1. B. Church
State Universitv of New York ot Buffalo Buffalo, New York 14214
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The Chemistry of Winemaking A n unique lecture demonstration
The chemistry of winemaking is not often viewed as material suitable for inclusion into many elementary chemistry courses, expccially as a lecture or lab demonstration. This paper proposes to show that the very complex series of chcmical reactions present in the preparation of wine can be used as the focal point to illustrate many other gcneral classes of chemical reactions and physical processes. The familiarity of most students with the finished product will help to capture and maintain their interest, and thus encourage them to learn and appreciate the chemistry being illustrated by the winemaking demonstration. Normally wine is started in the fall when the ripe grapcs arc harvestcd. I t is not ready to be tasted until at least the following May-the origin of the young, and often very harsh, "May Wine." This time sequence coincides with the typical school ycar and thus provides an ideal opportunity to have a continuing demonstration. As the vear Dasses and the course progresses, continued reference can be made to the various stages of the wine production. Depending upon local laws,' customs, and the stomaches of those involved in the experiment, the end of the year could be celebrated with the testing and tasting of the wine. As the following years pass, the wine could also serve to encouragc students to return to the school or department, to talk chemistry and see how their wine matures
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The suneested demonstration of makine " wine in the classroom and using it as the central point in many discussions of chemieal reactions has not been widely practiecd bccause of local custom, the lack of a near-by vineyard, and the necessary knowledge of just how to make wine. Thc last point is the purpose of this paper. The commcrcial availability of concentrated wine musts (which make a su~prisinglygood wine) no longer nccessitates the proximity of a vineyard. Concerning the first point: the author hopes that others will have more success in convincing their school or department to pay for the cost of this demonstration. In a state school, the standard excuse seems to be. ". . .and think what the legislators would say if they heard of this!" Since many excellent books have been written on wine (1-8) and winemaleng ( Q - ~ Q , every single detail of not be discussed, but rather a general making wine of the important chemical outline Mrillbe at tach step in sufficient detail to permita ~~
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novice to begin to appreciate the chemistry of winemaking. Wc will confine this discussion to wine from grapes, but the general principles can be applied to the wine of almost any other fruit, flower, or vegetable. Initial Preparation of the Must
The obvious first step in making winc is thc selection of the grape. While superior wines are usually the product of expensive grapes, some very good wines can be madc from rathcr inexpensive grapes. These are to be recommended for the beginner-a cheap failure is easier to rationalize than an expensive one. However, the common Thompson white seedless grapes are known to make a rathcr poor wine and should be avoided. The ripe grapes are crushed to frcc thc juice. The time honored method of barefeet in a washbasin is quite aceeptablc, hut very often a local farmer will provide the crushing..for.a.small fee. One gallon of wine is made from l l t 1 5 lb of ripe grapes. If a white wine is desired, the skins are immediately separated from the raw juice or must. Normally this is accomplished with a wine press. However, if small quantities of wine are involved, the skins can be placed in a large piece of washed muslin which is wrung tightly to squeeze all of the juicc from the skins. If a red or rose wine is preferred, the skins are kept in contact with the must for three or four d a m de~endinpw o n the color desired. At that time they ark separated as described above. The initial crushing and separating of the skins is best accomulished in sturdv nlastic basins or e a r b a ~ e~ a i l s . Metal should be avoided because trace amounts of the container might impart a metallic taste to the wine. After the skins have been removed the must is placed in a container where it will remain until bottled. We have found 5-gal glass carboys to be ideal. Although somewhat fragile, these are easy to obtain, inexpensive, and permit visual observations of the fermentation processes as they oecur. As will bc explained latcr, one extra carboy to the number in use a t any one time ... . in addition . . wlll be needed. The must of white wines can bc placed into the carboys immediately after separation of the skins. The must of red wines is best lcept in a large wide-mouth container (e.g., a garbage t ail) until the skins are separated, at which timeit may be transferred to the carboys. Before the fermentation starts. it is best not to fill the carboys more than 3/4 full because the must will expand and foam duringthe ensuing fermentation, The must is now analyzed,, and if necessary, adjusted in the concentration of dissolved sugars and acids. Thc Sugar concentration of the must will determine the final alcohol content of the wine. The yeasts which convert ~
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In the US.the production of wine is subject to the Federal Wine Ilegulation Law, Article 240.540 which states that up to 200 gal of wine may be produced snnually tax free, provided it is not far sale. Five days before any wine is started, Form 1541 must he filed wit,h the regional office of the Alcoholic and Tobacco Tax Ilivision of the Internal Service. Local authorities should also he consulted.
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the various sugars into alcohol and COzwill usually die in a solution which is greater than 12% by volume alcohol. Thus, fermentation will cease when either the yeasts die because of the 12% alcohol concentration, or the sugar is completely used up. If the latter occurs, a dry or non-sweet wine will result; on the other hand if excess sugar remains after fermeutation has ceased, a sweet wine will be produced. Thus, it is important to accurately determine the sugar content of the must prior to fermentation as this will greatly influence the type of wine which is produced. An ordinary hydrometer (or better still, a winemakers hydrometer) with the help of the table is usually used to determine the amount of natural sugars present in the must. If the specific gravity is lower than 1.070, then ordinary cane sugar (added as a syrup to ensure dissolution) is mixed into the must until a specific gravity in the region of 1.075 is obtained. On the other hand if a dry winc is desired from a must with specific gravity greater than 1.085, water is added to dilute the must. Sgecific Gravity Table
Before the sugar concentration is adjusted, it is wise to first determine the acid content of the must. The proper amount of several natural acids are necessary to assure a good wine. The most common acids found in grapes are tartaric, malic, and citric. The acidity of most wines is reported in terms of equivalents of titratable acid per liter of must. Usually 10.0 ml of must is diluted with 25 ml of water (more if the must is a dark red color), phenolphthalcin indicator added, and the solution titrated with 0.1 M NaOH. A satisfactory balance of acids is in the range of 0.10-0.17 meq of acid per ml of must. If the must measures lower than this, a mixture of tartaric, malic, and citric acids (in a ratio of 2 : 1 : 1) can be added to bring the acidity content into the desired range. While dilution with water or sugar solution is the most common way of lowering the acidity, it will have the effectof reducing the final flavor and aroma of the wine. After the acid and sugar concentrations have been checked and adjusted, a small quantity of sulfite solution is added to the must. The primary purpose of the sulfite is to protect the wine from bacteriological growth and subsequent infection. The bacteria, wild yeasts, and micro-organisms present in fresh musts have to be killed less they impart an off-flavor to the wine and promote spoilage or oxidation of the ethanol to acetic acid. The active chemical is SOz which is released when potassium metabisulfite is added to the slightly acidic solution.
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K&Or HzO 2KHSOa KHSOa H 3 0 + SO1 2H10
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releases 10 ppm SO,/gallon of must. Usually a dose of 50-100 ppm SOzis given to a new must prior to the onset of fermeutation. Fermentation
All of the above steps should be accomplished in fairly rapid succession before fermeutation starts. About 24 hr after the introduction of the sulfite, thc yeast is added. During this time the sulfite has gradually lost much of its potency and the yeast can quickly multiply in the must which is now free of the spoilagecausing organisms. Although the skins often contain many wild yeasts which invariably get mixed into the must, thc wincmaker is advised to add a known healthy strain of yeast suitable for inducing alcohol fermentation. The easiest strain of yeast to acquire is common dried baker's yeast,SaceharomycesCe~evisiae,whichissimply sprinkled onto the must (1 oz./S gal). Although many winemakers prefer to use S. Ellipsoideus, a strain of yeast specifically developed for winc, baker's yeast is a cheap, vigorous, and reliable strain which has a high alcohol toyerance, 12-14%. It is desirable to have the yeast multiply to a thriving colonv as auicklv as ~ossible. Aerobic conditions favor the g"rowth of the ;cast and often there is sufficient oxygen dissolved in the must to accomplish this desired rapid growth. However, anaerobic conditions are necessary for alcohol production and force the yeast to rely solely on the sugar and the fermentation process for its metabolic energy. When the fermentation starts (usually after a lag time of 2-3 dn), the must and the air spacc above it are quickly saturated with COXand no further oxygen is avilable to the yeast. The onset of active fermentation is noted by thc rising bubbles of COz. Since the yeast is quite temperature sensitive and the fermentation reactions exothermic, the temperature of thc must should not be permitted to rise above 27'C. I n general the lower the fermentation temperature, the longer the fermentation time, and the better the quality of winc. If a red wine is being made, the "cap" of skins and pulp which rise to the surface should be broken up arid vigorously stirred into the must twice a day. After the skins and pulp are separated, the red wine must be placed into a carboy and treated like a white wine. The chemical reactions which occur during the fermentation process are many in number and vcry complex in nature. They can be summarized by the overall reactiona
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C12H22011 H1O
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4CaHjOH
+ 4C02
(1)
But this one reaction is very misleading, for the many intermediates and minor reaction products help to give each wine its own distinctive body, bouquet, flavor, and appearance. Some of the more important fermentation reactions arc shown in the figure. Another important side reaction is the secondary fermentation which is the transformation of malic acid to lactic acid and occurs near the end of the fermentation process. This reaction causes a marked decrease in the total titratable acidity.
+Kt
The easiest way to work with the sulfite is to make a stock 0.5 M solution. One milliliter of this solution
2 A n interesting problem is to calculate the theoretical yield of ethsnol (as in the table) for a. known specific gravity and assuming reaction (1) is the only reaction of significance. Volume
49, Number 3, March 1972
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CHO
CHO
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fHOH
CHO
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CH20POIOHI
COOH
COOH
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CHOH
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L
0
COOH
C H O P O I O H 1 2
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CH20POIOH12
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COPOIOH);,
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CH20H
CH2
2-pholph0gly~eric mid
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phorpha-en01 psruvic acid
ADpd AT P
-C02
COOH
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COOH
The major route of sucrose fermentation. The flnal productr are underlined. Each step is enzyme catalyzed and the maior cofacton ore shown: ATP(odenorine triphosphotel, ADP l~denorinediphorphotel, NAD (micotinamide odenine dinucleotidel, ond NADH (NAD in the reduced form) 112, 171.
It is most pronounced in those wines produced from grapes with a high malic acid content such as from the Eastern U.S., Rhineland, Germany, and much of the Burgundy District of France; and is less important in the warm climate wines of Southern France, Italy, and California. As the fermentation proceeds, the sugar concentration slowly decreases while the alcohol content increases. This process can be followed by frequent measurements of the specific gravity. As the fermentation ratc slows down, the lees, a sediment of dead yeast cells and pulp, will begin to settle out. Near the end of the fermenta176
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tion period, usually 1-3 wk after its beginning, the very young wine is separated from the lees to avoid possible yeast cell hydrolysis which might impart an off-taste to the wine. This separation or racking, is accomplished with an elementary siphon. I n the first racking, but not in later ones, the wine is aerated by keeping the lower end of the hose above the level of the rising wine. The new air will help to reactivate the yeast colony and will use up any traces of sugar which remain. After racking the containers should be filled to the very top so as to minimize the surface to volume ratio, and a water trap attached so as to permit the escape of COX but prevent any air from coming in contact with the wine. This is important in order to prevent the oxidation of the ethanol to acetic acid and thus convert the wine to vinegar. As the fermentation ceases, a definite decrease in volume will he noticed. This shrinkage is due in part to the cooling of the must with the cessation of the exothermic fermentation reactions. To minimize the surface to volume ratio, i t is important that the containers he "topped-off" at least once per week with a similar type of wine. The total shrinkage seldom exceeds 100 ml/gallon of wine. Ageing and Bottling
The wine is now set in a cool, quiet place to start the long ageing process. Many subtle chemical reactions, such as the csterification between various alcohols and acids, will begin. This will reduce the harshness common in young wines, and give the wine its body and bouquet. The temperature of the wine during the winter months may he permitted to go rather low (down to 5°C) to encourage the precipitation of tartaric acid salts. Upon warming in the spring, secondary fermentation often resumes for a hrief time. The wine should be racked again in March and May. On these occasions the exclusion of air is important and 50 ppm SO2 is added to the wine to help prevent oxidation. After each racking the containers should he topped-off and the water traps replaced. If by April the wine still appears cloudy, the remaining sediment should he coagulated by a process called fining. Several fining agents are commercially available, hut an egg white can also be used. Using a white of egg/lO gal wine, i t is first beat into a froth with a small amount of water and then stirred into the wine. As it slowly settles, it removes the suspended material leaving a very clear wine. The wine is then rcmoved from the egg and sediment by raclcing. Once the wine has become clear and stable, it may be bottled. Any conventional winc bottle can he used, provided it is clcan and can he sealed air tight with either a wine cork or a plastic screw cap. The ageing or maturing process will continue in the bottle for at least several more ycars. Conclusion
The above description of winemaking is a very hrief outline of a most complex and well researched set of chemical reactions. For example, Chemical Abstracts currently lists about 300 entries a year under the subject of wine. Sub-entries include the analysis of, the biological and chemical reactions in, the propertics of,
and the preparation of wine. Despite this advanced state of knowledge and complexity, the chemistry of winemaking offers the possibility of a unique year-long demonstration; one which could he used at all levels of competance from the most advanced biochemical course down through high school. When compared to most chemical reactions used in demonstrations, few can bring the satisfaction of being able to taste and enjoy the final product in the way that winemaking can. Acknowledgment
The author appreciates the help offered by Professor
H. D. Durst. Literature Cited D., "The Commonsense Book of Wine," David McKay Co. Ino.. New York. 1964. (2) AMnnrwB. M. A,. Sei. Amer., 211 (No. 2). 46 (1964). (1) Anhue. L.
A n s ~ r w C., , "The Science of Wine." American Elaevier Publishing Co., New York, 1968. HOBNICKEL. E., "The Great Wines of Europe," G. P. Putman's Sons. New York, 1965. LICXINE,A,. "Encyclopedia of Winea and Spirits," Alfred A. Knopf, New York. 1967. O I I * ~ ,B.. "All About Wine," Thomas Y. Crowell Co.. New York, 1965. SCHOONMAKEB, F., "Encvlopedia of Wins," Hasting's Houae. New York, 1969. Wnuw. A,. "Wines and Spirits," Time Life Books, New York, 1968. AWERINE.M. A,. B m e , H. W., A N D CBTIEBB. W. V.. "The Technology of Wine Making," Ami Publishing Co., Westport. Conn., 1967. Boawsm, P., "Wine Makers Manual," Orange Judd Publishing Co., Ino.. New York. 1952. BFAVERI, H. E.. "Sueoessful Wine Making at Home." Arc Booka, Ino.. New York. 1961. Dn&a, P., A N D ACTON.B.. "Progredve Winemaking;' Published by "The Amateur Winemaker," Andover, Hsnts., Great Britain. 1967. H n n n m c ~ "Winemaking . a t Home." W. Funk. Ino.. New York. 1954. T*YLOR,W. S.. A N D VINE.R. P., "Home Winemaker's Handbook." Harper and Row. Publishers. Inc.. New York. 1968. TRITTON. S. M., "Am&teur Wine Making," Farber and Farber, Ltd.. London, 1968. W n o m n . P.M.. "Amsrioan Wines and Winemaking." Alfred A. Knopi. New York, 1967. MAHLER.H. R., A N D CORDES,E. H.. " B i o l o g i ~ ~Chemistry." l Hsrper and Row. Pubtiahem. New York. 1966.
Volume 49, Number 3, Morch 1972
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