Breeding Science 53 : 169-175 (2003) Note Development of Rapid Measuring Method on Rice-cake Hardness in Waxy Rice Kazuyuki Kobayashi*, Kazuhiko Ishizaki, Yukiko Kawai, Takaaki Matsui, Hironobu Kasaneyama and Toyokazu Hoshi Crop Research Center, Niigata Agricultural Research Institute , 857 Nagakura, Nagaoka, Ni igata 940-0826, Japan Key Words: Oryza sativa L., early generation testing, hardness of rice-cake, rice breeding, test of specific character, waxy (or glutinous) rice. The hardening of waxy (or glutinous) rice-cakes is one of the important processing properties in the manufacture of rice-cakes ( mochi ) and rice-crackers ( Arare and Okaki ), be- cause the hardness of rice-cakes is greatly influenced by the refrigeration period before cutting (Yanase et al. 1982b). Waxy rice-cake hardness is generally evaluated by the bend- ing degree of the rice-cake after cooling for a definite time (Egawa and Yoshii 1990, Aris aka 1994, Yamashita 1995). In addition to this method, penetration resistance measured with a tensipresser (Egawa and Yoshii 1990), hardness meas- ured with a texturometer (Yanase et al. 1982a) and with a fruit hardness tester (Okamoto and Nemoto 1998), cutting resist- ance (Saito and Baba 1966), etc. have been used to measure the hardness. However, these methods require a large amount of grains, at least 1kg of brown rice and long time, over 20 hours of refrigerating before measurement, and are unsuit- able for testing the hardness in early generation breeding programs when hundreds of i ndividuals are tested. Thus, to estimate the hardness of rice- cake more conveniently, the pasting temperature which wa s highly correlated with the hardness of rice-cakes (Yanase et al. 1982a, 1982b), has been measured with a viscograph (Arisaka 1994, Matsue et al. 2002). In this method, the sample amount of 50 g is suffi- cient, but about 100 min is necessary to measure one sample and the vessel must be cleaned up before the next test. The Rapid Visco Analyzer (RVA- 3D, Newport Scientific Pvt. Ltd., NSW, abbreviated as RVA hereafter) recently devel- oped can measure the pasting temperature simply, easily and rapidly compared with the viscograph. On the other hand, Ishiza ki (1994) developed a simple method to measure the hardne ss of rice-cake using a boiled rice-flour dough of 20 g. The validity of this method has been reported elsewhere (Kobayashi et al. 1999). However, special instruments and a great d eal of skill are required for preparing the rice-cake. A more efficient method for meas- uring rice-cake hardness in early generation breeding is ab- solutely necessary for breeding a superior waxy rice variety. In this study, a simple, rapid and easy method of pre- paring the rice-cake for measuring hardness was developed, and the hardness was evalua ted with a tensipresser. This method is denoted as the “rap id method” hereafter. The re- sults obtained by this meth od will be compared with the pasting temperature which is known to correlate with the rice-cake hardness. In addition, the method of evaluating the rice-cake hardness for individual plant will also be reported. Plant materials The stock seeds of Watabou shi and Koganemochi, cul- tivated at Niigata Crop Research Center in 2000 were used to determine measurement conditions. In order to confirm the effectiveness of the rapid method, we tested the rice-cake hardness of 39 varieties/stra ins cultivated in 2001 in the same field under the same crop management at the Center. Out of 39 varieties/strains, 4 varieties with different rice- cake hardness, i.e., Koganemochi (very high), Himenomochi (medium), Miyatamamochi (low) and Hiyokumochi (very low) (Ishizaki et al. 1995, 1996) were used for establishing the method for applying it to individual plant. Preparation and manufacture of rice-cake Brown rice kernels thicker than 1.85 mm were sampled, polished to reach the appa rent pearling grade of 89 ± 1 % and milled with a sample mill (UDY Co. Ltd., Colorado). Rice- cake was prepared from 40 g of rice-flour passed through a 300 μ m mesh screen. The moisture content of the flour was determined using a near-infrared spectrochemical analyzer (6250HON, Nireco Co. Ltd., Toky o), and distilled water was added to the flour to make the moisture content 43 %, which is the standard moisture of rice-cakes (Kobayashi et al. 2002b). The sample was mixed thoroughly by hand to pre- pare dough. The dough was fla ttened at a thickness of 8 mm using a rolling bar, and cut out a circle of 8 cm in diameter using a Mido-measuring frame (Toyo Rice Cleaning Machine Co. Ltd., Wakayama, denoted as “hardness-measuring frame” hereafter). This piece was put between the two attaching-plates and held wi th two clips (denoted as “hardness-measuring holder” hereafter), and heated in boil- ing water for 5 min in conformity with Saito and Baba (1966). After heating, the sample s in the hardness-measuring holder were cooled in flowing water for 10 min, and water drops on the holder were wiped off by a dried towel, then Communicated by K. Okuno Received June 26, 2002. Accepted November 11, 2002. *Corresponding author (e-mai l: [email protected] ref.niigata.j
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