Fats are invariably the mixture of different triglycerides and have different melting points. Therefore they do not melt at fixed temperature but over a range of temperature. The melting point is the temperature at which solid and liquid phases are in equilibrium. While melting point of fat is related to its physical and thermal properties. In addition, it provides the information about the purity and identity of the compound.
A pure solid has a sharp melting point whereas impurity decreases it.There are varieties of methods for the determination of fat. This includes capillary tube methods, slipping point, Wiley melting point, Mettler dropping point, photoelectric point etc; and each method gives different results depends on the condition under which tests are performed. But according to AOAC, Wiley method and capillary tube method are the official methods of analysis. So, let’s see in details about most of all methods used for Fat Melting Point Analysis.
Chemical Composition Of Fat
Triglycerides are the main component of edible fats and oil. Most of the fats are mixed triglycerides as they contain two or three different fatty acids. In addition, fat includes minor components like mono and diglycerides, free fatty acids (FFA), phosphatides, fatty alcohols like sterols, fat-soluble vitamins like vitamin E, carotenoids like pigments and other substances.
Open tube capillary-slip method | Fat Melting Point Analysis
This mostly used method is known by different names. It is described as slip point method in Great Britain, melting point rise in Germany and as a softening point method by AOCS. But overall it is known as open tube capillary method.
The melting point is the temperature at which fat softens and becomes sufficiently fluid to run or slip in a capillary tube.
- Melting point tubes –They are made of glass with rectangular cut edges, thin-walled, open at both the ends.
- Thermometer – Thermometer should be of suitable range and properly calibrated for determining error-free melting point
- Beaker – should have side tube heating arrangement
- Gas burner or spirit lamp – used as a heat source
- First of all melt the fat sample completely and filter it through filter paper to remove impurities and any moisture.
- Dip capillary tubes in the liquid samples; sample should rise to a 10mm high in tubes.
- Chill it using the piece of ice until it feels like the fat is solidified.
- Place the capillary tube in a beaker and keep it in a refrigerator at 4-100C for 16 hrs.
- After removing it from a refrigerator, attach it to a thermometer by using a rubber band. Care should be taken that thermometer bulb and a lower end of the capillary tube should be at the same level.
- Take distilled water in a beaker and suspend thermometer in that, the bottom of the thermometer is immersed in the water.
- Adjust the bath temperature to 8-100C and agitate it with a small stream of air.
- Continue to apply heat at the rate of 10C/min, slow the rate to 0.50C when the melting point is about to reach.
- Continue heating until fat column rises in the capillary tube. Note down the temperature at which column rise. Take at least two readings and calculate the average temperature. This is the melting point.
Advantages Of Open tube capillary-slip method | Fat Melting Point Analysis
This method is used in international trade for palm oil and hard fats. It is also the good method for coconut oil, hydrogenated fats and hard tallows.
Disadvantages Of Open tube capillary-slip method | Fat Melting Point Analysis
One of the disadvantages is 16 hrs stabilization time, as a result, it requires long processing time. This method is less satisfactory for lard compounds, animal fats, the mixture of hard and soft fats and emulsions.
Wiley Melting Point | Fat Melting Point Analysis
In this method, firstly disk of the fat is solidified and chilled. Then disk is suspended in the alcohol-water mixture. Furthermore, start the heating source and stirred with a rotating thermometer. The sample is melted by heating at a 220C increase per 10 min with agitation of air bubbles. The temperature at which the fat disk becomes completely spherical is known as the melting point.
This method is more reliable and reproducible. But it is the subjective determination of melting point due to spherical disk formation.
Mettler dropping point
This method determines the temperature at which fat sample become sufficiently fluid to flow in particular apparatus. A fat sample is first placed in the sample cup. Sample cup is then placed in a heated furnace. The falling of the first drop of fat is sensed photometrically and reading is digitally displayed. This is the mostly used melting point analysis method by domestic industries.
In this method, a capillary tube is dipped into the melted fat. Then it is cooled using ice water and solidified. It is placed in a beam of light of photoelectric instrument. When the fat melts, the temperature is raised and the transmittance of the sample increases. This transmittance then calculated as a melting point.
In this way, there are different methods for melting point analysis of fat; and they depend upon sample pre-treatment, instruments used and different endpoints.
Food analysis: Theory and Practices, by Yeshajahu Pomeranz, Clifton E. Meloan