Mash coolers Fermenter
Yeast culture and intermediate yeast propagator Fermented-mash holding vessel Stillage return system Stillage flow to recovery system Whisky separating column
29. Heat exchanger
31. Vent condenser
32. Product cooler
33. Selective distillation column
34. Product concentrating column
35. Aldehyde concentrating column
36. Fusel oil concentrating column
37. Fusel oil decanter
Figure 2. Material process flow, modern beverage spirits plant.
undergoes liquefication at a much lower temperature than corn, which avoids thermal decomposition of critical grain constituents adversely affecting the final flavor of the distillate. For that reason, many distillers mash rye separately.
Water is drawn at the rate of 35 L/m3 (28 gal/bu) and rye and malt meal are added. The mash is slowly heated to 54°C and held for approximately 30 min. Proteolytic enzymes, active at 43-46°C, aid in reducing the viscosity, and the optimum temperature for /^-amylase is 54°C. The mash is then heated to 63-67°C and held for 30-45 min to ensure maximum conversion. The mash (pH 6.0) is then cooled to the fermenting temperature 20-22°C. This process of converting small grains is called infusion mashing.
Corn. Although the starch in corn grains converts rather easily, higher cooking temperatures are necessary to make the starch available. Usually malt is not added at the beginning, but to reduce viscosity, premalt of 0.5% may be added before cooking, preferably at around 66°C. Thin stillage (the residual dealcoholized fermented mash from the whisky distillation process) is added by some producers to adjust pH to 5.2-5.4. For cookers operating at atmospheric pressure, a mashing ratio of 95-115 L (25-30 gal) of slurry (grain, water, and stillage mixture) per 0.03 m3 (1 bu) and a holding time of 30 min at 100°C are preferable. The mash is cooled to 67°C and malt is added. Primary conversion, the saccharification taking place during conversion, is in the order of 70-80% of the available starches. The remainder of the conversion to fermentable sugar takes place during the fermentation process and is referred to as secondary conversion. For batch cooking under pressure only 65-83 L (17-22 gal) of water are drawn, and the maximum temperature is 120-152°C. In continuous pressure cooking, water is drawn at a ratio of 30 L/m3 (24 gal/ bu) of meal and sufficient thin stillage is added to adjust the pH to 5.2-5.4. The mash is pumped through the continuous pressure cooker, where it is exposed to temperatures of 170-177°C for 2-6 min, and then into a flash chamber where it is cooled immediately by vacuum to the malting (conversion) temperature of 63°C. A malt slurry is continuously introduced and the mixture proceeds through the water cooling system to the fermenters.
In fermentation the grain sugars (largely maltose), produced by the action of malt enzymes (amylases) on gelatinized starch, are converted into nearly equal parts of ethyl alcohol and carbon dioxide. This is accomplished by the enzymes in yeast. Yeast multiples by budding, and a new cell is produced about every 70 min. Although yeasts of several genera are capable of some degree of fermentation, Saccharomyces cerevisiae is almost exclusively used by the distilling industry. It has the ability to reproduce prolifi-cally under normal growth conditions found in distilleries, has a high fermentation rate and efficiency, and can tolerate relatively high alcohol concentrations (up to 15-16 vol %). A great variety of strains exist and the characteristics of each strain are evidenced by the type and amount of congeners the yeast is capable of producing.
Alcoholic fermentation is represented by the following reactions:
C12H22On C6H1206 _> 2C2H5OH + 2C02 maltose dextrose ethyl carbon alcohol dioxide
A fermentation efficiency of 95% is obtained based on the sugar available. Of the starch converted to grain sugar and subsequently subjected to fermentation, 5-6% is consumed in side reactions. The extent and type of these reactions depend on: (1) yeast strain characteristics, (2) the composition of the wort, and (3) fermentation conditions such as the oxidation—reduction potential, temperature, and degree of interference by bacterial contaminants.
Secondary products formed by these side reactions largely determine the characteristics and organoleptic qualities of the final product. In the production of whisky, the secondary products (known as congeners) formed and retained during the subsequent operations include a number of aldehydes, esters, higher alcohols (fusel oils), some fatty acids, phenolics or aromatics, and a great many unidentified trace substances (Table 3). In the production of grain neutral spirits, the congeners are removed from the distillate in a complex multicolumn distillation system. Some distillers, however, retain a small portion of the low-boiling esters in the distillate when the grain neutral spirits are to be matured. Fermentation of grain mashes is initiated by the inoculation of the set mash with 2-3 vol % of ripe yeast prepared separately (see below) and followed by three distinct phases.
1. Prefermentation involves rapid multiplication of yeast from an initial 4-8 million/mL to a maximum of 125-130 million/mL of the liquid and an increasing rate of fermentation.
2. Primary fermentation is a rapid rate of fermentation, as indicated by the vigorous "boiling" of the fermenting mash, caused by escape of carbon dioxide. During this phase secondary conversion takes place, ie, the changing of dextrins to fermentable substances.
3. Secondary fermentation is a slow and decreasing rate of fermentation. Conversion of the remaining dextrins, which are difficult to hydrolyze, takes place.
The degree of conversion, agitation of the mash, and temperature directly affect the fermentation rate. Fermenter mash set at a concentration of 144 L (38 gal) of mash per bushel of grain 25.4 kg (56 lb) will be fermented to completion in two to five days, depending on the set and control temperatures. The set temperature (temperature of the mash at the time of inoculation) is largely determined by the available facilities for cooling the fermenting mash. If cooling facilities are adequate, temperatures of 27-30°C may be employed; otherwise, the set temperature must be low enough to ensure that the temperature will not exceed 32°C during fermentation. When no cooling facilities are provided, the inoculation temperature must be below 21°C. Excessive temperatures during the prefermen-tation phase retard yeast growth and stimulate the development of bacterial contaminants which are likely to produce undesirable flavors.
In the production of sour mash whisky U.S. federal regulations require that a minimum of 25 vol % of the fermenting mash must be stillage (cooled, screened liquid recovered from the base discharge of the whisky separating column, pH 3.8-4.1). In addition to producing a heavier-bodied whisky, this procedure provides the distiller with an economical means of adjusting the setting pH (4.8-5.2) to inhibit bacterial development. It also provides buffering action during the fermentation cycle, which is important because secondary conversion does not take place if the fermenting pH drops below 4.1 in the immediate stages. Thin stillage also provides a means of diluting the cooker mash to the proper fermenter mash concentration, 3,2203,870 L/m3 (30-36 gal/bu) of grain for the production of spirits, and 4,080-4,850 L/m3 (38-45 gal/bu) for making whisky. This concentration gives about a 12-16% soluble solids in the fermenting liquid, within the range used in beer fermentations but lower than that in wine fermentation.
Preparation of the yeast involves a stepwise propagation, first on a laboratory scale and then on a plant scale to produce a sufficient quantity of yeast for stocking the main mash in the fermenters. A strain of yeast is usually carried in a test tube containing a solid medium (agar slant). A series of daily transfers, beginning with the removal of some yeast from the solid medium, are made into successively larger flasks containing liquid media— diamalt (commercial malt extract) diluted to 15-20° Balling, malt extract, or strained sour yeast mash—until the required amount of inoculum is available for the starter yeast mash, called a dona. After one day's fermentation, the dona is added to a yeast mash normally composed of barley malt and rye grains, and representing approximately 2-3.5 wt % of the total grain mashed for each fermenter.
The yeast mash is generally prepared by the infusion mashing method and then soured (acidified) to a pH of 3.94.1 by a 4-8-h fermentation at 41-54°C with Lactobacillus delbrucki, which ferments carbohydrates to lactic acid. Satisfactory souring can be induced with an inoculum of approximately 0.25% of culture per volume of mash. The water-to-meal ratio of 2,580-3,000 L/m3 (24-28 gal/bu) attains a yeast mash balling of approximately 21°. Before inoculation with yeast, the soured mash is pasteurized to 71-87°C to curtail bacterial activity, then cooled to the setting temperature of 20-22°C. The sour mash medium offers an optimum condition for yeast growth and also has an inhibitory effect on bacterial contamination. In 16 h the yeast cell count reaches 150-250 million/mL. Some distillers use the sweet yeast method for yeast development. In this instance the lactic acid souring is not included and the inoculation temperature is usually above 26.6°C to insure rapid yeast growth.
Distillation separates, selects, and concentrates the alcoholic products of yeast fermentation from the fermented grain mash, sometimes referred to as fermented wort or distillers beer. In addition to the alcohol and the desirable secondary products (congeners), the fermented mash contains solid grain particles, yeast cells, water-soluble proteins, mineral salts, lactic acid, fatty acids, and traces of glycerol and succinic acid. Although a great number of different distillation processes are available, the most common systems used in the United States are (1) the continuous whisky separating column, with or without an auxiliary doubler unit for the production of straight whiskies; (2) the continuous multicolumn, system used for the production of grain neutral spirits; and (3) the batch rectifying column and kettle unit, used primarily in the production of grain neutral spirits that are subsequently stored in barrels for maturation purposes. In the batch system, the heads and tails fractions are separated from the product resulting from the middle portion of the distillation cycle.
Although most modern plants have various capacity whisky stills available, a whisky separating column is usually incorporated into the multicolumn system, thus acquiring a greater range of distillation selectivity, ie, the removal or retention of certain congeners. For example, absorptive distillation involving the addition of water to the upper section of a column in the whisky distillation system is a method of controlling the level of heavier components in a product. In the beverage distillation industry, stills and auxiliary piping are generally fabricated of copper, although stainless steel is also used. All piping that conveys finished products is tin-lined copper, stainless steel, or glass.
The whisky column, a cylindrical shell that is divided into sections and may contain from 14 to 21 perforated plates, spaced 56-61 cm apart. The perforations are usually 1-1.25 cm in diameter and take up about 7-10% of the plate area. The vapors from the bottom of the still pass through the perforations with a velocity of 6-12 m/s. The fermented mash is introduced near the top of the still, and passes from plate to plate through down pipes until it reaches the base where the residual mash is discharged. The vapor leaving the top of the still is condensed and forms the product. Some whisky stills are fitted with en-trainment removal chambers and also with bubble-cap plate sections (wine plates) at the top to permit operation at higher distillation proofs. Because whisky stills made of copper, especially the refinement section, supply a superior product, additional copper surface in the upper section of the column may be provided by a demister, a flat disk of copper mesh. The average whisky still uses approximately 1.44-1.80 kg/L (12-15 lb of steam/proof gal) of beverage spirits distilled. Steam is introduced at the base of the column through a sparger. Where economy is an important factor, a calandria is employed as the source of indirect heat. The diameter of the still, number of perforated and bubble-cap plates, capacity of the doubler, and proof of distillation are the critical factors that largely determine the characteristics of a whisky.
The basic continuous distillation system for the production of grain neutral spirits usually consists of a whisky separating column, an aldehyde column (selective distillation column), a product concentrating column (some times referred to as an alcohol or rectifying column, from which the product is drawn), and a fusel oil concentrating column. In addition, some distillers, to secure a greater degree of refinement and flexibility, may include an aldehyde concentrating column (heads concentrating column) or a fusel oil stripping column. Bubble-cap plates are used throughout the system (except in the whisky column, which may have some bubble-cap plates).
This distillation system offers a wide range of flexibility for the refinement of distilled beverage spirits. Figure 3 shows a five-column, continuous distillation system for the production of grain neutral spirits. A fermented mash (generally 90% corn and 10% barley malt) with an alcohol concentration of approximately 7 vol % is pumped into the whisky column somewhere between the 13th and 19th perforated plate for stripping. The residual mash is discharged at the base and pumped to the feed recovery plant; the overhead distillate [ranging in proof from 105 to 135° (52.5 to 67.5%)] is fed to the selective distillation column (also called the aldehyde column), which has over 75 bubble-cap plates. The main stream [10-20° proof (5-10%)] from the selective distillation column is pumped to the product concentrating column. A heads draw (aldehydes and esters) from the condenser is pumped to the heads concentrating column (also called the aldehyde concentrating column), and a fusel oil and ester draw is pumped to the fusel oil concentrating column. The product is withdrawn from the product concentrating column.
Some accumulation of heads, at the top of the product concentrating column, are removed at the condenser, and transferred to the aldehyde concentrating column, where the heads from the system are removed for disposal. The fusel oil concentrating column removes the fusel oil from the system. Figures 3, 4, and 5 show the distribution and concentration of alcohols and congeners through the selective distillation column, the product concentrating column, and the heads concentrating column.
By-Products. The discharge from the base of the whisky column is called stillage and contains in solution and in suspension substances derived from grain (except the starch, which has been fermented), and from the mashing and fermentation processes. The suspended solids are recovered by screening and then subjected to a pressing and a drying operation (dehydrating), usually in rotary, steam-tube dryers. The liquid portion, called thin stillage, is concentrated by a multieffect evaporator to a syrup with a solid content of 30-35%. This concentrate can be mixed and dried with the previously screened-out solids, or it can be dried separately, on rotary-drum dryers. These byproducts are known as distillers' dried grains and distillers' solubles and are used by the feed industry to fortify
From product concentrating column
To fusel oil column
Steam Proof (100%) Esters, aldehyde, g/100 L Fusel oil, g/100 L
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