BLBS102-c31 BLBS102-Simpson March 21, 2012 14:0 Trim: 276mm X 219mm Printer Name: Yet to Come
604 Part 5: Fruits, Vegetables, and CerealsSelection and Biochemistry of
Microorganisms in SourdoughSourdough breads based on rye or wheat flour or their mix-
tures enjoys a remarkable standing in many societies, either
as established, traditional products or as “innovative” develop-
ments for more natural products and a wider choice of flavors.
Well-functioning and popular sourdough starters that have been
maintained by simple rebuilding for decades, and the reestab-
lishment of the “same” stable starter over and 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 bi-
ological systems. In the following sections, metabolic events
and biochemical aspects of sourdough fermentation will be dis-
cussed. 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 comprehensive understanding.Carbohydrate MetabolismThe development of a mixture (1:1) of rye flour or even wheat
flour with water incubated some hours at 25–30◦C will almost in-
evitably lead to a microbiological 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 compo-
sition of the flour and the conditions for growth are not changed
much. Representative LAB and yeasts have been presented in
Table 31.2. These microorganisms have certain characteristics
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. sanfran-
ciscensis, Lb. pontis, Lb. reuteri,andLb. fermentum, harbor a
key enzyme, maltose phosphorylase, which cleaves maltose (the
phosphorolytic reaction) to glucose-1-phosphate and glucose.
Glucose-1-phosphate is metabolized heterofermentatively via
the phosphogluconate 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 experience 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 sour-
dough flora.S. cerevisiaeas baker’s yeast is, however, used at
the bread-making stage, but as an addition in the recipe. Addi-
tional yeast cells may also be necessary for fast and efficient CO 2
production, because the yeasts are relatively sensitive to acids,
particularly to acetic acid, which is excreted by the heterofer-
mentative lactobacilli that often dominate the LAB flora of the
sourdough.Candida milleri(syn.S. exiguus, Torulopsis holmii)
is common 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; itthus appears to be a near ideal partner forLb. sanfranciscensis
(Gobbetti and Corsetti 1997, Gobbetti 1998, Wood 2000,
Hammes and Ganzle 1998). ̈
Wheat and rye flour contain mainly maltose as a readily avail-
able carbohydrate, although rye flour has greater amylase activ-
ity and therefore has a greater potential for release of maltose.
Early work in the United States onLb. sanfranciscensisindi-
cated that this organism would only ferment maltose (Kline and
Sugihara 1971). However, strains isolated in Europe appeared
more diversified, and some of them would ferment up to eight
different sugars (Hammes and G ̈anzle 1998). Utilization of mal-
tose byLb. sanfranciscensis, Lb. pontis, Lb. reuteri,andLb.
fermentumthrough phosphorolytic cleavage with maltose phos-
phorylase is energetically very favorable (Stolz et al. 1993) and
shows increased cell yield and excretion of glucose when mal-
tose 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 exploit co-fermentations for
optimized energy yield (Gobbetti and Corsetti 1997, Hammes
and G ̈anzle 1998, Stolz et al. 1995, Romano et al. 1987).Lacto-
bacillus sanfranciscensis, Lb. pontisandLb. fermentumall have
mannitol dehydrogenase. Thus, fructose may be used as an elec-
tron acceptor for the reoxidation of NADH in maltose or glucose
metabolism, and then acetylphosphate may react on acetate ki-
nase 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, lower
FQ (Spicher 1983). Comparable regulation of acetate produc-
tion may be achieved by providing citrate, malate or oxygen as
electron acceptors, resulting in products like succinate, glycerol
and acetate (Gobbetti and Corsetti 1996, Condon 1987, Stolz
et al. 1993).Proteolysis and Amino CompoundsIn a sourdough, the flour contributes considerable amounts of
amino acids and peptides; however, in order to satisfy nutritional
requirements of growing LAB and provide sufficient amino com-
pounds, precursors, for flavor development, some proteolytic
action is necessary. The LAB have been suspected as the main
contributors of proteinase and peptidase activities for release of
amino acids in sourdoughs (Spicher and Nierle 1984, Spicher
and Nierle 1988, Gobbetti et al. 1996), although the flour en-
zymes may also have considerable input (Hammes and G ̈anzle
1998). In addition, lysis of microbial cells, particularly yeast
cells, add to the pool of amino acids; a stimulant peptide con-
taining aspartic acid, cysteine, glutamic acid, glycine, and lysine
that appears in the autolytic process ofC. millerihas also been
identified (Berg et al. 1981).Lactobacillus sanfranciscensishas
been found to have a regime of intracellular peptidases, endopep-
tidase, and proteinase, as well as a dipeptidase and proteinase
in the cell envelope (Gobbetti et al. 1996). Limited autolysis of