Biomedical Engineering Reference
In-Depth Information
minor amount of α-(1,6)-linkages as branching points in amylopectin. A
great variety of starch converting enzymes is produced in nature, but the
most important industrial enzymes belong to the group of endo-acting
α-amylases. As starch is only accessible after heating it to its gelatinization
temperature, usually in the range of 60 to 100°C, this requires thermostable
enzymes. Until now, only bacterial α-amylases are applied for this so-
called liquefaction in the production of starch hydrolysates, modifi ed
starches, glucose and fructose syrups. Fungal amylases are usually less
thermostable and therefore applied in bread making. Actually, this low
thermostability of fungal α-amylases is benefi cial for its performance in
bread making. It degrades the amylose that leaks out of the damaged
starch granules, immediately when fl our, water, salt and other additives
are mixed to prepare the dough. This results in the availability of more
fermentable sugars in the dough for the yeast, giving an easier-to-handle
dough, increased loaf volume, improved crumb structure and crust colour
of the bread (Pence 1953). The major fungal α-amylase used in bread
making originates from
A. oryzae
. This enzyme is quickly inactivated in
the range of 55-70°C, which is just at the onset of gelatinization of wheat
starch. Due to this minor starch degradation, the bread crumb stays soft
and elastic, whereas it will be gummy when more thermostable endo-
acting α-amylases are applied.
Another fungal starch converting enzyme applied in both sugar
syrup production and bread making is amyloglucosidase produced by
A. niger
. This enzyme is able to release glucose monomers from starch
hydrolysates and can even hydrolyse the α-(1,6)-linked glucose in branched
maltodextrins, although the hydrolysis of this linkage is slower than that of
the 1,4-linkage. In bread making this activity results in further improvement
of loaf volume, crust colour and to some extend it gives an anti-staling effect
in bread-crumb (van der Maarel et al. 2002).
Minor amounts of both enzymes are also applied in beer, alcohol and
fruit juice production to remove haze. In beer and alcohol production they
can supply suffi cient fermentable sugars for the yeast.
Xylanases
Xylan is only a minor component in most plant parts used for food, but
endoxylanases have many applications in food production. Due to their
extended rod-like structure and the abundance of hydroxyl-groups,
xylans cause problems as they bind a lot of water. In beer production
xylans slow down the fi ltration rate and give viscosity to the product.
In bread making the dense water-insoluble xylan particles are thought
to interfere with the gluten network and cause instability of the dough,
resulting in coarse crumb structure and low loaf volume. Xylanases that
