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A Novel Means of Analyzing the Soluble Acidity of Rice Grains

Dep. of Agronomy, National Taiwan Univ., Taipei, Taiwan 107;
Dep. of Food, Taiwan Province, Republic of China.

^* Corresponding author (lurhs{at}ms.cc.edu.tw).

Soluble acidity has been used as one of major indexes to reflect the changes of rice (Oryza sativa L.) grain quality during storage. The acidity is in general visually examined by pH dye indicators or directly measured by a pH meter. By combining the characteristics of pH dye indicator, 96-well microplate, and microplate reader, we have developed a procedure capable of effectively measuring the pH value of rice grains at a single-grain level. First, from a pH-dye system prepared with methyl red and bromothymol blue, we scanned the spectral response to pH and determined that a wavelength of 630 nm showed the maximum response to pH changes with storage age of rice grains. By means of a 96-well microplate and plate reader, a pH standard curve was then constructed with a series of pH standard buffers and absorbance at 630 nm. The pH value of a single grain in a microplate well can be measured by incubating the grain with dye solution and reading absorbance. The proposed procedure offers an efficient means of measuring the change of soluble acidity in rice grains during storage on an individual-grain basis. The method may also rapidly detect the uniformity or blending status of fresh/old grains in a sample.

Received for publication June 14, 1998.