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
- Slip Melting Point (SMP)
Since fats are mixtures of many compounds, they exhibit a broad melting range rather than a clearly defined melting temperature. The melting point of a fat is thus an empirical property related to the experimental method of determination and not a basic physical property, like the melting point of a pure compound.
Two common methods of determining the melting point (MP) of a fat are:
Slip Melting Point
The temperature at which fat in a capillary tube placed in water becomes soft enough to slip or rise up the tube. Also called open tube MP, rising point or softening point and exemplified by AOCS Method Cc 3-25. In the Slip Melting Point test the fat slips in the capillary tube when about 5% solid fat is present.
Wiley Melting Point
The temperature at which a disc of fat becomes spherical when heated in an alcohol-water mixture. AOCS Method Cc 2-38. The Wiley MP is very popular in the USA, but little used elsewhere. In the Wiley MP test the fat is sufficiently fluid to form a sphere only when the fat is almost fully liquid with about 1 % solid fat still present. Hence the Wiley Melting Point value is frequently several degrees higher than Slip Melting Point value.
The Slip MP is used commonly in Europe and has been adopted in Malaysia as the preferred method for palm and palm kernel oil. Because there are substantial differences in MP’s determined by different methods, it is important to distinguish carefully between the methods. A comparison between values of both methods is as below.
A comparison of Slip Melting Point (or Softening Point) for various types of oil as shown below:
- Solid Fat Content (SFC)
The amount of solid fat present in the oil at any given temperature is due to the process of crystallisation. The different types of triacylglycerols in the oil influence this process. The solid fat content (SFC) of oil is a measure of the amount (%) of solid fat present in the oil at any given temperature.
Solid Fat Content (SFC) is an important characteristic that can influence appearance, flavour release, melt rate, shelf life and stability of fat based food products. In the chocolate industry it is desirable to manufacture products with the ideal Solid Fat Content that will allow for chocolate to remain solid at room temperature, but still give consumers that “melt in your mouth” experience. In the speciality fats industry, the properties of various palm oil/ palm kernel oil fractions are manipulated to produce products with solid fat content similar to cocoa butter.
Below is an example of SFC data for palm oil and palm kernel oil. Palm kernel oil is significantly harder at low temperatures (below 20°C) but quickly melts above its melting point is therefore 10°C below the one of palm oil.
A comparison of the SFC for cocoa butter and palm kernel oil (PKO), palm kernel stearin (PKS), hydrogenated PKS of melting point 35ºC (HPKS 35) and Palm Oil (PO) as shown below.
A comparison of SFC between palm kernel oil and its fractions as shown below.
PKO – Palm kernel oil
PKS – Palm kernel stearin
PKOo – Palm kernel olein
SFC has been traditionally determined by dilatometry. In addition to not giving the real SFC but rather the solid fat index (SFI), the technique is time consuming, cumbersome and not applicable to fats with high solids content. Currently, new instruments are available based on pulsed NMR. This technique is now well established and is used extensively.
The most comprehensive study of the density of palm oil has been made by PORIM. Crude and RBD palm oil and olein were studied over the temperature range 25-75°C. Statistical analysis of the data showed no significant difference between the various oils, and the density could be expressed as density (g/ml) =0.9244-0.00067 T, where T is the temperature in degrees Celsius. Typical Malaysian crude palm oil has a moisture content of 0.25% and free fatty acid content of 3%. Thus, the difference in density between RBD and crude oils would be expected to be only 0.0004 g/ml, and this small difference was not statistically significant compared with experimental error and natural variability of the various oils studied.
The densities of palm oil and its fractions as shown below.
Viscosity is a measure of the internal friction of the oil molecules. The relatively high viscosity of oils is due to the intermolecular attraction between the long-chain structures of the Triacylglycerol molecules.
Oils and fats show the flow behaviour of true Newtonian liquids. Near the melting point non-Newtonian behaviour may occur due to the presence of fat crystals.
The viscosities of natural oils and fats do not differ greatly. Viscosity increases with molecular weight but decreases with increasing unsaturation and temperature. The viscosity of an oil decreases by approximately 30% for each 10°C rise in temperature.
The term includes mechanical impurities such as rust, carbohydrates, nitrogenous substances, various resins, calcium soaps, oxidised fatty acids, fatty acid lactones, alkali soaps, hydroxy fatty acids and their acylglycerols, etc. The impurities are defined as substances which remain insoluble and can be filtered off after the oil is dissolved in a specific solvent such as petroleum ether or diethyl ether.
- Refractive Index (RI)
The refractive index of oil is a measure of the extent to which a beam of light is refracted on passing from air into oil. This can be a useful characteristic in that the determination is carried out with ease, speed and precision, using small amounts of the oil. The RI can also be used for establishing oil purity. It is generalised that the refractive indices of oils increase with increase in length of the hydrocarbon chains and the number of double bonds i.e. with increasing unsaturation or IV. With increase in temperature, the refractive indices of oils decrease. The refractive index can also be influenced by oxidative damage to the oil. Refractive index is useful for identification of fats and the observation of progress in reactions during hydrogenation. Hydrogenation reduces both the iodine value and the refractive index of oils. The relationship between the iodine value and refractive index depends on the molecular weights of the glycerides, which is very nearly the same for most oils. The exceptions are the oils high in either lauric or erucic fatty acids. Correlation between iodine value and refractive index is not precise, but will be within one or two units, which should be adequate to monitor the hydrogenation reaction and indicate when to interrupt the reaction for more precise evaluations.
Refractive Index of various oils and fats as shown below:
Changes in the colour of fats and oils finished products are perceived as indicating poor-quality product, regardless of the reason or effect upon performance. Consumers may not consciously notice the colour of bottled oil unless it appears different from other products on the shelf. Marketing has successfully promoted lighter or whiter oil as being better for most salad oils and shortenings. Food processors usually have ingredient specifications that identify the allowable colour parameters because the fats and oils product may have the ability to enhance or diminish the appearance of the prepared food product. Product colours of fats and oils must be monitored as they are received to maintain both real and perceived product quality.
The Wesson method using Lovibond glasses is an abbreviated version of a method originally developed in England for measuring the colour of beer. Colour has three attributes, but the Wesson method ignores the brightness factor and is interested only in the degree of redness. Yellow is necessary to make colours look similar to allow assessment of redness, but the amount of yellow is considered unimportant for this method. The Wesson method is the principal colour method for the U.S. edible-oil industry
and has been utilized for many years primarily because of its simplicity; however, some difficulties arise as a result of such oversimplification: (1) apparent red values are reduced when chlorophyll is present in the oil, (2) brown pigments interfere with red and yellow comparisons, and (3) visual comparisons must be made.44 Visual colour measurement is less acceptable because the operator must be adept at matching colours and also must have good colour vision.
AOCS Method Cc 13e-92 utilizes a Lovibond Tintometer® that has become the standard in most countries other than the United States. The geometry and colour scales for the Wesson and Lovibond methods are different; consequently, the results are not compatible. The vital parts of the Lovibond Tintometer are the series of red, yellow, and blue permanently coloured glass standards. These standards vary from water-white colours to deep reds, yellows, and blues. Each standard colour is numbered and subtly different from the one preceding and following it.
PORAM standard for refined palm oil quality with the colour requirements as shown below.
- Specific Heat
Overall, there is little variation among the natural oils and fats. For practical purposes the specific heats of liquid oils and fats, including PO, may be taken as: 0.47 + 0.00073T kcal/kg where T is the temperature in degrees Celsius.
Note: 1 Btu/ lb =0.252 kcal/kg. There is no data available for Palm Kernel and Coconut oil, but the above equation is probably satisfactory for practical purposes since the variation of specific heat with molecular weight is small.
- Smoke point, Flash Point and Fire Point
In cooking, the smoke point of an oil or fat is the temperature at which, under defined conditions, enough volatile compounds emerge from the oil that a bluish smoke becomes clearly visible. At this temperature, volatile compounds, such as water, free fatty acids, and short-chain degradation products of oxidation come up from the oil.
Considerably above the temperature of the smoke point is the flash point, the point at which the vapours from the oil can first ignite when mixed with air.
The fire point of a fuel is the temperature at which the vapour produced by that given fuel will continue to burn for at least 5 seconds after ignition by an open flame. At the flash point, a lower temperature, a substance will ignite briefly, but vapour might not be produced at a rate to sustain the fire.
For Palm Oil
Smoke point: 245°C
Flash Point: 304°C
Fire Point: 315°C
Below is a comparison of the smoke points of various oils and fats.
|Fats or Oils||Description||Type of Fat||Smoke Point °F||Smoke Point °C|
|Almond Oil||Has a subtle toasted almond aroma and flavor.||Monounsaturated||420°F||216°C|
|Avocado Oil||Vibrant green in color with a soft nutty taste and a mild avocado aroma.||Monounsaturated||520°F||271°C|
|Butter||Whole butter is a mix of fats, milk solids, and moisture||Saturated||350°F||177°C|
|Butter (Ghee), clarified||Ghee has a higher smoke point than butter since clarification eliminates the milk solids (which burn at lower temps).||Saturated||375-485°F (depending on purity)||190-250°C (depending on purity),|
|Canola Oil (Rapeseed oil)||A light, golden-colored oil.||Monounsaturated||400°F||204°C|
|Coconut Oil||A heavy nearly colorless oil extracted from fresh coconuts.||Saturated||350°F||177°C|
|Corn Oil||A mild, medium-yellow color refined oil. Made from the germ of the corn kernel.||Polyunsaturated||450°F||232°C|
|Cottonseed Oil||Pale-yellow oil that is extracted from the seed of the cotton plant.||Polyunsaturated||420°F||216°C|
|Grapeseed Oil||Light, medium-yellow oil that is a by-product of wine making.||Polyunsaturated||392°F||200°C|
|Hazelnut Oil||The nuts are ground and roasted and then pressed in a hydraulic press to extract the delicate oil.||Monounsaturated||430°F||221°C|
|Lard||The white solid or semi-solid rendered fat of a hog. This was once the most popular cooking and baking fat, but has been replaced by vegetable shortenings.||Saturated||370°F||182 °C|
|Macadamia Nut Oil||This oil is cold pressed from the decadent macadamia nut, extracting a light oil similar in quality to the finest extra virgin olive oil.||Monounsaturated||390°F||199 °C|
|Olive Oil||Oils vary in weight and may be pale yellow to deep green depending on fruit used and processing.||Monounsaturated||Extra Virgin – 320°F||160°C|
|Virgin – 420°F||216°C|
|Pomace – 460°F||238°C|
|Extra Light – 468°F||242°C|
|Palm Oil||A yellowish-orange fatty oil obtained especially from the crushed nuts of an African palm.||Saturated||446°F||230°C|
|Peanut Oil||Pale yellow refined oil with a very subtle scent and flavor. Made from pressed steam-cooked peanuts. Used primarily in Asian cooking.||Monounsaturated||450°F||232°C|
|Rice Bran Oil||Rice bran oil is produced from the rice bran, which is removed from the grain of rice as it is processed.||Monounsaturated||490°F||254°C|
|Safflower Oil||A golden color with a light texture. Made from the seeds of safflowers.||Polyunsaturated||450°F||232°C|
|Sesame Oil||Comes in two types – a light, very mild Middle Eastern type and a darker Asian type pressed from toasted sesame seeds.||Polyunsaturated||410°F||232°C|
|Shortening, Vegetable||Blended oil solidified using various processes, including whipping in air and hydrogenation. May have real or artificial butter flavor added.||Saturated||360°F||182 °C|
|Soybean Oil||A fairly heavy oil with a pronounced flavor and aroma.||Polyunsaturated||450°F||232°C|
|Sunflower Oil||A light odorless and nearly flavorless oil pressed from sunflower seeds. Pale yellow.||Polyunsaturated||450°F||232°C|
|Vegetable Oil||Made by blending several different refined oils. Designed to have a mild flavor and a high smoke point.||Polyunsaturated|
|Walnut Oil||Medium-yellow oil with a nutty flavor and aroma. More perishable than most other oils.||Monounsaturated||400°F||204°C|
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
7) Speciality Fats vs Cocoa Butter, Wong Soon, 1991
8) Proceedings of AOCS World Conference on Processing of Palm, Palm Kernel and Coconut Oil, 1985
9) Determination of melting point of vegetable oils and fats by differential scanning calorimetry
(DSC) technique, Renata Tieko Nassu and Lireny Aparecida Guaraldo Gonçalves, 1998
10) Properties and Utilization of Palm Kernel Oil, February 2001 issue of INFORM.