2.3. Samples and the pretreatment
Totally 13 kinds of soy sauces were purchased from local markets,
10 of which are labeled with total nitrogen and amino acid
nitrogen contents higher than 1.2 g/100 mL and 0.48 g/100 mL,
respectively. Among the 10 soy sauces with nitrogen content
labels, 8 are from trustful makers and were claimed as pure fermented.
The remaining three are cheap and without information
regarding fermentation and nitrogen contents. The samples were
kept at 4 C after the first use.
Before the measurements, sample solutions were deproteinized
either by salting out or by alcohol precipitation. For the salting out
process, 10 mL of sample solution was added with 3.5 g of NaCl
and heated at 80 C for at least 3 h. The solution was filtered
through 0.22 lm membrane filter (MCE membrane, Merck Millipore,
USA) to remove the precipitation. For alcohol precipitation,
1 mL of sample solution was added with 4 mL of absolute alcohol
and then centrifuged (4000 rpm 5 min at room temperature).
The supernatant was filtered through the aforementioned
0.22 lm MCE membrane, and the filtrate was served as the deproteinized
sample solution.