Characterization of Vegetable Oils by DSC

Introduction 

Differential Scanning Calorimetry (DSC) is used to characterize very many different types of materials in research, product development and quality control.

The crystallization behavior of fats and oils has been studied by DSC methods for many years.

This article presents some interesting DSC measurements performed on vegetable oils. Vegetable oils (or fats, depending on their consistency) are plant constituents obtained from the seeds or fruit pulp by pressing and/or solvent extraction followed by evaporation of the solvent. Vegetable oils consist mainly of triglycerides. Chemically, triglycerides are tri-esters, formed by the esterification of glycerol with three long-chain carboxylic acids (fatty acids).

In natural oils, triglycerides contain many different fatty acids. A natural oil is therefore always a mixture of different triglycerides. In addition, the oil contains different amounts of partial glycerides (i.e. mono- and diglycerides) and other constituents (e.g. phospholipids, sterols, vitamins, etc.), depending on the origin of the oil and the type of processing.

The crystallization behavior of a fat or oil is very sensitive to its composition and can easily be measured by DSC. The DSC curve obtained from an oil sample is characteristic of the particular oil and serves as a “fingerprint”. DSC is in fact an excellent method for determining differences between oils, for example between refined and natural oils. 

Experimental Details

The measurements described in this article were performed using a DSC822e equipped with an IntraCooler. The samples were hermetically sealed in 40-µl aluminum crucibles and cooled from 50 °C to −75 °C at 1 K/min. The sample mass was typically 5−10 mg.

Measurements and Results  

Figure 1 shows a typical DSC cooling curve of a vegetable oil. As the oil cools, various components crystallize out at characteristic temperatures. These crystallization processes are observed on the DSC curve as exothermic peaks.

In general, raw vegetable oils are normally unfit for direct human consumption. The raw oils are cleaned and refined in processes that eliminate a large number of substances whose presence is detrimental to the taste, human health properties, shelf life or appearance of the oil. In a further process, the oils can be treated to give them more desirable properties. These processes include the so-called “hardening” or “hydrogenation” of oils, in which the content of unsaturated fatty acids is reduced. This increases the crystallization temperature of the oil.

Figure 2 shows DSC cooling curves of cold-pressed and refined walnut oil. Comparison of the two curves shows distinct differences. The native oil (cold-pressed) crystallizes at about −48 °C. Above this temperature, a certain amount of crystallization is visible but the corresponding peaks are small. In contrast, the refined walnut oil crystallizes at lower temperatures and the DSC curve shows several more pronounced peaks.

As already mentioned, the crystallization behavior of a vegetable oil depends mainly on its chemical composition. Vegetable oils and fats contain fatty acids with mostly even numbers of carbon atoms and chain lengths of 4 to 24 carbon atoms. Quantitatively, fatty acids with chain lengths of 14 to 18 carbon atoms predominate.

In general, oils composed mainly of saturated fatty acid triglycerides crystallize at relatively high temperatures, whereas oils with unsaturated fatty acid triglycerides crystallize at relatively low temperatures.

Depending on the proportions of fatty acid triglycerides present in the oils, several crystallization peaks appear in the DSC curve. Usually the curve exhibits one pronounced crystallization peak (as a rule at about −50 °C, −35 °C or 10 °C) and one or more smaller peaks.

This is also true for the refined oils (see Figure 4). Compared with native oils, crystallization occurs here at slightly lower temperatures. 

Conclusions

Vegetable oils exhibit characteristic crystallization behavior that is strongly influenced by the composition of the oil. The crystallization behavior can easily be studied by DSC methods.

The resulting measurement curves are used to identify unknown native and refined vegetable oils and to monitor and control refining processes. DSC is therefore an extremely useful technique for the quality assurance of vegetable oils.

_{Characterization of Vegetable Oils by DSC | Thermal Analysis Application No. UC 234 | Application published in METTLER TOLEDO Thermal Analysis UserCom 23}