Page 157 - Color Atlas of Biochemistry
P. 157
148 Metabolism
Fermentations lated by pyruvate decarboxylase [2], which
does not occur in animal metabolism, to pro-
As discussed on p. 146, degradation of glucose duce acetaldehyde (ethanal). When this is re-
to pyruvate is the only way for most organ- duced by alcohol dehydrogenase [3], with
isms to synthesize ATP in the absence of oxy- NADH being consumed, ethanol [3] is formed.
+
gen.The NADH+H that is also formed in this Yeasts, unicellular fungi that belong to the
process has to be constantly reoxidized to eukaryotes (3), rather than bacteria, are re-
NAD + in order to maintain glycolysis and sponsible for this type of fermentation. Yeasts
thus ATP synthesis. In the animal organism, are also often used in baking. They produce
this is achieved by the reduction of pyruvate CO 2 and ethanol, which raise the dough.
to lactate. In microorganisms, there are many Brewers’ and bakers’ yeasts (Saccharomyces
+
other forms of NAD regeneration. Processes cerevisiae) are usually haploid and reproduce
of this type are referred to as fermentations. asexually by budding (3). They can live both
Microbial fermentation processes are often aerobically and anaerobically. Wine is pro-
used to produce foodstuffs and alcoholic bev- duced by other types of yeast, some of which
erages, or to preserve food. Features common already live on the grapes. To promote the
to all fermentation processes are that they formation of ethanol, efforts are made to gen-
start with pyruvate and only occur under erally exclude oxygen during alcoholic fer-
anaerobic conditions. mentation—for example, by covering dough
with a cloth when it is rising and by ferment-
ing liquids in barrels that exclude air.
A. Lactic acid and propionic acid
fermentation
C. Beer brewing
Many milk products, such as sour milk, yo-
gurt, and cheese are made by bacterial lactic Barley is the traditional starting material for
acid fermentation (1). The reaction is the same the brewing of beer. Although cereal grains
as in animals. Pyruvate, which is mainly de- contain starch, they hardly have any free sug-
rived from degradation of the disaccharide ars. The barley grains are therefore first al-
lactose (see p. 38), is reduced to lactate by lowed to germinate so that starch-cleaving
lactate dehydrogenase [1]. Lactic acid fermen- amylases are formed. Careful warming of the
tation also plays an important role in the sprouting grain produces malt. This is then
production of sauerkraut and silage. These ground, soaked in water, and kept warm for
products usually keep for a long time, because a certain time. In the process, a substantial
the pH reduction that occurs during fermen- proportion of the starch is broken down into
tation inhibits the growth of putrefying bac- the disaccharide maltose (see p. 38). The
teria. product (the wort) is then boiled, yeast and
Bacteria from the genera Lactobacillus and hops are added, and the mixture is allowed to
Streptococcus are involved in the first steps of ferment for several days. The addition of hops
dairy production (3). The raw materials pro- makes the beer less perishable and gives it its
duced by their effects usually only acquire slightly bitter taste. Other substances con-
their final properties after additional fermen- tained in hops act as sedatives and diuretics.
tation processes. For example, the character-
istic taste of Swiss cheese develops during a
subsequent propionic acid fermentation. In
this process, bacteria from the genus
Propionibacterium convert pyruvate to propi-
onate in a complex series of reactions (2).
B. Alcoholic fermentation
Alcoholic beverages are produced by the fer-
mentation of plant products that have a high
carbohydrate content. Pyruvate, which is
formed from glucose, is initially decarboxy-
Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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