Abstract
Mid-infrared and particularly near-infrared spectroscopy is extremely useful for food analysis because they measure chemical and physical properties fast and non-destructively. The advancement of a supercontinuum light source covering the near-infrared and parts of the ultraviolet and mid-infrared regions, gives new possibilities in spectroscopy. The theoretical advantages are: (a) that the supercontinuum light does not illuminate in all directions and therefore do not heat the surrounding spectrometer components, (b) that the beam is collimated and therefore a high spectral brightness (W/(m2·nm)) can be obtained, (c) that the supercontinuum light is fiber compatible i.e. it can couple directly to fibers, and (d) that the fast repetition rate of the supercontinuum pulses makes it possible to do very fast measurements.
For these reasons, the supercontinuum light stands out from the commonly applied near- and mid-infrared incandescent light bulbs. This thesis aim to explore the utility of using a supercontinuum source in two food applications. (1) The supercontinuum light was applied for the first time to barley seeds in transmission mode in the long wavelength near-infrared region from 2260-2380 nm. From these spectra it was possible to predict the β-glucan content of individual seeds. The measurements of single seeds can be used in plant breeding to sort according to trait or in grain industries to sort according to quality or property. (2) The supercontinuum light has been used with a new wavelength separation method called dispersive Fourier transformation. Different wavelengths travel at different speed through a dispersive element, which in this case is a 10.6 km long silica fiber, and the polychromatic light pulses will therefore be separated by wavelength. The signal will then be transformed from the time-domain to a frequency domain. The spectrometer has no moving parts, which makes it insensitive to mechanical vibrations. A spectrometer with a wavelength separating fiber is therefore an obvious candidate for industrial process measurements. This thesis presents preliminary results on sucrose in water and melamine in milk powder using a supercontinuum light and a dispersive fiber from 1300-1650 nm. The possibility for the new spectrometer to measure β-glucan in barley and bread hardness in aging bread was evaluated by calculating Pearson’s correlation coefficients. The spectral correlation to β-glucan and bread hardness showed r2 values equal to 0.98 and 0.94, respectively.
The interpretation of near- and mid-infrared spectra can be difficult due to, for instance, overlapping absorbance bands, overtones, combination bands, coupling,
Fermi resonance and matrix effects. Two-dimensional correlation spectroscopy was applied to near- and mid-infrared spectra of aging bread to aid interpretation. The well-known information on starch recrystallization in aging bread was followed by mid-infrared spectroscopy and correlated to the near-infrared spectra to increase spectral interpretation. The unwanted physico-chemical changes that happen during bread aging, called staling, was quantified by bread hardness. The analysis revealed that the 1st overtone near-infrared region at 1688 nm, the long wavelength near-infrared region at 2288 nm and the mid-infrared at 1047 cm-1 exhibited the highest correlations to bread hardness. A principal component analysis (PCA) was carried out from 2150-2370 nm and 1100-850 cm-1, separately. The high correlation (r2 equal to 0.98) observed between the two 1st principal component scores indicates that near- and mid-infrared spectroscopy measure the same staling phenomena.
In conclusion, this work shows that it is possible to use a supercontinuum source for spectroscopy measurements on intact barley seeds in transmission mode in the long wavelength near-infrared region. The supercontinuum light has advantages in combination with a wavelength separating fiber. However, this application needs further experimental studies to be confirmed in its usefulness. The interpretation of near-infrared spectra can be enhanced by the correlation to other infrared spectra with a more clear interpretation.
For these reasons, the supercontinuum light stands out from the commonly applied near- and mid-infrared incandescent light bulbs. This thesis aim to explore the utility of using a supercontinuum source in two food applications. (1) The supercontinuum light was applied for the first time to barley seeds in transmission mode in the long wavelength near-infrared region from 2260-2380 nm. From these spectra it was possible to predict the β-glucan content of individual seeds. The measurements of single seeds can be used in plant breeding to sort according to trait or in grain industries to sort according to quality or property. (2) The supercontinuum light has been used with a new wavelength separation method called dispersive Fourier transformation. Different wavelengths travel at different speed through a dispersive element, which in this case is a 10.6 km long silica fiber, and the polychromatic light pulses will therefore be separated by wavelength. The signal will then be transformed from the time-domain to a frequency domain. The spectrometer has no moving parts, which makes it insensitive to mechanical vibrations. A spectrometer with a wavelength separating fiber is therefore an obvious candidate for industrial process measurements. This thesis presents preliminary results on sucrose in water and melamine in milk powder using a supercontinuum light and a dispersive fiber from 1300-1650 nm. The possibility for the new spectrometer to measure β-glucan in barley and bread hardness in aging bread was evaluated by calculating Pearson’s correlation coefficients. The spectral correlation to β-glucan and bread hardness showed r2 values equal to 0.98 and 0.94, respectively.
The interpretation of near- and mid-infrared spectra can be difficult due to, for instance, overlapping absorbance bands, overtones, combination bands, coupling,
Fermi resonance and matrix effects. Two-dimensional correlation spectroscopy was applied to near- and mid-infrared spectra of aging bread to aid interpretation. The well-known information on starch recrystallization in aging bread was followed by mid-infrared spectroscopy and correlated to the near-infrared spectra to increase spectral interpretation. The unwanted physico-chemical changes that happen during bread aging, called staling, was quantified by bread hardness. The analysis revealed that the 1st overtone near-infrared region at 1688 nm, the long wavelength near-infrared region at 2288 nm and the mid-infrared at 1047 cm-1 exhibited the highest correlations to bread hardness. A principal component analysis (PCA) was carried out from 2150-2370 nm and 1100-850 cm-1, separately. The high correlation (r2 equal to 0.98) observed between the two 1st principal component scores indicates that near- and mid-infrared spectroscopy measure the same staling phenomena.
In conclusion, this work shows that it is possible to use a supercontinuum source for spectroscopy measurements on intact barley seeds in transmission mode in the long wavelength near-infrared region. The supercontinuum light has advantages in combination with a wavelength separating fiber. However, this application needs further experimental studies to be confirmed in its usefulness. The interpretation of near-infrared spectra can be enhanced by the correlation to other infrared spectra with a more clear interpretation.
| Original language | English |
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| Publisher | Department of Food Science, Faculty of Science, University of Copenhagen |
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| Publication status | Published - 2017 |