27 Bakery and Cereal Products 625favorite among many peoples (Hammes and Gänzle
1998, Wood 2000).
ADVANTAGES OFMAKINGSOURDOUGHBREAD
- Sourdough bread does not have to contain high
levels of gluten for successful leavening. - Low pH inhibits amylase, and thereby,
degradation of starch is avoided. - Sourdough improves the water-binding capacity
of starch and the swelling and solubility of
pentosans. - Sourdough bread has very good keeping quality
and an excellent safety potential. - Less costly cereal flours can be used.
- A different variety of flavor and taste attributes
can be offered. - Sourdough bread can nutritionally compete with
regular bread. - Phytic acid is degraded by phytase in flour and
from lactic acid bacteria. This improves the
availability of iron and other minerals. - Bread volume is increased, crumb quality is
improved, and staling is delayed.
Rye flour is very low in gluten proteins, and in-
stead, starch and pentosans make an important con-
tribution to bread structure. The swelling and solu-
bility of pentosans increase when LAB fermentation
lowers pH. Gelatinization of starch occurs at about
55–58°C. Considering that the flour amylase has a
temperature optimum around 50–52°C, it is crucial
that the amylase is actually inactivated in the pH
range that is obtained during sourdough fermenta-
tion. When mixtures of wheat and rye flour are used
for bread making, a sourdough process is necessary
if the content of rye flour exceeds 20%.
Rye and wheat flour contain phytic acid that binds
minerals, particularly iron, that then become nutri-
tionally unavailable. However, these cereals also
contain phytases with pH optima around 5.0–5.5;
thus phytate degradation is very good in fermented
flour, where these phytate complexes are also more
soluble. Lactic acid bacteria also appear to have
some phytase activity.
As wheat flour is able to form gluten, some of the
considerations about amylases and starch are not
equally relevant when baking wheat bread. Never-
theless, wheat flour is often used in sourdough bread.
However, preferred qualities like improved keep-
ing and safety potential as well as the increased vari-
ety of flavors appeal to many consumers. These desir-
able qualities are also praised because they represent
an alternative natural preservation method (Gobbetti
1998, Hammes and Gänzle 1998, Wood 2000).MICROBIOLOGY OFSOURDOUGHAn established, “natural” sourdough is dominated
by a few representatives of some bacteria and yeast
species. This results from the selective ecological
pressures exerted in the (rye) flour-water environ-
ment. Rye flour is an appropriate choice for this
mixture because leavening of the dough is depend-
ent on sourdough development. At the start of fer-
mentation, a 50:50 (w/w) rye flour–water mixture at
25–30°C will harbor approximately the following:Mesophilic micro- 103 –10^7 cfu/g
organisms, aerobes
Lactic acid bacteria 10 to 5 102 cfu/g
Yeast 10–10^3 cfu/g
Molds 102 to 5 104 cfu/gAmong the mesophiles at the start, members of the
Enterobacteriaceae dominate. Microorganisms dom-
inating in the sourdough after 1–2 days at 25–30°C
are as follows:Lactic acid bacteria 109 cfu/g
Yeast 106 to 5 107 cfu/gSome important properties of the rye flour–water
environment determine that certain LAB and yeasts
will compete most favorably. Lactobacilli have a
superior ability to ferment maltose, they thrive de-
spite limited iron due to the presence of phytic acid,
and they are able to grow at about pH 5.0 and lower.
Reports show that different LAB may be isolated
from sourdoughs; however, Lactobacillus sanfran-
ciscencishas been found most often. Table 27.2
presents some of the other Lactobacillusspecies that
have been isolated from sourdough. The selection of
yeasts may be even narrower, with Candida milleri
often cited (Table 27.2). Several other species are
isolated occasionally (Spicher 1983, Gobbetti and
Corsetti 1997, Hammes and Gänzle 1998, Martinez-
Anaya 2003, Stolz 2003).STARTERSIt is traditional bakers’ practice to maintain a good
sourdough over time by regular transfer, for example,