2.1 Sample source

Some round-shaped japanica rice were purchased from the local supermarket. The samples were divided into 5 equal portions with 50 g for each, sealed in polythene (PE) bag and stored in a drier until use.

2.2 Irradiation treatment

Rice samples were exposed to 60-Cobalt source at an ambient temperature at the Irradiation Center, Zhejiang University, Hangzhou, China. The activity of gamma radiation was 2.96×10¹⁵ Bq. Five doses of irradiation, i.e. 0, 1, 2, 3, 5 kGy were used with the dose rate of 0.8 kGy/h. All samples were placed with the same horizontal level with the irradiation source and irradiated in the natural temperature.

2.3 Determination of starch pasting viscosity

The samples after irradiation were ground into powders and passed through a sieve of 100 mesh size. The starch pasting viscosity was determined in duplicate by a Rapid Visco Analyser (RVA, Model-3D, Newport Scientific Inc, Australian), which was controlled by the Thermo Cycle for Windows (TCW) software (American Association of Cereal Chemists (AACC), 1998)^[13]. Rice starch 3.0 g with 14%, moisture content was weighed into the sample cup and 25 mL distilled water was added just befor analysis, temperature inside the pot changed while stirring is as follows: first maintained at 50ºC for 1 minute, then raised to 95ºC at the rate of 12ºC/min, and maintained at 95ºC for 2.5 min, then cooled to 50ºC,and maintained at 50ºC for 1.4 min. The paddle will rotate at 960 rpm for the first 10 s, at 160 rpm till running out. The parameter values of starch pasting viscosity can be expressed by Rapid Visco Units (RVU), the following parameters such as peak viscosity (PV), hot viscosity (HV), cool viscosity (CV), break down (PV minus HV) and set back (CB minus PV) were used as the characteristic values of RVU profile.

2.4 Detection method

Two parallel samples were selected from the starch samples without gelatinization, one was added acid solution pH=2.5, the other was added distilled water (blank experiment). All samples were stirred to uniformity at the same conditions, starch pasting viscosity of the sample was tested on the Rapid Visco Analyser, and the dynamic changes of the RVA profile were monitored. The samples were determined to be irradiated if the RVA profile was higher at pH=2.5 compared to the samples added distilled water. The samples were determined to be free from irradiation treatment if the RVA profile was lower at pH=2.5 than the samples added distilled water.

3.1 Effect of different absorbed doses on starch pasting viscosity

The major parameters of RVA profile such as PV, HV and CV were considerably decreased with the increment of absorbed doses when the starch in rice was irradiated at different absorbed doses as listed in Table 1. It was obvious that the viscosity of non-irradiated sample was higher than that of irradiated samples. With increment of absorbed doses all of viscosity including PV, HV and CV decreased when the samples added with distilled water at pH 7. When added with the acid solution of pH 2.5, the PV was higher than that of added with the distilled water of pH 7. And there was also the PV decreased with the absorbed doses as listed in Table 2. The reductions in the values of starch pasting viscosity can be served as the reference whether the samples have been irradiated or not. Some reports suggest that starch degradation reaction resulting in the decline of the molecular weight is responsible for the decrease of starch’s viscosity when the amylose and amylopectin were involved in radiation degradation of rice^[14]. Radiation can cause the cleaving of polysaccharide chains, thereby generating degradable fragments of dextrin, which is the reason for decrease of starch content^[15]_.

3.2 Effect of the solution pH on RVA profile

According to the method described in 2.4, the samples were equally separated into two portions, one portion was added distilled water of pH 7, another was added the acid solution of pH 2.5. The RVA profile was determined by the Rapid Visco Analyser (RVA). As shown in Fig.1, RVA profiles at pH=2.5 were higher than those of at pH=7 for the non-irradiated rice.

Fig.1Full-screen View

Effect of the solution pH on RVA profile.

However, the values of starch pasting viscosity in the irradiated samples at pH=7 were lower than those of at pH=2.5, which was still less than those of the non-irradiated counterparts. The higher of the irradiation dose, the lower starch pasting viscosity. It can be explained that hydrogen ion can cut down small polysaccharide chains caused by irradiation and cleavage into large molecular, as a result, the molecular weight increasing compensates for the viscosity. Hence, we can easily and rapidly inspect whether starch in rice has been exposed to irradiation treatment or not with additive of acid solution to markedly alter the RVA profile.

Fig.2Full-screen View

Effect of storage time on RVA profile.

3.3 Effect of storage time after irradiation on RVA profile

To keep breathability of rice, both the irradiated and non-irradiated samples were put into the nylon bags, respectively. The nylon bags were placed on the experiment desk and set for the stored 360 days to investigate the changes in starch pasting viscosity. It was found that, 1.56% reduction in PV value was determined after 360 days storage time in the non- irradiated sample, meanwhile there were 6.58% reduction in the sample irradiated by the dose 1 kGy, 3.25% reduction by 3 kGy, 10.69% reduction by 5 kGy as listed in Table 3. The decrease in RVA profile was also clearly observed after 360 days storage time (Fig.2), indicating that the fragmentation of molecular chains still occurred in the irradiated macromolecules of amylose and amylopectin during the storage.

3.4 Relationship between viscosity and absorbed doses

With the regularities between PV, HV, CV and absorbed dosages, the correlation between logarithm of starch pasting viscosity (Y) and absorbed doses can be represented as follows: Y_pv=3.5012–0.1277X R²= 0.9920, Y_hv=3.3207–0.2139X, R²=0.9916, Y_cv= 3.5003 –0.1575X, R²=0.9868, and the absorbed dosage can be calculated^[16]. Here Y_pv, Y_hv and Y_cv are the logarithm of starch pasting viscosity with PV, HV and CV respectively, and X is absorbed doses (kGy). Thus the absorbed doses for irradiated rice samples can be calculated. When PV used, it can be suggested that it is relatively reasonable to estimate the actual absorbed doses based on the PV exponential correlation though the theoretical calculation is less than the value determined as shown in Table 4. On the other hand as shown in Table 3, the PV of irradiated rice sample with additive acid aqueous solution should be higher than with additive distilled water meanwhile the PV of non-irradiated rice sample with additive acid aqueous solution should be lower than with additive distilled water. So in the actual practice, we could distinguish whether the samples were irradiated. If PV of rice samples was higher at the neutral conditions than those at the acid conditions, the samples were not irradiated.

4.1 Mechanism for the decrement of starch pasting viscosity caused by irradiation

Starch is mainly consists of amylose and amylopectin, amylose is commonly called chain starch. Chain starch is composed of D-anhydroglucose units linked with α-1, 4 glycosidic bond and amylopectin is composed of D-anhydroglucose units connected by α-1, 6 glycosidic bond and α-1, 4 glycosidic bond, and the branch positioned in the α-1, 6 glycosidic bond where branching taking place ^[17]. The starch pasting viscosity decreased with the increment of absorbed doses (Tables 1 and 2). The irradiation with gamma rays was capable of degrading starch through the cleavage of glycosidic linkages and bonding peptide into smaller fragments, such as short aldehyde, ketone, acid chain and monosaccharide (glucose, maltose, and dextrin) et al., peptide derivatives as well as amino acid and some small molecular weight fragments, then reduced in the reduced starch pasting viscosity^[18]. Water contained in the irradiated samples can generate free radicals and hydrated electron free radicals, which can react with the group in the glycosidic bond and peptide as well as the radicals generated by protein after irradiation. The starch can be further cleaved into glycosidic bond and peptide chain through one chain reaction accelerate the decrease in starch pasting viscosity^[19]. The H· in the water can also react with the group on the aldehyde, ketone, acid, and lead to the polymerization of the short chains, which can compensate for starch pasting viscosity at certaint.

4.2 Error analysis for the estimated irradiation dosage

There was relative error between theoretically calculated value and actual irradiation dose according to the regression equations (Tab. 3), the relative error between theoretically calculated value and actual irradiation dose was highest with 7.03% and lowest with 2.74% when estimated by the PV exponential correlation, highest with 10.53% and lowest with 0.73% by HV, and highest with 10.93% and lowest with 4.28% by CV. The main reasons for the error can be described as follows: at first, the error is caused by irradiation. The absorbed dose designed in the experiment is not in accordance with the actual exposed dose to sample since the samples have certain volume instead of the dose just on one dot. Second, the static irradiation processes are chosen then the sample is being treated, the inaccuracy of the actual distance between the samples and the irradiation source compared to the calculated distance can also cause error. Third, it is necessary to correct the radioactivity of the irradiation source immediately when the samples are being irradiated. We need to make sure that the samples being irradiated should maintain on the same horizontal level with the irradiation source. Fourth, the error is caused by sampling time for detection. At last, the system error is caused by the measurement of starch pasting viscosity in the samples. Based on the analysis of the experimental data, the lower of the absorbed dose, the larger the error is, the higher of the absorbed dose is, the smaller the error is.