Cooh

Phospho-enolpyruvic acid ch3

COOH Pyruvic acid

(9) Needs enolase and Mg24 (10) Needs pyruvate kinase

(d) Formation of pyruvic acid Figure 18. Chemistry of alcoholic fermentation (continued).

perature, 0-3°C. Much the same cleansing action occurs, but the early transfer ensures the presence of sufficient yeast to cleanse the beer more efficiently. It takes 2-4 w. It has not found wide favor, probably because of the need to transfer the beer at times that could be inconvenient.

A third, and most elegant way, is to transfer the fer mented beer to another tank, chilling it less, and treating it with a small quantity of beer that just started to ferment a day earlier. This subjects the fermented beer not just to a specific quantity of yeast, but to very active yeast that very effectively cleanses the fermented beer. This process, called krauesening, is used by very fastidious brewers. It normally takes 3-5 w.

ch, cho

H+ nadh

C00H Pyruvic acid

Acetaldehyde + C02

NAD4

H+ nadh

CH3CH2OH Alcohol

(11) Needs carboxylase, Mg2"1" and thiamin

(12) Needs alcohol dehydrogenase and Zn2+

(e) Production of alcohol and carbon dioxide Figure 18. Chemistry of alcoholic fermentation (continued).

Table 6. Metabolism of Amino Acids

Extent of absorption

Acid

Group A

Almost completely absorbed within 24 hours Glutamic acid

Glutamine Aspartic acid Asparagine Serine Threonine Lysine Arginine

Group B

Absorbed gradually during fermentation Valine

Methionine Leucine Isoleucine Histidine

Group C

Absorbed only after Group A amino acids Glycine are gone Phenylalanine

Tyrosine Tyrptophane Alanine Ammonia

Group D

Absorbed only slightly

Proline

After any of these finishing processes, the beer is filtered, always in the cold, to remove insoluble particles and yeast. This filtration uses diatomaceous earth as the filter medium. The beer may be filtered again, and this filtration may be a sterile filtration to remove all yeast and lacto-bacilli or another diatomaceous earth filtration. A sterile filtration may involve cotton fibers, a porous plastic sheet, or a ceramic filter as the retaining barrier.

Beer Packaging

Beer is packaged in either cans or bottles or in large containers or kegs for use in restaurants and taverns. The packaging machinery for beer has become very sophisticated and fast. Because oxygen changes the flavor of beer (see later), modern fillers evacuate the bottle before it is filled and replace the air with C02. Then the beer enters against a counter pressure of C02. Bottles are being filled at more than 20/s, and cans at more than 30/s. A highspeed can filler is shown Figure 19.

If such packaged beer has not been sterile-filtered, it must be pasteurized, as beer is a fertile medium for many microbes. The pasteurization may be done just before filling, a so-called bulk pasteurization, or after filling in long tunnels with hot-water sprays. Bulk pasteurization takes about a minute, tunnel pasteurization about an hour.

Kegs may be filled with diatomaceous earth-filtered beer, in which case they are kept refrigerated; or they may be filled with sterile filtered or bulk pasteurized beer, in which case they do not need refrigeration.

Problems of Beer

The complexity of beer makes it prone to several problems. First and foremost is that the delicate flavor of beer is adversely affected by oxygen, so great pains are taken in the brewery and in the packaging operation to minimize the pick up of oxygen. When yeast is present, it will scavenge oxygen and keep the beer oxygen-free. But after filtration beer is very prone to pick up oxygen. At every transfer, steps are taken to avoid oxygen pickup, which is monitored closely. The filling operation has been mentioned earlier.

Correctly packaged beer has a flavor shelf life of about 4 m: if kept cold, it will last longer. The change in flavor with oxidation is difficult to describe because it is unique to beer. It has been described as papery, bready, or cardboard. But mainly it loses the fine appeal of fresh beer.

Another problem of beer is that it develops a peculiar, skunky aroma if packaged in clear or green bottles and is exposed to light. The problem has been eliminated with the

Figure 19. High-speed can filler. Source: Courtesy of H&R Inc.

discovery that brown bottles filter the offending wavelengths of light. The aroma is caused by the action of light on isohumulone, which splits off an unsaturated hydrocarbon, which in turn reacts with a trace of hydrogen sulfide in the beer to give a mercaptan.

A third problem is that beer is a good medium for some nonpathogenic microbes and so must be sterile-filtered or pasteurized. Either of these treatments effectively eliminate the problem.

Another shortcoming that has largely been eliminated is the tendency for beer to become cloudy when it is chilled. This haze is caused by a reversibly insoluble proteinpolyphenol complex. It is removed in one of several ways. The protein may be solubilized by the enzyme papain, or the polyphenols may be removed by absorption onto silica gel. These treatments are effective and widely used.

Light Beer

The enzymes present in barley malt cannot completely degrade starch to fermentable sugars. So all regular beers contain dextrins, fragments of starch held by 1,6' linkages and too large to be fermented by yeast. There is an enzyme, however, that can break up all the starch—amyloglucosi-dase.

A true light beer is one in which all the starch has been made fermentable. This is accomplished by the use of amy-loglucosidase, either in the brewhouse to work with the a-amylase and /¡-amylase or during fermentation to work after the a-amylase and /(-amylase.

Some light beers are made by merely extending the action of the a-amylase and ^-amylase. This produces a beer reduced in dextrins but not free of them. The legal definition of a light beer in this country permits this.

Analysis of Beer

Typical analysis of some American beers are shown in Table 7.

Table 7. Analysis of American Beers

Standard beer All malt beer Light beer

Water

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