2009_Book_FoodChemistry

724 15 Cereals and Cereal Products
15.4.2.2 Sour Dough Making
In sour dough making (lowering the pH to 4.0–
4.3) rye flour acquires the aroma and taste properties
so typical of rye bread (cf. 15.1.5).
Yeast (Saccharomyces cerevisae, Saccharomyces
minor and others), which are mainly responsible
for dough leavening, and a complex bacterial
flora in which lactic acid-forming organisms
dominate (Lactobacillus plantarum, Lactobac.
San Francisco and Lactobacillus brevis)arepresent
in sour dough.
Sour dough is prepared by various procedures
which differ considerably in the length of time required
(Fig. 15.38). A three-stage procedure takes
into account the optimum temperature and humidity
needs of yeast and bacteria. Yeast prefer to
grow at 26 ◦ C, while the bacteria of interest grow
best at 35 ◦ C.
In setting up a three-stage process, initially an
aqueous flour suspension is inoculated. This is the
first “full sour” build-up stage (Fig. 15.38). After
maturation, further amounts of flour and water
are added and the process is continued with
a “basic sour” stage at 35 ◦ Candthen,inasimilar
way, continued with an additional “full sour”
third stage at 26 ◦ C.
The incubation conditions given in Fig. 15.38 are
only the essential outline. Temperature deviations
influence the spectrum of fermentation products.
At warmer temperatures (30–35 ◦ C) lactic acid is
preferentially formed (Fig. 15.39), while at cooler
temperatures (20–25 ◦ C) more acetic acid is produced.
The desirable lactic acid: acetic acid ratio,
called the “fermentation ratio”, is close to 80:20.
A ratio with a higher acetic acid concentration
gives too sharp an acid taste. The portion of rye
flour in the end-product determines the amount
of rye sour (full sour) to be added to the dough
in the preparation stage. Thus, for rye bread the
sour dough to be added is 35–45%, while for a rye
mix bread it is 40–60% (on the basis of rye flour).
In the short sour method the growth of yeast is
negligible. Only a single sour stage, which lasts
about 3 h, is involved, yeast is added and the
dough is ready for use (Fig. 15.38). However, this
short method requires a relatively high content
of starter saved from a previous ripe sour. Additional
time can be saved by using dough acidifiers
(cf. 15.4.1.5.2 and Fig. 15.38). In short sour processes
all the organic acids needed for the sour
taste of the rye endproduct are present. However,
there is a lack of aroma compounds and precursors
from which odorants can be generated during
baking. In a three-stage rye sour procedure, part
of the flour proteins is hydrolyzed by proteinases
of the microflora into free amino acids which then
Fig. 15.38. Time requirement for various sour dough
development methods (according to Rothe, 1974).
1 A three step process, 2 short sour, 3 dough souring
agents used
Fig. 15.39. Acid formation in sour dough versus time
at 30 ◦ C (according to Rabe, 1980). 1 Malate, 2 pyruvate,
3 citrate, 4 acetate, and 5 lactate