nondestructive determination of sugar content in … · determination of sugar content in potato...
TRANSCRIPT
(Received 15 Sep. 2009: accepted 1 Feb. 2010)
†Fax: 018-872-1676,E-mail: [email protected]
1.Introduction
Potatoes are used in the food industry to produce food
products such as potato flakes, potato chips, and French
fries. In each instance, potatoes are presorted before
being introduced to the processing line. Consequently,
the composition of the potato must be measured for vari-
ables such as dry matter, carbohydrate, and reducing
sugar content (fructose and glucose) prior to determin-
ing its correct use, handling, and processing [1-3]. In
the fried products industry, the determination of the
reducing sugars content is par ticularly impor tant
because the presence of high levels of reducing sugars
not only causes browning, but also generates acrylic
amide by the Maillard reaction with asparagines during
the high temperature heating process. Acrylic amide is
believed to be carcinogenic and poisonous to the ner-
vous system and is therefore a highly undesirable attri-
bute for consumers.
Potato tubers are normally stored at low temperatures
to prevent sprouting and weight loss; however, during
storage sugar content increases. Freshly har vested
tubers normally have sugars in trace amounts. Very high
sugar levels may be reached when stored at low tempera-
tures. Removal of these potatoes with high reducing
sugar content is necessary in the fried food industry.
The reducing sugar content of potatoes is generally
determined by very accurate chemical methods, such as
high performance liquid chromatography (HPLC). These
methods are performed off-line, are time consuming and
usually require trained personnel for their execution.
Additionally, these methods destroy the material that is
being analyzed. Due to these reasons, chemical methods
are generally not feasible for on-line measurement appli-
cations in the food industry. Therefore, a rapid method for
determining the amounts of sugars in intact tubers would
be useful for monitoring sugar levels during extended
storage or for testing of shipments prior to processing.
Near infrared (NIR) spectroscopy offers a rapid and
nondestructive alternative to chemical measurement
techniques to evaluate compounds in food. NIR spectros-
copy has been used to nondestructively estimate the spe-
cific gravity of potato tubers [4] and dry matter in pota-
toes [5], as well as the main ingredients including the
carbohydrate content in potatoes [6-9]. In contrast, there
are no reports describing nondestructive measurement
of the sugar content in intact potatoes; however, a
destructive NIR analysis of the sugar content of potato
slices was reported [10,11]. The objective of this study
was to ascertain whether NIR spectroscopy could be
used to determine the reducing sugar content of intact
potato tubers.
2.Materials and methods
2.1 Experimental sample preparation
Potatoes of one cultivar (May-Queen) were purchased
Nondestructive Determination of Sugar Content in Potato Tubers Using Visible and Near Infrared Spectroscopy
Jie Yu CHEN1,†, Han ZHANG1, Yelian MIAO2, and Mitsunaka ASAKURA1
1Faculty of Bioresource Sciences, Akita Prefectural University 241-438 Kaidobata-Nishi, Shimoshinjo-Nakano Akita-shi, Akita 010-0195, Japan
2College of Food and Light Industrial Engineering, Nanjing University of Technology, Nanjing 210009, China
Near infrared (NIR) spectroscopy was investigated as a method for the nondestr uctive measurement of sugar content in intact potato tubers. The NIR spectra (400-1100 nm) of potato samples were acquired by fiber optics in the interactance mode. Calibration models for the prediction of the fructose and glucose contents were developed by a partial least squares statistical analysis method. The calibration model gave standard errors of prediction of 0.26 mg/g for fructose and 0.46 mg/g for glucose. The preliminary results showed that NIR spectroscopy gives a reasonable estimate that can be used for nondestructive sorting of potatoes according to their sugar content.Key words: nondestructive determination, sugar, potato, near infrared spectroscopy, interactance
◇◇◇ Original Paper ◇◇◇
Japan Journal of Food Engineering, Vol. 11, No. 1, pp. 59 - 64, Mar. 2010
Jie Yu CHEN, Han ZHANG, Yelian MIAO and Mitsunaka ASAKURA60
on the 7th of September, 2008 from the Akita fruits and
vegetables distribution market and were used as the
experimental samples. The potato samples were grown at
locations around Hokkaido, Japan. A total of 100 potato
samples were used. Potato samples were individually
separated into two groups. To provide a wide range of
sugar concentrations in the sample groups, one group
was stored at 5℃ for 4 months, and the other group was
stored at 25℃ for 4 months. At biweekly intervals, five
tubers were removed from each storage room for testing.
2.2 Spectra acquisition
NIR spectra were measured on all intact potato sam-
ples using a spectrophotometer (NIRSystems Model
6500) equipped with a fiber optics interactance probe as
shown in Fig. 1. The fiber optic probe consisted of a cen-
tral bundle (diameter: 7.6 mm) of Schott glass fibers and
a concentric ring of the fibers (outside diameter: 19 mm,
width: 0.64mm). The ring was separated from the central
bundle by a 5 mm thick metal barrier. To obtain the opti-
cal spectrum, an intact potato is placed on the optical
probe and the cover of the sample-housing is closed.
Monochromatic light is emitted by the ring, and "inter-
acts" with the tissue. Some of the non-absorbed light is
internally reflected to the central bundle. NIR spectra in
the wavelength range of 400-1100 nm at 2 nm intervals
were collected as absorbance measurements. A 2.5-mm
thick white ceramic plate was used as a standard optical
reference. Each spectrum was an average of 32 scans
and one spectrum was obtained per potato sample. Prior
to spectral measurements, each sample was placed in the
measurement room (25℃) for 5 hours.
Raw spectra obtained from three differently sized pota-
toes are shown in Fig. 2. The spectra clearly shift accord-
ing to the sample size. To reduce the sample-size effect,
we used the second derivative spectra. In addition, the
second derivative spectra in the 600–1100 nm wavelength
range showed higher resolution than the spectra in the
400–600 nm wavelength range. Thus, second derivative
spectra were used to estimate sugar content of stored
potatoes in this study.
2.3 Chemical analysis
After NIR spectra were measured, flesh of the potato
tuber samples was cut and mashed by a mixer, the mate-
rial was wrapped in gauze, and then the juice was
extracted by squeezing. This juice was used for sugar
analysis. The sugar compositions (fructose, glucose, sor-
bitol, sucrose) of the potato juices were analyzed using
an HPLC (Shimadzu, SCL-10A VP) method with a refrac-
tive index detector (Shimadzu, RID-10A). Sample mix-
tures were separated on a Shodex Asahipak NH2P-50 4A
column using an acetonitrile/water (3:1 v/v) solution as
the eluate. The operating conditions were as follows: flow
rate of 1 mL/min; column temperature of 40℃; injection
sample volume was 10μL. The raw potato juice samples
were centrifuged at 3000 rpm for 30 min, diluted 1:5 by
adding distilled water and subsequently passed through
a 0.45-μm pore-size filter.
2.4 Statistical Analysis
Data analyses were carried out using "Unscrambler"
(CAMO, Oslo, Norway) and Excel software (Microsoft
Office 2003). Second derivative spectra (Savitzky-Golay
algorithm, left and right averaging of 20 nm and 2nd
order polynomial) were used. The partial least square
(PLS) regression was used to develop calibrations for
analyzing the sugar compositions of intact potato tuber
samples. Calibrations were performed on glucose and
fructose. PLS regression was performed in the wave-
Fig. 1 The configuration of the interactance probe. Fig. 2 Raw visible and NIR spectra of three dif ferently sized tubers.
Potato
From light source
To detector
A A
A-A To detector
From light source
Determination of Sugar Content in Potato Tubers 61
length range of 600 to 1100 nm where the absorption
bands were assigned to the fourth, third, and second
overtones of O-H and C-H fundamental bands or their
combinations. Validation was performed by leave-one-
out full cross validation.
3.Results and discussion
3.1 Variations in the sugar content of potato
tubers during storage
Figure 3 shows the variations in the contents of glu-
cose and fructose of potato tubers stored at either 5 or
25℃, as measured by chemical methods. The sugar con-
tent values shown are averages of five potato tubers. In
tubers stored at 5℃, the glucose contents of the potato
tuber samples gradually increased after 1 week of stor-
age and continued to increase until 8 weeks of storage.
The sugar contents of the potato tuber samples
decreased after the 8th week of storage, suggesting that
the potato tubers had begun to germinate. Fructose lev-
els in the potato tuber samples also increased until the
8th week of storage. For potato tubers stored at 25℃,
there were no clear variations in the glucose and fruc-
tose contents at the beginning of storage, but clear
decreases were observed after storage for 6 weeks.
3.2 Variations of the NIR spectra during the
storage process
The average NIR second derivative spectra of 15 potato
tuber samples stored at 25℃ for 0, 2, 4, 6, 8, 10, 12, 14,
and 16 weeks are shown in Fig. 4(a). The variations in
the second derivative spectra that result from the chang-
ing composition of the stored tubers may be very subtle,
and therefore, can be difficult to observe. To clearly
observe and confirm the variations in spectral intensity,
the variance of these average spectra was calculated and
plotted as shown in Fig. 4(b). In the variance plot of the
second derivative spectra, the peaks around 970 nm and
760 nm were attributable to water. The differences in the
sizes of these peaks indicated that the potato tuber sam-
ples lost weight during the storage process. Other peaks
were observed around 670, 710, 798, 830, and 926 nm.
The peak at 670 nm is attributable to chlorophyll, which
is present in greater quantities in immature fruit than in
mature fruit [12]. However, we also observed an absorp-
tion peak of glucose and fructose around 670 nm in the
second derivative spectra of glucose and fructose (Fig.
5). Moreover, the NIR second derivative spectra of potato
tuber samples stored at 5℃ showed relatively large varia-
tions around 670 and 710 nm (Fig. 6(b)). Based on these
facts, the considerable variations around 670 and 710 nm
may be associated, either directly or indirectly, with the
sugar content of potato tubers (see Fig. 4(b) for compari-
Fig. 3 Changes in the amounts of reducing sugars in the potato tubers stored at 5℃ (a) or 25℃ (b).
Fig. 4 Variations in NIR second derivative spectra of potato tubers during storage at 25℃ for 16 weeks (a), and variance of NIR second derivative spectra (b).
0.00
0.50
1.00
1.50
2.00
2.50
0 2 4 6 8 10 12 14 16
Storage time (weeks)
Sug
ar c
onte
nt (m
g/g) Fructose
Glucose
0.00
0.50
1.00
1.50
2.00
2.50
0 2 4 6 8 10 12 14 16
Storage time (weeks)
Sug
ar c
onte
nt (m
g/g) Fructose
Glucose
(a)
(b)
Jie Yu CHEN, Han ZHANG, Yelian MIAO and Mitsunaka ASAKURA62
son). The spectra values at 710 nm and 934 nm are the
fourth and third overtones of C-H stretching [13]. The
values at 798 nm and 826 nm also may be related to fruc-
tose and glucose (Fig. 5).
3.3 PLS regression
PLS regression analysis was performed based on the
sugar (glucose and fructose) contents and NIR second
derivative spectra (600-1100 nm) of all potato tuber sam-
ples (total of 80: 40 stored at 5℃ and 40 stored at 25℃).
Figure 7 shows the calibration and validation results of
the fructose (a) and glucose (b) contents of the potato
tubers. The correlation coefficients of the calibration
were 0.71 and 0.65, respectively, with a standard error of
validation (SECV) equal to 0.26 mg/g and 0.46 mg/g,
respectively. As an index for determining the validity of
the calibration models, the RPD (ratio of standard devia-
tion of reference data in prediction sample set to SECV)
is usually employed [14]. A RPD value of 1.4-1.7 is
regarded as adequate for rough screening and a value
above 1.7 is regarded as satisfactory for screening. In this
study, the RPD values of 1.42 for fructose content and
1.32 for glucose (close to the value proposed by Williams)
were obtained. As such, the NIR calibration models are
suitable for rough screening of the sugar contents of the
potato tubers. Here, this nondestructive sorting approach
can roughly partition the potato tubers into two groups;
those with high values, and those with low values.
3.4 Discussion of the NIR Calibration Models
Regression coefficients can be used to discuss the con-
tributions of individual wavelengths to a PLS calibration
model, because a regression coefficient spectrum shows
characteristic peaks and troughs that can indicate which
wavelength range is important for the calibration model
[15,16]. Figure 8 shows the regression coefficients of the
PLS calibration models of the fructose (a) and glucose
(b) components, respectively. These regression coeffi-
cient spectra showed several marked peaks, such as
those observed at 670, 710, 798, and 830 nm. Variations
in the size of these peaks of the NIR second derivative
spectra were observed in potato tuber samples during
storage. We confirmed that these peaks were the absorp-
tion bands of glucose or fructose by comparison to sec-
ond derivative spectra of glucose and fructose standards
(Fig. 5). In particular, the peak around 798 nm is consis-
tent with the findings of Miyamoto and Kitano [17], who
reported a sugar absorbance band around 794 nm. The
results presented here are also consisted with those
reported by McGlone [18] and Abebe [19], who reported
a calibration model to estimate soluble solids content of
mandarin fruit. In addition, there is a small peak around
830 nm in the regression coefficient of the PLS calibra-
tion model that can only be accounted for by glucose, as
shown in Fig. 8. We confirmed that this peak represented
the absorption band of glucose by comparison to the sec-
ond derivative spectra of glucose and fructose standards
(Fig. 5). In other words, it is possible to differentiate glu-
cose from fructose on the basis of these small differences
in the calibration models. Furthermore, we confirmed
that the peak at 888 nm in the regression coefficient
Fig. 5 Second derivative spectra of standard chemical reagents of glucose and fructose.
Fig. 6 Variations in NIR second derivative spectra of potato tubers during storage at 5℃ for 16 weeks (a) and variance of these NIR second derivative spectra (b).
-0.006-0.005-0.004-0.003-0.002-0.001
00.0010.0020.0030.0040.005
600 700 800 900 1000 1100
Wavelength (nm)
log(
1/R
)
670
710
798
830888
918992
glucose
fructose
Determination of Sugar Content in Potato Tubers 63
spectra represented the absorption band of fructose, as
shown in Fig. 5. Consequently, the PLS calibration mod-
els for estimating the sugar components of potatoes were
established based on the specific absorption characteris-
tics of the various sugar components.
3.5 NIR Prediction Results
The PLS calibration models for estimating the sugar
components were applied to other independent potato
tuber samples. The potato samples were separated into
two groups; one was stored at 5℃ for 3 months, the other
was stored at 25℃ for 3 months. As in the calibration
analysis, five tubers were removed from each storage
room every 2 weeks, and their NIR spectra were evalu-
ated. Fructose and glucose contents of the potato tubers
were predicted based on the calibration models. The
changes in the glucose and fructose levels of the potato
tuber samples during storage are shown in Fig. 9. These
results were consistent with pattern of glucose and fruc-
tose values determined in the potato tuber samples using
chemical methods.
4.CONCLUSION
In this study, we roughly estimated reducing sugar
contents in potato tubers during storage using a simple
non-destructive approach that involved the acquisition of
NIR spectra (400-1100 nm) with fiber optics in the inter-
actance mode. The marked variations around 670, 798,
830, and 888 nm were associated with sugar content in
the potato tubers. Comparisons between variance plots of
the second derivative NIR spectra of potato samples
stored at 25 and 5℃ showed clear differences that were
attributable to sugar and water content. Based on the
variations around these major wavelength bands, calibra-
tion models to accurately estimate fructose and glucose
contents were developed by a partial least squares statis-
tical analysis method. This preliminary study demon-
strates that near infrared spectroscopy can be used for
Fig. 7 Scatter plots of sugar contents of potato tubers, and values predicted using NIR technique.
-1500
-1200
-900
-600
-300
0
300
600
900
600 700 800 900 1000 1100
Wavelength (nm)
Reg
ress
ion
coef
ficie
nts
710
(a) Fructose
-2500
-2000
-1500
-1000
-500
0
500
1000
1500
600 700 800 900 1000 1100
Wavelength (nm)
Reg
ress
ion
coef
ficie
nts
710
(b) Glucose
670
670
888
888
798
800
830
Fig. 8 Regression coefficients of PLS calibration models based on second derivative NIR spectra (600-1100 nm) for glucose (a) and fructose (b) contents of potato tubers.
0.0
0.3
0.6
0.9
1.2
1.5
1.8
0.0 0.3 0.6 0.9 1.2 1.5 1.8Fructose content (mg/g)
NIR
pre
dict
ed v
alue
(mg/
g)
R=0.71SEC=0.23 mg/gSECV=0.26 mg/g
0.0
0.4
0.8
1.2
1.6
2.0
2.4
2.8
0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8Glucose content (mg/g)
NIR
pre
dict
ed v
alue
(mg/
g)
R=0.65SEC=0.39 mg/gSECV=0.46 mg/g
(a) Fructose (b) Glucose
Jie Yu CHEN, Han ZHANG, Yelian MIAO and Mitsunaka ASAKURA64
nondestructive sorting of potatoes according to their
sugar content. However, the accuracy of this method
could still be improved, and further research is required
to develop the method to a level that is suitable for practi-
cal use.
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Fig. 9 Variations in reducing sugar contents predicted by NIR technique for potato tubers stored at 5℃ (a) or 25℃ (b).
0.00
0.50
1.00
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2.00
2.50
0 2 4 6 8 10 12 14
Storage time (weeks)
Sug
ar c
onte
nt (m
g/g) Fructose
Glucose
0.00
0.50
1.00
1.50
2.00
2.50
0 2 4 6 8 10 12 14
Storage time (weeks)
Sug
ar c
onte
nt (m
g/g) Fructose
Glucose
(a)
(b)
可視および近赤外分光法によるじゃがいもの糖成分の非破壊測定
陳 介余 1,†,張 函1,繆 冶煉2,朝倉 弘仲1
1秋田県立大学生物資源科学部,2中国南京工業大学食品与軽工学院
じゃがいもの糖成分の非破壊測定法の開発のため,可視および近赤外分光法の利用を検討した.光ファイバを利用したインタラクタンス法でじゃがいもの可視および近赤外スペクトル(400-1100 nm)を非破壊的に測定できた.測定されたスペクトルと糖成分の関係を検討した上で,PLS回帰分析法を用いて糖含量の予測モデルを開
発したところ,2次微分スペクトルとフルクトースおよびグルコース含量の間に有意の相関関係が認められ,標準誤差がそれぞれ 0.26 mg/gと 0.46 mg/gの予測モデルが得られた.本研究では,インタラクタンス測定法を用いた近赤外分光法がじゃがいもの糖含量の非破壊的測定法として利用の可能性があることを示唆した.
(受付 2009年 9月 15日,受理 2010年 2月 1日)
1 〒 010-0195 秋田市下新城中野字街道端西 241-438
2 〒 210009 中国南京市新模範馬路 5号
†Fax: 018-872-1676,E-mail: [email protected]
「日本食品工学会誌」, Vol. 11, No. 1, p. 65, Mar. 2010
◇◇◇ 和文要約 ◇◇◇