The identity characteristics of oils are usually used to determine authenticity or purity. These characteristics include the physical properties and chemical composition of the oil. The technical applications of vegetable oils and their uses in edible and non-edible products depend on physical properties such as melting behaviour, viscosity etc.
- Fatty acid Composition
Fatty acids, esterified to glycerol, are the main constituents of oils and fats. The industrial exploitation of oils and fats, both for food and oleochemical products, is based on chemical modification of both the carboxyl and unsaturated groups present in fatty acids. Palm oil and its derivatives can be clearly differentiated from other types of edible oil by its high content of C16 fatty acids. The tables below show the typical fatty acid composition of major oils.
Prior to the gas liquid chromatograph (GLC) development identification of fats and oils was limited to a combination of iodine value, relative density, refractive index, and saponification value. The GLC fatty acid composition analysis provides a rapid accurate means of determining the fatty acid distribution of fats and oils products.
This information is beneficial for all aspects of product development, process control, and marketing because the physical, chemical, and nutritional characteristics of fats and oils are influenced by the kinds and proportions of the component fatty acids and their position on the glycerol radical. The fatty acid composition results provides a large quantity of information with one analysis, such as: identification of individual fatty acids and quantities, saturate/unsaturate levels (calculated iodine value), identification of the unsaturated fatty acid isomers (cis, trans, conjugated, positional), provides the data to determine the source oil proportions and processing of a blended product, and it applies equally well to refined and unrefined oils.
In general, the procedure involves passing the methyl ester, or transesterified triglycerides, to be analyzed through a heated column by means of a carrier gas such as helium or nitrogen. The components of the mixture are eluted with the gas and detected and measured at the exit end of the column by a suitable means. The retention time is the time required for a given compound to pass through the column. The fatty acid esters exit in the order of saturation. The retention time is indicated on the horizontal axis of the chart and is a qualitative index of the substance, and the area under the curve is in each case a quantitative measure of the component. Separation of the fatty acids is based on chain length, degree of saturation, as well as the geometry and position of the double bonds.
The diagram below shows the chromatogram of palm oil fatty acid methyl esters. The different peaks refer to the different fatty acids with area under each peak representing the concentration.
- Triglyceride Composition (TAG)
The TAG profile of palm oil has been characterised by carbon-number gas chromatography (Table 1). The TAG of palm oil consists of C46 to C56 molecules in a near normal distribution, the major TAGs being of C50 and C52.
These carbon numbers represent the number of carbon atoms in the three acyl chains and exclude the glycerol carbon atoms. A more detailed profile of the TAGs is seen in Table 2. For TAG abbreviations, fatty acids are combined in groups of three, e.g. tripalmitin (PPP), triolein (OOO), palmitodiolein (POO), oleodipalmitin (POP) and tripalmitin (PPP). In palm oil, 4-8% of the TAGs are trisaturated (S3), 41-59% disaturated monounsaturated (S2U), 32-54% monounsaturated diunsaturated (SU2) and 3 – 12% triunsaturated (U3). The major constituents are palmitodiolein and oleodipalmitin, the central or 2 position of the Triacylglycerol molecule being mainly occupied by oleic acid. The dominant TAGs in palm oil are those containing palmitic acid, in particular POO and POP. This is consistent with the results for the FAC where palm oil has over 40% palmitic acid.
The polymorphic behaviour of a fat is determined to a large extent by the fatty acids within the TAGs. Fats which are composed of fatty acids predominantly of single chain length are most likely to be stable in the β form. Palm oil, containing C16 and C18 acids in most of its glycerol esters, is highly stable in the β’ form.
Palm oil is unique among vegetable oils in having a significant amount of saturated acids (10–15%) at the 2-position of its TAGs. The appreciable amounts of disaturated (POP and PPO) and monosaturated (POO, OPO and PLO) are apparent as high-melting and low-melting fractions in the differential scanning calorimetry (DSC) thermograms. The oil can be easily separated into two products, palm olein (low melting point fraction) and palm stearin (high melting point fraction). A wide range of fractions with different properties to suit requirements of the food industry is available through dry fractionation.
DSC melting and crystallisation thermograms of palm oil.
For melting thermogram, sample was cooled to −30◦C at rate of 40◦C/min, held for 10 mins and heated to 80◦C at 5◦C/min; for cooling thermogram, sample was melted to 80◦C and cooled to −30◦C at 5◦C/min. Lm: low melting fraction, Hm: high melting fraction.
The properties of the triacyglycerol itself is largely dictated by the properties of the fatty acids and the different types of fatty acids esterified on the glycerol’s 1,2,3 positions. This manifestation of properties can be illustrated as follows:
a) Unsaturated fatty acids with long carbon chain length – Results in higher melting points
b) Inversion of cis to trans isomers – Results in raising but steeping the melting point and increasing the oxidative stability
c) Increased in branched chain – Results in lowering of melting point and maintains high liquidity at sub-zero temperature.
d) Increasing in unsaturation – Results in lowering of melting points and more prone to oxidation
e) Unsaturated fatty acids with short carbon chain length – Results in lowering of melting point but faster rate of crystallization
3. Iodine Value (IV)
The iodine value is a chemical constant for a fat or oil. It is a valuable characteristic in fat analysis that measures unsaturation.
The iodine value is a measure for the average number of double bonds of a fat or oil. Double bonds add halogen, with each double bond consuming 1 mol of halogen. The average number of double bonds can then be concluded from the halogen consumption (here: iodine). Iodine numbers are often used to determine the amount of unsaturation in fatty acids.
Iodine value measure unsaturates or the average number of double bonds in a fat; therefore, it is logical that an iodine value can be easily calculated from a fatty acid composition analysis. The constants for the most common unsaturated fatty acids required for calculation of a triglyceride iodine value are shown below
The calculated iodine value is determined simply by multiplying the percentage of each unsaturated fatty acid by its constant and addition of the results. This procedure has not replaced the regular iodine value. It can be utilized as an audit for the chemical iodine value and does provide the capability of obtaining two results from one analysis.
Iodine value is a useful tool for process control and product specification. It gives an indication of the oil’s stability and health properties. Coconut oil has an iodine value of 10. This indicates that it contains a high amount of saturated fatty acids and a very small amount of unsaturated fatty acids. The higher the iodine value, the greater amount of unsaturation. As noted above, coconut oil is 92 percent saturated and 8 percent unsaturated. Soybean oil, in contrast, has an iodine value of 130. It contains only 15 percent saturated fatty acids with 85 percent unsaturated fatty acids, thus the reason for its high iodine value.
The higher the iodine value, the less stable the oil and the more vulnerable it is to oxidation and free radical production. High iodine value oils are prone to oxidation and polymerization. During heating, such as when used in cooking, oils with high iodine value readily oxidize and polymerize. Polymerization is an irreversible process which causes the fatty acids to become hard, insoluble, plastic-like solids.
Testing Methods: AOCS: Cd 8-53 and Cd 8b-90; DGF: C-V-1 la and b
- Carotene Content
Carotenoids are natural pigments of crude palm oil which has a rich orange-red colour due to its high content of carotene. The major carotenoids in palm oil are β and α carotene which account for 90% of the total carotenoids. Carotenoids are the precursors of vitamin A, with β-carotene having the highest pro-vitamin A activity. Carotene concentrations are low in most fats and oils except for palm oil, which contains 0.05 to 0.2% (500ppm to 2000ppm). Typical carotenoid composition of palm oil is shown in table below.
In crude palm oil, the carotene content provides an indication of quality as is shown by the use of DOBI, an index for determining the ‘bleachability’ of palm oil. As oils from a particular oil palm species will have carotene values within a narrow range, any dramatic lowering of carotene is due to degradation of the oil. Good quality oils not only have high carotene values, but also low secondary oxidation characteristics as measured by absorption at 269 nm. The DOBI value, which is the ratio of the absorbance at 446 nm (measurement of carotene) to 269 nm, is well correlated to the colour remaining after refining.
- Chlorophyll Content
The presence of green pigments or chlorophyll is of interest not only because of their impact on the colour of the finished product, but also because chlorophylls are photosensitizers; hence their presence leads to enhanced photo-oxidation of the oil. These pigments must be removed in the pre-bleaching process. AOCS Method Cc 13d-55 is used to determine the chlorophyll content (parts per million) of vegetable oils by spectrophotometric absorption measurements at 630, 670, and 710 nm.
High chlorophyll content in crude palm oil would indicate the presence of oil extracted from unripe fruits. On the other hand, CPO with too low a concentration of total chlorophyll also indicates bad quality oil due to (a) harvesting of over-ripe fruits or (b) poor handling of FFB and the production oil. It is postulated that there might be an optimal value of total chlorophyll content where the fruits are just ripe and the fruit quality not deteriorated due to delay in processing.
No general listing of chlorophyll/pheaphytin levels has been discovered but the following information has been gleaned from a range of sources.
- Olive oil: chlorophyll levels vary with the maturity of the olive and with the method of extraction. Unrefined oils contain 1±20 ppm of chlorophyll.
- Canola oil: levels of chlorophyll in crude oils (5±35 ppm) are much reduced (<50 ppb) by alkali-refining and bleaching.
- Soybean: low levels of chlorophyll in crude oil (1±1.5 ppm) are reduced to around 15 ppb after refining.
- Sunflower: refined oil contains <30 ppb of chlorophyll.
- Palm: crude palm oil contains 250±1800 ppb of chlorophyll (mean value 900 ppb). The level falls with increasing maturity of the palm fruit.
- Tocopherol Content
Crude palm oil, besides being rich in vitamin A, also has a high content of vitamin E, present as tocopherols and tocotrienols of which 70% are tocotrienols. Crude palm olein has a higher content of tocopherols and tocotrienols. Refined oils retain about 70% of the tocols, the amount varying depending on conditions of refining. Most of the loss occurs at deodorisation, and consequently palm fatty acid distillate (PFAD) has up to five to ten times the level in crude oil and PFAD is a good starting material for recovery of vitamin E.
There is considerable interest in the nutritional and physiological properties of vitamin E in palm oil, particularly the tocotrienols. Tocopherols and tocotrienols contents in palm oil as shown below.
- Saponification Value
The saponification value is useful in predicting the type of triacylglycerols in oil by measuring the alkali-reactive groups. The saponification value (SV) is a measure of the free and esterified acids present in fats and oils. In effect it measures the average molecular weight or the equivalent weight of fatty materials in the oil. It is defined as mg of potassium hydroxide (KOH) required to saponify one gram of fat or oil. The smaller the SV, the larger the average weight of the triacylglycerols present. Glycerol esters containing short-chain fatty acids have higher saponification values than those with longer-chain fatty acids. Typical saponification value of various types of oil as shown below.
- Unsaponifiable matter
The unsaponifiable matter of an oil serves as a check for contamination by foreign materials such as mineral oils and damage to the oil by oxidation. Highly oxidised oils contain polymerised fatty acids which are extracted together with the unsaponifiable matter. The unsaponifiable components in palm oil include the sterols, hydrocarbons, tocopherols, tocotrienols, alcohols, carotenoids or pigments and the unsaponifiable organic impurities such as mineral oil. Typical unsaponifiable matter content in palm oil as shown in table below.
- Chemistry of Oils and Fats, CRC Press, 2004
- Fats and Oils Formulating and Processing for Application, CRC Press, 2009
- Fats and Oils Handbook, AOCS Press, 1998
- Bailey’s Industrial Oils and Fats Products, John Wiley & Sons, 2005
- Palm Oil : Production, Processing, Characterization and Uses, AOCS Press, 2012
- Speciality Fats vs Cocoa Butter, Wong Soon, 1991