Lautet Tuns

Figure 16. Cut-out diagram of lauter tub. Source: Courtesy of Robert Morton.

The reactions that occur in the mash tub are very complex. At a low temperature, about 50°C, proteolytic and cytolytic enzymes in the malt are operative. The corn or rice are not present at this stage and don't need to be. Proteolysis is necessary to supply amino acids for the growth of yeast and also to break down large, insoluble proteins to simple soluble peptides for foam enhancement. Cytoly-sis is required to reduce the viscosity of the barley gums (soluble fiber) to accelerate the later filtration. After some 10 to 60 min, the temperature is raised by the addition of the boiled cooker (or decoction vessel). The resulting temperature may be held for 10-60 min, and the temperature raised again, finally ending at 75°C. During these elevated temperatures, two amylolytic enzymes, a-amylase and /?-amylase, cooperate to hydrolyze most of the starch to fermentable sugars. Beta-amylase, which operates best at about 63°C, attacks starch from its nonreducing end and splits off the disaccharide, maltose. It cannot proceed past a 1,6' linkage, and so depends on the concurrent action of a-amylase. Alpha-amylase, which operates best at 70°C, splits any 1,4' linkage at random, and so provides a supply of nonreducing ends for the ^-amylase to operate on. Cooperatively, the action of these two enzymes can result in the splitting of about 65-75% of the starch into mono-, di-, and trisaccharides.

The mash temperature is raised to 75-77°C when the desired action of the amylolytic enzymes has been achieved. This arrests any further enzymatic action and prepares the mash for the next step, lautering.

Lautering (Straining or Filtration). The converted mash is transferred to a vessel that will permit separation of the liquid (wort) from the insoluble solids (husk, fiber) of the malt and grains. Three different types of vessels accomplish this separation. The one most widely used is a lauter tub, a large cylindrical vessel with a slotted false bottom and a set of movable and adjustable flights. The mash is pumped into this tub, allowed to settle for a few minutes, and then liquid is allowed to flow to the next stage, the kettle. The first runnings are always cloudy and are run back onto the mash. When it runs clear, it is run to the kettle. After the clear liquid, called first wort, stops run

Figure 17. A row of brew kettles. Source: Courtesy of Miller Brewing Co.

Figure 17. A row of brew kettles. Source: Courtesy of Miller Brewing Co.

ning, fresh water (sparge water) is sprayed onto the grain bed to elute the wort adhering to the grains.

Sparging continues until the next vessel is full, the concentration of the wort has reached the desired value, or the solids in the effluent are too dilute to be worthwhile. In some breweries, these last runnings are recovered and used for a succeeding brew. At the end of sparging, the rakes are lowered, their angle changed, and the spent grains are pushed into now-open large holes at the bottom of the lauter tub. These grains have a ready market as a valuable animal feed.

Another device used to separate wort from grain is a mash filter, a plate-and-frame press with plastic cloth as the barrier. These are faster than lauter tubs, but lack flexibility to grain depth. They have found some use in the United States, but wider use outside that country.

A third device, patented by Anheuser-Busch, is a Strainmaster, a modified lauter tub with perforated tubes projecting into the grain bed that serve to collect the wort. It has fallen out of favor and is being replaced by the lauter tub. This also is more rapid than a lauter tub. They are used almost exclusively in the United States.

The Kettle. The clear wort running from any of the grain-separation vessels is collected in a large vessel, the kettle, and boiled. Heat is normally supplied by steam in a set of coils and a center-mounted percolator. Alternatively, the vessel may be heated by direct fire or by an external collandria heated by steam.

During the heating period hops are added, in one or several portions, and at varying times. The varieties, the quantity, and the duration of their boiling time, all affect the flavor of the finished beer.

The boiling of the wort serves many vital functions:

1. It extracts the resin from the hops.

2. It isomerizes the humulones to soluble isohumu-lones.

3. It volatilizes most, but not all of the volatile oils in the hops.

4. It stops all enzymatic action.

5. It sterilizes the wort.

6. It concentrates the wort.

7. It removes grainy odors from the malt and other cereals.

8. It darkens the wort and produces flavor-inducing melanoidin reactions between the amino acids and simple sugars.

At the end of the boil, the wort is passed through a strainer to remove hop leaves, if whole hops were used, or directly to a tank in which the wort is allowed to settle. Recently, tangential entrance to this tank was found to produce a rapid settling of the precipitate that always forms in the kettle. This precipitate, called trub, contains hop bitter resins, proteins, and tannins and needs to be eliminated from the beer.

After settling some 20-30 min the wort is passed through a heat exchanger and cooled to the temperature desired for fermentation. Immediately after cooling, the wort is aerated to provide oxygen for the yeast.

Yeast, from a preceding brew, is added (pitched) in measured amounts to the aerated wort. The usual quantity is 1 lb of liquid yeast per barrel of wort. This will give a count of about 12 million yeast cells per milliliter. The second stage of the brewing process now begins.

Fermentation

Yeast enters a cool solution containing oxygen, fermentable sugars, and various nutrients. The yeast quickly absorbs the oxygen as well as minor nutrients that it requires, such as phosphate, potassium, magnesium, and zinc. It then begins to metabolize sugars and amino acids. The glycolytic pathway that yeast uses is common to many higher organisms, and only the ability to split pyruvic acid into acetaldehyde and carbon dioxide makes yeast distinctive. The scheme for the fermentation of glucose is shown in Figure 18.

The temperature during fermentation is very rigidly controlled. Because fermentation itself is exothermic, the fermentation vessels in all beer fermentations are cooled by a refrigerated fluid circulating through coils in the tanks or through jackets surrounding the tanks. The process usually takes 5-9 d.

During this time the yeast also metabolizes amino acids and makes three to four times as much yeast as was pitched. The excess yeast, over that used for succeeding brews, is collected and sold for use in dietary supplements and flavors for soups and snack foods.

The metabolism of amino acids is orderly and proceeds in a specific sequence. Certain amino acids are metabolized first, followed by the others, as shown in Table 6.

S. carlsbergensis has a marked ability to flocculate when it completes fermentation. This tendency allows the yeast to settle quickly when fermentable sugars are exhausted and makes collection and reuse of the yeast fairly simple. Some brewers hasten this settling by using centrifuges to collect their yeast.

S. cerevisiae, on the other hand, first rises to the top of its fermenting tank, where it may be collected by skimming, but then settles to the bottom and may also be collected there. Certain strains of S. cerevisiae settle more quickly and have been chosen by many brewers of ales and stouts.

Finishing

The fermented beer may now be finished in one of several ways. The simplest and most widely used is merely to transfer the beer to another tank, chilling it en route. This stage, called ruh (rest), allows much of the still suspended yeast to settle and also removes some harsh, sulfury notes. Furthermore, the yeast removes some undesirable flavor compounds, notably diacetyl, which were produced during the earlier fermentation. Ruh normally takes 7-14 d.

An alternative way of finishing beer is to move it to another tank sometime before it is completely fermented. Again, it is chilled en route, and the so-called secondary fermentation is allowed to proceed at a much lower tem-

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