Biomedical Engineering Reference
In-Depth Information
virtually absent, while b-amylases are abundantly present, but the latter have little if any
activity on starch granules and are inactivated before starch gelatinization [
4
] .
In rye flour proteins are not capable of forming a network similar to wheat gluten,
here water-binding pentosans take over the function in the structure-forming process
[
21
]. Pentosans mainly consist of arabinoxylans (AX), which can be subdivided into
water extractable (WEAX) and water unextractable arabinoxylans (WUAX). WUAX
have a deleterious influence on bread quality, in contrast WEAX increase bread vol-
ume due to the high water-binding capacity and the ability to undergo oxidative
gelation [
31
] (see Chap.
2
). Acidification has a pronounced effect on the solubility of
AX due to acid hydrolyzation of WUAX, therefore increasing the proportion of
WEAX [
32
]. Furthermore, during sourdough fermentation pH optima for endoge-
nous rye enzymes like L-arabinofuanosidase, endo-xylanase, and xylosidase are
reached [
33
]. Also in wheat endoxylanases cause a reduction of WUAX and increase
the level of WEAX [
6
]. Thus, beneficial WEAX can increase during sourdough fer-
mentation. In rye doughs, this improves physical properties by increasing the elastic-
ity of the dough and the subsequent bread crumb. The bread crumb is characterized
by an improved mouthfeel, improved crumb structure, and volume [
7,
31,
33
] . In
wheat baking the increased solubility of pentosans in sourdough also contributes to
enhanced bread volume and improved softness [
6,
13,
34
] .
8.4
Enzymes from LAB Contributing to Dough
and Bread Structure
Proteolysis in sourdoughs is mainly based on the pH-mediated activation of endogenous
fl our proteases [
28,
29,
35
] (Table
8.1
). Lactobacilli used in sourdough fermentation
may also exhibit strain-specific proteolytic activity [
36,
37
], but appear to play a minor
role in the overall proteolysis. Although LAB do not influence overall proteolysis
when compared to aseptically acidified doughs, they affect the pattern of hydro-
lyzed products, increasing the amount of dipeptides and amino acids. In fact, the pro-
teolysis by LAB induced softening of the dough in comparison to chemically acidified
doughs [
37
]. Furthermore, the choice of a highly proteolytic starter culture, such as
Enterococcus faecalis
, substantially contributed to the gluten proteolysis [
38
] .
The significant role of enzymes produced by LAB has been proposed to explain
observed differences in the staling of sourdough breads [
12,
13
] . Sourdough breads
with comparable acidity levels had varying staling rates in terms of firmness and
starch retrogradation. LAB strains possessing proteolytic and amylolytic activities
were most effective in delaying staling [
12
] .
In addition to the pH-dependent cereal proteases and LAB-liberated proteases,
glutathione reductase expressed by heterofermentative lactobacilli contributes to
depolymerization of gluten protein [
39
] (Table
8.1
). Glutathione reductase reduces
extracellular oxidized glutathione (GSSG) to the reducing agent glutathione (GSH)
leading to increased SH groups in the gluten proteins. GSH undergoes thiol-
exchange reactions with gluten proteins and decreases intermolecular disulfide
