David W. McClure Portland State University Portland, Oregon 97207
I
I
The Chemistry of Winemaking and Brewing -
As part of the University Special Summer Programs, i t has been possible during the last three years, for the author to teach a rather unusual course entitled "The Chemistry of Winemaking and Brewing." The intent of the course was to present the practical and technical basis for winemaking and brewing to an audience with an interest in pursuing these subjects a t a non-professional level. As it turns out, the course offers a significant enrollment potential from both University students and qualified individuals in the non-academic community. Furthermore such a topic has the merit of enticing students with little interest in science beyond the minimum chemistry mandatory for graduation. The purpose of this paper is to descrihe the author's experience with the course, and to briefly indicate some of the topics presented. General Course Description
The course consisted of approximately 20 hours of lecture and 12 hours of laboratory. The laboratory afforded the novice a sound introduction to the practical aspects of amateur enology and brewing while the lecture dealt for the most part with the chemistry of the subject. Viticulture was not discussed. Prerequisite to the course was one year of colleee chemistw. ~ p p r i x i m a t e l y?5% of the course was devoted to enology (including subjects of mutual importance to brewing) and the remainder to topics peculiar to brewing alone. In the laboratorv each student made one eallon of wine from a concentratk, and one gallon of beer fGm a malt extract. Relevant laboratory analyses (1,2) done by each student included total and volatile acid by titration, reducing sugar with the aid of the commercial Clinitester, alcohol, actual and predicted by hydrometry and ebulliometry, and the Ripper analysis of free and fixed SO? by an iodometric titration. Paper chromatography was used to demonstrate the separation of tartaric, malic, and citric acids. Wlnemaking The Commercial Process
The first step in the winemaking process involves the picking of suitably ripe grapes which are then crushed and de-stemmed. If the wine is to be white, the crushed grape skins are immediately pressed to remove the remaining juice. The pressed pulp is then discarded and the juice (called the "must") is sent on to the fermentation vat. If the wine is to be red (or ros6), the must (usually the same color as in the case of the white wine must since the pigmentation of most wine grapes is confined to the skins alone), pulp, skins, and seeds are added together to the fermentation vat. Once the must is in the fermentation vat, sulfur dioxide is added to kill wild yeasts and bacteria prior to the addition of a pure yeast culture. Also certain analyses are performed, including total acid and sugar, with adjustments in these quantities being made if required. If the wine is to be red, fermentation of the juice-pulp mixture is continued until sufficient color has been extracted from the skins. The fermenting must is then separated from the skins, seeds, and pulp which are in turn pressed to 70 / Journal of Chemical Education
remove any remaining juice. The partially fermented pulpfree juice is then allowed to ferment to completion producing approximately 12-14% alcohol. In the case of white wines, the fermentation is uninterrupted because the skins and seeds are not present and thus pressing is unnecessary. After fermentation is complete, the wine is "racked" (separated) off of the yeast to avoid autolysis (decomposition of the yeast), and stored in containers for aging. Volatile acid and SO2 are monitored during this time to ensure freedom from undue oxidation and bacterial infection. Before bottling, stability is ensured by "fining" with a colloid such as bentonite and by filtration. Only the most basic steps have been described, since details do vary depending on the winery and the growing region. For further details see ref. ( 1 ), chap. 13-17. Organic Acids Table 1 lists the approximate range of the major constituents found in the prefermentation must and the resulting wine. Of particular interest to the enologist is the kind and concentration of the plant acids, which can he seen to total roughly 1% by weight. We note that tartaric and malic acids predominate. In addition, succinic acid (formed by reduction of oxalacetic acid), lactic acid (from reduction of pyruvic acid), and acetic acid (formed by oxidation of acetaldehyde) make their appearance in the finished wine as either fermentation by-products or as the result of processes which can occur after fermentation. The fact that the data cover a broad range reflects the effect of the growing conditions on grape composition. I t is important to have a well defined acid concentration in the must to ensure both resistance to bacterial infection and a proper balance tastewise in the finished wine. Practicallv, one does not attempt t o control the exact acid distrib&ion in the pre-ferme&tion must, but rather adjusts, if necessary, the total acid content. Determination of the actual acid concentration is done by titration, usualIv with NaOH (-0.1 N ) t o either the phenoohthalein endpoint or, potentiometrically to a pH of8.2. Once the number of equivalents of acid has been found by titration, the acid content is reported in terms of an arTable 1. Percentage Major Constituents in Must and Win@ constituent ~ u r t wine ... ~~~
70-85
15-25 8-13 7-12 0.08-0.20
Dextrose ~evulose Pentorer ~thyl Methyl Higher Glycelol ~ ~ ~
~
d
D-tartaric L-ma~ic L.CitliC
succinic lactic acetic Tannins Nitrogenous C o m ~ o u n d r Mine*a' ComPoundr n F r ~ r nRef. ( 4 ) . 0 . 114.
&
s
0.05-0.1 0.08-0.20
Trace
8-15
0.0 0.0
0.01-0.02 0.008-0.012
0.0 ~
80-90 0.1-0.3 0.1-0.5
Trace 0.3-1.5
0.2-1.0
0.1-0.8 0.01-0.05 0
o
0.00-0.02 0.01-0.10 0.03-0.17 0.3-0.5
0.3F1.40 0.001-0.050
0.3-1.1 0.1-0.6
0.0-0.6 0.0-0.05 0.05-0.15 0.1-0.5
0.03-0.05 0.01-0.30 0.01-0.09 0.15-0.40
bitrary acid reference as if the wine or must was composed of that acid alone. Thus, for example, in the American literature the total titratable acid content is reported in terms of either percent or parts per thousand tartaric acid while in France the reference is sulfuric. The ideal total acid content of musts varies from about 6-9 ppt tartaric depending on the type of wine desired. If the must is low in acid, adjustment is made hy addition, say, pure tartaric acid or possihly a mixture of tartaric and malic. The various calculations required to determine the total acid concentration, interrelate acid conventions, and finally to correct the acid content, if required, all afford the student practice in a situation where he appropriately reaps the results of his mistakes. Acids, which are usually present in sound wines a t only low concentration, and not at all in the must, such as acetic, formic, butyric, etc. are termed "volatile acids" since they are determined by steam distillation and titration. Wineries monitor the volatile acidity of finished wines to ensure compliance with federal regulations. Large amounts (0.12-0.14% or more) of acetic acid, for example, may reflect a bacterial (Acetobacter) infection. A laboratory analysis of a commercial wine, using a Cash electric still, affords the student an acquaintance with the technique. Insufficient acidity in the must, as indicated, is easily corrected, but the opposite problem of excess acidity, common to cool northern climates such as parts of Germany and the Willamette Valley in Oregon, presents much more difficulty. A variety of remedial techniques, both chemical and biological, are discussed in the course. Ionic eauilihrium and buffers can be reviewed at this stage by rkquesting a quantitative explanation of the fact that most wines are about 0.1 N in acid and have a p H in the range 3-4. The importance of buffering can he appreciated when a calculation of the theoretical p H of a 0.1 N tartaric acid solution yields a value of about 2.2. If, however, one bases the calculation on potassium bitartrate (0.02-0.04 M ) a p H of 3.4-3.6 is found, illustrating the buffering role of dibasic salts. In addition, students are usually impressed by the fact that there is often no monotonic relationship between p H and titratable acidity for different wines just as there is no simple relation between acid taste and pH. Carbohydrates
Grapes grown in many parts of the world require the addition of sugar to the must prior to fermentation in order to guarantee an adequate alcohol content. A hvdrometer is &ally used to determine the initial sugar content of the must from which one can ascertain the amount of sugar - to add, if necessary. Once fermentation of the students' wine is complete (this takes approximately 1-3 weeks), the wine is analyzed in the laboratory for actual alcohol by use of a Dujardin ebulliometer which measures the boiling point of the mixture. Other standard methods of alcohol determination discussed include dichromate titration and density measurements (2). Tannins
Tannins play an important role in both winemaking and brewing. New red wines contain approximately 0.2% or less tannin, which, interestingly, decreases significantly as the wine ages. White wines, which are not fermented on the skins and seeds, contain only about one-tenth as much tannin as that of red wines. Tannins are discussed from the viewpoint of their properties and their origin as condensation products of such precursors as catechin and quercetin. Included are important reactions such as those with sugars, other tannins, and, particularly, proteins. The closely related topic of the grape skin pigments, such as the anthocyanidins and their glycosides is also presented.
Sulfur Dioxide
Sulfur dioxide, as previously indicated, is used as a sterilant prior to innoculating the must with a yeast culture. Depending on the winery, the source is either liquid SOz or a sulfite salt such as K&05 (metabisulfite). In any case, it is known that the effective sterilizing agent is free S02. Reactions between SO2 and aldose sugars, certain organic acids (e.g., pyruvic), and particularly aldehydes are discussed. The familiar reaction between acetaldehyde and SO2 to form the bisulfite addition product is especially important and accounts for approximately 75-100% of the "bound S02" in a wine, that is, SO2 which is not free in solution as hvdrated SO7. The acetaldehyde-skfite reaction is also responsible for the production of dvcerol durine fermentation. 1; order to ensuie an adequate but not excessive SO2 content, it is necessary to periodically monitor its level up until the time of bottling. The most common method of analysis is the iodiometric "Ripper" procedure (2), for which the reaction is
The free SO2 can be titrated directly whereas the bound SO2 is first hydrolyzed with NaOH, acidified, and then titrated. A free SO2 content of approximately 20-50 ppm (mg/l) is usually considered optimum. The only difficulty that students experience with the analysis is the obscure endpoint in red wines. Yeasts. Enzymes, andFermentation
The problem with these topics, especially yeast and fermentation, are their vast and rather specialized nature. For more than any other subject covered, the treatment rendered will depend on the background of the class and instructor (the author is a physical chemist), and the time available. In the case of yeasts, we chose to limit the topic to an elementary discussion of wine and beer yeast classification, their growth phases, and nutritional requirements. In the laboratory, such elementary microbiological techniques as yeast sterilization and propagation are demonstrated. Enzymes, on the other hand, provide a splendid opportunity to introduce a broad variety of new concepts such as the definition of an enzyme, proteins, denaturation, etc. as well as a review of basic freshman topics, including reaction rates as a function of temperature and p H , catalysis, activation enerev. ... . etc. Finally, fermentation was dealt n,ith in an ~lemenrary farhiun bv delineatine the Embden-Meverhoff.I'nmaa pathway. Fining
Fining is the process in which a "fining agent" such as gelatin or bentonite is added to a wine or beer to remove, by aggregation, haze-causing substances. These colloidal substances inchding pectins, gums, and tannins are presumed to be negatively charged in solution. Addition of a positively charged fining agent can thus promote flocculation and ~ r e c i ~ i t a t i o n . ~ h e m i c n l l ~ ; t h iiss a nice topic as it pnwides the student u,ith an introduction to rolloidv including such conceots as electrical stability, isoelectric point, counter ions, protection, flocculation, etc. In conclusion, and, in order to give the reader some feel for the degree of research activity in the fields of Enology and Viticulture we note that ref. (31, which is a 109-page review article covering the two-year period from January 1969 to December 1970, contains some 848 references. According to ref. (3) there are some 150 current journals devoted entirely to the publication of research papers in Enology and Viticulture. A symposium on the chemistry of Volume 53. Number 2,February 1976 / 71
winemaking was held in 1973 a t the Dallas ACS meeting and has been recently published in the "Advances in Chemistry Series" (5).
Table 2. Names and Structures of t h e e and O Acids Name
Formula
Side Chain. R
Bsric Structure
Brewing
Introduction The origins of brewing are uncertain, but it is known that a form did exist in ancient Meso~otamiaconsiderablv before 3000 BC (6). Just how circukous the evolutionari development of hrewing must have been can be appreciated when one considers the modern process. Technicallv. brewine is rather more interestine than winemaking due t o thecomplexity of the pre-fermentation chemistrv. Our approach, in the class and here, is to discuss in turn
