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

Abstract

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. © 2013 Wageningen Academic Publishers.