edited bv:
chem I zupplement
MICHAEL R SLABAUGH
Weber State College Ogden. Utah 84408
The Production of Ethanol from Grain W. R. Oliver, R. J. Kempton, and H. A. Conner Northern Kentucky University, Highland Heights, KY 41076 Current enthusiasm for "gasohol"-90% gasoline, 10%ethyl alcohol-as a possible fuel source to reduce America's dependence on foreign oil has stimulated interest in one of mankind's oldest chemical processes: production of ethyl alcohol from sugars by fermentation. Although there are numerous alcohols in common use, the ubiquity of ethyl alcohol, C2H50H, is such that the word "alcohol" alone is usually understood to refer to ethyl alcohol. The IUPAC name is ethanol. As a consumer chemical, ethanol is the "active ingredient" in alcoholic beverages: beer, wine, and distilled liquors. In the not-too-distant past, it was the preponderant ingredient of many tonics, cough remedies, and other patent medicines and, undoubtedly, many of their therapeutic effects were due to this ingredient. The practice has not died; a currently popular nieht-time cold remedv contains 25% alcohol. bdustrially, ethanolis used as a solvent, as a germicide, and as an intermediate for the vrevaration of other chemicals such as ethyl acetate, ether, and vinegar. The two leading methods for the production of ethanol are hydration of ethylene and fermentation of sugars by yeast. The chief sources of sugars for fermentation are various starches from grain-thus the name "grain" alcohol-and molasses residue from sugar refining. The chief source of starch in the United States is corn. Europeans obtain most of their starch from potatoes while Asians use rice as their source. Brazil, the world leader in gasohol usage, obtains most of its starch from cassava roots, although sugar cane provides much of its alcohol. Archeological evidence indicates that fermentations have been carried out since at least 4200 B.C., although the first written evidence of the distillation of aqua vitae (literally, water of life) does not appear until the 12th century A.D. The first industrial scale alcohol distilleries were introduced in Europe in the 16th and 17th centuries. In the Americas, heverage spirits were first distilled from grain in 1640 on Staten Island in New York City, then the Dutch Colony of New Netherland. In recent years, total beuerage alcohol production in the United States has varied from about 6 to 8 X lo7 gallons per year. In contrast, 1.31 X SO9gallons of synthetic ethyl alcohol for industrial and laboratory use were produced in 1979, chiefly by the high pressure reaction between water and ethylene, eqn. (1). This method of ethanol production may hecome less desirable as the price of crude petroleum, the source of ethylene, rises. CHz=CHz
-
+ Hz0 BW',
10W-40W psi
acid catalyst
CHBCH~OH
(1)
Formation of Ethanol In Nature The ultimate source of the energy we derive from ethanol is the sun. The sun's energy is used to drive the photosynthesis of sugars, via what is called the Calvin cycle, named for Melvin
D Galactose
Maltose
Figure 1. The fermentable sugars.
Calvin. who received the Nobel Prize in 1961 for elucidatinr
runlight
hexose (CsHlzOs) + 1 8 Pi + l8ADP
+ 12NADPt
(2)
The most common hexose (a monosaccharide) is D-glucose, also called dextrose. D-glucose is fermentable to ethanol, as arc D-fructose, D-mannose, D-galactose, and the disaccharide, maltose (Fig. 1). Glucose, mannose, and galactose all have the same molecular formula and arrangement of atoms, hut differ in the way the hydroxyl groups are directed. These compounds are called stereoisomers. Sugars with the L configuration are the mirror image stereoisomers (enantiomers) of D sugars. Other disaccharides or polysaccharides must he converted to fermentable sugars by hydrolysis. Sucrose, the disaccharide found in beet sugar and molasses, is hydrolyzed to glucose and fructose by invertase, an enzyme found in yeast, eqn. (3) CnHzzOn
+ Hz0
mcrose
CsH12Ofi + CsH120e glucose fructose
(3)
While sucrose from cane molasses serves as the source of ethanol in much of the tropical and sub-tropical regions of the world, starch is the major source of fermentation ethanol in the temperate zones. I t is a polysaccharide which occurs in high concentrations in many cereal grains such as rice, corn, wheat, barley, and rye, and in certain tubers such as potatoes. Most starches contain two major components, amylose and amylopcctin. Both are polymers of D-glucose, hut they differ markedly in their physical behavior and in their reactions with enzymes and some chemical reagents. Volume 59
Number 1 January 1982
49
-
Figure 2. The arnylopectin fraction of starch. Arnylose does not have the a(1 6) branch points.
-
Amylose is a long unbranched polymer containing D-glucose units in a ( 1 4) linkages. (The orientation of the connecting oxygen at C-l in u linkares is below the ring. A 0 linkage has the oxygen above the ring.) Amylopectini~ig.2) contains relatively short chains of 20-25 1 4-linked a-D-glucose units, connected by 1 6 a-glucosidic bonds. Yeast does not ferment starch until after the starch has been gelatinized by cooking and then hydrolyzed to simple sugars. Both amylose and arnylopectin are converted to fermentable sugars-D-glucose and maltose-by the enzymes found in barley malt (sprouting or germinating barley). Fungal glucoamylase, a recently developed economical substitute for malt, yields only glucose. After starch has been converted to sugars, enzymes produced by yeasts convert the sugars to ethanol and carbon dioxide. Equation (4) summarizes alcoholic fermentation. an 11-sten process which occurs in yeasts. Except for the last two step;, which uroduce the ethanol, alcohol fermentation is identical to glycolysis, the metabolic pathway which produces lactic acid from glucose in other organisms.
-
-
2 ti 2 ADP
2 CH,-CH-COOH
I OH
+ 2 ATP +
2 H,O
Commercial Manufacture of Fermentation Ethanol The starch-containing grains, corn and rve, are cleaned bv screening in high velocity& andmilled (grolnd) to a specified particle size. Millinr breaks the outer cellulose protective wall around the kernel-and exposes the starch to the mashing process. The &shing process consists of cooking, for gelatinization of the starch, and conversion of the starch to surars-D-rlucose and maltose. The ground grain is slurriedwith water, stillage (the de-alcoholized liquid remaining from a previous distillation). (ore-conversion . , and a small amount of enzvme " .. enzyme) (Fig. 4). The pre-conversion enzyme (malt or microbial) reduces the viscosity of the mash by cleaving a portion of the gelatinized starch. The reduced viscosity is essential for efficient pumping and heat transfer. All of this enzyme activity is sacrificed during the subsequent cooking, but not before it has served its function. The addition of stillage is not always necessary; some producers add it to adjust the pH. Direct steam is admitted to the mixture which is cooked, with agitation, to yield a mash. Some distillers cook the mash in open (that is, unuressurized) vessels, a t about 100°C. while others shorten thecooking time by using closed tanks or tubular reactors under pressure with temperatures uu to 150°C. After a shok holding period, depending on the temperature of cooking, the mash is cooled to about 60°C and additional enzyme is added to hydrolyze the starch to fermentable sugars. After a conversion period of 30 minutes or less, the mixture is cooled to fermentation temperature, 18-35OC (Fig. 4), and yeast is added to convert the sugars to alcohol and carbon dioxide. Within 48-72 hours after the addition of the yeast to the mash, the hulk of the sugars will have been fermented, and the fermented mash, or "beer" as it is now called, is ready for distillation. (Note: This "beer" has no relationship to commercial beer which is brewed by an entirely different process.) A fermentation efficiency of 95% may he obtained, based on available surar. Side reactions account for -5% of the sugar. Factors affecting the side reactions are, inter alia, yeast characteristics, temperature, and bacterial contamination.
Lsctic acid 2 ADF
(4)
riuoholi rermm>Ir-
2 CH,CH,OII Ethanol
+ 2 CO, +
2
ATP
+
2 H,O
The most abundant carhohydrate, cellulose, has not sewed as a major source of ethanol. Cellulose (Fig. 31, a structural and cell-wall component of plants, is alinear polymer of D-glucose with P(1- 4) linkages. The only difference between starch and cellulose, both polymers of D-glucose, is that the linkages are d l 4) in starch and Oil 41 in cellulose. Cellulose is not degraded by the enzymksin malt or fungal glucoamylase, thus no fermentable sugars are formed. Severe conditions of acid hydrolysis can convert cellulose to sugars, but these can be fermented only after removal or neutralization of the acid from the hydrolysis mixture. Such a process has not been found to be economical, although the recent development of active cellulase enzymes which convert cellulose to sugars has led t o a renewed interest in cellulose as a source of ethanol. ~
-
-
U(1-4)
Figure 3.
50
bonds
Structure of cellulose.
Journal of Chemical Education
secondary prod& (known as-congeners) formed-during fermentation and retained during subsequent operations include a number of aldehydes, esters, higher alcohols (fuse1 oils), fatty acids, phenolics, aromatics, and a great many unidentified trace substances. The first distillation of the fermented mash is carried out in a beer still constructed with sieve plates and fitted with heat exchangers and condensers to condense the alcohol vapor and heat the incoming beer. Sophisticated regulatory devices maintain a constant flow of beer and steam to the still and cooline water to the condensers. Inflowine steam forces the ethanol vapor out of the top of the still intocondensers which convert the vapors to a liauid. A portion of the condensed vapors is returned to the still as reflux. The de-alcoholized beer, now called "stillage." is removed continuously from the bottom section of thestill by gravity or by pumping. Except for the starch, which has been converted to & h i 0 1 and carbon dioxide, all of the nutrients of the grain, plus those which have heen synthesized by the yeast, are found in the whole stillage. Part of the stillage may be returned to the grain cooker andlor to the fermentors, while the rest is dried and processed for sale as Distillers Dried Grain with Solubles (DDGS). The DDGS is widely used in dairy, beef. swine. and noultrv rations as a rich source of vroteins. be collected and marketed, essentiaiy none of the grain is wasted. The condensed product from a beer still, called "low wine," contains 55-75% ethanol, water, and less than 1% of other
Malt or Microbial Enzyme
other grains
which removes most of the acetaldehyde, acetal, and ethyl acetate, a fusel oil column which removes most of the fusel oil, and an extractive distillation column which removes additional quantities of these compounds. A rectifying column serves to reconcentrate the dilute ethanol from the extractive distillation to vield 95%-ethanol and at the Bame time removes additional quantities of impurities. Ethanol produced on such a system is quite pure as judged by its low absorbance toward ultraviolet light. This ethanol is used in the production of gin and vodka; production of ethanol suitable for gasohol requires removal of the last 5%of water to yield anhydrous (ahsolute) ethanol. This cannot be accomplished by simple distillation since the 95% ethanol-5% water forms an azeotrope, a constant-boiling mixture. The water is removed by adding benzene and distilling off a ternary azeotrope of water, benzene, and ethanol. Anhydrous ethanol is left in the still. This absolute ethanol is not consumed because of the likelihood that traces of henzene will be present. The alcoholic content of beverage alcohols is usually given in terms of proof, an archaic term used by early distillers of fermentation alcohol. Feed Supplement The "proof" of alcohol strength was demonstrated by pouring it over gunpowder and setting a match to it; at or above a concentration of eleuen parts of alcohol to ten parts of water by volume the gunpowder would ignite, thus urovidinr dramatic "proof" that the alcohol was potent. 1; the ~ n l ' t e dStates, the proof is twice the alcohol content by volume; thus 100 proof whiskey contains 50 percent ethanol. ~lcoholic beverages have long been subjected to regulations and taxation. The Babylonian Code of Hammurahi, -2000 B.C.. contained ~rovisionsfor the aualitv. sale. and use of fermented liquids. A clause in the Mag& cart; provided a standard of measurement for the sale of ale and wine. The English parliament first imposed a tax on distilled spirits in 1643, perhaps copying the action of the Direqtor General of New Netherland (New York), who imposed the first liquor tax in America in 1640, a date which coincides with this country's first distillation of ethanol. Tax-free ethanol may be p& chased by educational, scientific, and medical organizations, subject to approval of the U.S. Treasury, Bureau of Alcohol, Tobacco, and Firearms (BATF). Approval involves Use and Withdrawal Permits, a Bond, and the keeping of detailed records. Industries which use ethanol as a raw material purchase any one of a number of "denatured" ethanols, which are not subject to the excise tax., Denatured alcohol has been rendered unfit for heverage purposes by such additives as
'i Converter
A
+Residue
Low Wine
Rectifying Columns
I Stinage Tank
nigh Wine (Neutral Gram
I
Spirits)
1 1 Whiskey Barrels
Figure 4.
A
simplified diagram of a conventional whiskey distillery.
volatile organic compounds. The trace of impurities contains a multitude of compounds which are essential for taste and aroma, hut which must be removed for the production of vodka, gin, and alcohol for gasohol. For whiskey, the low wine is redistilled by a process called doubling. This process separates the alcohol from some of the high molecular weight cmnl~mu~ds: rhr I I M ~ alwnuls aud most uther cwngvurrs ~trrmeddurin: iermenration dist:ll uwr wit11the t:~hau~d.'l'hr rehu~rin~: r d d e s 3 "high wine" is then axxi in flew, charred, u hilt, m k barrels t o pmduw I h u r h m . 'l'hcw h3rrel, t ouier 11:rwr and calm to the pruduct. \\'hiskq cannor he vdllrd Buurbun unlwi it is agtd in new ~~~~~~~~~~oak l~ilrrel-.\Ian!. , i t h e uwd Buurhun la~rn.lidreahil~lretl t,~5,, t l u t d or Catlatli~ for the aging of other whiskeys. For gasohol and for beverage alcohol (Neutral Grain Spirits), a doubler is not used. Instead, the low wine is passed through a multicolurnn system to yield a 95% ethanol-5% water mixture containing only traces of other compounds (Fig. 4). The multicolumn system may contain a heads column
Volume 59
Number 1 January 1982
51
sodium and potassium iodide, methanol, esters such as ethyl acetate, or hydrocarbons such as benzene. Today, research continues on all aspects of ethanol production. In particular, extensive research is being directed toward improving the net energy balance in obtaining ethanol. This has been a matter of controversy; it appears that more energy is required to produce a gallon of ethanol than is obtained from its combustion. Efforts are being made to improve the distillation process to achieve a more favorable energy balance. Recombinant DNA techniques are also being investigated in an attempt to create more efficient strains of yeast. With this renewed interest in ethanol production, it can be safely stated that production of ethanol from grain will con-
52
Journal of Chemical Education
tinue to be an important process, not only for beverage and laboratory use, but for use as an energy supplement to petroleum. References
"Kirk-Othrner Encyclopedia alChrmicalTochnolnm? 3rd ed.,.lohn Wiley and Sons.. N.Y., 1978, Vol. 3, PO. 830-863. Ishninger. A. L.,"Biocherniatry? 2nd ed., Worth Fnbliiheri, New York.1975, pp. 418.419, pp. 62%6R6. Marsha1l.J. d.."TheStructure, Function and MetabolisrnufCereal Carhohydrutes; W o l ~ Ierrtrin Carnrnunicaiions. 35, RS(April.1972).
