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Rapid prediction of oil content of camelina seeds using near-infrared spectroscopy Ke Zhang, Zhenglin Tan, Chengci Chen, Xiuzhi Susan Sun, and Donghai Wang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b02762 • Publication Date (Web): 04 Apr 2017 Downloaded from http://pubs.acs.org on April 7, 2017
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Energy & Fuels
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Rapid prediction of camelina seeds oil content using near-infrared
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spectroscopy
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Ke Zhanga,#, Zhenglin Tana,b,#, Chengci Chenc, Xiuzhi Susan Sund, and Donghai
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Wanga*
5 a
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Department of Biological and Agricultural Engineering, Kansas State University, Manhattan,
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KS 66506, USA b
College of Tour and Hotel Management, HuBei University of Economics, Wuhan, 430205, P.R.
9 10 11
China c
Central Agricultural Research Center, Montana State University, Moccasin, MT 59462, USA d
Department of Grain Science and Industry, Kansas State University, Manhattan, KS 66506,
12 13 14
USA #
The first two authors contributed equally to this work
*Corresponding author. Telephone: 785-5322919, Fax: 785-5325825. E-mail:
[email protected].
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Abstract Camelina is a promising feedstock due to its ability to provide high quality edible oil and jet
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fuel. However, predicting its oil content currently requires time and labor intensive analysis.
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Near-infrared (NIR) spectroscopy provides a rapid, low-cost determination approach for oil seed
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characterization. The objective of this study was to develop NIR model to predict camelina oil
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content using 200 camelina seeds simples. Partial least squares regression (PLS) and principal
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component regression (PCR) were used to compare the performance of calibration models with
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full spectra range (4,000-1,000 cm-1). PLS regression showed better prediction performance than
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PCR. The optimal model provided excellent fitness with an R2 of 0.94 and root mean square of
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prediction error of 0.495%; madding the model useful in various applications, including quality
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assurance and screening. This study confirmed that the NIR method significantly reduces time
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(from 60 to 1 min) and cost (from 20 to 1 USD) required to determine camelina seed oil content.
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Last but not least, this study leverage a high-throughput, cost-effective prediction method of
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camelina oil content to facilitate plant breeding and genetics studies. Future work on the NIR
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model should focus on developing a model system to achieve rapid analysis of genomics at low
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cost to assist plant feedstock improvement.
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Keywords: Camelina; oil content; near-infrared; chemometric analysis; prediction model
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1. Introduction
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Camelina sativa (L), commonly as false flax, is an underutilized oil seed crop belonging to the
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Brassicaceae family. Camelina oil is a promising candidate for high quality edible oils because it
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contains up to 90% unsaturated fatty acid with high α-linolenic acid (18:3, omega-3) content 1. It
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also reduces cholesterol and provides resistance to stress 2. Recent studies have shown that
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camelina is a superior feedstock for biodiesel and jet fuel, specifically in fuel performance.
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Commercial airline and military fighter jet testing showed that jet fuel made from camelina oil
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has up to 80% reduction of greenhouse gas emissions compared to petroleum-based jet fuel 3.
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Camelina also offers many agronomic advantages over traditional commodity oilseed crops, such
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as low requirement for water, fertilizer, and lands; good tolerance to adverse environmental
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conditions, and high resistance to alternaria black spot and other diseases and pests 4, 5. In general,
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camelina contains 29.9% to 38.3% oil, 23% to 30% protein, 10% carbohydrates, and 6.6% ash
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based on % w/w, depending on variations in breeding conditions 6. Many camelina breeding
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programs have been established in order to improve camelina characteristics such as oil content,
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fatty acid composition, seed size, disease resistance, and yield7-10. These programs require large
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numbers of camelina seeds to be rapidly and cost-effectively screened for multiple traits, but the
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traditional wet chemical method to measure oil content is time-consuming. Near-infrared (NIR)
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spectroscopy, by contrast, provides a reliable and efficient prediction tool for food,
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pharmaceutical, and agricultural applications11-13.
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NIR is a fast approach based on spectrum absorption by molecular overtone and combination
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vibrations within a sample. NIR offers some advantages such as high-throughput, less-
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preparation, and non-destructive prediction and can be used in harsh industrial settings 14, 15.
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Previous research has shown that NIR is an effective method for motor oil classification in terms 3 ACS Paragon Plus Environment
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of base stock and viscosity-based classes 16. Azizian and Kramer 17 developed several useful FT-
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NIR models in order to classify 55 oil, fat, and oil/fat mixtures. In addition, NIR discriminate
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analysis has been conducted on edible oils 18, and many oil parameters (acidity and peroxide
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index) have been analyzed using NIR 19. Wang et al. 20 used NIR to discriminate soybean oil
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adulteration in camelina oils with an R2 of 0.992 and root mean standard error of cross validation
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(RMSECV) of 1.79 for a PLS model. Weinstock et al. 21 built an NIR model that predicted oil
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and oleic acid concentrations in individual corn kernels with a root mean square error of
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prediction (RMSEP) of 0.7% and 14%, respectively. NIR has also been successfully employed to
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model the oil contents of peanut 22, soybean seed 23, fish oil 24, and ground nuts 25.
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However, no published information exists on modeling the oil content of camelina seed using
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NIR. The objective of this research was to develop NIR models coupled with chemometrics in
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order to rapidly and cost-effectively predict camelina oil content.
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2. Materials and Methods
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2.1Materials
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The study used two hundred camelina whole seed samples from different fertility trials, crop
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rotation, and nitrogen treatment studies, provided by the Central Agricultural Research Center,
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Montana State University, Bozeman, Montana, United States. These camelina seed samples
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consisted of four cultivars planted in 2013 in unique environments in Montana with various
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fertility levels, resulting in large variations in oil contents.
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2.2 Camelina oil content
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Camelina seeds were grinded using Micro-Mill grinder (SP Scienceware, NJ, USA) to < 0.5
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mm powder and encapsulated in a plastic bag in a desiccator to homogenize moisture content.
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Oil content measurement of camelina seeds was based on AOAC 2003.5 26. In briefly, a 2.0 g
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grinded camelina seed sample was weighed in a single thickness cotton cellulose thimble (80
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mm external length by 33 mm internal diameter) and extracted in a Soxtec HT2 apparatus
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comprised of a 1045 Extraction Unit 1046 Service Unit (Tecator, Hoganas, Sweden) with 60 ml
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of hexane (boiling period: 20 min; rinsing period: 40 min). When almost all the hexane was
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collected, the extraction cup was dried for 30 min at 103 ℃. The cup was cooled to room
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temperature and weighed. Extracted oil was collected in a 2-ml centrifuge tube and stored at 7 ℃
for further use. Oil content is calculated as: =
!"#$ %&' '(&) *)"+&* ,*"-. − !"#$ %&' ,*"-. 0!1'(* ,*"-.
× 344 (3)
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2.3 NIR spectra
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An Antaris II FT-NIR analyzer (Thermo Scientific Incorporated, Madison, WI, U.S.) was
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applied to collect NIR spectra in reflectance mode. Intact camelina seeds were measured using a
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sample cup spinner combined with an integrating sphere to rapidly and accurately obtain bulk
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information by spinning the sample cup. Each spectrum was averaged with 32 accumulations at a
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resolution of 4 cm-1 in the wavelength range of 4,000–10,000 cm-1.
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2.4 Spectra treatment and chemometrics
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TQ Analyst 8.6.12 (Thermo Scientific Incorporated, Madison, WI, U.S.) was used to pretreat
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and analyze the spectra. Path length was calibrated using standard normal variate (SNV), which 5 ACS Paragon Plus Environment
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has a mean of 0, a standard deviation of 1, and multiplicative signal correction (MSC). MSC is
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another spectrum processing method by regressing a measured spectrum against a reference
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spectrum and then corrects the measured spectrum using the slope. The Savitzky-Golay (SG)
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filter was applied to a set of digital data points to increase the signal-to-noise ratio and reducing
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random noise. First and second derivatives were used as a treatment method to resolve spectra
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peak overlap and eliminate linear baseline drift. The first derivative was the rate of change of
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absorbance with respect to wavelength, whereas the second derivative corresponded to the
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curvature or concavity of the graph. First and second spectra formats were compared based on
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RMSEP, R2 and RPD of models.
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In order to eliminate bias in the subsets and carry out a calibration subset and prediction subset
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with a ratio of 3 to 1, all of 200 camelina seed samples was arranged as descending order based
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on measured value. One in every four spectra was randomly selected to the prediction subset (50),
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with remaining spectra as calibration samples (150). Both full spectra range (4,000 to 10,000 cm-
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1
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PCR methods described in the previous publications 27, 28 were applied for models building. The
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performance models were evaluated in terms of R2, RMSEP, and ratio of standard deviation of
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calculated subset (SDy) to RMSEP (RPD), calculated with the following equations:
) and reduced spectra range (7500-7000, 5600 – 5000, 4700-4250 cm-1) were used. PLS and
789:; = < 7;I =
B
C ∑?DE (ŷ? − @? )A
(2)
FG
9IJ 789:;
(3)
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where np is the number of samples in the prediction subset; yi and ŷi are the measured value and
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predicted value of the i th sample, respectively.
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The Chauvenet test was applied to remove outliers defined as points at distances greater than
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3.0 in the principal component (PC) space. Predicted residual error sum of squares (PRESS)
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diagnostic function was used to determine the number of factors necessary for calibration.
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3. Results and Discussion
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3.1 Sample statistics
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The camelina seed samples’ oil content range mean, and standard deviation as measured using
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reference methods, are summarized in Table 1. The results showed good range and distribution:
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25.9 to 37.7% oil content with a mean of 32.3% and standard deviation of 2.1% for the full set
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and a mean of 32.2% and standard deviation of 2.09% for the calibration subset. The range,
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mean, and standard deviation of the calibration subset covered those of the prediction subset,
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which was from 28.3 to 36.6% with a mean of 32.5% and standard deviation of 2.0%. The oil
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content range was consistent with published informaton1, 6. In addition, both subsets had similar
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and consistent distribution patterns for prosperous model development.
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3.2 Samples spectra
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NIR spectra of 200 camelina seed samples with the full range (4000-10000 cm-1) are
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demonstrated in Fig. 1. In general, absorption peaks were observed at 8250, 6800, 5830, 5765
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cm-1, with several small peaks between 4800 and 4550 cm-1. The first peak around 8250 cm-1
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was related to the second overtones of CH stretching vibration 24. The second peak was relatively
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large, from 7200 to 6600 cm-1, resulting from the OH stretch overtone 29. Peaks at 5830 and 5765
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cm-1 corresponded to the first overtones of CH stretching vibrations of –CH3, –CH2 and –
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HC=CH– 30. Small peaks between 4800 and 4550 cm-1 were attributed to the C=C and C–H
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stretch combination tone of cis unsaturated fatty acids (C18:1 and C18:2) 24.
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3.3 NIR model development for camelina oil content
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Principal component analysis (PCA), employed prior to NIR model development, derived 10
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PCs from the spectral data in order to analyze the relevant and interpretable structure in the data.
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In addition, five outliers were identified based on the Chauvenet test. Chauvenet test found a
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probability band, which contains all the statistically acceptable data. Although several dots in Fig.
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2 have a relative distance from the cluster, these samples were not strictly considered outliers
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because they represented real variation in industrial conditions. Both PCR and PLS were used to
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build calibration models; however, PLS consistently demonstrated better performance and
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statistic results than PCR in terms of R2 and RPD. PLS also showed more quantitative analytical
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power than PCR based on R2 and RPD (Table 2).
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The PCA score showed how each spectrum in the PLS was represented by each PC during
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model calibration. Each PC presented an independent factor from spectral variation in the data.
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Fig. 2 shows plots of PC analysis scores (PC1 versus PC2) of camelina oil content. PC1
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accounted for 98.73% of the variation explanation and described most variation in the calibration
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spectra. PC2 represented 1.06% of the variation in the spectra. The first two PCs accumulatively
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accounted for more than 99% of the variation among samples. The pattern of PC score dots
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seemed reasonably random, suggesting good representation for all camelina seed samples. In
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order to determine sufficient amounts of variation in the data and a useful spectral region, PC1
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loading was analyzed, as shown in Fig. 3. Orthogonal loading spectra showed strong loadings in
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PC1 from 7500 to 7000 cm-1 and from 5600 to 5000 cm-1. These spectral regions corresponded 8 ACS Paragon Plus Environment
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to the first overtone of OH and the CH2 overtone, both of which were representative absorption
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of fatty acid in oil 29.
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Spectra processes including SNV, MSC, SG, and derivatives were applied to optimize the
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models. The optimal model was developed by comparing the performance of every combination
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of derivative treatments, spectra processes, and quantitative methods 31. SNV enhanced the
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performance of prediction compared to MSC and SG, suggesting that SNV resolved the
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accentuation problems caused by the light scattering of a slightly uniform camelina seeds pile.
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First and second derivatives were used to further improve the model, and their performances are
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summarized in Table 2. First derivative significantly enhance the performance of calibration and
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prediction models, but no improvement was found for the second derivative, possibly because
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the second derivative brought more false information from mathematical artifacts when
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identifying smaller absorption signals 32. A reduced spectral range was also applied in order to
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enhance the goodness-of-fit of models.
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In Fig. 4, the mean spectrum (red line) shows the average of 200 camelina seed spectra. The
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variance spectrum (blue line), which shows variance in all spectra, was calculated from the
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square root of the spectral variance for each oil content across all spectra. The mean and variance
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spectra of 200 camelina seed samples revealed a similar trend, suggesting that the calibration
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model could be robust. The regression coefficient line as function of wavelength (black line)
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indicates wavelengths with large weights in the calibration model. In order to improve the model,
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three wavelength ranges (7500-7000, 5600-5000, 4700-4250 cm-1) were selected resulting from
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their high coefficient and high variance, which indicate high correlation and large variation. The
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reduced range demonstrated better goodness-of-fit for both calibration and prediction models
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than the full range models, with an R2 of 0.97, RMSEP of 0.495%, and RPD of 4 for the
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prediction sample subset, suggesting that this model provides an accurate prediction and could be
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used in most applications 33. The reduced wavelength region correlated best with samples’ oil
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content with reduced random noise. Previous studies have also reported that similar wavelength
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regions highly corresponded to the oil instauration index and cis/trans ratio of unsaturated fatty
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acids 34, 35.
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PRESS function was used to avoid overfitting the models. In Fig. 5, the R2 value increased
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sharply at three factors, peaked gradually at five factors, and then leveled off to a constant value.
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Fig. 6 plots chemical reference data against NIR-predicted values in the calibration and
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prediction subsets of the final model. This plot illustrates the relationship between the chemical
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data and NIR calibration models as well as the presence of outlier samples, suggesting that the
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optimal model was prosperous and did the terrific job on the prediction of camelina seed oil
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content. The conventional method to measure the oil content of oil seeds requires grinding the
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sample and extracting oil using chemical solvent, which is time consuming and labor intensive.
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Our NIR method, by scanning whole oil seeds using spectroscopy, significantly reduced the time
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(from an estimated 60 to 1 min) and cost (from an estimated 20 to 1 USD)to determine camelina
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oil content.
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4. Conclusion
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This study successfully developed models to predict camelina seeds’ oil content. The optimal
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model was achieved by PLS quantitative analysis, SNV, and first derivative with a prediction R2
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of 0.94, RMSEP of 0.495%, and RPD of 4. NIR models could improve the speed of oil content
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determination and can assist in crop management with increased efficiency.
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Acknowledgment 10 ACS Paragon Plus Environment
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This work was supported by Biomass Research and Development Initiative Program with grant
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number of 2012-10006-20230 from the U.S. Department of Agriculture National Institute of
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Food and Agriculture.
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Table 1. Oil content of 200 camelina seed samples measured using reference method.
211 Full set
Oil content (%)
Calibration subset
Prediction subset
Range
Mean
SD
Range
Mean
SD
Range
Mean
SD
25.9-37.7
32.3
2.1
25.9-37.7
32.2
2.1
28.3-36.6
32.5
2.0
212 213 214
Table 2. NIR calibration and prediction statistics for camelina oil content prediction. Full range (10000 - 4000cm-1)
Reduced range (7500-7000, 5600 – 5000, 4700-4250 cm-1)
Raw spectra
First derivative
Second derivative
Raw spectra
First derivative
Second derivative
0.83 0.80 0.886 2.23 2
0.89 0.88 0.668 2.96 4
0.79 0.67 1.19 1.66 1
0.93 0.91 0.606 3.27 5
0.97 0.94 0.495 4 5
0.87 0.81 0.952 2.08 4
0.79 0.67 1.21 1.63 2
0.78 0.68 1.17 1.69 10
0.67 0.67 1.15 1.72 2
0.81 0.85 0.732 2.71 2
0.83 0.88 0.666 2.97 2
0.77 0.74 0.974 2.03 2
PLS R2, cal. R2,val. RMSEP RPD Factor used
PCR R2, cal. R2,val. RMSEP RPD PC used
215 216
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Fig. 1. NIR spectra of 200 camelina seed samples.
219 220 221
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0.4
0.2
PC2 Score x 10 (1.06%)
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0.0
-0.2
-0.4
-0.6
-0.8 -0.10
-0.05
0.00
0.05
0.10
PC1 Score (98.73%) 222 223
Fig. 2. Principal component analysis scores of camelina seed sample oil content for PLS model.
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0.4
PC1 loading 0.3
0.2
Absorbance
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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0.1
0.0
-0.1
-0.2
-0.3 10000
9000
8000
7000
6000
5000
4000
-1
Wavenumbers (cm ) 225 226
Fig. 3. Loading for the first principal component of camelina seeds sample oil content.
227 228
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0.74
Regression coefficient spectra Mean spectra Variance spectra
0.72
1.4
0.14
1.2
0.12
1.0
0.70
0.8 0.68
Absorbance
Regression coefficient
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0.10
0.08
0.6 0.66
0.06 0.4
0.64
10000
0.04 9000
8000
7000
6000
5000
0.2 4000
-1
229 230
Wavenumbers (cm ) Fig. 4. Regression coefficient, mean, and variance spectra of oil content in PLS model.
231 232
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PRESS R2
1.0 500 0.9
400
0.8
0.7 300
R2
PRESS
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0.6 200 0.5 100
0.4 0
2
4
6
8
10
Factor 233 234
Fig. 5. PRESS and R2 vs. calculated factors in the prediction model.
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Calculation Prediction
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Calculated value (%)
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34
32
30
28
26 26
28
30
32
34
36
38
Measured value (%) 237 238 239 240
Fig. 6. Comparison of calculated versus measured camelina seed sample oil contents.
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