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J Sci Food Agric 79 :675–678 (1999)
Journal of the Science of Food and Agriculture
Influence of extrusion variables on the protein
in vitro digestibility and protein solubility of
extruded soy tarhana
Paul Ains worth,1 David Fuller,1 Andrew Plunkett1 and SÓ enol I0 banog‘ lu2*
1 Department of Food and Cons umer Technology , Hollings Faculty , The Manches ter Metropolitan Univers ity , Old Hall Lane , M14 6HR
Manches ter , UK
2 Department of Food Engineering , Engineering Faculty , The Univers ity of Gaziantep , 27310 Gaziantep , Turkey
Abstract : Tarhana, supplemented with 150 g kg—1 full-fat soy ýour, was extruded at diþ erent extrusion conditions (barrel temperature : 80–120ÄC ; screw speed : 100–300 rpm ; feed rate : 10–20 kg h—1)
using a twin-screw extruder. The eþ ect of extrusion conditions on the in vitro digestibility (PD) of the
protein and protein solubility (PS) was investigated using response surface methodology. Regression
equations for predicting PD and PS were developed. While the barrel temperature had a signiücant
eþ ect on PD (P Æ 0.1), feed rate was the most signiücant variable on PS of the samples (P Æ 0.05).
Since the protein solubility should be high for the instant properties of extruded soy tarhana soup, it is
suggested that soy tarhana should be extruded at low feed rates (ie high residence times) while high
barrel temperatures should be achieved for the inactivation of antinutritional factors present in the
soy ýour.
( 1999 Society of Chemical Industry
Keywords : extrusion ; soy tarhana; protein digestibility ; solubility
Tarhana is a fermented Turkish wheat ýour–yoghurt
mixture used in soup making. Traditionally, it is
prepared by mixing yoghurt, wheat ýour, yeast and a
variety of vegetables and spices followed by fermentation for one to seven days. The resulting slurry is
then sun-dried and used in soup making, giving a
product with high nutritional contents and vitamins.1
The traditional batch method of tarhana manufacture is not automated with manufacturing capacities
being low and highly labour intensive. Extrusion
cooking is a versatile process for producing starchand/or protein-based foods at a lower cost due to
more efficient use of energy and greater process
control, with greater production capacities.2 A continuous method to produce instant tarhana soup
powder by extrusion cooking has been developed.
Extrusion cooking can be used to improve the nutritional value of food products by denaturation of proteins (ie opening the protein structure) and/or
inactivation of antinutritional factors.4 However, it is
known that heat processing and shearing can cause a
decrease in the digestibilities of proteins and availability of essential amino acids through nonenzymatic browning reactions and thermal
Several researchers have replaced wheat ýour
totally6 or partially7 with soy ýour to improve the
nutritional value of tarhana. In this work, wheat
ýour in the traditional recipe of tarhana was partially
replaced by full-fat soy ýour (30% of wheat ýour)
and the eþect of extrusion variables (screw speed,
barrel temperature, feed rate) on protein in vitro
digestibility (PD) and protein solubility (PS) of
extruded soy tarhana were investigated.
* Corres pondence to : SÓ enol I0 banog‘ lu, Department of Food Engineering, Engineering Faculty, The Univers ity of Gaziantep, 27310
Gaziantep, Turkey
(Received 9 September 1997 ; accepted 17 Augus t 1998 )
The composition of soy tarhana, based on total
weight (wet basis), was as follows : wheat ýour,
350 g kg~1 ; full-fat soy ýour, 150 g kg~1 ; yoghurt,
250 g kg~1 ; onions, 120 g kg~1 ; tomato pure e,
60 g kg~1 ; salt, 40 g kg~1 ; yeast, 10 g kg~1 ; paprika,
10 g kg~1 ; lactic acid solution, 0.60 g kg~1 ; dill,
0.20 g kg~1 ; and mint, 0.20 g kg~1. The crude
protein of wheat ýour and soy ýour used were
12.30 g kg~1 and 36.80 g kg~1, respectively. Yoghurt
was made from cow’s milk with a fat content of
3.60 g kg~1. Yeast was baker’s yeast in active dry
form. Tomato pure e was double concentrated with a
solids content of 300 g kg~1. The spices used were in
( 1999 Society of Chemical Industry. J Sci Food Agric 0022-5142/99/$17.50
P Ainsworth et al
powder form. Food grade lactic acid solution was
used in the extrusion runs (860 g kg~1).
Extrusion of soy tarhana was performed in a corotating twin-screw extruder (Continua 37, Werner
and Pýeiderer, Germany) with a screw diameter of
37 mm and L/D \ 27. A screw proüle which is a
standard design for processing cereals and ýourbased products was used throughout this study,
which was made up of self-wiping elements except
for a section consisting of short reverse and forwarding elements.3 The barrel was heated using circulating oil from an independent heating unit with
the feed zone being cooled by tap water. Two circular dies, each of 4 mm diameter 19 mm length, were
Onions were peeled, cut into halves and placed
into a bowl chopper (Kilia Type 201, Kiel,
Germany) together with the rest of the ingredients,
other than the wheat and soy ýour, to obtain a slurry
which could be fed to the extruder using a Watson
Marlow (Falmouth, UK) peristaltic pump. The
wheat and soy ýour were premixed and fed into the
extruder using a volumetric twin-screw metering
screw feeder (Rospen, Gloucester, UK). The overall
moisture content and pH of tarhana (the ýours plus
the slurry) were 430.0 g kg~1 and 5.1, respectively.
When the extrusion system reached a steady state
as indicated by constant percentage torque, pressure
and constant material temperature samples were
taken. All samples collected were oven-dried for 48 h
at 50¡C before grinding in a laboratory mill (Retsch
GmbH, Haan, Germany) to a particle size of
\500 lm. Ground extrudates were stored in airtight glass containers until analysis.
Experimental design
A multifactorial composite rotatable design with six
replicates at the centre point8 was chosen to study
the contribution of the three independent variables :
screw speed, feed rate and barrel temperature.
Ranges of the selected independent variables are
shown in Table 1. This range of operating variables
was found to be feasible for soy tarhana extrusion.
Response surface methodology was applied to the
data using a commercial statistical package, DesignExpert version 4.0 (Stat-Ease Inc, Minneapolis,
USA). A ürst order polynomial was ütted to the data
to obtain regression equations. Statistical signiücance
of the terms in the regression equations were examined. Response surface plots were generated with the
same software.
Table 1. Independent variables
and experimental des ign levels
us ed
Screw s peed (rpm)
Feed rate (kg hÉ1)
Barrel temperature (¡C)
Analytical methods
Determination of protein in vitro digestibility (PD) and
protein solubility (PS)
The pepsin–pancreatin method9 was used to determine the PD values of the extruded samples and an
unextruded sample while PS was calculated according to the method described previously.10
The complementary and supplementary eþect of soy
proteins on wheat proteins have been reviewed.11
Lysine is the limiting amino acid in wheat proteins,
while it is present in excess amounts in soy proteins.12 Therefore, combining the two proteins in a
proper ratio would result in a mixture that is nutritionally superior to each other.13 It has been stated
that a mixture of 70 parts wheat and 30 parts soy
would lead to a product having a protein efficiency
ratio approximately equal to that of casein.14 Therefore, 30% (w/w) of the wheat ýour in the traditional
tarhana recipe was replaced with soy ýour (15%
(w/w) of the total recipe) to improve the biological
value of tarhana.
Protein in vitro digestibility (PD)
In vitro digestibility of the protein is an important
aspect deüning the nutritional quality of a protein
along with its amino acid composition and bioavailability.4 In vitro protein digestibility of food
products containing soy ýour could be low due to the
presence of trypsin inhibitors and other antinutritional factors associated with soybeans.15 Heat
and shear are known to be eþective in destroying
these antinutritional factors.5
It was observed that only the barrel temperature is
signiücant (P \ 0.1) aþecting the protein in vitro
digestibility of extruded soy tarhana among the other
variables studied (ie feed rate and screw speed)
(Table 2). Most of the extrusion processes are performed at relatively low moisture contents
(\200 g kg~1).14 However, the moisture content of
soy tarhana in this study was relatively high as compared to other extrusion processes (430 g kg~1) which
would reduce the possible positive shearing eþects
on PD values. Low feed rates at constant extrusion
conditions lead to high mean residence times during
tarhana extrusion.16 Although both residence time
and shearing have been stated as important factors
for destroying the antinutritional factors present in
soy products,5 the results of this study suggest that
temperature of the extrusion had the greatest impact
on the PD of soy tarhana than residence time and
[ 1 .682
]1 .682
J Sci Food Agric 79 :675–678 (1999)
Extrusion of soy tarhana
Coefficients b
Independent variables
Table 2. Regres s ion equation
coefficients for protein in vitro
diges tibility (PD) and protein
s olubility (PS) in extruded s oy
Cons tant
(Predicted R 2 \ 0 .73 )
] 0.32*
[ 0.63
] 0.03
[ 0.04
[ 0.43**
] 0.004
a X , barrel temperature (¡C) ; X , feed rate of tarhana (kg hÉ1) ; X , s crew s peed (rpm)
b *, s ignificant at P \ 0.1 ; **, s ignificant at P \ 0.05.
shear rate. It is seen from Fig 1 that an increase in
barrel temperature results in increased PD values at
300 rpm screw speed. Similar trends were observed
at other screw speeds (data not shown). It is seen
from Fig 1 that the PD of soy tarhana extruded at
300 rpm reaches a PD value of 75%, which is not
signiücantly diþerent from that of extruded tarhana
without soybean ýour (ie 80%) (P \ 0.05).17 This
would suggest that antinutritional factors in soy
ýour, incorporated into the tarhana formulation,
have been destroyed at high barrel temperatures.
In our previous work it was found that extrusion
did not have either a detrimental or a positive eþect
on the PD of traditional tarhana.17 Therefore, it is
possible to say that the increase in PD of extruded
soy tarhana (Fig 1) compared to that of unextruded
soy tarhana (56%) could result from the destruction
of antinutritional factors present in the soy ýour by
Protein solubility
Protein solubility was measured using a ‘fast stir’
method which is widely used in the food and feed
industries to characterise the protein quality of raw
materials.10 With this method it is possible to
measure the amount of proteins which are soluble in
the water. The PS value was used as a protein
Figure 1. Effect of feed rate and barrel temperature on protein in
vitro diges tibility (PD) of extruded s oy tarhana at 300 rpm.
J Sci Food Agric 79 :675–678 (1999)
(Predicted R 2 \ 0 .70 )
quality parameter during extrusion of raw peas.18 It
has been stated10 that protein in vitro digestibility
and protein solubility index can be correlated for
some raw materials such as soy and rape seed meals.
However, in this study no correlation was found
between the PD and PS values (R2 \ 0.2, data not
shown) which would suggest that PS should not be
used in evaluating the nutritional status of extruded
soy tarhana.
Extruded instant tarhana powder with traditional
composition has been produced in our previous
work.3 It is thought that extruded soy tarhana could
be used as an instant soup powder to be reconstituted with boiling water. Therefore, the solubility of
proteins in extruded soy tarhana is important for
instant properties of the resulting soup (ie consumer
Results indicate that extrusion increased the PS of
soy tarhana since the PS of unextruded soy tarhana
(16%) was signiücantly lower than that of the soy
tarhana samples extruded at high barrel temperatures
(P \ 0.05) (Fig 2). The results in Table 2 suggest
that the changes in feed rate are signiücant for the
changes in PS of extruded soy tarhana (P \ 0.05)
while screw speed and barrel temperature are not
signiücant. It is seen from Fig 2 that PS increases
Figure 2. Effect of feed rate and barrel temperature on protein
s olubility (PS) of extruded s oy tarhana at 300 rpm.
P Ainsworth et al
with decreasing feed rate at constant screw speed
(300 rpm), which would indicate the importance of
residence time for PS. Similar trends were observed
at other screw speeds (data not shown). As in the
case of PD, the high moisture content of soy tarhana
(430 g kg~1) could reduce the possible positive eþects
of shearing on PD values.
The proteins in tarhana are composed of milk
(from yoghurt), wheat and soy proteins, each have
diþerent functional and nutritional properties.
Therefore, a wide range of interactions between proteins themselves and/or other macromolecules such
as starch are possible (ie cross-linking, associationdisassociation reactions, formation of covalent and
noncovalent bonds, formation of disulphide bonds).
It can be suggested that the conditions applied in
this study could lead to the exposure of hydrophilic
sites in tarhana proteins resulting in increased PS
values. Proteins may also form soluble complexes
with other constituents of tarhana during extrusion
which could also lead to increased PS values.
The results suggest that antinutritional factors in
extruded tarhana supplemented with 15% (w/w) soy
ýour can be destroyed by extrusion within the range
of operating conditions applied in this study. While
barrel temperature is signiücant for PD values
(P \ 0.1), feed rate was found signiücant for protein
solubility (P \ 0.05). Therefore, it can be suggested
that a combination of high barrel temperatures and
low feed rates would lead to a product with high PD
and PS values, which are important for the nutritional value and instant properties of extruded soy
tarhana, respectively. As functional properties are
important for instant properties of protein-based
soup powders, further studies will be conducted to
evaluate the eþect of extrusion variables on the functional properties of the proteins in extruded soy
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