Postharvest Biology and Technology 18 2000 121 – 132
Light penetration properties of NIR radiation in fruit with respect to non-destructive quality assessment
Jeroen Lammertyn
a,
, Ann Peirs
a
, Josse De Baerdemaeker
b
, Bart Nicolaı¨
a
a
Katholieke Uni6ersiteit Leu6en, Flanders Centre for Posthar6est Technology, Willem de Croylaan
42
,
3001
He6erlee, Belgium
b
Katholieke Uni6ersiteit Leu6en, Department of Agrotechnique and -Economics, Kardinaal Mercierlaan
92
,
3001
He6erelee, Belgium Received 21 January 1999; accepted 11 October 1999
Abstract
Some issues related to the non-destructive measurement of apple quality attributes by means of NIR reflectance spectroscopy are addressed. A comparison was made between two optical configurations, which can be used to
perform NIR-spectroscopic measurements: the bifurcated and the 0°45° optical configuration. A relationship was established between the reflectance spectra 880 – 1650 nm and the soluble solids content by means of the partial least
squares technique. Depending on the data pre-processing method, correlation coefficients between 79 and 91 were obtained. The results obtained with the bifurcated fibre were only marginally better than those obtained with the
0°45° configuration. The apple skin reflectance and skin transmission properties with regard to NIR radiation were also investigated. The intensity of the light source was high enough to penetrate through the apple skin and gather
information about the apple parenchyma tissue. A method was developed to calculate the light penetration depth for each wavelength in the range from 500 to 1900 nm. This method was applied to measure the light penetration depth
in ‘Jonagold’ apple fruit tissue. The penetration depth is wavelength dependent: up to 4 mm in the 700 – 900 nm range and between 2 and 3 mm in the 900 – 1900 nm range. © 2000 Elsevier Science B.V. All rights reserved.
Keywords
:
Quality; PLS; Light penetration; Sugar; Optical configuration; Apple www.elsevier.comlocatepostharvbio
1. Introduction
In recent years research has been focused on the development of non-destructive techniques for
measuring quality attributes of apples such as pH, sugar content and firmness. The advantages of
these techniques include fast execution, limited sample pre-processing and easy use in process
control and grading systems. NIR-spectroscopy is one such technique, and NIR methods have al-
ready been used to detect bruises on apples e.g. Pen et al., 1985; Upchurch et al., 1994. Kawano
et al. 1992 studied the sugar content in peaches
Corresponding author. Tel.: + 32-16-322376; fax: + 32- 16-322955.
E-mail address
:
jeroen.lammertynagr.kuleuven.ac.be J. Lammertyn
0925-521400 - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 5 2 1 4 9 9 0 0 0 7 1 - X
with an optical fibre in interactance mode. Slaughter 1995 determined that visible and NIR-
spectroscopy could be used to measure non-de- structively the internal quality of peaches and
nectarines as characterised by their soluble solid SSC, sugar, sorbitol and chlorophyll contents.
Bellon-Maurel 1992 used the wavelength region between 800 – 1050 nm to built a model for sugar
measurement. Recently, Moons et al. 1997 and Lammertyn et al. 1998 established a relationship
between NIR spectra and apple fruit quality parameters such as acidity, pH, sugar content and
texture parameters.
For internal quality measurements, it is impor- tant that the NIR radiation penetrates the apple
tissue sufficiently, an issue not often discussed in the literature. Chen and Nattuvetty 1980 investi-
gated the effect of the distance between the light incident and detection points on the transmittance
and on the depth through which the detected light penetrated into the fruit. Hother et al. 1995
followed the changes in reflectance properties of unpeeled apple disks of varying thickness. They
found that, depending on the variety and the wavelength, the penetration depth varied between
0 and 7 mm. For ‘Jonagold’ apples, the maximum depth was 5.5 mm. However, the authors only
considered a wavelength range between 480 and 800 nm. It should also be noted that, even if the
radiation sufficiently penetrates the apple tissue, reflected radiation due to internal scattering needs
to be separated from that due to specular reflec- tion. Several optical configurations have been
used in the literature, including bifurcated light guides and 0°45° configurations. It is not clear
how the configuration affects the quality of the calibration models.
The objectives of this paper were i to compare two optical configurations for measuring internal
apple quality attributes by means of NIR reflec- tance measurements, ii to obtain fruit skin
parameters which describe the interaction of the skin and the incident radiation, and iii to deter-
mine the penetration depth values of NIR radia- tion in apple tissue for the wavelengths between
500 and 1900 nm.
2. Materials and methods
