SYMPOSIUM ON BREWING @ Presented before the Division of Industrial and Engineering Chernist7.y a t the 89th Meeting of t h e American Chemical Society, Xea York, N . Y . , April 22 t o 26, 1935.
Brewing Processes ROBERT SCHWARZ, Schwarz Laboratories, Inc., IVew York, N. T.
of the wort, and the temperatures suitable for fermentation are discussed. Fermentation is reviewed from the technical viewpoint, particularly with regard to the development of flavor and character of beer. Closed and open fermentation are compared, and the normal procedure of sound lager beer fermentation is given with time and temperature data. The three general methods of finishing lager beer, the krausening process, the bunging process, and carbonating are compared, and the slight variations in type and character which result from these three different practices are discussed. The final steps of bringing the beer into the trade packages are presented.
Brewing processes are divided into three groups of reactions taking place in (1)the brewhouse, (2) the fermenting cellar, and (3) the storage and finishing cellars. The first group of operations involves the conversion of starchy raw materials and malted cereals into soluble carbohydrates and the extraction of soluble nitrogenous degradation products obtained from these brewing materials. General types of equipment and specific procedure in the brewing of American-type draught beer are presented. The modern aspect of pH control of operations is shown. The significance of boiling the brewers’ wort with hops, the influence of correct procedure a t this point and during cooling
ORDER to give a clear and concise picture of brewing, this presentation will be confined to the most important processes and reactions involved in the production of the arerage bottle beer as brewed in what might be considered a representative American brewery.
0 1
r\r
Brewing Materials I n addition to barley malt, the brewing industry uses unmalted cereals, and products derived therefrom, brewing sugars, and sirups; they are grouped in a class commonly referred to by brewers as malt adjuncts. Malting barleys grown on the American continent almost invariably are of the six-rowed variety and contain an average protein content of about 12 per cent. European malting barleys, almost exclusively of the two-rowed variety, show an average protein content of 10 per cent. Thus, if American beers were brewed from barley malt exclusively as is generally the case in continental Europe, the greater protein content of 1031
the malt and the effects of relatively high protein content in the finished beers vould present special problems for the brewer. Long experience has shown that lager beers (bottom-fermented malt beverages) with comparatively high protein content are not as well suited t o climatic conditions and the beer-drinking habits of the people of this country as beers brewed from barley malt plus malt adjuncts. With respect to ale (the top-fermented malt beverage), the situation is somewhat different because of the fact that ale presents certain characteristics which tend to counteract satiating qualities introduced by high protein content. Thus the nature of the malt available and the conditions under which beer is consumed have established the necessity for the use of unmalted cereals, sugars, and sirups; their employment can in no way be looked upon as a substitution for a more valued beer ingredient or the lowering or quality thereof. Figure 1 shows that the first step in the brewing process which involves chemical reactions takes place in the water tanks. Conversion of the starches of malt and cereals and
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VOL.27, NO. 9
1
I
CCREAL GOVtRNMfNT BOTTLING T A N K
MALT STORAGE
RACLING TANK
1 FIGURE 1. FLOWSHEETOF BREWERY PROCESS
degradation of malt proteins is effected through enzyme action, which, as is well known, is exceedingly sensitive to the reaction-pH-of the medium in which the enzymes function. Thus the p H of the brewing water, its mineral content, and its hardness play an important part in brewing processes. Often the first step in making a brew consists of properly adjusting the pH and the hardness of the water used. I n general, practical experience in this country has shown that a p H of 6.0 to 6.5 and a total permanent hardness of 200 to 300 p. p. m. expressed as calcium carbonate represent correct conditions for the average beer.
Mashing The materials employed for brewing American bottle beers average about 35 to 38 pounds of barley malt and from 12 to 14 pounds of unmalted cereals (or their equivalent in brewing sugars and sirups) per U. S. beer barrel of 31 standard gallons. The hop rate is from 0.55 to 0.9 pound per barrel. Cleaning and grinding of the malt and, when necessary also of the unmalted cereal, constitute the preliminary steps preceding the preparation of the so-called cereal or converter mash wherein unmalted cereals representing from 20 to 35 per cent of the total material are mixed with substantially one-third of this quantity of ground malt. This composition is infused and subsequently boiled with amounts of water varying between 7.5 and 9 barrels per 1000 pounds of mixed materials. American lager beer practice has clung to the Reaumur
thermometer, imported from Germany almost 100 years ago, whereas ale brewing uses the Fahrenheit scale. The Centigrade scale is unknown in our breweries, except in those instances where a laboratory and a trained chemist have been installed. Therefore, in speaking of lager practices, Reaumur and Centigrade temperatures will be given, and, when discussing ale brewing, the Centigrade equivalents of the Fahrenheit temperatures will be presented. The so-called cooker or converter mash has for its object the conversion of the insoluble starch of the unmalted cereal into gelatinized and partly soluble starch, a t least partial degradation of the malt proteins, and the conversion of the soluble malt starch into dextrins and maltose sugar. Although many investigations have clarified the difference between the activities of the starch-dextrinizing amylase and the starch-saccharifying amylase, the brewing industry still lacks complete graphs and charts which will enable the brewer to reach a definitely predetermined proportion between fermentable sugar and nonfermentable dextrins by means of a charted time-temperature curve. A large number of variables exercise conflicting influences, but the definite knowledge that, in a brewer's mash, starch liquefaction is most rapid a t 56" to 60" R. (70" to 75" C.) and saccharification a t about 52" R. (65" C.) enables the practical brewer to conduct his cooker mash so that he can produce a high percentage of fermentable sugar from his malt starch when highly fermentable beers are desired, or restrict the production of such sugars when lowalcoholic beers or dark beers of the Munchner type are brewed. The cereal starch is subjected to little saccharifying action be-
STARCH
h Y
1
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INDUSTRIAL AND ENGINEERISG CHEMISTRY
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Boiling the Wort Fermented malt beverages owe some of their most pleasing characteristics to the hop extractives which they contain. The alpha and beta bitter resins and to some extent the essential oils of hops incorporate in ale or beer an attractive aroma and a pleasant, stimulating bitter taste. Thus one important purpose of boiling the wort with hops is properly to extract thcir bitter principles as well as tannins which assist in coagulating higher molecular nitrogenous substances that would not remain in solution in the fermented or finished b e e r . Thus extraction of the flavoring substances of the hops, proper coagulation of the precipitable proteins, sterilization of the wort, and concentration thereof to remove s o m e of the -water used in sparging, c o n s t i I tute the four major FINISHED woRr changes which t a k e p l a c e when PROTEOLr n c ENZ YNES the wort is boiled 9 - PROTEIffASE with hops. During - P€PTIDAS€ 5 - PEP?-/DAS€ this procedure the FIGURE 3. DEGRADATION OF PROTEINS p H of t h e w o r t DURINQ MASHIXG drops slightly. The average American lager beer wort, when ready to be cooled, shows a pH of 4.9 to 5.2. The time of boiling should not be prolonged unduly; 2.5 to 3 hours are ample in usual cases, about 1.5 to 2 hours of this time representing the period during which the wort is in contact with hops. Spent hops together with the proteins coagulated during boiling should be separated from the boiling wort as quickly as possible and with a minimum opportunity for oxidation. Vessels containing either a false bottom or a perforated basket are employed for this purpose. Since the spent hops retain about 3 barrels of wort (11" to 13" Balling) per 100 pounds of hops used, the hop strainer or older type of hop jack should be equipped with some form of sparging device so that the kettle wort retained in the hops can be leached out with boiling water and these sparging worts added to the main volume of the wort. Cooling Processes Preparing the wort for proper yeast action is by no means merely a matter of reducing the temperature or extracting heat units in the most efficient mechanical manner. The wort must absorb sufficient air to obtain a correct beer fermentation. I n addition, to further the clarification of the young beer after fermentation, coarse precipitation of proteins and hop resins which were soluble in the hot wort, but are insoluble a t fermentation temperatures, should be attained during the cooling process. Furthermore, air-borne infections through acidifying organisms and the so-called wild yeasts can readily take place during the process of cooling, bringing to this part of brewing operations the necessity for most careful consideration of the sanitary conditions under which the wort is cooled. For lager beer fermentations, the initial temperature ranges between 5.5' and 8" R. (7" to 10" C.) and for typical ales from 56" to 58" F. (13" to 14' C). I n most cases de-
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VOL. 27, NO. 9
crease in temperature starts when the wort, after leaving the hop jack or hop strainer, is transferred to a hot wort tank generally located above the cooler where it remains for a settling period of 30 to 60 minutes. The wort then drains over a horizontal battery of tubes through which cold water or brine and, in the lower section, direct-expansion ammonia flow as refrigerants. This type of cooler automatically effects aeration. Countercurrent closed coolers likewise are used; the inner tube contains the wort, and the outer is the conduit for the refrigerant. This type of cooler requires forced aeration which generally is supplied by admitting filtered air to the hot wort a t a point slightly beyond the entrance to the cooler and also to the cold wort before it passes from the cooler and into the hose or pipe line leading into the starting cellar. A slight degree of cooling and therefore a limited amount of coagulation and sedimentation take place in the collecting tank. Khen, however, this unit is replaced by a surface cooler (a large shallow vessel) designed to contain the wort a t a liquid depth of about 12 inches and located in an airconditioned room, gradual cooling to 50-60" R. (144.5167" F., or 62.5-75' C.) is readily attained. This effects precipitation of insoluble proteins and hop resins in coarse, flocculent form in which they settle readily and remain behind in the surface cooler, permitting a relatively clear wort to be run over the open (Baudelot) or through the closed countercurrent cooler. This modern method of cooling avoids carrying undesired wort constituents into the cellars, thus definitely favoring the attainment of quality factors and, to some extent, shortening the period of time necessary for cellar operations. It is important to note that the surface cooler as well as the open-type pipe cooler effects a certain amount of evaporation which not only introduces a cooling effect but likewise results in concentration of the wort. Where such types of coolers are in operation, the brewer generally strikes his kettle with a gravity from 0.5" to 0.8' Balling less than the figure set for the cellar wort. Air-conditioning of cooler rooms and scrupulous care to protect the wort from every possible source of infection during the cooling process are exceedingly important; many cases of deterioration in beer quality during storage in the cellars have been definitely traced to cooling of the wort without ample protection against acidifying germs and foreign yeasts invariably present in the atmosphere. rls the cooled wort leaves the brew house, the first part of brewing operations may be considered complete. Brew house efficiency is determined by comparing theoretical yield from materials as disclosed by their laboratory analyses with the amount of extract actually obtained in the starting tub in the refrigerated fermenting or starting cellar. Accurate measurements of wort volume as well as its gravity are most important. I n many instances, inefficient layout and worn out or antiquated equipment are producing such low yields that their replacement with efficient, modern units, correctly installed, could be amortized within a short time and a losing * brewing enterprise could be changed into a profitable bu siness. Fermentations Yeast, the unicellular organism belonging to the genus Saccharomyces cerevisiae, plays a far more important role in the development of beer character than most laymen and even some brewers realize. The significance of yeast in ale brewing is perhaps even greater than in the production of bottom-fermented beer. The frequently presented simple formula, CeHIZOs == 2CzHsOH 2C02
+
gives but a fraction of the involved chain of reactions which take place in a brewer's fermenting tub wherein a practically
SEPTEMBER, 1935
INDUSTRIAL AND ENGINEERING CHEMISTRY
unpalatable wort is converted into the attractive beverage, beer. Besides producing ethyl alcohol, yeast fermentation develops a large number of other organic substances such as aldehydes, ketones, esters, organic acids, and higher alcohols, all of which produce definite and marked effects upon the sense of taste and therefore are most important factors in creating the characteristic taste or flavor of an ale or beer. It is well recognized by brewery technologists and fermentation biologists t h a t yeast is the dominating factor in governing beer character and that the strain of yeast selected is of more importance than the fermentation procedure followed. The cooled m-ort is mixed with yeast, which has been derived from the crop of a previous brew, either in the cooler pan or in the line leading to the starting tub. From 0.75 to 1 pound of liquid yeast is used per barrel of lager beer. Starting cellar temperatures (3' to 5' R., or 4" to 6.25' C.) are somewhat lon-er than wort temperatures so that during the first 20 to 24 hours, known as the dead period of fermentation, development of foreign germs present through accident a l infection may be retarded until the time when yeast activity definitely sets in as shown by a white foam ring along the edge of the starting tub. At this point, the Tvort is designated a' having reached the krausen stage. I t is then pumped or dropped to the fermenting vessel, leaving behind a corer of further separated hop resins and a sediment of precipitated proteins and weakened or dead yeast cells which may have been present in the pitching yeast. From t h a t point on, fermentation proceeds with the development of thick, creamy covers caused by a vigorous production of (,arbon dioxide, and generation of heat which increases the temperature u p to 10-11" R. (12.5-14' C.) despite a cellar temperature of 4" to 5" R. (5" to 6" (3.). After 4 to 5 days in the fermen$er, during which time the saccharometer indication drops from 1.5" to 2' Balling, each 24 hours, the activity of fermentation slackens, the foam covers start to collapse, and the temperature of the beer naturally begins to derline. I n most American breweries cooling is furthered during the later stages of fermentation by circulating a refrigerant through attemperators in the fermenti n g v e s s e l s . During the period of receding fermentation-that is, from the fourth or fifth day on-periodic a k i m m i n g m a y also take place, depending upon the quantity of precipitated nitrogenous substances and hop resins still being thrown out and collecting on the top of the beer. It is during thih time that the yeast, which has practically completed its work, settles to the bottom of the tub; at the end of 7 to 10 days, depending upon the original gravity of the ~ o r tnature , of the yeast, and temperatures of f e r m e n t a tion, the fermented beer is ready to be vatted. At this stage it should show a cover of fine foam, a 1 5 - 2 5 inch layer of sedimented yeast on the bottom, and an opalescent appearance in the liquid which. however, produces an almost black appearance when observed from allore after the foam cover has been pushed aside. Practical brewers lay much stress upon the correct appearance of the fermented beer when i t is ready to be transferred to the storage tanks. The saccharometer indication of the
1035
fermented beer is from 2.5' to 4" Balling, the temperature from 2" to 5' R., and the p H from 4.3 to 4.8, showing a considerable increase in hydrogen-ion concentration during the process of fermentation. A variation of the above procedure involves transferring the beer in the krausen stage to closed fermenters connected with carbon dioxide compressors. I n these vessels fermentation proceeds until all air has been displaced by fermentation carbon dioxide. Then the outlet of the fermenter is connected with a compressor system, and the carbon dioxide is collected, compressed to 250 pounds per square inch pressure, cooled, washed, and stored in metal pressure tanks in a cold cellar. The carbon dioxide gas so recovered is subsequently used for finishing the storage beer by what is known as the carbonating process. When beer leaves the fermenting cellar, it contains in suspension appreciable amounts of yeast, insoluble nitrogenous hubstances, and hop resins. I t possesses certain taste characteristics which the brewer designates as "young" beer flavors and a somewhat puckering bitterness, largely due to the gamma or hard hop reqins which a t thi. point are still dissolved in the beer. The objectives to the so-called storage cellar operations therefore are (I) clarification of the beer, ( 2 ) elimination of bitter hop resins a t reduced temperatures, and (3) mellowing of the beer as a result of chemical reactions between its constituents. Maturing and Finishing Beer Three processes are followed in maturing the fermented beer and impregnating i t with the amount of carbon dioxide necessary to impart "life" and to enable it to maintain a creamy, lasting head of foam when poured into the glass. These processes, in the language of the brewer, are known as (1) krausening process, (2) bunging process, and (3) carbonating process. The first is the oldest method and begins with storing the fermented beer in a cold cellar at 0" to 2" R. (0' to 2.5" C.) for 2 to 4 weeks. Within this period clarification,
precipitation of hard resins, mellowing, and development of flavor take place. The brewery tanks or vats used for this purpose need not be pressure-tight. When ready lor finishing, such storage beer is transferred to pressure casks or tanks where it is mixed with 12 to 15 per cent of beer in the firat stage of fermentation-that is, krausen, which actually
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INDUSTRIAL A S D ENGISEERING CHEMISTRY
means wort in the stage that exists 20 to 24 hours after it has been pitched with yeast. Since the krausen has a temperature of about 8" R. (10" C.) and the cellar temperature ranges between 2.5' and 4" R. (3" and 5' C.), the young beer ferments slowly, impregnating the contents of the cask or tank with carbon dioxide gas up to a concentration of 0.45 t o 0.52 per cent by weight. Greater carbon dioxide content unfavorably affects beer, causing it to be wild when filled into a shipping package or bottle. Therefore the pressure casks are connected with a blowoff valve known, in the language of the brewer, as a bunging apparatus. Correlating bunging pressure with tlie cellar, and therefore also with the beer temperature, enables the brewer to regulate the carbon dioxide gas cont'ent of the beer, for he %as available exact tablesshowing the percent,ageof carbon dioxide by weight and volume for any given temperature and pressure. When this secondary or after-fermentation has consumed all the fermentable sugar introduced by the krausen, yeast action stops and clarification sets in. This, in s0rr.e instances, is aided by the addition of inert suspensions or vegetable or animal gelatins. ilfter the beer has thus carbonated itself (over a period of 3 to 4 weeks), it is subjected to a further storage period of 3 to 8 weeks, eit'her left in the same container or pumped over into a clean unit in a colder, so-called chilling cellar. I n the bunging method, which is commonly used in Central Europe and is being employed in a few instances in America, the fermenting beer is transferred to a fresh cask or tank while it still contains about 2 per cent of fermentable extract, and its temperature ranges between 3 " and 4" R. (3.5' to 5" (2.). The bunging apparatus is attached to the tank, and the carbon dioxide gas generated by yeast fermenting the remaining fermentable sugar is ret'ained by the beer under conditions of temperature and counter pressure which build up a carbon dioxide content of 0.45 to 0.52 per cent-that is, the amount necessary for life and foam-keeping capacity. When fermentation subsides, sedimentation and clarification ensue in a manner similar to that which takes place in the krausening process, but it generally is found that beer finished off by the bunging process shows a heavier yeast sediment than krausened beer. These beers therefore are more susceptible t'o the development of a yeast flavor and, as a general rule, should be pumped over and filtered as soon as clarification has reached a point where substances remaining in suspension are not present in such quantity as to interfere with the usual filtration procedure as commonly followed in brewing practice. Carbonating is a distinctly American procedure, and was invented and perfected in this country some 45 years ago. When followine-this mocedure. the beer is allowed to com. plete its fermentation with the attainment of as much sedimentation as possible in the fermenting tub. Either through the use of the attemperators or an enclosed countercurrent cooler, the beer is brought into the storage cellar a t a temperature as close as possible to 0 " R. Here it is stored for 3 to 6 weeks, when the usual processes of clarification, separation and precipitation of hard resins, and maturing and mellowing of taste take place. At, the end of the storage period the beer is pumped through a pulp filter and impregnated with carbon dioxide gas drawn from one of the pressure tanks referred to earlier. Various ingenious forms of carbonating apparatus have been devised and invented. Many are combinabions of pumps and gas feeders while others operate on the principle of letting the beer rain through a n atmosphere of carbon dioxide gas whereby the absorption of gas is commensurate with the beer temperature and the pressure existing in the system. -4prefiltered and carbonated beer is pumped into pressure tanks where a counter pressure is maintained which is but slightly lower than the head pressure of the incoming beer.
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The above are the physical aspects of the processes involved. Absorption of carbon dioxide gas, howel-er, is not merely a mechanical process. Ample evidence exists that this gas is both absorbed and adsorbed, and that rather complicated physico-chemical reactions take place. S o t alone tlie quantity of carbon dioxide gas retained in the beer but, to a marked extent, the quantity and nat'ure of the nitrogenous substances present (particularly those of higher molecular weight and those existing in colloidal state) govern the foam-retention property of the beer. Carbohydrates of the dextrin group and gums' assist in sustaining the foambuilding properties of nitrogenous s u b s t a n c e s e v e n though any attempt to produce a beer-like foam from dextrin and carbon dioxides in the absence of beer proteins gives decidedly negative result's. Attempts have been made to carry out quantitative studies of the chemical changes which take place during the cellar treatment or so-called maturing processes which beer undergoes. These changes, however, while marked in the effect upon the taste and olfactory nerves, are so slight chemically that not much progress has been made in this direction. One indicator, however, rather definitely shows that certain amounts of esters must be formed, for the pH slowly rises after the end of fermentation, indicating t,hat acids, as such, disappear. Since free bases other than alcohols are entirely absent in beer, this decrease in pH, no doubt, is the result of ester formation by the combination of organic acids and alcohols. These esters naturally improve the flavor. X study of the maburing of ale where the aromatic substances increase far more noticeably during the storage period, strengthens this conclusion with regard to the important changes which take place during the storage and maturing processes. Finishing and Racking The American public has come to demand brilliant beers showing a distinctive foam retention when poured into the glass. Natural clarification in the cellars does not produce beer of the required brilliancy. Various forms of cotton pulp and other types of filters have been developed to meet the needs of brewery practice n-hich involves filtration of a carbonated beverage containing substances of gummy nature in suspension. The brewer a190 must guard against ultrafiltration which robs the beer of colloids that sustain foam head. On the other hand, in the case of unpasteurized draught beers, filtration must, as efficiently as possible, remove yeast cells and any other microorganisms which may be present, in order to produce a beer which will remain clear and brilliant in the keg for a t least one week. Filtering operations constitute an important part of the cellar treatment of beer, for the question of filter efficiency also ent'ers into the problem. Depending upon the condition of the beer, the same filter may handle from 200 to 1200 barrels of beer, showing how wide may be the range of usefulness of filter cakes under varying conditions of the beer. The so-called racking cellar operations which involve the final filtering, chilling, carbonating (when necessary), and filling of the beer into trade packages constitute the last mechanical processes. which bring the beer to the shipping platform of the brewery. It is important that the beer remain a t low temperatures until it reaches the distributor,. and for this reason, as well as for the prevention of foaming and loss of carbon dioxide when filling into packages, it is common practice to chill the beer to about 0 " R. (32" F.). The counterpressure principle of handling carbonated beverages must be applied to beer. The keg filler, or "racker," operates on this principle, and many ingenious mechanical devices have been built into this type of equipment to give it increased capacity. There are now available
SEPTElZBEH, 1935
INDUSTRIAL AND E U C l Y E E R l N G CHEAIISTRY
three-arm rackerc: which nil1 deliver up to two hundred forty half-barrel< per hour n ithout foaming or other loss oi beer. The washing and cleaning of the kegs are an important part of the finiihing operatiom. Theae containers are &ject to all kind< of contamination, particularly those packages which hare been used for shipping trade. Recently great progress has been inade in developing more efficient and automatically operating keg-washing machine.. These function on the .pray principle whereby streams of hot and cold water a t preq-ures up to 60 pounds per square inch are injected into the package to dislodge and wash out dried beer remain.: and other accumulated foreign substanceb. A sound, clean trade package i i just as important as the maintenance of sanitary conditions in other parts of the brewery. Economic lspects Unmalted cereals, and products derived therefrom, furnisli to the brewers' wort colids or extract in amount.: which are almost equal to the laboratory or theoretical yield. The brewery yield obtained from barley malt, the main ingredient of beer, varieq between 62 and 72 per cent for malt< produced from American six-rowed barley, depending upon the quality of this brewing material and the nature and condition of the brewhouse equipment. With the conventional combination ma\h and lauter tub, the brewhouqe yield from malt rnay be 4 to 8 per cent below the laboratory figure. With the separate mash and lauter tubs, laboratory yield may be approached more closcly, and with a mash filter i t is often exceeded. The iecond econoinic aspect is presented by process shrink-
1035
age during cellar operations. Beer is lost by being admixed with yeast crops in tank or cask iedinients, by entrainment, and by admixture with witer used to flush out conduits, such as pipe and hose lines, and in filters. Process shrinkage to rome extent depends upon the size of units coi1,qtituting the hrewery equipment, and therefore is smaller in large breweries and greater in the small plants. I n general, a wellconducted brewery will turn out about 97 barrels of beer per 100 barrels of wort brought into the starting tub, whereas in smaller plants a final yield of 88 to 90 barrels generally is obtained. d third economic Triewpoint involve,? the engineering services required for brewery operations-that is, production of steam, electric power, refrigeration, and the maintenance of wat'er and air supplies. Much has been done during the last few years to correlate and coordinate the development of t,hese services with breming procedure, but the so-called engineering departments of breweries still offer a splendid field for the mechanical and chemical engineer. As in any factory using process steam, the brewery offers many opportunities for steam to do considerable work before and after i t has been used in the actual brewing processes. Many mechanical improvements can be introduced into existing breweries to reduce operating costs without in any way reducing quality of finished product. Acknowledgment Acknowledgment is made of the helpful assistance of S. Laufer and A. R. Erda in preparing material for this paper. RECEIVED .ipril 26, 1935.
Chemistry of Alcoholic Fermentation LEONOR MICHAELIS The Rockefeller Institute for Medical Research, New York, N. Y.
catalytic reaction* could easily he separated from the living LCOHOLIC fermentation has h e n known cells simply by extraction with water. and utilized from times immeniorial, but amroaches to the understanding of the chemical and biological processes underlying it are of rather ~i~~~~~~~of a Cell-Free Fermenting .iigent aecent date. I t is unnecessarv to dwell uDon the stages represented by such names as Schwann, Gay-Lussac, Liebig, Bucliner ( I ) dixovered in 1897 that by special methods t'he Wohler, Pasteur. The results of their works may be sumfermenting agent of the yeast cell can also be separated froin iiiarized as follows: Fermentation is caused by a living orthe living cell. This discovery made it clear that alcoholic ganism, the yeast cell, and confermentation, like ot'her enzysists mainly in the conversion matic reactions, is caused by a of sugar into a l c o h o l and carc h e m i c a l substance or rather bon dioxide although other end A brief reviewis given Of what is hewn at by a complicated s y s t e m of product,s may also be forined present about the chemical processes inchemical substances which can in smaller q u a n t i t i e s . T h e valved in alcoholic fermentation. The by no means be c a l l e d l i v i n g essential difficulty in a further large field ofa~coho~ic fermentation is Withm a t t e r . The achievement' of the living cell is to produce these approach consisted to the the out boundary and is interlinked with all the Hut once problem in theOf fact that no chemical agent endowed adjacent territories of chemistry and biproduced, these agents act in a ology. The alluring feature is the fact that v-ith the faculty of causing ferpurely chemical way, even in t,he any progress in its exploration has always absence of the living material by mentation could be isolated from the living yeast cell. This was meant a simultaneous progress in the field xhich they were produced. in c o n t r a s t t o t h e f a c t that Buchner's method cells consisted of general metabolism both of animals and grinding with many other agents, now called e n z y m e s , which bring about Plants, and vice versa. sand and Kieselguhr and, after o t h e r biologically i m p o r t a n t coniplet'e crushing of the cellh I
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