Oxygen Dependent Browning of Soy Sauce

It is well known that soy sauce darkens during storage in contact with atmospheric oxygen. Kato (30) suggested that 3-deoxyglucosone (3-DG) was an active precursor in a sugar-amine browning reaction, and Okuhara et al. (31) reported the participation of peptides in the browing of soy sauce. Motai and Inouue (32) isolated melanoidin from soy sauce and examined its oxidative polymerization and physicochemical properties. Although caramel is used as an additive to adjust the color of soy sauce products, the most preferable color of soy sauce is red-brown soon after pasteurization. The browning of soy sauce is considered to be due to the amino compounds, peptides, sugars, and intermediates. There is little browning of soy sauce without oxygen, but that browning is very significant (about 10 times more) in the presence of oxygen (33).

Amadori compounds composed of a neutral amino acid and glucose have been isolated and characterized from soy sauce. These Amadori compounds were demonstrated to be important in oxidative browning of soy sauce. Oxygen decomposes Amadori compounds to produce glucosome and liberate amino acid. Dicarbonyl compounds derived from these Amadori compounds may polymerize to form melatonin. The browning of all the Amadori compounds increased very significantly when oxygen and iron were present. Okuhara et al. (34) suggested the significant positive correlation between initial color [amount of melanoidin] and oxidative browning of soy sauce. Hashiba (35) reported the polymerization and the change of color tone of melanoidin on oxidation. A high proportion of the iron in soy sauce was suggested to be present in high molecular state and could not be removed by dialysis. A large part of iron is bound to melanoidin in soy sauce. Moromi samples of traditional Thai soy sauce were used to investigate the browning and chemical changes related to the Maillard reaction. 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 results suggested that the browning is enhanced with the proper size of proteins and peptides (36). Table 5 lists the chemical reactions that occur during manufacture of fermented soy sauce.

Table 5 Chemical Reactions Occurring During Manufacture of Fermented Soy Sauce

Reactions

Steps

Raw material

Function

Denaturation

Steaming

Soy protein

Increase digestivity

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