Determination of the Geographical Origins of Rice
by Stable Isotopes and Elemental Compositions
Analysis
INTRODUCTION
Rice (Oryza sativa L.) is an important staple food for more than half of the world’s population, and is said to
have more than 40,000 varieties in the world. Different varieties of rice can have different grain size, texture,
aroma, appearance, taste and nutritional compositions. These properties have become contributing factors in
the prices of rice across the globe. The continuing demand for rice helps to sustain the rice trade and in turn,
produces an effect on the rice export prices. The introduction of specialty rice in the market also sees a price
differentiation between the regular long grain and specialty rice, resulting in a growing importance in
determining the geographical authenticity and traceability in these commodities.
EXPERIMENTAL
Samples
200 different brands and/or varieties of rice samples purchased from the retailers and wholesalers were used
in this study. Examples of the rice varieties obtained include Indica rice, parboiled rice, Basmati rice, Jasmine
rice, Japonica rice, brown rice, glutinous rice, wild rice and broken rice.
Authors
1,3
Angela Li
[email protected]
Joanne Chan Sheot Harn
1
2
Brendan Keely
3
Malcolm Baxter
3
Simon Kelly
Addresses
1
Health Sciences Authority,
11 Outram Road, Singapore
169078
2
The University of York,
Heslington, York, YO10 5DD.
UK.
Figure 1: Top row (left to right): Indica rice, parboiled rice, Basmati rice, Jasmine rice and Japonica rice. Bottom row (left to right): Jasmine brown rice, Japonica brown rice,
glutinous rice, wild rice and broken rice
3
Instrumentations
(1) Homogenisation of rice samples
The rice samples were first pulverised with a ball mill
(Retsch Mixer Mill MM 400). The milling parameters
were programmed at a frequency of 25 Hz and a
milling time of 30 seconds.
(2) Coupled Plasma Mass Spectrometry (ICP/MS)
analysis
The multi-elemental content in rice was carried out
with the microwave acid digestion followed by the
ICP/MS analysis. A mixture of concentrated nitric acid
and hydrochloric acid is used to digest a small amount
of rice sample in a quartz vessel at a pressure of 75
bar. The application of hydrogen or helium gas mode
was used to eliminate the polyatomic effects for
selected elements in the ICP/MS analysis.
(3) Isotope Ratio Mass Spectrometry (IRMS) analysis
The stable isotope ratio analysis was performed by
IRMS to obtain the respective δ values for carbon
13
12
15
14
2
1
( C/ C), nitrogen ( N/ N), hydrogen ( H/ H) and
18
16
oxygen ( O/ O). A small amount of powdered rice
sample is weighed into a tin or silver capsule and
introduced into the Elemental Analyser for the
respective combustion or pyrolysis process. The
combustion mode converts the sample, via a series of
oxidation and reduction processes, into carbon dioxide
and nitrogen gases for detection by the mass
spectrometer, in order to obtain the carbon and
nitrogen isotope ratios respectively. Alternatively, the
sample undergoes high temperature conversion in the
absence of oxygen to form carbon monoxide and
hydrogen gases to attain the oxygen and hydrogen
isotope ratios respectively in the pyrolysis mode.
The Food and Environment
Research Agency,
Sand Hutton, York,
YO41 1LZ. UK.
References
1
IsoPrimeTM EA Training
Manual Version 1 – April 2005.
2
The Principles of ICP-MS.
Available at
http://www.chem.agilent.com/
en-US/Products/Instruments/
atomicspectroscopy/icpms/
pages/gp455.aspx.
Accessed on 12 February 2011.
Figure 2. Principle of operation for the ICP/MS. Adapted from the Agilent
2
Technologies
Figure 3. Principle of operation for the Elemental Analyser in the combustion
1
mode. Adapted from the IsoPrime Limited
RESULT AND DISCUSSION
Using the results obtained from the IRMS and ICP/MS analysis, a supervised pattern recognition type of
multivariate analysis, the Canonical Discriminant Analysis (CDA), can be applied. CDA is a useful tool to provide
information for differentiating between rice from the various countries in this study. In addition, this allows for
the development of empirical models to identify the geographical origin of unknown rice samples across the
different rice varieties.
Fera is an Executive Agency of Defra
www.defra.gov.uk/fera