Biomedical Engineering Reference
In-Depth Information
On the basis of a similar identification scheme as described above, sourdough
yeasts can be preliminarily identified by a phenotypic approach consisting of cell
and colony morphology, presence and number of spores, carbohydrates, organic
acids, alcohols and nitrite fermentation/assimilation tests using commercial kits
(e.g. ID32C, API 20C AUX, RapID
Yeast Plus System
,
Rapid Yeast Plus
), growth at
different temperatures and salt concentrations (
44
). As for lactic acid bacteria,
chemotaxonomy based on fatty acids, proteins and polysaccharides can be success-
fully applied to yeast identification. The genotypic approach is often based on the
sequencing of various ribosomal genes (e.g. 18S, 5,8S, 26S, 5S, and spacer frag-
ments) (
45
), as well as on the PCR-RFLP of the rDNA 5,8S-ITS (
Internal Transcribed
Spacers
) (
34
). As for lactic acid bacteria, yeast strain typing mainly relies on the
PCR-RAPD technique.
Both lactic acid bacteria and yeasts can be identified and monitored during sour-
dough fermentation by culture-independent systems, for example by PCR-DGGE
(
Denaturing Gradient Gel Electrophoresis
), generally directed toward the
amplification of specific variable regions of 16S or 26S rRNA genes of lactic acid
bacteria and yeasts, respectively. A similar approach was applied by Meroth et al.
(
14,
15
) to evaluate the effect of type I and type II sourdough fermentation on micro-
bial population dynamics.
4.6.3
Determination of the Physico-Chemical Parameters
The balance between the communities of lactic acid bacteria and yeasts and the
composition of the microbial communities in terms of species and strains markedly
influences the sourdough performances and the overall quality of related leavened
baked goods. The microbial community is influenced by some sourdough
characteristics and, in turn, it modifies the chemical composition and physical
parameters of the sourdough, which is reflected in the overall quality of the prod-
ucts (Fig.
4.3
). An overview of the physico-chemical characteristics of the sour-
dough, which are related to its performance, is determined through the evaluation
of the following parameters:
-
Dough Yield (DY)
-
Dough Acidity (pH and Total Titratable Acidity, TTA)
-
Fermentation Quotient (FQ).
Dough Yield (DY)
. The ratio between water and flour in the dough is indicated as
dough yield and it deals with the dough consistency. Considering that different
flours have different capabilities to absorb water, doughs of various consistency are
obtained having the same DY. DY is calculated as follows: DY = (flour weight + water
weight) × 100/flour weight. To consider other ingredients in the formula, the expres-
sion is modified as follows: DY = dough weight × 100/flour weight.
Overall, a firm wheat flour sourdough has a DY of 150-160. A liquid sourdough
shows values close to 200. Intermediate values of DY indicate soft dough (
12
) . It
has been observed that both DY and temperature of fermentation markedly influence
