Chemistry Reference
In-Depth Information
the reaction the presence of oxygen is required. Oxygen serves as an electron acceptor.
In this reaction hydrogen peroxide is formed. One explanation of the reaction mechanism
is that hydrogen peroxide, in the presence of endogenous peroxidase, naturally occurring in
flour, promotes the oxidation of sulfhydryl ( SH) groups to disulphide (S S) bridges in the
gluten network, as well as the formation of a gel from the water-soluble pentosans.
123, 124
Another explanation is that hydrogen peroxide reduces the level of reduced glutathione,
which normally has a weakening effect on the gluten network formation.
125
The increase in gluten network results in increased strength of the gluten structure in
the dough. This leads to improved dough stability, reduced dough stickiness and improved
dough machinability. These improvements in turn lead to increased volume, improved crumb
structure and better softness of the baked product. However, there are also studies which do
not support these theories, since no evidence could be found for increased or changed gluten
structures.
126
The main difference between glucose oxidase and hexose oxidase is that the
latter can use various monosaccharides and even oligosaccharides as a substrate.
Lipoxygenase
converts polyunsaturated fatty acids, more specifically those containing
a
cis
,
cis
-1-4-pentadiene moiety, to fatty acid peroxy radicals. This reaction also requires
the presence of oxygen. The free radicals react further to yield monohydroperoxides with
conjugated double bonds and these compounds can react further with a wide variety of
products.
121, 127
In bread making, lipoxygenase from soya bean flour has been used for decades not only
for its bleaching effect, resulting in a whiter crumb,
128
but also for its improving effect
on dough rheology (viscoelasticity), on mixing tolerance, loaf volume and on the gluten
stability.
129-132
The hydroperoxides react with the naturally occurring yellow carotenoid
pigment in wheat flour, leading to a reduction of the yellow colour. Furthermore, it has been
claimed that lipoxygenase has a direct oxidizing effect on gluten formation.
121, 133,134
This
effect cannot be ascribed to hydroperoxides, since addition of lipid hydroperoxides did not
show any effect.
135, 136
Most of the oxygen uptake by wheat dough during mixing is due to the
oxidation of free and esterified polyunsaturated fatty acids (PUFAs).
137-141
The improving
effect of lipoxygenase may thus be due to the oxidation of gluten proteins through the
co-oxidation of accessible thiol groups of the gluten protein by the enzymatically oxidized
lipids.
142-146
Wheat flour itself also contains lipoxygenase, but the activity of this enzyme is confined
to free linoleic acid, linolenic acid and monoglycerides containing these fatty acids.
147
Currently there are no other commercial sources of lipoxygenases other than enzyme-
active soya bean flour and, to a lesser extent, flour from other beans (e.g. faba beans). With
the current trend towards liquid bread improvers, either water based or oil based, the usage of
soya bean flour is gradually reducing as a consequence of the limited solubility of soya bean
flour. This increases the necessity for a microbial lipoxygenase. However, it seems extremely
difficult to find a lipoxygenase with exactly the right specificity. Also in soya beans there are
three distinct isoforms of lipoxygenase (LOX1, LOX2 and LOX3), of which only LOX1 and
LOX3 have a positive effect on loaf volume. LOX2 is mainly responsible for undesirable
aroma formation in bread dough.
148
Polyphenol oxidases
(PPO) are normally involved in enzymatic browning reactions. Enzy-
matic browning is the enzymatic oxidation of phenols leading to the formation of pigments.
The colour of those pigments varies widely in colour and intensity.
149
Several names are in use for PPO, including phenolase, creolase, tyrosinase, diphenolase,
catecholase, laccase, etc. According to the official nomenclature, two kinds of enzymes are
classified under the trivial PPO name. The first group, catechol oxidases (or diphenol oxidase)
