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Determination of key quality parameters of palm oil, namely free fatty acids (FFA), iodine value (IV), moisture content, deterioration of bleachability index (DOBI), and carotene require the use of several different analytical methods, which are laborious and can lack in accuracy.

This application note demonstrates that the XDS RapidLiquid Analyzer operating in the visible and near infrared spectral region (Vis-NIR) provides a cost-efficient and fast solution for the determination of these quality control parameters in palm oil. With no sample preparation or chemicals needed, Vis-NIR spectroscopy allows for the analysis of palm oil in less than a minute and can be used by anyone.

XDS RapidLiquid Analyzer and a palm oil sample present in a 8 mm disposable vial.
Figure 1. XDS RapidLiquid Analyzer and a palm oil sample present in a 8 mm disposable vial.

Palm oil samples (crude palm oil) were measured in transmission mode with a XDS RapidLiquid Analyzer over the full wavelength range (400–2500 nm). Reproducible spectrum acquisition was achieved using the built-in temperature control (at 60 °C) of the XDS RapidLiquid Analyzer. For convenience, disposable vials with a path length of 8 mm were used, which made cleaning of the sample vessels unnecessary. The Metrohm software package Vision Air Complete was used for all data acquisition and prediction model development.

Table 1. Hardware and software equipment overview
Equipment Metrohm number
XDS RapidLiquid Analyzer 2.921.1410
Disposable vials, 8 mm diameter, transmission 6.7402.000
Vision Air 2.0 Complete 6.6072.208

The obtained Vis-NIR spectra (Figure 2) were used to create prediction models for quantification of the individual key parameters. The quality of the prediction models was evaluated using correlation diagrams, which display the correlation between Vis-NIR prediction and primary method values. The respective figures of merit (FOM) display the expected precision of a prediction during routine analysis.

Figure 2. Selection of palm oil Vis-NIR spectra obtained using a XDS RapidLiquid Analyzer and 8 mm disposable vials. For display reasons a spectra offset was applied.

Result free fatty acid

Figure 3. Correlation diagram for the prediction of the result free fatty acid in palm oil using a XDS RapidLiquid Analyzer. The free fatty acid lab value was evaluated using titration.
Table 2. Figures of merit for the prediction of the free fatty acids in palm oil using a XDS RapidLiquid Analyzer. 
Figures of merit Value
R2 0.835
Standard error of calibration 0.266%
Standard error of cross-validation 0.270%

Result iodine value

Figure 4. Correlation diagram for the prediction of the iodine value (IV) in palm oil using a XDS RapidLiquid Analyzer. The iodine lab value was evaluated using titration.
Table 3. Figures of merit for the prediction of the iodine value in palm oil using a XDS RapidLiquid Analyzer. 
Figures of merit Value
R2 0.911
Standard error of calibration 0.184 mg/100 g
Standard error of cross-validation 0.201 mg/100 g

Result moisture content

Figure 5. Correlation diagram for the prediction of the moisture content in palm oil using a XDS RapidLiquid Analyzer. The moisture lab value was evaluated using Karl Fischer (KF) titration.
Table 4. Figures of merit for the prediction of the moisture content in palm oil using a XDS RapidLiquid Analyzer. 
Figures of merit Value
R2 0.638
Standard error of calibration 0.046%
Standard error of cross-validation 0.047%

Result deterioration of bleachability index

Figure 6. Correlation diagram for the prediction of the deterioration of bleachability index (DOBI) in palm oil using a XDS RapidLiquid Analyzer. The DOBI lab value was evaluated using photometry.
Table 5. Figures of merit for the prediction of the deterioration of bleachability index (DOBI) in palm oil using a XDS RapidLiquid Analyzer. 
Figures of merit Value
R2 0.842
Standard error of calibration 0.17
Standard error of cross-validation 0.19

Result carotene content

Figure 7. Correlation diagram for the prediction of the carotene content in palm oil using a XDS RapidLiquid Analyzer. The carotene lab value was evaluated using photometry.
Table 6. Figures of merit for the prediction of the carotene content in palm oil using a XDS RapidLiquid Analyzer. 
Figures of merit Value
R2 0.677
Standard error of calibration 22.9 ppm
Standard error of cross-validation 23.4 ppm

This application note demonstrates the feasibility of NIR spectroscopy for the analysis of the FFA content, iodine value, moisture content, DOBI, and carotene content in palm oil. In comparison to wet chemical methods, running costs are significantly lower when using NIR spectroscopy (Table 7 and Figure 8).

Table 7. Comparison of running costs for the determination of the hydroxyl number with titration and NIR spectroscopy.
  Lab method NIR method
Number of analyses per day 10 10
Cost of operator per hour $25 $25
Costs of consumables and chemicals (FFA, IV, moisture, DOBI, carotene) $9 $1
Time spent per analyses (FFA, IV, moisture, DOBI, carotene) 22 min 1 min
Total running costs per year $42,900 $2,063
Figure 8. Comparison of the cumulative costs over three years for the determination of key quality parameters in palm oil with titration/photometry and NIR spectroscopy.
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