Crystallization of Vegetable Oils

Sample

Rape seed oil, pressed Soybean oil, Homa brand Olive oil, Dante brand, Italian first pressing, Palmoil

Conditions

Measuring cell: DSC30

Pan: Aluminum standard 40 µl, hermetically sealed

Sample preparation: No special sample preparation

DSC measurement: Cooling from 50 °C to -100 °C at 10 K/min

Interpretation

Olive oil crystallizes below -10 °C. The triglyceride fraction, with in part saturated fatty acids, crystallizes between -10 °C and -35 °C. The main fraction of olive oil, the triglyceride with 3 oleic acid units (70%), crystallizes at lower temperatures and is recognizable as a crystallization peak at α-form 

Commercial palm oil crystallizes below +15 °C. It has a high percentage of saturated fatty acids (50% C12, 18% C14) and hence a high melting point; 8% unsaturated oleic acid lowers the melting point to values below room temperature.

Soybean oil contains a significant fraction of saturated fatty acids (10% C16) and also a high percentage of monobasic and dibasic unsaturated fatty acids (nutritionally valuable). Triglyceride specimens analyzed by HPLC show fractions with predominantly saturated fatty acids, with some unsaturated fatty acids and also with highly unsaturated fatty acids. These 3 triglyceride fractions are visible in the DSC curve at different crystallization temperatures. The high fraction of linolenic acid in soybean oil (50%) leads to a high fraction of glycerides with three linolenic acid units (melting point -45 °C), which is shown as a peak at -42 °C.

Rape seed oil contains virtually only unsaturated fatty acids (60% C₁₈ monobasic, 25% C18 dibasic, 10% C18 tribasic) and crystallizes at a very low temperature. Only 5% of the fatty acids are saturated and this is indicated by ‘small peaks’ between -20 °C and -40 °C. Even though DSC cannot clearly identify fatty acids or triglyceride fractions, rapid characterization of the oils/fats is possible.

Conclusion

DSC is a rapid means of investigating the crystallization behavior of edible oils. For comparison purposes approximately identical sample sizes should be taken, e.g. 20 ± 5 mg. The sample size may influence the supercooling: the smaller the sample size the greater the degree of supercooling.

_{Crystallization of Vegetable Oils | Thermal Analysis Application No. HB 1001 | Application published in METTLER TOLEDO TA Application Handbook Food}