1. Introduction
Legumes, such as soybean Glycine max L. are the main source of isoflavone. A research on the potential health benefit of soybean isoflavone have
widely been published, especially associated with its anti-carsinogenicity, anti- oxidant activity, as well as its ability to prevent or inhibit heart attack,
osteoporosis and menopause symptoms Liu 1997; Oomah Hosseinian 2008; Fukushima 2001.
In soybean seeds, isoflavone is mainly in the form of β-glucoside and its
aglycone free from glucose molecule. Isoflavon mainly consists of daidzein, genistein and glycitein Collison 2008; Liu 1997; Rostagno et al. 2005.
Isoflavone content of soybean varies as affected by degree of processing and also depends on the type variety of soybeans Nakajima et al. 2005; Carrao-Pinizzi
Kitamura 1995; Niamnuy et al. 2012.
Application of ionizing radiation at cereal products has been applied to reduce toxic and anti-nutritional compound Arvanitoyannis Stratakos 2010;
Sommers et al. 2006; Byun et al. 2006; Siddhuraju et al. 2002 and to achieve sanitary and phytosanitary purposes Hallman 2011; IAEA 2004. For example of
legumes including soybeans. For example, gamma radiation at dose level of 1 kGy low dose has been applied for quarantine treatment of legumes including
soybeans. Higher dose of irradiations have been applied for fresh meat products, dry or frozen fresh vegetables, and ready to eat food product to increase safety,
maintain the quality and prolong the shelf life of the food Tanhindarto et al. 2005; 2011.
Variyar et al. 2004 reported that isoflavone of soybean irradiated with gamma irradiation at the dose level of 0.5-5 kGy is more in the form of aglicones.
This suggests that irradiation may attack the glycosidic bond of soy isoflavone glycosides genistin, daidzin, and glycitin to produce aglycones genistein,
daidzein, and glycitein. Gamma irradiation at the doses level of up to 10 kGy, however, did not affect total isoflavone content of soybean Yun et al. 2012. Our
previous research reported that gamma irradiation at the same radiation dose showed different effect on the decrease anti-nutritional concentration and the
changes of color of soybean as influenced by the dose-rate used Tanhindarto et al. 2013. At the irradiation dose; the higher the dose-rate the lower the
irradiation time is more effective in decreasing concentration of anti-nutritional compounds, but less affective in reducing the quality of soybean color.
Specifically, this research is will evaluate the influence of the dose-rate selection on the rate of change of isoflavones of soybean during gamma
irradiation.
2. Materials and methods 2.1. Materials
Soybean Glycine max L. was from mitani variety obtained from plant breeding section of PATIR, BATAN. Isoflavone standard reagents daidzein
and genistein were purchased from Sigma Chemical Co, methanol HPLC, chloride acid, ammonium acetat from Merck, acetonitril and water solvent from
JT. Baker, respectively.
2.2. Equipment A gamma source radiation facility of Natural Rubber Irradiator IRKA,
was used to irradiate sample materials at 122.9 kCi activity. IRKA is at PATIR BATAN, Pasar Jumat, Jakarta. A shimadzu liquid chromatograph equipped with
LC 20 AD type pump, SPD 20A type UV detector and RF 10AXL type flourescene, water-bath Napco model 220A, USA, centrifuse IEC Centra 8
Centrifuge, USA, magnetic stier Velp Scientifica type ate, Italy and the other supporting equipments.
2.3. Sample preparation Soybean was used as a sample material. The sample was weighted into
100 g, packed in polietilen plastic bag for each treatment. The samples were prepared in triplicates.
2.4. Dosimetry Radiation dose measurement was done using harwell amber 3042 and
harwell red 4034 dosimeter Harwell Dosimeters Co. Ltd., Oxfordshire, UK by using spectronic spectrophotometer IAEA 2002.
2.5. Irradiation The sample was placed in the irradiation area with different locations; to
get irradiation treatment with specific dose-rates. Dose-rates used were previously determined at preliminary study. Four 4 locations in irradiation
area have been identified to have dose-rates of 1.30; 3.17; 5.71 and 8.82 kGyh. Irradiation treatments were done at room temperature 28 ± 2 °C for each of the
samples with exposure time ranging from 0.5-55 h; depending on its dose-rate.
2.6. Determination of isoflavone Isoflavone concentration was determined according to the method of
Wang et.al. 1990. The two grams of ground sample was extracted with hexane for 6 h to remove fat. The isoflavone content of non-fat sample was analyzed
using a high performance liquid chromatography equipped with an UV detector at wavelength of 254 nm and each flourescene of excitation and emission
wavelength was 365 nm and 418 nm. Agilent XDB-C18 column
5 μm, 4.6 x 15 cm length was used. A mobile phase was a mixture of metanol : 1 mM
ammonium asetat, 6:4 at a flow rate of 1.0 mlmin.
3. Results
The initial concentration of isoflavones in soybean sample used in this study was
81.39 μgg and 367.42 μgg for free and total daidzein and 62.27 μgg and 232.55
μgg for free and total genistein, respectivelly. Wang et al. 1990 reported that free and total daidzein contents in soybean were
25.6 μgg and 330.6 μgg, and for free and total genistein were 28.4 μgg and 461.6 μgg. The
difference in isoflavone concentration among soybeans used could probably due to the varieties, planting location and harvesting time Tepavcevic et al. 2011.
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