Vegetable oil is an important widely used lipid source for food preparation. A variety of different vegetable oils is available for cooking, but no pure oil simultaneously combines high nutritional properties, mild taste, and oxidative stability. Soybean oil is the second most consumed oil, but it degrades when heated. To overcome this issue, blending different oil types together is a common practice in the food industry. The determination of the iodine value (IV) allows the oil blending process to be monitored and adjusted accordingly. Oil blends with lower IV contain fatty acids with a large number of saturated bonds and are therefore less susceptible to oxidation. Standard analysis techniques to determine the degree of saturation in oils include titration or gas chromatography. Both methods are time-consuming and need trained personnel. In contrast to these methods, the IV can be analyzed via near-infrared spectroscopy (NIRS) without any sample preparation or toxic chemicals.
In total, 21 soybean-palm oil blends with an iodine value from 60 g/100 g to 130 g/100 g were measured on the Metrohm NIRS DS2500 Liquid Analyzer (Figure 1). To ensure all mixtures were liquified, the samples were preheated at 60 °C using the NIRS XDS Vial Heater. The spectra were collected in transmission mode using 8 mm disposable vials at the same temperature to ensure consistent measurement performance. Data acquisition and prediction model development were performed with the Metrohm software package Vision Air Complete.
Table 1. Hardware and software equipment overview.
| Equipment | Article number |
|---|---|
| DS2500 Liquid Analyzer | 2.929.0010 |
| NIRS XDS Vial Heater | 2.921.9010 |
| DS2500 Holder 8 mm vials | 6.7492.020 |
| Vision Air 2.0 Complete | 6.6072.208 |
The measured Vis-NIR spectra (Figure 2) were used to create a prediction model for quantification of iodine value. The performance of the prediction models was evaluated using correlation diagrams which display a very high correlation (R2 > 0.999) between the Vis-NIR prediction and the standard reference method. The displayed standard error of cross validation (SECV) shows the expected accuracy during routine analysis in QC laboratories (Figure 3).
| Figures of Merit | Value |
|---|---|
| R2 | 0.999 |
| Standard Error of Calibration | 0.20 g/100 g |
| Standard Error of Cross-Validation | 0.22 g/100 g |
Monitoring the iodine value in edible oil blends is crucial to produce vegetable oils with the desired properties for preparing food. This Application Note displays the benefit of using the Metrohm NIRS DS2500 Liquid Analyzer for quality control in food laboratories. Compared to other conventional methods, measurements performed with NIR spectroscopy according to the AOCS Cd 1e-01 standard save time and avoid the production of chemical waste. A sample measurement can be performed within one minute. This ultimately leads to a workload reduction (Table 2) and minimization of running costs in the laboratory as a result.
Table 2. Time to result overview for the measurement of iodine value in edible oil blends by a titration method.
| Parameter | Method | Time to result |
|---|---|---|
| Iodine value | Titration (AOCS Cd 1b-87) | ∼1 to 2 hours |
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Internal reference: AW NIRS SG-0003-052018