Biomedical Engineering Reference
In-Depth Information
can selectively and partially degrade these water-insoluble xylan particles
improve dough handling, stability and fi nal product characteristics (Maat
et al. 1992, Courtin and Delcour 2002). However, if the xylanase starts to
degrade soluble and solubilised xylans to some extent, dough can become
sticky and is unacceptable for processing. For that reason endoxylanases
of the glycosyl hydrolase (GH) family 11 are preferred over those family
10 (Courtin and Delcour 2002), the former having preference for water-
insoluble xylans and the latter for the water soluble xylans. Fungal
xylanases applied in food industry are mainly produced from A. niger , T.
reesei, T. longibrachiatum , Humicola insolens and Thermomyces lanuginosus,
either produced by the organism of origin or expressed in Aspergillus or
Trichoderma species (Polizeli et al. 2005). Accessory enzymes, which can
release the branches of xylans, such as arabinose, are usually present in
the classical products as side-activity. Yet, arabinofuranosidase enriched
products are also produced separately. But the benefi t in food application
is very much dependent on the raw materials and the specifi c process.
Arabinofuranosidase of A. niger is commercialized for such purposes
(Souppe 1999).
Xylanases are also applied in biscuit making, leading to lower energy
consumption, as less water can be used to make the dough and the water
can be more easily removed during baking. Furthermore, xylanases are
applied in the wheat industry for starch-gluten separation (Polizeli et al.
2005). Future applications of fungal xylanases in food industry can be in
production of xylo-oligosaccharides, which were shown to have prebiotic
effect (Rycroft et al. 2001), or for the production of xylitol, a sweetener,
from enzymatic hydrolysed xylan (Polizeli et al. 2005). Xylose as raw
material for xylitol is mainly obtained from acidic hydrolysis and requires
purifi cation before it can be processed in food-grade xylitol (Polizeli et al.
2005). Endoxylanases for such processes can be either from mesophilic or
thermophilic origin. Thermophilic fungal xylanases have been described
for Talaromyces emersonii, Th. lanuginosus and Humicola insolens (Polizeli et
al. 2005).
Cellulases
In some cereals, such as barley, a relative high content of non-cellulosic
β-glucans is present, in combination with cellulose. The non-cellulosic
β-glucans of barley are quite similar to cellulose, in having glucose linked
to each other by β-(1,4)-linkages, but every 3-7 glucose units a β-(1,3)
linkage is present. This renders these β-glucans readily soluble in hot
water, whereas cellulose is insoluble under these conditions due to the
dense and hydrogen bond-stablilised inter- and intrachain packing of the
different chains. The solubility of these so-called mixed linkage β-glucans
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