Chemistry Reference
In-Depth Information
Xylan
O
-
O
O
MeO
α
-
D
-glucuronidases
HO
O
Acetyl-xylan-
esterases
OH
O
O
O
O
HO
R
3
O
O
O
O
O
R
1
HO
O
OH
O
O
O
α
-
L
-arabinofuranosidases
OH
Endoxylanases
OH
R
2
O
OMe
Ferulic acid
esterases
O
Plant xylan structure with bound ferulic acid.
26
Fig. 6.3
One possible mechanism is through dimerization of FA residues on adjacent arabinoxylan
chains. Proteins also participate in the gelation process, since the gel fraction contains about
25% protein and is dissolved by proteases.
27, 35
This indicates that FA is also coupled to
the protein, most likely to tyrosine and/or cysteine residues. In total three different possible
mechanisms were suggested as explanation for this phenomenon
35, 39
: Cross-linking of two
FA moieties through their aromatic rings; cross-linking of a FA reside with the ring structure
of tyrosine amino acid moieties; or cross-linking of two tyrosine rings.
In feruloylated AX, laccase catalyzes gelation by dimerization of feruloyl esters. Although
there was no evidence for a coupling of thiol compounds to phenoxyl radicals, it has been
proposed that these phenoxyl radicals, oxidized by laccase from that FA, are involved in
the conversion of the thiol compounds into disulphide bonds and thus influence the overall
dough firming which is seen upon use of laccase.
36, 40
6.1.6 Lipids
Wheat flour lipids form a highly heterogeneous group of molecules with different chemical
structures and compositions. The lipids can be divided into free lipids and bound lipids.
41
Both fractions contain polar and non-polar components. Polar lipids can be divided into gly-
colipids and phospholipids. The glycolipids are predominantly composed of monogalactosyl
diglycerides (MGDG) and digalactosyl diglycerides (DGDG), whereas lysophosphatidyl-
cholines (LPC) and phosphatidylcholines (PC) are major components of the phospholipid
class of lipids. Both classes are also highly heterogeneous with respect to the position and
the structure of the esterified fatty acids. The non-polar lipid class is mainly composed of
triglycerides.
42
The fatty acids in the various lipids are dominated by linoleic acid, whereas
other fatty acids, such as palmitic and oleic acid are found in lower levels.
43
Bound lipids are mainly bound to starch and to a lesser extent to proteins. Starch lipids
(around one third of total) comprise mostly LPC. These lipids form inclusion complexes
with amylose during gelatinization, whereby the LPC fatty acid chains form complexes with
the hydrophobic interior of the amylose
-helices.
44
α
Such complexes may already exist in
