In the capacity of some countries to

In recent
years, the baking industry has undergone very important changes in its
productive processes. Some of the major changes have been brought about by an
increasing mechanization in its processing unit operations. This fact has
contributed to an increased demand for strong wheat flours, yielding doughs
with high tolerance to handling and mixing, and able to remain stable during
fermentation. Functional properties of flours greatly depend on the gluten
proteins. On the other hand, the quality of gluten is dependent on diverse
factors such as wheat variety and growing conditions. For this reason, the
capacity of some countries to produce high-quality flours is limited. In this
context, the treatment of flours with functional additives must be considered.
Chemical improvers have been used for decades in breadmaking as a way of
adjusting the variations in flour properties and baking conditions. Nowadays,
the baking industry is deeply involved in research for alternatives to chemical
compounds because of their potential hazards. The enzymatic treatment of wheat
flours is an interesting alternative to generate changes in the structure of
the dough and in consequence, for improving functional properties of flours.
They are generally recognized as safe (GRAS) and do not remain active in the
final product after baking. Therefore, enzymes do not have to appear on the
label, which is an additional commercial advantage. The intentional inclusion
of enzymes in bread formulas dates back to more than one century .Today, a wide
range of enzymes produced especially for bread-making is available for bakers.
A variety of aims may be pursued by enzyme addition, for example, to achieve a
partial gluten hydrolysis for improving machinability, to obtain enough sugars
for fermentation by means of starch hydrolysis, to attain a certain amount of
lipid peroxidation for dough strengthening, or to reduce retrogradation and
crumb firming through gelatinised starch hydrolysis. Gluten cross-linking
enzymes play an important role in current baking processes. Through different
biochemical mechanisms (the oxidative coupling of thiol groups, the cross-link
of tyrosine residues due to the action of intermediate reactive compounds such
as hydrogen peroxide, the acyl-transfer reaction between amino acid residues),
these enzymes promote the formation of covalent bonds between polypeptide
chains within a protein or between different proteins, improving functional
behaviour of dough during the bread-making process. Transglutaminase (TG) (EC is a transferase able to yield inter- and intramolecular ?-N-(?-glutamyl)lysine
cross-links. Its addition causes structural changes in gluten proteins, being
high molecular weight (HMW) glutenin subunits the most affected protein
fraction. TG may also lead to the formation of disulfide bridges by oxidation
due to the proximity of sulphur containing amino acids. Because of these
effects, TG has been widely used to improve wheat dough functionality and bread
quality. The possibility of using this enzyme to alleviate some of the
detrimental effects of frozen storage of puff pastry and croissants, as well as
to solve the damage promoted by the insect attack of wheat has been proposed.
The results obtained with wheat flour have been also extrapolated to other
cereals, allowing an improvement in the viscoelastic properties of the rice
dough and therefore in the ability of rice flour to retain the carbon dioxide
produced during proofing. Recently, the possibility that TG in wheat-based
baked products may generate the epitope associated with the coeliac response
has been suggested 27, although there is no experimental evidence to support
this postulate. Glucose oxidase (EC (GO) is an oxidative enzyme that
catalyses the oxidation of ?-d-glucose to ?- d-gluconolactona and hydrogen
peroxide. Disulfide bond interchange and the gelation of pentosans promoted by
hydrogen peroxide action are the most widespread theories to explain the
strengthening effect of the GO. Furthermore, it has been related with the
formation of non-disulfide covalent intermolecular bonds in the gluten proteins
by GO treatment, either among glutenins or between albumins and globulins. GO
modifies the functional properties of dough, increasing its tenacity and
elasticity. Gujral and Rosell revealed even an increase in the elastic and
viscous moduli of rice flour dough. As a result of such changes in dough
behaviour, GO showed positive effects on bread quality, yielding improved
specific volume, bread texture and crumb grain. Through a similar oxidative
mechanism, hexose oxidase (EC (HO) has been also suggested as an efficient
bread improver. When this enzyme is added to dough model systems, it induces
the formation of disulphide bridges between proteins and the gelation of
pentosans, increasing dough strength and bread volume. HO was found to be more
effective than GO because of its ability for using several monosaccharides and
oligosaccharides as substrates and its higher affinity for glucose. Since Si  proposed laccase (LAC) (EC as dough
and bread improver as a result of its oxidant effect on dough constituents,
numerous studies have been developed to analyse the effects and applications of
this oxidoreductase. LAC is a type of polyphenol oxidase able to gel water
soluble arabinoxylans by coupling feruloyl esters of adjacent chains into
dehydrodimers. The probable development of a protein–arabinoxylan network by
LAC action has been hypothesized. Even though Figueroa-Espinoza et al. and
Labat et al. have concluded that gluten and arabinoxylans form two distinct
networks, Oudgenoeg et al. proposed a mechanism by which tyrosine-containing
proteins cross-link with arabinoxylans. Because of the simultaneous
arabinoxylans gelation and oxidative action, LAC addition significantly
improves gluten quality and leads to changes in the rheological properties of
dough, slightly diminishing dough extensibility, increasing dough consistency ,
reducing time to maximum consistency and accelerating dough breakdown during
mixing. Improvement in the quality of bread elaborated with LAC has been also
reported. The functional properties of bread dough greatly depend on the
proteins forming the gluten network. Strengthening enzymes affect different
protein fractions (glutenins, gliadins, albumins or globulins) depending on
their particular action mechanism. The type of protein being crosslinked
appears to be more important than the cross-linking agent or type of cross-link
formed and it is highly correlated with the character of qualitative changes in
the final product. Thus, while HMW glutenin subunits are correlated with
several macroscopic properties of dough and baked products (such as strength of
gluten network and volume), the albumins and globulins play an important role
in textural and crumb grain properties. For this reason, association of
different gluten modifying enzymes could be an excellent option to improve
overall quality of baked products. Besides the gluten network, another
secondary crosslinks among minor compounds of flour such as arabinoxylans and
pentosans can be promoted. The combined use the aforementioned enzymes with
non-starch polysaccharide degrading enzymes could induce synergistic effects on
dough behaviour or product quality. Combinations of hemicellulase/GO/?-amylase,
TG/amylase/hemicellulase and TG/pentosanase/?- amylase 53–55 have been
reported as bread quality enhancers. Amylolytic enzymes have been also proposed
as active contributors towards fresh bread quality and staling behaviour during
storage. The objective of this study was to analyse the individual and
synergistic effects of a wide range of enzymes currently used in bread-making
processes. In order to improve the response of some of the most representative
enzymes, the effect of combined use of gluten cross-linking.


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