Evaluation on the quality of Malaysian re ﬁ ned palm stearin

– The quality of Malaysian palm stearin was monitored through a twelve-month survey in 2018, by participation of palm oil re ﬁ neries and oleochemical plants from Peninsular Malaysia and Sarawak. Quality parameters requirement as listed in the Palm Oil Re ﬁ nery Association of Malaysia (PORAM) speci ﬁ cation such as moisture, impurities, free fatty acids, slip melting point, peroxide value and iodine value were determined. Other identity characteristics analyses as detailed in the palm stearin speci ﬁ cation namely Malaysian Standard MS 815:2007 and Codex Alimentarius CXS 210-1999 documents were also analyzed, which were apparent density, refractive index, fatty acids composition, triacylglycerols, etc. Results obtained from this survey indicated that the quality of Malaysian palm stearin complied with the requirements speci ﬁ ed in MS 815:2007 and Codex CXS 210-1999 documents. The iodine value determined was ranged from 28 g I 2 /100 g oil to 48 g I 2 /100 g oil, with 60.9% of the palm stearin tested were in the range of 30 g I 2 /100 g oil to 35 g I 2 /100 g oil. Meanwhile, some deviations are found in the average of parameter such as refractive index, apparent density, slip melting point, oleic and palmitic acids of the palm stearin produced. These deviations could be due to the improvement in current fractionation technologies causing less olein entrainment in stearin products, which resulted of harder stearin fraction in the sample itself, giving a more representative reading of the parameters.


Introduction
Oil palm (Elaeis guineensis) was first introduced as an ornamental plant in 1870, and well ahead, it has become the most important commodity crop in Malaysia. The valuable oil derived from oil palm mesocarp is naturally semi-solid at room temperature and has distinct physical and chemical properties after undergoing several fractionation processes. It also possessed distinctive properties of fatty acids and triacylglycerols (TAG) profile making it suitable for numerous food applications. The initial fractionation separates the palm oil into two different fractions, namely palm olein (liquid form) and palm stearin (solid form). Palm stearin is derived from the crystallization process at controlled temperatures, and it is widely used in edible and nonedible applications (Tang and Pantzaris, 2009). In 2021, the palm stearin production in Malaysia recorded was 2.522 million tonnes, and the stearin price has increased up to 64.2% from RM 2801 per tonnes in 2020 to RM 4598 per tonnes in 2021 (Parveez et al., 2022;MPOB, 2022).
The quality of palm stearin produced depends on both crystallization and separation steps. A wider range of stearins are being produced resulted from the developments and technology in crystalliser design, cooling programmes, filtration technology as well as the addition of crystals aids (Saw et al., 2015).
Palm stearin contains more saturated fatty acids and a TAG profile with a higher melting point of 48°C to 50°C (Siew, 2011). It is a natural source in providing solid fat functionality without the need for hydrogenation, and hence strengthens oil structure that enhances the plasticity of the products (Kellens et al., 2007;Pande et al., 2012;Siti Hazirah et al., 2012). Further fractionation stage of palm stearin produces palm mid fractions (PMF). Palm stearin is directly used in blends with other vegetable oils to produce suitable functional products such as margarine, shortenings and vanaspati (Ornla-ied et al., 2016;Noor Lida et al., 2017). Moreover, palm stearin plays a significant role in the process of film formation and as a coating substitution material and a useful natural hard stock for making trans-free fats (Edy and Rizki, 2020).
Apart from food application, palm stearin also possesses suitable properties for non-edible products such as formulation of soaps and animal feeds (Norliza et al., 2012;Ogan et al., 2015). It is also an excellent feedstock for oleochemicals products, production of biodiesel, and lubricant (Theam et al., 2016;Farhanah and Syahrullail, 2016;John et al., 2021). Figure 1 shows the fractionation of palm oil, resulted of different type of palm stearin with different iodine value (IV) properties. The IVof palm stearin is varied with a maximum of 48 g I 2 /100 g oil, producing of soft stearin and hard stearin category. The IV of soft stearin is found higher (40 to 42 g I 2 / 100 g oil), while hard stearin normally owned a lower IV (17 to 21 g I 2 /100 g oil) (Podchong et al., 2018). Soft stearin fractions are commonly used as the main components in shortening and margarine blended with liquid oils, while the hard palm stearin is the structural fats that act as the backbone of margarine and shortening (Kim et al., 2014). The multiple-step of dry fractionation to manufacture higher added-value products known as super stearin was reported for the production of palm stearin with very low IV (IV 12 to 14 g I 2 /100 g oil) (Gibon, 2012). Super stearin is an excellent hard stock to substitute hydrogenated products and is also usable in the formulation of zero or low trans margarine and shortening.
Palm stearin is traded by either mutual contract between buyer and seller or generic-trading specifications following PORAM standard for processed palm oil products (Tab. 1) (PORAM, 2012). In demand to facilitate trade as well as to provide tools for the market to create enthusiasms in improving the overall product quality, the requirements for quality, composition, chemical and physical characteristics of palm stearin were indicated in specifications documents, i.e. Malaysian Standard;MS 815:2007(Department of Standard Malaysia, 2007 and Codex Standard for Named Vegetable Oils; CXS 210-1999210- (Codex, 1999. Malaysian Palm Oil Board (MPOB), as the custodian of Malaysian palm oil, is committed in monitoring the quality performance of palm oil and its products, as well to ensure these products are within the specification requirements. In view of this, a one-year palm stearin survey has been conducted from January to December 2018 to gather the latest information on the characteristics of palm stearin, specifically to refined palm stearin, to ascertain whether the quality is within the specification requirement. A similar survey was previously carried out in last 1992, and hence a new survey was deemed timely for data update (Siew et al., 1992). Therefore, this paper elaborates the results and findings from the quality monitoring of palm stearin activity, which voluntary participated by selected palm oil refineries and oleochemical plants in Malaysia.

Collection of palm stearin samples
A total of 140 palm stearin samples were obtained from 11 refineries and 3 oleochemical plants in Peninsular Malaysia and Sarawak on monthly basis throughout the survey period. The samples were stored in a cool (∼ 5°C) dry place prior to the analyses.

Quality parameter analyses
Palm stearin samples were analyzed for quality parameter analyses, namely moisture, impurities, free fatty acids, and peroxide value. The sources of analytical methods used are varies, based on which applicable the best to palm stearin samples. All of the analyses were carried out in duplicate. Palm stearin samples were heated at 60°C in the oven prior to the analyses and stirred well to ensure samples are fully melted and homogenized.
Moisture content. The moisture content in the palm stearin sample was measured using a volumetric titration Karl Fischer moisture analyzer (Mettler Toledo, Switzerland) according to ISO 8534:2017 method (International Organization for Standardization, 2017a). Samples were dropped into the validated Aqualine ® reagent (Fisher Scientific) in which have been certified with standard water solutions over the necessary range.
Impurities content. The impurities were determined using the oven method according to ISO 663:2017 (International Organization for Standardization, 2017b). The palm stearin  sample was filtered using Whatman Paper No.4, followed by washing a few times with petroleum ether. The filter paper with the filtrate was then heated in the oven at 103°C ± 1°C for 2 h. The residue weight was measured and calculated as percentage. Free fatty acids content (FFA). FFA content was analyzed according to acid-based titration procedure following American Oil Chemists' Society (AOCS) Official Method Ca 5a-40 (American Oil Chemists' Society, 2017a). Five grams of palm stearin sample was dissolved in neutralised iso-propanol and the free acids were neutralised with 0.1 M sodium hydroxide solution. Phenolphthalein was added as an indicator to observe and detect the end point. The FFA was expressed in percentage as palmitic acid.
Peroxide value (PV). PV was determined by titration technique, referring to the acetic acid: iso-octane method, ISO 3960:2017 (International Organization for Standardization, 2017c). Five grams of palm stearin sample was first dissolved in a 50 ml mixed solution of acetic acid and iso-octane at a ratio of 3:2. Next, approximately 0.5 ml of saturated potassium iodide solution was added into the mixture and gently stirred for 1 min, followed by the addition of 30 ml deionized water. Similar to IVanalysis, an automated titration system is used for titration. The mixture was then titrated with 0.01 M sodium thiosulphate (Na 2 S 2 O 3 ) solution until the equivalence point was reached potentiometrically. The blank sample was determined concurrently in duplicate.

Identity characteristics analyses
Palm stearin samples were also analyzed for identity characteristics parameter, i.e. iodine value, refractive index, apparent density, fatty acids composition, TAG composition, and slip melting point. All of the analyses were carried out in duplicate. Palm stearin samples were heated at 60°C in the oven prior to the analyses and stirred well to ensure samples are fully melted and homogenized.
Iodine value (IV). The IVof the oil sample was determined using the Wijs method according to ISO 3961:2018 (International Organization for Standardization, 2018). As an alternative to manual titration, an automated titration system T90 (Mettler Toledo, Switzerland) was used for the titration based on potentiometric measurement. About 0.4 g of palm stearin sample was weighed in an amber beaker (300 ml). A mixture of iso-octane and cyclohexane (1:1) solvent was added, followed by the addition of Wijs solution. The mixture was then incubated for 60 min. After the incubation period, potassium iodide (10%) solution and 100 ml deionized water was added. The mixture was titrated with 0.1 M thiosulphate solution until the equivalence point is reached. Blank determination was also performed concurrently in duplicate.
Refractive index (RI). RI is measured by using refractometer Abbemat 300 (Anton Paar, Austria) at the temperature of 60°C according to ISO 6320:2017 method (International Organization for Standardization, 2017d). The refractometer was calibrated against deionized water. A drop of palm stearin sample was introduced onto the measuring prism and the measurement was performed.
Apparent density (AD). The AD of the palm stearin was measured using densitometer DMA 4500 M (Anton Paar, Austria) according to ISO 18301 method (International Organization for Standardization, 2014). The densitometer is equipped with a temperature-controlled heating inlet to ensure the samples are fully melted and well homogenized throughout the measurement process. The densitometer was calibrated against air and water prior to the measurements. Palm stearin sample was introduced to the U-tube cell by using a syringe and the measurement was occurred.
Slip melting point (SMP). SMP was measured according to AOCS Official Method, AOCS Cc 3-25 (American Oil Chemists' Society, 2017b). Palm stearin is solidified in an open capillary tube and tempered at 4°C to 10°C for 16 h. Then, the tube is heated in a water bath until the temperature at which the fat column in the tube begins to rise.
Fatty acids composition (FAC). FAC was measured by conversion of the oil sample to their fatty acid methyl esters (FAME) according to ISO 12966-Part 2 (International Organization for Standardization, 2017e). The FAME was prepared by adding 1 ml of hexane into 0.05 g of palm stearin in the vial. Next, 0.2 ml sodium methoxide solution was added and the mixture was vortex for 60 s. The solution was allowed for 5 min before addition of sodium chloride solution. The clear upper layer of FAME was collected into a vial and dried over 0.5 g of sodium hydrogen sulphate. The FAC was determined using a Gas Chromatography (Agilent GC 7890, Agilent Technologies, Germany) equipped with a fused silica capillary column SP TM 2560 (100 m Â 0.25 mm film thickness) and flame ionization detector. The column temperature was set at 110°C and held for 7 min, with temperature increment rate of 4°C min À1 an increment up to 240°C and hold for further 7 min. Nitrogen was used as the carrier gas with a flow rate of 1 ml min À1 .
Triacylglycerols (TAG) composition. TAG composition was determined according to AOCS Official Method Ce 5c-93 by using reversed-phase high performance liquid chromatography (HPLC) (Agilent 1100 Series, Agilent Technologies, Germany), equipped with a refractive index detector and a Lichrospher ® 100 RP-18 column, 250 mm Â 4.6 mm with 5 mm particle size (Darmstadt, Germany) (American Oil Chemists' Society, 2017c). The oil samples were weighed and diluted in acetone for injection into the HPLC column. The mobile phase used was a pre-mixed acetone and acetonitrile with a ratio of 3:1, with the flow rate of 1 ml min À1 . Identification of the TAG peaks was based on the reference standard of refined bleached and deodorized palm oil.

Data analysis
Basic statistical analyses such as mean, standard deviation, the minimum, and the maximum value were calculated using Microsoft Office Excel 2007 (Microsoft Corporation, Redmond, WA). Other statistical analyses such as data distribution were performed using Minitab 16 software.

Quality parameters
The quality parameters obtained were as shown in Table 2 and compared with the data reported in 1988 (Siew et al., 1988).
In general, the quality of palm stearin is not much different and was well within the specification indicated in MS 815:2007 document. The moisture content and PV were slightly lower compared to that of the previous survey, while the FFA showed an increase in the average value despite being within the specification. This indicated that the basic product handling and storage procedures in the Malaysian palm oil industry are well practiced (Berger, 2010).

Trend of palm stearin quality from different states in Malaysia
Figure 2 demonstrated the palm stearin quality at different participated states in Malaysia. From the figure, it clearly showed that the quality characteristic of palm stearin samples from different state, referring to moisture, impuritries, FFA and PV parameters were within the MS 815 requirement as specified in Table 1. However, only the sample from Pahang showed a higher PV; as the number of samples is not equal, no conclusive difference can be made. The PV for that sample could have increased during the transportation process.

Identity characteristics
Physical and chemical characteristics of the palm stearin indicated the characters, properties and the appearance of the oil, which is unique. It is useful for the purity and identification of the oil. Table 3 showed the identity characteristics of palm stearin with a comparison to the reported survey in 1992. In the current survey, the average AD has declined to 0.8815 g ml À1 from 0.8823 g ml À1 as reported in the 1992 survey with narrower range, meanwhile, the average RI has decreased from 1.4493 as reported in 1992 survey to 1.4486, respectively. The average of IV was also decreased from 37.9 to 34.3 g I 2 /100 g oil. On the other hand, the average of SMP is slightly increased. The upward trends in SMP are due to the appearance of membrane filter presses in the industry allowing effective stearin squeezing, but also to the growing demand for super stearin with a very low IV for specific application, i.e. substitutes for hydrogenated products and specialty fats. These four parameters are co-related to each other and mainly associated to the unsaturation properties of the oil. The changes of these parameters are suspected could be due to the changes in fatty acid composition properties, which will be discussed in the separate subtopic. Pearson correlation was applied to identify the presence of a relationship between IV, AD, and RI parameters of the palm stearin samples. Results showed that a fairly strong positive relationship occurred between RI and AD, RI and IV, and IV and AD with the correlation of 0.878, 0.888, and 0.822, respectively. This is in agreement with the observations discussed in the previous chapters.
Iodine value. The IV of palm stearin is diversified to a maximum of 48 g I 2 /100 g oil depending on the fractionation process (Deffense, 1985;Gibon, 2012). Data obtained revealed that the IV of palm stearin are diversified from 28 to 48 g I 2 / 100 g oil, which with mod value of 33 g I 2 /100 g oil. Figure 3 displayed the pie chart of the percentage of palm stearin in this survey, which 93.5% are within the IV range from 30 to 45 g I 2 / 100 g oil, while 5.1% is less than 30 g I 2 /100 g oil and 1.4% is more than 45 g I 2 /100 g oil. The IV results obtained indicated that palm stearin produced in Malaysia is within the requirement set by the relevant standards, which is maximum of 48 g I 2 /100 g oil. The common usage of palm stearin in oleochemicals products has IV ranging from 30 to 35 g I 2 /100 g oil, which encountered of 60.9% from the total of palm stearin samples analysed. As the sampling condition for this survey requires the collection of the common production samples, which is refined palm stearin, most of the palm stearin samples obtained are in the soft stearin form. The production of hard (IV < 20 g I 2 /100 g oil) or super stearin (IV < 14 g I 2 /100 g oil) at lower IV usually needs a special or additional fractionation profile.
Fatty acids composition. A wide variation was seen in the FAC of the palm stearin (Tab. 4). The major saturated fatty acids found in the samples were palmitic (45.2% to 66.4%) and stearic (3.9% to 5.7%) acids and the main unsaturated fatty acids were oleic (22.3% to 39.5%) and linoleic (4.8% to 9.7%) acids. Only a trace amount of linolenic acid was detected (0.1% to 0.5%). Generally, the ranges for the major acids, palmitic and oleic, were fairly similar to those in the previous survey. The upper range of palmitic is slightly decreased; meanwhile the upper range of oleic is slightly increased. However, the range reported were based on individual results, when total up the average of unsaturated acids and saturated acids, it showed that the saturated acids had increased and unsaturated acids had decreased, compared to the previous survey data. This is in agreement with the increase of SMP results, which resulted from the unsaturation that enhance the fluidity of fatty acids (Berg et al., 2002).
Triacylglycerol. TAG are basically structural components of fats, thus, variation in TAG composition will lead to major changes in the characteristics of the functionality of the fats. TAG composition determines the characteristics of the fats with respect to their macroscopic properties such as overall texture,      (Kanagaratnam et al., 2020). The variation of TAG composition gives significant changes to the overall functionality and application of the fats in the final food products. Palm stearin had trisaturated TAG (such as PPP) and monounsaturated TAG (such as POP) as the main constituents. Palm stearin consisted mainly of POP (29.07%), PPP (24.85%) and POO (13.36%) as shown in Table 5. The total monosaturated TAG is 21.65%, with POO is the major content ranging from 9.8% to 24.5%. Meanwhile, the percentage of total disaturated TAG is 42.53%. The total of trisaturated TAG is 32.4%, with the main species of tripalmitoyl-glycerol (PPP) ranged from 6.1% to 33.1%. Data obtained are not comparable as there were no previous survey data are available. However, the TAG data obtained from this survey are not much difference to those reported by Bangun (2009).

Conclusion
Data generated from this survey found that most of the palm stearin produced in Malaysia are well within the average and ranges specified in the Malaysian Standard document, MS 815:2007, in terms of quality, physical, and identity characteristics. Most samples of exhibit IV ranging from 30 to 35 g I 2 / 100 g oil, which represented by 60.9% of palm stearin analyzed. The deviations found in the average of RI, AD, IV, SMP, oleic and palmitic acids of the palm stearin produced could be due to the improvement in current fractionation technologies causing less olein entrainment in stearin products which resulted of harder stearin fraction in the sample itself, giving a more representative readings of the parameters. Data obtained from this study will be useful in updating the relevant standards and specifications for palm stearin, as well as to understand the changes in the quality of palm stearin, in relation to the improvement of processing technology of palm stearin. Further investigations are needed, in order to find the actual factors that contributed to the deviation.

Conflicts of interest
The authors declare that they have no conflicts of interest in relation to this article.