Roles of the maillard reaction in browning during moromi process of thai soy sauce

Moromi samples of traditional Thai soy sauce from two conventional manufacturers A and B were used to investigate the browning and chemical changes related to the Maillard reaction (i.e. reactive amino compound, soluble protein, 5% trichloroacetic acid precipitated protein, reducing sugar, and 5-hydroxymethyl-2-furaldehyde). The samples were collected at specific intervals during the 60-day process of moromi fermentation. The browning (OD 420 nm; y) was expressed as powered regression with fermentation time-course (day; x), y=0.5191 * x^0.2367, r²=0.978 and y=0.6188 * x^0.1596, r²=0.821 for moromi A and B, respectively. The browning rate was high at the first 3 days and declined in the later stage, while the accumulation of 5-hydroxy-2-furaldehyde, which is the Maillard reaction intermediate, increased linearly. The size of proteins and peptides observed on sodium dodecyl sulphate-polyacrylamide gel electrophoresis and the measurement of the ratio of 5% trichloroacetic acid precipitated protein to total soluble protein (PP/SP). This PP/SP ratio showed the linear correlation to browning rate with r²=0.921 and 0.965 for moromi A and B, respectively. The results suggested that the rate of browning of the moromi fermentation did not entirely depend upon the Maillard reaction rate. It appeared that the browning is enhanced with the proper size of proteins and peptides.