Biomedical Engineering Reference
In-Depth Information
that maintained sucrose concentrations at 10% throughout fermentation [
113
] . The
addition of acceptor carbohydrates other than sucrose also influences HoPS yields
in sourdough fermentations. Maltose is a stronger acceptor for glucansucrases
compared to glucose [
97,
115
]. Correspondingly, dextran yields in wheat sourdoughs,
characterized by high maltose concentrations throughout fermentation, were
substantially lower compared to yields by the same strains in sorghum fermentations,
which are characterized by low maltose and high glucose concentrations [
15
] .
Maltose, xylose, and arabinose also act as alternative acceptor carbohydrates for
levansucrase activity [
95,
96
] .
Exopolysaccharide-producing sourdough starter cultures have found industrial
application to improve the textural properties of bread [
116
]. In addition to the func-
tion of HoPS as hydrocolloids in baking applications, specific HoPS were found to
prevent pathogen adhesion to eukaryotic cells [
117
] and to exhibit prebiotic activity
[
92,
118
]. HoPS-producing lactic acid bacteria thus enable the formulation of baked
goods with specific health properties.
7.6
Antimicrobial Compounds from Sourdough
Lactic Acid Bacteria
Leavened baked goods can become contaminated by spores of the genus
Bacillus
, which
survive baking, yeasts, mainly belonging to the genera
Pichia
and
Zygosaccharomyces
,
which colonize the surface and negatively affect the sensory properties, and, especially,
by moulds, mainly belonging to the genera
Penicillium
,
Aspergillus
and
Cladosporium
which alter the color and the sensory properties, and, in some cases, synthesize myco-
toxins. More than 40 species of fungi were described as contaminants of baked goods.
Although chemical preservatives (e.g., sorbate and propionate, ethanol) are routinely
used for preventing the contamination of leavened baked goods, sourdough lactic acid
bacteria show a number of natural bio-preservative features that are complementary to
chemical preservatives, or can even substitute their use. Antimicrobial compounds from
sourdough lactic acid bacteria were additionally shown to influence sourdough micro-
biota, and to contribute to the stability of individual strains.
The inhibitory activity of sourdough lactic acid bacteria is generally attributable
to rapid consumption of oxygen and fermentable carbohydrates, and the formation
of lactate with concomitant reduction of the pH. Additional metabolites with specific
antimicrobial activity include diacetyl, hydrogen peroxide, acetate and other short-
chain fatty acids, and reuterin. Acetate formation by heterofermentative lactobacilli
in sourdough is readily adjusted by addition of sucrose or pentoses [
39
] , and con-
tributes to shelf-life extension of bread (see below). The odor threshold of diacetyl,
butyrate, and caproate is substantially lower than the concentrations required for
antimicrobial activity; these compounds can thus not be accumulated in bread with-
out adverse effects on the sensory bread quality. Although individual sourdough
lactic acid bacteria are capable of reuterin synthesis, reuterin formation has not been
achieved in cereal fermentations [
119
] .
