Overview - ACS Symposium Series (ACS Publications)

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Overview

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Ethanol and Education: Alcohol as a Theme for Teaching Chemistry Roger Barth*,1 and Mark A. Benvenuto2 1Department

of Chemistry, West Chester University, West Chester, Pennsylvania 19383 2Department of Chemistry and Biochemistry, University of Detroit Mercy, Detroit, Michigan 48221-3038 *E-mail: [email protected].

Fermented beverages have been in use since early prehistory. Fermentation itself as well as the processes before and after it all have important chemical implications. The subdisciplines of chemistry play significant roles in the preparation, quality, and packaging of fermented beverages. Several key concepts in chemistry and related technical disciplines have origins in fermented beverage production.

Introduction Although the alcoholic beverage industry cultivates an image of traditional craftsmanship passed down the generations, it is in fact a highly technical endeavor. Production processes are constantly refined for efficiency, quality, and consistency. Each step is rich in chemistry, engineering, and biotechnology. Each offers opportunities for students to learn about aspects of chemistry. Every subdiscipline of chemistry is heavily represented. Because of its relevance and its ability to arouse student interest, teaching about the chemistry of fermented beverages is becoming more widespread in higher education. © 2015 American Chemical Society In Ethanol and Education: Alcohol as a Theme for Teaching Chemistry; Barth, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


History Alcohol goes back to the origins of humanity. It seems likely that when modern humans crossed the Red Sea from Africa to Asia, they were already equipped with the skills to ferment sugary liquids to alcoholic beverages (1). Chemical evidence for the deliberate production of alcoholic beverages dates from 9000 years ago in Neolithic China (2). Evidence for grape wine is reported dating at least 7000 years ago in the northern Mesopotamian region (3), but since grapes originate at least 500 km to the north, it is likely that wine is actually older (4). Barley beer has been found dating from 5500 years ago, also in the Mesopotamian region (5). When the invention of writing opened the book on history 5000 years ago, beer was already distributed throughout the Mesopotamian society (6). Distillation came much later. It was mentioned by Abu Musa Jabir ibn Hayyan “Geber” (721-815), an ancient chemist from the Moslem golden age whose writings influenced the teaching and practice of chemistry for centuries. Distilled beverages came into regular use in Germany in the late 1400’s (7). The production of alcoholic beverages has been a driving force for chemistry from its origins as a science. Antoine Lavoisier (1743-1794), in whose analytical balance chemistry took its modern form, described the fermentation reaction as grape must = carbonic acid + alcohol. Lavoisier is the author of the first modern chemistry textbook (8). The fascination with alcoholic fermentation and alcoholic beverages has not subsided; a search for “alcoholic beverage” on the Amazon® book site turns up 22,898 results.

Classification of Alcoholic Beverages All alcoholic beverages are produced by fermentation of simple sugars by microorganisms, most commonly a yeast, Saccharomyces cerevisiae or one of its close relatives, S. uvarum or S. pastorianus. A major classification is whether, the alcohol is significantly concentrated, either by distillation or freezing. A second major classification is the source of sugar for fermentation. The sugar can be provided by a natural source of sugar, such as fruit, honey, or sugar cane, or it can derive from the hydrolysis of starch. A myriad of secondary distinctions emerge when post-processes, such as addition of flavors or aging in barrels are considered.

Non-Concentrated Beverages The key distinction in this class of beverages is the type of the carbohydrate, sugar or starch. Some examples of non-concentrated beverages derived from sugar are given in Table 1. There are many flavored and hybrid varieties, each with its own name, and often more than one name.

2 In Ethanol and Education: Alcohol as a Theme for Teaching Chemistry; Barth, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Table 1. Non-Concentrated Beverages from Sugar


Sugar source

beverage

Fruit

wine, cider (apples), perry (pears)

Sugar

basi

Honey

mead

Milk

kumis

Tree sap

toddy, palm wine, maple wine

Cactus

pulque

Distinctions among non-concentrated beverages from starch take into account both the source of the starch and that of the hydrolysis enzymes. Some examples are given in Table 2. Sometimes, especially for beer, additional sources of starch or sugar are added.

Table 2. Non-Concentrated Beverages from Starch Starch source

hydrolysis enzymes

beverage

Barely, wheat, rye

malt

beer

Rice

mold

sake, huangjiu

Maize (corn)

saliva or malt

chicha

Manioc

saliva

masato

One feature of beverages from starch is the importance of water. The character of the beverage can be significantly affected by the trace compounds in the water used to hydrolyze the starch and extract the sugar. Often the chemistry of the water, especially its hardness and alkalinity, determine the type of beverage that can be optimally produced. Water chemistry, especially acid-base chemistry, is a branch of inorganic/physical chemistry that is a universal part of the chemistry curriculum.

Concentrated Beverages Some examples of concentrated beverages prepared from sugar sources are given in Table 3. 3 In Ethanol and Education: Alcohol as a Theme for Teaching Chemistry; Barth, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Table 3. Concentrated Beverages from Sugar Sugar source

beverage

Sugar, molasses

rum

Fruit

brandy, aquavit

Apples, conc. by freezing

applejack

Cactus

tequila, mezcal


Concentrated beverages from starch are sometimes distinguished by preparation technique in addition to starch source. Some examples are given in Table 4.

Table 4. Concentrated Beverages from Starch Starch and other details

beverage

Barley, maize, rye, wheat

whiskey

Grain or potatoes, distillation or filtration regulated to remove flavor compounds

vodka

Rice

baiju, shochu

Preparation of Fermented Beverages The details of preparation of fermented beverages vary widely, but a few broad generalizations can be made. A mixture of water and fermentable sugars is prepared. The mixture is allowed to come into contact with fermentation organisms, usually one of a few species of yeast. (9). The organism consumes the sugar and releases ethanol, carbon dioxide, and various side products that can be important to the flavor of the beverage. After fermentation, the beverage can be drunk directly out of the fermentation vessel, but more often there is post-fermentation processing of varying degrees of complexity.

Preparation of Fermentation Mixture For sugar-based beverages, such as wine, mead, or rum, the preparation of a fermentable mixture is largely mechanical. The plant material often needs to be ground or crushed, and sometimes cooked. The sugar may need to be dissolved or diluted to provide a suitable concentration for the fermentation organism. In a few cases the sugar is concentrated by boiling or freezing. The determination of the amount and nature of the sugars can be important in the production process, and can serve as practical problems for students. 4 In Ethanol and Education: Alcohol as a Theme for Teaching Chemistry; Barth, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


Starch-based beverages require more complex processing to effect the enzymatic hydrolysis of the starch to fermentable sugar. For many Western beverages the source of fermentation enzymes is malt. To make malt, seeds of grain are steeped in water to cause them to sprout. During sprouting, the seeds need moisture and oxygen. Cellular respiration releases heat which must be allowed to escape. During sprouting the embryo releases hormones that stimulate the production of amylolytic (starch hydrolyzing) and other enzymes. At an appropriate stage of sprouting the seeds are dried in a kiln, killing the embryo and stabilizing the malt. Malt can be stored for months before use (10). To make the fermentation solution (called wort) the malt is crushed and mixed with water, and possibly other sources of starch, at about 65-70° C. If beer is being prepared, the sugary liquid is separated from the spent grain and boiled, usually with hops, then chilled before fermentation. If a distilled beverage is being prepared, the wort may be fermented without separation. For Eastern rice beverages, the rice is cooked, then seeded with koji (Aspergillus oryzae), a mold that releases amylolytic enzymes. Often, in the case of rice beverages, the hydrolysis and fermentation take place simultaneously. Additional cooked rice is inoculated with a mixture of koji-bearing rice and yeast. After the hydrolysis/fermentation process, the liquid is squeezed out through cloth (11). A third method of starch hydrolysis is the exposure of ground grain to human saliva, which contains amylolytic enzymes. This is the traditional method of making chicha from maize. Although this process is sometimes called mastication, it does not involve chewing. The ground maize is dampened, put into the mouth to absorb saliva, then removed in the form of a wad, called muko, which is sun-dried for later use. When chicha is to be made, muko and other sources of starch are treated with hot water to hydrolyze the starch. After a separation process, the liquid is boiled, chilled and fermented (12). Fermentation The alcohol in alcoholic beverages is produced by fermentation of simple sugars by microorganisms, most commonly a yeast, Saccharomyces cerevisiae or one of its close relatives, S. uvarum or S. pastorianus. Other yeasts and even bacteria are occasionally used (13). In addition to ethanol and water, fermentation produces a variety of minor products that greatly influence the character of the beverage. Fermentation temperature is the most important variable that needs to be controlled. Sugar concentration, alcohol concentration, pH, hydrostatic pressure, and other factors play significant roles. For some beverages, including cider and some wines, a malo-lactic fermentation may take place subsequent to alcoholic fermentation. In this step, bacteria decarboxylate malic acid (hydroxybutanedioic acid) to the less sour lactic acid (2-hydroxypropanoic acid) (14). Alcoholic fermentation is a classic example of the glycolysis pathway of biochemistry. Some form of glycolysis occurs in every living cell. Alcoholic fermentation in the teaching laboratory is inexpensive, easy to implement, and provides samples for several analytical techniques as the process proceeds over a period of several days. 5 In Ethanol and Education: Alcohol as a Theme for Teaching Chemistry; Barth, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Post-Fermentation Processing


Carbonation Nearly all beer, cider, and a few varieties of wine are carbonated. Carbon dioxide is dissolved in the beverage under pressure. Most beverages are carbonated by the direct addition of food-grade carbon dioxide during packaging. Some are carbonated by the controlled addition of sugar before the package is sealed. The added sugar undergoes alcoholic fermentation, producing the dissolved carbon dioxide. Beverages carbonated in this way have yeast in the container. When present, carbon dioxide is a primary flavor compound (15). Carbonation provides a practical example of Henry’s law and of the van’t Hoff equation.

Aging/Conditioning Most alcoholic beverages undergo some sort of maturation process. Often clarifiers are added followed by filtration or sedimentation. Maturation can involve interaction with yeast, bacteria, or with wooden containers. It can be a matter of a few days to decades. Maturation for more than a few days becomes a dominant factor in the cost of the beverage. Lager beer is an interesting case. During low temperature fermentation, the yeast produces butanedione (diacetyl), and 2,3-pentanedione, which contribute a buttery off-flavor to the beer. Three weeks or more of conditioning in the presence of live yeast under refrigeration are needed for the yeast to convert these compounds to nearly flavorless alcohols. The brewery thus needs chilled maturation tanks for three weeks of production. This is a huge capital outlay for a relatively inexpensive product. Finding ways to reduce the conditioning time is a significant preoccupation of beer chemists. Some wines are aged for months or years. The aging reduces sourness and astringency, to some extent as a result of esterification of acids and precipitation of phenolic compounds. Distilled beverages are often aged in wooden barrels which, in addition to contributing flavors from the wood, also absorb undesired flavor compounds, such as higher alcohols.

Distillation Distillation is a critical factor in the character of beverages that use it. The process typically involves two runs through the still (or two stills). In the first run, the distillation is run until all volatile components have distilled over. The distillate is distilled again (more than once in some cases). The low boiling fraction (foreshots) and the high boiling fraction (feints) of the second distillation are discarded or reprocessed. The boiling point range for the accepted fraction determines the flavor components in the beverage. 6 In Ethanol and Education: Alcohol as a Theme for Teaching Chemistry; Barth, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Distillation is an exercise in vapor-liquid equilibrium, an important topic in physical chemistry. The effect of the configuration of the still on the extent of reflux of the distillate can explain the effect of different configurations on the character of the beverage.


Packaging Except in the case of brewpubs, alcoholic beverages are packaged in some way. Carbonated beverages must be packaged under pressure. In all cases the beverage must be sealed to prevent exposure to oxygen and microbes in the air and to prevent loss from evaporation. Hopped beer must be protected from light, which causes a photochemical reaction giving a skunky odor. In addition to protecting the product, commercial beverages must be packaged in ways that appeal to potential consumers. Glass, despite its high cost and weight, is preferred by many consumers. Wine packaged in anything else is perceived to be of low quality. Quaint packaging materials, like natural cork, which can be a significant threat to the quality of the wine, are still in use. Most beer is packaged in epoxy-lined aluminum cans, but the more expensive craft beers go into bottles.

Conclusion Fermented beverages involve every area of chemistry, including the history of chemistry. Many key issues in chemistry are readily addressed in the context of fermented beverages. Several familiar methods or concepts in chemistry originated with alcoholic beverage production. These include Kjeldahl’s method for proteins, the statistical Student’s T, and the concept of pH (16).

References 1. 2.

3. 4. 5. 6. 7. 8. 9.

McGovern, P. Uncorking the Past; University of California: Los Angeles, 2009; pp 15−17. McGovern, P.; Underhill, A. P.; Fang, H.; Luan, F.; Hall, G. R.; Yu, H.; Wang, C.; Cai, F.; Zhao, Z.; Feinman, G. M. Chemical Identification and Cultural Implications of a Mixed Fermented Beverage from Late Prehistoric China. Asian Perspect. 2005, 44 (2), 249–275. McGovern, P.; Glusker, D. L.; Exner, L. J.; Voigt, M. M. Neolithic Resinated Wine. Nature 1996, 381, 480–481. McGovern, P. Ancient Wine; Princeton University: Princeton, NJ, 2007. Michel, R. H.; McGovern, P.; Badler, V. R. The First Wine and Beer. Anal. Chem. 1993, 65 (8), 408A–413A. Standage, T. A History of the World in 6 Glasses; Walker: New York, 2005. Gately, I. Drink; Gotham: East Rutherford, NJ, 2008. Barnett, J. A. A History of Research on Yeasts 1. Yeast. 1998, 14, 1439–1451. Hornsey, I. S. Alcohol and its Role in the Evolution of Human Society; Royal Society of Chemistry: London, 2012; Chapter 2. 7 In Ethanol and Education: Alcohol as a Theme for Teaching Chemistry; Barth, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.


10. Hough, J. S. The Biotechnology of Malting and Brewing; Cambridge: New York, 1998; Chapter. 3. 11. Frost, G.; Gauntner, J. Saké pure + simple; Stone Bridge, Berkeley, CA, 2003; pp 20−33. 12. Buhner, S. H. Sacred and Herbal Healing Beers; Siris Books: Boulder, CO, 1998; pp 94−95. 13. Hornsey, I. S. Alcohol and its Role in the Evolution of Human Society; Royal Society of Chemistry: London, 2012; Chapter 2. 14. Hornsey, I. S. Alcohol and its Role in the Evolution of Human Society; Royal Society of Chemistry: London, 2012; pp 454−455. 15. Barth, R. The Science of Carbonation. Zymurgy 2013, 36 (6), 65–69. 16. Barth, R. The Chemistry of Beer: The Science in the Suds; Wiley: New York, 2013; Chapter 1.

8 In Ethanol and Education: Alcohol as a Theme for Teaching Chemistry; Barth, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Overview - ACS Symposium Series (ACS Publications)

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