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Abstract

For the production of "snack" foods, maize, wheat, rye and rice are used as basic ingredients. With the development of extrusion technology, special attention is focused on the enrichment of extruded products with different ingredients like proteins, dietary fibre or bioactive compounds. Physical and sensory properties of the extrudates are strongly affected by adding ingredients rich in proteins or fibre. Extrusion parameters like temperature, screw speed and water content are crucial for obtaining an acceptable product. In this paper, review of the newest research and achievements in incorporating various raw materials that improve nutritional value of the extruded food products is presented.
Journal of Food and Nutrition Research (ISSN 1336-8672) Vol. 53, 2014, No. 3, pp. 189–206
© 2014 Národné poľnohospodárske a potravinárske centrum (Slovakia) 189
Extrusion is predominantly a thermomechani-
cal processing operation that combines several
unit operations, including mixing, kneading, shear-
ing, conveying, heating, cooling, forming, partial
drying or puffing, depending on the material and
equipment used [1]. Food extruders belong to the
family of HTST-equipment (high temperature
short time), capable of performing cooking tasks
under high pressure. This is advantageous for vul-
nerable food as exposure to high temperatures for
only a short time will restrict the unwanted de-
naturation effects on proteins, amino acids, vita-
mins, starch and enzymes [2]. The most used raw
materials in the extrusion process are starch- and
protein-based materials, which form the struc-
ture of the extruded products. Most products,
such as breakfast cereals, snacks and biscuits, are
formed from starch, while protein is used to pro-
duce products that have meat-like characteristics
and that are used either as full or partial replace-
ments for meat in ready meals and dried foods [3].
Extruded foods are composed mainly of cereals,
starches and/or vegetable proteins. The major role
of these ingredients is to give structure, texture,
mouth feel, bulk and many other characteristics
desired for the specific finished products. While
maize starch provides all the features for produc-
tion of highly acceptable extruded snack foods, its
nutritional value is far from satisfying the needs of
health-conscious consumers [4].
Functional food is any fresh or processed
food that is claimed to have a health-promoting
and/or disease-preventing property beyond the
basic nutritional function of supplying nutrients.
These foods may help prevent disease, reduce
the risk of developing disease, or enhance health
[5]. Functional foods can also be useful in making
nutrients more available by providing particular
dietary components in foods that will increase
their availability and palatability beyond that
which might normally be consumed [6]. Functional
foods represent one of the most interesting areas
of research and innovation in the food industry. In
Europe, functional foods sales have increased sig-
nificantly, although demand for functional foods
within European Union varies considerably from
country to country mainly due to food traditions
and cultural heritage [7]. Extrusion is flexible in
Improvement of nutritional and functional properties
of extruded food products
VALENTINA OBRADOVIĆ – JURISLAV BABIĆ – DRAGO ŠUBARIĆ
ĐURĐICA AČKAR – ANTUN JOZINOVIĆ
Summary
For the production of “snack” foods, maize, wheat, rye and rice are used as basic ingredients. With the development of
extrusion technology, special attention is focused on the enrichment of extruded products with different ingredients like
proteins, dietary fibre or bioactive compounds. Physical and sensory properties of the extrudates are strongly affected
by adding ingredients rich in proteins or fibre. Extrusion parameters like temperature, screw speed and water content
are crucial for obtaining an acceptable product. In this paper, review of the newest research and achievements in incor-
porating various raw materials that improve nutritional value of the extruded food products is presented.
Keywords
extrusion; extrudate; protein; dietary fibre; anthocyanin; carotenoid
Valentina Obradović, Agricultural Department, University of Applied Sciences in Požega, Vukovarska 17, 34 000 Požega,
Croatia.
Jurislav Babić, Drago Šubarić, Đurđica Ačkar, Antun Jozinović, Subdepartment of Technology of Carbohydrates, Faculty
of Food Technology, University of Osijek, Franje Kuhača 20, 31 000 Osijek, Croatia.
Correspondence author:
Babić Jurislav, tel. +385 31 224 333, e-mail: jbabic@ptfos.hr
REVIEW
Obradović, V. et al. J. Food Nutr. Res., 53, 2014, pp. 189–206
190
foods, which might expose new sites for enzyme
attack [14]. Disulfide bonds are involved in sta-
bilizing the native tertiary configurations of most
proteins. Their disruption during extrusion aids
in protein unfolding and thus digestibility [1]. Di-
sulfide bonds break and reform, while new elec-
trostatic and hydrophobic interactions promote
aggregate formation [18]. The aggregation of soya
protein subunits in extruded samples showed that
hydrophobic interactions, hydrogen bonds, di-
sulfide bonds and their interactions collectively
hold the structure of extrudate; and the impor-
tance of non-covalent bonds outweighs covalent
bonds. Increasing the food moisture content could
increase the interactions between disulfide bonds
and hydrogen bonds, and between disulfide bonds
and hydrophobic interactions, reduce the degree
of aggregation and the difference in protein-pro-
tein interactions and protein subunits among dif-
ferent zones within the extruder [19]. Increasing
specific mechanical energy can enhance the ex-
tent of breakdown of soya protein aggregates and
increase the proportion of the smaller fraction,
which indicates that protein was disassociated/
depolymerized by mechanical shear in extrusion
cooking [20].
One major constraint in the utilization of pro-
tein-rich crops is the presence of a number of an-
tinutritional compounds, in particular the trypsin
inhibitor, phytic acid and tannins. Extrusion cook-
ing is one effective method of inactivating the
trypsin inhibitor and other antinutrients in food
[21, 22].
Addition of soya protein and proteins
of other legumes
A reappraisal effect of legume seed dietary
intake is currently taking place. Soya proteins
have been widely used as a functional ingredient
in many processed foods because of their abil-
ity to form gels with high nutritional, sensory
and physio logical qualities [23]. The use of plant
proteins, such as soya protein, instead of animal
proteins is a cheaper and more viable interven-
tion strategy to reduce the risk of coronary heart
disease [24]. Animal and human studies have in-
dicated that the presence of soya in the diet im-
proves cardiovascular health and has particularly
beneficial effects for those with elevated low den-
sity lipoproteins and hyperlipidemia [25, 26]. In
recognition of the health-promoting properties
of soya foods, the Food and Drug administration
(FDA) allowed in 1999 food companies to use
health claims on soya-derived foods containing at
least 6.25 g of soy protein per serving [27]. Since
FDA approval linking soya protein consumption
the production of new products, such as cereal
baby foods, breakfast cereals, snack foods, bakery
products or pasta. In order to combine the need
of ready-to-eat products with the need for the con-
sumption of high-value products, beneficial ingre-
dients are added to the extruded mixtures [8].
A relevant issue is whether the consumers are
willing to accept functional foods that taste worse
than the corresponding conventional foods, and if
so, what is their profile and what are the determi-
nants of their willingness to compromise on taste
[9]. Addition of high-fibre, high-protein alternate
ingredients to starch was demonstrated to signifi-
cantly affect the texture, expansion and overall
acceptability of extruded snacks [4]. A scheme for
the development of the extruded functional food
was published by GUY [10]: Extruded products
can promote weight loss by adding dietary fibre
and resistant starch to the products, or can impact
heart health by adding antioxidants and minimiz-
ing fat, or can promote immune protection by add-
ing inulin and dairy protein.
Reviews of various chemical changes during
extrusion were published by CHEFTEL [11],
CAMIRE et al. [12], AREAS [13], SINGH et al. [14],
BRENNAN et al. [15] and ROBIN et al. [16]. These
studies explain the nutritional changes of certain
compounds. The aim of this paper is to review
the research that cover different aspects of the ex-
trudates fortification, with the accent not only on
nutritional improvement, but also on physical and
sensory properties of the extrudates, which are
crucial for the actual acceptance. This paper con-
tains an attempt to improve different nutritional
characteristics: protein, fibre and bioactive com-
pounds content.
IMPROVEMENT OF THE PROTEIN
CONTENT OF THE EXTRUDATES
Raw ingredients usually used for extrusion
contain considerable protein amounts, which vary
from 6.0% to 10.3% in various types of flours such
as barley, maize, rice, rye and wheat flour [17].
Soya and whey proteins are used for protein forti-
fication of extrudates.
Protein nutritional value is dependent on the
quantity, digestibility and availability of essen-
tial amino acids. Digestibility is considered as the
most important determinant of protein quality in
adults. The nutritional value in vegetable proteins
is usually enhanced by mild extrusion cooking con-
ditions, owing to an increase in digestibility. It is
probably a result of protein denaturation and inac-
tivation of enzyme inhibitors present in raw plant
Improvement of nutritional and functional properties of extruded food products
191
to reductions in cholesterol level, there has been
increased production, marketing and consumption
of soybean and its products [24, 28]. It should be
pointed out that European Food Safety Authority
did not approve this health-claim [24].
Soybeans contain approximately 42% pro-
teins, 20% lipids, 33% saccharides and 5% ash on
dry basis [29]. The important issue is whether the
addition of soya proteins makes the physical and
sensory properties of the extrudates acceptable for
consuming. Fortification of cereal-based snacks
with soybean naturally has a positive effect on
chemical properties. On the other hand, there is
a negative effect on the physical and consequently
sensory characteristics. Some examples are shown
in Tab. 1. Addition of soybean to the extrusion
mix led to poor product texture and, as a result,
lowered the consumer acceptance. However, some
conflicting findings have also been reported. The
porous texture and crispness of the samples were
improved by soya flour. Thus, the consumer panel
showed better purchasing intent of the samples
made from a mixture fortified with soybean.
Soya protein concentrate forms small uni-
form pores in the extruded products after being
squeezed out of the die, as soya protein concen-
trate can work as high-quality emulsifier between
hydrophilic and hydrophobic materials by expos-
ing the hydrophilic and hydrophobic groups to
their respective phases. The thickness of the wall
of the pores becomes thinner when the amount of
soya protein concentrate increases and the soya
protein concentrate absorbs high amounts of wa-
ter. Thus, it is logical to expect an increasing bulk
density of the extrudates with increasing the pro-
tein and moisture contents. High density product
naturally offers high breaking stress because air
cell membrane of the extrudates becomes harder
due to the high soya protein concentrate content
[34].
The role of starch and proteins in compounded
formulations for expanded snacks should also be
considered [35]. Starch gelatinization during ex-
trusion processing has a big influence on bulk den-
sity of extrudates. The low processing temperature
decreases the extent of gelatinization, which leads
to low swelling, low volume and high bulk densi-
ty [34]. Starch-protein interactions also probably
play an important role in affecting the expansion
either indirectly through specific mechanical ener-
gy (SME), or directly by disrupting the continuous
starch matrix and thus reducing the extensibility of
cell walls. Water absorption index (WAI) and wa-
ter solubility index (WSI) are related to the degree
of starch fragmentation. Higher WAI indicates the
presence of larger starch fragments, while higher
WSI implies that starch has been dextrinized. In
general, WAI decreases with increasing the soya
protein concentrate level, mainly because of a re-
duction in the starch content. WSI appeared to in-
crease with increasing soya protein content, which
was confirmed by gel permeation chromatogra-
phy. The addition of soya protein concentrate
also depressed the SME, in particular at levels up
to 10%. At that level, a drop in melt viscosity due
to the lipid and fibre contents of the soya protein
concentrate caused a reduction in SME. Beyond
10% soya protein concentrate, protein interaction
effects had an increased contribution, counteract-
ing the effects of lipids and fibre [35].
Mixing different ingredients to make a puffed
ready-to-eat product using the extrusion process is
difficult. Oat bran, for example, has a high level of
lipids and soluble gum. Combination of oat bran,
soya flour and maize starch is good for obtaining
high-fibre, high-protein extrudate, but reduces
the starch level in the mixture, which is undesir-
able due to the resulting increase in hardness. The
addition of inulin showed a positive effect on the
extrusion of the mixture. Inulin has low degree
of polymerization and can provide a lubricating
effect, which is essential to impart flow to mixtures
during extrusion [36].
Lysine is the limiting essential amino acid in
cereals [10]. Extrusion of a soya-sweet potato sys-
tem might favour Maillard reaction and of course
lysine loss due to the presence of both reducing
saccharides and the epsilon-amino group of lysine.
Losses were more pronounced at increasing levels
of soya addition, as it has a higher lysine content
than sweet potato. Increase in screw speed in-
creased the lysine retention owing possibly to
reduced residence time of the mixture in the ex-
truder [37]. During extrusion of rice-based snack
fortified with protein, increasing the raw material
moisture and reducing the barrel temperature en-
hanced lysine retention, but the best expansion
was at low moisture and high barrel temperatures.
Interestingly, the protein and moisture contents of
raw material and barrel temperature had no sig-
nificant influence on cysteine and methionine con-
tents [31].
High barrel temperatures and low moisture
promote Maillard reactions during extrusion. Re-
ducing saccharides, including those formed during
shear of starch and saccharose, can react with
lysine, thereby lowering the protein nutritional
value [10].
Incorporation of disaccharides into soya-based
formulations for extrusion resulted in a decrease
of SME values, and the colour of the product
changed depending on the employed disaccharide.
Obradović, V. et al. J. Food Nutr. Res., 53, 2014, pp. 189–206
192
Tab. 1. Examples showing the effect of protein content and extrusion conditions on physical properties of the extrudates.
Raw materials Process conditions Critical factors Physical properties Reference
Increase Decrease
White maize
Partially defatted soybean (10–30%)
Feed moisture 20%
Screw speed 31.4 rad·s-1
Temperature 200 °C
Increase of soybean content Density
Breaking strength
Hardness
Expansion ratio [30]
Rice flour
Wheat gluten
Toasted soya grits
Feed moisture 200–300 g·kg-1
Screw speed 41.9 rad·s-1
Temperature 150–180 °C
Increase of protein content Expansion [31]
Increase of temperature Expansion Breaking strength
Increase of moisture content Breaking strength
Yellow maize flour
Soybean flour (10–40%)
Moisture content 21–23%
Screw speed 20.9–36.6 rad·s-1
Temperature 24 °C, 110 °C, 127 °C,
150 °C
Increase of soya flour content Specific volume Hardness [32]
Increase of moisture content Hardness Specific volume
Increase of screw speed Specific volume Hardness
Maize flour
Powder lycopene
Soya protein concentrate (0–30%)
Moisture content 20–30%
Temperature 100–150 °C
Increase of soya protein content Hardness [33]
Increase of moisture content Hardness Expansion index
Increase of temperature Hardness
Maize flour
Soya protein concentrate (322–666 g·kg-1)
Feed moisture 316–484 g·kg-1
Temperature 126.4–193.6 °C
Increase of soya protein content Bulk density
Breaking stress
Expansion ratio [34]
Increase of moisture content Bulk density
Breaking stress
Expansion ratio (then
decrease)
Expansion ratio (at
first increase)
Increase of temperature Expansion ratio (then
decrease)
Bulk density
Expansion ratio (at
first increase)
Maize starch
Soya protein concentrate (0–20%)
Screw speed 24.1–34.6 rad·s-1 Increase of soya protein concentrate level Bulk density Expansion ratio [35]
Maize starch
Soya flour
Oat bran
Temperature 130–160 °C
Screw speed 7.33 rad·s-1
Increase of starch content Expansion ratio Hardness [36]
Increase of temperature Expansion ratio Hardness
Maize : „hard to cook“ bean (60 : 40) Moisture content 15.5–19.5 g·kg-1
Temperature 155–185 °C
Screw speed 13.6 rad·s-1
Increase of temperature Density Expansion index [41]
Increase of moisture content Density Expansion index
Brown rice
Whole maize
Lathyrus seeds (15%)
Moisture content 14%
Screw speed 15.7 rad·s-1
Temperature 175 °C
The addition of Lathyrus seeds Density Expansion [42]
Improvement of nutritional and functional properties of extruded food products
193
The degree of Maillard reaction was higher for
materials with lactose than for those with saccha-
rose, due to the presence of free hydroxyl group in
the anomeric carbon of lactose. In the early stage
of the reaction, the formation of protein-saccha-
ride conjugates leads to highly coloured and insol-
uble polymeric compounds [38].
Little is known of the effects that soya pro-
tein concentrate and acid-hydrolysed vegeta-
ble proteins, as ingredients, have on the odour
of extruded cereal-based foods. The retention
of compounds responsible for these ingredient
odours could influence the acceptability of the
final product. Furthermore, the interaction of the
cereal base with non-volatile components, such as
amino acids and fatty acids, during extrusion could
lead to the production of additional odours. SOLI-
NA et al. [39] demonstrated how combinations of
ingredients, such as soya protein concentrate, and
acid-hydrolysed vegetable protein (aHVP), at 1%
level affect the odour of starch-based extrudates.
For example, 39 compounds were identified in the
starch/soya protein isolate/aHVP feedstock, with
lipid-derived compounds dominating the volatile
profile. Thirty-eight compounds were identified
in the extrudate obtained at 150 °C, and 51 were
found in that obtained at 180 °C. Lipid-derived
compounds qualitatively dominated the volatile
profile of both extrudates, followed by the Streck-
er aldehydes. Additionally, eight Maillard reaction
products were identified in the extrudate obtained
under extreme conditions. Qualitative sensory as-
sessment of the extrudates showed that those ob-
tained at 180 °C had relatively “stronger” odours.
Legumes are a cheap and valuable potential
source of good quality protein. The nutritive value
of legume proteins is low in comparison to animal
proteins. This has been attributed to poor digest-
ibility, deficiency of sulphur amino acids and the
presence of antinutritional compounds. Legume
extrusion cooking allows reduction of antinutri-
tional factors and therefore improves the nutri-
tional quality at a cost lower than other heating
systems, due to a more efficient use of energy. Ex-
trusion cooking of legumes such as fava bean, pea,
chickpea and kidney bean was shown to improve
the in vitro protein digestibility, enhance phospho-
rus availability, reduce tannins and polyphenols,
and eliminate trypsin and -amylase inhibitors.
Treating soaked legumes by extrusion at 140 °C
or 180 °C at 22% moisture improved the nutritive
value of the studied legumes [40]. Extrusion cook-
ing is of special interest for incorporating “hard-
to-cook” beans into cereals. A blend of quality
protein maize and “hard-to-cook” bean (60 : 40,
w/w) was extruded at 155 °C, 170 °C and 185 °C
and moisture content 155 g·kg-1, 175 g·kg-1 and
195 g·kg-1. This combination provided the blend
with almost 15% protein, producing extrudates
with good nutritional quality. Increased protein
digestibility was observed at both temperatures
tested (155 °C and 170 °C). Available lysine con-
tent decreased by 17.3% at 155 °C and by 26.9%
at 170 °C [41]. Chemical composition of cereal-
legume blends depends upon the cereal type as
well. For example, changes in leucine and valine
contents depend on the type of cereal: maize sam-
ples had a higher content of leucine than the rice
samples, but the addition of wild legumes did not
significantly affect the value. On the other hand,
changes in lysine content depend both on the type
of cereal and the type of legume. Rice samples
showed higher lysine values than maize samples,
and the addition of legume significantly increased
lysine content [42].
Addition of whey protein to extrudates
Whey proteins are globular molecules with
a substantial content of -helix motifs, in which
the acidic/basic and hydrophobic/hydrophilic ami-
no acids are distributed in a fairly balanced way
along their polypeptide chains [43]. The various
proteins in whey in the order of abundance are
-lactoglobulin, -lactalbumin, proteose, peptone,
immunoglobulins, bovine serum albumin, lacto-
ferrin and lactoperoxidase [44]. -Lactoglobulin
is more sensitive to extrusion treatment than
-lactalbumin, especially with increasing moisture
content [45]. These proteins have many biological
activities: cancer prevention, tumour cell vulner-
ability increase, antimicrobial activities and immu-
nomodulation [43]. Whey proteins and amino acid
supplements have a strong position in the sports
nutrition market based on the purported quality
of proteins and amino acids they provide. Several
studies support the notion that only indispensable
or essential amino acids are necessary to stimulate
muscle protein synthesis and suggest that proteins,
which provides a high portion of these amino
acids, will be efficient in promoting muscle growth
[46]. Not only is whey protein a good source of
amino acids, but it is also a rich source of bioactive
peptides generated during its digestion. Peptides
shorter than four residues can cross intercellu-
lar junctions and reach the bloodstream, whereas
larger peptides can be transported via peptide
transporter-mediated transport system [44].
The high-protein, low-saccharide diet trend
may be contributing to increased utilization of
whey protein. Extruded whey protein has unique
properties as a food ingredient. Compared to
spray-dried whey protein, it is more amenable to
Obradović, V. et al. J. Food Nutr. Res., 53, 2014, pp. 189–206
194
introduction into food products [47].
Research of extrudates containing whey pro-
teins mostly focuses on physical properties, since
they directly influence the acceptance of the
product. Twin-screw extrusion is preferred for
starch-protein blends [48]. ONWULATA et al. [49]
showed that the addition of whey proteins reduces
expansion and consequently increases breaking
strength. This has been ascribed to protein-protein
interactions at higher levels of protein content.
The protein fractions reinforce the product cell
wall and increase breaking strength. Incorpora-
tion of whey products, whey protein concentrate
(WPC) and sweet whey solids (SWS) at 25% or
50% in maize meal at high and low shear extru-
sion conditions was studied. Higher shear resulted
in higher moisture loss, and in an increase in melt
temperature from 120 °C to 128 °C. SWS reduced
post extrusion product moisture. On the other
hand, incorporation of the same whey products
in potato flour gave different results. The product
containing SWS and WPC at 50% bound consider-
ably more water at higher shear.
Moisture content of the maize-whey protein
concentrate blends also affects expansion. Es-
pecially hard extrudates were obtained at water
intake 14.3 l·h-1 when the levels of whey protein
concentrate were 15% and 22.5%. These samples
showed uneven distribution of water and pro-
teins after extrusion since the regular structure
of maize flour, water and proteins could not be
formed [50]. The effect of extrusion processing on
extrudate expansion depends to a large extent on
the flour, and the extrudate properties are gener-
ally unpredictable when milk proteins are incor-
porated in flours. ONWULATA et al. [51] showed
that substituting WPC (250 g·kg-1 and 500 g·kg-1)
for maize, potato or rice flours reduces the expan-
sion, but some earlier works reported that substi-
tution of WPC (200 g·kg-1) for rice flour increased
the extrudate expansion. The same author showed
that, by reducing the moisture and adding reverse
screw elements, specific mechanical energy was
increased, which increased the product expan-
sion. The negative textural indicators associated
with the inclusion of whey products can be im-
proved significantly by adding wheat bran fibre at
125 g·kg-1. Addition of fibre improved SME along
with the improvement of quality characteristics of
the product [52].
Particle size also affects the extrusion of whey
protein blends. The use of maize meal fractions
that approximated the particle size of added whey
protein concentrate resulted in viscosity increase
in the extruder, which enhanced the expansion
ratio and the porosity of the puffed product. The
hardness of the expanded extrudates decreased,
making the snack more easily broken. Under tem-
peratures of 105–130 °C, screw speed 31.4 rad·s-1
and moisture 8.5%, the expansion of the smallest
fraction (< 250 μm) was equivalent to, or greater
than, the extrudate made from maize meal alone
[53].
The overall colour of the extrudates tended to-
wards brownish with increasing the whey protein
content. It is known that the main reason for the
colour difference is the Maillard reaction between
the reducing saccharides (lactose, dextrinized
starch) and the whey proteins [48]. Addition of
whey products at 25% to maize, potato and rice
flour tended to increase total colour difference;
addition of whey products at 50% lightened the
colour [49].
IMPROVEMENT OF DIETARY FIBRE
CONTENT OF EXTRUDATES
A problem in defining dietary fibre arises due
to the lack of a universally accepted method to
quantify all components of dietary fibre. American
Association of Cereal Chemists defined dietary
fibre as the edible parts of plants or analogous
saccharides that are resistant to digestion and ab-
sorption in human small intestine, with complete
or partial fermentation in large intestine [54]. The
types of plant materials that are included within
the definition may be divided into two forms based
on their solubility: insoluble dietary fibres, which
include cellulose, hemicelluloses and lignin, and
soluble dietary fibres, which include -glucans,
pectin, gums, mucilages and some hemicellu loses.
This definition includes only non-starch poly-
saccharides, but resistant starch also may be con-
sidered as a component of dietary fibre because it
is determined within the total dietary fibre when
measured by the approved AOAC method [54].
Dietary fibre decreases the risk for type 2 dia-
betes, cardiovascular disease and colon cancer by
reducing the digestion and absorption of macro-
nutrients and decreasing the contact time of car-
cinogens within the intestinal lumen [55]. Sup-
plementation with dietary fibre can result in
fitness-promoting foods, low in energy cholesterol
and lipids. According to current recommenda-
tions, the average daily requirement of dietary
fibre is 21–25 g per day for women and 30–38 g per
day for men [56]. Dietary fibre has also important
health benefits in childhood, especially in promot-
ing normal laxation. Studies also suggest that die-
tary fibre in childhood may be useful in preventing
and treating obesity, and also at lowering blood
Improvement of nutritional and functional properties of extruded food products
195
cholesterol level, both of which may help reduce
the risk of future cardiovascular disease [57].
In general, refined cereal flours contain a low
amount of fibre (between 2% and 5%). Whole
grain flours contain a higher amount of fibre
(between 10% and 15%). The highest quantity of
fibre is found in the bran part of cereals (20–90%).
In cereals, dietary fibre is mostly insoluble except
for oat, in which about 50% of the fibre is soluble
[58]. Cereal, fruit and vegetable by-products can
be recovered and used as value-added products
[56, 59].
Addition of dietary fibre from cereals
Barley plays a minor role in human nutrition,
but products with new functional and nutritional
properties are a precondition for higher accept-
ance of barley. -Glucan is an important nutrition-
al component of this cereal [60]. It was shown that
-glucan from barley is hypocholesterolemic, and
this property may be a result of its ability to in-
crease viscosity of the intestinal content. It is also
a potent inductor of humoral and cell-mediated
immunity [61].
Depolymerization of polysaccharides during
extrusion is affected by increased shear stress.
The molecular weight of -glucan extracted from
barley meal is 160 000. -Glucan extracted from
extruded barley preparations prepared at 22.5%
moisture showed, depending on the extrusion tem-
perature, the following molecular weights: 110 000
(130 °C), 125 000 (150 °C) and 80 000 (170 °C).
Binding and immobilization of water is an impor-
tant function of soluble dietary fibre. Due to their
cellulose-like structure, -glucans are only partly
soluble. Conditions during extrusion increase wa-
ter retention with the increase in the extrusion
temperature. Extrusion also leads to a higher
solubil ity of the barley material. In order to obtain
good textural properties of the barley extrudate,
moisture of 20.0–22.5% is preferable [60].
The formation of resistant starch during barley
extrusion cannot be generalized. FARAJ et al. [62]
showed that extrusion at 100 °C increased the re-
sistant starch content but, at lower temperatures
(60 °C and 80 °C), resistant starch content de-
creased. This suggests that starch fragmentation
readily occurs at 100 °C, leading to formation of
amylose chains that could be incorporated into
the crystalline structure of resistant starch type 3.
Resistant starch type 3 is defined as retrograded
starch fraction formed after cooking and storage
[63]. VASANTHAN et al. [64] reported the forma-
tion of this type of starch in extruded high-amy-
lose barley flour, but did not observe any resistant
starch type 3 in the extruded low-amylose barley
flour. HUTH et al. [60] reported that generation
of resistant starch during extrusion was distinctly
influenced by technical parameters. Highest con-
tents of resistant starch were obtained by using
a mass temperature of approximately 150 °C, and
moisture of approximately 20%. Different results
obtained in different studies can be explained in
several ways:
After relaxation of the cereal-based material
when leaving die, retrogradation and re-crys-
tallization occurred, which are a precondition
for the formation of enzyme-resistant starch
type 3 [60].
– The extrusion conditions, especially the shear-
ing action of the extruder screw, may have
caused degradation of the amylose into mole-
cules of a smaller polymerization degree that
could not be incorporated into a crystalline
structure of resistant starch type 3.
Flour is a complex system and other compo-
nents such as proteins, -glucans and/or pen-
tosans may interfere with the formation of re-
sistant starch type 3 [62].
Extrusion of barley flour may be, in a certain
way, unacceptable for consumers because extru-
dates may have “bran” flavour. Mixing of barley
with tomato pomace gave extrudates with a high
preference level, especially with tomato pomace
at a level of 10%. High extrusion temperature
(160 °C) was preferable because a decrease in die
temperature increased the product hardness [65].
For economic reasons, it may be particular-
ly interesting to utilize by-products from other
branches of the food industry, which are sources
of components of high nutritional value. Brew-
ers spent grain is an example of such a material,
which contains around 52% of insoluble dietary
fibre and 2.5% of soluble dietary fibre. An addi-
tion of brewers spent grain to maze grits at a level
of 5–20% significantly increased the contents of all
dietary fibre fractions. Addition of 10% increased
almost three times the contents of all fractions of
dietary fibre. The highest proportions were re-
corded for fractions of hemicellulose and cellulose
[66]. Higher content of dietary fibre after extru-
sion in comparison to their theoretical contents
was observed [64, 66, 67]. This could be explained
by the formation of resistant starch and interac-
tions of partly degraded substances, leading to
formation of new complexes resistant to digestion
[66].
Fibre increases the hardness of the extruded
products, and decreases the expansion [51, 68, 69],
as a result of its effect on cell wall thickness [70].
Thickening of cell walls results in a decreased air
Obradović, V. et al. J. Food Nutr. Res., 53, 2014, pp. 189–206
196
cell size in the microstructure of the extrudate.
Another explanation could be that the break-
down of components into smaller particles, which
might interfere with bubble expansion, reduced
the extensibility of the cell walls and caused pre-
mature rupture of steam cells in the extrudate
microstructure [71, 72]. At higher addition levels
(20%), brewers spent grain caused almost two-fold
increase in the bulk density, and very low expan-
sion. Also, thus obtained product was character-
ized by a specific after-taste and aroma of brewers
spent grain, so it required an addition of flavour-
ing agents [66].
Wheat milling by products can also be used
in “all-bran” breakfast extruded products. These
products contain almost exclusively insoluble
dietary fibre. The mechanical stress during the
extrusion process should be responsible for the
breakdown of polysaccharide bonds, leading to the
release of oligosaccharides and, therefore, to the
increase of soluble dietary fibre [73]. The increase
of soluble fibre during extrusion cooking of maize
fibre was also observed, due to transformation of
some insoluble fibre components into soluble fibre
during extrusion. Insoluble dietary fibre and total
fibre contents slightly decreased at the same time
[74]. ESPOSITO et al. [73] showed the increase of in-
soluble dietary fibre also, which could be explained
by gelatinization and retrogradation of the starch,
which occurred during extrusion, and part of it
could be changed into non-degradable polysac-
charides. Besides that, the Maillard reaction that
took place during extrusion led to the formation of
a protein-polysaccharide complex, which was re-
sistant to enzymatic degradation. According to the
definition, these products cannot be considered as
dietary fibre, but they behave as dietary fibre at the
analytical determination and also physiologically.
On the other hand, Vasanthan et al. [64] showed
that the content of insoluble dietary fibre from two
different types of barley increased in one case and
decrease in another during extrusion. Extrusion of
the oat bran also led to the increase of soluble die-
tary fibre, from 89 g·kg-1 to 95–142 g·kg-1, depend-
ing on extrusion conditions. Higher temperature
(140 °C compared to 100 °C), and lower moisture
(10% compared to 30%) caused a higher increase
in the soluble dietaryfibre content [75].
Addition of dietary fibre from legumes
In recent years, cereals were used to prepare
soluble dietary fibre in a number of studies, but
the information about soybean or other legumes
is scarce. Soybean residue, which is the main by-
product from soymilk and tofu production, repre-
sents a good dietary fibre source. The content of
total fibre in soybean residue is around 60%, while
the soluble dietary fibre content is only 2–3%.
Under an extrusion temperature of 115 °C, mois-
ture of 31% and screw speed of 18.8 rad·s-1, the
soluble dietary fibre content increased by 10.6%
compared with the unextruded soybean residue.
Heating also modified the structural character-
istics of the fibre, hence enhancing its water and
oil uptake abilities [76]. Increased levels of bean
flour resulted in a significant decrease in expan-
sion, compared to fibres of cereal origin. However,
the type of bean significantly influenced the ex-
pansion. Navy bean flour fortification of maize-
based extrudates produced slightly less expanded
products than small red beans. This was probably
due to the higher amount of fibre from seed coats
in the flour of navy beans (navy: 15.2 g, small red
27.2 g, based on the weight of 100 seeds). The
research showed that the replacement of maize
starch by bean flour, regardless of cultivar, was
feasible at a level of 30% [4].
Food and Agriculture Organization recom-
mends a 30 : 70 ratio of leguminous and cereal
flours. However, the sensory characteristics of cer-
tain blends may be negatively affected in combina-
tions following this ratio [77]. Studies of the usage
of some other legumes like chickpea, lentil and
fenugreek have also been conducted [78–80]. The
hardness of the extrudates decreased as chickpea
proportion increased from 50% to 70%. The ex-
truded product containing 80% chickpea flour was
harder than that containing 70% chickpea flour.
The extruded products made from 70 : 30 blends of
chickpea and rice flour had highest expansion and
lowest hardness value [78]. The chickpea-based
snack products with high expansion ratio and low
bulk density and hardness were obtained at low
moisture, high screw speed and medium to high
barrel temperature within the range of 15.3–18.7%
moisture, screw speed 23.7–36.0 rad·s-1 and a tem-
perature of 143–177 °C [79]. The addition of fenu-
greek, however, may not be suitable even at very
low inclusion levels (2%) due to the pronounced
bitter taste. However, the product containing 15%
fenugreek polysaccharide was acceptable [78].
In order to prepare soluble dietary fibre from
legumes, twin screw extrusion is preferable, since
it is possible to extrude at lower temperatures,
which require less energy. Besides that, twin-screw
extrusion may reduce the extrusion time by in-
creasing the screw speed [76].
Addition of dietary fibre from fruits and vegetables
The addition of broccoli and olive paste to
maize extrudates was examined by BISHARAT et al.
[8]. The independent variables of the extrusion
Improvement of nutritional and functional properties of extruded food products
197
process (temperature, screw speed, feed moisture
content, broccoli or olive paste content) signifi-
cantly influenced the extrudate structural charac-
teristics. Increase in temperature and screw speed
caused a reduced mixture viscosity and increased
starch gelatinization, leading to more porous
products. Increase in moisture had a negative im-
pact on starch gelatinization and reduced the po-
rosity of the extrudates. The increase in the addi-
tion level of broccoli and olive paste reduced the
porosity of the products due to their high fibre and
protein contents. Extrudate expansion decreased
as moisture content and additon level increased,
while the increase in the screw speed resulted in
more expanded products. The most appropriate
conditions that produced products with higher
expansion were 14% initial moisture content, 4%
material content and 26.2 rad·s-1 screw speed. For
maize-broccoli extrudates, the optimum tempera-
ture was 140 °C and for maize-olive paste extru-
dates it was 180 °C.
STOJCESKA et al. [68] demonstrated that the
hardness of the extruded products was not relat-
ed to the level of cauliflower, which was the only
source that increased the level of dietary fibre.
Sensory evaluation of same samples showed that
extrudates containing 0–10% cauliflower were
judged to be significantly more acceptable than
samples containing 15–20% cauliflower. STOJ-
CESKA et al. [71] increased the level of total
dietary fibre in gluten-free products by incorpo-
rating a number of fruits and vegetables such as
apple, beetroot, carrot, cranberry and gluten free
teff (Eragostris tef) flour cereal. All the samples
showed an increase in dietary fibre under all test-
ed conditions. Increasing temperature increased
the level of total dietary fibre while decreased
lateral expansion. This was not in agreement with
study of STOJCESKA et al. [68], where the decrease
of total dietary fibre was detected after extrusion.
The decrease was probably a result of solubiliza-
tion and degradation of pectic substances.
The extrusion of orange pulp led to an increase
in total pectin. Longer residence time (lower
screw speed of 14.3 rad·s-1) and moisture content
of 27–33% were sufficient to cause alterations in
the structure of protopectin, allowing both solu-
bilization and release of the pectin [81]. Higher
levels of the soluble fraction in total dietary fibre
were found in concentrates of vegetables than in
cereals, because total dietary fibre from vege-
tables has a greater affinity for water than cereal
bran [67]. The extrusion conditions increased the
amount of total dietary fibre in a blend of maize
flour and red cabbage, but decreased in a blend of
wheat flour and red cabbage. This was probably
because maize starch has high amylose content,
which resulted in resistant starch formation [67].
The extrusion increased the amount of total and
soluble dietary fibre of sweet potato, especially
compared to freeze-dried and hot air-dried sam-
ples [82].
ADDITION OF BIOACTIVE COMPONENTS
Phytochemicals as the bioactive non-nutrient
compounds in fruits, vegetables, grains and other
plant foods have been linked to the reduction in
the risk of major chronic diseases. More than
5 000 phytochemicals have been identified, but
a large number still remains unknown. However,
convincing evidence suggests that the benefits of
phytochemicals in fruits and vegetables may be
even greater than is currently understood because
oxidative stress induced by free radicals is involved
in the etiology of a wide range of chronic diseases
[83, 84]. For example, flavonoids possess antimi-
crobial, antiviral, anticarcinogenic and vasodilato-
ry effects [85]. There is a rapidly growing body of
literature covering the role of plant secondary me-
tabolites in food and consumers are increasingly
aware of diet health problems, therefore demand-
ing natural ingredients that are expected to be safe
and health-promoting [86].
Bioactive compounds in cereal grains
Cereal grains contain a large variety of sub-
stances that are biologically active, including
antioxidants. The major portion of phenolic com-
pounds is located in the outer parts of grains,
where they are involved in the defence against
ultraviolet radiation, pathogen invasion and in
modification of mechanical properties [87]. The
dominant phenolic acid found in wheat, barley and
rye is ferulic acid, whereas in oat it is cumaric acid.
Rye and oat contain also small quantities of si-
napic and caffeic acids. The ester-bound phenolic
acids are dominant when compared to free acids.
The highest contents of both forms were found in
rye (54.6 mg·kg-1) and oat (30.1 mg·kg-1).
The extrusion (20% moisture content, screw
speed 52.4 rad·s-1, barrel temperature 120–200 °C)
caused an increase in all analysed free and ester-
bound phenolic acids, except for sinapic and caf-
feic acids. The latter was not found in the hydro-
thermically processed grains [88]. SHARMA et al.
[89] reported a significant decrease in total phe-
nolic content during barley flour extrusion. The
reduction in total phenolic content may be at-
tributed either to the decomposition of phenolic
compounds due to the high extrusion temperature
Obradović, V. et al. J. Food Nutr. Res., 53, 2014, pp. 189–206
198
or alteration in the molecular structure of phe-
nolic compounds that may lead to reduction in the
chemical reactivity of phenolic compounds or de-
crease their extractability due to a certain degree
of polymerization. It has also been reported that
the phenolics may interact with the proteins and
may not exhibit their actual value. An increase in
phenolic content during extrusion indicates that
extrusion conditions may liberate phenolic acids
and their derivatives from the cell walls. Then, as
a result, the liberated phenolic acids may contri-
bute to a higher antioxidant potential [88].
Oats are unique among the common cereal
grains by having a high lipid and protein contents,
their lipolytic enzymes being 10–15 times more ac-
tive than those of wheat. Endogenous phenolics in
oats provide some protection, but processing prior
to extrusion may damage those compounds, re-
ducing their antioxidant effects. Natural phenolic
compounds added to grains prior to extrusion may
synergize and protect the endogenous antioxidants
[90]. It may lead to formation of new antioxidants
as well [91]. These added antioxidants would be
evenly dispersed within the food matrix and be less
likely to sublimate than butylated hydroxytoluene
(BHT) or butylated hydroxyanisole, resulting in
a delayed onset of lipid oxidation. Ferulic acid and
benzoin at levels of 1 g·kg-1 were effective in delay-
ing the onset of oxidation, while chlorogenic acid
was ineffective, perhaps due to complex formation
with iron from the screw wear [90]. Addition of
cinnamic acid and vanillin protected maize snacks
against lipid oxidation better than BHT without
impairment of physical characteristics [91].
Incorporation of brewers spent grain to maize
and wheat starch has been previously mentioned.
It was found that the addition of brewers spent
grain to the formulation has no significant effect
on the phenolic content and antioxidant proper-
ties of the samples [67, 69, 70]. ESPOSITO et al. [73]
showed that antioxidant activity of durum wheat
by-products was comparable to that of fruits and
vegetables, because of the presence of fibre-bound
phenol compounds.
Bioactive compounds from legumes
The significant occurrence of bioactive
phenolic compounds, the relevant antioxidant
capacities along with the interesting functional
proper ties of dehydrated bean flour, make them
useful for effective inclusion in the human diet
[92]. Total polyphenols content in whole bean
flour found by DELGADO-LICON et al. [93] varied
from 4.3 g·kg-1 to 17.4 g·kg-1 on dry matter basis.
Whole bean flour and nixtamalized maize were
mixed in a 60 : 40 proportion and extrusion was
performed in different moisture (14.5–18.0%)
and temperature (150–190 °C) conditions. The re-
sults showed that, after the extrusion, the contents
of polyphenols remained high (15.1 g·kg-1 on dry
matter basis, expressed as gallic acid equivalents)
in the mixture extruded at 142 °C and a moisture
of 16.3%. A correlation was observed between the
best extrusion procedure, the contents of bioactive
compounds and the antioxidant capacity of the
end product.
The effect of extrusion on the total phenolic
content of beans depends on bean cultivar [4, 94].
KORUS et al. [94] found that Rawela cultivar (dark-
red cultivar of Phaseolus vulgaris) showed a 14%
increase in the amount of phenolics in extrudates
compared to raw beans, while Tip-Top (black-
brown cultivar of P. vulgaris) and Toffi (cream
cultivar of P. vulgaris) exhibited a decrease by
19–21%, respectively. The same authors showed
that the beans of all three cultivars extruded at
a lower temperature (120 °C) retained a higher
content of phenolics in total than those extruded
at 180 °C. At both extrusion temperatures, smaller
losses were noted in samples of higher initial mois-
ture. The addition of navy and red bean flours to
maize starch gave denser, less expanded and hard-
er extrudates. Total phenols and antioxidant ac-
tivities determined in the cooked products showed
significant variation with respect to bean flour
content and bean cultivar. Bean flour addition
had a positive impact on the levels of these phy-
tochemicals. However, fortification with small red
bean flours was, to a great extent, more effective in
producing extrudates with higher nutritional func-
tionality. Extrusion (15.7 rad·s-1, 22% moisture,
160 °C) caused the reduction of total polyphenols
in both cultivar mixtures [4]. In another study [95],
changes in screw speed, feed rate and moisture
content of the feed had no effect on the content of
total phenolic compounds in a mixture of chickpea
flour (30%), maize flour (20%), oat flour (20%),
maize starch (15%), carrot powder (10%) and
hazelnut (5%). Although the total phenolic com-
pounds did not change during extrusion, the anti-
oxidant capacity values were affected, probably as
a result of degradation of antioxidant compounds
other than the phenolic compounds.
Bioactive compounds from fruits and vegetables
Anthocyanins
Anthocyanins are water-soluble pigments
responsible for the red, blue and purple colours in
many food crops [96]. They are well-known alter-
natives to synthetic dyes [97]. Natural colours have
several disadvantages such as a high price, extrac-
Improvement of nutritional and functional properties of extruded food products
199
tion difficulty and discolouration during process-
ing. Artificial colours are inexpensive and are su-
perior to natural extracts in tinctorial strength, hue
and stability. Although the consumer awareness of
health-related risks of artificial colour additives
has increased, artificial colours are still used more
frequently than natural colours in many processed
foods [98].
Breakfast cereals coloured with natural fruits
may appeal to consumers interested in healthy
food. Whole yellow maize is a good source of
phenolic compounds, but milling may remove
some endogenous antioxidants such as phenolics
and thus addition of other antioxidants to maize
should improve the shelf life [99]. Moreover, as for
flavonoids and related phenolics, both antiradical
and antioxidant activities contribute to explain-
ing the protective effect of vegetable-rich diets on
coro nary diseases [97].
When blueberry, cranberry, raspberry powder
and Concord grape juice concentrate at a level of
1% were mixed with white maize meal, saccharose
and citric acid, a significant loss of the phenolics
occurred during extrusion. Considering the an-
thocyanin content of the fruit powders, there was
an apparent loss of about 90% of the pigments in
all fruits except for raspberry. However, blueberry
cereals had the highest content of antho cyanins
and phenolics. Antioxidant activity was not sig-
nificantly correlated with either anthocyanin or
phenolic contents. Possible explanation is that
Maillard browning during extrusion and/or storage
was suppressed by fruit powders, thus reducing
one source of antioxidants [99]. Similar results
were obtained by CAMIRE et al. [96], who studied
blueberry and grape juice concentrates mixed
with maize meal. Extrusion caused a decrease in
the content of blueberry anthocyanins by 90% and
grape anthocyanins by 74%. Extrusion also de-
creased colour density in blueberry samples (by
78%) and grape samples (by 70%). Blueberry sam-
ples had a higher anthocyanin content (40 mg·kg-1,
dry basis) and presented darker and redder colour
than grape samples (26 mg·kg-1, dry basis).
Sensory evaluation showed that overall accept-
ability was highly correlated with sweetness, hard-
ness and flavour. Blueberry samples received the
lowest score on overall acceptability. Blueberry
concentrate provided tartness that may have inter-
fered with the sweetness of the product. Thus, if
bright colour is major objective, blueberry should
be used, but improvements in sweetness and fla-
vour are needed [96]. KHANAL et al. [100] showed
that temperature and screw speed, but not their
interaction, affected total anthocyanin contents
during extrusion of grape pomace and white sor-
ghum flour at a ratio of 30 : 70. Increasing the
extrusion temperature decreased anthocyanin
content linearly, and similar effect was found by
WHITE et al. [101]. Increase in the screw speed
from 10.5 rad·s-1 to 20.1 rad·s-1 reduced the resi-
dence time of the material inside the extruder
barrel, thus minimizing the exposure to high tem-
peratures. The reduction in total anthocyanins
content was between 18% and 53% [100].
The extrusion of cranberry pomace with maize
starch in ratios of 30 : 70, 40 : 60 and 50 : 50, at tem-
peratures 150–190 °C and screw speed 15.7 rad·s-1
and 20.9 rad·s-1 showed that anthocyanin losses
were highly dependent on the level of pomace.
The minimum loss in anthocyanins was observed
in the mixture containing only 30% pomace, with
only 50% of anthocyanins lost. Extrudates con-
taining 50% pomace had only 35% retention. This
suggests possible protection by the starch present
in the extrudate mixture. The antioxidant ca pacity
of the extrudates increased at higher tempera-
tures due to the formation of Maillard reaction
products, which possess reducing capacity [101].
DURGHE et al. [102] showed that, besides tempera-
ture and screw speed, moisture content has a posi-
tive effect on anthocyanin retention.
Sensory evaluation of the extrudates obtained
from red carrot powder (1–3%, w/w) and rice
flour showed good acceptance of all samples. The
best sensory score was not in correlation with the
colour retention of the anthocyanins. The addi-
tion of 2% citric acid increased the colour reten-
tion from 41% to 63%, but the extrudates were
very sour and sensorially unacceptable. The ad-
dition of 1% citric acid was also useful (retention
59%), and showed much better results. The ad-
dition of ascorbic acid (0%, 0.1% and 1%) to the
mixture containing maize meal, saccharose (15%)
and blueberry concentrate (17%) showed that for-
tification by ascorbic acid accelerated anthocyanin
degradation during extrusion. This might be due
to interaction of ascorbic acid oxidation products
with anthocyanins or to a direct condensation and
enhanced polymeric pigment formation. The sen-
sory evaluation of samples showed that the fruit
flavour was not strong enough, but the mild acidity
provided by ascorbic acid contributed to the im-
pression of fruit flavour [103].
Carotenoids
-Carotene is not only an important and safe
source of vitamin A, but also a useful food colour.
There is considerable evidence that -carotene,
being a highly active singlet oxygen quencher, may
play an important role in the prophylaxis of free
radical-mediated diseases [104].
Obradović, V. et al. J. Food Nutr. Res., 53, 2014, pp. 189–206
200
since the incorporation of tomato derivatives lu-
bricated the melt and therefore dropped SME and
torque, the expansion was also decreased. Rice
flour produced the most expanded products due
to the high starch and lower fibre and lipid con-
tents compared to maize grits and wheat semo-
lina. Although lycopene retention in products
containing tomato skin was much higher than for
products containing tomato paste, the mean value
in products containing tomato skin was only about
15% that of products containing tomato paste, due
to much higher initial content of lycopene in the
tomato paste. The degradation of lycopene was
greatest for extruded products containing wheat,
which had a lower starch content that might pro-
vide some protection to lycopene. The effect of
temperature (140–180 °C) on expansion was not
significant [109]. Different results were obtained
by ALTAN et al. [65] during extrusion of barley
flour and tomato pomace. Sectional expansion
index decreased when the temperature was in-
creased (133.2–166.8 °C). This could be attributed
to increased dextrinization and structure weaken-
ing.
Sensory evaluation showed that extrudates with
10% tomato pomace had the highest level of ac-
ceptance for colour, texture and overall acceptabil-
ity. However, the tomato flavour was perceived as
weak for the highest level of pomace. HUANG et al.
[110] incorporated tomato powder in maize grits.
Results showed that moisture content (10–16%)
had a significant effect on the expansion, the best
expansion being obtained at a medium moisture
level and at medium levels of tomato powder. The
addition of tomato powder linearly increased the
hardness of the extrudates.
INFLUENCE OF PROCESS CONDITIONS
ON EXTRUDED FOOD PRODUCTS
As mentioned earlier, extrusion belongs to
the family of HTST processes, which is advanta-
geous for vulnerable food as exposure to high
temperatures for only a short time will restrict un-
wanted effects on proteins, amino acids, vitamins
and starch [2]. In the extrusion process, there are
generally two main energy inputs to the system:
energy transferred from the rotation of the screws
and the energy transferred from the heaters [1].
Effects of various process variables (moisture con-
tent, temperature, screw configuration and rota-
tion) on extrusion behaviour of extrudate compo-
nents have been extensively studied [45, 53, 65, 93,
111, 112]. Tab. 2 presents changes of the extrudate
nutrients taking place during the extrusion and
In order to investigate processing losses due
to the sensitivity of phytochemicals, -carotene
was incorporated into an extrusion-cooked cere-
al-based products. Process-induced stresses were
varied by using different dosing points, screw
speeds and barrel temperatures. Results showed
degradation of -carotene by oxidation driven by
thermal and mechanical stresses. When the solu-
tion was incorporated at the end of the extruder,
the -carotene molecules were exposed to me-
chanical and thermal stresses for a shorter time,
which resulted in by 10% higher retention of the
total content as compared to an application prior
to starch plastification. Increasing the melt tem-
perature from 135 °C to 170 °C did not show any
influence on the -carotene retention. Increasing
the screw speed from 31.4 rad·s-1 to 52.4 rad·s-1 in-
creased the retention significantly by about 25%.
These results suggested that -carotene losses
were mainly caused by the generated mechanical
stress rather than thermal stress [105].
SHIH et al. [106] compared the -carotene
losses from sweet potato, during extrusion cook-
ing, hot-air drying and freeze drying. For orange
and yellow sweet potato, the -carotene contents
of hot air-dried and extruded samples were signifi-
cantly lower than those of freeze-dried samples.
The mixing of sweet potato flour with the rice
flour showed that losses of carotenoids were lower
in mixed flours (2.6–3.8% for the orange variety
and 9.9–16.2% for the cream variety) than in the
single sweet potato flour (20.8–27.9% for the
orange variety and 41.0–60.4% for the cream one).
Proteins and lipids from rice flour form a caro-
tene-lipid-protein net protecting carotenoids from
thermal denaturation. Losses of total caro tenoids
were higher in the extruded product with low
feeder flow rate and low screw speed [107].
The addition of different antioxidants (BHT,
rosemary oleoresin, -tocoferol) in -carotene-
maize starch extrudates was investigated by
BERSET et al. [108]. In order to reach the level
of BHT efficacy, natural antioxidants had to be
used at high contents. In the case of aromatic
herbs such as rosemary, the oleoresin at a dosage
1 000 mg·kg-1 imparted a strong and undesirable
taste to snacks.
Tomato derivatives (tomato skin powder, paste
powder) were incorporated in maize, wheat and
rice extruded snacks. Incorporation reduced the
expansion values of up to 25% compared to the
controls [109]. This was also confirmed by ALTAN
et al. [65], in whose study increasing the level of
tomato pomace (0–12.7%) in barley flour resulted
in a decrease in expansion index of extrudates. Ex-
pansion was positively correlated with SME and,
Improvement of nutritional and functional properties of extruded food products
201
leads to deterioration of the nutritional charac-
teristics of proteins. This phenomenon is primarily
due to Maillard reactions. Increasing temperature
and pressure, or lowering moisture, promotes
Maillard reactions during extrusion [2].
CONCLUSIONS
Enrichment of the extruded snacks with nu-
tritionally valuable ingredients is increasingly
practised. Incorporation of protein- or fibre-rich
ingredients influences physical properties of the
extrudates, but the influence cannot be general-
ized because it strongly depends upon the cereal
and the additional material used. Optimization
of the process parameters such as temperature,
moisture content or screw speed, is the key for
developing nutritious extruded products with the
adequate consumer acceptance. Although this re-
view summarizes results of many studies on this
topic, there is still a long way from research to
commercial production of this kind of products.
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Increase of
process component
Nutrient content
Protein (lysine) Starch Vitamin
Gelatinization Depolymerization B1B2C
Temperature + + – x + x
Moisture content + + * + – x – **
Screw rotation – x – x
Screw geometry +
Die diameter + + x x +
Torque, extrusion pressure + x
(+) – increase, (–) – loss, (x) – no effect, (*) – high temperature, (**) – high temperature and low moisture content.
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Hull-less barley flours from CDC-Candle (waxy) and Phoenix (regular) were extrusion cooked under various combinations of temperature (90, 100, 120, 140 and 160 °C), moisture content (20%, 25%, 30%, 35% and 40%) and screw speed (60, 80 and 100 rpm). The effect of processing on the formation of resistant starch (RS3) in the extruded flour was determined by a technique that involved a step in which the flour were heated at 100 °C in the presence of thermostable α-amylase which was expected to destroy all the RS1 and RS2. Literature on resistant starch has suggested that RS3 forms only during heat treatment of moist starchy materials. In contrast, when determined by using the same methodology, the native (unprocessed) flours that is supposed to be free of RS3, showed resistant starch value of 40–60 mg/100 g. This indicated that RS1 and RS2 are not totally destroyed by the heat treatment step of the methodology used in determining RS3. Extrusion cooking did not significantly (P<0.05) alter the RS3 content of native flours, the RS3 contents generally decreased. Refrigeration at 4 °C for 24 h before oven drying of extruded flour samples slightly increased the RS3 content. An attempt was made to isolate the RS3 from the extruded flour using enzymes. The methodology involved sequential enzymatic treatments to flours with lichenase, β-glucosidase, protease, thermostable α-amylase and amyloglucosidase. The RS3 content in the isolates ranged between 6–34 mg/100 g which was lower than that of the raw materials (extruded samples) before enzymatic isolation. This was unexpected but indicated that the RS3 did not concentrate, suggesting that hydrolysis of the other grain components such as β-glucan and proteins may have exposed the RS1 and RS2, that initially escaped hydrolysis to α-amylase.
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To examine the effects of protein source and isoflavones on triglyceride (TG) fatty acid (TGFA) and cholesterol biosynthesis, subjects (>50 years, LDL cholesterol >130 mg/dl) underwent a four-phase randomized cross-over feeding trial. Diets contained either isolated soy protein or common sources of animal protein (25 g/1000 kcal), without or with isoflavones (49 mg/1000 kcal) and were each fed for 6 weeks. Blood samples from 20 hyperlipidemic subjects (6M, 14F, 62±9 years, BMI 26±3 kg/m², LDL cholesterol >160 mg/dl after feeding animal protein without isoflavones) were selected to measure TGFA fractional synthetic rate (TGFA-FSR) and free cholesterol fractional synthetic rate (FC-FSR) over 24 h as deuterium oxide uptake into TGFA and free cholesterol. Soy protein reduced TG by 12.4% (P<0.0001), total cholesterol by 4.4% (P<0.001), and LDL cholesterol by 5.7% (P=0.003) compared to animal protein. The TGFA-FSR was reduced by13.3% (P=0.018) and FC-FSR was increased by 7.6% (P=0.017) after the soy protein relative to the animal protein. Isoflavones had no significant effect on TG and TGFA-FSR. Isoflavones reduced total cholesterol levels by 3.1% (P=0.009) but had no significant effect on LDL, HDL cholesterol levels, or FC-FSR. These data demonstrate that dietary protein type modulates circulating TG and cholesterol levels in hypercholesterolemic individuals by distinct mechanisms.
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The second edition of the Food Processing Handbook presents a comprehensive review of technologies, procedures and innovations in food processing, stressing topics vital to the food industry today and pinpointing the trends in future research and development. Focusing on the technology involved, this handbook describes the principles and the equipment used as well as the changes ? physical, chemical, microbiological and organoleptic ? that occur during food preservation. In so doing, the text covers in detail such techniques as post-harvest handling, thermal processing, evaporation and dehydration, freezing, irradiation, high-pressure processing, emerging technologies and packaging. Separation and conversion operations widely used in the food industry are also covered as are the processes of baking, extrusion and frying. In addition, it addresses current concerns about the safety of processed foods (including HACCP systems, traceability and hygienic design of plant) and control of food processes, as well as the impact of processing on the environment, water and waste treatment, lean manufacturing and the roles of nanotechnology and fermentation in food processing. This two-volume set is a must-have for scientists and engineers involved in food manufacture, research and development in both industry and academia, as well as students of food-related topics at undergraduate and postgraduate levels. From Reviews on the First Edition: "This work should become a standard text for students of food technology, and is worthy of a place on the bookshelf of anybody involved in the production of foods." Journal of Dairy Technology, August 2008 "This work will serve well as an excellent course resource or reference as it has well-written explanations for those new to the field and detailed equations for those needing greater depth." CHOICE, September 2006.
Article
The aim of this study was to determine the artificial colours in foods and evaluate the dietary intake of artificial colour additives in foods commonly consumed by different age groups in Korea. The content level of artificial food colour was experimentally determined by high performance liquid chromatography with a photodiode array detector. Of the 643 food items analysed, 503 (approximately 78%) contained artificial colorants. Consumers aged 13-19 years displayed the greatest consumption of colour additive-containing foods, but the amount was much lower than the acceptable daily intakes (ADI) established by FAO/WHO Joint Expert Committee on Food Additives (JEFCA). Both mean and high (95th percentile) intakes of permitted artificial colour additives for all population groups were markedly lower than the AD!, because the Korean food industry has widely substituted natural colour additives for artificial colorants. However, the issue of consumption of colour additives by targeted groups such as children is of concern, so further studies to provide data on dietary intake of artificial colour additives on such vulnerable groups are needed.
Article
β-carotene mixed with corn starch before extrusion cooking furnishes products whose color diminishes continuously with storage time. Butyl hydroxytoluene, rosemary oleoresin and dl α-tocopherol protect the color of these model snacks to varying degrees and increase their shelf life. The color change of products can be easily followed with tristimulus colorimetry or reflection spectrophotometry. The difference spectra of samples measured in reflection suggest the existence of slight qualitative differences in the mode of action of the different antioxidants.
Article
Incorporation of phytochemicals, such as lipophilic bioactives, into starch based food products via extrusion has become a very attractive process in the last decades. However during extrusion cooking, phytochemicals are exposed to high temperatures and high mechanical stresses accelerating oxygen or light induced as well as other chemical reactions or structural changes (i.e. isomerisation). In order to investigate processing losses due to the sensitivity of phytochemicals, oil-dispersed beta-carotene as a model for lipophilic phytochemicals (e.g. carotenoids, tocopherols) was incorporated into an extrusion cooked cereal based product. Process induced stresses were varied by using different dosing points, screw speeds and barrel temperatures. An initial loss of about 30% beta-carotene due to oxidative/thermal degradation was found for all process conditions investigated. Maximum retention was achieved, if the beta-carotene was incorporated at the end of the extruder. Increasing the melt temperature from 135 degrees C to 170 degrees C didn't show any influence on the beta-carotene retention. Increasing the screw speed from 300 to 500 1/min increased the retention significantly (P < 0.05) by about 25%. These results suggest that beta-carotene losses are mainly affected by the generated mechanical stress in extrusion rather than by thermal stress.