The table below shows the parameters for Identity and Quality characteristics of palm oil.
Quality is an important attribute of edible oil products and it is a very important attribute from the trade point of view. With the growing innovations of product developments and improvements, there is an increasing demand for quality improvement from consumers and end-users of palm oil products. Hence, it can be expected that with time, trading specifications will become wider and more stringent albeit on a willing buyer/willing seller basis for the more discerning end-user. This will require reliable and commonly agreed methods of analysis to ensure that these contractual specifications are met. It will also enable analysts to have recourse to validated methods to meet trade dispute requirements. Noncompliance to quality specifications could involve huge discounts or rejection of the consignment if it cannot be used for the intended purpose. To meet this growing need for method standardization, there are many organizations around the world which are involved with development and publication of standard methods of analysis for oils and fats, some of which are listed below.
List of Standard Organizations Involved in Development and Publication of Standard Methods of Analysis for Oils and Fats.
1. British Standards Institute (BSI)
2. Association Francaise de Normalisation (AFNOR)
3. Nederlands Normalisatie-Institut (NNI)
4. Deutsche Gessellschaft fur Fettwissenschaft (DGF)
5. American Oil Chemists Society (AOCS)
6. Federation of Oils, Seeds and Fats Association (FOSFA)
7. International Association of Seed Crushers (IASC)
8. International Standards Organization (ISO)
9. Oils, Fats and Derivatives Commission VI 3 of the International Union of Pure and Applied Chemistry (IUPAC)
10. Association of Official Analytical Chemists (AOAC)
11. Codex Alimentarius Commission for Oils and Fats
12. European Economic Commission (EEC)
13. International Dairy Federation (IDF)
The MPOB Methods is a compilation of test methods where the special requirements of palm oil and palm kernel oils had been considered. Where there are any shortcomings in the standard methods for oils and fats in general, collaborative studies or cross checks were carried out to resolve these shortcomings. In general, the MPOB test methods are based more on the International Standards Organization methods for oils and fats analysis rather than the American Oil Chemists Society (AOCS) methods.
Below are some explanations on the Quality Characteristics of Palm Oil.
1. Peroxide Value (PV)
Oxidation of lipids is a major cause of their deterioration, and hydroperoxides formed by the reaction between oxygen and the unsaturated fatty acids are the primary products of this reaction. Hydro peroxides have no flavour or odours but break down rapidly to form aldehydes, which have a strong, disagreeable flavour and odours. The peroxide concentration, usually expressed as peroxide value, is a measure of oxidation or rancidity in its early stages. Hence this parameter gives a measure of the extent to which an oil sample has undergone primary oxidation Peroxide value (PV) measures the concentration of substances (in terms of milliequivalents of peroxide per 1000 grams of sample) that oxidize potassium iodide to iodine.
2. Ultraviolet Absorption at 233nm and 269nm (E233 and E269)
During the oxidation of polyunsaturated fatty acids such as linoleic acid (C18:2), displacement of the double bonds and formation of conjugated bonds (alternating single and multiple bonds) takes place. The latter absorb highly in the ultraviolet region at 233 nm. The secondary oxidation products (aldehydes and ketones) and trienes absorb in the region of 269 nm. Thus, ultra-violet measurements at 233 and 269 nm are related to conjugated dienes, trienes and keto-dienes formed during oxidation of oils and fats.
3. Anisidine Value
The primary oxidation products (hydroperoxides) are unstable and susceptible to decomposistion. A complex mixture of volatile, nonvolatile, and polymeric secondary oxidation products is formed through decomposition reactions, providing various indices of lipid oxidation (5). Secondary oxidation products include aldehydes, ketones, alcohols, hydrocarbons, volatile organic acids, and epoxy compounds, among others. The p-anisidine value (p-AnV) method measures the content of aldehydes (principally 2-alkenals and 2,4-alkadienals) generated during the decomposition of hydroperoxides.
The method is based on the fact that in the presence of acetic acid, p-anisidine reacts with the aldehydic compounds in an oil, producing yellowish reaction products. The color intensity depends not only on the amount of aldehydic compounds present, but also on their structure.
4. DOBI Value
Deterioration of Bleachability Index (DOBI) was added as a quality parameter for measurement of crude palm oil quality at the turn of the millennium. The DOBI index was developed by Dr. P.A.T Swoboda in MPOB (then PORIM) in the 1980s. The DOBI value was used as a guide as to how easily a sample of crude palm oil can be bleached. The DOBI value is a numerical ratio of the UV absorbance of the sample at 446 nm to the absorbance at 269 nm. Hence, it is interplay between the amounts of carotene (UV 446nm) present in the crude palm oil to the amounts of secondary oxidation products (UV 269nm) present in the oil. This is because the carotenes present in crude palm oil will be the first to be sacrificed in protecting the oil from oxidation, due to its high number of double bonds. The higher the DOBI value, the easier it is to bleach the oil through heat and absorptive cleansing bleaching.
Crude palm oil over 3 is considered to be good while average quality palm oils have DOBI values above 2.3.
The main causes of a low (poor) DOBI are:
• High percentage of black (unripe) fruit bunches
• Delay in processing, especially during rainy season
• Contamination of CPO with steriliser condensate
• Contamination of CPO with badly oxidised sludge oil
• Prolonged sterilisation of fruit bunches
• Overheating (> 55C) of CPO in the storage tank
There are other causes, but these are less significant compared with the above causes. These include splashing/aeration of hot oil, delay in processing due to temporary machinery breakdown, high-temperature crude oil clarification and high-temperature processing at other stages.
Picture: Fresh fruit bunches showing three categories of ripeness.
The black bunch on the left has oil with the lowest DOBI.
The bunch in the centre has oil with the highest DOBI.
The oil extracted from black bunches can have DOBI < 1.5 whereas that from fruit bunches with optimum ripeness can have DOBI > 3.5. In practice, DOBI > 3.0 can be achieved with a little effort in harvesting and processing.
DOBI method is described in ISO 17932:2005 as “Animal and vegetable fats and oils—Determination of the Deterioration of Bleachability Index (DOBI)”, while the corresponding method under the MPOB Test Method is MPOB p2.9: Determination of the Deterioration of Bleachability Index (DOBI).
5. Trace Metal
Throughout the processing of edible fats and oils, metals can be encountered, many of which reduce the efficiency of the process or cause deterioration of the product quality. The most notable of these metals are copper, iron, calcium, magnesium, sodium, lead, zinc, and nickel. Initially, wet chemical analyses were the only quality control analyses available, but improved trace metal determination procedures have been introduced; flame atomic absorption spectroscopy was improved by replacing the flame with a graphite furnace, and more recently plasma emission spectroscopy, inductive coupled plasma (ICP) has been introduced.
6. Phosphorus Content
Deodorizer feedstock with phosphatide content above 20 ppm will cause high deodorized oil colours. The phosphatides must be removed in refining and bleaching before the deodorization process. Some of the phosphatides and their associated metal complexes are not easily hydratable. These complexes require a phosphoric acid pretreatment for their removal in degumming or refining. Hence it is important to eliminate phosphorus as much as possible and to prevent phosphoric acid slip-through.
Levels of residual phosphorus in refined and bleached oils over 1.0 ppm can poison hydrogenation catalyst and/or cause off-flavours. Periodic trace metal analysis can effectively monitor crude oil receipts quality.
7. Free Fatty Acid Content and Acid Value
Hydrolytic rancidity occurs as a result of a splitting of the triglyceride molecule at the ester linkage with the formation of free fatty acid (FFA), which can contribute objectionable odour, flavour, and other characteristics. The flavours resulting from FFA development depend on the composition of the fat. Release of short-chain fatty acids, such as butyric, caproic, and capric acid, cause particularly disagreeable odours and flavours, whereas the long-chain fatty acids (C-12 and above) produce candle like or, at alkaline pH, soapy flavours. Both acid value and FFA are measures of the free fatty acid content of fats and oils.
Both acid value and FFA are measures of the free fatty acid content of fats and oils. Acid value is the amount of potassium hydroxide required for neutralization, whereas FFA utilizes sodium hydroxide for neutralization.
FFA results may be expressed in terms of acid value by multiplying the FFA percent by 1.99. FFA is calculated as free oleic acid on a percentage basis for most fats and oils sources, although for coconut and palm kernel oils it is usually calculated as lauric acid and for palm oil as palmitic acid.
Crude vegetable oils may have abnormally high FFA levels if the seed has been field damaged or improperly stored. Seed and fruit enzyme lipases are activated by moisture, and hydrolysis is initiated, which increases the FFA content. Higher crude oil FFA levels equate to higher refining losses. High FFA results for deodorized oils indicate a poor deodorizer vacuum, inadequate steam sparging, or air leaks if the product colour is high with an oxidized oil flavour.
8. Oxidative Stability Index (OSI)
During lipid oxidation, volatile organic acids, mainly formic acid and acetic acid, are produced as secondary volatile oxidation products at high temperatures, simultaneously with hydro peroxides In addition, other secondary products, including alcohols and carbonyl compounds, can be further oxidized to carboxylic acids. The oil stability index (OSI) method measures the formation of volatile acids by monitoring the change in electrical conductivity when effluent from oxidizing oils is passed through water. The OSI value is defined as the point of maximal change of the rate of oxidation, attributed to the increase of conductivity by the formation of volatile organic acids during lipid oxidation (70). However, this method requires a somewhat higher level of oxidation (PV > 100) to obtain measurable results than other methods in which hydro peroxides are the most important products formed and detected (71). Therefore, to determine oil stability in the laboratory, especially for some oils that are stable under normal conditions, the oxidation process is accelerated by exposing oil samples to elevated temperatures in the presence of an excess amount of air or oxygen. The OSI method differs from ambient storage conditions by using a flow of air and high temperatures
to accelerate oxidation. The OSI is an automated development of the active oxygen method (AOM), because both employ the principle of accelerated oxidation. Two pieces of commercially available equipment, the Rancimat (Metrohm Ltd.) and the Oxidative Stability Instrument (Omnion Inc.) are employed for determining the OSI value. Although the OSI method is useful for quality control of oils, it is not recommended for measurement of antioxidant activity for certain reasons. The high temperatures used do not allow reliable predictions of antioxidant effectiveness at lower temperatures.
9. Discriminant Function (DF) or Quality Index
DF is used to distinguish quality of crude palm oil based on its oxidative status as measured by E269, DOBI and PV shown in the equation below:
0.3(E269) + 16(DOBI) + 0.13(PV) – 27.29
The above equation was derived from a set of three equations used to define the quality and categories of palm oils (PORIM Information Series No. 18). Crude palm oil of various grades and sludge palm oil can be categorised on an arbitrary scale based on DF (Siew et al., 1989a) as shown below:
10. Moisture & Impurities
This parameter determines the amount of moisture and impurities found in crude palm oil. High moisture and impurities will result in high processing loss as the water will be converted to water vapour and removed from the refinery plant. Moisture is also undesirable because it will support microbial growth and facilitate lipid hydrolysis. Impurities are meal, dirt, seed fragments, and other substances insoluble in kerosene or petroleum ether.
Examples of MPOB and AOCS Analytical Methods for the above quality parameters.
NA – Not available
1) Chemistry of Oils and Fats, CRC Press, 2004
2) Fats and Oils Formulating and Processing for Application, CRC Press, 2009
3) Fats and Oils Handbook, AOCS Press, 1998
4) Bailey’s Industrial Oils and Fats Products, John Wiley & Sons, 2005
5) Palm Oil : Production, Processing, Characterization and Uses, AOCS Press, 2012