ESR Dosimetry of Gum Arabic - DCM

ESR Dosimetry of Gum Arabic
Sergio J. Leonor
1
1,2
1
3
, Jorge Antonio Gómez , Angela Kinoshita , and Oswaldo Baffa
1*
Departamento de Física e Matemática, FFCLRP-USP, 14040-901 Ribeirão Preto, São Paulo, Brazil
2
Universidade Federal do Tocantins, Arraias, Tocantins, Brazil.
3
Universidade do Sagrado Coração, Bauru, São Paulo, Brazil
*Corresponding author: baffa@usp.br
Abstract
Gum Arabic (GA), also called Acacia gum, is a dried exudation obtained from the stems and branches of some acacia trees belonging to Leguminosae family. This gum is widely used in food, pharmaceutical, and cosmetic industries; in the food industry it
is used in confectionery, baked goods, beverages and packaged products. Being a natural product and considering that the growth areas of the acacia trees have a favorable climate for growth of microorganism, thus decontamination is necessary. The
common decontamination method uses a chemical treatment with ethylene dibromide, which is extremely harmful to the ozone layer, its marketing is forbidden in several countries and its manufacture will be banned by 2015. Searching for alternative
methods for decontamination of food and medical products containing GA, Zaied et al showed that GA can be effectively decontaminated by irradiation without adverse effects on the physical properties of the final products. Therefore, there is a need
for an appropriate method of measurement and control of irradiation doses of products with GA decontaminated by radiation. This work reports a study of the electron spin resonance (ESR) signals of irradiated GA showing the feasibility of ESR as an effective measuring method of its irradiation doses. Industrial grade powdered GA samples of 50 mg were packed and placed between 5 mm thick acrylic plates and then irradiated to doses ranging from 0.5 to 5 kGy in a 60Co irradiator. All ESR measurements were carried out in a JEOL FA-200 X-Band ESR spectrometer at room temperature. Irradiated GA samples have ESR spectra with g-factors ~2 and showed response of the peak to peak amplitude of the signals with the doses at determined times after irradiation. A study of the temporal stability of the ESR signals of irradiated GA showed a composed spectrum with two decay times. Decay curves of the value of the double integral of the ESR signals were fitted with a bi-exponential curve. According
to this fitting, at least two species of free radicals with halve-times of 3.3 and 125.4 hours are produced in the GA by the radiation. Simulated ESR spectra composed by two different radicals agrees with the experimental results. This study shows that the
ESR dosimetry is an effective method for control and measurement of irradiation doses in gum Arabic.
glucuronic
acid

rhamnose
Gum Arabic consists mainly of high molecular mass polysaccharides and their calcium, magnesium
and potassium salts, which on hydrolysis yield arabinose, galactose, rhamnose and glucuronic acid [16]. This gum is widely used in food, pharmaceutical, and cosmetic industries, for example, in the food
industry the gum Arabic is used in confectionery, baked goods, beverages and packaged products [713]. Being a natural product and considering that the growth areas of the acacia trees have a favorable
climate for growth of microorganisms, the decontamination of gum Arabic is typically performed using
a chemical treatment with ethylene dibromide, which is extremely harmful to the ozone layer. The
marketing of ethylene dibromide is forbidden in several countries, and its manufacture will be
banned by 2015 around the world [14].
In search for alternative methods for decontamination of food and medical products with gum
Arabic, Zaied, Youssef, Desouky, & El Dien in 2007 performed a complete study of the effects of decontamination of commercial gum Arabic with g-rays and electron beams, using microbiological
analysis, optical spectrometry, scanning electron microscopy, and rheological and color measurements. Using an optimal dose of a few kGy, they showed that gum Arabic used in food and medical
industries can be effectively decontaminated by irradiation without adverse effects on the physical
properties of the final products [15]. Therefore, there is a need for a suitable method of dose
measurement and control of irradiation of gum Arabic products.
Since the 1980’s, radiation treatment of food has been used to reduce spoilage and improve hygienic quality; thus, ionizing radiation has been used effectively in numerous applications in the
food and medical industries: medical products sterilization, hospital waste and sewer treatment,
modification of some polymers properties, and irradiation of foodstuff [16,17]. In order to facilitate
trade in irradiated foods, there is interest in using simple and reliable methods to detect foods
treated by irradiation. Electron spin resonance (ESR) spectroscopy is one of the leading methods
that has gained wide acceptance as a standard dosimetry method for high irradiation doses, such
as those used in industrial radiation processing applications [18-22]. Irradiation produces free radicals, some of which are quasi-stable because they can be trapped by impurities or inherent defects. The unpaired spins can be detected in an ESR spectrometer where the resulting paramagnetic absorption intensity is proportional to the absorbed radiation dose. We report here a comprehensive study that demonstrates the feasibility of using ESR as an effective method of measuring radiation dose in gum Arabic.
galactose
Results and Discussions
arabinose
Introduction

Fig. 3 shows the decay of the relative concentrations (CREL) of free radicals following a radiation of 3 kGy. It can be seen that a fit of
two exponential decay curves gives good agreement with the experimental results. At least two species of free radicals with halftimes of 3.3 and 125.4 hours are produced in the gum Arabic by the radiation. The observation of two groups of radicals in irradiated gum Arabic is in agreement with the results of [24].
 DI ESR ( D, t ) 
CREL ( D, t )  

 DI ESR ( D  0) 
Figure 3.
Figure 2.
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
Experimental Setup

Fig. 2 shows a composed ESR spectrum (3 kGy ) and a fast decay of the signal at short times after irradiation. Peak-to-peak signal
amplitude decays 53% in the first ten hours, reaching 90% at 1037 hours after irradiation.
ESR measurements were carried out in a JEOL FA-200 X-Band ESR spectrometer at room temperature. Table 1 shows the spectral
parameters used to acquire the ESR signals of the gum Arabic. To calibrate the signals, the Mn2+ marker of the spectrometer was used. This ESR marker sample is made by thermally dispersing Mn2+ in MgO, and gives a sharp hyperfine structure of Mn2+ (I= 5/2).
Simulations were performed using SimFonia Bruker software. According to the
spectral feature shown in Fig. 2, there are two main groups of radicals that decay.
One group is present only at short times after irradiation, characterized by a
wider line (Radical 1), while the other persists after ten hours (Radical 2). To characterize Radical 1, the spectrum at 10.3 hrs was normalized and subtracted from
the spectrum at 1 hr (Fig. 4A). The result suggests a hyperfine interaction. The
Spin Hamiltonian parameters of Radical 1 determined by simulation are g= 2.0046
and A= 1.2 mT. The line of Radical 2 can be represented by a line with axial symmetry and Spin Hamiltonian parameters: gx= gy= 2.0062, gz= 2.0025. This model
fit well with the recorded spectra at different times from 1 to 1037 hrs. Fig. 4B
shows the experimental, Radical 1, Radical 2, and the composed (Radical 1 + Radical 2) spectrum for t=1 hr. The calculated radical kinetics of Radical 1 and Radical
2 gives half-lives of 3.3 and 121 hrs respectively, in agreement with the model of
two exponential decay functions shown in Fig. 3.
As mentioned in the introduction, gum Arabic is a natural product that consists
mainly of high-molecular weight polysaccharides. Typically, the structures of polysaccharides are linear, consisting of slight modifications of the repeating unit,
which in the case of the gum Arabic are arabinose, galactose, rhamnose and glucuronic acid, forming complex branched chains. Other irradiated disaccharides,
such as lactose and dextrose, have spectral characteristics [25] similar to those of
irradiated gum Arabic, conferred by the radical group Radical 1.
Figure 4.

Figure 5.
Figure 5 shows the effect of hydration in irradiated gum Arabic. Milli-Q water
was added to irradiated gum, which was then allowed to dry at room temperature.
Notice how the ESR spectrum of gum Arabic irradiated with 5 kGy, recorded 60
days after irradiation, returns to its initial state before irradiation. Thus, we observe that water permits the recombination of free radicals induced by radiation.
This is an interesting finding, especially for applications in the food industry, implying that no radiation induced radicals will be present after hydration.
Conclusions


Gum Arabic was obtained from Vetec Quimica Fina Ltda. (cod. 629, lot
0704307). Powdered 1 g gum samples were packed in a sachet and placed
between acrylic plates of 5 mm of thickness, and were then subjected to
doses ranging from 0.5 to 5 kGy.
We observed a linear relationship between the absorbed dose and the intensities of the ESR spectra, peak-to-peak amplitudes, and
double integrals. Experimental results of the observed kinetics of the radicals generated by irradiation indicated the existence of at
least two different radical groups with half-times of 3.3 and 125.4 hours, in agreement with spectral simulations. Hydration of irradiated gum Arabic leads to a recombination of the free radicals induced by radiation. The results show that ESR is a simple and reli-

The irradiations were made using an industrial 60Co irradiator (Gammacell)
in static mode with a dose rate of about 2.47 kGy/hr. The gum samples were irradiated with half of the total dose delivered on one
side (0° in relation to the radiation beam direction) and the other half delivered on the opposite side (180°) in order to obtain a
more homogenous dose distribution (Farrar Iv, 1995).
able method of analysis of irradiated gum Arabic up to 60 days after initial radiation with doses of 5 kGy.
References
Results and Discussions
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

Figure 1 shows the ESR spectra of Arabic gum before irradiation
and after irradiation with different doses between 0.5 and 5 kGy.
All ESR signals were measured approximately 140 hours after being irradiated and have a g-factor around 2.
Inset in Fig. 1 shows the linearity of App and DIESR ESR intensities as
function of irradiation dose can be observed at specific times following irradiation. The correlations (R-square) of the linear regressions
Acknowledgments
Figure 1
The authors are grateful to FAPESP and CNPq for partial financial support.