Food Sci. Technol. Res., 8 (1), 8–9, 2002 Note

Discrimination of Cooked Mochiminori and Koshihikari Grains by Observation of Internal Hollows Using Light Transmittance Photography

Masahiro SUZUKI, Toshiyuki KIMURA, Kenji YAMAGISHI and Hiroshi SHINMOTO

Department of Upland Farming, Tohoku Agricultural Experiment Station, Ministry of Agriculture, Forestry and Fisheries, 50 Harajuku Minami, Arai, Fukushima city, Fukushima 960-2156,

Received April 3, 2001; Accepted December 12, 2001

We compared cv. Mochiminori as and cv. Koshihikari as non-glutinous rice using light transmit- tance photography and clarified that the maximum hollow width/grain width ratios at the hollow (hollow ratio) of Mochiminori and Koshihikari were different. The Mochiminori rice grain had a small hollow ratio or undetectable internal hollows, but the Koshihikari had a large hollow ratio. This agreed with the result by nuclear magnetic reso- nance micro imaging. In addition, grains from mixed cooking of the two were identified as Mochiminori or Koshihikari rice using light transmittance photography instead of the iodine dyeing method. The remaining iodine solution and the dyed grain samples were post-treated safely, because the iodine is toxic. Light transmittance photog- raphy easily discriminated Mochiminori or Koshihikari rice grains.

Keywords: light transmittance, internal hollow, Mochiminori, Koshihikari

In a study of water distribution in cooked rice grains using Results and Discussion nuclear magnetic resonance (NMR) micro imaging, Horigane et Light transmittance photography cannot measure hollow vol- al. (1999) found internal hollows formed within the rice (Oryza ume, because it produced only a projected image as opposed to sativa cv. Koshihikari) during cooking and hypothesized there the three-dimensional data of NMR micro imaging. Neverthe- was a hollow formation mechanism. The Japanese judge rice pal- less, the difference of hollow volume could be simulated as the atability mainly correct meaning by its texture (Okabe, 1979). difference of hollow shape for the comparison of two cultivars. The hollows make the endosperm of the grain less homoge- The cooked rice grains of the two cultivars were photographed neous, so the number and size of hollows can be important fac- (Fig. 1). In Fig. 1, Koshihikari has wide hollows (left), and tors for rice palatability. NMR micro imaging is difficult to Mochiminori has narrow (middle) or undetectable hollows operate, and the NMR instrument is expensive. Light transmit- (right). The hollow depths, length of the minor axis, of the two tance photography can be easily used for detection of hollows cultivars were similar, but the widths, length of the major axis, and requires no special techniques or expensive machines (Suzu- were different (Fig. 2). ki et al., 2000). Horigane et al. (2000) demonstrated that the hol- The maximum hollow width/grain width ratio at the maxi- low volume of five cultivars varied with cooking. We noticed that mum hollow (hollow ratio) is illustrated in Fig. 2. The histogram Mochiminori rice grains had small hollows and Koshihikari rice of hollow ratios of two cultivars are plotted in Fig. 3. Although grains had large ones. In this paper, we studied how to distin- the two cultivars partly overlapped, Mochiminori and Koshi- guish Mochiminori and Koshihikari rice grains using light trans- hikari cooked rice grains were distinguishable using light trans- mittance photography as one application of hollow analysis. mittance photography. The criterion for determination was a In the rice market, several rice cultivars are sometimes blended hollow ratio of 2/3. This determination method was compared to modify the texture. Furthermore, it is widely known that palat- with the iodine dyeing method. The data comparing iodine dye- ability of non-glutinous rice can be improved by adding gluti- ing with hollow ratio observation are summarized in Table 1. The nous rice. Recently, a new regulation in Japan directed producers, agreement and the standard deviation were satisfactory for a non- wholesalers and retailers to indicate the composition of mixed destructive method. Hollow observation is useful for judging rice rice cultivars, harvest year, and areas. This act does not apply to grains such as Mochiminori or Koshihikari without using toxic processed rice, but with the expanding processed rice market, reagents and specific machines. many consumers will require more information about blending. One-grain analysis is adequate to identify the blending ratio of Materials and Methods glutinous and non-glutinous rice. For this purpose, the iodine-dye- Rice ( cv. Koshihikari and Mochiminori) grown ing method is easy, rapid and accurate, but toxic iodine must be safe- in Ibaraki prefecture was harvested in September 1999, and the ly post-treated. In this paper, we investigated light transmittance was stored at 4 to 6˚C before the experiment. The photography to distinguish Mochiminori and Koshihikari rice. grains were milled to about 90% before cooking. Water was added to form a 1:1.5 ratio of rice to water by weight. Ten grams E-mail: [email protected] of rice was presoaked for 30 min at room temperature (about Discrimination of Mochiminori and Koshihikari Rice 9

Fig. 1. Photographs of Koshihikari (non-glutinous, left) and Mochiminori (glutinous, middle and right). Arrows indicate hollows.

Fig. 3. Hollow ratio vs. frequency for Koshihikari (non-glutinous) and Mochiminori (glutinous) rice grain.

Table 1. Cooked rice grains were separated based on a hollow ratio (hol- low width/grain width) of 2/3. Separated grains were districted by iodine dyeing. Agreement Standard deviation ratio (%) (%) Hollow ratio over 2/3 Mochiminori grain 86.3 4.2 Hollow ratio under 2/3 Koshihikari grain 97.8 4.7

and grain width at the hollow (Fig. 1). The hollow ratio was then Fig. 2. Hollow ratio mean width of maximum hollow (A) / grain width at calculated as the hollow width/grain width, and the hollow ratios the hollow (B). were plotted for each (Fig. 3). Rice samples blending equal weights of Koshihikari to Mochiminori were cooked, then grains belonging to each group 23˚C) and then cooked in a cooker-steamer (RCK-5ET, Toshiba were separated by the criterion described in the Discussion. Sep- Co, Ltd., Tokyo) with a capacity of 0.54 l (Okadome et al., arated samples were washed with hot water to remove water sol- 1996). When the cook cycle was completed, the cooker automat- uble starch, and then inspected by iodine dyeing (2% potassium ically shifted to the “warm” setting. Cooked rice was then held iodide and 0.2% iodine in water). Mochiminori and Koshihikari for an additional 10 to 15 min. rice grains were stained reddish-violet and violet, respectively. Samples were held on a copy stand (Type 3, Asahi Opt. Co., Ten batches of rice were cooked, and 20 grains were sampled for Tokyo) and subjected to light transmittance. The light source, each batch. Results are summarized in Table 1. five 10 W fluorescent lights (Neoline white FL10W, Toshiba Co, Ltd., Tokyo), was set under the samples and covered by two 3 References mm thick translucent plastic plates to irradiate samples equally. Okabe, M. (1979). Texture measurement of cooked rice and its rela- 4 tionship to the eating quality. J. Texture Stud., 10, 131–152. Irradiation energy was 7.2¥10 lux at the sample table, measured by an illuminometer (Photocell Illuminometer SPI-5, Toshiba Okadome, H., Toyoshima, H. and Ohtsubo, K. (1996). Many-sided evaluation of physical properties of cooked rice grains with a single Co, Ltd., Tokyo). The camera was a Nikon F-2 with a 35 mm apparatus. J. Jpn. Soc. Food Sci. Technol., 43, 1004–1011. lens and an extension tube. Photographs of the cooked rice were Horigane, A.K., Engelaar, W.M.H.G., Toyoshima, H., Ono, H., Sakai, made on Provia ASA100 film (Fuji Photo Films Co. Ltd., Tokyo) M., Okubo, A. and Nagata, T. (2000). Differences in hollow vol- and converted to digital data by an image scanner (Seiko Epson umes in cooked grains of rice cultivars with various amylose con- tents determined by MRI micro imaging. J. Food Sci., 65, 408–412. Corporation, GT-9500, Tokyo). Digital photographs were image- Horigane, A.K., Toyoshima, H., Hemmi, H., Engelaar, W.M.H.G., processed by computer software (Adobe Systems Inc., Adobe Okubo, A. and Nagata, T. (1999). Internal hollows in cooked rice Photoshop 5.0, San Jose, CA) to provide high contrast images. grains (Oryza sativa cv. Koshihikari) observed by NMR micro Two hundred cooked rice grains of both cultivars were mea- imaging. J. Food Sci., 64, 1–5. sured. Ten batches of rice were cooked, and 20 grains were sam- Suzuki, M., Horigane, A.K., Toyoshima, H., Yan, X., Okadome, H. and Nagata, T. (2000). Detection of internal hollows in cooked rice pled from each batch. Unbroken grain samples were photo- using a light transmittance method. J. Food Sci., 64, 1027–1028. graphed and measured to determine the maximum hollow width