Process for the Hydrolysis of Starch and the Continuous Fermentation of the Sugars Obtained Therefrom to Provide Ethanol

This application is related to commonly assigned copending U.S. patent application Ser. Nos. 043,191 and 043,190, each filed May 29, 1979 and entitled "process for the Hydrolysis of Starch and Hydrolysates obtained therefrom" and "Fermentation Process", respectively.

1. Field of the Invention

This invention relates to processes for hydrolyzing starches and more particularly, to such processes especially adapted to provide substrate sugars for the fermentation of ethanol.

2. Description of the Prior Art

With the ever-increasing depletion of economically recoverable petroleum reserves, the production of ethanol from vegetative sources as a partial or complete replacement for conventional fossil-based liquid fuels becomes more attractive. In some areas, the economic and technical feasibility of using a 90% unleaded gasoline-10% anhydrous ethanol blend ("gasohol") has shown encouraging results. According to a recent study, gasohol powered automobiles have averaged a 5% reduction in fuel compared to unleaded gasoline powered vehicles and have emitted one-third less carbon monoxide than the latter. In addition to offering promise as a practical and efficient fuel, biomass-derived ethanol in large quantities and at a competitive price has the potential in some areas for replacing certain petroleum-based chemical feedstocks. Thus, for example, ethanol can be catalytically dehydrated to ethylene, one of the most important of all chemical raw materials both in terms of quantity and versitility.

The various operations in processes for obtaining ethanol from such recurring sources as cellulose, cane sugar, amylaceous grains and tubers, e.g., the separation of starch granules from non-carbohydrate plant matter and other extraneous substances, the chemical and/or enzymatic hydrolysis of starch to fermentable sugar (liquefaction and saccharification), the fermentation of sugar to a dilute solution of ethanol ("beer") and the recovery of anhydrous ethanol by distillation, have been modified in numerous ways to achieve improvements in product yield, production rates and so forth. For ethanol to realize its vast potential as a partial or total substitute for petroleum fuels or as a substitute chemical feedstock, it is necessary that the manufacturing process be as efficient in the use of energy as possible so as to maximize the energy return for the amount of ethanol produced and enhance the standing of the ethanol as an economically viable replacement for petroleum based raw materials. To date, however, relatively little concern has been given to the energy requirements for manufacturing ethanol from biomass and consequently, little effort has been made to minimize the thermal expenditure for carrying out any of the discrete operations involved in the manufacture of ethanol from vegetative sources.

The substitution of alcohol for at least a portion of petroleum based fuels is particularly critical for developing economies where proven domestic petroleum reserves are limited, such as in India and Brazil and these nations have therefore increasingly emphasized the production of alcohol from vegetative sources. The most common such operation employs cane sugar in a fermentation-distillation operation which conveniently utilizes the bagasse by-product as a fuel source. Cassava or manioc (Manihot utilissima Pohl) as a source of starch has also been considered for conversion into alcohol (see "Brazil's National Alcohol Programme", Jackson, ed. Process Biochemistry, June, 1976, pages 29-30; "Ethyl Alcohol from Cassava", Teixeira et al. Industrial and Engineering Chemistry pp. 1781-1783 (1950); and United Kingdom Patent Specification No. 1,277,002). However, since manioc lacks the equivalent of sugar cane's bagasse, the fuel for alcohol conversion must come from an external source. Thus, to make manioc root an economically attractive source of ethanol, it is essential to achieve rapid and high levels of conversion of the starch content to fermentable saccharide and of the fermentable saccharide to ethanol with high levels of thermal efficiency and at conservative plant investment and operating costs.

Processes for the liquefaction and saccharification of starch to provide fermentable saccharides are well known (viz., U.S. Pat. Nos. 2,219,668; 2,289,808; 2,356,218; 2,431,004; 2,676,905; 2,954,304; 3,308,037; 3,337,414; 3,423,239; 3,425,909; 3,551,293; 3,565,764; 3,591,454; 3,592,734; 3,654,081; 3,720,583; 3,910,820; 3,912,590; 3,922,196; 3,922,197, 3,922,198; 3,922,199; 3,922,200; 3,922,201; 3,969,538; 3,988,204; 3,922,261, 3,966,107; 3,998,696; 4,014,743; 4,016,038; 4,017,363; 4,028;186; and 4,032,403; see also, Novo Industri A/S (DK-2880 Bagsvaerd, Denmark) brochures entitled "Dextrose and Starch Sugar", "Conversion of Starch" and "glucose Syrups"). The hydrolysis of manioc root starch with mineral acid preparative to fermentation of the resulting sugar to produce ethanol has been investigated (see "Tapioca as a Source of Alcohol", Krishnamurti, #.G. 1960, Current Science 9:346-348). However, the hydrolysis resulted in the consumption of acid and additional hydrolysis could not be effected without the addition of fresh acid. Moreover, under the conditions employed (heating at 50-60 p.s.i. with 2% sulfuric acid for 4 hours), the hydrolyzed starch solution developed a dark color and a burnt smell which would signal saccharide degradation. Hydrolysis of the starch with lower amounts of sulfuric acid, i.e., 0.5% and 1.0% respectively, under the foregoing conditions failed to provide complete hydrolysis.

Processes for the continuous fermentation of sugars to provide alcohol are well known (viz., U.S. Pat. Nos. 2,155,134; 2,371,208; 2,967,107; 3,015,612; 3,078,166; 3,093,548; 3,177,005; 3,201,328; 3,207,605; 3,207,606; 3,219,319; 3,234,026; 3,413,124; 3,528,889; 3,575,813; 3,591,454; 3,705,841; 3,737,323; and 3,940,492; "Process Design and Economic Studies of Alternative Fermentation Methods for the Production of Ethanol", Cysewski, et al. Biotechnology and Bioengineering, Vol. xx, Pp. 1421-1444 (1978)). In a typical continuous fermentation process, a stream of sterile sugar liquor and a quantity of yeast cells are introduced into the first of a battery of fermentation vessels wherein initial fermentation takes place, generally under conditions favoring rapid cell growth. The partial fermentate admixed with yeast cells is continuously withdrawn from the first fermentation vessels wherein fermentation is carried out under conditions favoring the rapid conversion of sugar to ethanol. The yeast in the last fermentation vessel can be recovered by suitable means, e.g., centrifugation or settlement, and recycled. In such a system, the ability of the fermentation organism to produce ethanol is affected by the ethanol and sugar concentrations. As a rule, a yeast which gives high conversion rates of sugar to ethanol in a low-ethanol, high-sugar fermentation medium will only sluggishly produce ethanol under the opposite conditions, i.e., at high-ethanol level, low-sugar concentrations.

The composition of manioc is similar to other tropical starchy roots in that the bulk of the dried matter is carbohydrate, about 66-72% of which is starch in the form of granules of about 5 to 35.mu. in dimension. Starch granules comprise amylose, a straight chain polymerized maltose and amylopectin, a branched chain polymerized maltose. However, cassava starch is distinguished from common sources of starch by its relatively low content, e.g., 17% of amylose as compared to potato starch (22%) and corn starch (27%). Its corresponding relatively large percentage of branched chain amylopectin imparts different properties; and yet it is not a typical amylopectin starch, rendering its treatment as in saccharification, somewhat unique.

Accordingly, there has heretofore existed a need for a process for hydrolyzing manioc root starch and starches from other sources at rapid and high levels of conversion without any significant degradation of the resulting saccharide and at only a modest expenditure of thermal energy and of utilizing the saccharide in a thermally efficient, rapid continuous fermentation process to provide industrial ethanol at competitive prices.

In accordance with the present invention, an aqueous slurry of starch is first liquefied, i.e., converted to a pumpable partial hydrolysate, employing a liquefying agent selected from the group consisting of strong acid and liquefying enzyme, the partial hydrolysate is thereafter saccharified in a primary saccharification vessel or vessels in the presence of a saccharifying enzyme for a period of time sufficient to convert from about 60 to about 80 weight percent of the original starch present (calculated on a dry basis) to fermentable sugar with the remaining portion of the original starch being present in the form of partial hydrolysate, the fermentable sugar is introduced, with or without the partial hydrolysate therein having been previously further saccharified in a secondary saccharification vessel or vessels, into a series of fermentation vessels wherein fermentation of sugar to ethanol by yeast and the saccharification of partial hydrolysate, if present, to fermented sugar takes place and wherein the ethanol content of the fermentation medium in each fermentation vessel is progressively increased as the sugar content of the fermentation medium is consumed, there being at least two strains of yeast selected for the fermentation, one of which provides a high rate of ethanol production in a fermentation medium containing a relatively low concentration of ethanol and a relatively high concentration of fermentable sugar and the other of which provides a high rate of ethanol production in a fermentation medium containing a relatively high concentration of ethanol and a relatively low concentration of fermentable sugar.

Liquefaction is rapidly accomplished at elevated temperature and pressure employing a strong acid such as hydrochloric acid to effect partial hydrolysis, a liquefying enzyme such as alpha-amylase or a two-step procedure employing acid for initial liquefaction and liquefying enzyme for further liquefaction. Following the cooling and reduction in pressure of the liquefied starch, saccharification is carried out in the presence of a dextrogenic enzyme such as amyloglucosidase until conversion of starch to fermentable sugar has reached a level of from about 60 to 80 weight percent.

When acid is used in the liquefaction step, neutralization of the partially hydrolyzed starch medium prior to the addition of any enzymes which effect further liquefaction and/or saccharification is generally necessary as the enzymes work best only at higher levels of pH. It is a feature of the present invention to employ at least one acid selected from the group consisting of nitric acid, sulfuric acid, hydrochloric acid and phosphoric acid in the liquefaction stage and neutralizing the acid with ammonia prior to the addition of enzyme to accomplish further hydrolysis of the starch. The ammonium salt which is formed as a result of the neutralization of the liquefying acid is retained in the fermentable sugar liquid and serves to satisfy part of the nutritive requirements of the yeast employed in the fermentation of the sugar liquid to ethanol.

While the process of the invention herein is especially advantageous for converting manioc root starch to fermentable sugar, the process is equally applicable to the hydrolysis of starch from other sources such as corn, sorghum, wheat, potatoes, rice, mylo, and the like.

The term "strong acid" as employed herein refers to any of the inorganic acids having a pKa value of at least about 2.5 or less. The term "fermentable sugar" should be understood as referring to a single fermentable sugar such as dextrose, maltose or isomaltose but more commonly will be applicable to these and similar fermentable saccharides in admixture.

The process herein also contemplates the adjustment of temperature and/or pH in each fermentation vessel as required to maintain optimum fermentation activity therein. To conserve raw materials and direct yeast metabolic activity to the production of ethanol rather than cell growth and propagation, a portion of the yeast is continuously recycled and additional fresh yeast is added only as is necessary to replace dead cells. The aqueous ethanol or "beer" containing as much as about 12 weight percent ethanol which is obtained by the foregoing process can be concentrated employing any of the known and conventional techniques and is advantageously concentrated by the anhydrous distillation process disclosed in commonly assigned copending U.S. patent application Ser. No. 043,189, filed May 29, 1979. The stillage effluent obtained from the rectifying column employed in the aforesaid distillation process contains soluble proteins and amino acids of the original beer feed and provides an excellent source of nutrient for yeast employed in the fermentation process herein.

Employing two or more organisms which maintain high rates of ethanol production in the presence of different concentrations of ethanol and fermentable sugar provides a faster, more efficient fermentation than that attainable employing a single strain of yeast in each fermentation vessel as is the current practice. As such, the fermentation process of this invention is particularly well suited for the production of ethanol which is price competitive with ethanol produced from non-vegetative sources.

In the accompanying drawings,

FIG. 1A is a diagrammatic flow sheet illustrating the starch hydrolysis stage of the invention and

FIG. 1B is a diagrammatic flow sheet illustrating the fermentation stage of the invention.

Referring to FIG. 1A, a concentrated aqueous slurry of manioc root starch which contains from about 20 to about 50 weight percent dry substance (D.S.), and preferably from about 30 to about 40 weight percent D.S., and which may also contain other components of the root such as water soluble proteins, fats, sugars and minerals and/or water insoluble materials such as fiber, minute amounts of gravel, etc., is delivered from starch slurry tank 10 by pump 11 through line 12 to steam jet mixer 13 where it is combined with steam and thereafter passed through starch liquefier 14. Pump 11 provides a discharge pressure which is substantially greater than the saturation pressure of steam at the temperature to which the slurry is heated in starch liquefier 14. Prior to introduction into steam jet mixer 13, the starch slurry is combined with a strong acid as partial, but preferably, exclusive liquefying agent to provide a pH of from about 1.0 to about 2.5, and preferably from about 1.2 to 2.2. Suitable acids include nitric acid, sulfuric acid, hydrochloric acid and phosphoric acid. The acid is supplied from storage vessel 15 where it is moved by pump 16 through line 17 to be mixed with the starch slurry passing through line 12. When employing acid as the liquefying agent, the amount of steam introduced into the acidified starch slurry through steam jet mixer 13 is sufficient to provide a temperature in starch liquefier 14 which is in the range of from about 160.degree. F. to about 350.degree. F., in which range the pressure of the steam can vary from about 25 psig to about 250 psig. Preferably, starch liquefier 14 is operated within the range of from about 200.degree. F. to about 250.degree. F. Residence time of the acidified starch slurry in liquefier 14 to effect partial hydrolysis and sterilization of the starch can vary from about 2 to about 15 minutes and preferably from 5 to about 10 minutes. Emerging from liquefier 14, the acidified starch is neutralized with liquid or aqueous ammonia conveyed from tank 18 by pump 19 through line 20. When initial starch liquefaction is accomplished with acid followed by a further liquefaction with enzyme, the pH of the neutralized slurry will be adjusted to a level which promotes optimum enzymatic activity in the liquefying enzyme which is later added to the initially liquefied starch in flash tank 21 or holding tank 27 to complete liquefaction. For many liquefying enzymes, the pH of the neutralized liquefied starch is advantageously within the range of from about 3.0 to about 6.0 and preferably from about 4.0 to about 5.0. The ammonium nitrate, sulfate, chloride and/or phosphate which is produced by neutralization of the acid liquefying agent(s) is retained in the product fermentable sugar liquid produced by the process herein in order to satisfy a nutritional need of the yeast which is used for the conversion of the sugars to ethanol. Before neutralization, the initially liquefied starch is introduced into flash tank 21 where steam is flashed to adiabatically cool the liquid mass, preferably to about 212.degree. F. The vapors discharged from flash tank 21 through line 22 contain a small amount of hydrogen cyanide gas generated from the hydrolysis of cyanogenic glucosides present in the manioc root. These vapors may, if local regulations permit, be discharged therefrom to the upper atmosphere with the aid of steam. Alternatively, the vapors are passed through line 23 into direct contact water jet condenser 24 supplied with cold water through line 25, with the liquid condensate passing to sump sewer 26. In the event the cooled depressurized liquefied starch from flash tank 21 requires an enzymatic treatment to complete liquefaction, it is conveyed to holding tank 27 which may be integrated with atmospheric flash tank 21 to form a single unit or, as shown, may be provided as a separate vessel where enzymatic liquefaction is permitted to take place. The liquefied starch is combined with from about 0.3 to about 2.0 lb. per 1000 lb. of dry starch, and preferably from about 0.5 to about 1.0 lb. per 1000 lb of dry starch, of liquefying enzyme such as alpha-amylase delivered from storage vessel 28 by pump 29 through line 30(b) to flash tank 21 or through line 30(c) to holding tank 27. The enzyme-containing partial hydrolysate is held at about 212.degree. F. in holding tank 27 for from about five minutes to about three hours or until a level of from about 12 to about 24 dextrose equivalent (D.E.), and preferably from about 16 to about 20 D.E., has been attained. It is, of course, within the scope of this invention to employ acid as the sole starch liquefying agent in which case there will be no need to add enzyme to the cooled depressurized starch obtained in flash tank 21 nor will there by any need to provide a holding vessel such as holding tank 27.

It is further within the scope of this invention to employ liquefying enzyme exclusively for the liquefaction step thereby dispensing with an acid neutralization procedure. In such a case, the entire amount of liquefying enzyme, e.g., from about 0.3 to about 3.0 lb. per 1000 lb. of dry starch, and preferably from about 0.5 to about 1.0 lb. per 1000 lb. of dry starch, can be added to the starch slurry prior to the passage of the slurry through starch liquefier 14. Starch liquefier 14 is advantageously maintained at a temperature of from about 160.degree. F. to about 250.degree. F., and preferably at from about 200.degree. F. to about 230.degree. F. It is preferred to adjust the pH of the starch slurry, either before, with or following addition of the liquefying enzyme, but before significant liquefaction has occurred, to a pH level of from about 3.0 to about 6.0, and preferably from about 4.0 to about 5.0, advantageously employing any of the aforementioned strong acids. However, since some of the liquefying enzyme may be inactivated or destroyed at the highe operating temperature of starch liquefier 14, it is preferred to add only as much enzyme at this location as is needed to obtain a pumpable partially liquefied starch at the discharge end of the liquefier, e.g., from about 1/4 to about 1/3 the total amount, with the remaining amount of liquefying enzyme being added to the partially liquefied starch in flash tank 21 through line 30(b) or in holding tank through line 30(c).

The liquefied starch from flash tank 21 (if an acid liquefaction alone was enmployed) or in holding tank 27 (if enzymatic liquefaction or combined acid-enzymatic liquefaction was employed) is conveyed by pump 31 through line 32 to vacuum flash tank 33 where the liquid mass is further cooled to from about 130.degree. F. to about 160.degree. F., and preferably from about 140.degree. F. to about 145.degree. F. The vapors discharged from flash tank 33 are condensed in direct contact water jet condenser 34 supplied with cold water through line 25 to maintain a vacuum of from about 1 to about 5 psig, and preferably from about 2 to about 4 psig, in flash tank 33. The liquid condensate from water jet condenser 34 passes to sump sewer 26. From vacuum flash tank 33, the liquefied starch passes through line 35 where it is combined with a saccharifying enzyme such as amyloglucosidase, preferably containing a saccharification catalyst such as a source of calcium ion, delivered from storage vessel 36 by pump 37 through line 38 and the saccharifying enzyme-containing liquefied starch is then delivered by pump 39 through line 40 into temperature regulated, agitated primary saccharification vessel 41. Prior to addition of the saccharifying enzyme, it is preferred to adjust the pH of the liquefied starch (to promote maximum enzyme activity) to a level of from about 4.0 to about 5.0, and more preferably to a level of from about 4.3 to 4.7. This will generally require the addition of base since the liquefied starch will usually be somewhat more acidic than the aforestated pH ranges. It is preferred to employ ammonia or aqueous ammonia to accomplish the pH adjustment since the resulting ammonium salt will be a useful nutrient for the yeast which is used to convert the sugar liquid herein to ethanol. Saccharification in vessel 41 is advantageously maintained at a temperature which is most favorable to maximum enzyme activity, generally in the range of from about 140.degree. F. to 145.degree. F. The saccharification is permitted to proceed in vessel 41 only until such time as about 60 to about 70 weight percent fermentable sugar is obtained. Depending upon the saccharification conditions, this level of fermented sugar can be reached within about two to about ten hours and more usually, four to eight hours, of saccharifying time. In contrast to this, known and conventional saccharification processes are carried out in the same vessel until the maximum amount of fermentable sugar is obtained, i.e., about 92 to 96 weight percent. Since the rate of saccharification falls off rather abruptly after only about eight hours, such a high conversion level of liquefied starch to fermentable sugar can only be achieved over a fairly long period of time, generally from about 24 to 72 hours. These lengthy saccharifying times have not been of concern to the sugar and alcoholic beverage industries where starch conversion is widely practiced since the quality of the end product is of paramount concern. However, such saccharifying times are a serious obstacle to realizing an efficient and rapid process for providing low cost fermentable sugar which in turn will provide low cost ethanol upon fermentation. Since the saccharified liquid herein containing from about 60 to about 70 weight percent fermentable sugar can be conveyed from saccharification vessel 41 by pump 42 through line 43 directly into the series of continuous fermentation vessels of FIG. 1B where ethanol production side-by-side with saccharification of the remaining partial hydrolysates of the starch takes place, from three to nine times as much starch can be effectively processed in accordance with the invention herein as in the processes of the prior art without requiring a multi-fold increase in plant equipment expenditure and operating costs. If desired, all or a part of the sugar liquid from primary saccharification vessel 41 can be conveyed by pump 42 through line 44 into one or more temperature regulated, agitated secondary saccharification vessels 45a and 45b where further saccharification can be carried out. The total volume V.sub.2 of secondary saccharification vessels 45a and 45b must be at least equal to:

wherein V.sub.1 is the total volume of fermentation medium in primary saccharification vessel 41, x is the time period (e.g. number of hours) of saccharification in vessel 45a (which will be the same for vessel 45b) and y is the time period of saccharification in vessel 41. Thus, for example, when saccharification is carried out in vessel 41 for a period of eight hours and the hydrolysate therefrom is further saccharified in vessel 45a for a period of sixteen hours (i.e., for a fermentable sugar content of from about 85 to about 90 weight percent), the total volume of vessels 45a and 45b must be at least twice that of vessel 41 in order to accommodate the volume of liquid processed by primary saccharification vessel 41 over a period of sixteen hours. The combined sugar liquid from secondary saccharification vessels 45a and 45b is conveyed by pump 46 through line 47 either to the series of fermentation vessels of FIG. 1B for ethanol production or to temporary storage prior to introduction into the aforesaid fermentation vessels. If storage is contemplated, the sugar liquid should be maintained at least about 140.degree. F. to inhibit any repolymerization of partial hydrolysates contained therein (i.e., "starch retrogradation"). If a starch slurry is employed in the foregoing hydrolysis stage which contains insoluble matter, such matter should be separated therefrom employing known and conventional techniques, e.g., filtration, centrifugation, etc, prior to the use of the sugar in fermentation in order to prevent the accumulation of such matter in the fermentation vessel(s).

The starch hydrolysis stage of this invention is further illustrated by the following examples:

1.0 ml of Termanyl 60 L (an alpha-amylase liquefying enzyme from Novo Industri A/S) was added to 120.degree. F. tap water. A 30 weight percent aqueous slurry of manioc root starch was added to the enzyme all at once, pH was adjusted to 6.5 and the mixture was heated from 200.degree. F. to 220.degree. F. for two hours. Samples of the liquefied starch were taken at various time intervals, the amounts of reducing sugar and dextrose being measured as follows:

The above liquefied starch samples were saccharified at 140.degree. F. and a pH of 4.5 with 3 ml AMG 150 L (an amyloglusidase saccharifying enzyme from Novo) per 1000 g. starch D.S. Samples of the saccharified starch were taken at various intervals, the amounts of reducing sugar and dextrose being measured as follows:

The starch liquefaction procedure of Example 1 was repeated except that the starch was added, at equal amounts, in three steps, at times 0, 25 and 45 minutes. (The next portion of starch was added in dry form, when the viscosity of the dextrin in the reaction vessel had decreased). Assay was as follows:

Following the saccharification procedure of Example 1, the above liquefied starch samples were saccharified with the following results:

Referring to FIG. 1B, the sterile aqueous solution of fermentable sugar resulting from the aforedescribed hydrolysis stage, containing from about 10 to about 40 weight percent sugar, and preferably from about 15 to about 25 weight percent sugar, is taken from vessel 50 which can be a storage vessel or a saccharification vessel, and is delivered by pump 51 through line 52 to a first temperature regulated, agitated fermentation vessel 53 provided with pH control and means for introducing nutrients and the small amounts of oxygen conventionally employed for maintaining proper yeast metabolism during fermentation. In the event the sugar solution contains more than 20 weight percent sugar, it is preferable to dilute the solution to about this level of sugar, advantageously with the nitrogen-rich stillage obtained from an ethanol distillation unit such as described in the aforesaid Ser. No. 043,189, filed May 29, 1979. The use of stillage when available possesses the two-fold advantage of recycling nitrogen to the fermentation system which would otherwise be lost upon concentration of the ethanol during distillation, and reducing process water consumption by avoiding water build-up in the still bottoms. The hydrolysate may, in addition to sugar, contain significant amounts of partial starch hydrolysates (e.g., up to about 40 weight percent of the total carbohydrate present) which can be saccharified to fermentable sugar under the influence of the saccharifying enzyme produced by the fermenting yeast and/or added saccharifying enzyme. A pumpable slurry of ethanol-producing yeast organisms free of contaminating organisms is conveyed from yeast storage tank 54 by pump 55 through lines 56 and 57 into fermentation vessel 53. The yeast selected for introduction in fermentation vessel 53 is one which provides high rates of ethanol production in the presence of relatively low concentrations of ethanol and relatively high concentrations of fermentable sugar. Yeasts which will perform in this manner can be selected employing known microbiological techniques. Thus, for example, several strains of yeast can be introduced into a laboratory or large scale fermentation vessel (e.g., a chemostat) in which initial ethanol, sugar and nutrient concentrations are noted and predetermined levels of temperature and pH are accurately maintained so as to simulate the conditions present in a commercial fermentation unit. As the different strains of yeast compete with one another for survival over a prolonged period which can be several weeks or even months, only one or a few strains will have survived, the surviving organisms being optimal producers of ethanol under the conditions selected for the operation of the fermentation unit. Using the same procedure, the mutation of a single yeast organism to provide an optimal ethanol producer under the fermentation conditions selected can be induced. The foregoing screening procedure can also be used to evaluate and isolate selected strains of yeast produced by techniques of induced mutation, e.g., those employing ultraviolet radiation, gamma rays, etc., to accelerate the incidence of mutation. Other useful techniques for obtaining different strains of yeast for evaluation as ethanol producers under predetermined fermentation conditions include cross breeding of two different strains to yield a third and genetic engineering in which genetic materials from two different strains are recombined to form a completely new genetic "blueprint". A yeast which has been found to provide especially good rates of ethanol production at relatively low concentrations of ethanol and relatively high concentrations of fermentable sugar is Saccharomyces bayanus. The yeast in fermentation vessels 53 and 67 can be present at a level of from about 2 to about 8 weight percent of the fermentation medium (based on dry weight of yeast) and preferably is present at from about 3 to about 6 weight percent. Once continuous fermentation has started and a steady state has been achieved, there will be no need to add more yeast other than those amounts necessary to make up for cells which die. The temperature of each fermentation vessels is advantageously regulated at a level which favors maximum ethanol production, i.e., generally from about 68.degree. F. to about 104.degree. F. and preferably from about 86.degree. F. to about 99.degree. F. The pH of each fermentation vessel is similarly regulated and can range from about 3.5 to about 5.5 and preferably from about 4.0 to 4.6 Dilute ethanol produced in fermentation vessel 53 containing a portion of the yeast cells therein is conveyed by pump 58 through line 59 to yeast separator/recovery unit 60 which separates substantially all of the yeast cells from the aqueous ethanol stream. Unit 60 can be a micro-filtration device, centrifuge, etc. Since fermentation is exothermic, a portion of the fermentation medium passing through line 59 is diverted through line 61 into cooler 62 and returned to fermentation vessel 53. The yeast cells recovered in unit 60 are conveyed as a pumpable slurry or "cream" containing from about 10 to about 50 weight percent dry yeast and preferably from about 20 to 40 weight percent dry yeast by pump 63 through lines 64 and 57 into fermentation vessel 53. The ethanol-containing fermentation medium thus freed of yeast cells is delivered by pump 65 through line 66 into fermentation vessel 67 which is essentially similar to fermentation vessel 53. A pumpable slurry of ethanol-producing yeast organisms essentially free of contaminating organisms is conveyed from yeast storage tank 68 by pump 69 through lines 70 and 71 into fermentation vessel 67. The yeast selected for introduction in fermentation vessel 67 is one which provides high rates of ethanol production in the presence of relatively high concentrations of ethanol and relatively low concentrations of fermentable sugar. Strains of yeast satisfying these requirements can be isolated in the manner described above. A yeast which has been found to provide especially good rates of ethanol production at relatively high concentrations of ethanol and relatively low concentrations of fermentable sugar is Saccharomyces cerevisiae (Distillers Active Dry Yeast from Red Star Yeast). The dilute aqueous ethanol (approximately 10 to 12 weight percent ethanol) containing yeast cells is withdrawn from fermentation vessel 67 and conveyed by pump 72 through line 73 to yeast separator/recovery unit 74. A portion of the fermentation medium passing through line 73 is diverted through line 75 into cooler 76 and returned to fermentation vessel 67. The yeast cells recovered in unit 74 are conveyed as a pumpable slurry (similar in fluid characteristics to the yeast slurry recovered from unit 60) by pump 77 through lines 78 and 71 to fermentation vessel 67. The cell-free ethanol solution from yeast separator/recovery unit 74 is delivered by pump 79 through line 80 directly to an ethanol concentration unit, e.g., anhydrous distillation apparatus, and/or to a storage facility. It is also within the scope of this invention to employ both types of yeast herein in each fermentation vessel with only one yeast separation/recovery unit (receiving the fermentation medium from the last fermentation vessel in the series) being provided. Metabolically evolved carbon dioxide gas containing ethanol is conveyed from each of fermentation vessels 53 and 67 through common line 81 and by means of blower 82 is introduced into the bottom of ethanol absorption unit 83. Water at ambient temperature entering the top of the absorption unit through line 84 and flowing downwardly, absorbs substantially all of the ethanol vapor rising through the unit. The aqueous solution of ethanol withdrawn from the base of ethanol absorption unit 83 through line 85 is conveyed to line 80 where it is combined with the bulk of the flow from the last fermenter. Vent gases are discharged from ethanol absorption unit 83 through atmospheric vent line 86.