Determination of γ-Polyglutamic Acid in "Natto" Using Cetyltrimethylammonium Bromide

Studies on "Natto" Part V

Abstract

Asimple and rapid spectrophotometric method has been established for determination of γ- polyglutamic acid (γ-PGA) which is the sticky material in fermentated soybean food "natto". The method is based on the complex formation between γ-PGA and cetyltrimethylammonium bromide (CET). The complex could be determined by the measurement of absorbance at 400nm. The linear relationship was obtained between absorbance and γ-PGA concentration in the range from 0 to 54.85μg per 6ml of a reaction mixture. The complex formation with CET was not observed on glutamic acid, glutamylglutamic acid, levan and pectin. When γ-PGA prepared from "natto" was decomposed by γ-glutamyldepolymerase induced by Bacillus natto bacteriophage infection. viscosity of the reaction mixture decreased rapidly, while the absorbance of the complex with CET was not decreased. Therefore, it was confirmed that values obtained by the method was not affected by the molecular weight of γ-PGA. We demonstrated the formation of γ-PGA during manufacturing process of "natto" and its content of the commercial products by the method. γ-PGA was detected at 10h from the beginning of the fermentation and then increased. The mean value of the γ-PGA content and standard deviation in 34 commercial "natto" were 328mg±120.99 per 100g on wet basis.