Water Diffusion in Buckwheat Noodles and Wheat Noodles during Boiling and Holding as Determined from MRI and Rectangular Cylinder Diffusion Model

Abstract

Magnetic resonance imaging (MRI) was employed to observe water diffusion within two kinds of buckwheat noodles (marunuki, sarashina) and one kind of wheat noodle during boiling and holding. The apparent diffusion coefficients for water were statistically estimated with Fick's second law using a rectangular cylinder model, and the changes in moisture distribution in buckwheat and wheat noodles were compared quantitatively. Apparent diffusion coefficients of water in noodles during boiling were 4 to 7 × 10 -6 cm 2 /sec. The diffusion coefficients of water in buckwheat noodles during boiling were higher than those in wheat noodles. For each noodle, the diffusion coefficient during holding after boiling was 2 to 3 × 10 -7 cm 2 /sec and constant through the holding time, from 30 to 120 min. The diffusion coefficients in buckwheat noodles during holding were lower than those in wheat noodles. These results show that, as compared with wheat noodles, buckwheat noodles cooked more rapidly and lost favorable texture during holding.Keywords: buckwheat noodle, water diffusion, diffusion coefficient *To whom correspondence should be addressed. E-mail: ikukom@affrc.go.jp IntroductionSoba, noodles made from buckwheat flour, and udon, noodles made from wheat flour, are popular and traditional noodles in Japan. Soba is particularly favored for its flavor and texture and the nutritional advantages associated with its rich protein, mineral, and functional contents, including rutin (Yasuda, 2000). Buckwheat flour contains about 10% protein by weight. Compared to wheat flour, buckwheat flour has double or more the ash content (Kanetani, 2000). De-hulled buckwheat grains can be ground entirely into "marunuki" flour without discarding germ, which is necessary when grinding wheat grains (Shibata, 2000). Another way of grinding buckwheat grains is to separate flours into three types during the grinding process: the first flour, or "sarashina" flour, the second flour, and the third flour (Shibata, 2000). The first flour, which contains about 77.6 % carbohydrate, is derived from the inner part of de-hulled buckwheat grains, mainly endosperm, as endosperm is most easily ground into flour. The second and third flours are derived mainly from the intermediate and outer layers of de-hulled buckwheat grains, respectively. They contain more protein, fat, and ash than the first flour, and their carbohydrate contents are therefore lower (Kanetani, 2000).To serve a good dish of soba, the characteristic flavor of buckwheat and the texture of the boiled soba noodles are important factors, as in the case of other noodles such as udon (Kojima et al., 2001). The texture of noodles and pasta, such as lasagna (Gonzalez et al., 2000) and spaghetti (Irie et al., 2004), is significantly affected by moisture content and moisture distribution in the matrices. Starch in foods is gelatinized and becomes more digestible upon application of sufficient water and proper heating; for this reason, boiling in water is the most popular method for cooking noodles. During...

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