Chemistry Reference
In-Depth Information
a need for wheat flours to be supplemented with
-amylase. These can be added in the form
of malt flour or fungal amylases. Since the 1960s, bakers have supplemented the naturally
occurring enzymes in wheat flour to minimize natural differences caused by, for example,
weather conditions.
α
6.2.3
Amylases in bread making
α
-1,6
linkages. Amylases can act only on damaged or gelatinized starch, since these are suscepti-
ble to enzymatic attack. The amount of damaged starch is dependent on wheat variety and
especially on milling conditions. Standard UK flour has a higher percentage of damaged
starch in order to increase water binding in the dough. Suitable dosages of fungal amylase
lead to the desired improvement of dough and the final product. However, extensive degra-
dation of damaged starch due to too high levels of
-Amylases are endoglucanases. This means that they hydrolyze random
α
-1,4 and
α
-amylase leads to sticky dough.
In Figs 6.5(a) and (b) the effect of increasing levels of a fungal
α
-amylase on volume,
crumb structure and stickiness is shown
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using two different flour qualities.
The volume and crumb structure (manual scores) improve with increasing levels of
amylase added to the flour. This effect is seen with different flour types, although the extent
of the effect is flour dependent. Even though the positive effects increase with the amylase
dose rate, there is an optimum dose level, since the stickiness of the dough also increases,
leading, in this case, with flour 1 to an unworkable dough at higher amylase dose levels.
α
6.2.4 Other amylases
Pullulanase and isoamylase are the two best-known debranching enzymes. Both enzymes
are capable of hydrolyzing
-1,6 glucosidic linkages, thereby releasing side chains from the
branched amylopectin molecule.
β
α
-1,4 glu-
cosidic linkages at the non-reducing end of linear chains in the starch molecule, thereby
catalyzing successive removal of
-Amylase and amyloglucosidase are typical exo-acting enzymes, cleaving
α
-glucose, respectively.
9, 56
β
-maltose and
β
β
-Amylase is
stopped by
-1,6 linkages, whereas amyloglucosidase can bypass the side chains and thus, in
theory, can completely degrade starch to
α
-glucose.
57
These latter four classes of amylases
all have a limited effect on dough properties and bread quality.
β
6.2.5 Anti-staling enzymes
Bread rapidly looses its freshness and is subject to microbial spoilage. Changes in flavour
and texture, other than due to microbial spoilage, taking place during storage are commonly
called staling. This phenomenon, which makes bread hard and dry, is often attributed to
starch retrogradation. Changes that are observed are crumb firming, increased crumb-texture
harshness, increase in opacity of the crumb, loss of crust crispness, disappearance of fresh
bread flavour and appearance of stale bread flavour.
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All these factors result in a loss of
consumer acceptance of the product.
About 85 million ton of wheat flour is used every year to bake bread. By adding specific
agents, such as emulsifiers or enzymes, bread stays fresh longer. It is assumed that 10-15% of
bread is thrown away because it not longer fulfils the consumer demands for quality, crumb
