Metachromasia of Acid Red by Binding with Soybean Proteins at Acidic pH Regions

Abstract

Binding of Acid Red (an artificial food color) with soybean proteins was observed by changing pH from 6.8 to 1.0. The absorption maximum of Acid Red (at 566 nm in aqueous solution) showed a “red shift” by binding with soybean water-extracted protein and with acid-precipitated protein. The “red shift” was increased with decreasing pH from 6.8 to 2.2. Owing to acid-induced conformation changes in the soybean proteins, binding of Acid Red with the proteins was accelerated by decreasing pH and it became the greatest at pH 2.2. Difference spectra of Acid Red caused by binding with the proteins varied greatly with a decrease in pH from 3.8 to 2.2. However, no marked changes in “red shift” difference spectra and binding of Acid Red with the soybean proteins were observed at pH 1.0_??_2.2.