Validation of the dye colour chart method for pH determination of rice grains

Main Article Content

R.J.-C. Hsu
S. Lu


dye-colour chart method, freshness of rice kernels, rice kernels, rice pH, validation of pH


During storage of rice, acidity is gradually generated in the rice grains. This increase in acidity results from an accumulation of compounds including tricarboxylic acid cycle metabolites, amino acids, and free fatty acids, and is used as an index for the length of storage or the freshness of rice grains. The acidity can be conventionally determined and expressed using a pH meter. A dye colour-chart method has been developed and is currently used to determine the pH of rice grains by using a bromothymol blue-methyl red dye solution and a pH colour chart. The method is low cost and convenient. To validate the repeatability and reproducibility of the dye colour-based method in measuring the pH of rice grains, an international ring test including 14 laboratories was performed. The high consistency and low standard deviation of repeatability and reproducibility showed that this method was acceptable and worthy to promote. The results show that the dye colour chart method is conditionally acceptable and highly comparable to the conventional pH meter method. In addition, training in this method is suggested for reducing variation.

Abstract 24 | PDF Downloads 62


Chang, Y.J., Wang, C.Y. and Liu, A.N., 2000. Application of rice freshness analysis for rice quality control. Agriculture Policy and Review 92: 69-71.
Dhaliwal, Y.S., Sekhon, K.S. and Nagi, H.P.S., 1991. Enzymatic activities and rheological properties of stored rice. Cereal Chemistry 68: 18-21.
Grant, E.L. and Leavenworth, R.S., 1980. Statistical quality control (5th Ed.). McGraw Hill Book Company, New York, NY., USA, 81 pp.
International Organization for Standardization (ISO), 1994. Accuracy (trueness and precision) of measurement methods and results - Part 2: Best method for the determination of repeatability of a standard measurement method. ISO 5725-2:1994. ISO, Geneva, Switzerland, 42 pp.
Kumagai, C., Hagiwara, Y., Yamamoto, T. and Akiyama, H., 1978. Detecting old rice. Journal of the Brewing Society of Japan 73: 773-736.
Lii, L.J., Wang, C.Y. and Lur, H.S., 1999. A novel means of analyzing the soluble acidity of rice grains. Crop Science 39: 1160-1164.