Rheo-Raman | Combining Rheology and Raman Spectroscopy: Crystallinity of Palm Oil

The combination of rheology and Raman spectroscopy offers unique opportunities in measuring both physical and chemical properties of a system simultaneously. Monitoring of the crystallization in palm oil demonstrates the capabilities of such an instrument combination.

The second most important renewable lipid source after soybean oil is palm oil and can be found in products such as lipstick, pizza dough, instant noodles, shampoo, detergent, ice cream, chocolate, biodiesel, and packaged bread to name just a few.

Palm oil consists mostly of different varieties of triacylglycerols (TAGs), i.e. glycerol esterified with three of the following major fatty acids: myristic, palmitic, stearic, oleic, or linoleic acid.

The constituent fatty acids differ in their physical properties. At room temperature, palm oil is a solid or semi-solid slurry, a state which is caused by many tiny crystalline domains that float in a liquid matrix. Because the different TAGs have slightly different melting points, the separation of the oil phases is rather easy and can be used in large-scale fractionation processes in the refinement of raw palm oil. Here, the crystallized components are separated into a high melting fraction called stearin, mostly used in oleochemical applications (soaps, emulsifiers, lubricants), and a low melting fraction (olein) which is used primarily for cooking and frying.

Rheometers are excellent tools for following the change in the viscoelastic parameters during the melting and solidification. In the case of palm oil or other vegetable oils, further insight can be gained by a simultaneous acquisition of chemical information using a Raman spectrometer. While the rheometric data gives insight into the physical properties of the bulk sample, the Raman spectrum probes the vibrational states of the molecules reflecting the chemical structure and microenvironment.

The melting process of palm oil involves a gradual change in viscosity and elasticity as a direct consequence of the transition from a solid into a liquid sample. The combination of rheology and Raman spectroscopy presented here offers the advantage of simultaneous measurements on the same sample. 

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