VERTICAL FARMING Leafy vegetables shelf life - NIREOS
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HERA IPERSPETTRALE - APPLICATION NOTE VERTICAL FARMING
Leafy vegetables shelf life
The monitoring of the degradation process of leafy vegetables enables
the determination of their shelf life: a crucial characteristic to be studied
for food safety.
Through the use of the HERA hyperspectral camera it’s possible to
detect senescence or illness signs in the sample’s spectra in advance
with respect to visual monitoring of the plant. Interestingly, information
about the chemical and nutrient composition of the sample can be
derived from the spectral read-out, such as chlorophyll, anthocyanin
and phenol concentration. Samples for this study were provided by
Agricola Moderna, a vertical farm based in Milan.
These features can be obtained by using both reflectance and
fluorescence hyper spectral measurements.
ADVANTAGES OF USING HERA
• Staring technique (ideal for static
objects)
• High sensitivity
• Fluorescence measurements
• High spatial and spectral resolution
• Plug & Play
• Non destructive measurements
HERA IPERSPETTRALE - APPLICATION NOTE VERTICAL FARMING
Leafy vegetables shelf life
EXPERIMENTAL SETUP
The experimental setup is shown in Figure 1.
The HERA camera is set on a tripod placed
above the sample and the light source is
composed of two 20 W halogen lamps.
The HERA is connected via USB to a computer,
in which the acquisition software enables the
user to set measurement parameters such as
integration time, number of averages, and
spectral resolution.
The focus and the iris aperture can be finely
adjusted manually by using two sliders directly
on the camera.
Figure 1: Experimental setup for shelf
life monitoring.
RESULTS
The degradation over time of mustard wasabina leaves was analyzed. During the
overall measurements period (19 days post-harvest), the samples were kept in a
box and stored in the refrigerator at 4°C. The experimental conditions as described
in the previous paragraph were maintained constant throughout all the senescence
process.
1
0.8
Intensity [a.u.]
0.6 Day 1
Day 12
0.4 Day 19
0.2
0
500 600 700 800 900 1000
Wavelengths [nm]
Figure 2: Reflectance spectra referring to a leaf in different days after harvesting.
HERA IPERSPETTRALE - APPLICATION NOTE VERTICAL FARMING
Leafy vegetables shelf life
The spectra in Figure 2 are referred to the 1st, 12th and 19th day after harvesting.
Senescence-related changes in the spectra are clearly visible. Firstly, the
growth of the peak between 500 nm and 700 nm relates to the leaves’ hue shifting
from green to yellow, as visually confirmed.
Secondly, it’s possible to observe that the absorption in the infrared region around
970 nm with respect to the plateau observed in the range 825 - 875 nm diminishes
with time. Since one of the absorption peaks of water resides in that region this is
most likely the sign that the leaf undergoes a process of dehydration during
degradation, as it is actually confirmed by visual inspection of the sample (Figure
4).
To confirm this trend, a NIR index has been developed (Equation 1) comparing the
intensity of the spectrum at the plateau and the absorption around 970 nm. The
results confirm the reduced absorption around 970 nm (Figure 3).
ρ[825−875nm]
NIRindex =
ρ[960−1000nm]
Equation 1: NIR index formula.
1.14
1.13
Day 1
1.12 Day 5
Day 12
Day 16
1.11 Day 19
NIR index
1.1
1.09
1.08
1.07
1 5 12 16 19
Days after harvesting
Figure 3: NIR absorption index.
Day 1 Day 12 Day 19
Figure 4: RGB reconstruction of the monitored leaves.
The colored squares refer to the areas analyzed for the NIR index calculation in Figure 3.
HERA IPERSPETTRALE - APPLICATION NOTE VERTICAL FARMING
Leafy vegetables shelf life
FLUORESCENCE
HYPERSPECTRAL IMAGING
Thanks to its high sensitivity, it is very
easy to measure fluorescence
hyperspectral images with the HERA
camera. The same samples used for
reflectance measurements were
excited via a 20 W blue LED at 400
nm and a long pass filter was
employed in detection to cut off the
excitation light (experimental setup is
shown in Figure 5). Fluorescence
spectra related to the 1st, 12th and
1 9 th d a y a fte r h a r v e s ti n g a r e
displayed hereafter (Figure 6). Figure 1: Experimental setup for fluorescence
measurements.
The principal peak at around 750 nm is related to the presence of chlorophylls in
the leaf and, clearly, its degradation over time is detected. Instead, the secondary,
smaller peak, is likely connected to phenols concentration. Phenolic compounds
are responsible for the antioxidant properties of lettuce; however, their production
is also connected to plant stress and senescence. Indeed, as displayed in Figure
6, the phenolic peak increases over time as the sample in consideration degrades.
Chlorophylls-related
Peak
Day 1
Day 12
Intensity [a.u.]
Day 19
Phenols-related
Peak
300 400 500 600 700 800 900 1000 1100
Wavelengths [nm]
Figure 6: Fluorescence spectra of the leaf in different days after harvesting.
HERA IPERSPETTRALE - APPLICATION NOTE VERTICAL FARMING
Leafy vegetables shelf life
The results regarding the diminished chlorophyll concentration over time
obtained with fluorescence hyperspectral measurements were also validated
through analysis of the reflectance data. An index correlated to the chlorophyll
concentration was developed (Equation 2) and a reconstruction, based on this
index, of the analyzed leaves is displayed hereafter (Figure 7). As expected by
fluorescence measurements, the chlorophyll-related index shows that the
concentration of this molecule drops as the leaves degradate.
ρ[660−680nm]
Chlindex = 1 −
ρ[760−800nm]
Equation 2: Chlorophyll index formula.
1
1 1
1 1
1
0.95 0.95 0.95
50
0.9
0.9
50
0.9
0.9 50 0.9
0.9
100 0.85 0.85 0.85
0.8 0.8 0.8
100
0.8 0.8 100 0.8
150
0.75 0.75 0.75
150
200 0.7
0.7
0.7
0.7 150 0.7
0.7
0.65 0.65 0.65
200
250
0.6
0.6
0.6
0.6
200 0.6
0.6
300
Day 1 0.55 250 Day 12 0.55
Day 19 0.55
20 40 60 80 100 120 140 160 180 200
0.5
220
0.5
20 40 60
0.5
80 100 120 140 160 180 200 220
0.5
20 40 60 80 100 120 140 160
0.5
0.5
Figure 7: Chlorophyll index calculated over the monitored leaves.
More info on Agricola Moderna at https://agricolamoderna.com/
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