Visible/Near Infrared (Vis/Nir) Spectroscopy As An Optical Sensor For Evaluating Olive Oil Quality

INTRODUCTION

The olive tree (Olea europaea L.) has enormous cultural and economic importance in the Mediterranean region, particularly in Palestine, where it forms a large part of the local agricultural sector. Olive cultivation is vital to the livelihoods of approximately 100,000 Palestinian families and contributes significantly to the local economy. Moreover, olive oil is an important product that is consumed primarily locally; exports occur primarily in surplus years. However, growing global concern about food quality and authenticity has increased the need to ensure that olive oil is of high quality and properly sourced, which is essential to maintaining the reputation of Palestinian olive products. Nevertheless, testing the quality of olive oil using traditional chemical analysis is expensive, laborious, and time consuming. As a result, optical sensors were used as an alternative method. For example, Visible/Near Infrared Spectroscopy (VIS/NIR) is used to check quality, detect adulteration, and determine the geographical origin of olive oil.

METHOD

VIS/NIR spectroscopy is a non-destructive, low-cost, and time-saving method for measuring chemical compounds in heterogeneous products like olive oil. This method collects the absorption spectrum of olive oil. The obtained experimental dataset of olive oil was analyzed with smart algorithms like classifying olive oil samples based on their spectral data, and predicting chemical parameters (such as acidity and peroxide value) from the spectral data. The performance of the predicted model was assessed using metrics. For instance, correlation coefficients (R²), root mean square errors (RMSE) and the ratio of performance to deviation (RPD). Models with RPD values greater than 2.0 were considered reliable for predicting the quality of olive oil.

Figure 1 VIS/NIR (400–1100 nm) spectral curves representing olive oil groups

One study collected 48 olive oil samples from various regions in Palestine and stored in dark, tightly closed bottles at 17°C to prevent oxidation. The VIS/NIR spectral analysis showed distinct absorption peaks at certain wavelengths (450.5, 532, 601, 673, 932 and 977 nm). Extra virgin and virgin olive oils had higher absorbance than regular virgin olive oil in almost all spectral ranges. This method was able to successfully differentiate the olive oil samples (extra virgin, virgin, regular virgin and lampante) based on their quality.

About the predicted result, the R² for acidity was 0.88 for the calibration and validation sets, and the RMSE was about 0.35. The RPD for the calibration set was 2.78 and for the validation set was 3.08, indicating good prediction accuracy. The R² for peroxide values were 0.83 (calibration) and 0.86 (validation), with an RMSE of approximately 1.6. The RPD for both sets was 2.2, with a relative error of less than 10%, indicating reliable model performance.

SYSTEM

A USB2000 + XR1-ES (Ocean Insight) spectrometer was used as a VIS/NIR sensor, with a halogen lamp HL-2000-FHSA as a light source. Also, optical fibers were used for conducting the absorbance light through to the samples.

The USB2000 + XR1-ES spectrometer is a versatile and compact device widely used in various spectroscopic applications, including food quality assessment, environmental monitoring and biomedical research. The device was equipped with a 2048-element linear silicon CCD array detector, which is known for its reliability and performance. It covers a broad spectrum from UV to Near-Infrared (NIR), making it suitable for diverse applications. Moreover, it offers good sensitivity and provides high resolution. Low power consumption makes it ideal for portable and field applications.

Figure 3 USB2000+XR1-ES Spectrometer

The HL-2000-FHSA from Ocean Insight is a high-powered halogen light source designed for spectroscopic applications. Its key features include a 20-watt tungsten halogen lamp with an integrated attenuator for controlling light output and a TTL-shutter for automated control. The light source covers a broad spectral range from 360 to 2400 nm and is suitable for use in VIS-NIR spectroscopy. That is why it provides stable, high-intensity illumination, making it ideal for applications such as reflectance and absorbance measurements.

Figure 4 HL-2000-LL-FHSA Light Source

CONCLUSION

These findings demonstrate the potential of the spectrometer as a rapid, non-destructive tool for quality control in the oil industry. This measure provides a significant increase in efficiency and reliability compared to the traditional method. It confirms the value of using a spectrometer, specifically the USB2000 + XR1-ES, for measuring the quality of olives via NIR spectroscopy (400-1100 nm). The spectrometer had a high degree of accuracy in predicting the key quality parameters of interest, including acidity and peroxide. Additionally, it had a successful distinction between different types of olives.