Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
several, mainly mammalian, α-amylases, but also in microbial α-amylases. 28 In
contrast to the endo-acting α-amylases, some α-amylases such as maltogenic amylase,
maltotriohydrolase, maltotetraohydrolase, and maltohexaohydrolase synthesized
mainly by Bacillus strains (Table 10.4) preferentially attack one end of the
glucan chain and liberate selected oligosaccharides only.
Mesophilic α-amylases, derived from Bacillus species and filamentous fungi
(mainly Aspergillus species), which dominated industrial starch processing in its
infancy, have been currently replaced with their thermophilic counterparts such as
B. stearothermophilus or B. licheniformis enzymes. They provide satisfying liquefaction
of the polysaccharide at 100 to 105°C, that is, under conditions causing rapid
denaturation of mesophilic proteins. However, the most thermoactive α-amylases
are those produced by the archaea Pyrococcus woesei and P. furiosus, active up to
130 and 120°C, respectively. 30
Attention of technologists has been also drawn to raw starch-digesting α-amylases.
They reduce costs of amylolysis because they degrade starch granules without
prior energy-consuming gelatinization. These α-amylases posses an additional raw
starch-binding E-domain, providing adsorption on glucan granules. 31 The enzymatic
digestion of uncooked starch is much slower than that of gelatinized glucan, and
light microscopy and scanning electron microscopy observations demonstrated that
starch-adsorbable α-amylases and glucoamylases produce pits or pores, penetrating
the granule surface toward its center. The diameter and depth of pores as well as
the composition of water-soluble digestion products hinge on the enzyme used. The
pitted starch granules remain insoluble in cold water, but are soluble in warm water
and are more readily degradable. Such granules are useful in production of food,
cosmetics, and pharmaceuticals.
10.4.1.3 Exo-Acting Enzymes
The exo-acting starch-digesting enzymes comprise β-amylases and glucoamylasesas
well as enzymes of the α-amylase family, such as α-glucosidases, and oligo-1,6glucosidases.
2 All these exo-hydrolases attack their substrates from the nonreducing
ends.
α-Glucosidases hydrolyze the α-1,4 and, more slowly, also the α-1,6 glycosidic
bonds and release glucose molecules from disaccharides, oligosaccharides, and arylglucosides.
In nature, they participate in the final steps of starch hydrolysis into
glucose. They are much less important in industrial starch processing. Oligo-1,6glucosidases
liberate terminal α-1,6-linked glucose from isomaltose and starch and
glycogen dextrins. They degrade exclusively short-chain substrates.
In contrast to the α-amylase family enzymes, α- and gluco-amylases cause
inversion of anomeric configuration on hydrolysis of the α-glycosidic bond. Active
site of β-amylases (Family 14 of GH) is included as a part of the (α/β) 8 barrel,
whereas that of glucoamylase (Family 15 of GH) is localized in the (α/α) 6 fold.
Some glucoamylases are also classified in Family 31 of GHs.
β-Amylases liberate maltose and split exclusively the α-1,4 linkages. High-molecular-weight
limit dextrins are the by-products of starch digestion with β-amylases The
latter can neither cleave nor omit the α-1,6 branchings. β-Amylases are widely