Genetic diversity in chestnuts of Kashmir valley
Pak. J. Agri. Sci., Vol. 50(3), 421-425; 2013
ISSN (Print) 0552-9034, ISSN (Online) 2076-0906
http://www.pakjas.com.pk
THE EFFECT OF REFINING STEP ON THE CHANGES IN VISCOSITY
VALUES OF VEGETABLE OILS
Pelin Günç Ergonul
Food Engineering Department, Engineering Faculty, Celal Bayar University, Muradiye Campus, Manisa, Turkey
*
Corresponding author’s e.mail: [email protected]
In this work, the viscosity values of chemically refined vegetable oils (sunflower, corn, soybean and rapeseed) and physically
refined vegetable oils (olive and palm) were determined during refining processes. At this point of view, fatty acid
compositions and viscosity values of oil samples were determined. The edible vegetable oils presented Newtonian behavior
in shear rates at ranges 6.28-20.93 s-1. It was observed that palm oil is more viscous than the others. During physical refining,
the effect of both oil type and refining steps were significantly important, whereas in chemical refining only the effect of oil
type was found statistically important (p<0.01). It was observed that correlation among fatty acid compositions and viscosity
values of the samples showed differences according to oil type.
Keywords: fatty acid composition, rheological properties, vegetable oils, viscosity
INTRODUCTION
Viscosity is one of the most important parameter which
characterizes rheological properties of liquid foods. From
the physicochemical point of view, viscosity means the
resistance of one part of the fluid to move relative to another
one (Abramovic and Klofutar, 1998). The estimation of
viscosity of a vegetable oil is essential in the design of unit
processes such as distillation, heat exchangers, piping and
reactors (Rodenbush et al., 1999). Viscosity of vegetable oils
is affected by some physical and chemical factors such as
temperature, oil density, molecular weight, melting point
and degree of unsaturation. Viscosity is known to be
variable due to the fatty acid composition. Oils or fats
containing a greater proportion of fatty acids of relatively
low molecular weight are slightly less viscous than ones of
an equivalent degree of unsaturation but containing a higher
proportion of high-molecular-weight acids (Lawson, 1995).
Fresh vegetable oils show Newtonian flow behavior because
of their long chain molecules (Maskan, 2003; Santos et al.,
2004). In vegetable oils, viscosity increases with chain
length of triglyceride fatty acids and decreases with
unsaturation (Abramovic and Klofutar, 1998; Santos et al.,
2005). For this reason, viscosity is a function of molecules
dimension and orientation (Santos et al., 2005). One of the
factors that greatly affect the viscosity of oils is temperature.
It has been reported that the viscosity of oils and fats
decreased linearly with temperature (Igwe, 2004). Generally
the flow behaviors of vegetable oils were characterized by
steady shear measurements at 25˚C (Kim et al., 2010).
From that point of view, the aim of this study was to
investigate the viscosity of six different vegetable oils
(sunflower, corn, soybean, canola, olive and palm oils) from
each step of refining processes at 25˚C constant temperature
and then correlated with their fatty acid composition. Also,
the effect of the type of vegetable oils was investigated.
MATERIALS AND METHODS
Sunflower, corn, soybean and canola oils were obtained
from processing lines of factories using chemical refining
including degumming-neutralizing, bleaching, winterizing
and deodorizing steps. Olive oil and palm oil samples were
obtained from processing lines using physical refining
including bleaching and deodorizing steps. All samples are
taken after extraction and each refining steps in duplicate.
Rheological measurement: The flow behaviors of vegetable
oils were measured by using a Brookfield viscometer (DV-I
Viscometer, Middleboro, USA) with a small sample adapter,
spindle 3, which permits the use of 600 mL of oil in each
analysis. Measurements were done at room temperature
(25˚C), in different shear rates at ranges 6.28-20.93 s-1.
Temperature was controlled using a water bath with
precision of ±2˚C.
Sample preparation and measurement of fatty acid
composition: Fatty acid composition of vegetable oils was
investigated by using 6890 N model Agilent Technologies
GC (Palo Alto, USA). Fatty acid methyl esters (FAME)
were prepared according to Anonymous (1997). 100 mg of
oil was placed into a centrifuge tube and 0.5 mL of 2 N
methanolic potassium hydroxide and 2.5 mL of n-hexane
(Merck, Darmstadt, Germany) were added. The mixture was
then mixed vigorously and centrifuged in 6000 rpm for 10
min by using a centrifuge (Hettich EBA 8S, UK). The upper
clear supernatant, the FAME, was transferred to a screwcapped vial. After methylation of oil samples, the fatty acid
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methyl esters were injected into a capillary DB-23 fused
silica capillary column (60 m × 0.25 mm i.d., 0.25 μm film
thickness; Supelco Inc., Bellefonte, PA, USA). The
temperature of GC was 180˚C. The injector and detector
temperatures were 220˚C (15). Hydrogen was used as the
carrier gas at a flow rate of 1 mL/min. The fatty acids were
identified by comparing their retention times with the
retention times of fatty acid standards in chromatograms
obtained.
Statistical analysis: Factorial design was used in statistical
analyses. Two replications were done. The analysis of
variance was done using the PROC GLM procedure of SAS
(version 8.2., SAS Institute, Cary, NC, USA 2001).
LSMEANS for treatments were generated and separated
when significant (p<0.0001) using the Duncan procedure.
Correlation coefficients among viscosity values and fatty
acids composition of samples were also calculated using
PROC-CORR procedures of SAS (SAS, 2001).
RESULTS AND DISCUSSION
Results of chemically and physically refined vegetable oils’
viscosity values determined after each refining steps are
presented in Table 1 and in Table 2. The highest viscosity
was observed in palm oil which was followed by olive,
rapeseed, corn, sunflower and soybean oils. Kim et al.
(2010) was observed the highest viscosity in hazelnut oil,
followed by olive, canola, corn, soybean, sunflower and
grapeseed oils. Generally, liquid food viscosity depends on
its composition and temperature. The oil viscosity has a
direct relationship with some chemical characteristics of the
lipids, such as the degree of unsaturation and the chain
length of the fatty acids that constitute the triacylglycerols
(Abramovic and Klofutar, 1998). According to the results, it
was observed that vegetable oils’ viscosity increased due to
the removing of free fatty acids and with polymerization and
decreased with unsaturation. In case of polymerization, the
increase in chain size leads to the increase in the electron
number in the molecule, increasing London forces and, one
more time, intermolecular forces. Besides this, longer chains
are more difficult to move, leading to a higher viscosity
(Santos et al., 2005).
As seen as in Fig. 1, the viscosity of corn and sunflower oils
increased after neutralization and bleaching steps then
decreased after winterization step. It is thought that this
situation can be explained by the removal of compounds
having short carbon chain, especially free fatty acids during
neutralization and bleaching steps from sunflower and corn
oils which are rich in wax content. By removal of these
compounds, concentration of waxes having higher viscosity
values, increases in these oils. The degree of neutralization
of the acid is of critical importance for the efficiency of the
degumming process. If the degree of neutralization is too
low, the viscosity of the phosphatides is rather high, which
often makes the continuous discharge from a centrifugal
separator problematical (Greyt and Kellens, 2000). In the
case of rapeseed oil, the viscosity declined rather limitedly
after neutralization step and after bleaching step showed a
little increase and then remained almost similar. But in
general, the viscosity value of rapeseed oil has not changed
during refining process. Also, there is no statistically
significant change in the viscosity value of soybean oil was
determined during refining process. Generally the viscosity
values of chemically refined of all oils did not change after
winterization step and remained stable. It is obvious that,
variety of the oil is an important factor affecting the changes
in wax content of the samples. It was observed that changes
in wax content of the oils having more wax amount were
significant. It can be said that the rheological behaviors of
oils did not change significantly in constant room
temperature. Among the physically refined oils, the viscosity
values of palm-olein increased significantly after bleaching
Table 1. Viscosity of chemically refined vegetable oils at 25˚C in different steps of refining processes
Viscosity (Pa.s)
Refining
Steps
SunflowerA
Soybean B
Corn A
Rapeseed A
Crude
7.03
6.24
7.52
8.00
Neutralized
7.18
6.16
8.30
7.36
Bleached
8.00
6.20
8.67
7.71
Winterized
6.67
6.20
7.34
7.79
Deodorized
6.58
6.10
7.37
7.72
Table 2. Viscosity of physically refined vegetable oils at 25˚C in different steps of refining processes
Viscosity (Pa.s)
Refining
Steps
Olive Oil a
Palm Oil b
A
Crude
8.79
12.03
Bleached B
8.12
17.02
Deodorized C
7.96
10.76
422
Changes in viscosity values of vegetable oils
step and decreased significantly after deodorization step
(Fig. 2). It is thought that this prominent change is due to the
high saturated structure of palm oil. The viscosity values of
olive oil has been shown a consistently decrease during
refining (Fig.2). Average viscosity value of all oil samples
decreased when compared with the wax content of crude oil.
10
viscosity (cp)
8
6
4
2
ize
d
De
od
or
ed
W
in
te
r iz
Bl
ea
ch
ed
Ne
ut
ra
liz
ed
Cr
ud
e
0
viscosity (cp)
Figure 1. The changes of viscosity values of chemically
refined oils
18
16
14
12
10
8
6
4
2
0
olive oil
palm oil
Crude
Bleached
Deodorized
Figure 2. The changes of viscosity values of physically
refined oils
The statistical results demonstrated that in chemically
refining only the oil types were significantly affected the
viscosity values (p<0.01). The differences between oil types
in terms of viscosity are indicated by different letters in
Table 1. In different refining steps, the viscosity values of
oils did not change significantly due to the applied constant
temperature. But the oil type was prominently affected the
viscosity (Kim et al., 2010). In physically refined oils, both
of oil type and refining steps are statistically important
(p<0.0001) and they are indicated also by different letters by
themselves (Table 2).
For all the oils analyzed in this study, the percent amount of
total saturated fatty acids (SFA) and unsaturated fatty acids
increased from 7.3% to 44.8% and 11.3% to 69.9%
(Table 3-4). In general, when the monounsaturated fatty
acids (MUFA) contents of oils decreased, polyunsaturated
fatty acids (PUFA) contents increased and when the MUFA
contents of oils increased, PUFA contents decreased during
refining processes. It was determined that refining steps and
oil types were not affected statistically the fatty acid
composition of oil samples in chemically refining (p>0.05)
whereas in physically refining, only the oil types was
statistically important effect on the changes in fatty acid
compositions of the samples (p<0.0001). The differences
between oil types are indicated by different letters in
Table 3. This is because, in physical refining no oil
degradation occurred as in chemical refining so the viscosity
decreased significantly by removing saturated components
during physical refining. The viscosity of olive oil was
positively correlated with the amounts of SFA (r=+0.54) and
negatively correlated with the amounts of MUFA (r=-0.44).
For palm-olein, viscosity was positively correlated with the
amounts of SFA (r=+0.85) and MUFA (r=+0.95) and
negatively correlated with the amounts of PUFA (r=-0.80).
Among the chemically refined oils, the viscosity of
sunflower oil was positively correlated with the amounts of
saturated fatty acid (SFA) (r=+0.97) and monounsaturated
fatty acid (MUFA) (r=+0.56) and negatively correlated with
the amounts of polyunsaturated fatty acid (PUFA) (r=-0.62).
The viscosities of rapeseed and soybean oils were positively
correlated with the amounts of SFA (r=+0.85 and r=+0.84)
and negatively correlated with the amounts of MUFA (r=0.47 and r=-0.45). The correlation the viscosity of corn oil
with the amounts of SFA was quite low (r=+0.27). Santos et
al. (2005) was determined that viscosity of oils was better
related to the concentration of PUFA chains than to MUFA
Table 3. Fatty acid composition of physically refined vegetable oils in different steps of refining processes
Fatty acids (%)
Oil Type
Saturated
Monounsaturated
Polyunsaturated
Olive oil A
Crude
17.34
69.92
12.76
Bleached
17.21
70.23
12.6
Deodorized
17.11
70.08
12.82
Palm oil B
Crude
44.77
44.01
11.25
Bleached
43.98
44.55
11.5
Deodorized
43.82
44.53
11.66
423
Ergonul
Table 4. Fatty acid composition of chemically refined vegetable oils in different steps of refining processes
Fatty acids (%)
Oil Type
Saturated
Monounsaturated
Polyunsaturated
Sunflower oil
Crude
10.34
49.87
39.78
Neutralized
10.09
50.07
39.93
Bleached
10.00
50.17
39.85
Winterized
10.03
48.70
41.10
Refined
9.95
48.26
41.81
Corn oil
Crude
15.82
38.81
45.51
Neutralized
15.37
38.33
46.28
Bleached
15.20
37.95
46.89
Winterized
14.64
37.32
46.48
Refined
14.60
38.83
47.49
Rapeseed oil
Crude
8.21
64.22
27.61
Neutralized
7.73
64.34
27.94
Bleached
7.71
64.46
27.84
Winterized
7.33
64.38
29.31
Refined
7.32
63.72
29.49
Soybean oil
Crude
16.55
26.93
56.52
Neutralized
17.18
27.84
55.01
Bleached
16.35
28.20
55.47
Winterized
15.44
29.30
55.36
Refined
15.29
29.51
55.37
chains. Kim et al. (2010) was not observed any positive
correlation between viscosity and total saturated or
unsaturated fatty acids, in their study. But they indicated
positive correlation between the viscosity and both of oleic
and linoleic acids.
Conclusion: Changes in viscosity values of chemically
refined oils during refining process are similar then the
physically refined oils. From that point of view, it is thought
that average viscosity values of the oil samples obtained
from different steps of refining process are similar at
ambient temperature 25˚C. But, for all oils samples it was
determined that viscosity value decreased at the end of the
deodorization step when compared to viscosity value of
crude oil. The results demonstrated that there was a high
positive correlation between oil viscosities and SFA, and
low negative correlation between oil viscosities and MUFA
or PUFA. Since the value of viscosity did not change
significantly, there were no important correlation was
determined among viscosity and MUFA and PUFA values at
25˚C.
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