review gluten free
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REVIEW PAPER
Possibilities to increase the quality in gluten-free breadproduction: an overview
Andreas Houben • Agnes Hochstotter •
Thomas Becker
Received: 9 November 2011 / Revised: 16 March 2012 / Accepted: 21 March 2012 / Published online: 15 June 2012
� Springer-Verlag 2012
Abstract The market for gluten-free products is
increasing. Owing to better diagnostic methods, more and
more people are identified to have coeliac diseases. Pro-
duction of bakery products that do not harm these people is
a big challenge for bakers and cereal scientists in the
twenty-first century. The use of different cereals and flours
makes it necessary to find possibilities to take over the task
of gluten by other flour ingredients, by the addition of
different components, by different flour and dough treat-
ment or by changing the method of baking. The purpose of
this review is to give an overview about the various pos-
sibilities to increase the baking quality of gluten-free
bakery products, increasing their water-binding capacity,
uniform the crumb structure and increase the final bread
volume. All the listed methods and ingredients are already
in single use helpful to increase the quality in gluten-free
bread production.
Keywords Gluten-free � Hydrocolloids � Rheology �Dough � Bread � Emulsifiers � Sourdough � Enzymes
Introduction
Baking without gluten is a big challenge for all bakers and
cereal researchers. The task of gluten to form a three-
dimensional protein network during dough preparation has
to be taken over by other ingredients in gluten-free baking.
In the recent years, owing to the increasing numbers of
people with coeliac diseases, the market for gluten-free
products has been increasing speedy. This haste is mostly
based on the improvement in the diagnosis of coeliac dis-
ease [1, 2]. The increasing market pushes the cereal
industry to increase its output of high-class gluten-free
products. To supply the market with high-quality products,
new developments and knowledge have to be aimed in
research and development [3, 4].
The absence of gluten in dough production shows high
influence on dough rheology, the production process and
the quality of the final gluten-free product. The gluten-free
doughs are much less cohesive and elastic than wheat
dough. They are highly smooth and difficult to handle; they
are more sticky, less elastic and pasty; and it is more like
handling the batter of a cake [5]. In literature, these gluten-
free doughs are often called batters instead of dough. The
doughs are not really kneaded by a lot of energy input, but
mostly mixed in mixing machines [6, 7].
The final products show some deficits in quality when
compared to French bread; their texture is crumble and
their crumbs are lighter colour [8, 9], and because of their
low carbon dioxide binding activity during raising, the
volume of the products are mostly lesser [10]. Based on the
missing interactions, the water molecules are not really
stiffly bounded in the crumb and they diffuse much faster
into the crust; this leads to a firmer crumb and softer crusts
[9]. A short shelf life, particles detection in the mouth
during consumption, a dry mouth feeling and a not really
satisfying taste are also some of the disadvantages of glu-
ten-free bread [11]. Development of new technologies and
the use of gluten-free flours, starches, hydrocolloids and
novel food ingredients will make it possible to find alter-
natives for the traditional bakery products. Especially,
changing the gas-binding capacity and the stabilization of
the starch gel during baking is the most important aspect
for being successful in reaching these aims [3]. The natural,
A. Houben (&) � A. Hochstotter � T. Becker
Lehrstuhl fur Brau- und Getranketechnologie, TU Munchen,
Weihenstephaner Steig 20, 85354 Freising, Germany
e-mail: [email protected]
123
Eur Food Res Technol (2012) 235:195–208
DOI 10.1007/s00217-012-1720-0
synthetic and biotechnological hydrocolloids, because of
their high water-binding capacity and their structure-cre-
ating behaviour, are mostly used in the different recipes for
replacing the gluten network and its functionality. Other
trials to replace the gluten are the use of other food proteins
like the one from soybean, eggs or milk [1]. Also the use of
enzymes can increase the gluten-free dough behaviour
required for shelf life and quality [10]. Because most of the
recipes are based on flours and starches that are by nature
poor in their nutritional level, use of different fibres,
wholemeal flour, addition of vitamins and minerals lead to
an increase in the nutritional level of the gluten-free
products [12].
Positive is also the use of a very traditional bakery
ingredient, of sourdough, because of it textural and sen-
sorial advantages [13]. All these treatments and ingredients
allow making gluten-free bakery products better in quality.
The aim of all these changes is to reach a final product
close to French bread quality [1].
Gluten-free flours and starches
Most non-gluten-free bakery products are based on wheat,
rye, barley or even oats. Among these, oats should be
gluten-free by nature but due to its planting and breeding
procedure, in literature it is often counted as the non-glu-
ten-free grains. So all these grains and flours are forbidden
in the gluten-free bread production. Only gluten-free
cereals and the so-called pseudocereals are allowed to be
taken as raw materials.
Gluten-free cereals are, for example, rice, maize and
millet. Up to now, two different rice species—Oryza sati-
va, originating from Asia, and Oryza glaberrima, culti-
vated in Africa [14]—and different subspecies of zea mays,
millet species and Sorghum bicolor L. Moench [15], teff
(Eragrostis tef (Zuccagni) Trotter), finger millet (Eleucine
coracana L. Gaertn.), pearl millet (Pennisetum glaucum
(L.) R. Br.) and foxtail millet (Setaria italica (L.)
P. Beauv.), are used in the production of gluten-free bakery
products [16]. Next to these grains and grasses, the
pseudocereals amaranth, buckwheat and quinoa are often
taken for gluten-free bakery products. The pseudocereals
do not belong to the monocots, like the cereals do, but
belong to the dicots. Some species used in human nutrition
are buckwheat (Fagopyrum esculentum Moench), tartary
buckwheat (Fagopyrum tataricum Gaertner), some ama-
ranth spp. like Amaranthus caudatus L., Amaranthus
cruentus L. and Amaranthus hypochondriacus and Che-
nopodium quinoa [17].
Next to gluten, pseudocereals are mainly taken as an
ingredient in gluten-free products because of their nutri-
tional level, high protein value, essential amino acids and
fatty acids and high mineral content [18]. The functionality
of the flours made from these grains and pseudocereals
depends on their particle size, the particle distribution, the
milling yield and the flour treatment. Also the growing
conditions and the plant species influence the composition
of the ingredients and by this the final product quality
[17, 18].
Next to gluten-free grains and pseudocereals, flours
made from legumes like chickpea, field bean, soya bean
and French bean, from cassava [18–20], chestnut, coconut,
linseed and from plantain are also used as starch additives
because of their water-binding capacity in the recipes of
gluten-free bakery products.
There is mostly a mixture of different gluten-free flours
and starches. Especially, the starches, the most important
stored carbohydrates in plants, have a high influence on the
dough parameters, the texture, the moisture retention and
the final quality [21].
The role of starch during baking is to bind the water and
create a gas-permeable structure [22]. Commercial gluten-
free starches are mostly obtained from rice, cassava,
potatoes and maize [1, 7, 20, 23]. Since 2008, there is also
a gluten-free wheat starch available on the market, whose
gluten content is beyond 20 mg kg-1, the borderline given
by the codex alimentarius; it does not harm most coeliac
patients [24].
The differences between the listed starches in their
thermal behaviour, gelatinization process, gel-forming
behaviour and final texture are based on individual com-
position and chemical structures [21]. Its functionality is
mostly influenced by its main components, the glucose
polymers amylose (linear, 1–4 glycolysed a-D-glucose
units) and amylopectin (complex bounded 1–4 and 1–6
glycolysed a-D-glucose units). These components are
associated with hydrogen binding into crystal and form an
insoluble kernel shape in cold water. By reaching a high
temperature during baking, the starch kernels irreversibly
swell by the uptake of water. The needed temperature, the
so-called gelatinization temperature, is a characteristic
parameter for every starch. The intermolecular hydrogen
bonds are broken up, and the hydroxyl groups released are
immediately hydrated. The special relation of the hydroxyl
groups of water is the basis of gelatinization. A further
increase in temperature leads to higher molecular activity
and by this to partial softening of the intermicellar network.
Some parts of the amylose molecules are colloidal dis-
solved, and the increase in hydration finally creates a paste
similar to starch mixture. The starches form a composite
gel network, consisting of swollen amylopectin, filling an
interpenetrating amylose gel matrix [25]. 21, 26By lower-
ing temperature and molecule activity, the starch paste
confirms again [21, 26]. The swelling behaviour, the
maximum water-binding capacity and the needed
196 Eur Food Res Technol (2012) 235:195–208
123
gelatinization temperature of the starch are influenced by its
origin, the species, its combination, the concentration and its
particle size, the amylose/amylopectin ratio, the starch treat-
ment, the moisture content, the pH value of the water and other
molecules in the mixture (e.g., sugar, fat, proteins, salt) and
hence, the rheological behaviour of the dough and also the final
bread crumb structure [21, 26–29]. Also the starch granule
shape, its molecular weight and particle size distribution are
responsible for the process of gelatinization [30]. A higher
degree of crystallinity results in requiring a higher gelatiniza-
tion temperature of the starch [31]. From nature, the amylose/
amylopectin ratio differs a lot; the amylose content is inversely
proportional to the gelatinization temperature [31, 32]. All for
gluten-free production used starches show to have a shear
thinning behaviour (pseudo plastic) [33]. The biggest influence
of the natural origin of starch is shown during the baking
process and in the dough density. In baking, the highest dough
density is reached by the use of wheat starch; all other starches
create doughs with more air included [25, 33–35]. Also the
highest bread volume is reached by the use of wheat starch,
while creating the dough with the lowest flow index and hence
with the best gas-expanding possibility [33, 36]. Higher starch
gelatinization temperatures lead to higher final bread volume,
because the change from batter and dough, as a fluid, aerated
emulsion to a solid, porous structure, takes place later and
allows increasing the volume for a longer time [37–39]. The
most similar in behaviour to gluten-free wheat starch is the rice
starch [33]. But if the gelatinization, because of a very high
gelatinization temperature, is not totally finished, the volume
is decreased after cooling down [26]. The addition of other
ingredients like shortenings, eggs or proteins to the dough
influences the gelatinization temperature as well [25, 33].
Next to the native starches, modified starches are used in
the food industry. They are made out of native starches by
chemical, enzymatical, mechanical and/or thermal treat-
ment and can be used for reaching specified aims in the
textural of bread baking [29, 40].
In gluten-free bread production, the most used modified
starches are the cross-linked ones like distarch phosphate
and distarch adipate, starch esters like monostarch phos-
phate and starch acetate, partial complex bounded or pre-
gelatinized starches and mechanical treated or extruded
starches. These modified starches are able to change the
dispensability, the water absorption, the swelling behav-
iour, the gelatinization temperature and the viscosity of the
dough. They are used as a thickening agent; they stabilize
the crumb structure and can decrease retrogradation [27].
Hydrocolloids and gums
In gluten-free baking, hydrocolloids can also be used as a
gluten replacer because of their character to stabilize the
products and increase their texture. Their chemical struc-
ture makes them mostly belonging to the polysaccharides.
They are often used as a thickening agent, helping in
swelling, for stabilization, for gelatinization and as a
humectant agent. In the gluten-free baking, hydrocolloids
used can be classified into the one from plant origin and the
chemical synthetic created ones. The plant origins can be
(a) from marine algae like agar–agar and carrageen,
(b) plant extracted like pectin and oat b-glucan, (c) plant
exudate like gum arabic and tragacanth, (d) seed mucilage
like locust bean gum, guar gum and psyllium, (e) starches
and modified starches and other natural hydrocolloids like
konjak. Next to these hydrocolloids, the chemically or
biochemically synthesized cellulose derivatives such as
hydroxypropyl methylcellulose (HPMC), carboxymethyl-
cellulose (CMC) and methylcellulose (MC) and microbial
biosynthetic hydrocolloids such as xanthan and proteins—
casein, soy protein and egg albumin—are also used [41–
43].
Polysaccharide hydrocolloids are formed by glycosidic-
bounded monosaccharide molecules in a linear and/or
cross-linked structure [27]. In gluten-free dough prepara-
tion, they are often used for creating the viscoelastic and
cohesive behaviour of gluten [44] and to increase the gas-
binding capacity by raising the viscosity [45, 46]. They
also interact with the swelling, the gelatinization and gel-
ling properties of the dough and the retrogradation of the
starch [47]. In dough, all hydrocolloids work together with
the water molecules included in dough; they reduce their
diffusion and support the stability of the system. Xanthan
gum, guar gum and CMC are soluble in cold water; indeed,
the hydrocolloids carrageenan, locust bean gum and most
alginates need hot water for their full hydration. The water
molecules are bound to the hydrocolloids in three different
ways: via hydrogen bounds, embedded in inter- or intra-
molecular openings or immobilized by structuring [48].
The hydrocolloids with a branched, tangled ball-shaped, or
chained structure are taken as thickening agents (hydration
of the macromolecule); indeed, hydrocolloids with a
thread-like, linear structure are taken as gel-forming
agents. The gel-forming process of hydrocolloids is real-
ized by connecting the fibril polymer molecules or polymer
molecule bunches, which are intermolecularly fixed to each
other by hydrogen bonds or by cross-linking of anionic
molecules by multivalent cations (calcium ions or pro-
teins). A three-dimensional network is by further linking of
these limited ordered structures formed out; the resulting
cavities have a defined water-holding capacity. When there
are regular and irregular sequence segments and the inter-
chain interaction of the sequence segments with normal
conformation is interrupted by irregular sequences in the
chain, gel forming is possible. In contrast to the gluten
network, where both fibril and film formations occur, the
Eur Food Res Technol (2012) 235:195–208 197
123
hydrocolloid networks show only to have a fibrillar char-
acter. The dominant type of bounding is also different.
Hydrogen bonds, cationic cross-links and hydrophobic
interactions are common in hydrocolloid networks; in the
gluten networks, there are mostly covalent disulphide
bonds, electrostatic, van der Waals and dipole–dipole
interactions, hydrogen bonding and hydrophobic associa-
tions. Because the sulfhydryl groups in hydrocolloids are
absent in gel formation, the presence of reducing agents
and oxidation agents is not necessary. In gluten-free bak-
ing, hydrocolloids are often used singly or in combination,
always based on their technological effect on dough and
the final product [27, 42, 49–51].
However, there exist specific characteristics and effects,
for example, cold and hot solubility, of each hydrocolloid
for all kind of doughs. For example, xanthan (linear,
anionic and substituted polymer) is able to create, over a
huge temperature range, a constant high viscosity and
finally forms a weak, cold-set gel out [52–55].
The modified cellulose derivative HPMC (linear and
neutral polymer) has, because of its hydrophilic character,
a high water-binding capacity and also has, in its structure,
hydrophobic methyl and hydrophilic hydroxypropyl groups
located, which makes HPMC an interface activity in the
dough system during the resting period (promoting dis-
persion/preventing coalescence of the gas bubbles). The
molecular structure of HPMC is shown in Fig. 1; its foam
forming is shown in Fig. 2 [56, 57].
HPMC can create a reversible, heat-set gel network
[56, 57] that leads to an increase in dough viscosity and
stabilization of the boundaries of the expanding gas cells.
During baking, the gas-binding capacity is increased and
higher volume can be, as shown in Fig. 3, reached in the
final bread [43].
The influence of the hydrocolloid on the dough rheology
and the bread quality, especially the final volume and the
crumb texture, depends on the specific options of the
hydrocolloid used, like molecular mass, molecular struc-
ture, chain length and bonds and chemical modification;
the added amount; the flour and starch raw material used;
other recipe ingredients; and the process parameters used
like pH value, temperature, shearing, ionic bonds and the
attendance of ions [21, 61–63]. The results by the addition
of a hydrocolloid are based on the increase in stiffness due
to a decline in the starch swelling and the decrease in
dissolved amylose. By inhibiting the inter-particle inter-
actions of the swollen starch granules, the hydrocolloid
softens the starch network [63].
The functionality of the different hydrocolloids in the
system dough is created and influenced by the interactions
with the other added food polymers, like starch and pro-
teins. It is well known that these ingredients influence each
other and the final bread quality, but up to now, there is less
knowledge about the mechanism of these interactions [21].
Proteins
To form a network similar to what gluten does in bread
production, proteins can also be added during dough
preparation [7, 64, 65]. The proteins used can be like milk
proteins and egg albumins from animal origin or like the
proteins of soya, taken from plants.
Milk proteins have a high nutritional level and are quite
often used because their chemical structure is quite similar
to the one of gluten proteins [66]. They pretend to swell in
a high level, and they are also able to build up a network
[47, 64]. In Fig. 4, the addition of milk protein to a gluten-
free flour mixture is shown in comparison to a French
bread with a three-dimensional gluten network and a
commercial gluten-free mixture.
Depending on the kind of milk protein, a specific change
of the product quality can be reached. Caseinate is a good
emulsifier and is able to stabilize a batter; isolated and
concentrated whey proteins can form gels, and high tem-
perature skim milk powders have a high water-binding
capacity [67]. The functionality of the quite heat-stable
hydrophobic caseinate molecules is linked to their aggre-
gated status as caseinate complex or caseinate micelles.
Because of their flexibility, there is nearly no cysteine and
cysteine, open and on the environment-based conforma-
tion, the monomers are different from other milk proteins.
The micelles are not stiff pressed, but they are very porous,
and so, they are dissolved in water. The main whey
Fig. 1 Molecular structure of
HPMC (b-(1-4)-glycosidic
bounded glucose units, partial
substituted by methyl or
hydroxypropyl groups [58]
198 Eur Food Res Technol (2012) 235:195–208
123
Fig. 2 Foam-forming behaviour of HPMC (left, q = 0.62 g/cm3) and xanthan gum (right, q = 0.83 g/cm3) in a level of 2 % added to water;
scale bar 5 mm [59]
Fig. 3 Yeasted bread made from wheat starch without the addition of
hydrocolloids (reached total volume 1650 ml; received volume out of
1 g flour: 3,9 cm3) left hand side; yeasted bread made from wheat
starch and addition of 2 % xanthan (2000 ml, 3,5 cm3) middle;
yeasted bread made from wheat starch and addition of 2 % HPMC
(2490 ml, 5,3 cm3) right hand side (original procedure of Jongh)
[3, 60]
Fig. 4 Comparison of different bread types. On the top, the surface
and a typical slice view; beyond the demonstration of pictures
received in CLSM (benchmark scale 50 lm); wheat bread sample (a);
gluten-free bread from a commercial gluten-free flour mix (b); gluten-
free bread sample without the addition of milk protein (c); gluten-free
bread with added milk protein (d) [7]
Eur Food Res Technol (2012) 235:195–208 199
123
proteins, the a-lactalbumin (four disulphide bonds) and the
b-lactoglobulin (one free thiol group and two disulphide
bonds), which can be a monomer, dimer and an oligomer
depending on pH value, ionic strength and temperature,
have a globular structure and hydrophobic, compact folded
polypeptide chain. The heath sensitivity of the b-lacto-
globulin can be used via temperature treatment for partial
up-folding and aggregation of the whey proteins by thiol
disulphide interactions and hydrophobic bonding. The
resulting porous structures absorb and immobilize the water,
increase the water uptake, give stability to the dough and the
final product, and elongate shelf life [27, 47, 68, 69].
Next to its functional benefit, the addition of milk pro-
teins and essential amino acids like lysine, methionine and
tryptophan also increases the nutritional level of the gluten-
free bakery products [70]. During dough production, the
addition of dairy-based ingredients increases the water-
binding capacity, lowers the dough stickiness and makes
dough behave more plastic [1]; in the final bread, it
increases the volume and improves the texture, taste, crust
colour and the shelf life [70–74]. The addition of high-
protein and poor lactose milk-protein ingredients shows a
clear quality increase [64]. The colouring of the crust is
based on different maillard and caramelization reactions
supported by milk products including lactose [64, 67]. But
there are also some disadvantages in the use of milk pro-
teins. First, for people with small intestine inflammation
because of coeliac disease, there is often a link also to
lactose intolerance. These people cannot metabolize the
lactose because of a secondary lactose intolerance, when
the enzyme lactase, which is normally located in the small
intestine mucosa, is nearly absent because of the villous
atrophy [75].
The second reason is that milk proteins can also be the
activator of an allergenic reaction. And in low-protein
diets, bakery products with a low allergenic potential are
taken [3]. Because of their composition and their way of
production in the market, existing milky ingredients for the
bakeries have different functionality [70, 73].
Next to milk proteins, the proteins of surimi, soya beans
and egg proteins can also be used for the addition of protein
in gluten-free bakery products [1]. Surimi has a high
functionality and shows a good gel-forming behaviour
[76], so it is used to replace protein in gluten-free baking
[77]. Its creation of stiff, cohesive gels is linked to the high
content of highly elastic actomyosin and myosin com-
plexes or myosin molecules [78–80]. The thermo-irre-
versible hydrogels are created by the thermal denaturation
and dissociation of, for example, the actomyosin com-
plexes, and reforming of intermolecular hydrophobic and
covalent bonds. The water molecules are immobilized by
clathration [81]. The dissociation of actomyosin and
myosin complexes is supported by the addition of salt and
increases the cohesive and elastic texture of the gel [82].
The use of four different surimi preparations was tested in a
gluten-free recipe, based on rice and potato starch [77].
Three of these surimi products were able, in comparison
with a standard recipe, to enhance the crust colour, create a
softer crumb and reach a higher bread volume. Only one
sample was able to enhance the taste of the bread. Besides
all the positive effects detailed so far, it is up to now not
known whether the customer will accept the fish taste
proteins in gluten-free bakery products [3]. And, next to the
milk proteins, the fish, the soya and the egg proteins can
also be responsible for allergenic reactions. Also these
ingredients cannot be allowed in bakery products for a low-
protein diet [3].
Soya protein, also high in its essential amino acid lysine
content, can be added in the form of high-protein soya flour
or as a soya protein isolate and can lead to an increase in
crumb texture and bread volume [20, 83, 84].
Legume proteins show strong gel-forming behaviour
and can be used for the production of emulsions and foams.
Their functionality depends on the environmental param-
eters like pH value, ionic strength and temperature. The
soya proteins are divided into the two heterogeneous
groups, globulin (90 % of the total amount) and albumin
(10 %). The polymeric main components of the globulin
fraction, the 7S and 11S globulins, are formed by 3 or 6
subunits, which are glycolysed or have one disulphide bond
[27]. In the amino acid composition of soya globulin, there
is a high content of asparagine, aspartic acid, glutamic acid
or glutamine. Stabilization of the subunits and the overall
structure is reached by hydrophobic interactions. By ther-
mal treatment, the intramolecular interactions are stopped
and intermolecular interactions and aggregation via
hydrogen and ionic bonds start [85].
As a gluten replacer, egg proteins can also be used. Due
to their border areas activity, they are considered for bak-
ing as a foaming agent, as a crumb stabilizer and for cre-
ating a good shape. Especially, the heat coagulation of the
protein and the egg yolk contained phospholipids and
lipoproteins as emulsifiers facilitate the dispersion and
stabilization of gas bubbles in the gluten-free dough sys-
tems [3, 86]. The swelling of the egg proteins in gluten-free
dough lead to a viscous fluid that shows a similar network
protein structure function than the one known from gluten
[7, 87]. During the addition of egg white build-up, thermo-
reversible protein gels are based on hydrophobic interac-
tions. Except the nine thermostable disulphide bonds
including ovomucin, all other egg white proteins are
responsible for the gel-forming process [83]. These phe-
nomena form the protein structure and are able to give
stability to the dough.
Other sulphur containing egg white proteins, especially
the main egg white protein, ovalbumin (54 %), which
200 Eur Food Res Technol (2012) 235:195–208
123
includes four thiol and one disulphide group, stabilize the
gel by polymerization via the thiol disulphide exchange.
Also the coagulation of the egg yolk during thermal
treatment creates similar thermo-irreversible gels. In the
baking industry, the egg yolk is often used as an emulsi-
fying agent [27, 88]. An increase in the bread volume and
the amount of pores per square centimetre because of the
addition of full egg powder was shown by Moore [89].
Figure 5 shows that the positive effect of an addition of egg
powder can be increased further by the additional use of the
enzyme TG; here a layer-like protein structure is received
by covalent isopeptide bonds.
Jonagh et. al. [90] showed that egg albumin can increase the
gas-binding capacity by connecting the starch granules. In
comparison with the other protein sources, the best crumb
texture of gluten-free bread was reached by the addition of full
egg powder [89]. By the addition of already foamed egg, it was
possible to increase the gas binding during dough preparation
and stabilize the bread structure [91].
The ovalbumin inside the egg white denatures and
aggregates during pitching because of the increase in the
interface of liquid and air. Stable gas foam is created,
which looses the dough and stabilizes the dispensation of
further ingredients. The egg white protein ovomucin sta-
bilizes the gas bubbles by forming fibrillar structures, and
is very effective. During baking, the protein network
coagulates and prevents the coincidence of the bread [27].
Next to this, the protein network is able to reduce the
swelling and gel forming of the starch.
Enzymes
A lot of enzymes are naturally included in the raw mate-
rials, like in most flours. But not to influence negatively the
final product quality by destroying the crumb structure or
decreasing the volume, they are mostly inactivated during
the various production steps [47]. In gluten-free and also in
gluten-contending recipes, enzymes are very often added to
improve the dough-handling properties and to increase the
final baking quality. Depending on the enzyme activity, the
water-binding capacity, the shelf life, the retrogradation
and the crumb softness can be influenced positively [10].
Some of those enzymes that are often used in gluten-free
bread production are the starch-modifying amylase,
cyclodextrin glycosyltransferases (CGTase, EC. 2.4.1.19)
or the protein-connecting transglutaminase (TG, EC.
2.3.2.13). Also glucose oxidase (GO, EC. 1.1.3.4), laccase
(EC. 1.10.3.2) and proteases can be found in the recipes
[10, 89, 92–95]. Some of these enzymes are essential for
reaching higher quality in gluten-free bread.
In protein cross-linking, one of the reactions catalysed
by the enzyme transglutaminase (TG) is the important
trans-acylation reaction between, as an acyl donor, the
c-carboxamide group of a protein- or peptide-bound glu-
tamine and primary amino acyl-acceptor. Inter- or intra-
molecular covalent isopeptide bonds are formed if this
acceptor is a e-amino group of a lysine residue [94, 96, 97].
In Fig. 6, the differences by TG catalysed reactions are
shown.
The transglutaminase used in baking is always from
microbiological origin. The enzyme shows different
activity depending on the accessibility of glutamine and
lysyl residues in the proteins [99, 100]. A high TG activity
can be expected by the composition of the caseinate and
soya proteins, but also of some fractions of the egg, wheat,
meat and whey proteins [97, 101]. It is possible to create,
by the addition of transglutaminase, a network in high-
protein-content, gluten-free baking products whose stabil-
ity depends on the protein origin, its thermal compatibility
Fig. 5 CLSM pictures of gluten-free bread crumb; addition of egg
powder (a), addition of egg powder and 1 U TG per gram protein (b);
A denser and more branched network is formed out by the addition of
TG; the crumb was coloured with safranin, size was 63 times
magnified, size scale is 50 lm [89]
Eur Food Res Technol (2012) 235:195–208 201
123
[102] and the dosage of the enzyme. The addition of skim
milk powder, an egg protein powder, forms with protein
networks and increases the stiffness and the sappiness of
the crumb. A lower baking loss can additionally be
reached.
In combination with skim milk powder, a higher addi-
tion of TG leads to a high increase in crumb firmness but
also to an increase in the amount of pores per square
centimetre. In rice flour-based bread, the addition of TG
leads to cross-linking [103]. In Fig. 7, an example for
increasing baking quality by the addition of TG is shown
[104]. In this study, it was worked out that special buck-
wheat and brown rice flour pretend to be good TG sub-
strates, whereas sorghum, teff and oat flour did not work
out that good. Buckwheat and rice naturally include high
amounts of glutamine and lysine [84]. The doughs based on
maize flour increase their baking quality by TG addition
not by protein cross-linking but by deamination of the
glutamine residues.
The enzyme GO received from fungus catalyses the
oxidation of b-D-glucose to D-gluconolactone or D-glucon
acid and hydrogen peroxide. The hard oxidizing agent
hydrogen peroxide interacts as well with the very reactive
thiol groups of the proteins by forming disulphide bonds
[10]; moreover, it creates the cross-linking of the water-
insoluble pentosans via oxidation of the ferula acid [105]
and creation of other non-disulphide bonds like dityrosine
and dehydroferulic acid-protein bonds [103, 105, 106]. The
enzyme laccase catalyses by a similar effect the oxidation
of phenolic substances under reduction of oxygen. By
cross-linking proteins and proteins with arabinoxylans,
laccase is able to stabilize the dough structure [107]. The
cleavage of the a-1, 4 glycosidic bonds and the sometimes
Fig. 6 TG catalysed reactions. a acyl group transfer between the
c- carboxamide group of a protein or peptide bound Gln residue and a
primary amine; b cross-linking between protein bound Gln and Lys
residues to form a e-(c-glutamic)-lysine isopeptide bond; c deamina-
tion of the Gln residue by water [98]
Fig. 7 Some slices of gluten-free bread based on buckwheat flour (BW), brown rice flour (BR) and maize flour (CR) after addition of different
levels of the enzyme transglutaminase; 0, 0.1 and 10 U TG per gram protein [104]
202 Eur Food Res Technol (2012) 235:195–208
123
simultaneous cyclization of the resulting fragments inside
the starch molecules is catalysed be the enzyme CGTase
[108]. These cyclodextrins give, by the formation of
inclusion complexes with lipids or hydrophobic proteins,
emulsifying properties to the dough and can lead to an
increase in the crumb structure and a better gas binding in
gluten-free rice bread and finally aim in a higher specific
volume [93]. Also staling of gluten-free bread because of
the cyclodextrins reportedly decreased, based on interac-
tion with retrogradation of the amylopectin and other
interactions between proteins and starch [109].
In the past, most enzymes were sold immobilized on wheat
flour or wheat starch as carrier, but this phenomenon no longer
exists [10]. For using combinations of different enzymes in
gluten-free baking, presence of any antagonistic effects must
be checked first. The existence of the needed enzymes for
gluten-free products is the only limiting factor.
The use of sourdough
Sourdough fermentation is one of the most used and most
effective way of increasing flavour, taste, shelf life and the
structure of all kinds of bread [110–112]. Next to traditional
use in rye- and wheat-based bread production, it is also
widespread in gluten-free bread production [13, 110, 113]. It
has already been proved [13] that small amounts of lactic acid
bacteria-fermented sourdough in gluten-free dough produc-
tion, in comparison with chemical acidified and non-acidified
dough, can increase the viscosity, homogenize the crumb
structure, elongate the shelf life and create a stronger flavour
[13, 114, 115]. There is no substrate and activity limitation for
the lactic acid bacteria in gluten-free flours, but its fermen-
tation can differ between the strains used. There is always a
need for a compatibility test of substrate and starter strain
[113, 116–119] [114]. Next to the changes in bread quality,
the microbiological stability of gluten-free bread is also
increased by the use of sourdough [120]. The production of
antifungal organic acids (lactic acid, phenyl lactic) and cyclic
dipeptides (cyclo (L-Leu-L-Pro), cyclo (L-Phe-L-Pro)) can
delay the spoilage of bread [13]. During fermentation build-
up, organic acids and free amino acids, depending on the
selected starter culture, are responsible for the increase in
taste by the use of sourdough [121]. Next to the already listed
benefits of the use of sourdough in gluten-free bread pro-
duction, there can also be a helpful enzyme and exopoly-
saccharide production in the selected lactic acid bacteria.
During the fermentation of sucrose or raffinose, exopolysac-
charides and oligosaccharides are formed and lead to an
increase in the water-binding capacity. This optimizes the
starch gelatinization, slows staling and improves shelf life of
gluten-free bread [122–124]. The homopolysaccharides are
important in sourdough. The most common ones are dextran
and levan; glucan and fructan polymers that are built from
glucose and fructose units, and the synthesis is catalysed by
extracellular glucosyltransferases. Some of the homopoly-
saccharides (e.g., levan of L. sanfranciscensis) show a pre-
biotic effect and support the bifidobacteria in the intestine
[125]. The positive effect of sourdough addition on bread
volume and crumb structure is shown in Fig. 8 [126]. Next to
the use of sourdough, the use of chemical acids can also have
positive effects in gluten-free bread production [127].
Fat, oil and emulsifiers
The stabilization of the gas bubbles in bread dough is often
reached by the addition of fat. During kneading, the fat crys-
tals adsorb at the gas bubbles interface inside the dough, and
during baking, they melt and give the gas bubbles the
Fig. 8 Slice from gluten-free sorghum bread, the used flour was a mixture of 70 % sorghum and 30 % potato flour and an addition of 2 %
HPMC; a no use of sourdough b use of 70 % sorghum sourdough [126]
Eur Food Res Technol (2012) 235:195–208 203
123
possibility to expand without destruction [128, 129]. Next to a
decrease of the starch gelatinization and the starch solubility in
gluten-free bread production, the addition of margarine leads
to an increase in the gas-binding capacity [91]. By the addition
of vegetable oil, the kneading resistance is decreased and also
the swelling of the starch granules is reduced by complexation
of amylose with the monoacyl lipids [93]. The vegetable oil
results in a softer crumb and an increase in the specific volume
too [93]. An additional increase in the dough stickiness was
reported by Schober et al. [130].
Emulsifiers can be used to increase the dough stiffness,
improve the bread structure and decrease the speed of
staling. Due to their amphiphilic nature and their ability to
migrate to interfaces, they can reduce surface tension and
produce stable dispersions for the crumb [131]. By inter-
action with the starch molecules, they retard retrogradation
and inhibit the migration by immobilization of the water
[132]. They increase the gas absorption of the dough by
reducing the gas bubble surface tension [133]. Defloor
et al. [19] showed that an increase in the bread volume, a
change of the dough viscosity and a decrease in drying-out
process of the crumb are reachable by the use of glycerol
monostearate. By the addition of emulsifiers and fat, it
should always be kept in mind that overdosage cannot lead
to a total loss of crumb structure. Not only the right
emulsifier and fat but also the right dosage is important [3].
Conclusion and outview
The various listed-up methods show that not just one single
ingredient in gluten-free bread production can replace the
gluten and its functionality. The use of one enzyme, of one
additional starch, natural or modified, of just sourdough or
the addition of just one hydrocolloid can already increase
dough and bread quality. But a costumer-satisfying struc-
ture, a high volume and a good taste can only be reached by
a composition of replacers. The complexity of these addi-
tives shows the big challenge of cereal technology in the
twenty-first century. The next step is to work out the
interactions of these functional additives with the basic
ingredients. Not all functional additives show the same
activity and the same result for all starches and flours, so
there are no general statements possible.
A great benefit can be further use of sourdough. It
increases enzymatic activity and gives next to functional
benefit also an additional aroma to the product and elon-
gates shelf life. Great use in dough handling and crumb
softness can be expected by the addition of protein-con-
necting and also starch-crashing enzymes in the right
dosage.
Next to changes in the recipe, changes in dough prep-
aration and the use of dough pretreatment forecast to be a
good way in gluten-free bread production. A lot of work is
already done in the last years in high-pressure (HP) treat-
ment for polymer gluten-free ingredients. High-pressure
treatment is a non-thermal step to modify or even dena-
turize the proteins and change the properties of carbohy-
drates and fats [134]. There is a decrease in the quality of
oat bread made by high-pressure treatment above 350 MPa
[135]. This HP works out covalent bondages in the protein
network and leads to a gelatinization of the starch; finally,
it comes to an increase in dough stiffness and elasticity.
The final bread is smaller in size, and its crumb is harder
Fig. 9 First line oat dough samples with different pressure treat-
ments; on the left side the control without any special treatment; in
the other doughs 20 % of hp treated dough is added (200/350/
500 MPa); in the second line bread samples of the final oat bread.
Left side, bread made out of the dough without treatment, the other
breads are made by the addition of 10 % hp treated dough to the basic
recipe (200/350/500 MPa) [135]
204 Eur Food Res Technol (2012) 235:195–208
123
than without HP (Fig. 9). Smaller pressure levels weaken
the protein network by dissociation and weakening
hydrophobic and electrostatic bondages. The reallocation
of water and changes in the protein starch interactions
finally lead to homogeneous crumb pores. The addition of
10 per cent hp (200 MPa) oat dough to the basic dough
leads to an increase in the volume and a retardation in
staling of the final bread [135, 136].
For the hp treatment, next to the right pressure level, it is
necessary to find the optimal ratio between increase in
bread quality and decrease in staling.
The development in gluten-free baking products still
goes on. Further investigations and research are necessary
to increase quality and offer coeliac disease patients high-
quality baking products. With an increase in the request
for gluten-free baking products by non-coeliac disease
patients, the speed of developments will raise and the
prices for the final products on the market will go down.
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