Analytical Chemistry Insights 2008:3 127–133 127
ORIGINAL RESEARCH
Correspondence: Azmil Haizam, Analytical and Quality Development Unit, Malaysian Palm Oil Board, 6, Persiaran
Institusi, Bandar Baru Bangi, 43000 Kajang, Selangor, Malaysia. Tel: (603) 87694443; Fax: (603) 89221742;
Email: azmil_haizam@mpob.gov.my
Copyright in this article, its metadata, and any supplementary data is held by its author or authors. It is published under the
Creative Commons Attribution By licence. For further information go to: http://creativecommons.org/licenses/by/3.0/.
Palm-Based Standard Reference Materials for Iodine Value
and Slip Melting Point
Azmil Haizam Ahmad Tarmizi, Siew Wai Lin and Ainie Kuntom
Analytical and Quality Development Unit, Malaysian Palm Oil Board, 6, Persiaran Institusi, Bandar
Baru Bangi, 43000 Kajang, Selangor, Malaysia.
Abstract: This work described study protocols on the production of Palm-Based Standard Reference Materials for iodine
value and slip melting point. Thirty-three laboratories collaborated in the inter-laboratory profi ciency tests for characteriza-
tion of iodine value, while thirty-two laboratories for characterization of slip melting point. The iodine value and slip melting
point of palm oil, palm olein and palm stearin were determined in accordance to MPOB Test Methods p3.2:2004 and
p4.2:2004, respectively. The consensus values and their uncertainties were based on the acceptability of statistical agreement
of results obtained from collaborating laboratories. The consensus values and uncertainties for iodine values were 52.63 ± 0.14
Wijs in palm oil, 56.77 ± 0.12 Wijs in palm olein and 33.76 ± 0.18 Wijs in palm stearin. For the slip melting points, the
consensus values and uncertainties were 35.6 ± 0.3 °C in palm oil, 22.7 ± 0.4 °C in palm olein and 53.4 ± 0.2 °C in palm
stearin. Repeatability and reproducibility relative standard deviations were found to be good and acceptable, with values
much lower than that of 10%. Stability of Palm-Based Standard Reference Materials remained stable at temperatures of –20 °C,
0 °C, 6 °C and 24 °C upon storage for one year.
Keywords: palm-based standard reference materials, iodine value, slip melting point, MPOB Test Methods p3.2: 2004 and
p4.2:2004, consensus values, uncertainties
Introduction
Palm oil is one of the most important sources of revenue for Malaysia. The total export earnings increased
by 41.8% in 2007 compared to that in 2006 (Mohd. Basri, 2008). In fact, palm oil contributes to almost
30% of the vegetable oils production worldwide, in which 60% of the share accounts for the overall
global export (Carter et al. 2007). Palm oil has now gained worldwide acceptance due to its unique
properties and versatile applications as well as the competitive traded price over other vegetable oils
(Choo et al. 2007). As one of the leading countries in the palm oil business, attempts should be taken
to ensure that palm oil is of good quality.
Some of the indicators used to characterize palm oil are iodine value (IV) and slip melting point
(SMP). The IV measures the degree of unsaturation or double bonds of oils and fats. It also indicates
the ease of oxidation of oils and fats (Guided Wave Incorporated, 2008). Meanwhile, the SMP is widely
used to characterize the melting and solidifi cation properties of oils and fats. It changes with the chain
length of fatty acids, unsaturation ratios, trans fatty acid content and the position of the fatty acids in
the glycerol backbone (Karabulut et al. 2004).
Harmonization of the test methods for these characteristics is crucial in producing palm oil within
the traded specifi cations. Thus, a comparison of measurements should be made with the certifi ed values
of the standard reference materials to assure the reliability and robustness of the methods used. With
the emphasis on the quality aspects, our research group has been engaged in the development of Palm-
Based Standard Reference Materials for palm oil analyses due to the unavailability of such standard
reference materials in the market. This is to facilitate the use of these standard reference materials for
calibration and validation of analytical measurements as well as to assess the capability of analysts in
performing the measurements in the palm oil sector.
Currently, Palm-Based Standard Reference Materials for fatty acids composition, solid fat
content, IV and SMP have been produced in-house. However, only two characterization works of fatty
acids composition and solid fat content have been published. Thus, this paper reports the outcomes of

128
Azmil Haizam et al
Analytical Chemistry Insights 2008:3
the inter-laboratory profi ciency tests of the standard
reference materials from palm oil, palm olein and
palm stearin for IV and SMP. Stability of the stan-
dard reference materials produced upon storage was
also evaluated and further discussed in this paper.
Materials and Methods
Production of Palm-Based Standard
Reference Materials
Refi ned, bleached and deodorized palm oil, palm
olein and palm stearin were obtained from a local
supplier. The antioxidant, tert-butylhydroquinone
(97% purity) and 5-mL dark amber glass ampoules
were purchased from Sigma-Aldrich (Steinheim,
Germany) and Scherf Praezision (Meiningen-
Dreissigacker, Germany), respectively.
A batch of palm oil, palm olein and palm stea-
rin, respectively were heated up to 70 °C to liquefy
the solid fat prior to the addition of 200 mg kg
−1

of tert-butylhydroquinone. These solutions were
then mixed thoroughly to ensure homogeneity.
Each batch of oil produced about 3000 ampoules
of standards. The usage of 5-mL dark amber glass
ampoules helps to avoid color changes and photo-
oxidation of oil standards upon storage. Portions
of 5 mL of homogenized oils were pipetted into
the ampoules prior to fl ushing with nitrogen and
fl ame sealed. The oil standards were then labelled,
packed in fabricated boxes and stored at −20 °C
until dispatch.
Characterization exercises
Thirty-three and thirty-two of local and overseas
laboratories collaborated in the inter-laboratory
profi ciency tests for IV and SMP, respectively,
which include Analytical and Quality Development
Unit, Malaysian Palm Oil Board, Selangor,
Malaysia; Kek Seng Berhad, Johor, Malaysia;
Research and Development Laboratory, Golden
Jomalina Food Industries Sdn. Bhd., Selangor,
Malaysia; Aarhuskarlshamn Sweden AB, Analys-
Centrum, Karlshamn, Sweden; PT Multimas Nabati
Asahan, Sumatera Utara, Indonesia; Southern
Edible Oil Industries Sdn. Bhd., Selangor, Malaysia;
Wilmar International Ltd., Singapore; School of
Sciences and Food Technology, Universiti Kebang-
saan Malaysia, Selangor, Malaysia; Quality Assur-
ance Laboratory, Golden Jomalina Food Industries
Sdn. Bhd., Selangor, Malaysia; PT Asianagro
Agungjaya, West Java, Indonesia; Kempas Edible
Oils Sdn. Bhd., Johor, Malaysia; PGEO Edible Oils
Sdn. Bhd., Johor, Malaysia; IOI Edible Oils Sdn.
Bhd., Sabah, Malaysia; Oleochemical Products
Services Unit, Malaysian Palm Oil Board, Selangor,
Malaysia; Pan-Century Edible Oils Sdn. Bhd., Johor,
Malaysia; Edtech Associates Sdn. Bhd., Penang,
Malaysia; Chemara Laboratory Sdn. Bhd., Negeri
Sembilan, Malaysia; ITS Testing Services Sdn.
Bhd., Selangor, Malaysia; Kuala Lumpur-Kepong
Berhad, Selangor, Malaysia; Biochem Laboratories
Sdn. Bhd., Penang, Malaysia; Allied Chemists Sdn.
Bhd., Johor, Malaysia; Felda-Johore Bulkers
Sdn. Bhd., Johor, Malaysia; Chemsain Konsultant
Sdn. Bhd., Sarawak, Malaysia; Lotus Labora-
tory Services Sdn. Bhd., Johor, Malaysia; SGS
Laboratory Services Sdn. Bhd., Johor, Malaysia;
Alami Technological Services Sdn. Bhd., Selangor,
Malaysia; Chemical Laboratory Sdn. Bhd., Johor,
Malaysia; Kuala Lumpur-Kepong Edible Oils Sdn.
Bhd. (East Malaysia), Sabah, Malaysia; Testing
Services (Sabah) Sdn. Bhd., Sabah, Malaysia; MM
Vitaoils Sdn. Bhd., Selangor, Malaysia; PGEO
Edible Oils Sdn. Bhd. (Prai Division), Penang,
Malaysia; KL-Kepong Edible Oils Sdn. Bhd., Johor,
Malaysia; Lam Soon Edible Oils Sdn. Bhd.,
Johor, Malaysia.
Four ampoules of each oil standards (palm
oil, palm olein and palm stearin), which corre-
spond to four replications, were sent to the
collaborating laboratories. The IV and SMP
measurements should be conducted using
MPOB Test Methods p3.2:2004 and p4.2:2004
(Ainie et al. 2004), respectively. The MPOB Test
Methods p3.2:2004 was technically equivalent
to ISO 3961:1996. The MPOB Test Methods
p4.2:2004 was originated from AOCS Offi cial
Method Cc 3-25 (Firestone, 1998).
In the determination of IV, about 0.2 g of oil
sample, in a 20 mL mixture of cyclohexane (Merck,
Darmstadt, Germany) and glacial acetic acid
(Systerm, Shah Alam, Malaysia) was reacted with
25 mL of Wijs reagent (Merck, Darmstadt,
Germany) followed by addition of 20 mL of
100 g L
−1
potassium iodide (Systerm, Shah Alam,
Malaysia) and distilled water after storage in the
dark for 1 h. The liberated iodine was titrated with
0.1 M sodium thiosulfate (Univar, Seven Hills,
Australia) until the yellowish iodine color disap-
peared. A small amount of starch solution (Merck,
Darmstadt, Germany) was then added to the solu-
tion as an indicator and the titration continued until

129
Palm-Based Standard Reference Materials for Iodine Value and Slip Melting Point
Analytical Chemistry Insights 2008:3
the blue color has also disappeared. The IV is
calculated by the following equation :

IV g g
CV V
m
/
.
100
12 69
12
()
=
−
()

where,
C is the concentration of sodium thiosulfate
solution (mole L
−1
);
V
1
is the volume (mL) of sodium thiosulfate
solution used for the blank test;
V
2
is the volume (mL) of sodium thiosulfate
solution used for the sample; and
m is the mass (g) of the sample
Wijs reagent, which contains iodine monochlo-
ride in acetic acid, can be also prepared manually
in the laboratory. The ratio between iodine and
chlorine of the Wijs reagent shall be within the
limits of 1.10 ± 0.1. However, the preparation of
the reagent is time consuming and hence is highly
recommended to use the commercially available
Wijs reagent.
SMP was measured using the following
procedure. At least three clean capillary tubes were
initially dipped into a completely melted oil sample
to a depth of 10 mm. The tubes were then chilled
until the oil sample was solidifi ed prior to placing
them in a test tube and held in a beaker of water
equilibrated at 10 °C for 16 h in a thermostat water
bath (Huber, Offenburg, Germany). The capillary
tubes were subsequently removed from the test tube
and attached to a thermometer with a rubber band
such that the lower ends of the tubes were at the
same level as the bottom of the mercury bulb of
the thermometer. The thermometer was suspended
in a beaker containing 400 mL of boiled distilled
water. The thermometer should be immersed in the
water to a depth of 30-mm. The initial temperature
of the thermostat water bath was adjusted between
8 to 10 °C below the expected SMP of the oil
sample. The water bath was agitated using a
magnetic stirrer and heat was supplied at the rate
of 1 °C min
−1
and reduced to 0.5 °C min
−1
. The
temperature at which the sample in the tubes started
to melt and become clear is defi ned as the SMP.
The difference between values of the measurement
carried out by the same analyst on the same test
sample shall not exceed 0.8 °C for palm oil and
0.5 °C for palm olein and palm stearin.
Apart from the oil standards, each collaborator
was also supplied with detailed instructions of the
study protocol and reporting cards to compute their
analysis results. The IV and SMP analyses should be
carried out within two month’s time before sending
out the results to the Malaysian Palm Oil Board.
Statistical evaluation
Inter-laboratory proficiency tests results were
assessed using the SoftCRM Version 1.2.0 software
(Bonas, 1997). The software is particularly applied
to evaluate the standard reference materials data
as well as to document the quality of the standard
reference materials produced. Consensus values
for IV and SMP of each oil standard were generated
at 95% confi dence interval (CI). Outlying data or
extreme values were discarded based on Grubb and
Cochran tests. Grubb test determines outlying mean
values (variability) between laboratories, whereas
Cochran test identifies data variability within
laboratories (Pocklington and Wagstaffe, 1987;
ISO 5725-2 1994; Azmil Haizam et al. 2008).
Repeatability relative standard deviation, RSD
r

(relative standard deviation within laboratory) and
reproducibility relative standard deviation, RSD
R

(relative standard deviation between laboratories)
were also calculated for IV and SMP (ISO 5725-2
1994). The RSD
r
is determined from the test
results generated under repeatability conditions of
the same method on identical test items in the same
laboratory by the same analyst using the same
instrument within short intervals of time. Mean-
while RSD
R
is identifi ed from the test results
generated under reproducibility conditions in
which the results are obtained using the same
method on identical test items in different
laboratories and analysts using different instru-
ments (ISO 3541-1 1993).
Stability test
Stability of the Palm-Based Standard Reference
Materials was monitored for 12 months at four
storage conditions of –20 °C, 0 °C, 4 °C and 24 °C.
The oil standards were randomly analyzed for their
IV and SMP at predetermined storage intervals.
Results and Discussion
Characterization of lodine value
Statistical evaluations for IV in palm oil, palm
olein and palm stearin are tabulated in Table 1.
The consensus values (IV) and their uncertainties
were calculated at 95% CI using the SoftCRM

130
Azmil Haizam et al
Analytical Chemistry Insights 2008:3
1.2.0 software. Data of thirty laboratories were
retained for IV characterization in all palm
products, while only three laboratories were
discarded. The RSD
r
of the characterized IV in the
three oils ranged from 0.40 to 0.64%, while RSD
R

of 0.79%, 0.64% and 1.50% was achieved in palm
oil, palm olein and palm stearin, respectively. Both
RSD
r
and RSD
R
were observed to be more than
5-fold lower than that of 10%, which signifi ed
that the IV analysis performed was within the
acceptable variability. Example of intra- and inter-
laboratory variability of IV in palm oil is illustrated
in Figure 1. The bar graph consists of laboratory
codes with their individual means and standard
deviations after elimination of outliers through
Cochran and Grubb tests at 95% CI. From that, the
overall mean of means and its standard deviation
was then determined from the accepted values. All
the values were obtained according to ISO
Guide 35 (1989).
Characterization of slip melting point
Table 2 summarizes the statistical evaluations of
SMP in palm oil, palm olein and palm stearin.
Table 1. Statistical evaluation of iodine value in Palm-Based Standard Reference Materials.
Standards P
a
N
b
Consensus value
(Uncertainty)
c
s
r
d
RSD
r
e
R
f
S
R
g
RSD
R
h
R
i
Palm Oil 30 120 52.63 (0.14) 0.21 0.40 0.58 0.42 0.79 1.16
Palm Olein 30 120 56.77 (0.12) 0.17 0.31 0.49 0.37 0.64 1.02
Palm Stearin 30 120 33.76 (0.18) 0.22 0.64 0.61 0.51 1.50 1.41
a
Number of laboratories retained after eliminating outliers.
b
Number of accepted test results (replicates).
c
Consensus value and uncertainty (Wijs) generated as 95% confi dence interval.
d
Repeatability standard deviation.
e
Repeatability relative standard deviation.
f
Repeatability limit.
g
Reproducibility standard deviation.
h
Reproducibility relative standard deviation.
i
Reproducibility limit.
Figure 1. Example of bar graph for characterising iodine value in palm oil. The results plotted correspond to four replications. Mean of means
designates the average result of total individual laboratory means at 95% confi dence interval (CI).
L4–
L6–
L8–
L11–
L13–
L15–
L17–
L19–
L21–
L23–
L25–
L27–
L29–
L31–
L33–
95% CI
51.800 52.628 53.700
Mean of means = 52.628
Accepted laboratory means at 95% CI
0

131
Palm-Based Standard Reference Materials for Iodine Value and Slip Melting Point
Analytical Chemistry Insights 2008:3
Table 2. Statistical evaluation of slip melting point in Palm-Based Standard Reference Materials.
Standards P
a
N
b
Consensus value
(Uncertainty)
c
s
r
d
RSD
r
e
R
f
S
R
g
RSD
R
h
R
i
Palm Oil 29 116 35.6 (0.3) 0.27 0.76 0.76 0.91 2.57 2.56
Palm Olein 25 100 22.7 (0.4) 0.13 0.59 0.38 0.93 4.08 2.60
Palm Stearin 26 104 53.4 (0.2) 0.14 0.27 0.40 0.46 0.87 1.30
a
Number of laboratories retained after eliminating outliers.
b
Number of accepted test results (replicates).
c
Consensus value and uncertainty (
o
C) generated as 95% confi dence interval.
d
Repeatability standard deviation.
e
Repeatability relative standard deviation.
f
Repeatability limit.
g
Reproducibility standard deviation.
h
Reproducibility relative standard deviation.
i
Reproducibility limit.
The RSD
r
and RSD
R
were also identifi ed for the
characterized SMP. Three out of 32 laborato-
ries were eliminated for characterization of
SMP in palm oil while 25 and 26 laboratories
were retained in palm olein and palm stearin,
respectively. In general, both RSD
r
and RSD
R

were found to be less than 10%. The RSD
r
of palm
oil, palm olein and palm stearin was 0.76%, 0.59%
and 0.27%, respectively. These values were almost
comparable with that of RSD
r
for IV characteriza-
tion. The RSD
R
, however, exhibited slightly
higher values for palm oil (2.57%) and palm olein
(4.08%) in comparison to that of IV characteriza-
tion. The higher RSD
R
is generally due to
run-to-run variations, which may be caused by the
variation in the instrument sensitivity, change of
environmental conditions or even uncontrolled
change of instrument parameters (Azmil Haizam
et al. 2007; Azmil Haizam et al. 2008). Such
effects are important, which could infl uence the
RSD
R
when the samples were analysed by differ-
ent laboratories. Only palm stearin produced a
lower RSD
R
of 0.87%. Furthermore, both RSD
r

and RSD
R
were still well below 10% and hence
the SMP results were acceptable. Similar to IV,
the SMP assessment can also be expressed in the
form of bar graphs. Figure 2 shows an example
of within- and between- laboratory variability of
SMP in palm oil.
Stability of Palm-Based Standard
Reference Materials
Results of storage stability tests for IV and SMP
are showed in Table 3 and Table 4, respectively.
The test results of the four storage conditions
(−20 °C, 0 °C, 4 °C and 24 °C) were compared with
oil standards that were initially prepared and stored
at –20 °C (t = 0). No detectable changes of IV and
SMP were perceived even at 24 °C. As the
Palm-Based Standard Reference Materials of these
characteristics were observed to have good stability,
their shelf-life could be extended for more than
one year.
Conclusion
From the characterization exercises, it can be
concluded that the production of Palm-Based
Standard Reference Materials (palm oil, palm olein
and palm stearin) for iodine value and slip melting
point is achievable through inter-laboratory profi -
ciency tests. The establishment of the consensus
values (certifi ed values and their uncertainties) at an
acceptable level of 95% confi dence interval has been
attained using the SoftCRM 1.2.0 software. Consen-
sus values of these Standard Reference Materials
remained stable after one year of storage and hence
their shelf-life could be prolonged for another
one year.
Acknowledgments
The authors are grateful to the Director-General of
the Malaysian Palm Oil Board for approval to
publish this research work; Director of Product
Development and Advisory Services Division and
Head of the Analytical and Quality Development
Unit for their valuable suggestions; collaborators
of the inter-laboratory profi ciency tests for their
contributions; and laboratory staff of the Analytical
and Quality Development Unit for their technical
assistance.

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Azmil Haizam et al
Analytical Chemistry Insights 2008:3
L3–
L5–
L7–
L10–
L12–
L14–
L16–
L18–
L20–
L22–
L24–
L26–
L28–
L30–
L32–
95% CI
33.800 35.591 38.400
Mean of means = 35.591
Accepted laboratory means at 95% CI
0
Figure 2. Example of bar graph for characterising slip melting point in palm oil. The results plotted correspond to four replications. Mean of
means designates the average result of total individual laboratory means at 95% confi dence interval CI.
Table 3. Changes of iodine value in Palm-Based Standard Reference Materials.
Standards T
a
(
o
C)
Storage period (month)
b
0 2 4 6 8 10 12 Mean SD
c
Palm Oil −20 52.25 ± 0.19 52.65 52.55 52.15 52.55 52.40 52.50 52.47 0.18
0 52.55 52.60 51.90 52.20 52.40 52.70 52.39 0.30
6 52.60 52.60 51.70 52.45 52.50 52.55 52.40 0.35
24 52.70 52.60 52.05 52.45 53.30 52.60 52.62 0.41
Palm Olein −20 57.20 ± 0.08 56.80 56.65 56.15 56.20 56.25 55.40 56.24 0.49
0 57.00 56.70 56.25 56.55 56.00 56.00 56.42 0.40
6 56.80 56.80 56.15 56.60 56.20 56.15 56.45 0.32
24 56.05 56.45 56.20 56.65 56.10 55.90 56.23 0.28
Palm Stearin −20 34.18 ± 0.32 33.90 34.70 33.70 34.10 34.20 34.20 34.13 0.34
0 33.95 34.70 33.40 34.20 34.30 34.20 34.13 0.43
6 34.10 34.70 33.65 34.20 34.15 34.25 34.18 0.34
24 33.90 34.70 33.90 34.20 34.30 34.30 34.22 0.30
a
Storage temperature.
b
Mean of duplicate (Wijs).
c
Standard deviation.

133
Palm-Based Standard Reference Materials for Iodine Value and Slip Melting Point
Analytical Chemistry Insights 2008:3
Table 4. Changes of slip melting point in Palm-Based Standard Reference Materials.
Standards T
a
(
o
C)
Storage period (month)
b
0 2 4 6 8 10 12 Mean SD
c
Palm Oil −20 36.4 ± 0.1 36.4 36.3 35.7 36.0 35.8 36.1 36.05 0.27
0 36.1 35.8 36.1 35.8 36.0 35.9 35.95 0.14
6 36.0 36.3 35.9 36.0 35.9 36.3 36.07 0.19
24 36.2 35.8 36.0 35.7 36.2 36.0 35.98 0.20
Palm Olein −20 22.4 ± 0.2 22.4 22.4 22.9 22.6 23.1 22.3 22.62 0.32
0 22.7 22.5 22.4 23.0 22.6 22.8 22.67 0.22
6 22.5 22.9 23.0 23.2 22.5 22.6 22.78 0.29
24 22.4 22.2 22.8 23.0 22.8 23.1 22.72 0.35
Palm Stearin −20 53.2 ± 0.1 53.4 53.1 53.0 53.4 53.0 53.2 53.18 0.18
0 53.0 52.9 53.4 53.5 53.3 54.0 53.35 0.39
6 53.2 53.4 53.7 53.1 53.0 52.9 53.22 0.29
24 53.4 53.3 53.2 53.1 53.5 53.0 53.25 0.19
a
Storage temperature.
b
Mean of duplicate (
o
C).
c
Standard deviation.
Disclosure
The authors report no confl icts of interest.
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