628 Part VI: Fermented Foods
over again from a constant quality flour, are both
expressions of stable ecological conditions. The
simplicity of the procedures may be somewhat
deceiving with respect to the actual complexity of
these biological systems. In the following sections,
metabolic events and biochemical aspects of sour-
dough fermentation will be discussed. Attention will
be drawn to some of the more clear points about the
metabolic events and other biochemical facts. The
near future should bring us closer to a comprehen-
sive understanding.
Carbohydrate Metabolism
The development of a mixture (1:1) of rye flour or
even wheat flour with water incubated some hours at
25–30°C will almost inevitably lead to a microbio-
logical population consisting of lactic acid bacteria
(LAB) and yeasts. It may need rebuilding several
times in order to stabilize it, but from then on the
composition of the microflora may be constant for
years, provided the composition of the flour and the
conditions for growth are not changed much. Re-
presentative LAB and yeasts have been presented in
Table 27.2. These microorganisms have certainchar-
acteristics in common. First, the selected LAB are
very efficient maltose fermenters, a prime reason
why they competed so well in the first place. Several
lactobacilli in sourdoughs, e.g. Lb. sanfranciscensis,
Lb. pontis, Lb. reuteri,and Lb. fermentum, harbor a
key enzyme, maltose phophorylase, which cleaves
maltose (the phosphorolytic reaction) to glucose-1-
phosphate and glucose. Glucose-1-phosphate is me-
tabolized heterofermentatively via the phosphoglu-
conate pathway, while glucose is excreted into the
growth medium. Glucose repression has not been
observed with these lactobacilli. Most of the yeast
species identified in sourdoughs are, per se, maltose
negative, and will thus prefer to take up glucose
when it is available. Other microorganisms may ex-
perience glucose repression of the maltose enzymes,
to the benefit of the sourdough lactobacilli. Among
the yeasts, S. cerevisiae, which is maltose positive
and transports maltose and hexoses very efficiently,
cannot take up maltose due to glucose repression
and will, as a consequence, be defeated from the
sourdough flora. S. cerevisiaeas baker’s yeast is,
however, used at the bread-making stage, but as an
addition in the recipe. Additional yeast cells may
also be necessary for fast and efficient CO 2 produc-
tion, because the yeasts are relatively sensitive to
acids, particularly to acetic acid, which is excreted
by the heterofermentative lactobacilli that often
dominate the LAB flora of the sourdough. Candida
milleri(syn. S. exiguus, Torulopsis holmii) is com-
mon in sourdoughs for San Francisco French bread.
This yeast tolerates the acetic acid from heterolactic
fermentation and thrives on glucose and sucrose in
preference to maltose; it thus appears to be a near
ideal partner for Lb. sanfranciscensis(Gobbetti and
Corsetti 1997, Gobbetti 1998, Wood 2000, Hammes
and Gänzle 1998).
Wheat and rye flour contain mainly maltose as a
readily available carbohydrate, although rye flour
has greater amylase activity and therefore has a
greater potential for release of maltose. Early work
in the United States on Lb. sanfranciscensisindicat-
ed that this organism would only ferment maltose
(Kline and Sugihara 1971). However, strains isolat-
ed in Europe appeared more diversified, and some of
them would ferment up to eight different sugars
(Hammes and Gänzle 1998). Utilization of maltose
by Lb. sanfranciscensis, Lb. pontis, Lb. reuteri,and
Lb. fermentum through phosphorolytic cleavage
with maltose phophorylase is energetically very
favorable (Stolz et al. 1993) and shows increased
cell yield and excretion of glucose when maltose is
available. In these conditions the cells have very low
levels of hexokinase.Co-metabolismLactobacilli in sourdough production are not only
specialized for maltose fermentation they also ex-
ploit co-fermentations for optimized energy yield
(Gobbetti and Corsetti 1997, Hammes and Gänzle
1998, Stolz et al. 1995, Romano et al. 1987). Lacto-
bacillus sanfranciscensis, Lb. pontisand Lb. fermen-
tumall have mannitol dehydrogenase. Thus fructose
may be used as an electron acceptor for the reoxida-
tion of NADH in maltose or glucose metabolism,
and then acetylphosphate may react on acetate
kinase to yield ATP and acetate (Axelsson 1993).
The lactobacilli gain energetically and more acetic
acid may contribute to the desirable taste and flavor
of bread. In practical terms addition of fructose is
used to increase acetate in the products, that is, low-
er FQ (Spicher 1983). Comparable regulation of
acetate production may be achieved by providing
citrate, malate or oxygen as electron acceptors, re-