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Effects of Frying and Antioxidants on French Fries and Oil Quality

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The objective of this study was to investigate the effects of frying temperature, time and antioxidant addition on the quality of French fries (FF) and frying oil (FO). The antioxidants tested were synthetic tert-butylhydroquinone, TBHQ, and natural pandan leaf. The L* value reduction of FF and FO occurred on the increasing of frying temperature (160-190 o C) and time (1-7 min.). A remarkable reduction of a* value in FO was found in the sample with pandan leaf, while the a* value in FF slightly differed from the control sample (without antioxidant). Using both synthetic and natural antioxidants increased the oil absorption in FF, which was indicated by lower FF shear force values than the control. The oxidation reaction measured by conjugated dienes in FO and oil absorbed in FF showed a significant decreasing value when TBHQ and pandan leaf were present. The viscosity of FO was used for indicating the extent of the polymerization reaction, which reduced in the samples containing antioxidants.
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Effects of Frying and Antioxidants on French Fries and Oil Quality

Kornpaka Arkanit*, Wiriyapong Tanawuttipong, Sutthirak Nuntarith

Department of Food Technology, Faculty of Engineering and Agro-Industry, Maejo University,
Nongharn, Sansai, Chiangmai 50290, Thailand

________________________________________________________________________

Abstract

The objective of this study was to investigate the effects of frying temperature,
time and antioxidant addition on the quality of French fries (FF) and frying oil (FO). The
antioxidants tested were synthetic tert-butylhydroquinone, TBHQ, and natural pandan leaf.
The L* value reduction of FF and FO occurred on the increasing of frying temperature
(160-190oC) and time (1-7 min.). A remarkable reduction of a* value in FO was found in
the sample with pandan leaf, while the a* value in FF slightly differed from the control
sample (without antioxidant). Using both synthetic and natural antioxidants increased the
oil absorption in FF, which was indicated by lower FF shear force values than the control.
The oxidation reaction measured by conjugated dienes in FO and oil absorbed in FF
showed a significant decreasing value when TBHQ and pandan leaf were present. The
viscosity of FO was used for indicating the extent of the polymerization reaction, which
reduced in the samples containing antioxidants.
Keywords: Antioxidant, Pandan, TBHQ, French fries, Oxidation
























________________________________________________________________________
* Corresponding author
E-mail address: k_arkanit@hotmail.com

Introduction

Deep-fat frying is extensively used both at home and on a commercial scale to
enhance the organoleptic properties of foods. A number of chemical changes occur both in
the frying medium and the product fried. These affect the quality of the oil and the fried
food product (Varela, 1988). All chemical changes of frying fats and oils and the product
at elevated temperatures originate in oxidation, hydrolysis, polymerization, isomerization
or cyclization reactions. All these reactions may be promoted by oxygen, moisture, traces
of metal and free radicals (Gertz, 1996). These processes may reduce the amount of
antioxidants in the oil, decrease its stability and produce new products which are
responsible for loss of the nutritional value and quality of the oil (odor, flavor, absorption,
etc). In some countries, such as USA, oil and fat manufacturers normally treat the refined
oils with antioxidants to retard the undesirable changes during storage and frying
operations and, in eventuality, to prolong the shelf-life of the fried products. It is believed
that the antioxidants protect the fat from oxidation during the time that the oil is exposed
to high temperatures (Augustin and Berry, 1983). To avoid or delay the lipid oxidation in
food processing, antioxidants have been used for over 50 years (Cuvelier et al., 1994). The
antioxidants play an important role in the manufacturing, packaging and storage of fats
and fatty foods and have been proven to retard oxidation (Lin et al., 1981).
Currently, synthetic antioxidants are often used to improve the storage behavior of
fats and oils. The most commonly used antioxidants are butylated hydroxyanisole (BHA),
butylated hydroxytoluene (BHT) and tert-butylhydroquinone (TBHQ). They are added to
a wide variety of foods in the market (Chang et al., 1977). However, their use is
increasingly contested or even banned in certain countries. The recent consumer interest in
“natural” products drives the need for natural antioxidative substances to replace the
conventional antioxidants (Cuvelier et al., 1994). BHA and BHT are quite volatile and
easily decomposed at high temperatures. Consequently, they are only effective for certain
products and are not effective for such common food products as French fries and potato
chips. Furthermore, it has been stated that BHA and BHT are not effective in vegetable
oils at preventing the development of initial off-flavor (Chang et al., 1977). Recently, the
use of natural antioxidants for frying purposes has increased (Bracco et al., 1981). Plant
extracts, especially antioxidants obtained from herbs and spices, have been proposed for
stabilizing frying oils.
Plants belonging to the genus Pandanus (Pandanaceae) are palm-like evergreen
trees or shrubs, widely distributed in the moist tropics from Africa to the Pacific Islands.
In Thailand, the leaves of Pandanus amaryllifolius Roxb. are widely used for flavoring
foods and beverages. In some cases, pandan leaves are added to frying oil for flavoring
fried foods and retarding oxidation.
The objective of this study was to investigate the effects of frying temperature,
time, and antioxidant addition on the quality of French fries (FF) and frying oil (FO). The
antioxidants tested were synthetic tert-butylhydroquinone, TBHQ, and natural pandan leaf.

Materials and Methods

Materials

Palm olein oil, pandan leaf, and frozen potatoes strips (5 x 5 x 60 mm) were
purchased in a local supermarket.
Deep-fat Frying Procedure

Before frying, the potatoes strips were thawed and allowed to reach room
temperature. 200 ml oil was poured into the pan fryer and heated to 160, 170, 180 or

190oC. Then, 150 g of potato strips were added to the fryer and fried at a time for 1, 3, 5 or
7 min. After frying, the French fries were removed from the oil and allowed to cool to
room temperature. The frying oil was allowed to cool and filtered through cheesecloth.
The French fries and frying oil were collected, flushed with nitrogen, and kept below-20oC
until analyzed. Frying procedures were replicated three times for each experiment. The
antioxidant-treated oils were prepared by adding 100, 200 ppm TBHQ, or 5, 10% (w/w
oil) pandan leaf into the cold oils before frying processes.
Extraction of Oil from French Fries

The method for extraction of oil from French fries was modified from the method
of Rafecas (1998). To 50 ml chloroform/methanol (2:1, v/v), a 25 g sample ground French
fries was added. The solution was mixed using magnetic stirrer for 30 min and then
filtered (Whatman No.1 filter paper). The solvent extraction steps were repeated twice.
The filter paper was washed with 25 ml solvent to recover the residual oil on the filter
paper. All of the extraction solvents were then collected in separatory funnel. 35 ml
saturated NaCl solution was added to the funnel. The chloroform layer was separated and
filtered through filter paper with an anhydrous Na2SO4 layer. The chloroform was
evaporated at 35oC under vacuum. The extracted oil from French fries was collected,
flushed with nitrogen, and kept below -20oC until analyzed.
Physical Properties

The color of French fries and frying oil were measured using a Tri-stimulus
colorimeter, Model JC801, Tokyo, Japan. Viscosity of frying oil was measured using a
Brookfield Viscosimeter, Model DV III, Stoughton, MA, USA.
Textural Properties

Shear force of French fries was determined using a Lloyd universal testing
machine, Model LR10K, Fareham Hampshire, UK. The full scale load was set at 1 kN
with the cross head speed set at 100 mm/min. Data were recorded as the average of
three samples per treatment. The measured parameter was the maximum cutting force.
Chemical Properties
Oil absorption in French fries was determined according to AOAC (2000).
Conjugated dienes contents of oil absorbed in French fries and frying oil were determined
according to AOCS (1997).

Results and Discussion

The L* values were used to indicated the lightness of the FF and FO. As shown in
Fig. 1 the lightness of FF and FO decreased with the increasing of frying temperature and
time. Frying foods at elevated temperature in the presence of water caused lipolysis.
Antioxidant addition helped in preventing the darkening of the frying oil by retarding
oxidation.
In general, an increase in a* values is undesirable because that means a more red
product, which is not acceptable for FF. As shown in Fig. 2, the a* values of FF and FO
increased when increase the frying time which is negative for color of FF and FO (become
more red). Addition of antioxidants resulted slightly changes of a* values in FF and FO,
except for natural pandan leaf addition reduced the red color in FO significantly due to
dissolved green pigment from pandan leaf in FO.
In general higher b* values give more yellow products, which is desirable for fried
products. The b* value decreased with the increasing of frying time (Fig. 3). Antioxidants
did not effect changes of b* value in FF and FO.

80
4
(a)
(b)
160oC
160oC
60
3
* 40
L
* 2
L
20
1
0
0
0
2
4
6
8
0
2
4
6
8
Time (min.)
Time (min.)


80
3
170oC
170oC
60
2
* 40
*
L
L
1
20
0
0
0
2
4
6
8
0
2
4
6
8
Time (min.)
Time (min.)


80
2.0
180oC
180oC
60
1.5
* 40
* 1.0
L
L
20
0.5
0
0.0
0
2
4
6
8
0
2
4
6
8
Time (min.)
Time (min.)


80
0.50
190oC
190oC
60
* 40
L
*
L 0.25
20
0
0.00
0
2
4
6
8
0
2
4
6
8
Time (min.)
Time (min.)
control
100 pp m TBHQ
200 p pm TBHQ
control
100 ppm TBHQ
200 ppm TBHQ
5% Pandan leaf
10% Pandan leaf
5% Pandan leaf
10% Pandan leaf


Fig. 1 L* values of French fries (a) and frying oil (b) at different frying temperature, time,
and antioxidants

8
(a)
(b)
20
160oC
160oC
6
10
4
a
*
a
*
2
0
0
-10
-2
0
2
4
6
8
0
2
4
6
8
Time (min.)
Time (min.)


8
170oC
170oC
20
6
4
10
a
*
a
*
2
0
0
-10
-2
0
2
4
6
8
0
2
4
6
8
Time (min.)
Time (min.)


8
20
180oC
180oC
6
10
4
a
*
a
*
2
0
0
-10
-2
0
2
4
6
8
0
2
4
6
8
Time (min.)
Time (min.)


8
190oC
190oC
20
6
10
4
a
*
a
*
2
0
0
-10
-2
0
2
4
6
8
0
2
4
6
8
Time (min.)
Time (min.)
control
100 pp m TBHQ
200 ppm TBHQ
control
100 ppm TBHQ
200 pp m TBHQ
5% Pandan leaf
10% Pandan leaf
5% Pandan leaf
10% Pandan leaf


Fig. 2 a* values of French fries (a) and frying oil (b) at different frying temperature, time,
and antioxidants

(a) 50
28
(b)
160oC
26
160oC
40
24
30
22
*
*
b
b 20
20
18
10
16
0
14
0
2
4
6
8
0
2
4
6
8
Time (min.)
Time (min.)


50
28
170oC
170oC
26
40
24
30
22
*
b
*
b
20
20
18
10
16
0
14
0
2
4
6
8
0
2
4
6
8
Time (min.)
Time (min.)


50
28
180oC
26
180oC
40
24
30
22
*
*
b
b 20
20
18
10
16
0
14
0
2
4
6
8
0
2
4
6
8
Time (min.)
Time (min.)


50
28
190oC
26
190oC
40
24
30
22
*
b
*
b
20
20
18
10
16
0
14
0
2
4
6
8
0
2
4
6
8
Time (min.)
Time (min.)
control
100 p pm TBHQ
200 ppm TBHQ
control
100 ppm TBHQ
200 pp m TBHQ
5% Pandan leaf
10% Pandan leaf
5% Pandan leaf
10% Pandan leaf


Fig. 3 b* values of French fries (a) and frying oil (b) at different frying temperature, time,
and antioxidants


25
25
160oC
170oC
20
20
e
d
e
d
15
r
b
15
r
b
s
o
s
o
i
l

a
b
10
o
i
l

a
b
10
o
%
%
5
5
0
0
0
2
4
6
8
0
2
4
6
8
Time (min.)
Time (min.)


25
25
180oC
190oC
20
20
e
d
e
d
15
r
b
15
r
b
s
o
s
o
i
l

a
b
i
l

a
b
o 10
10
o
%
%
5
5
0
0
0
2
4
6
8
0
2
4
6
8
Time (min.)
Time (min.)

control
100 pp m TBHQ
200 p pm TBHQ
5% Pandan leaf
10% Pandan leaf

Fig. 4 % Oil absorption in French fries at different frying temperature, time, and
antioxidants


During deep-fat frying, oil is absorbed into the fried materials and absorbed on its
surface, which is undesirable due to oily appearance and high caloric content of such
products (Mellema, 2003). The addition of synthetic TBHQ and natural pandan leaf
increased the oil absorption in FF (Fig. 4).















30
160oC
30
170oC
)
)
20
20
r
c
e

(
N
r
c
e

(
N
e
a
r

f
o
e
a
r

f
o
h
h
S
S
10
10
0
0
0
2
4
6
8
0
2
4
6
8
Time (min.)
Time (min.)


180oC
190oC
30
30
)
)
20
20
r
c
e

(
N
r
c
e

(
N
e
a
r

f
o
e
a
r

f
o
h
h
S
S
10
10
0
0
0
2
4
6
8
0
2
4
6
8
Time (min.)
Time (min.)

control
100 ppm TBHQ
200 ppm TBHQ
5% Pandan leaf
10% Pandan leaf

Fig. 5 Shear force values of French fried at different frying temperature, time, and
antioxidants


Shear force indicated the textural quality of the products. In case of FF, shear force
values lower than 10 N indicates soft, 10-15 N indicates crispy, and higher than 15 N
indicates hard. As shown in Fig. 5, shear force values of FF increased with the increase in
frying temperature and time. The appropriate combinations of frying temperature and
time, that is combinations giving shear force values between 10 and 15 N, were 160oC 5
min., 170oC 3-5 min., 180oC 3-5 min., and 190oC 3 min. Antioxidant additions decreased
the shear force values, which was a result of increased oil absorption in FF.















2.5
2.5
(a)
(b)
160oC
160oC
2.0
2.0
m
m

n

n
3
3
3
3
1.5
1.5
c
e

a
t

2
c
e

a
t

2
a
n 1.0
a
n 1.0
r
b
r
b
s
o
s
o
b
b
A
A
0.5
0.5
0.0
0.0
0
2
4
6
8
0
2
4
6
8
Time (min.)
Time (min.)


2.5
2.5
170oC
170oC
2.0
2.0
m
m

n

n
3
3
3
3
1.5
1.5
c
e

a
t

2
c
e

a
t

2
a
n
a
n 1.0
1.0
r
b
r
b
s
o
s
o
b
b
A
A 0.5
0.5
0.0
0.0
0
2
4
6
8
0
2
4
6
8
T ime (min.)
Time (min.)


2.5
2.5
180oC
180oC
2.0
2.0
m
m

n

n
3
3
3
3
1.5
1.5
c
e

a
t

2
c
e

a
t

2
a
n
a
n
1.0
1.0
r
b
r
b
s
o
s
o
b
b
A
A
0.5
0.5
0.0
0.0
0
2
4
6
8
0
2
4
6
8
Time (min.)
Time (min.)


2.5
2.5
190oC
190oC
2.0
2.0
m
m

n
3

n
3
3
3
1.5
1.5
c
e

a
t

2
c
e

a
t

2
a
n 1.0
a
n
r
b
1.0
r
b
s
o
b
s
o
b
A 0.5
A 0.5
0.0
0.0
0
2
4
6
8
0
2
4
6
8
Time (min.)
Time (min.)
control
100 ppm TBHQ
200 ppm TBHQ
control
100 ppm TBHQ
200 ppm TBHQ
5% Pandan leaf
10% Pandan leaf

5% Pandan leaf
10% Pandan leaf

Fig. 6 Conjugated dienes values of oil absorbed in French fries (a) and frying oil (b) at
different frying temperature, time, and antioxidants


Conjugated dienes concentration was used as a measure of the amount of primary
oxidation products present throughout the deep-fat frying processes. Results shown in Fig.
6 indicate that frying temperature and time had a significant effect on conjugated dienes
formation in FF and FO in all of the samples. The conjugated dienes increased with the
increased frying temperature and time as a result of oxidation reaction occurred during
frying. The use of both synthetic TBHQ and natural pandan leaf during frying had
excellent antioxidant activities as shown by giving remarkable reductions of conjugated
dienes in FF and FO.


70
70
160oC
170oC
65
65
i
s
e
)
i
s
e
)
o
o
t
i
p
t
i
p
60

(
c
e
n
60

(
c
e
n
s
i
t
y
s
i
t
y
i
s
c
o
i
s
c
o
55
V
V 55
50
50
0
2
4
6
8
0
2
4
6
8
Time (min.)
Time (min.)


70
70
180oC
190oC
65
65
i
s
e
)
i
s
e
)
o
o
t
i
p
t
i
p
60
60

(
c
e
n

(
c
e
n
s
i
t
y
s
i
t
y
i
s
c
o
i
s
c
o
55
55
V
V
50
50
0
2
4
6
8
0
2
4
6
8
Time (min.)
Time (min.)

control
100 ppm TBHQ
200 ppm TBHQ
5% Pandan leaf
10% Pandan leaf

Fig. 7 Viscosity of frying oil at different frying temperature, time, and antioxidants


The viscosity of FO was used for indicating the extent of the polymerization
reaction caused by oxidation of unsaturated fatty acids. These polymeric materials are
mainly responsible for the increase in viscosity in cooking oils (Berger, 1984). As shown
in Fig. 7, the viscosity of FO increased with the increase of frying temperature and time.
The addition of antioxidants significantly lowered the viscosity of FO by retarding
polymerization reactions during frying.

Conclusion


Frying processes at elevated temperature affected the quality of FF and FO.
Oxidative reactions during frying are the major cause of the product quality deterioration.
The use of synthetic TBHQ and natural pandan leaf as antioxidants had the effects of
improved color in FF, but a lower textural quality. Increased oil absorption in FF was
observed when antioxidants were used in FO. Natural pandan leaf was shown to be a

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