Non-destructive inspection of food and technical oils by terahertz spectroscopy

Mindaugas Karaliūnas,Andrzej Urbanowicz,Svajus Asadauskas,Dalia Bražinskienė,Irmantas Kašalynas,Gintaras Valušis,Kinan E Nasser

doi:10.1038/s41598-018-36151-3

Mindaugas Karaliūnas, Andrzej Urbanowicz + Show 5 more

Open Access

https://doi.org/10.1038/s41598-018-36151-3

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Abstract

Quality control and non-destructive monitoring are of notable interest of food and pharmaceutical industries. It relies on the ability of non-invasive inspection which can be employed for manufacturing process control. We hereby apply terahertz (THz) time-domain spectroscopy as non-destructive technique to monitor pure and degraded oils as well as hydrocarbon chemicals. Significant differences in the spectra of refractive index (RI) and absorption coefficient arising from the presence of ester linkages in the edible and technical oils were obtained. Explicit increase from 1.38 to 1.5 of the RI in all THz spectrum range was observed in hydrocarbons and mono-functional esters with the increase of molar mass. This fact is in contrast of RI dependence on molar mass in multi-functional esters, such as Adipate or vegetable oils, where it is around 1.54. Degradation products, Oleic Acid (OA) and water in particular, lead only to some changes in absorption coefficient and RI spectra of vegetable oils. We demonstrate that complex colloidal and supramolecular processes, such as dynamics of inverse micelles and oil hydrolysis, take part during oil degradation and are responsible for non-uniform dependence of optical properties on extent of degradation.

Highlights

Terahertz (THz) imaging and spectroscopy is a well-known technique for detection and identification of concealed metal objects and chemical substances due to the unique properties of THz waves penetrate through most of the materials opaque to visible light, such as plastic, paper or cloths
The results show complicated chemical processes of fatty acid (FA)-water complexes as inversed micelles governing the optical properties of oils above the critical micelle concentration (CMC)
The experimental set-up was carefully calibrated and its dispersion features were examined by measuring transient waveforms of the THz field propagated through free optical path, empty high density polyethylene (HDPE) cuvette and unrefined Sunflower oil in the cuvette

Summary

Introduction

Terahertz (THz) imaging and spectroscopy is a well-known technique for detection and identification of concealed metal objects and chemical substances due to the unique properties of THz waves penetrate through most of the materials opaque to visible light, such as plastic, paper or cloths. Gorenflo et al used normalized integrated area under the RI and absorption coefficient curves to draw the dependence on water concentration in the lubricating oils as the oil spectra lack pronounced spectral features[21]. Jiusheng in his study attributed tentatively the spectral features of absorption spectra of vegetable oils in the range between 1.1 and 1.5 THz to the inter-molecular vibrations of hydrogen-bond bending in hydrocarbon chains[23]. We enrich the family of the investigated edible oils including Hemp, Fish, Flax and Camelina oils, few lubricating oils as well as several lighter hydrocarbon chemicals in the THz range using THz-TDS and determined the relation of optical properties to its chemical composition. The chemical changes of heavily deteriorated oils lead to significant impact on the optical properties in the THz range that can be revealed using THz-TDS methods

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