Stability Studies of Fish Oils by High-Pressure DSC

How can you quickly measure the quality of a sample of fish oil? What are the optimum storage conditions? Can DSC measurements under pressure (HP-DSC) give a quick answer to such questions?

This article describes DSC measurements performed on untreated and stabilized fish oils at increased temperatures under oxygen. The results are compared with classical test methods [1].

Introduction

Fish oils with a high content of essential omega-3 fatty acids are widely used as dietary supplements in foodstuffs. Unfortunately, the oils are very sensitive to oxidation as soon as they are extracted and rapidly become rancid. This makes their further use problematic (unpleasant taste and smell, presence of free radicals, etc.).

This problem can be overcome by adding antioxidants as stabilizers and using microencapsulation. This delays the oxidation process and improves the stability of the oils. Manufacturers of such fish oil products need a reliable test method to determine the quality of their products and to optimize storage conditions. The method should be rapid and easy to perform.

The aim of this study was to compare the results obtained from high-pressure DSC measurements with those from other analytical methods such as the peroxide value and gas phase extraction-gas chromatography. 

Experimental Details

Different fish oils were investigated that had been enriched with 50% (DHA50) or 70% (DHA70) docosahexaenoic acid (DHA, C₂₂H₃₂O₂) and that contained antioxidant (AA) or no antioxidant (SA). A mixture of tocopherols (vitamin E) was used as the antioxidant.

The peroxide value is a measure of the level of oxidation of an oil containing polyunsaturated fatty acids. The primary products of the oxidation reaction are peroxides. Determination of the peroxide value therefore allows one to understand the effect of oxygen on the stability of the fish oils. The peroxide value is determined by titrating the iodine liberated in the reaction between the peroxides and potassium iodide with sodium thiosulfate [2].

The SPME-GC method [3] involves solid phase microextraction (SMPE) followed by gas chromatography (GC). The method allows you to follow the formation of secondary products of the oxidation. The formation of volatile components can be quantitatively measured as a function of time.

The sample to be analyzed is stored in a sealed vial under well-defined conditions until the gas and liquid phase are in equilibrium (incubation time). The volatile products in the gas phase are then absorbed on a suitable fiber system (extraction time). Finally, the products are desorbed in the injector of a gas chromatograph without a separation column and measured (desorption time). The peak areas of the detector signals are directly proportional to the total amount of the volatile components and hence the oxidation products.  

This method can also be used to measure individual components in order to elucidate reaction mechanisms. This requires the use of separation columns and internal standards. The following parameters were used for SPME-GC: 1 mL sample in a 22 mL vial; incubation time: 30 min at 90°C; extraction time: 15 min; desorption time 2 min. 

The results obtained for samples using the METTLER TOLEDO high-pressure DSC (HP-DSC827e ) [4] yield information about the isothermal induction time for oxidation (OIT) and the temperature at which oxidation begins on heating under oxygen pressure (OOT). Both values provide global information on the accelerated oxidation and indicate how long a stabilizer is effective. 

The dynamic OOT method (Oxidation Onset Temperature) has the advantage that it is quick. The isothermal OIT method (Oxidation Induction Time) however yields better reproducibility than the OOT method.

The measurements were performed at oxygen pressures between 0 and 40 bar and at different temperatures. The furnace purge rate was 100 mL/min oxygen (referred to normal pressure).

The oxidation measurements were carried out using about 5 mg of fish oil in a 40-µL aluminum crucible without a lid. The oxygen pressure was set at room temperature before the start of the measurement and held constant by means of a pressure controller. In the OIT methods, the sample was heated rapidly at 90 K/min to the isothermal temperature. 

Results

OIT and OOT Measurements Using High-Pressure DSC 

Figure 1 shows the typical course of the OIT and OOT curves of the DHA-50-SA fish oil sample (fresh oil with 50% DHA but no antioxidant) at 35 bar oxygen pressure. The OIT was measured at 90°C and the OOT at 10 K/min. In the OIT curve (lower diagram), the onset time of oxidation (OIT) is determined as the point of intersection of the two tangents (extrapolated onset time), as shown in the diagram. In the OOT curve, the onset temperature of the oxidation (OOT) is determined as the extrapolated onset temperature.

Conclusion and Outlook

The experiments performed in this work clearly show that dissolved oxygen and temperature have a dominant influence on the oxidation, especially on the initiation and the rate of propagation of the reaction (radical formation and propagation).

Equally important is the information on the indispensable role of antioxidants in the commercialization of fish oils. Microencapsulation does not protect the oils from becoming rancid because the matrix is permeable to oxygen. It does however improve the manipulation and storage of the products as well as their commercial acceptance by removing unpleasant smells.

The investigations also show that measurements using HP-DSC yield important information that helps one to understand the phenomena associated with the stability of microencapsulated and non-encapsulated fish oils. In combination with classical methods, OIT and OOT measurements are a valuable aid for analyzing and monitoring the quality of products. A further aim of the use of high-pressure DSC is its possible use as an accelerated test method in the manufacture of fish oil products. 

^{Stability Studies of Fish Oils by Highpressure DSC | Thermal Analysis Application No. UC 322 | Application published in METTLER TOLEDO Thermal Analysis UserCom 32}