Typical NIR reflectance spectra of some fruit. The NIR reflectance spectra were recorded using a Corona 45 Vis/NIR diode array spectrophotometer (Carl Zeiss Jena Gmbh, Jena, Germany) as described in Nicolaı et al. (2006a). 

Contexts in source publication

Context 1

... infrared (NIR) radiation covers the range of the elec- Near infrared radiation was discovered by Friedrich Wilhelm tromagnetic spectrum between 780 and 2500 nm. In NIR Herschel in 1800 ( Davies, 2000, Davies, 2000) and covers by definition the spectroscopy, the product is irradiated with NIR radiation, and wavelength range from 780 to 2500 nm. When radiation hits the reflected or transmitted radiation is measured. While the radi- a sample, the incident radiation may be reflected, absorbed or ation penetrates the product, its spectral characteristics change transmitted, and the relative contribution of each phenomenon through wavelength dependent scattering and absorption pro- depends on the chemical constitution and physical parameters cesses. This change depends on the chemical composition of of the sample. the product, as well as on its light scattering properties which Reflection is due to three different phenomena. Specular are related to the microstructure. Advanced multivariate sta- reflection causes gloss, whereas external diffuse reflection is tistical techniques, such as partial least squares regression are induced by rough surfaces. Both only provide information about then applied to extract the required information from the usually the surface of the sample. Scattering results from multiple refrac- convoluted spectra. tions at phase changes inside the material. The main scattering NIR spectroscopy was first used in agricultural applications elements in fruit and vegetables are the cell wall interfaces since by Norris (1964) to measure moisture in grain. Since then it has they induce abrupt changes in refractive index (McGlone et al., 1997, McGlone et al., 1997), been used for rapid analysis of mainly moisture, protein and but suspended particles, such as starch granules, chloro- fat content of a wide variety of agricultural and food products plasts and mitochondria may also induce scattering caused by (Davies and Grant, 1987; Gunasekaran and Irudayaraj, 2001). diffraction at the particle surface where the refractive index is Early applications in horticulture focussed on dry matter con- different from that of the surroundings (Il’yasov and Krasnikov, 1991, Il’yasov and Krasnikov, 1991). tent of onions (Birth et al., 1985), soluble solids content (SSC) The scattering is also dependent on the size, the shape of apples (Bellon-Maurel, 1992) and water content of mush- and microstructure of the particles. Scattering may also appear rooms (Roy et al., 1993), but since then many other applications due to heterogeneities, such as pores, openings, capillaries that have followed. As the propagation of NIR radiation in fruit and are randomly distributed through the sample. Multiple scat- vegetable tissue is affected by their microstructure, it was soon tering events largely determine the intensity of the scattered discovered that NIR spectroscopy could also be used to measure light that is emitted (McGlone et al., 1997, McGlone et al., 1997). The scattering pro- microstructure-related attributes, such as stiffness (Lammertyn et al., 1998), cess affects the intensity level of the reflected spectrum rather internal damage (Clark et al., 2003a,b), and even than the shape; the latter is more related to the absorption sensory attributes (Mehinagic et al., 2004). Recent developments process. which extend the potential of NIR spectroscopy further include Most absorption bands in the near infrared region are overtone multi- and hyperspectral imaging techniques which also provide or combination bands of the fundamental absorption bands in the spatial information (Martinsen and Schaare, 1998; Lu, 2003) infrared region of the electromagnetic spectrum which are due and time-resolved spectroscopy which allows measurement of to vibrational and rotational transitions. In large molecules and absorption and scattering processes separately (Cubeddu et al., 2001). in complex mixtures, such as foods, the multiple bands and the effect of peak-broadening result in NIR spectra that have a broad The increasing importance of NIR spectroscopy in posthar- envelope with few sharp peaks. In Fig. 1, typical NIR reflectance vest technology is obvious from the recent increase in numbers spectra of some fruit are shown (see also Sharpe and Barber, 1972, Sharpe and Barber, 1972 of publications, as well as from the fact that many manufacturers for other typical spectra). The spectra are clearly very sim- of on-line grading lines have now implemented NIR systems to ilar and are dominated by the water spectrum with overtone measure various quality attributes. The objective of this review bands of the OH-bonds at 760, 970 and 1450 nm and a combina- is to give a comprehensive overview of NIR spectroscopy for tion band at 1940 nm (Polessello and Giangiacomo, 1981, Polessello and Giangiacomo, 1981). This measuring quality attributes of fruit and vegetables. We will pay similarity is the reason why sophisticated multivariate statistical attention to optics and chemometrics as well as applications, and will try to identify areas where more research is required. Near infrared radiation was discovered by Friedrich Wilhelm Herschel in 1800 (Davies, 2000) and covers by definition the wavelength range from 780 to 2500 nm. When radiation hits a sample, the incident radiation may be reflected, absorbed or transmitted, and the relative contribution of each phenomenon depends on the chemical constitution and physical parameters of the sample. Reflection is due to three different phenomena. Specular reflection causes gloss, whereas external diffuse reflection is induced by rough surfaces. Both only provide information about the surface of the sample. Scattering results from multiple refrac- tions at phase changes inside the material. The main scattering elements in fruit and vegetables are the cell wall interfaces since they induce abrupt changes in refractive index (McGlone et al., 1997), but suspended particles, such as starch granules, chloro- plasts and mitochondria may also induce scattering caused by diffraction at the particle surface where the refractive index is different from that of the surroundings (Il’yasov and Krasnikov, 1991). The scattering is also dependent on the size, the shape and microstructure of the particles. Scattering may also appear due to heterogeneities, such as pores, openings, capillaries that are randomly distributed through the sample. Multiple scattering events largely determine the intensity of the scattered light that is emitted (McGlone et al., 1997). The scattering process affects the intensity level of the reflected spectrum rather than the shape; the latter is more related to the absorption process. Most absorption bands in the near infrared region are overtone or combination bands of the fundamental absorption bands in the infrared region of the electromagnetic spectrum which are due to vibrational and rotational transitions. In large molecules and in complex mixtures, such as foods, the multiple bands and the effect of peak-broadening result in NIR spectra that have a broad envelope with few sharp peaks. In Fig. 1, typical NIR reflectance spectra of some fruit are shown (see also Sharpe and Barber, 1972 for other typical spectra). The spectra are clearly very similar and are dominated by the water spectrum with overtone bands of the OH-bonds at 760, 970 and 1450 nm and a combination band at 1940 nm (Polessello and Giangiacomo, 1981). This similarity is the reason why sophisticated multivariate statistical techniques are essential to extract useful information from an NIR spectrum. An NIR spectrophotometer consists of a light source (usually a tungsten halogen light bulb), sample presentation acces- sory, monochromator, detector, and optical components, such as lenses, collimators, beam splitters, integrating spheres and optical fibers. Spectrophotometers are conveniently classified according to the type of monochromator. In a filter instrument, the monochromator is a wheel holding a number of absorption or interference filters. Its spectral resolution is limited. In a scanning monochromator instrument a grating or a prism is used to separate the individual frequencies of the radiation either entering or leaving the sample. The wavelength separator rotates so that the radiation of the individual wavelengths subsequently reaches the detector. Fourier transform spectrophotometers use an interferometer to generate modulated light; the time domain signal of the light reflected or transmitted by the sample onto the sample can be converted into a spectrum via a fast Fourier transform. Often a Michelson interferometer is used, but also polarisation interferometers are used in some spectrophotometers. In photodiode array (PDA) spectrophotometers, a fixed grating focuses the dispersed radiation onto an array of silicon (350–1100 nm) or InGaAs (Indium Gallium Arsenide, 1100–2500 nm) photodiode detectors. Laser based systems do not contain a monochromator but have different laser light sources or a tunable laser. Acoustic optic tunable filter (AOTF) instruments use a diffraction ...

Context 2

... different from that of the surroundings (Il’yasov and Krasnikov, 1991, Il’yasov and Krasnikov, 1991). tent of onions (Birth et al., 1985), soluble solids content (SSC) The scattering is also dependent on the size, the shape of apples (Bellon-Maurel, 1992) and water content of mush- and microstructure of the particles. Scattering may also appear rooms (Roy et al., 1993), but since then many other applications due to heterogeneities, such as pores, openings, capillaries that have followed. As the propagation of NIR radiation in fruit and are randomly distributed through the sample. Multiple scat- vegetable tissue is affected by their microstructure, it was soon tering events largely determine the intensity of the scattered discovered that NIR spectroscopy could also be used to measure light that is emitted (McGlone et al., 1997, McGlone et al., 1997). The scattering pro- microstructure-related attributes, such as stiffness (Lammertyn et al., 1998), cess affects the intensity level of the reflected spectrum rather internal damage (Clark et al., 2003a,b), and even than the shape; the latter is more related to the absorption sensory attributes (Mehinagic et al., 2004). Recent developments process. which extend the potential of NIR spectroscopy further include Most absorption bands in the near infrared region are overtone multi- and hyperspectral imaging techniques which also provide or combination bands of the fundamental absorption bands in the spatial information (Martinsen and Schaare, 1998; Lu, 2003) infrared region of the electromagnetic spectrum which are due and time-resolved spectroscopy which allows measurement of to vibrational and rotational transitions. In large molecules and absorption and scattering processes separately (Cubeddu et al., 2001). in complex mixtures, such as foods, the multiple bands and the effect of peak-broadening result in NIR spectra that have a broad The increasing importance of NIR spectroscopy in posthar- envelope with few sharp peaks. In Fig. 1, typical NIR reflectance vest technology is obvious from the recent increase in numbers spectra of some fruit are shown (see also Sharpe and Barber, 1972, Sharpe and Barber, 1972 of publications, as well as from the fact that many manufacturers for other typical spectra). The spectra are clearly very sim- of on-line grading lines have now implemented NIR systems to ilar and are dominated by the water spectrum with overtone measure various quality attributes. The objective of this review bands of the OH-bonds at 760, 970 and 1450 nm and a combina- is to give a comprehensive overview of NIR spectroscopy for tion band at 1940 nm (Polessello and Giangiacomo, 1981, Polessello and Giangiacomo, 1981). This measuring quality attributes of fruit and vegetables. We will pay similarity is the reason why sophisticated multivariate statistical attention to optics and chemometrics as well as applications, and will try to identify areas where more research is required. Near infrared radiation was discovered by Friedrich Wilhelm Herschel in 1800 (Davies, 2000) and covers by definition the wavelength range from 780 to 2500 nm. When radiation hits a sample, the incident radiation may be reflected, absorbed or transmitted, and the relative contribution of each phenomenon depends on the chemical constitution and physical parameters of the sample. Reflection is due to three different phenomena. Specular reflection causes gloss, whereas external diffuse reflection is induced by rough surfaces. Both only provide information about the surface of the sample. Scattering results from multiple refrac- tions at phase changes inside the material. The main scattering elements in fruit and vegetables are the cell wall interfaces since they induce abrupt changes in refractive index (McGlone et al., 1997), but suspended particles, such as starch granules, chloro- plasts and mitochondria may also induce scattering caused by diffraction at the particle surface where the refractive index is different from that of the surroundings (Il’yasov and Krasnikov, 1991). The scattering is also dependent on the size, the shape and microstructure of the particles. Scattering may also appear due to heterogeneities, such as pores, openings, capillaries that are randomly distributed through the sample. Multiple scattering events largely determine the intensity of the scattered light that is emitted (McGlone et al., 1997). The scattering process affects the intensity level of the reflected spectrum rather than the shape; the latter is more related to the absorption process. Most absorption bands in the near infrared region are overtone or combination bands of the fundamental absorption bands in the infrared region of the electromagnetic spectrum which are due to vibrational and rotational transitions. In large molecules and in complex mixtures, such as foods, the multiple bands and the effect of peak-broadening result in NIR spectra that have a broad envelope with few sharp peaks. In Fig. 1, typical NIR reflectance spectra of some fruit are shown (see also Sharpe and Barber, 1972 for other typical spectra). The spectra are clearly very similar and are dominated by the water spectrum with overtone bands of the OH-bonds at 760, 970 and 1450 nm and a combination band at 1940 nm (Polessello and Giangiacomo, 1981). This similarity is the reason why sophisticated multivariate statistical techniques are essential to extract useful information from an NIR spectrum. An NIR spectrophotometer consists of a light source (usually a tungsten halogen light bulb), sample presentation acces- sory, monochromator, detector, and optical components, such as lenses, collimators, beam splitters, integrating spheres and optical fibers. Spectrophotometers are conveniently classified according to the type of monochromator. In a filter instrument, the monochromator is a wheel holding a number of absorption or interference filters. Its spectral resolution is limited. In a scanning monochromator instrument a grating or a prism is used to separate the individual frequencies of the radiation either entering or leaving the sample. The wavelength separator rotates so that the radiation of the individual wavelengths subsequently reaches the detector. Fourier transform spectrophotometers use an interferometer to generate modulated light; the time domain signal of the light reflected or transmitted by the sample onto the sample can be converted into a spectrum via a fast Fourier transform. Often a Michelson interferometer is used, but also polarisation interferometers are used in some spectrophotometers. In photodiode array (PDA) spectrophotometers, a fixed grating focuses the dispersed radiation onto an array of silicon (350–1100 nm) or InGaAs (Indium Gallium Arsenide, 1100–2500 nm) photodiode detectors. Laser based systems do not contain a monochromator but have different laser light sources or a tunable laser. Acoustic optic tunable filter (AOTF) instruments use a diffraction based optical-band-pass filter that can be rapidly tuned to pass various wavelengths of light by varying the frequency of an acoustic wave propagating through an anisotropic crystal medium. Finally, liquid crystal tunable filter (LCTF) instruments use a birefringent filter to create con- structive and destructive interference based on the retardation, in phase between the ordinary and extraordinary light rays passing through a liquid crystal. In this way they act as an interference filter to pass a single wavelength of light. By combining several electronically tunable stages in series, high spectral resolution can be achieved (Stratis et al., 2001). There is definitely a shift towards PDA systems because their high acquisition speed (the integration time is typically 50 ms but can be as low as a few milliseconds) and the absence of moving parts enables them to be mounted on online fruit grading lines. Miniaturised versions are available from companies, such as Ocean Optics (Dunedin, FL, USA), Zeiss (Jena, Germany), Oriel (Stratford, CT, USA), and Integrated Spectronics (Baulkham Hills, Australia). Guidelines for selecting an appropriate spectrophotometer are given by Walsh et al. (2000). Optical instruments should be handled with care. Manipula- tion may break optical fibers and decrease the signal to noise ratio. Excessive bending of a fiber optic cable may affect its spectral response. Dust is to be avoided by all means, and it is important to keep the white reference clean for obvious reasons. The light source should be switched on well in advance because the spectral characteristics change during the warming up period. The integration time for PDA detectors should ide- ally be sufficient to obtain a detector response of about 50% of saturation. Three different measurement setups for obtaining near infrared spectra are shown in Fig. 2. In reflectance mode (Fig. 2a), light source and detector are mounted under a specific angle, e.g., 45 ◦ , to avoid specular reflection. In transmittance mode the light source is positioned opposite to the detector, while in interactance mode the light source and detector are positioned parallel to each other in such a way that light due to specular reflection cannot directly enter the detector. This can be achieved by means of a bifurcated cable in which fibers leading to the source and detector are parallel to each other and in contact with the product, or by means of a special optical arrangement (e.g., Greensill and Walsh, 2000a; McGlone et al., 2002a). In both reflectance and transmittance mode integrated spheres may also be used to collect light and increase the signal to noise ratio. In selecting the measurement setup, it is important to know that the penetration of NIR radiation into fruit tissue decreases exponentially with the depth (Lammertyn et al., 2000; Fraser et al., 2000; Greensill and Walsh, 2000a). Lammertyn et al. (2000) found a penetration depth of up to 4 mm in the 700–900 nm range and ...