in high concentrations has also been studied by subjecting the residue from the doubler still to hydrogenation. In these high concentrations the yeasty or amine-like odors are completely removed by a small amount of hydrogen. These results are in striking contrast to the natural aging process in which three to four years are required to remove all of the objectionable green tastes and odors. The observation has also been made that, in as short a time as a year and a half, the content of congeners is within 60 to 70 per cent of that of four-year-old whisky. This indicates that a palatable nrhisky would result if initial greenness could be removed. This point has been repeatedly proved in this work, In a typical experiment to substantiate this point, a batch of whisky was hydrogenated and aged under identical conditions Kith the same spirits untreated. At the end of nine months the two whiskies were examined. The whisky which was hydrogenated had a considerably better aroma and taste than the untreated whisky. In summarizing, the hydrogenation process offers a distinctly new and advantageous method for removing greenness from freshly distilled spirits, and by this removal considerab1,y accelerates the natural aging process.
Acknowledgment The authors wish to take this opportunity of expressing their appreciation to H. E. Talbott, N. S. Talbott, and the National Pure Spirits Corporation, who financed this work., To William H. Carmody the authors are indebted for much of the experimental work.
Literature Cited Crampton, C. A,, and Tolman, L. M., J . A m . C'hem. SOC.,30, 98 (1908). Fain, J. M., and Snell, F. D., IND.ENQ.CHEJI..News E d , 12. 120 (1934). Hewitt, J. T., J. Soc. Chem. Ind., 21, 99 (1902). Raney, M., U. S. Patent 1,628,190 (May 10, 1927). Schidrowitz, Philip, and Kaye, F., J. SOC.Chem. Ind., 24, 585 (1903. Thorpe; Edward, "Dictionary of Chemistry," Vol. 7, p. 467, London, Longmans Green & Co., 1927. RECEIVED October 21, 1936. Presented before the 28th Annual hleeticg of the .4merican Institute of Chemical Engineers, Columbus, Ohio, November 13 to 15, 1935.
