Applied_Spectroscopy_Practica_01_01 - 37

Nakashima et al.
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due to lycopene based on the literature.11 However, its band
position is at a higher wavelength than the literature data in
organic solvents around 500 nm.23 This will be discussed
later.
A large and wide band around 670 nm is present at the
early stage of ripening and decreased at the later stage
(Figs. 1e and 3c). This band is due to chlorophylls24,25 and
will be discussed later.
Discussion
Absorption Band Positions of Chl a and Lycopene
The band at 670 nm of the mini tomato was considered to be
due to Chl a in this study, since a 670-680 nm band was generally
reported in direct spectroscopic measurements on
plants including tomatoes.6,7,21,25,26 However, the absorption
maxima of Chl a in organic solvents such as diethyl ether
and acetone-hexane are reported to be around 430, 578,
and 662 nm, with the longest one around 660 nm.1,24 This
band is reported to shift to the longer wavelength (redshift)
in the presence of charged polar components.27 The redshift
of Chl a in the plant tissue (in vivo) can be originated from
the presence of water and the binding of Chl a in the photosynthetic
complexes (photosystems).
The band at 565 nm in the mini tomato was assigned to
lycopene in this study, since this position was generally
reported in direct spectroscopic measurements on tomatoes.8,11,17,18
The absorption maxima of lycopene in organic
solvents such as n-hexane and methanol are reported at
360, 442, 468, and 499 nm, with the longest wavelength
peak around 500 nm.23,28 These bands can be redshifted by
up to about 30 nm in nonpolar solvents having larger refractive
indices for carotenoids including lycopene.23 The redshift
can also be originated from the binding of the carotenoids by
photosynthetic complexes (photosystems).23 The redshifts
by up to 40 nm of these carotenoid bands have been
reported in polar solvents.29 Therefore, the redshifted
band around 565 nm of lycopene in the plant tissue (in
vivo) can be explained by the presence of water and the binding
of lycopene in the photosynthetic complexes.
Changes with Time in Band Areas During Mini Tomato
Ripening
In Vis-NIR spectroscopic measurements of tomatoes, several
spectral analyses including chemometrics have been conducted
to obtain good indicators having good correlations to contents
of representative chemical components such as chlorophylls,
carotenoids, lycopene, sugars, and soluble solids (Brix).5-
12,17-22,25 However, in this study, it is impossible to conduct
conventional chemical analyses of the same mini tomato sample
at the different ripening stages. Therefore, the absolute
contents of chemical components cannot be evaluated. Since
the same position of the same sample was measured by the
same method, changes with time of band intensities such as
band areas of chlorophylls, carotenoids, and lycopene can provide
at least relative changes in amounts of these components.
Therefore, the following band areas were determined (Fig. 3c).
The band area around 490 nm due to carotenoids was
determined by a linear baseline from 460 to 520 nm for avoiding
overlaps with other bands in the shorter and longer wavelength
regions (Fig. 3c). The band area around 565 nm due to
lycopene was determined by a linear baseline from 545 to
600 nm for avoiding overlaps with the bands of carotenoids
and chlorophylls (Fig. 3c). The band area around 670 nm
due to chlorophylls was determined by a linear baseline
from 640 to 710 nm for avoiding overlaps with the lycopene
band (Fig. 3c). Since this narrow region corresponds to the
absorption band of Chl a with only small contributions from
chlorophyll b,24 this band is taken as the Chl a band. The
band area around 970 nm due to water and sugars was determined
by a linear baseline from 920 to 1050 nm (Fig. 3b).
The 640-710 nm band area of Chl a decreased
quasi-exponentially with time throughout the measurement
period (Fig. 4a). This Chl a decrease will be analyzed later.
The 545-600 nm band area of lycopene is almost zero at
the early stage, and increased rapidly from 36 to 44 days,
then decreased slightly but stayed relatively constant from
50 to 68 days. It increased and fluctuated at the final stage
(Fig. 4b). These changes in the lycopene band are very similar
to changes with time of a* values (green/red; Fig. 2f).
Therefore, the lycopene band is considered to control the
reddish color of the mini tomato; this band increased from
36 to 44 days and will be analyzed later.
The 460-520 nm band area of the carotenoid decreased
quasi-exponentially during the early stage until 40 days but
became fluctuated afterward (Fig. 4c).
The 920-1050 nm band due to water and sugars fluctuated
throughout the period (Fig. 4f). Since water contents
in tomatoes are very high around 95 wt%, this band area
around 970 nm corresponds to the relatively constant
water contents during the tomato ripening period. This
trend is quite different from that of Brix (Fig. 2d).
Therefore, even though this small band includes contributions
from sugars, it cannot be used as a good indicator of
sugar contents.
Chlorophyll a Decrease Rate During Mini Tomato
Ripening
The decrease of the 640-710 nm band area due to Chl a
appeared quasi-exponential with time throughout the measurement
period (Fig. 4a). This trend was fitted by the following
equation assuming the first-order reaction:
C = C0 exp (−kt) + C1
(3)
The chlorophyll decrease data were very well fitted by this
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