At the start of fermentation, a low-salt solution is added. Alcohol and a large amount of yeast are added to replace salt to prevent mold and inhibit spoilage. Condiments like soy sauce, being liquid, are relatively easy to reduce their salt content using various methods.
Electrodialysis:
This method uses an ion-exchange membrane for desalination. After 3 hours of desalination, the desalination rate is 70.9%, with an amino acid loss of 17.9%, and other components also lost.
Selective Dialysis Membrane (PVA Hollow Core Membrane Desalination):
500 ml of soy sauce is passed through the membrane at a rate of 12 liters over 1 hour. Simultaneously, 500 ml of distilled water is passed through the membrane at a rate of 4 liters over 1 hour. As dialysis time increases, water mixes into the soy sauce. 150 minutes of dialysis can increase the volume by approximately 40%, achieving a desalination rate of 25%, while amino acid loss is approximately 10%, and lactic acid and acetic acid losses are 6% and 18%, respectively.
Desalting by Vacuum Concentration
Soy sauce is heated to approximately 80°C. Under reduced pressure, volatile alcohols and acetic acid evaporate, causing salt to precipitate. In a laboratory setting, this salt can be removed while still hot using a suction filter. After desalting, the highly concentrated liquid has a total nitrogen content of 4.62% and a salt content of 22.9%, becoming a viscous substance. Compared to the composition before concentration, salt content has increased to 7.6%, lactic acid remains almost unchanged, and acetic acid has decreased significantly. One method to replenish these losses is to adjust the concentration by mixing volatile components (distillate).
Adding Amino Acid Solution
High-concentration amino acid solution contains approximately 3% total ammonia and 20% salt. If the salt content is diluted to 9.2%, the total nitrogen is approximately 1.37%. If this diluted amino acid solution (40%) is mixed with raw soy sauce (1.55% total nitrogen, 17.5% salt) and fermented for one month, a new type of high-grade low-sodium soy sauce with a salt content of 14.2% and a total nitrogen of 1.47% can be produced.
Using Re-fermentation:
When producing low-sodium soy sauce using the re-fermentation method, since the total nitrogen of the re-fermented soy sauce is 1.4%, by adding 1200 liters of 19 Baume fermentation brine to a koji (made by blending defatted soybeans, wheat, and wheat bran in 55%, 35%, and 10% proportions respectively) for re-fermentation, the total nitrogen can reach over 2.5%. Therefore, simply diluting the concentrated soy sauce obtained from the re-fermentation with water to a certain concentration yields low-sodium soy sauce.
Low-salt brewing method:
In the past, the concentration of brine in soy sauce brewing could not be lower than 15% within one month after fermentation. This was because soy sauce brewing is an open fermentation process, during which harmful microorganisms could be introduced. Therefore, a higher concentration of brine was added to inhibit the growth of harmful bacteria. With the advancement of technology, by adding alcohol and a large amount of yeast, the growth of harmful microorganisms can be inhibited even when using brine concentrations below 15%.
